Symposium Organizers
R. Joseph Kline National Institute of Standards and Technology
Iain McCulloch Imperial College London
Garry Rumbles National Renewable Energy Laboratory
Alberto Salleo Stanford University
E1: Organic Light Emitting Diodes
Session Chairs
Monday PM, November 29, 2010
Room 312 (Hynes)
9:30 AM - **E1.1
Highly Efficient White OLEDs Based on Phosphorescent and Fluorescent Blue Emitters.
Bjoern Lussem 1 , Thomas Rosenow 1 , Sebastian Reineke 1 , Karl Leo 1
1 Institut für Angewandte Photophysik, TU Dresden, Dresden Germany
Show AbstractDuring the last decades, organic light emitting diodes (OLEDs) have experienced a tremendous increase in efficiency and stability. Nowadays, first OLED products such as displays for mobile phones, TVs and first luminaries are commercially available. In particular for lighting applications, OLEDs can take advantage of their outstanding properties such as high power efficiency, good color quality and new design possibilities such as illumination by flat light sources.\nTo obtain highly efficient white OLEDs, it is essential that conversion between injected charge carriers and generated photons is almost perfect, i.e. that an internal quantum efficiency of almost 100% is reached. In this contribution, new results on two approaches that have the potential to reach such a high internal quantum efficiency are presented: the all-phosphorescent concept and the triplet-harvesting approach.In the all-phosphorescent concept, white light is generated by combining phosphorescent red, blue, and green emitters. By optimizing the organic layer stack and applying different techniques to enhance the light outcoupling from the OLED, we have recently reached a luminous efficacy of 90lm/W [1]. This result demonstrates the potential of this approach, but a drawback of the all-phosphorescent approach is the instability of the blue phosphorescent emitter. These concerns can be overcome by the triplet-harvesting approach: It allows to replace the phosphorescent blue emitter with a fluorescent one [2,3]. When the triplet energy of the blue emitter is located above the triplet level of the red emitter, triplets formed on the fluorescent blue layer can diffuse to the red emitter and can be harvested there. Thus, in principle, all excitons can be utilized and an internal quantum efficiency of 100% is possible – leading to a high luminous efficacy. In this contribution it will be shown that a luminous efficacy of 47 lm/W using a flat outcoupling enhancement structure and of up to 90 lm/W using a large hemisphere to couple out all generated light can be obtained.[1] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, and K. Leo, Nature 459, 234 (2009).[2] G. Schwartz, S. Reineke, T.C. Rosenow, K. Walzer, K. Leo, Adv. Funct. Mat. 19, p.1319, 2009.[3] T.C. Rosenow, M. Furno, S. Reineke, S. Olthof, B. Lüssem, K. Leo, submitted.
10:00 AM - E1.2
Construction of 3D Nanostructures in Hybrid Light Emitting Devices.
Eugenia Martinez-Ferrero 1 , Amparo Forneli 1 , Clement Sanchez 2 , Emilio Palomares 1
1 , Institut Catala d'Investigacio Quimica, Tarragona Spain, 2 Laboratoire de Chimie de la Matiere Condensee, Universite Pierre et Marie Curie, Paris France
Show AbstractHybrid Light Emitting Devices (HyLEDs) are the product of a recent research in which inorganic charge carrier transporters are combined with organic lumophores, which also participates in the hole transport. The recombination of electron and holes at the hybrid interface results in the emission of light that can be tuned depending on the polymer properties. This recent technology allows the construction of devices that present competitive luminance and efficiency values compared to the well-known polymeric organic LED technology. The resulting devices are cheap, environmentally friendly and stable to air and oxygen moisture under operation, without the need for encapsulation making them very interesting for technological applications. However, most of the research in this new area has been focused on the design, construction and characterization of the hole injecting and the electroluminescent layers. Improvement on the electron injection layers has just been focused on the test of different metal oxides as thin films or the use of Cs derivatives. In this communication, we present the result of our investigations on the inorganic electron injection layer. We have studied its morphology dependence with the preparation methods and the related performance of the devices. Moreover, we have also designed and prepared new 3D architectures and combinations of transition metal oxides in order to improve the energetic level correspondences, the interfacial organic-inorganic area and the electron injection, which is positively reflected in the luminance and efficiency values of the resulting devices.
10:15 AM - E1.3
Improved Lifetime of Liquid Desiccant for Encapsulation of Large-sized Active Matrix Organic Light Emitting Diodes.
Byoungduk Lee 1 , Yoon-Hyung Cho 1 , Minho Oh 1 , Yong Tak Kim 1 , So Young Lee 1 , Yun Ah Chung 1 , Seung Yong Song 1 , Jong Hyuk Lee 1 , Sung Chul Kim 1
1 OLED Precede Technology Team, Samsung Mobile Display, Yongin-city, Gyeonggi-Do, Korea (the Republic of)
Show AbstractFor small-sized active matrix organic light emitting diode (AMOLED) devices, edge sealing encapsulation with inorganic frit material heated locally by laser is enough to fabricate reliable panels. However, for large-sized AMOLED devices, edge sealing with the frit has serious problems such as detachment from glasses and mechanical strength under external stress. In order to prevent those drawbacks, new sealing technique, epoxy and liquid desiccant was developed. The liquid desiccant was prepared by mixing nano-sized calcium oxide powders and silicone binder including polyalkylalkenylsiloxane, polyalkylhydrogensiloxane and platinum compound. The calcium tests (transmittance of the calcium layer is changed by penetrating moisture and oxygen.) were carried out. After storing calcium layer sealed with epoxy sealant and liquid desiccant for 720 hrs at 85 oC and 85 % R.H, the transmittance increased abruptly. Also, the test cells encapsulated with only epoxy sealants printed at the edge of the cell developed dark spots within 120 hrs, which grew larger with time at 85 oC and 85 % R.H. On the other hand, the test cell sealed with epoxy sealant and liquid desiccant showed no dark spots and retained 97% of its initial luminance even after being stored for 800 hr at 85 oC and 85 % R.H. Furthermore, the accelerating storage lifetimes of 31-inch bottom-emitting AMOLED with epoxy sealant and liquid desiccant showed about 1000hrs. These results suggest that the liquid desiccant can be applied to encapsulation of large-sized AMOLED TV.
10:30 AM - E1.4
Ultra-broad Optical Gain, Two-colour Amplified Spontaneous Emission and White Electroluminescence in Binary Blends of Insulated Molecular Wires.
Sergio Brovelli 1 2 , Marta Mroz 3 , Giuseppe Sforazzini 5 , Tersilla Virgili 3 , Alberto Paleari 6 , Francesco Meinardi 6 , Harry Anderson 5 , Guglielmo Lanzani 4 , Franco Cacialli 2
1 Chemistry and Applied Spectroscopy, Los Alamos National Laboratory, Los Alamos , New Mexico, United States, 2 London Centre for Nanotechnology, and Department of Physics and Astronomy, University College London, London United Kingdom, 3 IFN-CNR c/o Politecnico di Milano, Dipartimento di Fisica, Politecnico di Milano, Milano Italy, 5 Department of Chemistry, University of Oxford, Chemistry Research Laboratory, University of Oxford , Oxford United Kingdom, 6 Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, Milano Italy, 4 , Center for Nanoscience and Technology of IIT @ POLIMI, Milano Italy
Show AbstractThe ability to produce polymeric materials with broad optical gain spectra and large stimulated emission cross sections is fundamental to their use as active gain media in all-plastic lasers,1 optical switches2 and broadband amplifiers.3 Supramolecular encapsulation of conjugated polymers4-7 is an effective means of suppressing intermolecular interactions, which leads to enhanced luminescence quantum yields, blue-shifted emission, and to suppression of long-lived excited states. Furthermore, energy transfer (ET) in a blend of semiconducting polymers can be strongly reduced by non-covalent encapsulation of one constituent, ensured by threading of the conjugated strands into functionalized cyclodextrins. Such macrocycles control the minimum intermolecular distance of chromophores with similar alignment, at the nanoscale, and therefore the relevant ET rates, thus enabling fabrication of white-light-emitting diodes (CIE coordinates: x=0.282, y=0.336).5 Here, we show that all such properties of conjugated polyrotaxanes combine into a high impact photonic application: ultra-broad band optical gain in a binary polymer blend that can be further exploited for two-colour lasing.8 We study the ultrafast optical properties of a binary blend of a green-emitting polyfluorene copolymer and a polyrotaxane consisting of a polyfluorene derivative threaded into functionalized β-cyclodextrin macrocycles, by means of femtosecond transient absorption experiments and amplified spontaneous emission (ASE) measurements. The morphology of the blend is investigated by microRaman imaging, atomic force microscopy, fluorescence mapping, and fluorescence lifetime microscopy. We ascribe the ultra-broad optical gain spectrum (>850 meV), and the concomitant ASE for both blends constituents, to the dual effect of reduced polaron formation and suppressed energy transfer in our polyrotaxanes-based blends. The composite material investigated here represents only the first proof of principle of this strategy and we expect that the optical properties can be improved further by blending encapsulated compounds only. These polymeric composite materials have highly controlled optical properties and could realistically lead to disrupting technologies such as single broadband optical amplifiers covering the entire visible region, to be applied for data transmission by wavelength division multiplexing in plastic optical fibres.REFERENCES1 Laquai, F. et al. Advanced Functional Materials 18, 3265-3275, (2008).2 Vishnubhatla, K. C. et al. Applied Physics Letters 94, 041123, (2009).3 Virgili, T. et al. Physical Review Letters 94, 117402, (2005).4 Brovelli, S. et al. Nano Letters 8, 4546-4551, (2008).5 Brovelli, S. et al. Advanced Functional Materials 20, 272-280, (2010).6 Frampton, M. J. et al. Advanced Functional Materials 18, 3367–3376, (2008).7 Petrozza, A. et al. Advanced Materials 20, 3218-3223, (2008).8 Brovelli, S. et al. Advanced Materials in press, (2010).
10:45 AM - E1.5
A Unifying Model for the Operation of Light-emitting Electrochemical Cells.
Stephan van Reenen 1 , Piotr Matyba 2 , Andrzej Dzwilewski 1 , Rene A. J. Janssen 1 , Ludvig Edman 2 , Martijn Kemerink 1
1 Applied Physics, capaciteitsgroep: Molecular materials and nanosystems, Technische Universiteit Eindhoven, Eindhoven, Noord-Brabant, Netherlands, 2 The organic photonics and electronics group, department of physics, Umeå University, Umeå Sweden
Show AbstractLight-emitting electrochemical cells (LECs) differ from conventional OLEDs by the presence of mobile ions in the organic layer. These ions enable dynamic doping, significantly enhancing carrier injection and facilitating relatively large current densities. Mitigation of detrimental device properties like short life-times and long turn-on times requires an unambiguous understanding of the LEC device physics. The mechanism and effects of doping in LECs are, however, still far from being fully understood, as evidenced by the existence of two hotly debated, competing models that seem physically distinct: the ‘electrochemical doping model’ (ECD) and the ‘electrodynamic model’ (ED).By combining various experimental techniques (scanning Kelvin probe microscopy, doping and light emission profile detection) with numerical modeling and an analytical study, two operating regimes in LECs were identified, one ECD-like and one ED-like. Their prevalence depends on the ability of the device to form non-injection-limited ohmic contacts. In case such ohmic contacts are formed, the LEC follows the electrochemical doping model, characterized by the formation of a dynamic p-n junction in the bulk of the device. Anions and cations then become fully spatially separated across the junction, forming electric double layers at the contacts and doped regions in the bulk. In the case injection of electronic charge carriers is limited, doping becomes less pronounced and due to ion redistribution the electric double layers increase until the bulk is screened from the external electric field. In this injection-limited regime, the device strikingly follows the other competing model, the electrodynamic model, and the electronic current is dominated by diffusion. In contrast, in the ohmic injection regime drift and diffusion were found to contribute equally. Additionally, characteristic current transients were obtained and studied by calculations and experiments for both operation modes.These results imply that the electrochemical doping operation mode, i.e. without contact limitations, is the preferred operational mode for LECs, as it gives the highest current densities and the highest electron-hole recombination rates. They also imply that any degradation in the contact area, either by electrochemical side reactions or by contact oxidation may cause a transition to the electrodynamic operation mode and hence a reduction in current and light output. The often stated independence of LEC operation on contact material should thus be reconsidered. Finally, our results show that the operational mode of an LEC-type device may be concluded from the shape of the current transient, and does not per se require elaborate SKPM experiments.
11:00 AM - E1: OLEDs
BREAK
11:30 AM - **E1.6
Roll-to-Roll Wet-Coated OLEDs for Lighting Applications.
Jie Liu 1
1 , GE Global Research, Niskayuna, New York, United States
Show AbstractIn this presentation, we will first review attributes such as efficacy and cost of OLEDs necessary for lighting applications. Then some general design rules and considerations for improving OLED technology will be discussed. Finally we will present latest results and progress made on addressing each attributes.
12:00 PM - E1.7
High Reverse Intersystem Crossing Efficiency Using Exciplex Formation.
Kenichi Goushi 1 , Kou Yoshida 2 , Keigo Sato 2 , Chihaya Adachi 1 2
1 Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Fukuoka Japan, 2 Center for Future Chemistry, Kyushu University, Fukuoka Japan
Show AbstractOrganic light-emitting diodes (OLEDs) can open the door to new optoelectronics including flat-panel displays and lighting applications. To enhance electroluminescence (EL) efficiency of OLEDs, various emission materials based onto fluorescence and phosphorescence have been widely developed. Although OLEDs using fluorescent materials have achieved a high reliability, the internal EL quantum efficiencies are limited to approximately 25 % under electrical excitation due to the exciton branching ratio of singlet excited states. In contrast, OLEDs using phosphorescent materials have achieved the internal EL quantum efficiency of almost 100%. However, the selection of practically useful phosphorescent materials has been limited to Ir and Pt complexes. Therefore, since both fluorescence and phosphorescence used OLEDs have advantages and disadvantages, respectively, the use of a novel light-emitting mechanism has been expected to avoid the shortcomings.Recently, we proposed potential mechanism to enable the internal EL quantum efficiency of 100% without phosphorescent materials by using up conversion from triplet to singlet excited states.1) To realize the highly reverse intersystem crossing efficiency, the small energy gap between singlet and triplet excited states is required. It can be achieved by separating the highest occupied molecular orbital (HOMO) from the lowest unoccupied molecular orbital (LUMO). In this study, we demonstrate that radiative-exciton production efficiency exceed the fluorescent limitation of 25% by using high reverse intersystem crossing efficiency of exciplex states.We investigated the transient photoluminescence (PL) decay curves of 50mol% 4,4’,4”-tris[3-methylphenyl(phenyl)amino] triphenylamine (m-MTDATA) doped into 2-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) films. In addition, we measured the current – voltage – luminescence characteristics of OLEDs, consisted of ITO / m-MTDATA / 50mol% m-MTDATA doped into PBD / PBD / LiF / Al.The PL peak wavelength of a m-MTDATA:PBD composite film located at around 540 nm, which is significantly red-shifted compared to those of the m-MTDATA and PBD neat films, respectively. This is due to the exciplex formation at the m-MTDATA/PBD interface. We obtained the EL quantum efficiency of 2 %, while the PL quantum efficiency was 20 %. These results suggest that the radiative-exciton production efficiency of 50 % is realized due to the high reverse intersystem crossing efficiency of the exciplex states.Reference1.A. Endo, M. Ogasawara, A. Takahashi, D. Yokoyama, Y. Kato, and C. Adachi, Adv. Mater. 4802- 4806, 21 ( 2009).
12:15 PM - E1.8
Highly Stable ``Jumbo'' Quantum Dots.
Xiaofan Ren 1 , Keith Kahen 1 , John Minter 1
1 , Eastman Kodak Company, Rochester, New York, United States
Show AbstractFor solid-state lighting applications, the fastest route to high-efficiency white LEDs is to combine blue or violet LEDs with appropriate phosphors. Quantum dot (QD) phosphors have many advantages, such as greatly reduced scattering, ease of color tuning, improved color rendering index (CRI), an inexpensive deposition process and a broader absorption spectrum. Despite these advantages, quantum dot phosphors have not been introduced into the marketplace due to some major shortcomings such as poor temperature stability. In addition, due to their high surface-volume ratio, QDs are sensitive to surface conditions. As such, this effect would be mitigated through the use of larger sized QDs. We report here the synthesis and characterization of highly luminescent ternary ZnxCd1-xSe QDs with sizes in the range of 15–20 nm. QDs with similar sizes have been reported previously where the synthesis took five days, and the resulting dots suffered from wide size distributions and low photoluminescence (PL) efficiencies. In contrast, using the synthetic method developed here, it only took two hours to reach the desired size. The PL quantum yields of the synthesized QDs are over 40%, with the FWHM of the emission spectra being as narrow as 20–25 nm. The most striking feature of these large-sized QDs is their unusual stability at high temperatures: the emission intensity loss, relative to that at room temperature, was 19% at 100 C, and 28% at 150 C, while the emission color red shifted only a few nanometers over the entire temperature range. These features can be accounted for by the compositional profile of the QDs, as detailed by STEM and EELS images. Moreover, these large-sized QDs are unusually robust, as demonstrated by high-temperature (270 C) annealing experiments performed on films consisting of ligand-free dots (emission intensity measured before and after). We believe that this new type of QD will find many applications in electronics and photonics.
12:30 PM - E1.9
Both Luminous Efficiency and Lifetime Enhancement in Blue Fluorescence OLED Devices by Modifying Molecular Structure of Hole Transporting Materials.
Yoonhyun Kwak 1 , Hyein Jeong 1 , Daeyup Shin 1 , SunYoung Lee 1 , Seok-Hwan Hwang 1 , Young-Woo Song 1 , JongHyuk Lee 1 , Sungchul Kim 1
1 , Samsung SMD, Yongin-City Korea (the Republic of)
Show AbstractBoth luminous efficiency and lifetime in blue fluorescent OLED device have been improved by modified HTMs with higher LUMO energy levels. The LUMO energy levels of HTM were increased by modifying substituent in HTM molecules. Two HTMs with ortho and meta biphenyl substituent and one HTM with thiophenyl substituent were synthesized via palladium catalyzed amine coupling reactions to compare with a para biphenyl substituent HTM as a standard molecule. According to TDDFT calculations, these three modified HTMs showed 0.05-0.1 eV higher LUMO energy levels compared to the para biphenyl substituent HTM. The luminous efficiency and the lifetime (LT90) of OLEDs using modified HTMs at 500 cd/m2 have been enhanced up to 20% and 52%, respectively, compared to the device using standard HTM.
12:45 PM - E1.10
Interfacial Dipole-assisted Charge Generation in Transition Metal Oxide Based Interconnecting Units of Stacked OLEDs.
Sami Hamwi 1 , Jens Meyer 2 , Michael Kroeger 4 , Freya Gnam 4 , Thomas Winkler 1 , Thomas Riedl 3 , Antoine Kahn 2 , Wolfgang Kowalsky 1
1 Institute for High-Frequency Technology, Technical University of Braunschweig, Braunschweig Germany, 2 Department of Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 4 , InnovationLab GmbH, Heidelberg Germany, 3 Institute of Electronic Devices, University of Wuppertal, Wuppertal Germany
Show AbstractThe concept of stacking several organic light emitting diodes (OLEDs) on top of each other enables longer lifetimes of these devices, since each light emitting unit is stressed significantly less to achieve a given luminance level. Interconnecting units that serve as charge generation layers (CGL) are required when driving OLED stacks as two-terminal devices. Various concepts for CGL structures have been published so far. Besides junctions between electrochemically p- and n-doped charge transport layers, the insertion of transition metal oxides (TMOs) like WO3 or MoO3 into the interconnecting units has been shown as an attractive concept. It reduces the complexity of electrochemical doping, since due to the TMO p-type doping seems not to be necessary at the CGL. The reason for this can be derived from the charge generation mechanism in TMO based CGLs which differs from that of p-n junctions.First, we will demonstrate an in depth analysis on the minimum thickness of the CGL components required in p-n structures using CBP based complementarily doped homojunctions. As a result, we will show that the charge generation mechanism in organic p-n homojunctions relies on the formation of a space charge region due to the Fermi level alignment, Accordingly, the depletion width depends on the built-in voltage and charge carrier densities of the doped layers and determines their minimum thicknesses for an efficient electric field-assisted charge generation in stacked OLEDs. In the second step, we will demonstrate via electro-optical measurements on stacked OLED devices combined with Kelvin probe analysis and photoelectron spectroscopy on the separate CGL components that the charge generation in TMO based interconnecting units, consisting of BPhen:Cs2CO3 (16 wt%)/WO3/TCTA occurs at the heterointerface between the TMO and the adjacent non-doped hole transporting layer. Instead of the formation of a space charge region observed in the p-n junctions, an interfacial dipole between WO3 and TCTA arises, leading to a small energetic difference between the conduction band minimum of WO3 and the HOMO of TCTA of only 0.8 eV which allows for an efficient charge generation in stacked OLEDs. Consequently, the n-type doped electron transport layer (BPhen:Cs2CO3) is only used to facilitate the electron injection from the TMO into the adjacent light emitting unit.
E2: Organic Photovoltaics: Device Physics
Session Chairs
Garry Rumbles
Henry Snaith
Monday PM, November 29, 2010
Room 312 (Hynes)
2:30 PM - E2.1
Exciton-polariton Dynamics in Fabry-Pérot Microcavities of J-Aggregates.
Benoit Gosselin 1 , Jelissa De Jonghe 1 , Carine Berteli-Cardoso 1 , Paul-Ludovic Karsenti 1 , Richard Leonelli 1 , Carlos Silva 1
1 Department of Physics, Université de Montréal, Montréal, Quebec, Canada
Show AbstractExciton-polaritons are quasi-particles that result from strong (non-perturbative) photon-exciton coupling in optical microcavities. These can be viewed as superpositions of excitons in the active semiconductor layer and cavity photons in the photonic device. Detailed investigation of their dynamics is of fundamental importance because polariton devices are a test-bed for understanding of light-matter spontaneous coherence, and as composite bosons, they are compelling candidates for solid-state Bose-Einstein condensation. Furthermore, these devices promise applications in low-threshold lasers and light-emitting diodes. In this talk, we investigate dynamics of a polariton gas in Fabry-Pérot microcavities (λ/2 active medium thickness) where a film of double-walled tubular J-aggregates of the amphiphilic cyanine dye 3,30-bis(2-sulphopropyl)-5,50,6,60-tetrachloro-1,10-dioctylbenz-imidacarbocyanine (C8S3), sandwiched between distributed Bragg reflectors fabricated with twenty-four alternating layers of Ta2O5 and SiO2. These organic semiconductor nanostructures exhibit sharp optical transitions assigned to strongly-bound Frenkel excitons. Atomic-force-microscopy measurements demonstrate that the tubular architectures persist over lengthscales of up to several micrometers. The exciton resonance is centred at 2.175 eV (570 nm), slightly detuned from the cavity resonance at normal incidence (2.067 eV, 600 nm). Angle-resolved reflectivity and photoluminescence (PL) measurements reveal the presence of strongly-coupled exciton polaritons. To probe the evolution of photon-exciton coherence, we will present temperature-dependent, ultrafast PL and transient absorption measurements with <50 fs time resolution to compare exciton-polariton dynamics with those of incoherent excitons in thin films of the aggregate outside of the microcavities. We will also probe microcavity exciton-polariton mediated resonance Raman scattering to explore phonon-mediated polariton scattering. With these combined optical probes we address the dynamics of coherence dissipation, and the possibility of achieving Bose-Einstein condensation in organic semiconductor microcavities.
2:45 PM - E2.2
Dynamics of Charge Transfer Excitons Recombination in Polymer/Fullerene Solar Cells.
Markus Hallermann 1 , Josef Berger 1 , Enrico Da Como 1
1 Department of Physics, LMU, Munich, Munich Germany
Show AbstractElectron-hole recombination represents a severe loss mechanism for organic bulk-heterojunction solar cells. Besides the recombination in the form of tightly bound Frenkel excitons, before those reach the donor-acceptor interface for charge transfer, recombination at the heterojunction interface can occur, impacting the majority of photocarriers. This type of recombination exhibits photoluminescence via charge transfer excitons (CTEs), which are electrons and holes bound at the interface by the mutual Coulomb potential [1]. Because of the low energetic position of this recombination near infrared (NIR) photoluminescence spectroscopy can be used to give unique insights into the nature of the CTE and its correlation with solar cell photocurrent [2, 3]. To understand and limit this loss channel a direct access to the dynamics of recombination is crucial. In this communication, we present a study correlating the blend morphology, from electron microscopy, and the CTE emission dynamics probed by NIR time-resolved PL spectroscopy. The study considers the two model polymer/fullerene blends, MDMO-PPV/PCBM and P3HT/PCBM. Experiments spanning three orders of magnitude in time (ns to μs) allow for distinguishing between the prompt recombination and the delayed diffusion-controlled recombination. The two regimes are clearly separated by looking at the decay dynamics, which has an exponential decay of few nanoseconds for the prompt and a power-law (t -α) for the delayed recombination. In addition, by performing the same experiment upon application of a transient external electric field, we monitor those CTEs which separate and later recombine as non-geminate electron-hole pairs. Both the power-law dynamics and the exponential decay are independent of morphology in the absence of the external field, whereas the power-law shows differences up to a factor of two in the exponent of the power-law, α, as a function of blend morphology. Our results, therefore, provide experimental evidence for the role of morphology in controlling non-geminate recombination, while the prompt geminate recombination seems to be determined by the molecular characteristics of the polymer and the fullerene. [1] M. Hallermann, S. Haneder and E. Da Como Appl. Phys. Lett., 93, 053307 (2008)[2] M. Hallermann, I. Kriegel, E. Da Como et al. Adv. Funct. Mater. 19, 3662 (2009)[3] M. Hallermann, E. Da Como et al. Appl. Phys. Lett., in press (2010)
3:00 PM - E2.3
Formation Dynamics of Fullerene Cation in Low-bandgap Polymer Solar Cells Studied by Transient Absorption Spectroscopy.
Shunsuke Yamamoto 1 , Hideo Ohkita 1 2 , Hiroaki Benten 1 , Shinzaburo Ito 1
1 Graduate School of Engineering, Kyoto University, Kyoto Japan, 2 PRESTO, JST, Saitama Japan
Show AbstractFullerene and its derivatives are an excellent electron acceptor and hence widely used as an electron-transporting material in organic optoelectronic devices. Recently, we have discovered the formation of fullerene cations in blend films of a phenylenevinylene conjugated polymer (MDMO-PPV) and a fullerene derivative (PCBM), which suggests that PCBM can serve as a hole-transporting material as well.In this study, the charge carrier composition in blend films of a low-bandgap conjugated polymer (PCPDTBT) and a fullerene derivative (PCBM) was investigated by transient absorption spectroscopy to clarify the formation mechanism of fullerene cations. In the nanosecond time domain, PCPDTBT polarons and PCBM anions were observed but no PCBM cation was found in the blend. On the other hand, in the sub-microsecond time domain, the fraction of PCPDTBT polarons decreased and instead that of PCBM cations increased with time. Finally, the hole carrier composition became constant in the microsecond time domain: PCPDTBT polarons : PCBM cations = 3:1. Furthermore, the decay constant of PCPDTBT polarons was in agreement with the rise constant of PCBM cations. We therefore conclude that PCBM cations are generated by the hole injection from PCPDTBT polarons to the PCBM domain.
3:15 PM - E2.4
Fluorescence Spectral Imaging Applied to Hybrid Materials for Electronics and Photonics.
Clara Santato 1 , Dilek Isik 1
1 , École Polytechnique Montréal, Montréal, Quebec, Canada
Show AbstractOrganic/inorganic hybrid materials, attractive for their multi-functional properties, have to be studied with suitable experimental techniques.We explored the use of the fluorescence spectral imaging technique (spatial resolution of 0.6 micrometers and spectral resolution of 1 nm) to study the distribution as well as the photophysical interactions in organic/inorganic hybrid materials for electronics and photonics. The materials we investigated are based on (Er3+/Yb3+)-doped up-converting NaYF4 nanoparticles [1] blended with well-investigated organic electronics polymers such as MEH-PPV and P3HT. Up-conversion can be defined as a multistep process where two or more IR photons are absorbed sequentially to produce one UV-Vis high energy photon.These hybrid materials are interesting for applications such as solution-processable IR sensors and photovoltaic cells with improved absorption in the IR portion of the solar spectrum.The fluorescence spectral imaging study, complemented by atomic force microscopy and scanning electron microscopy investigations, indicated a homogeneous distribution of the nanoparticles within the organic polymer matrix. This observation was enabled by the possibility to spectrally “unmix” the organic and inorganic fluorophore signals, even if spectrally contiguous.The homogeneous distribution of the nanoparticles in the polymer matrix was confirmed by the charge carrier transport measurements in field effect transistor (FET) configurations performed on the organic/inorganic hybrid thin films. Here, values of hole FET mobility comparable with state-of-the-art values for P3HT and MEH-PPV based FET were indeed measured.Besides that, the fluorescence spectral imaging technique permitted us to study the photophysical interactions between the up-converting nanoparticles and the organic polymers. As an example, we studied the possible energy transfer from the up-converting nanoparticles to the organic polymers. Indeed, a polymer that absorbs in the emission region of the up-converting nanoparticles may be able to harvest by energy transfer the up-converted photons. This energy transfer could be used to improve the absorption of organic photovoltaic devices in the IR region of the solar spectrum. [1] G.S. Yi and G. M. Chow, Adv. Funct. Mat. 2006, 16, 2324.
3:30 PM - **E2.5
Carrier Transport and Recombination in Organic Solar Cells.
Robert Street 1
1 , Palo Alto Research Center, Palo Alto, California, United States
Show AbstractAn understanding of the physics of organic bulk heterojunction solar cells requires knowledge of the electronic transport and recombination mechanisms. We show that time-of-flight (TOF) transient photoconductivity can be used to obtain a wide range of information about the opto-electronic processes in the cells, including carrier mobility, recombination lifetimes and recombination mechanisms. Measurements of carrier mobility complicated because the solar cells are optically thin. The usual TOF model has to be adapted to carriers created throughout the material with both electrons and holes contributing to the transport. We show that the two carriers can be distinguished and their mobility measured except when the response is limited by the RC time constant of the measurement. Dispersive mobility is observed and we discuss the relation of the mobility to the material disorder. When the internal voltage is small, recombination dominates and the transient photoconductivity provides a measure of the recombination time. The transient photoconductivity measurements are able to rule out geminate recombination as a significant mechanism and together with dc photoconductivity indicate that recombination through interface states is the dominant process in the P3HT/PCBM and PCDTBT/PCBM cells measured.
4:00 PM - E2: OPV Physics
BREAK
4:30 PM - **E2.6
Ultrafast Exciton Dynamics and Dissociation in Nanocrystal Heterostructures.
Gregory Scholes 1 , Shun Lo 1 , Haizheng Zhong 1
1 Dept of Chemistry, university of toronto, Toronto, Ontario, Canada
Show AbstractI will describe our recent progress in synthetic strategies that allow for material and architectural modulation of nanoheterostructures, as well as the experimental work that provides insight into the photophysical properties of type-II heterostructures. The effects of external factors, such as electric fields, temperature and solvent have been explored in conjunction with exciton and multiexciton dynamics and charge transfer processes typical for type-II semiconductor heterostructures. I will discuss our recent studies of charge separation dynamics, control of charge separation using structure and solvent, and applications of nanocrystal heterostructures in photovoltaic devices.
5:00 PM - E2.7
Ultrafast Charge Separation at a Single-walled Carbon Nanotube – Polymer Molecular Junction.
Samuel Stranks 1 , Christian Weisspfennig 1 , Patrick Parkinson 1 , Michael Johnston 1 , Laura Herz 1 , Robin Nicholas 1
1 , University of Oxford, Oxford United Kingdom
Show AbstractWe report the observation of an ultrafast (~ 430 fs) charge transfer process at the interface between a single-walled carbon nanotube (SWNT) wrapped by a semi-conducting polymer, poly(3-hexylthiophene) (P3HT), creating free polarons on both materials. Remarkably, we demonstrate that the addition of excess P3HT as a surrounding network allows these free polarons to be long-lived, achieving a charge separation efficiency comparable to a P3HT-fullerene blend.It has been shown experimentally and theoretically that a type-II heterojunction exists between small diameter semi-conducting SWNTs and P3HT. Together with large aspect ratios and high carrier mobilities, these composites are promising candidates for use in organic photovoltaic (OPV) devices. Despite such favourable properties and band alignments, OPV devices to date have shown poor performances. In this work, we study purified P3HT-SWNT nanohybrid structures consisting of a SWNT coated with a monolayer of regioregular P3HT. We use photoluminescence measurements to observe the quenched polymer emission. Femtosecond up-conversion spectroscropy is used to monitor the quenched time-decay of the polymer emission and transient absorption studies are used to probe for charge on the P3HT resulting from charge separation processes.The photoluminescence and transient absorption measurements show that photoexcitation of a monolayer coating of P3HT leads to an ultrafast (~ 430 fs) charge transfer between the materials, with a bound charge-transfer complex ultimately being formed across the heterointerface. However, in the presence of additional P3HT, we observe a long-term charge separation. Photoexcitation in the surrounding P3HT leads to a free-charge rise time of 15 ps corresponding to the migration of excitons to a SWNT-P3HT junction. Ultrafast dissociation again occurs at the interface but the excess P3HT facilitates spatial separation of the created charges.Our results suggest that the SWNT-P3HT interface can offer a low threshold for charge-generation in photovoltaic devices, provided that small-diameter tubes are individually embedded in a matrix comprising a sufficient excess of P3HT. Previous device studies have only utilized larger diameter nanotubes which do not establish a type-II heterojunction with P3HT. These findings explain the poor polymer-SWNT device behaviour to date and provide a promising route to incorporate SWNT-polymer blends into OPVs.
5:15 PM - E2.8
The Ideal Diode Equation for Organic Solar Cells.
Noel Giebink 1 2 3 , Stephen Forrest 2 , Gary Wiederrecht 1 3 , Michael Wasielewski 1 3 4
1 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, United States, 2 Depts. of Electrical Engineering, Materials Science, and Physics, University of Michigan, Ann Arbor, Michigan, United States, 3 Argonne-Northwestern Solar Energy Research Center, Northwestern University, Evanston, Illinois, United States, 4 Dept. of Chemistry, Northwestern University, Evanston, Illinois, United States
Show AbstractThe current-voltage characteristics of organic heterojunctions are often modeled using the generalized Shockley equation derived for inorganic diodes. However, since this description does not rigorously apply to organic semiconductor donor-acceptor (D-A) heterojunctions (HJs), the extracted parameters lack a clear physical meaning. Here, we derive the current density-voltage (J-V) characteristic specifically for D-A HJ solar cells and show that it predicts the general dependence of dark current, open-circuit voltage (Voc), and short-circuit current (Jsc) on temperature and light intensity as well as the maximum Voc for a given D-A material pair. We propose that trap-limited recombination due to disorder at the D-A interface leads to the introduction of two temperature-dependent ideality factors, and show that this describes the dark current of copper phthalocyanine (CuPc)/C60 and boron subphthalocyanine chloride (SubPc)/C60 cells at low temperature, where fits to the generalized Shockley Equation break down. We identify the polaron pair recombination rate as a critical factor for cell power efficiency, and provide direct measurements of this process using intensity modulated photocurrent spectroscopy and emission from interfacial Pc/C60 exciplex states. These results provide a general physical framework for interpreting the J-V characteristics and understanding the efficiency of both small molecule and polymer organic solar cells.
5:30 PM - E2.9
Voltage Dependence of the Internal Quantum Efficiency in Bulk-heterojunction Solar Cells.
Eric Hoke 1 , George Burkhard 1 , Michael McGehee 2
1 Applied Physics, Stanford University, Stanford, California, United States, 2 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractThe internal quantum efficiency of a solar cell is the number of collected charge carriers divided by the number of photons absorbed in the active layer. Knowing the internal quantum efficiency helps one determine how much better a solar cell could be if problems associated with charge transport and recombination could be solved. However, the internal quantum efficiency is very rarely reported in studies of organic solar cells. When it is, absorption in the electrodes is typically ignored even though it can easily account for 10% of the light absorption. We present a simple and accurate method for measuring the internal quantum efficiency that involves measuring the total absorption in the solar cell and then subtracting the absorption in the electrodes, which we estimate using a transfer matrix model. We show that if one were to try to calculate the absorption in the active layer directly from optical modeling it would be difficult to obtain accurate results because of uncertainty in optical constants. One does not need to know the optical constants with high precision to calculate the absorption in the electrodes because a 10% error in this measurement only causes the internal quantum efficiency to be off by approximately 1%. This method has enabled us to discover that the internal quantum efficiency in P3HT:PCBM solar cells is higher when the polymer absorbs light than when the fullerene absorbs it.The voltage dependence of the photocurrent of an organic solar cell is very useful in diagnosing losses due to space charge effects as well as bimolecular and geminate recombination. It is commonly assumed that the photocurrent can be found by subtracting the dark current from the current measured when the solar cell is illuminated. We have observed cases where the photocurrent measured in this way would imply that the internal quantum efficiency exceeds 100% at reverse bias because photoconductivity has been ignored. We show that a more accurate method for determining the photocurrent involves measuring the internal quantum efficiency as a function of voltage with an oscillating weak probe beam and a continuous white light bias. The voltage dependence of the photocurrent for several bulk heterojunction solar cells are compared to the predictions of Onsager-Braun theory and charge trapping models.
5:45 PM - E2.10
Relation of Open Circuit Voltage to Charge Carrier Concentration in Organic Bulk Heterojunction Solar Cells.
Daniel Rauh 1 , Alexander Wagenpfahl 2 , Carsten Deibel 2 , Vladimir Dyakonov 1 2
1 , ZAE Bayern, Div. Functional Materials for Energy Technology, Würzburg Germany, 2 , Experimental Physics VI, Julius-Maximilians University of Würzburg, Würzburg Germany
Show AbstractOne of the key parameters in organic bulk-heterojunction solar cells is the open circuit voltage Voc. The understanding of the factors limiting Voc is therefore crucial. We investigated the temperature and light intensity dependence of Voc for different solar cells consisting of poly(3-hexylthiophene) (P3HT) blended with various fullerene based acceptors by current-voltage measurements. Additional charge extraction measurements under the same conditions allowed us to determine the charge carrier densities n in the operating device.We observed two different temperature regimes, a linearly increasing Voc with decreasing temperature in the range between room temperature and approx. 200 K, and a saturation regime at lower temperatures. The former can be explained by an increase of n, as revealed by the charge extraction measurements. We find our data to be in accordance with analytic models of Voc [1,2], and allows us to determine the effective bandgap of the bulk semiconductor as well as the dominant recombination mechanism. However, these Voc models fail to describe the behaviour of Voc at low temperatures. With the aid of our macroscopic simulations, we find that Voc is limited by the built-in potential due to the electrode work functions, resulting in a saturation of Voc.[1] Koster et al., Appl. Phys. Lett. 86, 123509 (2005) [2] Cheyns et al., Phys. Rev. B 77, 165332 (2008)
E3: Poster Session: Organic Light-emitting Diodes
Session Chairs
Tuesday AM, November 30, 2010
Exhibition Hall D (Hynes)
9:00 PM - E3.1
Spontaneous Proximity Doping and Transport in a Pyrenyl Carbazole.
Farman Ali 1 , Meghan Patankar 1 , K. Narasimhan 1 , N. Periasamy 1
1 , Tata Institute of Fundamental Research, Mumbai, Maharashtra, India
Show AbstractIn this paper, we present studies on sandwich and planar transport in a 3,6-di(pyrenyl)-N-hexylcarbazole (P2NHC). The sandwich structures reported here are A. ITO/F4TCNQ/TPD/ P2NHC /BCP/LiF/Al and B. ITO/TPD/ P2NHC /AlThe measured zero bias capacitance (10kHz) of structure B is equal to the geometrical capacitance of the device. In contrast, the zero bias capacitance of structure A is much larger than the geometrical capacitance at room temperature. Application of reverse bias to A decreases the capacitance and approaches the geometrical capacitance at -1V. This constitutes direct evidence for doping in the P2NHC layer. The doping gives rise to a depletion region in the P2NHC layer. With increase in reverse bias, the depletion region extends through the thickness of the P2NHC layer. These doping effects are not seen in samples of type B. We hence conclude that the doping in P2NHC is due to proximity effects of adjacent layers and not due to impurities in P2NHC.The room temperature capacitance is independent of frequency over the measured frequency range of 10Hz-10kHz. The capacitance is hence due to modulation of free carrier density at the edge of the depletion region and not due to deep levels as in the case of TPD doped with F4TCNQ [1]. This implies that P2NHC behaves like a classical semiconductor. From the slope of 1/ C^2 vs V, we estimate the carrier density using the profiler formula [2] and find it to be 1.1E17 cm^-3 at 300K and 7.7E16 cm^-3 at 240K.We have also measured the temperature dependence of the planar conductivity of the same structure A (without the top Al contact) between 240K and 300K. The electrical conductivity (σ) is σ = neμ, where n is the carrier concentration and μ the mobility. Using the carrier concentration obtained earlier, we estimate the room temperature carrier mobility to be 7E-5 cm^2/V-s. We show that, the mobility is thermally activated and can be written as μ = μ0exp(-ΔE/kT) with an activation energy of 0.28 eV and μ0 = 3 cm^2/V-s -characteristic of extended state transport in disordered semiconductor [3]. We finally conclude by demonstrating in structure A, an efficient blue electroluminescence (EL) (with a low EL turn-on voltage of 2.8V) peaked at 455 nm, and a current efficiency of 2.1 cd/A for a brightness of 10^2–10^4 cd/m^2 with a low droop for higher brightness up to 16,000 cd/m^2 References: 1. Debdutta Ray and K. L. Narasimhan J. App. Phys. 103, 093711, 2008 2. S. M. Sze, Physics of Semiconductor Devices, 2nd edition, Wiley India Pvt. Ltd. 3. N. F. Mott and E. A. Davis, Electronic Processes in Non-Crystalline Materials, Clarendon Press,Oxford 1971
9:00 PM - E3.10
Synthesis of Multicolor Fluorescent Conjugated Polymer with Variable Monomer Feed Ratios.
Jong Ho Kim 1 , Geun Seok Jang 1 , Dai Geun Kim 2 , Na Young Kwon 1 , Seong Won Seo 1 , Taek Seung Lee 1
1 Department of Advanced Organic Materials and Textile System Engineering, Chungnam National University, Daejeon Korea (the Republic of), 2 Department of Nanotechnology, Chungnam National University, Daejeon Korea (the Republic of)
Show AbstractOrganic and polymeric light-emitting diodes (OLEDs and PLEDs) have attracted considerable attention in recent years due to their potential applications in full-color flat panel displays, illumination light source, and backlight for liquid crystal display. The device based on polymers has the advantages of solution processing and tunable emission color through molecular design, which are important requirements for large-area display devices. There are several strategies to fabricate the multicolor emission devices, including the use of polymer blends, incorporating chromophores into polymer chain, and mixing polymer with phosphorescent complexes. We investigated multicolor emitted conjugated polymer via incorporating fluorophores into polymer chain. This polymer has a poly(ρ-phenylene) backbone containing green and red emitting exciton trap sites. We used exciton migration phenomenon for realization of multicolor emission. For this reason, we controlled feed ratios of 2,1,3-benzothiadiazole as green emitting site and 4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole as red emitting site. We introduced alkoxy side chain on phenylene backbone to improve solubility. These polymers are studied using gel permeation chromatography (GPC), 1H NMR, elemental analysis, UV-vis absorption, photoluminescence. The synthesized polymers are expected to have multicolor emission.
9:00 PM - E3.11
Adhesion in Organic and Hybrid Organic/Inorganic Solar Cells and Light Emitting Devices.
Tiffany Tong 1 2 , Wali Akande 1 2 , Jing Du 1 3 , Wole Soboyejo 1 3
1 Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, New Jersey, United States, 2 Department of Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 3 Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey, United States
Show AbstractThis paper presents the results of an experimental study of adhesion between layers that are relevant to organic and hybrid organic/inorganic structures that are relevant to solar cells and light emitting devices. A combination of atomic force microscopy (AFM) and interfacial fracture mechanics is used to study the robustness of interfaces between relevant bi-material interfaces. The measurements of adhesion are then incorporated into adhesion and fracture mechanics models that link the nano-scale AFM measurements to the micro-/macro-scale interfacial fracture mechanics measurements. The implications of the results are then discussed for the design of robust organic and hybrid organic/inorganic electronic structures.
9:00 PM - E3.12
Current Density Dependence of Transient Electroluminescence in Phosphorescent Organic Light-emitting Diodes.
Hirotake Kajii 1 , Noriyoshi Takahota 1 , Yadong Wang 1 , Yutaka Ohmori 1
1 Center for Advanced Science and Innovation, Osaka Univ., Suita, Osaka, Japan
Show AbstractEmploying phosphorescent materials exhibits high efficiencies because breaking the spin conservation rule indicates that both the singlet and triplet excitons contribute to emission. However, the response time of phosphorescent materials such as Ir complexes is slower than that of fluorescent ones due to their long emission lifetime. The study of the transient electroluminescence of organic light-emitting diodes (OLEDs) has been also reported. The applied voltage dependence of fundamental transient properties in these devices are mostly investigated. In this study, we studied the current density dependence of transient electroluminescence of phosphorescent OLEDs with Ir complex as the emissive layer. For a green phosphorescent OLEDs based on tris(2-phenylpyridine)iridium(III) (Ir(ppy)3), the maximum luminance of approximately 45,000 cd/m2 and maximum current efficiency of 34 cd/A were obtained. We investigated the current density dependence of the rise and decay times of a green phosphorescent OLED with Ir(ppy)3 as an emissive dopant by applying the rectangle voltages. The decay (rise) time is defined as the time required to change the optical response from 90 (10) to 10 % (90 %) of its total intensity change.At a lower current density, the rise time was longer than the phosphorescence lifetime of Ir(ppy)3. For a green OLED, the rise time gradually decreased with increasing applied voltage. The decay time of a green OLED was approximately 2 μs at lower current densities. While, both the rise and decay times of photoluminescence (PL) of host:Ir(ppy)3 film were approximately 2 μs by measuring the transient response time in PL with a 408 nm blue semiconducting laser diode. The decay time of EL is almost same as the that of PL in Ir(ppy)3 at lower current densities.At higher current densities, the increase of the nonradiative process results in the decrease in phosphorescence recombination lifetime. The device exhibited gradual decrease in quantum current efficiency due to the triplet-triplet annihilation at the high current density. At higher current density, the reduced rise and decay times may also be due to high-density triplet excitons related to the enhanced triplet-triplet annihilation. Therefore, both the rise and decay times were observed to be approximately 1 μs. This green phosphorescent OLED can be expected to be utilized as one of the light sources driven at less than 0.5 MHz.We also studied the transient electroluminescence of phosphorescent OLEDs using pulses of alternating current sine-waves with various frequencies. The frequency response of the device was improved by increasing applied current densities. The cut-off frequencies of the typical device were approximately 10 and 100 kHz at 1 and 100 mA/cm2, respectively.
9:00 PM - E3.13
Surface Morphologies in ITO/PANI Films Analyzed Using Minkowski Functional.
Gemirson Reis 1 , Gabriel Goncalves 1 , Gislayne Herculano 1 , Mirela Santos 2 1 , Rodrigo Bianchi 1 , Sukarno Ferreira 2 , Maximiliano Munford 2 , Andrea Bianchi 1
1 Physics Department, University Federal de Ouro Preto, Ouro Preto, Minas Gerais, Brazil, 2 Physics Department, University Federal of Viços, Viçosa, Minas Gerais, Brazil
Show AbstractThe conduction in polymers have been used as active layer in electronic devices, such as diodes, transistors, photovoltaic cells and sensors. However, many fundamental aspects related to the influence of morphological properties of polymer films in the electrical properties of such devices remain unclear, and the reported results are some what controversial. This work presents the investigation of morphological properties of nanocomposite of polyaniline/indium-tin oxide films using image processing techniques, since microscopic features plays an important role in the understanding of the material behavior. The films were obtained by casting from solutions of PANI (6%) in N-2-methyl pyrrolidone (NMP) with different weight ITO/PANI percentages, 2.5%, 10%, 25%, 50%. They were mixed in the solution of PANI for four hours, deposited on glass substrates at 55 °C and characterized by atomic force microscopy (AFM). The scanning area was 5µm2 at a frequency of 1.0Hz in intermittent contact mode. As described earlier, the morphology studies in hybrid films contributes to the understanding of films formation and growth, and are closely related to modifications in optical and electrical properties of devices. The Minkowski functional technique, based in image analysis framework, establishes features about the geometry and morphology of patterns observed on the surface of film. They provide information such as, fraction of covered area, shape and connectivity, identifying a finite number of measures that qualitatively compares different morphologies, nevertheless to roughness and dominant length. The integral geometry quantities were mainly used to described phase separating, but recently have been used to analyzed AFM surface patterns in terms of three Minkowski measures plotted as a function of height. The AFM images of ITO/PANI films with different proportions of ITO in PANI matrix presents increase. Some previous results show that the area coverage is directly proportional to concentration and inversely to the perimeter and Euler characteristic; such behaviors indicate that the films present flatter and more connected regions for lower nanoparticles concentrations. In addition, it was observed that dc conductivity of the films increased clearly with the increase of ITO/PANI content, whereas the alternating conductivity presents an almost universal behavior in which the logarithm of the real component exhibits two frequency regions, one plateau at low frequencies (the dc plateau) followed by a region of increasing conductivity. The sample is considered to be constituted of semiconductor regions (ITO nanoparticles) sparsely distributed in a disordered, that is, dispersive matrix (PANI). An effective medium approximation is then used to analyze the conductivity of such nonhomogeneous medium. This research was supported by Fapemig, Capes and CNPq, CNPq/INEO.
9:00 PM - E3.15
Singlet Exciton Quenching by Radical Cations of Aromatic Diamines as an Electron Donor in Organic Electroluminescent Devices.
Munkhbat Battulga 1 , Sharavsambuu Baasanjav 1 , Chimed Ganzorig 1
1 Center for Nanoscience and Nanotechnology and Department of Chemical Technology, Faculty of Chemistry, National University of Mongolia, Ulaanbaatar Mongolia
Show AbstractIn this study, we report the luminescence quenching by radical cations of aromatic diamines used as a hole transport layer (HTL) in organic electroluminescent (EL) devices. The EL characteristics of green organic EL devices with a electron transport layer (ETL) as an emitter i.e. ITO/TPD HTL/Alq3 ETL/Al is studied. Here, ITO, TPD, and Alq3 are abbreviations for indium-tin-oxide, N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine, and tris(8-hydroxyquinoline)aluminum, respectively.UV-visible absorption and electrochemical data indicate the formation of radical cations in thin film and solution of TPD after chemical and electrochemical oxidation. We find that the EL luminance increases less than linearly with an increase in current for the EL devices studied in this study.The luminance loss in the devices is attributed to quenching of singlet excited states by large excess radical cations of TPD accumulated in the emission zone due to large overlap between a flourescence spectrum of Alq3 and an absorption spectrum of radical cations of TPD.
9:00 PM - E3.16
Spectroelectrochemical Study of the Formation of Radical Cations of 4,4’-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (NPD) as a Hole Transport Semiconductor Material.
Sharavsambuu Baasanjav 1 , Munkhbat Battulga 1 , Chimed Ganzorig 1
1 Center for Nanoscience and Nanotechnology and Department of Chemical Technology, National University of Mongolia, Ulaanbaatar Mongolia
Show AbstractSpectroelectrochemical study on a new absorption band of radical cations of 4,4’-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (NPD) as an electron-donor hole-transporting material used in organic electronics is reported in this work. UV-visible spectroscopic and cyclic voltammetric (CV) properties for NPD in thin film and in solution are also examined. We find that the results are attributed to quenching process for blue fluorescence from NPD by a excess NPD radical cations accumulated in the emission region in the organic light-emitting devices related to a relatively large overlap between the fluorescence spectrum of NPD and the absorption spectrum of NPD radical cations.The band gap energies for NPD are calculated and compared from the UV-visible spectroscopic and CV data. The details of experimental results will be discussed and concluded at the proceedings paper.
9:00 PM - E3.17
Investigation of Optical Induced Degradation Phenomena of Blue Phosphorescent Emissive Materials for Organic Light Emitting Diodes.
Ruben Seifert 1 , Ines Rabelo de Moraes 1 , Sebastian Scholz 1 , Bjoern Luessem 1 , Karl Leo 1
1 Physics, Institut für Angewandte Photophysik, Technische Universität Dresden, Dresden, Sachsen, Germany
Show Abstract Since the invention of the first efficient organic light emitting device (OLED) by Tang and VanSlyke [1], there has been intense research in this field. White OLEDs with an “all phosphorescent” emitter stack already exhibit a better performance in efficiency than fluorescent tubes [2]. However, there are still problems concerning the long-term stability, especial for blue phosphorescent materials. In this contribution we will compare two common highly efficient phosphorescent materials iridium(III) bis(4’,6’-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6) [3] and bis(2-(4,6-difluorophenyl)pyridyl-N,C2’)iridium(III)picolinate (FIrpic) [4] by studying the degradation of single layers of emitting molecules, optically excited at the absorption maximum. In this contribution we will show that both materials can be easily destroyed by optical generated excitons. We will show that FIrpic is more stable than FIr6 and the degradation of FIr6 causes a red emitting degradation product. Using the laser-desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS) [5], a chemical degradation pathway for the FIr6 emitter is proposed.[1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913.[2] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, K. Leo, Nature 459 (2009) 234.[3] S.-H. Eom, Y. Zheng, E. Wrzesniewski, J. Lee, N. Chopra, F. So, J. Xue, Org. Electronics. 10 (2009) 686.[4] N. Seidler, S. Reineke, K. Walzer, B. Lüssen, A. Tomkeviciene, J.V. Grazulevicius, K. Leo, Appl. Phys. Lett. 96 (2010) 093304.[5] S. Scholz, K. Walzer, K. Leo, Adv. Funct. Mater. 18 (2008) 2541.
9:00 PM - E3.18
Organic Magnetoresistance Based onto Exciplex States.
Kenichi Goushi 1 , Chihaya Adachi 1
1 Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Fukuoka Japan
Show AbstractIn organic light-emitting diodes (OLEDs), magnetic-field-dependent current has been observed even with a low external magnetic field, named as organic magnetic-resistance (OMAR). The mechanism has been still hotly debated, and several groups have proposed the different models. Here, we focus on one model of them, in which the magnetic-field-dependent generation of secondary charge carriers is responsible for magnetic-field-dependent current.1, 2) These carriers are caused by the dissociation of bound electron-hole pairs via hyperfine interaction1, 2), forming further space-charges in organic semiconductors. In the framework of the model, the dissociation efficiency from carrier-pair states should be an important factor to determine OMAR, since the dissociation process occurs mainly from carrier-pair states and not from excitonic states due to the strong exciton binding energy. In this study, we investigated the OMAR devices based on exciplex formation in which large magnetic-field effects can be expected due to an increased duration of stay in the carrier-pair states, since exciplex states provide rather efficient carrier dissociation process due to the smaller binding energy of exciplex states.We investigated magnetic field effects on current in OLEDs consisted of ITO / m-MTDATA doped into Alq3 / Ca / Al. The m-MTDATA/Alq3 interface is well-known for the exciplex formations.3)The MR based onto the exciplex states is higher than that based onto Alq3 excitonic states. These results suggest that exciplex states provide large OMAR. To investigate the validity of our proposed concept, the dissociation efficiency is estimated from the electrical field dependence of photoluminescence decay curves. As a result, dissociation efficiency of exciplex states is higher than that of neutral excited states. Therefore, we concluded that the higher MR based onto exciplex states is due to the weak binding energy of exciplex sates.References1. V. N. Prigodin, J. D. Bergeson, D. M. Lincoln, and A. J. Epstein, Synth. Met. 2006, 156, 757-761.2. B. Hu, and Y. Wu, Nature Materials, 2007, 6, 985-991.3. N. Matsumoto, M. Nishiyama, and C. Adachi, J. Phys. Chem. C, 2008, 112, 7735-7741.
9:00 PM - E3.19
Horizontal Orientation of Planar Type-hole Transport Molecules and Their Application for Organic Light-emitting Diodes Aimed for Low Driving Voltage.
Kim JunYun 1 2
1 Applied chemistry, Kyushu university, Fukuoka Japan, 2 , Japan Center for Organic Photonics and Electronics Research (OPERA), Fukuoka Japan
Show Abstract Oganic light-emitting diodes (OLEDs) continue to be of great interest because they realize not only high energy conversion efficiency but also mechanically flexible and lightweight display and lighting applications. 1) Recently, large optical anisotropies were identified in films of hole and electron transport materials such as TPT1 and Bpy-OXD having rather long and planar backbone structures.2) The method of wide-range variable angle spectroscopic ellipsometry (VASE) clarified that the molecules having a long rod-like structure show horizontal orientation on any underlying layers.3) Also, high performance optical and electrical characteristics were demonstrated, based on the enhancement of π-π interaction between adjacent molecules In this study, we further developed novel molecular structures of B-DDP, T-DDP, BT-DDP aimed for the enhancement of horizontal orientation by introduction of our idea of two-dimensional planar structures having rather intense π-π interaction, leading to further low driving voltage in OLEDs. S is orientation order parameter (S=−0.5: completely parallel, S=0: randomly oriented, S=1: completely perpendicular to the surface). In the DDP derivatives, the order of S is BT-DDP (−0.23) < T-DDP (−0.18) < B-DDP (−0.11) < α-NPD (−0.01). In the OLED characteristics, compared with α-NPD, the use of DDP derivatives resulted in lower driving voltage. In particular, BT-DDP showed the lowest driving voltage which is consistent with the VASE result. We clarified that the ITO/ BT-DDP interface provides small energy for hole injection probably due to the planar orientation of BT-DDP on an ITO surface. [1] S. R. Forrest, Nature (London) 428, 911 (2004). [2] D. Yokoyama, A. Sakaguchi, M. Suzuki, C. Adachi, Appl. Phys. Lett. 93, 173302 (2008). [3] D. Yokoyama, A. Sakaguchi, M. Suzuki, C. Adachi, Org. Electron. 10, 127 (2009).
9:00 PM - E3.20
Correlation of the Structure of TIPS-pentacene/Polymer Blends and Light Emission.
Kittiyaporn Singsumphan 1 , Yang Choo Chua 2 , Maxim Shkunov 3 , Joao Cabral 2 , Joseph Keddie 1 , Thomas J. Hosea 1
1 Physics, University of Surrey, Guildford, Surrey, United Kingdom, 2 Chemical Engineering, Imperial College London, London, London, United Kingdom, 3 Advanced Technology Institute, University of Surrey, Guildford, Surrey, United Kingdom
Show Abstract6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) has one of the highest charge carrier mobilities of all small organic molecules. In comparison to pentacene, TIPS-pentacene has better solubility in organic solvents and also has better chemical stability. Moreover, there have been efforts in establishing the use of TIPS-pentacene in high performance transistors. The encapsulation of TIPS-pentacene within a polymer matrix has generated interest as a means to combine the carrier mobility of the small molecule and the attractive properties of polymers. Whereas most previous work has considered pentacene for transistor applications, this current research investigates the properties of TIPS-pentacene/polymer blends as candidate materials for lasers. Specifically, we correlate the photoluminescence (PL) of blends of TIPS-pentacene and poly(styrene) with microstructure as it is influenced by processing conditions and the selection of solvent. Poly(styrene) and TIPS-pentacene were blended in various ratios in an organic solvent, 1,2,3,4-tetrahydronaphthalene (tetralin) with one of two different grades: anhydrous and reagent-grade. Films were deposited by drop-casting. It was discovered that the grade of the solvent and the age of the solution prior to film deposition have a pronounced impact on the degree of TIPS-pentacene crystallinity. In turn, there is a strong correlation between the blend microstructure and the intensity of the PL signal. Film deposition from solutions in the anhydrous solvent resulted in large, needle-like TIPS-pentacene crystals, as observed with optical microscopy. Differential scanning calorimetry and wide angle X-ray scattering confirmed that crystallization of the expected crystal structure had occurred. On the other hand, the use of the reagent grade-solvent leads to a loss in crystallinity. The size and amount of crystals decrease as the reagent-grade solution ages over time, and there is no crystallinity in films cast from 22-hour old solution. Optical microscopy indicates the presence of particles of a second phase, which might be amorphous TIPS-pentacene. Nanoparticle tracking analysis of the solvent reveals the presence of nanoparticle impurities, ranging in size from 35 nm to 300 nm. The mechanism by which crystallization is disrupted is currently under investigation. The loss of crystallinity in the blends was found to correlate strongly with an increase in the PL intensity. Specifically, the PL intensity from films cast from reagent-grade solutions (aged for 22 hours) was significantly stronger (by a factor of 100) than a solution of the same age cast from anhydrous solvent. These results provide a means to adjust the microstructure of polymer/TIPS-pentacene blends and thereby to adjust their performance in optoelectronic devices, especially in lasing applications.
9:00 PM - E3.21
Spin-dependent Transport Properties in a Ferromagnet / Organic Semiconductor / Ferromagnet Heterojunction.
Yu Jeong Bae 1 , Nyun Jong Lee 1 , Jeahyoung Lee 2 , Hyunduck Cho 3 , Changhee Lee 3 , Tae Hee Kim 1
1 Department of Physics, Ewha Womans University, Seoul Korea (the Republic of), 2 , PARTRON Co., LTD, Hwaseong Korea (the Republic of), 3 School of Electrical Engineering and Computer Science, Seoul National University, Seoul Korea (the Republic of)
Show AbstractOne of the key requirements for engineering organic spintronic devices is the efficient injection of spin-polarized charge carriers from a ferromagnetic electrode into an organic semiconductor (OSC) interlayer. Therefore, the understanding of structural and electrical properties at ferromagnet / organic film interfaces is of great importance to improve device performance as well as to design novel organic devices. Here, we report the effect of the hybrid barrier consisting of the insulator MgO and the OSC Cu-phthalocyanine (CuPC). We explored the spin-dependent transport phenomena in the Fe/CuPC/Co and Fe(100)/MgO(100)/CuPC/Co tunnel junctions. The molecular-beam-epitaxy-grown Fe(100) film was prepared on the MgO buffered Si(100) substrates. (200) textured CuPC films of β-phase were grown beyond the epitaxial MgO(100)/Fe(100)/MgO(100) films by the vacuum deposition technique. The structural analysis of the interface or surface was performed by X-ray diffractometer (XRD), transmission electron microscopy (TEM) and atomic force microscopy (AFM). Temperature-dependent current-voltage measurements were also carried out to characterize electrical properties of the tunnel junctions by the conventional dc four-probe method. We observed a strong asymmetry of the I-V curves at room temperature and the magneto-resistance (MR) effect when the electrons tunnel from the Fe/MgO(100) to the Py electrode. However no MR effect was observed when the electrons tunnel from the Py to the Fe/MgO(100).Our results suggest novel approaches and the new possibilities to realize organic spintronic devices.This work is supported by the Korea Science & Engineering Foundation through the Quantum Meta-Materials Research Center and the Korea Research Foundation Grant (No. 2008-0062239, NRF-2006-531-C00026).* Corresponding author:
[email protected]9:00 PM - E3.22
Electrode Dependent Spin Injection in a Ferromagnet/Organic Semiconductor Heterostructure.
Nyun Jong Lee 1 , Yu Jeong Bae 1 , Jeahyoung Lee 2 , Hyunduck Cho 3 , Changhee Lee 3 , Atsufumi Hirohata 4 , Luke Fleet 5 , Tae Hee Kim 1
1 Department of Physics, Ewha Womans University, Seoul Korea (the Republic of), 2 , Partron Co., Hwaseong Korea (the Republic of), 3 School of Electrical Engineering and Computer Science, Seoul National University, Seoul Korea (the Republic of), 4 Department of Electronics, University of York, York United Kingdom, 5 Department of Physics, University of York, York United Kingdom
Show AbstractThe fundamental knowledge about spin injection and transport in the organic semiconductors(OSCs) is required for realizing of spintroinics devices based on OSCs. The efficient carrier injection from a metal electrode into an organic film is one of key processes in the OSC devices. It is great significant to understand the structural and electrical properties at M/O interfaces not only for designing novel organic devices but for improving device performance. In this work, we consider a heterostructure system consisting of a highly-qualified interface between the ferromagnet Fe and the OSC Cu-phthalocyanine(CuPc). We focus on interfacial effects on spin-polarized carrier injection, transport, and recombination properties of the organic layers by using the light emitting diodes(LEDs) and tunneling devices with different electrodes, such as polycrystal Fe, epitaxial Fe(001) and epitaxial Fe(001)\MgO(001). Both the LEDs with CuPc and Tris(8-hydroxyquinolinato)aluminium(Alq
3) organic films and the tunneling devices with CuPc films grown beyond the polycrystal Fe, epitaxial Fe(001) and epitaxial Fe(001)\MgO(001) films were prepared by the vacuum–deposition at a base pressure of 10
-8 Torr. The molecular-beam-epitaxy- grown Fe(001) films were prepared on MgO bufferd Si(100) substrate. The microstructural analysis was performed by TEM. From the difference magnetoluminescence with an epitaxial Fe(100)\MgO(001) electrode (Fe(001)\MgO(001)\OSCs\Py) shows the polarized spin injection compared to Fe(001) electrode(Fe(001)\OSCs\Py). This work is supported by the Korea Science & Engineering Foundation through the Quantum Meta-Materials Research Center and the Korea Research Foundation Grant (No. 2008-0062239, NRF-2006-531-C00026). * Corresponding author:
[email protected] 9:00 PM - E3.23
Study for Work Function Modification of Various Surfaces due to Adsorption of Highly Electron Withdrawing Molecule.
Ji-Hoon Kim 1 , Jae-Won Seo 1 , Hyeseung Kang 1 , Jeong-Kyu Kim 1 , Young Mi Lee 1 , Jeong Won Kim 2 , Hangil Lee 3 , Young-Kyun Kwon 1 , Yongsup Park 1
1 physics , Kyung Hee University, Seoul Korea (the Republic of), 2 , Korea Research Institute of Standards and Science, Daejeon Korea (the Republic of), 3 , Sook Myung Women's University, seoul Korea (the Republic of)
Show AbstractUsing both experimental and theoretical approaches, we have investigated the structural and electronic properties at the interface between HAT-CN and Cu (111) surface as well as those between HAT-CN and graphene. HAT-CN is an organic molecule used as an efficient hole injection layer in OLEDs. We observed an unusual phenomenon from x-ray photoelectron spectroscopic measurements that the work function decreases with the coverage of HAT-CN on Cu surface, but it increases back and becomes higher than that of pristine Cu surface with higher HAT-CN coverage. On the other hand, work function of HAT-CN on graphene surface monotonically increased. Our first-principles density functional calculations revealed that the change in work function results from a competition between the geometrical deformation and the bond dipole formed at the interface due to charge redistribution. At low coverage of HAT-CN, at only Cu substrate, the former reduces the work function significantly by pulling down the vacuum level. While the latter, at both substrate, tends to push up the vacuum level resulting in the work function increase. As more layers of HAT-CN are deposited on Cu surface, the effect of the charge redistribution becomes dominant and thus the work function increases even higher than that of pristine Cu surface. On graphene, absorbed HAT-CN is not distorted, and therefore the work function is dominated by bond dipole effect.
9:00 PM - E3.24
Modeling Plasmon Fluorescence Enhancement and Energy Transfer Mechanisms in Light-harvesting Systems.
Mikolaj Schmidt 1 , Alexander Govorov 2 , Sebastian Mackowski 1
1 Institute of Physics, Nicolaus Copernicus University, Torun, kujawsko-pomorskie, Poland, 2 Department of Physics and Astronomy, Ohio University, Athens, Ohio, United States
Show AbstractTheoretical background for non-coherent coupling between light-harvesting antennas and metallic nanoparticles seem to be well understood and consistent with results obtained from single-molecule experiments [1-2]. For certain geometries and taking into account many approximations, analytical solutions are known. Also, computational tools for handling the general problem have been established. Recently, the challenge of creating hybrid light-harvesting systems by combining organic light-harvesting systems like PCP or LH2 with semiconducting and metallic nanoparticles has been attempted [3-4]. Although theoretical formalisms were already applied to hybrid light-harvesting systems [4-5], more theoretical work is clearly needed to understand and predict complex optoelectronic and light-harvesting properties of these structures. We present the results of calculations of light absorption efficiency for hybrid nanostructures comprising light-harvesting complexes and metallic nanoparticles. Modelling is based upon elementary processes, such as FRET, absorption enhancement, fluorescence quenching due to non-radiative energy transfer to metallic nanoparticles leading to Ohmic losses and numerically derived light scattering on metallic nanoparticles. Theoretical results obtained for PCP and LH2 light-harvesting complexes conjugated to metallic nanoparticles of various geometries are compared to the experimental data. In detail we study a very interesting case of prolate nanoparticles, characterized with strong field enhancement and low Ohmic losses rates [6]. We also examine the optical properties of hybrid nanostructures containing modified PCP, reconstituted with different chlorophyll molecules. This work is supported by the WELCOME project "Hybrid nanostructures as a steping-stone towards efficient artificial photosynthesis” awarded by the Foundation for Polish Science. [1] Alexander O. Govorov, Garnett W. Bryant, Wei Zhang, Timur Skeini, Jaebeom Lee, Nicholas A. Kotov, Joseph M. Slocik, Rajesh R. Naik, Nano Lett. 6 (2006), 984-994. [2] Joel Gersten, Abraham Nitzan, Chem. Phys. Lett. 104 (1984), 31-37. [3] Sebastian Mackowski, J. Phys.: Condens. Matter 22 (2010), 193102. [4] Sebastian Mackowski, Stephan Wormke, Andreas J. Maier, Tatas H. P. Brotosudarmo, Hayk Harutyunyan, Achim Hartschuh, Alexander O. Govorov, Hugo Scheer, Christoph Brauchle, Nano Lett. 8 (2008), 558-564. [5] Alexander O. Govorov, I. Carmeli, Nano Lett. 7, 620-625 (2007). [6] H. Mertens, A. Polman, J. App. Phys 105 (2009), 044302.
9:00 PM - E3.25
Degradation and Stability Comparison of M-PPV Polymers.
Giovana Ferreira 1 , Eduardo deAzevedo 2 , Rodrigo Bianchi 1
1 Department of Physics, Federal University of Ouro Preto, Ouro Preto, MG, Brazil, 2 , Physics Institute of São Carlos, São Carlos, São Paulo, Brazil
Show AbstractSince the discovery of electroluminescence properties of organic crystals and conjugated polymers, a considerable number of researches have been done in order to investigate the semiconducting properties of this class of materials and their related applications in optoelectronic devices. Recently efforts have focused on improving the lifetimes of these devices, but the photoxidation process of the organic materials is yet an obstacle for many of their commercial applications. This effect reflects the possibility to design and develop dosimeters where the influence of visible radiation on the optical properties of conjugated materials is more important then improving the luminance and life-time of light-emitting devices made from them. In present work, we compare the stability of M-PPVs polymers with the blue-radiation exposure. We studied the variation of absorption and fluorescence spectra of MEH-PPV, BDMO-PPV and MDMO-PPV solutions in (i) toluene, (ii) chloroform and (iii) chloroform in presence of presence of free radical scavengers (hydroxyquinoline). The results show that PPVs solutions using chloroform as a solvent presents a more accentuated change in its optical properties than the solutions prepared with toluene. This effect is attributed to the presence of free radicals obtained from irradiated chloroform which may acts in the sense to accelerated the MEH-PPV photoxidation process. The optical properties of this system were investigated by photoluminescence, UV-Vis absorption spectroscopy and fluorescence microscopy. The changes in chemical structure of PPV due blue-light irradiation have been investigated by FTIR, 1H NMR and 13C NMR spectroscopy. These results show a direct evidence of main-chain scission and the oxidation of vinyl double bounds induced by the irradiation process as well as the conformation changes in polymers chain. All these effects may be responsible for the blue-shift observed in absorption and photoluminescence spectra after blue-ligth radiation exposure and this result confirms our assumptions that chlorine free radical may be acts accelerated the photoxidation process. The present work was sponsored by FAPEMIG, CNPq, CNPq/INEO and Capes.
9:00 PM - E3.26
A Novel Push-and-pull Λ-shape Molecule as an Emissive Material for Organic Light-emitting Diode.
Thanasat Sooksimuang 1 , Somboon Sahasithiwat 1 , Siriporn Kamtonwong 1 , Laongdao Menbangpung 1
1 , National Metal and Materials Technology Center (MTEC), Klong Luang, Pathumthani, Thailand
Show AbstractA new Λ-shape molecule, 3,12-dimethoxy-7,8-dicarboxylic anhydride-5,6,9,10-tetrahydro [5]helicene (MCTH) was synthesized in good yield by a convenient method and was successfully utilized as a novel emissive material for organic light-emitting diode (OLED). The compound showed good thermal stability with melting temperature at 283 °C. The energy levels of the lowest unoccupied molecular orbital (LUMO, -3.2 eV), the highest occupied molecular orbital (HOMO, -5.8 eV) and the energy band gap (2.6 eV) of this compound were determined using cyclic voltammetric technique. The photoluminescence (PL) spectrum of a dilute MCTH solution in chloroform showed a peak at 504 nm with the full-width at half-maximum (FWHM) of 75 nm and the fluorescence quantum yield was 0.63. The compound was employed as an emitter for an OLED with a configuration of ITO/PEDOT:PSS (35nm)/MCTH (50nm)/Alq3 (30nm)/LiF (1nm)/Al (100nm). The OLED exhibited a turn-on voltage of 4.8 V and the maximum brightness of 14,000 cd/m2 at 10.5 V. The maximum current efficiency of 5.78 cd/A and the power efficiency of 2.79 lm/w were detected at 6.5 V. The device produced a green electroluminescence (EL) emission with CIE coordinates of (x = 0.36, y = 0.56). The EL spectrum of the device was observed a peak at 540 nm with the FWHM of 95 nm. Since MCTH contains a well push-and-pull structure and exhibits very promising properties for emitter in OLED, it opens new windows for molecular structural design and synthesis of other new [5]helicene derivatives as a new family for organic emissive material.
9:00 PM - E3.28
The Durability of Thin-film Encapsulation for Organic Electronics.
Yongjin Kim 1 , Hyungchul Kim 1 , Parisa Pour Shahid Saeed Abadi 1 , Samuel Graham 1 2
1 Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractThe development of thin film barrier layers for the encapsulation of organic electronics is seen as a critical technology to their long reliability. When deposited on flexible substrates, the possibility of mechanically induced failure of the barrier films must be considered which places a limit on the application of these devices. Thus it is critical to understand the performance of barrier layers under mechanical deformation and how this relates to their fundamental mechanical properties. In this work, we present a study of the permeation and mechnical properties of thin film encapsulation barriers. The films consisted of alternating layers of SiOx/SiNx deposited by plasma enhanced chemical vapor deposition as well as Al2O3/ZnO deposited by atomic layer deposition onto 125 μm thick polyethylene terephthalate (PET) substrates. These films were chosen due to their potential to create films with effective water vapor transmission rates below 10-4 g/m2/day. By changing the thickness of the dyads used in the multilayer from 5 -25nm, the impact on permeation rate was determined using the Ca corrosion test method. Mechanical properties were measured using nanoindentation and tensile testing to determine elastic modulus, adhesion, and residual stress. Samples were then flexed in a two point bend fixture to determine the impact of cyclic loading on barrier performance after 10,000 cycles. The performance of the barrier films were analyzed by sealing the coated PET lids over the Ca sensors and testing the corrosion rate of the Ca samples. Water vapor trasmission rates show that the flexible lids have the ability to maintain barrier performance when flexed below a critical radius of curvature. Finally, the addition of a PMMA polymer layer to the barrier architecture is shown to improve the durability of the films.
9:00 PM - E3.29
Rational Design of Charge-transporting Materials for High-efficiency Blue OLEDs.
Asanga Padmaperuma 1 , Evgueni Polikarpov 1 , James Swensen 1 , Lelia Cosimbescu 1 , Philip Koech 1 , Amber Von Ruden 1 , Liang Wang 1
1 , Pacific Northwest National Laboratory, Richland, Washington, United States
Show AbstractOrganic light emitting devices with high quantum efficiencies and low operating voltages have the potential to generate solid state white lighting with high power conversion efficiency. Using previously developed computational tools, we have designed new charge-transporting materials compatible with blue phosphorescent devices. These materials have triplet energies exceeding that of the emitter allowing exciton confinement and thus maximum efficiency and charge transport properties analogous to state of the art transport materials. Device structures designed for optimum charge balance were fabricated to evaluate these materials. Design strategies, chemical and device properties will be discussed.
9:00 PM - E3.3
A Step by Step Strategy for Solution-processed Quantum Dots Light Emitting Diodes.
Helene Bourvon 1 , Stephanie Le Calvez 1 , David Vaufrey 1
1 LETI/DIHS/LTCV, CEA grenoble, Grenoble France
Show AbstractSolution-based printing and coating processes have the potential to dramatically reduce production costs of organic light emitting diodes. This is particularly true for Quantum Dots Light Emitting Diode (QD LEDs), the newborn in the field of LEDs, due to quantum dots price prohibiting wastage. Here, we report on the development of solution-processed QD-LEDs. We have implemented a layer by layer strategy, from a whole small molecule OLED to an hybrid QD LED developed by wet techniques for the first layers and by evaporation for the last ones. Intermediate steps are discussed here. First, we have worked on PEDOT:PSS deposit. PEDOT:PSS solution formulation for inkjet printing and spin coating have been optimised. Wettability on an ITO substrate, jettability of the inkjet formulation and baking conditions have been studied. Surfactant and ethylene glycol have been added to the commercial inkjet grade solution to achieve a better deposit. Anisotropic conductivity of PEDOT:PSS has been noticed and is reported here. Ethylene glycol has demonstrated a strong ability to improve the parallel conductivity by several orders of magnitude, but not the vertical one which is not impacted. Inkjet-printed and spin-coated device performances are compared to complete this first study. Hybrid devices with an efficacy of 12cd/A at 4V have been obtained, with 2.17 % of EQE, and a luminance of 4000 cd/m2 at 4V. Then, next stack optimisation steps have been developed, notably a study of interface effects between PEDOT:PSS and poly-TPD by impedance spectroscopy. Finally, we have succeeded in the development of our first QD-LED based on CdSe core/ CdS-ZnS shell quantum dots emitting at 620nm. Quantum dots were inkjet printed, in order to waste as little as possible this very expensive material.
9:00 PM - E3.30
The Electronic Structure of Tetracyanoquinodimethane (TCNQ) Measured with Synchrotron-based Soft X-ray Spectroscopies and Aided by Density Functional Theory.
Alexander DeMasi 1 , Sang Wan Cho 1 , Andrew Preston 1 , Louis Piper 1 , Kevin Smith 1
1 , Boston University, Boston, Massachusetts, United States
Show AbstractThe organic molecule tetracyanoquinodimethane (TCNQ) has been the subject of study for a number of years, due to its role as an electron acceptor in many charge-transfer complexes. The electronic structure of TCNQ has been measured using synchrotron radiation-excited resonant x-ray emission spectroscopy (RXES) and x-ray absorption spectroscopy (XAS). RXES in particular is useful, as a technique that provides orbital- and site-specific information regarding the bulk electronic structure below the Fermi level. Samples were in the form of thin films, grown in-situ in an organic molecular beam deposition chamber attached to the spectrometer system. Our measurements are compared to the results of density functional theory (DFT) calculations. An understanding of the core-level and valence electronic structure, encouraged by consistency between experiment and theory, is achieved.This work was supported in part by the NSF under CHE-0807368. Experiments were undertaken at the NSLS, which is supported by the U.S. DOE.
9:00 PM - E3.31
Electron Transporting/Injection in Organic Electronic Devices.
Lixin Xiao 1 , Jiaxiu Luo 1 , Zhijian Chen 1 , Bo Qu 1 , Qihuang Gong 1
1 Department of Physics, Peking University, Beijing China
Show AbstractConsidering the electron mobility in organic light emitting device (OLED) is much lower than that of hole mobility, n-type doping was used to improve the performance of organic electronic device. A facile way to fabricate highly efficient OLED with insulator MnO as an electron injecting and transporting material was devised, which eliminates the problem of the oxidation of reactive dopants. The power efficiency (PE) of 1.1 lm/W by inserting 3-nm-thick MnO as the electron injecting layer was obtained, higher than the 0.8 lm/W efficiency for the reference device with 0.5-nm-thick LiF. A thermal co-evaporation layer containing 10% weight of MnO and tris(8-hydroxyquinolato)aluminum as the electron transporting layer showed more efficient electron transport ability, with turn-on voltage of 3.8 V, lower than 7.4 V for the intrinsic Alq3. Meanwhile, the insertion of thin MnO layer between organic photoactive layer and inorganic metal electrode significantly improved performance and stability of organic solar cell compared to device without it. The power conversion efficiency (PCE) of 2.91% by inserting 3-nm-thick MnO was obtained, higher than the 0.91% efficiency for the device without it, and 2.59% for the device with 0.5-nm-thick LiF. Charge transport of rhenium trioxide (ReO3) in organic electronic devices was investigated. The hole injection/transport was blocked and the electron injection/transport was enhanced with doping of ReO3 in organic electronic devices. Thus the charge balance and efficiency of OLED were improved, 2.7 cd/A of current efficiency (CE) at 20 mA/cm2 for the device with ReO3 was higher than 1.5 cd/A for the device without it. In the case of organic photovoltaic cells (OPV), the open-circuit voltage (Voc), 0.58 V, was higher compared to the device without ReO3 (0.44 V) due to the improvement of interface properties. The PCE was increased to 2.27% by the combination of ReO3 (increasing Voc) with poly(3,4-ethylene dioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) (improve hole transport to increase Jsc) on the modification of the anode, higher than 1.85% for the device without ReO3. Keywords: Electron transport; n-Type; OLED; OPV; Metal OxideReferences[1]J. Luo, L. Xiao, Z. Chen, Q. Gong. Highly efficient organic light emitting devices with insulator MnO as an electron injecting and transporting material. Appl. Phys. Lett. 93, 133301 1-3, (2008).[2] Jiaxiu Luo, Lixin Xiao, Zhijian Chen, Bo Qu, Qihuang Gong. Insulator MnO: highly efficient and air stable n-type doping layer for organic photovoltaic cells. Organic Electronics, 11, 664-669 (2010).[3]L. Xiao, S.-J. Su, Y. Agata, H. Lan and J. Kido. Nearly 100% Internal Quantum Efficiency in an Organic Blue Electrophosphorescent Device Using a Weak Electron Transporting Material with Wide Energy Gap. Adv. Mater. 21, 1271 (2009).
9:00 PM - E3.34
1,5 μm Emitting Solution Processed OLEDs Based on Novel Er-doped Organic Complex.
Carmen Coya 1 , Maria Martin 1 , Angel Luis Alvarez 3 , Jesus Martin Gil 2 , Carlos Zaldo 4
1 Teoria de la Señal y Comunicaciones, Rey Juan Carlos University, Madrid Spain, 3 Tecnología Electrónica, Rey Juan Carlos University, Madrid Spain, 2 Tecnología del Medio Ambiente, Valladolid University, Palencia Spain, 4 Materiales Fotónicos, Instituto de Ciencia de Materiales (CSIC), Madrid Spain
Show AbstractThe field of organic electronics evolves rapidly moving in the past few years from a basic research level to the development of new generation devices, in large-scale mass production. In fact, the global organic electronic market is expected to exert a tremendous impact in the future semiconductor industry by their compatibility with flexible substrates, low-cost and low power consumption.[1] However several fundamental and technological challenges have still to be addressed, such as performance, stability, large area processing or even understanding of different factors involved in transport and mobility of organic semiconductors. In this sense, manufacturing cost-effective methods are demanded, as solution-processed techniques that can be easily scaled to large area fabrication. Among those applications of organic devices with an added value, we can cite large-area near-infrared emitters (NIR) over flexible substrates. In the field of Health Sciences, the analgesic and anti-inflammatory properties over human skin of NIR radiation of type A (760 – 1400 nm), or proximal radiation, are well known. OLED panels would allow that rolled, flexible screens may be used around a damaged member. Also, NIR OLEDs have potential applications as photodetectors and emitters in optical communications, specifically for integrated optics by hybrid systems in which light emitted by OLED is coupled via optical waveguides into Si-based electronic devices. This is a field where IR emitters as those mentioned above would play an interesting role. In this work, we describe the fabrication and characterization of solution processed OLEDs based on novel near-infrared emitting erbium(III) complexes, consisting of three perfluoroalkyl-β-diketone ligands and one chelating N,N-donor molecule (2,2-bipyridine, 5-NO2-1,10-phenanthroline, bathophenanthroline). The function of N,N molecules is to saturate the coordination sphere of the erbium ion and to harvest excitation light that can be transferred to the excited states of the erbium ion. The devices have been fabricated by spin-coating, using 1 %wt methanol solutions as active layer precursor solutions. These Er-complex forms very uniform thin films. The OLED structure is ITO/PEDOT:PSS/Er-complex/Ca/Al. The good electrical response, with low threshold voltages (a few volts), together with the very uniform thin films formed, made these complex promising for IR emitting displays.[1]- http://www.strategyr.com/Organic_Electronics_Market_Report.asp.
9:00 PM - E3.35
Highly Fluorescent Dithieno[3,2-b:2',3'-d]pyrrole-based Materials: Synthesis, Characterization, and OLED Devices.
Sean Evenson 1 , Matthew Mumm 2 , Konstantin Pokhodnya 2 , Seth Rasmussen 1
1 Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota, United States, 2 Center for Nanoscale Science and Engineering, North Dakota State University, Fargo, North Dakota, United States
Show AbstractOne approach to tuning conjugated polymers is the introduction of fused aromatic units into the conjugated backbone, resulting in materials exhibiting enhanced carrier mobilities and lowered band gaps. One such fused-ring building block that has found recent popularity are the N-alkyl- and N-aryl-dithieno-[3,2-b-:2',3'-d]pyrroles (DTPs). Since their introduction, these building blocks have been incorporated into various polymeric, oligomeric, and molecular materials to give high carrier mobilities, high solution and solid-state fluorescence, and materials with reduced and low band gaps. One limitation of the current DTP building blocks, however, is the high energy of the HOMO, which limits stability and the effective application to various devices. As a solution to this limitation, we have successfully synthesized a new class of DTPs incorporating N-acyl groups that significantly stabilize the HOMO and LUMO energy levels. The synthesis and photophysical characterization of DTP-based materials utilizing both N-alkyl and N-acyl sidechains will be presented. Initial application of these materials to OLED devices will also be presented.
9:00 PM - E3.36
Laser Action in a Surface-structured Free-standing Membrane Based on a pi-Conjugated Polymer-composite.
Allan Mackintosh 1 , Peter Skabara 1 , Alex Kanibolotsky 1 , Richard Pethrick 1 , Yujie Chen 2 , Benoit Guilhabert 2 , Erdan Gu 2 , Nicolas Laurand 2 , Martin Dawson 2 , Johannas Hernsdorf 2 , Ifor Samuel 3 , Graham Turnbull 3
1 Pure and Applied Chemistry, University of Strathclyde, Glasgow United Kingdom, 2 Institute of Photonics, SUPA, University of Strathclyde, Glasgow United Kingdom, 3 School of Physics and Astronomy, University of St Andrews, St Andrews United Kingdom
Show AbstractSince the realization of organic light-emitting diodes (OLEDs), great effort has been put into the investigation of light amplification in organic materials for laser applications. Attractive features of organic compounds for the fabrication of lasers and other active photonic devices include the potential for low-cost, high-throughput processing and the possibility to obtain ‘plastic-like’, mechanically flexible structures that have desired amplifying characteristics. Such structures could be employed in conformal photonic devices and integrated circuits for a range of potential applications in fields as varied as flexible displays, lighting, sensors and bio-sensing/monitoring. If their fabrication can be made cost-effective enough and their lifetime under typical operating conditions extended, they could also form the backbone for a range of ‘disposable’ visible laser sources. In this paper, we report a simple and potentially cost-effective-way to produce all-organic flexible composite membrane lasers. These membrane lasers operate under photo-pumping, in air and at room-temperature, and use a recently introduced organic semiconductor polymer system as the gain element. In the paper, we describe the concept of our device, its fabrication and present the results of photo-pumping experiments.
9:00 PM - E3.37
Non-resonantly Pumped High-quality-factor Lasers.
Carmel Rotschild 1 , Matthew Tomes 2 , Hiroshi Mendoza 1 , Tal Carmon 2 , Marc Baldo 1
1 EECS, MIT, Cambridge, Massachusetts, United States, 2 EECS, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractMany optical applications, including spectroscopy, sensing, and optical communications, require high-quality factor (high Q) lasers. Currently, high-Q lasers can only be resonantly pumped by a second high-Q laser, which significantly increases the cost, and results in oscillations in the lasing output. In this work, we show a new pumping technique for high-Q lasers that is independent of the coherence of the pump source. This technique is based on efficient cascaded energy transfer from incoherent to coherent light. It overcomes the poor modal-overlap associated with prior approaches to non resonant pumping of high-Q lasers. Experimentally, we deposit a combination of three organic dyes on micro-ring-resonators. The micro-ring-resonators have excellent optical properties and can support Q-factor of 10^8. With such resonators, the optically active materials limit the total Q-factor. To reduce absorption at the lasing wavelength, we employ energy transfer between the three dyes with a total Stokes-shift of about 300nm. The Q-factor reaches 10^6 as the Stokes-shift increases. The energy-transfer starts with AlQ3 molecules. A pulse of UV pump light at 350 nm, is absorbed by the AlQ3 at a sub-micron thickness, and energy is transferred non-radiatively to DCJTB molecules doped into the AlQ3 at a concentration of 1%. DCJTB lases at about 650 nm. The large Stokes-shift and relatively small concentration of DCJTB enables a Q-factor of about 10^5. Next The DCJTB radiatively transfer the energy to Terrylene, which in turn lases at about 680nm wavelength with a Q-factor exceeding 10^6. The concentration of the Terrylene is about 0.001%. Lasing in the presence of such a low concentration of the emissive dye, enables the propagation of the lasing photons for long distances in the cavity, which is reflected at the high Q-factor that we observe. This new pumping technique has no instability associated with conventional resonant pumping of high Q lasers, and may be implemented with incoherent solar light, enabling non-tracking solar powered lasers.
9:00 PM - E3.38
Electroluminescence Characteristics of Inorganic (p-GaN/MgO)-organic (Alq3) Hybrid p-n Junction Light Emitting Diodes.
Akihiko Kikuchi 1 , Tomoyuki Tsuji 1
1 Department of Engineering and Applied Sciences, Sophia University, Tokyo Japan
Show AbstractInorganic-organic hybrid p-n junctions are attractive materials for novel electronic and photonic device application [1, 2]. For example, adequate combination such as high carrier transport property of inorganic materials and low-cost and high optical properties of organic materials is expected to bring about next generation light emitting devices with low-cost and high-performance.In this study, we demonstrated a good current rectifying characteristic and electroluminescence (EL) of the inorganic/organic hybrid junction light emitting diodes (LEDs) of p-GaN/Alq3 (tris-8-hydroxyquinoline aluminum) and p-GaN/MgO/n-Alq3. By the insertion of thin MgO layer between p-GaN and n-Alq3 junction interface, green EL emission from Alq3 was clearly enhanced due to electron blocking effect of MgO which has large conduction band offset to lowest unoccupied molecular orbit (LUMO) of Alq3. The results suggest that band-engineering is effective technique for improve the performance of inorganic-organic hybrid junction devices.Two kinds of sample A and B were fabricated by following procedure. Mg-doped 3μm-thick p-GaN layer grown on (0001) sapphire substrates were used as inorganic part of the hybrid LED. For the sample A, Alq3 solution was directly spin coated on the p-GaN to form ~50nm-thick Alq3 layer then 220nm-thick Al circular (1mm in diameter) n-electrodes were formed by ion-beam sputtering through metal mask. For the sample B, 10nm-thick MgO layer was deposited on the p-GaN by ion-beam sputtering as electron-blocking-layer (EBL) then Alq3 and Al-electrode were formed by the same routine. InGa p-electrodes were formed on p-GaN. The both type of electrodes were confirmed to be ohmic contact. The current-voltage (I-L) and EL characteristics were evaluated at room temperature in atmosphere.Both samples showed almost similar I-L curves in spite of MgO-EBL and had good rectifying characteristics with forward threshold voltage of 3~4V and forward current of 3mA at 20V. EL emission was observed under forward current injection. For both samples, EL spectra had strong UV emission peaked at 371~378nm which cosidered to be emission from p-GaN layer. For the sample B, obvious green emission peaked at around 500nm which coinside with PL emission of Alq3 was also observed. This result indicates that holes injected from valence-band of p-GaN to highest occupied molecular orbit (HOMO) of Alq3 and electrons accumulated in LUMO of Alq3 due to MgO-EBL effectively recombine radiatively.The enhanced green emission from organic material by band-engineering technique is an important result for realization of high-performance inorganic/organic hybrid LED.Acknowledgement: The authors thank to Prof. Kishino of Sophia University for his support. This study was partly supported by Grant-in-Aid for Scientific Research (B) #21310087.References: [1]H. Kim et al. phys. stat. sol. (c) 4 (2007) 2411. [2]J.H. Na et al. Appl. Phys. Lett. 95 (2009) 253303.
9:00 PM - E3.39
Displacement Current Measurement of MIS Devices with Ionic Liquids to Explore Carrier Behaviors in Model Interfaces of Organic Devices.
Taiki Yamada 1 , Yutaka Noguchi 1 2 , Yukio Ouchi 3 , Hisao Ishii 1 2
1 Graduate School of Advanced Integration Science, Chiba University, Chiba Japan, 2 Center for Frontier Science, Chiba University, Chiba Japan, 3 Department of Chemistry, Graduate School of Science, Nagoya University, Nagoya Japan
Show AbstractInterfaces play crucial roles to the device performance of organic electronic devices such as organic electroluminescent devices (OELs) and organic field effect transistors (OFETs). So, in order to understand and improve the performance of organic devices, carrier behavior at organic/organic and organic/electrode interfaces should be experimentally clarified. Displacement current measurement (DCM) has been developed as a good tool to monitor the carrier behavior in organic devices. In this technique, a triangular wave voltage is applied to a device and its current response is measured. Since the displacement current depends on the effective capacitance of the device, useful information on carrier injection and accumulation can be observed. In the previous work of DCM, a metal-insulator-semiconductor (MIS) structure which involves a model interface to be investigated is measured. But, there are some problems: (i) the voltage across an organic semiconductor layer cannot be specified since the voltage drop across an insulating layer is significant,(ii) the process to deposit an insulating layer on an organic layer is cumbersome and sometimes induces damages. In order to overcome these problems, we propose to use a MIS structure with ionic liquids (ILs). IL, which shows a liquid phase at room temperature, is a salt consisting of pairs of an anion and cation. Because of their excellent properties, such as non-volatility, non-flammability, and electrochemical stability, ILs are expected to use as the electrolyte in batteries and the gate dielectric layer of OFETs. A drop of IL on the organic layer will serve as a good insulating layer with less damage. Because IL works as an insulator with very high dielectric constant, the voltage drop across the IL is negligible, and the external voltage is efficiently applied to the organic layer in the device layer. In this study, we examined the carrier behaviors in a device of ITO/ 4 ,4,-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (α-NPD) / IL / Pt by using DCM. The change of the displacement current due to the hole injection and accumulation was successfully observed. The relation between the current and the voltage across the organic layer enables us to analyze the carrier injection mechanism. This proposed method is expected to apply to a wide range of the model interfaces of organic devices.
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Luminescent Materials Based on Lanthanide Quinolinolates: Chemical and Photophysical Studies.
Flavia Artizzu 1 , Paola Deplano 1 , Maria Laura Mercuri 1 , Luca Pilia 1 , Angela Serpe 1 , Francesco Quochi 2 , Giovanni Bongiovanni 2 , Andrea Mura 2
1 Department of Inorganic and Analytical Chemistry, University of Cagliari, Cagliari Italy, 2 Department of Physics, University of Cagliari, Cagliari Italy
Show AbstractSince the first report by Tang and Van Slyke[1] of efficient green electroluminescence from aluminum quinolinolates (AlQ3, Q=8-quinolinolato), extensive work has been performed to improve the properties of organic light emitting diodes (OLEDs) based on “small molecules”, toward the development of flat panel displays and other emitting devices. Lanthanide ions were then proposed as electroluminescent and/or photoluminescent materials for OLEDs and doped optical fibers for display technology and telecommunications since their emission, arising from intra-shell, parity-forbidden, f-f transitions, is long- lived and can be considered as nearly monochromatic. The weak absorptivity of lanthanide ions and the very low solubility of their salts in all inorganic/organic matrices can be overcome by encapsulating them with a “light harvesting antenna” organic ligand which allows optical pumping and provides good solubility in the host material. Such lanthanide complexes could enable the realization of a new generation of low-cost optical amplifiers, featuring superior optical and mechanical figures of merit, and are suitable for the fabrication of photonic integrated circuits.We have reinvestigated a series of NIR-emitting lanthanide complexes with the Q-ligand and its 5,7-dihalo-derivatives.[2] The results of a combined photophysical-structural study on well characterized compounds have allowed drawing up a reliable description of the non-radiative decay processes induced by resonant energy transfer (Förster mechanism) to the oscillating quencher groups of the organic ligands (CH, OH) and have provided the guidelines to design new lanthanide complexes with improved luminescence efficiency.[3] Studies on the excited-state dynamics of these complexes are showing that quinolinolato ligands are particularly effective as “light harvesting antennae”, since they can transfer absorbed energy to the lanthanide ion with high efficiency allowing metal emission enhancement.We are currently addressing our efforts in the processing procedure to incorporate lanthanide quinolinolates into sol-gel glasses and to obtain Ln-doped polymeric films, as promising low-cost materials for optical applications.References: [1] C. W. Tang, S.A. Van Slyke, Appl. Phys. Lett. 1987, 51, 913; [2] F. Artizzu et al. Adv. Funct. Mat., 2007, 17, 2365; [3] F. Quochi et al. J. Appl. Phys., 2006, 99, 0535204; F. Quochi et al. J. Phys. Chem. Lett., 2010, 1, 141.The Regione Autonoma della Sardegna is gratefully acknowledged for financial support through POR Sardegna FSE 2007-2013, L.R.7/2007 “Promozione della ricerca scientifica e dell’innovazione tecnologica in Sardegna”.
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Self-assembled Nanolayers of Conjugated Silane with Π-Π Interlocking.
Jinyue Jiang 1 , Li Tan 1
1 , University of Nebraska, Lincoln, Nebraska, United States
Show Abstract Highly ordered, freestanding semiconducting layers with a unit thickness as small as a few nanometers are currently receiving wide attention from physical science researchers and are actively being investigated by the organic electronics and sensor communities. The surface-directed growth of monolayers has reached a level of maturity where many functional molecules, such as alkane thiols or π-conjugates, can be regulated by a S-Au bond or electrostatic interactions. While multistack construction using monolayers is possible, the widely practiced fabrication method of using a hydroxylated surface limits ordering in the upper layers. Freestanding and one dimensional structures (1D) can be constructed by using substrate-free pathways, achieved most commonly in a liquid environment through Π-Π stacking or hydrogen bonding. Grouping them further to produce a sheet-like object is less common due to the complexity in creating strong interactions or growth mechanisms in organic molecules in two different directions. While it is difficult to pack many organic molecules into a freestanding object due to their weak intermolecular forces, a layered stacking of organosilane (formula of R-SiX3, R is alkane and X can be halogen or alkoxy) is feasible and there have been attempts to provide organosilanes with electronic functions by replacing the alkyl moiety with a bulky π-conjugated unit. The resulting substrate-free assembly, however, revealed tubular structures rather than the expected planar objects. This was due, very likely, to the cross-linked (tetrahedral) configuration of the siloxane inside the two dimensional (2D) network, where close packing or an ordered grouping of π-units in plane became unstable. Presumably, when this very strained packing is partially relaxed by releasing the siloxane network to spatially aligned polysiloxane chains, a freestanding and highly ordered 2D object from the conjugated silane molecules may still be possible; and that is the focus of this presentation. To demonstrate our concept of planar construction, anthracene, a simple acene with three fused benzene rings, was used to represent the conjugated moiety on organosilane; and only two active terminals were stitched on a silicon atom. This design led to the building block, namely (12-(anthracen-2-yl)dodecyl)diethoxy(methyl)silane, for our nanostructure. A linear polymer chain is produced following silane self-assembly and the subsequent polycondensation. As a result, hydrophobic branches plus a noncovalent Π-Π interlocking between the molecules promote planar packing and continuous stacking along the surface normal. In contrast to conventional Π-Π stacking or hydrogen bonding pathways in a fibrous construct, multistacked nanolayers with coexisting Π-Π and herringbone interlocking can provide unmatched properties and processing convenience in molecular electronics.
9:00 PM - E3.41
Hybrid Organic-inorganic Light Emitting Diodes.
Henk Bolink 1 , Michele Sessolo 1 , Hicham Brine 1
1 Instituto de Ciencia Molecular, University of Valencia, Paterna Spain
Show AbstractRecently, transition metal oxides (TMO) have been employed as charge transport and particularly as electron injection layers in hybrid organic-inorganic light emitting diodes (HyLEDs), demonstrating the possibility to prepare air-stable electroluminescent devices.[1, 2] The use of an unreactive metal oxide as the cathode is appealing as this would allow the preparation of OLEDs having no or only a simple encapsulation, hence significantly reducing the cost. Such a cost reduction greatly enables the use of OLEDs in display and especially lighting applications.Promising luminance values, >30000 cd/m2, at low voltages have been reported for F8BT based HyLEDs.[1-3] A simple model was described that showed that the efficiency is determined by the device ability to block holes at the metal oxide-organic light emitting interface.[4] The use of thin interlayers of Cs2CO3 that are deposited in between the metal oxide and the organic luminescence layer, effectively block the holes and increases the device efficiencies.[5, 6] Additionally, with this interlayer wider bandgap light emitting materials can be used in allowing for the achievement of white light emitting[7] and phosphorescent HyLEDs.[8] We will report on further advances in HyLEDs obtained using solution processed interlayers and high bandgap metal oxides. The possibility of processing from solutions together with an increased efficiency improves the versatility of the devices making this class of devices a real alternative to the established OLEDs.[1]K. Morii, M. Ishida, T. Takashima, T. Shimoda, Q. Wang, M. K. Nazeeruddin, M. Graetzel, Appl. Phys. Lett. 2006, 89, 183510.[2]H. J. Bolink, E. Coronado, D. Repetto, M. Sessolo, Appl. Phys. Lett. 2007, 91, 223501.[3]D. Kabra, M. H. Song, B. Wenger, R. H. Friend, H. J. Snaith, Adv. Mater. 2008, 20, 3447.[4]H. J. Bolink, E. Coronado, D. Repetto, M. Sessolo, E. Barea, J. Bisquert, G. Garcia-Belmonte, J. Prochazka, L. Kavan, Adv. Funct. Mater. 2008, 18, 145.[5]K. Morii, T. Kawase, S. Inoue, Appl. Phys. Lett. 2008, 92, 213304.[6]H. J. Bolink, E. Coronado, J. Orozco, M. Sessolo, Adv. Mat. 2009, 21, 79.[7]H. J. Bolink, E. Coronado, M. Sessolo, Chem. Mater. 2009, 21, 439.[8]H. J. Bolink, H. Brine, E. Coronado, M. Sessolo, Adv. Mater 2010, 22, 2198.
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Organic Light Emitting Diode Having a Liquid Emitting Layer Doped with an Organic Salt.
Heo Hyo Jung 1 , Shuzo Hirata 1 , Korefumi Kubota 1 , Osamu Hirata 1 , Kenichi Goushi 1 , Masayuki Yahiro 1 , Chihaya Adachi 1
1 Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Fukuoka Japan
Show Abstract Recently, we reported a unique OLED based on a neat liquid emitting layer (liquid OLED) which will open another novel possibility of OLED applications1). For example, it is expected that liquid OLEDs are useful for high reliable flexible display applications because the detachment between a liquid emitting layer and electrodes does not occur in these flexible devices. Further, liquid emitters degraded by the long use of OLEDs can be replaced by fresh organic liquid emitters, resolving the device degradation ascribed to the decomposition of career transport materials and emitters. However, liquid OLEDs have serious problems such as low maximum luminance, low efficiency, and high applied voltage. Here we report improvement of external EL quantum efficiency (EQE), the maximum luminance, and turn-on voltage in the liquid OLEDs. We designed a new liquid OLED structure consisting of a neat liquid semiconducting layer doped with an organic salt as an emitting layer and titanium oxide (TiO2) as a hole blocking layer. 9-(2-ethylhexyl)carbazole (EHCz) and tetrabutylammonium hexafluorophosphate were used as a liquid host and an organic salt, respectively. A fabricated liquid OLED structure is indium-tin oxide (ITO) (anode)/PEDOT:PSS [40 nm]/0.1 wt%-organic salt, 16.7 wt%-guest compound showing green emission, EHCz [1100 nm]/TiO2 [10 nm]/ITO(cathode). The driving voltage is decreased drastically and EQE was increased by doping of only 0.1 wt%-organic ionic salt into a neat liquid emitting material. Further, the carrier balance was improved by the insertion of hole blocking layer such as TiO2. This novel designed liquid OLEDs showed EQE of 0.4 % and the maximum luminance higher than 100 cd/m2, which are 10 times and 100 times higher than previous report1), respectively.1) D. Xu and C. Adachi, Appl. Phys. Lett. 95, 053304 (2009).
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Integration of Organic Nanofibers as Active Components into Electronic Devices.
Jakob Kjelstrup-Hansen 1 , Kasper Thilsing-Hansen 1 , Luciana Tavares 1 , Roana Melina de Oliveira Hansen 1 , Horst-Guenter Rubahn 1
1 Mads Clausen Institute, University of Southern Denmark, Sønderborg Denmark
Show AbstractMuch research within organic materials focuses on large-scale applications due to their competitive price and straight-forward processing on large substrates. However, organic nanoparticles have recently received much attention as they can add increased functionality to small-scale systems for example within optoelectronics. Organic nanostructures such as nanofibers can be prepared by self-assembly of suitable molecules such as phenylene-based oligomers [1,2]. This particular self-assembly process takes place by vapor deposition typically onto a special crystalline substrate such as muscovite mica. For device applications where the nanofibers should be interfaced to surrounding circuitry, such substrates are impractical, and the challenge therefore lies in the integration, i.e. controlled positioning and interfacing. We have previously demonstrated how individual nanofibers can be interfaced to metal electrodes, however, using an integration strategy that is not suitable for high-volume fabrication [3].In this work, we report on two different strategies for integration of organic nanofibers on a field-effect transistor platform. The first method relies on nanofiber growth on a conventional substrate (muscovite mica) and subsequent transfer to a device platform by a soft stamping technique. This method can both enable bottom contacts by transfer onto pre-fabricated electrodes and top contacts by electrode deposition through a stencil mask after the nanofiber transfer. The second method involves the use of an alternative growth substrate, on which the nanofibers are formed directly on metal electrodes on a pre-fabricated device substrate. This requires the use of a special substrate with surface structures that can guide the nanofiber growth [4,5]. We have studied the electrical properties of nanofibers integrated with both methods and found that electrical connection can be established, however, with different device characteristics caused by the different integration schemes. References[1] F. Balzer and H.-G. Rubahn, Avd. Func. Mater., 15 (2005) 17[2] M. Schiek, F. Balzer, K. Al-Shamery, J. R. Brewer, A. Lützen, and H.-G. Rubahn, Small, 4 (2008) 176[3] H. H. Henrichsen, J. Kjelstrup-Hansen, D. Engstrøm, C. H. Clausen, P. Bøggild, and H.-G. Rubahn, Org. Electron., 8 (2007) 540[4] R. M. de Oliveira Hansen, J. Kjelstrup-Hansen, and H.-G. Rubahn, Nanoscale, 2 (2010) 134[5] R. M. de Oliveira Hansen, M. Madsen, J. Kjelstrup-Hansen, and H.-G. Rubahn, submitted to Nanotechnology
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A Solution-processed Alternative for the Electrode Bilayer in Organic Electronics.
Taner Aytun 1 , Ayse Turak 2 , Grzegorz Halek 3 , Iain Baikie 3 , Cleva Ow-Yang 1
1 Materials Science & Engineering, Sabanci University, Istanbul Turkey, 2 , Max Planck Institute for Metals Research, Stuttgart Germany, 3 , KP Technology Ltd., Caithness, Scotland United Kingdom
Show AbstractLithium fluoride (LiF) is commonly used to enhance charge injection and extraction at electrode bilayers in organic electronics. However, the conventional processing of LiF typically requires high vacuum, such as thermal evaporation. We report the development of an ambient, solution-processable alternative, in which polymeric reverse micelle reactors are used to synthesize LiF particles. The role of the micelles are two-fold, to control the synthesis of LiF nanoparticles, as well as to assist in the deposition of the nanoparticles into a well-ordered, 2-D layer during spin coating on indium tin oxide (ITO). Oxygen plasma etching is subsequently used to remove the polymer and expose the sub-monolayer film of LiF particles. To control the density of LiF covering the substrate surface, the spin-coating and O2-plasma etching of LiF-loaded micelles were repeated 3-5 times. To assess the performance of the solution-processed LiF fabricated as a part of the device electrode, a sub-monolayer film of LiF was also deposited onto ITO by thermal evaporation, and the work function of all samples was characterized by scanning Kelvin Probe. While thermal evaporated LiF lowers the ITO surface work function, the solution-processed LiF increases that of bare and O2-plasma etched ITO. The work function increase also scales with the LiF coverage of the ITO surface. These findings support the suitability of solution-processed LiF for electrode bilayers in organic solar cells, additionally allowing the engineering or tuning of the electrode bilayer work function. By minimizing the electrode work function offset from the organic semiconductor valence band edge, the overall device efficiency would be enhanced for harvesting solar energy.
9:00 PM - E3.45
Self Assembled Monolayer of Alkanethiols on Pt (111): A Density Functional Theory Study.
Yenny Cardona Quintero 1 , Hong Zhu 1 , Rampi Ramprasad 1
1 Chemical, Materials & Biomolecular Engineering , University of Connecticut, Storrs, Connecticut, United States
Show AbstractThe alkylthiolate (S-(CH2)n-1-CH3) self assembled monolayers (SAMs) on metals have been widely studied due to a broad range of applications. One of these metals is Pt, in which some studies with SAMs have been performed, but there are still unanswered questions about the stable adsorbed geometry and the extent to which differently functionalized SAMs control the work function. Here, we present a study of alkylthiolates and fluorinated alkylthiolates on Pt (111) for the full coverage of (√3 x √3) R30° using density functional theory (DFT). In this study we have included two kinds of orientations for the thiol molecule, namely, with the thiol tilted with respect to the normal of the metal and with the thiol vertical to the metal surface. Our study focuses on the stable geometries of the SAMs on the Pt surface and the metal surface work function shifts due to these molecules. Different chain lengths and different amounts of fluorine were considered in an attempt to identify trends in work function shifts. We found that the stable position of the thiol molecule in Pt (111) is the hollow position for both fluorinated and non-fluorinated thiols and that the tilted molecules are favored compared to the vertical ones. Moreover, we established that the fluorinated SAMs shift the work function to larger values (relative to the clean metal surface value), while the non-fluorinated thiols have the opposite effect.
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Theoretical and Experimental Study of Singlet Generation Fraction in Organic Light-emitting Diodes.
Changgua Zhen 1 , Yanfeng Dai 2 , Zhikuan Chen 2 , John Kieffer 1
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 , Institute of Materials Research and Engineering, Singapore Singapore
Show AbstractAlthough organic light-emitting diodes (OLEDs) using phosphorescent emitters (PhOLEDs) have shown ~20% external quantum efficiency (EQE), the efficiency roll-off at high current density is severe, which limits their application when high brightness is required, such as for outdoor lighting. On the other hand, OLEDs using fluorescent emitters (FOLEDs) exhibit better efficiency roll-off due to shorter exciton lifetime and less bimolecular quenching. However, the FOLEDs can only use singlet excitons, to convert electrical energy to light, which limits the EQE to 5%, because the fraction of singlet is 25% based on the simple statistics. In contradiction to this, there have been many reports in which fluorescent OLEDs have EQE larger than 5%. Our survey of the OLED data published in the last twenty years indicates that a maximally 25% singlet generation fraction is in disagreement with most of these experimental results. In fact, using a simplified model, the singlet generation fraction of the emitters was calculated to be between 40% and 70%, which fits the OLED data well.Our simulation-based predictions have already been translated into experiment. By optimizing molecular structures and device configuration, we achieve deep blue FOLED with a maximum EQE of 10.4% (at 47 mA/cm2, 3650 cd/m2) and CIE coordinates of (0.15, 0.09). Even at very high injection current density of 132 mA/cm2 (9170 cd/m2), the EQE remains at 9.3%.
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A Novel, Water-soluble Hole Injection/Extraction Layer for Improving Efficiency and Lifetime of Organic Optoelectronics.
Mi-Ri Choi 1 , Tae-Hee Han 1 , Kyung-Geun Lim 1 , Seong-Hoon Woo 1 , Dal Ho Hur 2 , Tae-Woo Lee 1
1 Department of Materials Science and Engineering, POSTECH, Phohang Korea (the Republic of), 2 Research Institute of Chemical & Electronic Materials, Cheil Industries INC, Uiwang-si Korea (the Republic of)
Show AbstractIn organic optoelectronic devices, charge injection/extraction layers play an important role in achieving good performance in terms of device efficiency and lifetime. In particular, solution processable conducting polymers are a very good candidate for hole injection/extraction buffer layers(HILs/HELs) in organic light-emitting diodes(OLEDs) and organic photovoltaic cells(OPVs). To date, many researchers have been focused on the water-dispersible conducting polymers such as poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate)(PEDOT:PSS) and polyaniline-poly(4-styrenesulfonate)(PANI:PSS). Here, we introduce a novel, water-soluble, self-doped conducting polyaniline graft copolymer(PSS-g-PANI) for HILs/HELs in small-molecule OLEDs and OPVs. We systematically controlled the self-organized surface layer of the conducting buffer layer by incorporating a perfluorinated ionomer(PFI) into the PSS-g-PANI solutions. Then we investigated the correlation between the surface work function of layers and hole injection/extraction capabilities. Upon spin-coating PSS-g-PANI/PFI solutions, the PFI is self-organized to form PFI-enriched surface layer in the film, which increases the surface work function. In OLEDs, we clearly observed nearly ohmic-contact and high hole injection efficiency of PSS-g-PANI/PFI layers by using trap-free space-charge-limited current(SCLC) model and a Poole-Frenkel equation in hole-only devices. The green fluorescent OLED devices employing the PSS-g-PANI/PFI composition as a HIL exhibited a much higher luminous efficiency of ~20 cd/A than that using a small molecular HIL(~12 cd/A), which is attributed to improved hole-injection because of high work function and improved electron-blocking due to PFI-enriched surface layer. We also found that the layers of PSS-g-PANI/PFI remarkably improved OLED device lifetime by more than 36 times than that of PEDOT:PSS because of the higher doping stability of PSS-g-PANI and the good blocking properties against the In and Sn diffusion from the anode into the emitting layer. In OPVs, the work function of HELs did not make a meaningful effect on hole extraction because the higher work functions of the HELs are pinned to the HOMO level of a photoactive donor. The power conversion efficiency of OPVs using PSS-g-PANI and PSS-g-PANI/PFI(~3.5%) was larger than that using PANI/PSS(~3.2%) and similar to that using PEDOT:PSS(~3.5%). We significantly enhanced the lifetimes of devices using water-soluble and stable self-doped PSS-g-PANI compositions. The half-lifetime of OPVs using PSS-g-PANI/PFI(>500 hrs) was greatly improved compared with those using PSS-g-PANI(~267 hr) and PEDOT:PSS(~156 hr). In conclusion, water-soluble PSS-g-PANI compositions (PSS-g-PANI or PSS-g-PANI/PFI) in place of conventional PEDOT:PSS and PANI:PSS can be a good candidate for HIL/HELs in organic optoelectronic devices because they possess tunable work function, high doping stability and good barrier against atomic diffusion.
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High Efficiency Near-infrared Phosphorescent Organic Light-emitting Devices.
Yixing Yang 1 , Jonathan Sommer 2 , Kenneth Graham 2 , Sang-Hyun Eom 1 , John Reynolds 2 , Kirk Schanze 2 , Jiangeng Xue 1
1 Department of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 Department of Chemistry, Center for Macromolecular Science and Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractNear-infrared (NIR) organic light-emitting devices (OLEDs) are attracting more and more interests due to their potential applications in a number of areas including defense, biomedical sectors, and telecommunications. Previous NIR OLEDs have been mostly based on lanthanide-based complexes, which generally have low external quantum efficiencies (EQE) (<0.1%). Only recently were high efficiency NIR OLEDs reported by using some novel classes of NIR emitters. Thompson and co-workers have reported NIR OLEDs based on a Pt-porphyrin complex, Pt-TPTBP, with peak emission at λ ≈ 770 nm and a maximum EQE up to 8.5%.1 We have also reported OLEDs based on NIR emitting fluorescent donor-acceptor-donor oligomers, and achieved peak emission up to 815 nm with a maximum EQE of 3.1%.2,3Here we report high efficiency phosphorescent NIR OLEDs based on a variety of expanded conjugation Pt-porphyrins including benzo-, naphtho-, and anthroporphyrin. First, a series of differentially substituted Pt-benzoporphyrins was studied. The structural factors including the steric effect of meso-aryl bulky end groups and the planarity of molecules, were demonstrated to influence the phosphorescence and electrophosphorescence efficiencies of Pt-benzoporphyrins. By applying the structure modification to the molecules, their quantum yield in the solution can be increased to as high as 87%, almost doubled compared to Pt-TPTBP reported by Thompson group.1 In the NIR OLEDs based on this series of molecules, we obtain a maximum EQE up to 9.2%, while the peak emissions were maintained at 770 nm. It was found the EQE improvement in OLEDs is less than the corresponding quantum yield improvement in the solution (twice). This inconsistence can be explained by the results of transient electroluminescence study, which shows the quantum yield improvement in the solid state is less pronounced than in the solution.The emissions were further red shifted to longer wavelengths by extending the conjugation of Pt-benzoporphyrins by replacing the benzo-groups with naphtho- and anthro-groups (Pt-TPTNP and Pt-Ar4TAP, respectively). In the Pt-TPTNP-based OLEDs, we obtain peak emission wavelength at 892 nm and maximum EQE of 3.8%. The emissive wavelength for Pt-Ar4TAP based devices was peaked at 1005 nm, although the device EQE was significantly lower, only 0.25%, which was related to the significantly lower quantum yield of the emitter.1 Y. Sun et al., Appl. Phys. Lett. 90, 213503 (2007). 2 Y. Yang et al., Appl. Phys. Lett. 93, 163305 (2008).3 Y. Yang et al., J. Appl. Phys. 106, 44509 (2009).
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Sub-Micrometer Patterning of Amorphous and beta-Phase Morphology in a Crosslinkable Poly(9,9-dioctylfluorene) Derivative – Demonstration of a Two-Wavelengths Laser.
Alexander J. Kuehne 1 , Markus Kaiser 1 , Allan Mackintosh 2 , Dirk Hertel 1 , Richard Pethrick 2 , Klaus Meerholz 1
1 Department of Chemistry, University of Cologne, Cologne, NRW, Germany, 2 WestCHEM, University of Strathclyde, Glasgow, Scotland, United Kingdom
Show AbstractManipulating the morphology of conjugated polymers is one of the key issues for improving organic optical and optoelectronic devices. Thin films of poly(9,9-dioctylfluorene) (PFO) that are spin coated from chloroform are amorphous and can be treated with toluene vapor to induce enhanced chain planarity along the fluorene backbone. Samples with a significant amount of polymer chain segments in this conformational morphology are termed beta-phase. The beta-phase is metastable and is hence being erased when heated above the respective transition temperature or when exposed to a polar solvent, which is undesirable for subsequent thin film processing. Samples of beta-phase PFO show an increased photoluminescence quantum yield at ambient temperatures, polarized light emission, enhanced charge carrier transport and superior optical gain properties compared to the amorphous phase. Beta-phase PFO is considered one of the most promising materials to achieve electrically pumped organic lasers. Here, we report on a novel vinyl ether-functionalized polyfluorene that may be crosslinked in the solid state. We demonstrate that crosslinking of the vinyl ether moieties locks in the previously induced amorphous or beta-phase morphology in the resulting polyfluorene structure. This crosslinking process enables subsequent wet thin-film processing and opens up the possibility of patterning multiple layers of PFO in either amorphous or beta-phase, which would not be possible using regular PFO. We show that electron-beam lithography (EBL) provides a means for sub-wavelength patterning of the crosslinkable polyfluorene films. As a specific application, optically pumped second-order distributed-feedback lasers utilizing corrugated bilayer thin films are demonstrated. Gratings consisting of amorphous polyfluorene are initially written by EBL. After a development step the crosslinked gratings are then backfilled with the same vinyl ether-functionalized polyfluorene, which is subsequently transferred into the beta-phase. The resulting bilayer device consists of two interpenetrating grating layers of amorphous and beta-phase morphology. The EBL crosslinking process can be used to adjust the grating periods of these double layer devices as desired. When tuned towards the corresponding grating period for laser emission of either the amorphous or the beta-phase, the devices exhibit lasing at two wavelengths in accordance with the two layers of different morphologies.
9:00 PM - E3.5
Low Band Gap Polymers for Near Infrared Light-emitting Diodes.
Sybille Allard 1 , Dietrich Breusov 1 , Seyfullah Yilmaz 1 , Penglei Li 2 , Oliver Fenwick 2 , Franco Cacialli 2 , Ullrich Scherf 1
1 Macromolecular Chemistry and Institute for Polymer Technology, University of Wuppertal, Wuppertal Germany, 2 London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London United Kingdom
Show AbstractRecently, polymeric materials with absorption and emission in the near infrared have attracted attention for their potential use in photovoltaic devices[1], photodetectors[2] and light-emitting diodes[3].Here we present the synthesis and characterization of three different low band gap polymers for applications in near infra-red light emitting diodes and infra-red sensitized solar cells.The first polymer synthesized is poly[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene-2,6-diyl)-alt-(2,3-diphenyl-5,7-dithiophen-2-yl-thieno[3,4-b]pyrazine-5,5'-diyl], Ph-pyrazine-CPDT (Mn = 5000 g/mol, Mw = 8000 g/mol) with an optical band gap of 1.64 eV. The electroluminescence spectra (Fig.2) show that the emission of pristine Ph-pyrazine-CPDT is in the near-infrared range with a peak at 957nm and a tail extending beyond 1100nm. With 5 wt% of Ph-pyrazine-CPDT blended into poly[(9,9’-dioctyl-9H-fluorene-2,7-diyl)-alt-(benzo[c][1,2,5]thiadiazole)], F8BT, the emission peak is 75nm blue-shifted with respect to pristine Ph-pyrazine-CPDT indicating decreased aggregation in the blend. Although the F8BT emission was mostly quenched in the blend, the overall efficiencies were low, maybe suggesting that remaining aggregation effects in the Ph-pyrazine-CPDT were reducing the efficiency. We synthesized another two polymers, poly[(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene-2,6-diyl)-alt-(benzo[c][1,2,5]selenadiazole-4,7-diyl)], BS and poly[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene-2,6-diyl)-alt-(4,7-dithiophen-2-yl-benzo[c][1,2,5]selenadiazole-5,5'-diyl)], TBST. Both polymers incorporate benzoselenadiazole units, and differ from each other by thiophene spacers (in TBST). The polymers can be considered as analogues to previously reported copolymers containing cyclopentadithiophene (CPDT) and benzothiadiazole units, which have been successfully used in bulk heterojunction solar cells with PCBM. Polymer BS (Mn = 18700 g/mol, Mw = 31300 g/mol) has an optical band gap of 1.74 eV, the optical band gap of polymer TBST (Mn = 10300 g/mol, Mw = 20000 g/mol) is 1.63 eV. Light- emitting diodes were fabricated with each of the two polymers in blends with F8BT. We observe nearly complete quenching of F8BT emission >99 % of the electroluminescence from BS or TBST at just 5 wt.% in the blend and external quantum efficiencies of ~ 0.14 % (BS) and ~ 0.24 % (TBST). At 2 % concentrations of TBST we obtain slightly increased efficiencies of ~ 0.30 % without significant increase in visible emission.[1] J. Peet et al., Nat. Mater., 2007, 6, 497[2] P. Pneumans, et al., J. Appl. Phys. 2003, 93, 3693[3] A. C. Arias, Chem. Rev., 2010, 110, 3
9:00 PM - E3.50
Single Molecules of MEH-PPV are Highly Polarized.
Girish Lakhwani 1 , Matthew Traub 1 , Joshua Bolinger 1 , Paul Barbara 1
1 Nano Science and Technology, University of Texas at Austin, Austin, Texas, United States
Show AbstractExcitation and emission polarization experiments done simultaneously over single molecules of MEH-PPV reveal high average anisotropies above 0.6. The difference in the anisotropies between excitation and emission is centered at 0.1 with latter being higher for the majority of the molecules. The average change in the in-plane transition dipole moment direction between the excitation and the emission is found to be 0.15 radians, where the molecules of lower excitation anisotropies show relatively large differences. The results strongly indicate that the alignment of the emitting sites reflects that of the overall molecule. An efficient resonant energy transfer process between chromophores resulting in the fluorescence from a few highly polarized emitting sites is proposed and modeled. These findings at a single molecule level are of practical importance in order to develop a bottom-up approach in understanding the photophysics of the conjugated polymer systems in bulk devices.
9:00 PM - E3.52
Enhancement in Quantum Efficiency and Stability of III-V Quantum Dots by the Composition Gradient ZnSexS1-x Shells.
Jaehoon Lim 1 , Wan Ki Bae 3 , Donggu Lee 3 , Min Ki Nam 2 , Joohyun Jung 2 , Changhee Lee 3 , Kookheon Char 1 , Seonghoon Lee 2
1 School of Chemical and Biological Engineering, Seoul National University, Seoul Korea (the Republic of), 3 School of Electrical Engineering and Computer Science, Seoul National University, Seoul Korea (the Republic of), 2 School of Chemistry, Seoul National University, Seoul Korea (the Republic of)
Show AbstractInP quantum dots (QDs) prepared by wet chemistry have been regarded as promising alternatives to replace Cd-based II-VI QDs due to low toxicity and bandgap tunability covering the near-infrared region. However, the achievement of their full potentials has been hampered by poor quantum efficiency and stability during intensive purification, hybridization, and device fabrication, which are essential for practical applications. In order to overcome the difficulties mentioned above, we introduced a thick ZnSexS1-x shell in the form of the composition gradient (InP@ZnSexS1-x) to protect the InP cores from oxidative degradation and also to minimize the formation of defect sites due to the lattice mismatch. The composition gradient of Se in the shell phase was easily realized by one-pot approach with a simple addition of both Se and S precursors all at once. The composition gradient is such that Se is dominantly located close to the InP cores while S content increases along the radial direction of the shell. The spatial variation of Se content was verified by the spectral evolution in photoluminescence and energy dispersive X-ray analysis. We confirm that a significant enhancement in quantum efficiency was achieved only if a small amount of Se was reacted. This implies that the uniform growth of ZnSexS1-x shells is guaranteed by the reduction of lattice strain by Se as well as by the thermodynamically controlled shell growth. With the systematic investigation on the relationship of shell nanostructure with the stability against UV irradiation as well as ligand exchange, it is shown that thick gradient shells exhibit the most improved stability against the shell degradation. This enhancement is attributed to the efficient confinement of exciton wavefunctions from the QD surface, otherwise, shell oxidation could be initiated. Based on the enhanced stability against intensive purification process, we were able to demonstrate green- and red-emitting III-V QLED devices.
9:00 PM - E3.53
New Cationic Iridium Complexes and Their Application in Organic Electroluminescent Devices.
Lei He 1 , Lian Duan 1 , Juan Qiao 1 , Liduo Wang 1 , Yong Qiu 1
1 Department of Chemistry, Tsinghua University, Beijing, Beijing, China
Show AbstractLight-emitting electrochemical cells (LECs) are attracting wide-spread interest because of their notable features such as single-layer, solution-process, air-stable cathodes and low operating voltages [1-2]. The widely used emitting materials in LECs are cationic iridium complexes (CICs) which possess tunable light emission color and high phosphorescent efficiencies [2]. Highly efficient LECs based on CICs are promising candidates for solid-state lighting applications.Conventional CICs used for LECs contain bpy-type ancillary ligands (bpy is 2,2’-bipyridine). For these complexes with bpy-type ancillary ligands, it is difficult to achieve blue light emission [2]. We proposed a new molecular design strategy to achieve blue-light-emitting CICs, that is, to insert electron-donating atoms into the rings of ancillary ligands. By replacing bpy with 2-(1H-pyrazol-1-yl)pyridine, we observed a more than 100 nm blue-shift of the emission of CICs. By replacing bpy with 2-(1-phenyl-1H-pyrazol-3-yl)pyridine, we observed a similar more than 100 nm blue-shift of the light emission of CICs, which demonstrates the great feasibility of the molecular design strategy we proposed. LECs based on new blue-emitting CICs showed an electroluminescent (EL) peak at 460 nm and Commission Internationale de L’Eclairage (CIE) coordinates of (0.20, 0.28), which represent the bluest CICs-based LECs reported so far.Beside pyrazole-type ancillary ligands, we used imidazole-type ancillary ligands to obtain new cationic iridium complexes. By replacing bpy with 2-(1-phenyl-1H-imidazol-2-yl)pyridine, we observed a nearly 30 nm blue-shift of the light emission of CICs. In addition, by adjusting the conjugation length of imidazole-type ancillary ligands, we obtained blue-green, green, yellow, orange and red-emitting CICs. LECs based on these CICs showed highly efficient blue-green to red electroluminescence. White LECs have been fabricated with CIE coordinates of (0.37, 0.41), a color-rendering index value up to 80 and a peak power efficiency of 11.2 lm/W, which represents the most efficient white LECs reported so far.We also used CICs as dopants in solution-processed organic light-emitting diodes (OLEDs). Highly efficient blue to red and white electroluminescence has been achieved with peak current efficiencies of 24.3 and 20.7 cd/A for the blue-green and white OLEDs, respectively, which demonstrates the great potential for the use of CICs in organic electroluminescent devices.[1] Q. B. Pei, G. Yu, C. Zhang, Y. Yang, A. J. Heeger, Science. 1995, 269, 1086.[2] J. D. Slinker, J. Rivnay, J. S. Moskowitz, J. B. Parker, S. Bernhard, H. D. Abruña, G. G. Malliaras, J. Mater. Chem. 2007, 17, 2976.
9:00 PM - E3.54
Employing Photo-assisted Ligand Exchange Technique in Layered Quantum Dot LEDs.
Wenjia Hu 1 2 , Shuai Gao 1 2 , Ting Zhu 1 , Paras Prasad 3 , Jingkang Wang 2 , Jian Xu 1
1 Engineering Science and Mechanics, The Pennsylvania State University, University Park, Pennsylvania, United States, 2 Chemical Engineering and Technology, Tianjin University, Tianjin China, 3 Institute for Lasers, Photonics and Biophotonics, University at Buffalo, The State University of New York, Buffalo, New York, United States
Show AbstractColloidal semiconductor quantum dots (QDs), by virtue of the large tunability in their bandgap, high luminance efficiency, narrow spectral emission, and high photo-stability, are promising lumophores and sensitizers in next-generation optoelectronic devices, such as light emitting diodes (LEDs) [1], photodetectors [2], and solar cells [3]. However, the surface of the QDs is often capped with long-chain ligand molecules, which are electrically insulating and thus hinder the electronic applications of the QDs. Ligand exchange with shorter and more conductive surfactant molecules is necessary for improved device performance [4]..We present in this conference a photo-assisted ligand exchange approach where light will be introduced to facilitate the replacement of oleic acid (OA) ligand molecules over PbSe QDs. The ligand-exchanged QDs were used to fabricate quantum dot light-emitting-diodes (QD-LEDs), which outperform the QD devices without ligand-replacement. Tert-Butyl N-(2-mercaptoethyl) carbamate (tBOC) was used, for the first time, to replace OA molecules over as prepared PbSe QDs [5-6], which was confirmed with FTIR spectra. Since the chemical bond between carbonyl and secondary amine of tBOC is unstable, the chemical bond will break upon UV light exposure. As a result, the OA ligand of 18 carbon-chain will be replaced with a new ligand containing only 4 carbon atoms in the molecule. The replacement of short-chain molecules over PbSe QDs could potentially enhance the carrier injection into QDs in light of the increased tunneling probability. QD-LEDs containing layers of ligand-exchanged PbSe QDs were fabricated and measured to exhibit an electroluminescence signal. In contrast, no electroluminescence was observed from the layered QD-LEDs fabricated with the as prepared QDs..[1] Q. Sun, Y. A. Wang, L. S. Li, D. Wang, T. Zhu, J. Xu, C. Yang, Y. Li, Nat Photon 1, 717 (2007).[2] D. F. Qi, M. Fischbein, M. Drndic, S. Selmic, Applied Physics Letters 86, (Feb 28, 2005).[3] I. Gur, N. A. Fromer, M. L. Geier, A. P. Alivisatos, Science 310, 462 (Oct 21, 2005).[4] M. Law, J. M. Luther, Q. Song, B. K. Hughes, C. L. Perkins, A. J. Nozik, J Am Chem Soc 130, 5974 (2008).[5] W. W. Yu, J. C. Falkner, B. S. Shih, V. L. Colvin, Chemistry of Materials 16, 3318 (Aug 24, 2004).[6] I. Moreels, B. Fritzinger, J. C. Martins, Z. Hens, J Am Chem Soc 130, 15081 (Nov 12, 2008).
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Degradation Studies of Colloidal Quantum Dot Light-emitting Diodes.
Jie Liu 1 , Chuang Xie 1 2 3 , Yu Zhang 1 4 , Wenjia Hu 1 3 , Shawn Pickering 1 , Guanjun You 1 , Andrew Wang 2 , Jian Xu 1
1 Engineering Science and Mechanics, The Pennsylvania State University, University Park, Pennsylvania, United States, 2 , OceanNanotech LLC., Springdale, Arkansas, United States, 3 Chemical Engineering, Tianjin University, Tianjin China, 4 Electronic Science and Engineering, Jilin University, Changchun China
Show AbstractRecent studies suggest that the low-cost, colloidal semiconductor quantum dots (QDs) can be used to replace organic emitting materials in solution-processed light-emitting diodes (LEDs) [1]by virtue of their superior optical properties, including high quantum yield, narrow emission bandwidth, and size-tunable wavelength over a broad spectral regime. Nevertheless, little has been done to understand the stability and degradation mechanisms of QD-LEDs when compared to their organic-LED (OLED) counterparts [2-3]. We report in this conference the results of our preliminary study in this regard.In the present work, the temporal evolution of the electroluminescence (EL) intensity of CdSe/ZnS QDs was measured under the condition of constant bias current. The degradation of the output intensity of QD-LEDs was correlated to the change of the exciton recombination rate that was measured with photoluminescence (PL) characterization. The steady-state and time-resolved PL of aged and fresh QD-LEDs were compared, showing a pronounced decrease in PL intensity and a prolonged PL relaxation for the aged devices. Three degradation mechanisms were proposed for QD-LEDs, i.e., (i) thermal instability, (ii) photochemical instability, and (iii) luminescence quenching by the diffusion of the hole-transport molecules.Experiments were designed to weight the impacts of the respective mechanisms. Temporal evolution of PL of thin QD films was measured at different temperatures to reveal the thermal instability of the emissive nanoparticles. The photochemical instability of QDs was evaluated by pumping the QD films with a 488nm laser. Finally, PL intensities of QD films doped with different concentrations of hole-transport molecules were compared to uncover the effect of HTM quenching in QD-LEDs. We have concluded that the thermal instability of QDs is the dominant force driving the degradation behavior of QD-LEDs, and suggest that the aging performance of QD LEDs can potentially be improved by employing heat dissipation design in QD-LED configurations .[1] Q. Sun, Y. A. Wang, L. S. Li, D. Y. Wang, T. Zhu, J. Xu, C. H. Yang, and Y. F. Li, Nature Photonics 1, 717 (2007).[2] H. Aziz and Z. D. Popovic, Chemistry of Materials 16, 4522 (2004).[3] Z. D. Popovic and H. Aziz, Ieee Journal of Selected Topics in Quantum Electronics 8, 362 (2002).
9:00 PM - E3.56
Full-color Emitting Dye-bridged Siloxane Hybrid Materials for Luminescence Converter Application.
Seung-Yeon Kwak 1 , SeungCheol Yang 1 , Jae Hong Kim 2 , Byeong-Soo Bae 1
1 Lab. Optical Materials & Coating (LOMC), Dep. of Materials Science & Engineering, KAIST, Daejeon Korea (the Republic of), 2 Dep. of Display & Chemical Engineering, Yeungnam University, Gyeongsan Korea (the Republic of)
Show AbstractFull-color emitting luminescent alkoxysilanes have been synthesized for luminescence conversion materials. We have obtained various luminescences by changing chromophoric system of organic dyes. Novel hydroxyl functional DCM derived from (2,6-dimethyl-4H-pyran-4-ylidene)malononitrile is synthesized and 2,5-diamino-3,6-dicyanopyrazine is prepared as raw materials for luminescent alkoxysilanes. Reaction of hydroxyl functional DCM with 3-(triethoxysilyl)propyl isocyanate gives red-emitting alkoxysilane that mainly emits at 640 nm. Green-emitting alkoxysilane is synthesized by amidine link between amino group of (3-aminopropyl)trimethoxysilane and cyano group of 2,5-diamino-3,6-dicyanopyrazine and its maximum emission is 520 nm. Addition of 3-(triethoxysilyl)propyl isocyanate to 3-(triethoxysilyl)propyl isocyanate initiates the reaction of ureido function and it forms five-membered imidazolone ring. It causes blue-emitting alkoxysilane with 443 nm emission. Finally, reaction of amino group of green-emitting alkoxysilane and cyclohexanone gives yellow-emitting alkoxysilane (577 nm). These various luminescent alkoxysilanes participate in sol-gel reaction to synthesize dye-bridged siloxane hybrid materials (dye-bridged hybrimer). Because of the covalently bridged structure and robust hybrimer matrix, the dye-bridged hybrimer shows superior thermal stability and anti-photobleaching characteristics compare to conventional dye/epoxy mixture system. Blended various luminescent dye-bridged hybrimers give full-color emission covering the whole visible range by changing irradiated UV wavelength. And white emission is obtained by changing the ratio of dye-bridged hybrimers. The luminescence converter using the dye-bridged hybrimer has high color rendering index, fine color tuning, easy processability and cost merit over inorganic phosphor based luminescence converter. Also, the dye-bridged hybrimer is a good LED encapsulant material, so that it can be used as luminescence converter/encapsulant one body system without additional epoxy encapsulant for white LEDs.
9:00 PM - E3.58
Evaluation of Scalable Electrode Materials for BulkHeterojunction Solar Cells.
S. Tuladhar 1 , P. Atienzar 1 5 , D. Credgington 2 , T. Kirchartz 1 , M. Voigt 1 , S. Haque 2 , D. Bradley 1 , J. Durrant 2 , J. Nelson 1 , S. Richards 3 , M. Hill 3 , K. Molloy 3 , T. Manning 4
1 Department of Physics, Imperial College London, London United Kingdom, 5 , Universidad Politecnica de Valencia, Valencia Spain, 2 Department of Chemistry, Imperial College London, London United Kingdom, 3 Department of Chemistry, University of Bath, Bath United Kingdom, 4 Pilkington Technology Management Limited, European Technical Centre, Lancashire United Kingdom
Show AbstractLow cost, high performance and scaleable electrodes are a high priority in the development of organic photovoltaic technology. In particular, the high financial and energy cost of indium tin oxide (ITO) makes this commonly used electrode incompatible with very low cost manufacture. At the same time, other widely used materials such as polystyrenesulfonate doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) and low work function metal cathodes suffer from poor stability in ambient conditions. In this work, we investigate a range of alternative electrode materials for use in organic bulk heterojunction solar cells. These include indium free transparent conducting oxides as alternatives to ITO, and metal oxide layers as high (e.g. NiO) or low (e.g. TiO2) work function interlayer materials. The metal oxide layer is deposited either via atmospheric chemical vapour deposition (ACVD) method or processed from solution. In addition we investigate conducting organic layers as alternatives to ITO. Preliminary studies show that metal and indium free devices yield comparable performance to standard structures. To investigate further any differences in electrode performance, we study the device physics of standard bulk heterojunction solar cells and inverted devices with oxide cathodes using transient voltage and current measurements, with the aid of a theoretical model. We demonstrate that oxide electrodes need not degrade device performance relative to the standard structures.
9:00 PM - E3.59
Analysis of Chemical Degradation Mechanism within Sky Blue Phosphorescent Organic Light Emitting Diodes by Laser-desorption/ionization Time-of-flight Mass Spectrometry.
Ines Rabelo de Moraes 1 , Sebastian Scholz 1 , Bjoern Luessem 1 , Karl Leo 1
1 Physics, Institut für Angewandte Photophysik, Technische Universität Dresden, Dresden, Sachsen, Germany
Show AbstractOrganic Light Emitting diodes (OLEDs) have attracted much interest in the past years due to their potential application in flat panel displays and general lighting sources. Although a remarkable improvement in efficiency and lifetime of p-i-n phosphorescent OLEDs has been reported recently [1-4], the short lifetime of blue emitting devices still the major limitation for a broad application of this promising technology. Despite the importance of the deep understanding of the intrinsic degradation mechanisms involved in the aging process of blue phosphorescent OLEDs only little information about the nature of the chemical reaction associated to the low stability of the blue emitters is known.In our contribution we will present the successful investigation of the intrinsic chemical degradation mechanisms of phosphorescent OLEDs based on the common sky blue emitter bis(2-(4,6-difluorophenyl)pyridyl-N,C2’)iridium(III)picolinate (FIrpic). The investigation was done with the chemical analysis technique laser-desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS) [5]. The devices were treated under normal OLED operation conditions and therefore aged under at a constant current density until the luminance decreased to about 5% of the initial luminance. The comparison between LDI-TOF-MS spectra collected for aged and unaged OLED samples allows the identification of reaction product fragments related mainly to the chemical dissociation of FIrpic molecules during the OLED operation. In the proposed degradation reaction pathway for FIrpic molecules, the dissociation indicates that the short lifetime of the OLEDs may be related to the irreversible dissociation of the FIrpic molecule by the loss of carbon dioxide (CO2). Additionally, the formation of chemical complexes between the fragments of the FIrpic molecules with the neighbour materials is visible. Finally, the formation of cesium adducts of FIrpic indicates a possible contribution of the dopant to the OLED degradation process.[1] R. Meerheim, K. Walzer, M. Pfeiffer, K. Leo, Appl. Phys. Lett. 89 (2006) 061111.[2] J. Birnstock, T. Canzler, M. Hofmann, A. Lux, S. Murano, P. Wellmann, A. Werner, SID Symposium Digest of Technical Papers 38 (2007) 1193.[3] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, K. Leo, Nature 459 (2009) 234.[4] N. Seidler, S. Reineke, K. Walzer, B. Lüssen, A. Tomkeviciene, J.V. Grazulevicius, K. Leo, Appl. Phys. Lett. 96 (2010) 093304.[5] S. Scholz, K. Walzer, K. Leo, Adv. Funct. Mater. 18 (2008) 2541.
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Transparent and Highly Conducting Polypyrrole Thin Film for Optoelectronic Applications: Synthesis and Characterization.
Concepcion Arenas 1 , Gustavo Molina 3 , Genoveva Hernandez 2 , Domingo Rangel 1 , Victor Castano 1
1 Ingeneria Molecular de Materiales, Centro de Física Aplicada y Tecnología Avanzada, Queretaro Mexico, 3 Licenciatura en Tecnologia, Centro de Física Aplicada y Tecnología Avanzada, Querétaro, Querétaro, Mexico, 2 Departamento de Nanotecnología, Centro de Física Aplicada y Tecnología Avanzada, Querétaro, Querétaro, Mexico
Show AbstractConducting polymers are characterized by their high flexibility, low cost of preparation and its environmental stability, enabling them to be used in optoelectronic applications. Polypyrrole (Ppy) is one of highly conductive polymers with excellent environmental stability, however is a dark polymer and a low transmittance of this polymer film is far from satisfactory for optoelectronic applications. In this work highly transparent films were synthesized by chemical bath deposition in the presence of sodium dodecyl sulfate (SDS) as doping agent, and a mixture of APS-FeCl3 (1:1) as oxidizing agent. In order to investigate the effect of the surfactant and the binary oxidizing agent in the properties of Ppy films, optical transmittance, electrical conductivity and surface morphology by AFM were analyzed. PPy-SDS films are more homogeneous, transparent and thinner compared with the dark Ppy as prepared, at the same time deposition. The conductivity of the films is in the range of 3.64 to 5.84 S/cm with an optical transmittance about 80%. According with AFM studies, the surface morphology of Ppy-SDS film is composed of agglomerates and it presents a roughness about 8 nm in a micrometer of area. Ppy films presents an optical coefficient in the order of 1E4 1/cm and an energy band gap about 2 eV, indicative that its a good candidate to be used as transparent absorbing material in a solar cell.
9:00 PM - E3.7
Deep-level Optical Spectroscopy Investigation of Intrinsic Degradation in Alq3- and Alq3:Qd-Based OLEDs.
Yoshitaka Nakano 1
1 Institute of Science and Technology Research, Chubu University, Aichi Japan
Show AbstractOrganic light-emitting diodes (OLEDs) have attracted a great deal of attention because they can be used in full-color flat-panel displays. In general, OLEDs are easily subject to the degradation through a long-term intrinsic decrease in electroluminescence (EL) efficiency during operation. Although the lifetime and the EL efficiency of OLEDs have been recently reported to be extended by doping Alq3 with quinacridone (Qd), the intrinsic degradation of OLEDs is still an open issue [1]. So, it is important to understand the origin of this degradation from the viewpoint of band gap states. In our previous study, we have clarified electronic band gap states in the Alq3 emissive region of the most well-known Alq3(:Qd)/α-NPD-based OLEDs by using modified deep-level optical spectroscopy (DLOS) [2]. In this study, we have applied this modified DLOS technique to these OLEDs before and after the intrinsic degradation, and have investigated degradation-induced variation in emissive interface states with and without the Qd doping into Alq3.Two kinds of α-NPD/Alq3(:Qd)/LiF/Al OLED samples were fabricated on ITO-coated glass substrates. The active area is 3.0x3.0mm2. The samples were degraded through the constant-current operation of 44.4mA/cm2 for 2x105min. The final luminances decreased down to 30 and 63% of the initial values (L0 : 1830 and 1633cd/m2) for non-doped and Qd-doped OLED samples, respectively.Both the OLED samples before the degradation showed a discrete trap level (I) located at ~1.77eV below the conduction band of Alq3 in the emissive interface region from DLOS measurements [2]. However, a significant difference in the band gap states can be seen between non-doped and Qd-doped samples after the degradation. For the non-doped sample, the interface trap I and the near-band-edge (NBE) transitions significantly red-shifted to the corresponding same levels as an Alq3 single layer [3]. These variations in the band gap states are probably induced by the degradation and indicate that initial molecular structures characteristic of the Alq3 emissive zone are transformed into the bulk-like relaxed ones through the degradation. On the other hand, the Qd-doped sample showed the interface trap I as it is and/or its red-shifted bulk level in a complicated manner, together with a significant variation in the Qd doping level (E2). So, in the case of the Qd-doped OLEDs, the degradation is considered only to induce a partial deformation of the Alq3 emissive zone due to the sufficient charge transfer to the Qd doping level E2.[1]H. Aziz et al.: Science 283 (1999) 1900.[2]Y. Nakano et al.: Jpn. J. Appl. Phys. 47 (2008) 464.[3]Y. Nakano: Appl. Phys. Express 2 (2009) 092103.
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The Quest For Quantum Cutting Via Sm3+, Eu3+, Gd3+ and Er3+ Ions Codoped Rare Earth Phosphates.
Hachani Souad 1 , Moine Bernard 2 , Ferid Mokhtar 3
1 , LPCML, Villeurbanne France, 2 , LPCML, Villeurbanne France, 3 , Laboratoire des Matériaux, Tunis Tunisia
Show AbstractThis work is concerned with the quest for new materials with quantum yield greater than 1 under VUV excitation; these materials will be used for lighting, in fluorescent tubes when we substitute the mercury by xenon, or for manufacturing the plasma display panels.We start by the Ln3+ 4f energy levels study and the calculation of their 5d band position [1]. According to quantum cutting ‘downconversion’ [2] conditions, which low to material to emit more than one photon for each photon absorbed, the ions couples chosen are (Gd3+, Eu3+) [3], (Sm3+, Eu3+) and (Sm3+, Er3+) codoped rare earth phosphates. Thus we have synthesized and characterized the monophosphates TRPO4: Ln3+ (1%, 10%) (TR = La, Gd and Y, Ln3+ = Sm3+, Eu3+, Gd3+ and Er3+), the ultraphosphates LaP5O14: Ln3+ (1%, 5%) (Ln3+ = Sm3+, Eu3+, Gd3+ and Er3+) and the polyphosphates LaP3O9: Ln3+ (1%, 2%) (Ln3+ = Sm3+, Eu3+ and Er3+). Their UV and VUV spectroscopy has been achieved; Ln3+ luminescence is few powerful in rare earth phosphates. No quantum cutting process has been observed in the case of (Gd3+, Eu3+) couple because of the position of Eu3+ CTB. The monophosphates are relatively efficient under excitation at 147 nm and could be good red phosphors. The energy transfer Sm3+ → Eu3+ occur in YPO4, LaP5O14 and LaP3O9 and the VUV excitation energy transfer matrix → Eu3+ CTB occur too in LaP5O14 and LaP3O9, these two matrices should be potential phosphors for vacuum ultraviolet excitation.References[1] P. Dorenbos, J. Lumin, vol. 91, p. 155-176, 2000.[2] R.T. Wegh, “Vacuum ultraviolet spectroscopy and quantum cutting for trivalent lanthanides”, PhD thesis, Ultrecht University, 1999.[3] R. T. Wegh, H. Donker, K. D. Oskam and A. Meijerink, J. Lumin, vol. 82, p. 93-104, 1999.
9:00 PM - E3.9
Fabrication of Large-area Hybrid Nanowires Arrays as Novel Field Emitters.
Huibiao Liu 1 , Yongjun Li 1 , Yuliang Li 1
1 CAS Key Laboratory of Organic Solids, Institute of Chemistry , Chinese Academy of Sciences, Beijing China
Show AbstractA hybrid nanomaterial composed of ZnO nanoparticles (NPs) and CuTCNQ (copper 7,7,8,8-tetracyanoquinodimethane) nanowires (NWs) was successfully fabricated in a feasible solution method. The electron field emission properties of the hybrid NWs were dramatically improved over the raw CuTCNQ NWs. The maximum current density of ZnO-CuTCNQ NWs was six times than that of the CuTCNQ NWs array. Importantly for ZnO-CuTCNQ NWs, no obvious degradation of current density was observed and the emission current fluctuation was less than 10% during the period of over 4000 seconds. It is of significance that the coating layer of ZnO NPs produced by our approach has great contribution to stabilization of organic field emitters and enhancement of their field emission properties. The high emission stability can be also observed for In2O3-CuTCNQ NWs which is also fabricated by the same method. It is confirmed that the moderate approach presented herein is of practical significance in stabilization of organic field emitters avoiding decomposition of the core organic materials.References1S. Cui, Y. Li, Y. Guo, H. Liu, Y. Song, J. Xu, J. Lv, M. Zhu, D. Zhu, Adv. Mater., 20, 309-313 (2008).2H. B. Liu, Q. Zhao, Y. L. Li, Y. Liu, F. S. Lu, J. P. Zhuang, S. Wang, L. Jiang, D. B. Zhu, D. P. Yu, L. F. Chi, J. Am. Chem. Soc., 127, 1120-1121 (2005).3C. S. Huang, Y. Zhang, H. B. Liu, S. Cui, C. R. Wang, L. Jiang, D. P. Yu, Y. L. Li, D. B. Zhu, J. Phys. Chem. C., 111, 3544-3547 (2007).
Symposium Organizers
R. Joseph Kline National Institute of Standards and Technology
Iain McCulloch Imperial College London
Garry Rumbles National Renewable Energy Laboratory
Alberto Salleo Stanford University
E8: Poster Session: Organic Photovoltaics
Session Chairs
Wednesday PM, December 01, 2010
Exhibition Hall D (Hynes)
E6: Organic Photovoltaics: Morphology Characterization
Session Chairs
Nikos Kopidakis
Peter Skabara
Wednesday PM, December 01, 2010
Room 312 (Hynes)
9:30 AM - **E6.1
Nanostructured Organic Photovoltaics: From Nanoscale Morphology to Light Harvesting with Nanoparticles.
David Ginger 1
1 , University of Washington, Seattle, Washington, United States
Show AbstractOrganic photovoltaics (OPV) are a potential low-cost alternative to conventional inorganic photovoltaics. The most successful organic solar cells to date rely on nanoscale phase separation between different components as the basis for their operation. Understanding and controlling this nanoscale film structure is a central challenge that affects every aspect of the field, from the optimization of new low-bandgap polymers to early efforts to scale up manufacturing. Our group has pioneered several scanning-probe methods, including time-resolved electrostatic force microscopy (trEFM) and photoconductive atomic force microscopy (pcAFM), as tools to characterize the influence of nanoscale film morphology in active organic solar cells. The resulting data show there is room to improve performance by better control of morphology, even in some of todays "best" model systems, and underscore the need to model these devices as fully 3D heterogeneous networks, rather than as homogeneous 1D or 2D semiconductor slabs. The importance of nanostructures to OPV doesn’t end at morphology—I will also discuss the use of colloidal materials as solution processable complements to organic bulk heterojunctions, wherein plasmonic near-field excitation enhancements of metal colloidal nanoparticles or absorption in low bandgap semiconductor quantum dots can be used to improve light harvesting in thin film photovoltaic devices.
10:00 AM - E6.2
Miscibility in Polymer Solar Cells: Do We Need a New Paradigm Describing Device Function?
Brian Collins 1 , Lewis Guignard 1 , Fred Stevie 2 , Jaewook Seok 2 , Eliot Gann 1 , Chris McNeill 3 , Harald Ade 1
1 Physics, North Carolina State University, Raleigh, North Carolina, United States, 2 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 3 Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, United Kingdom
Show AbstractTo date, the device function of polymer bulk heterojunction (BHJ) excitonic solar cells has been commonly interpreted to arise from two interpenetrating pure phases of donor and acceptor materials with excitonic charge separation occurring at discrete interfaces within the bulk. However, the paradigm of pure phases and discrete interfaces in BHJ devices has been called into question in some recent work [1-3]. To further probe this possibility and its implications, the miscibility between poly(3-hexylthiophene) (P3HT) or poly(2-methoxy-5-(3’,7’-dimethyloctyloxy)-p-phenylene vinylene) (MDMO-PPV) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) has been determined for temperatures of 100-200°C in thin films brought to equilibrium by measuring the composition of the polymer rich phase via near edge x-ray absorption fine structure spectroscopy in a scanning transmission x-ray microscope. As crystalline and amorphous regions of P3HT are not readily distinguishable with this method, the level of P3HT crystallization has been controlled by using various grades of the polymer including a non-crystallizing regio-random grade and regio-regular grades with varying molecular weights and quantified independently with x-ray diffraction. It has been found that all grades of P3HT and MDMO-PPV contain significant concentrations of PCBM in the polymer-rich phase. For example, in regio-random P3HT the miscibility of PCBM is measured to be 20-25% with miscibility levels scaling inversely with crystallinity as measured by x-ray diffraction. Furthermore, the miscibility increases for higher temperature, implying an upper critical solution temperature. To connect this with the morphological evolution under current device fabrication methods, P3HT has been floated onto layers of PCBM, and depth profiles using dynamic SIMS have been measured on unannealed, postannealed and preannealed P3HT layers. This study reveals that even short annealing (5 min at 150°C) causes significant interdiffusion of both materials, which shows that under no conditions do pure phases of amorphous P3HT exist in BHJ devices. We therefore propose that the present paradigm of device function needs to be modified.[1] B. Watts, W. J. Belcher, L. Thomsen, H. Ade, and P. C. Dastoor, Macromolecules 42, 8392 (2009). [2] C. R. McNeill, B. Watts, L. Thomsen, W. J. Belcher, A. L. D. Kilcoyne , N. C. Greenham, and P. C. Dastoor, Small, 2, 1432 (2006).[3] N. C. Cates, R. Gysel, C. Beiley, C. E. Miller, M. F. Toney, M. Heeney, I. McCulloch, M. D. McGehee, Nano Letters, 9, 4153 (2009).
10:15 AM - E6.3
Influence of Nanomorphology and Interface Structure in Polymer-polymer Heterojunction Photovoltaic Thin Films Probed with Resonant Soft X-ray Scattering.
Hongping Yan 1 , Sufal Swaraj 1 , Cheng Wang 4 , Inchan Hwang 2 , Neil Greenham 2 , Chris Groves 3 , Harald Ade 1 , Christopher McNeill 2
1 , NCSU, Raleigh, North Carolina, United States, 4 , Lawrence Berkeley National Laborratory, Berkeley, California, United States, 2 , University of Cambridge, Cambridge United Kingdom, 3 , Durham University, Durham United Kingdom
Show AbstractThe bulk nano-morphology of organic bulk heterojunction devices, particularly of all-polymer devices, is difficult to characterize with conventional methods due to the limited electron density contrast between the constituent materials. Resonant soft x-ray scattering can overcome this challenge and is used to show that the morphologies in chloroform cast and subsequently annealed polyfluorene copolymers poly(9,9’-dioctylfluorene-co-bis(N,N’-(4,butylphenyl))bis(N,N’-phenyl-1,4-phenylene)diamine) (PFB) and poly(9,9’-dioctylfluorene-co-benzothiadiazole) (F8BT) blends exhibit impure domains in as-cast films. Even optimized cells using present fabrication methods are found to have a dominant domain size much larger than the exciton diffusion length. Consequently, the morphology is far from ideal for efficient solar cell operation and very different from those achieved in high efficiency, fullerene based devices. Our results strongly imply that lack of morphological control contributes to the relatively poor performance of the all-polymer PFB:F8BT devices and may be problematic for all-polymer devices in general. We have also studied, through controlled annealing of planar heterojunction (bilayer) PFB/F8BT devices, the influence of interface roughness and interdiffusion on the generation and separation of electron-hole pairs at the donor/acceptor interface. The observed decrease in device efficiency with annealing is attributed to decreased interfacial charge separation efficiency, partly due to a decrease in the bulk mobility of the constituent materials upon annealing but also (and significantly) due to the increased interface roughness. Monte Carlo simulations are able to reproduce the experimental current-voltage curves by taking the increased interfacial roughness and decreased carrier mobility into account. Our results show that the sharpness of heterojunction interfaces should be considered a requirement for high performance devices. Novel processing strategies will have to be employed to harness the full potential high open circuit voltage polymer:polymer devices offer.
10:30 AM - **E6.4
Imaging and Modeling of Nanostructures for Solution-processed Photovoltaics.
Neil Greenham 1
1 , University of Cambridge, Cambridge United Kingdom
Show AbstractBetter understanding of the three-dimensional nanostructure of solution-processed blends is vital for improving photovoltaic device performance. I will describe the use of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) tomography to image the structure of blends of conjugated polymers with CdSe nanoparticles. This technique allows the three-dimensional arrangement of nanoparticles, including rod and tetrapods, to be determined. Blends of spherical nanoparticles with OC1C10-PPV show aggregated regions of nanoparticles with good connectivity, surrounded by less dense regions. As the overall nanoparticle concentration is reduced the size of the aggregates is reduced but the aggregates remain internally well-connected. Analysis of the connectivity of the nanoparticle networks will be presented and its implications for device operation will be discussed. I will also present recent modeling of the impact of nanostructure on charge separation at real polymer/polymer heterojunctions.
11:00 AM - E6: OPV Charact
BREAK
11:30 AM - **E6.5
The Structure and Thermodynamics of Polymer-fullerene Bimolecular Crystals.
Michael McGehee 1 , Roman Gysel 1 , Nichole Cates Miller 1 , Chad Miller 2 , Michael Toney 2 , Eunkyung Cho 3 , Chad Risko 3 , Jean-Luc Bredas 3
1 Materials Science & Engineering, Stanford University, Stanford, California, United States, 2 , Stanford Synchrotron Radiation Lightsource, Menlo Park, California, United States, 3 Chemistry & Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractMany of the fullerene derivatives that are used as electron acceptors in polymer bulk heterojunction solar cells are able to intercalate between the side chains of the polymer. We have used synchrotron x-ray diffraction and molecular mechanics simulations to determine the unit cell of PBTTT:PCBM bimolecular crystals and will discuss the implications that this structure has for exciton dissociation, charge transport and recombination. We will also present thermodynamic data and explain the factors that determine which molecules can intercalate between the side chains.
12:00 PM - E6.6
Use of Nucleating Agents for Microstructure Manipulation of Organic Photovoltaic Blends.
Jennifer Nekuda Malik 1 2 , Paul Smith 2 3 , Natalie Stingelin 1 2
1 Department of Materials, Imperial College London, London United Kingdom, 2 Centre for Plastic Electronics, Imperial College London, London United Kingdom, 3 Department of Materials, Eidgenössiche Techniche Hochschule ETH Zürich, Zürich Switzerland
Show AbstractPoly(3-hexylthiophene) (P3HT) / phenyl-C60-butyric acid methyl ester (PCBM) bulk heterojunction (BHJ) blends are a promising organic photovoltaic system. However, lack of understanding of relevant fundamental relationships between the nano- and microstructure of the blend and electronic processes is hampering further improvement in device performance, and renders straight-forward implementation of even the most simple fabrication protocols to other BHJ binaries challenging. The origin of this complexity is the fact that P3HT/PCBM active photovoltaic layers, as well as many other BHJ systems, consist of a multiphase architecture comprising: 1) phase-separated crystalline P3HT and crystalline PCBM phase regions, and 2) a highly intermixed P3HT/PCBM amorphous phase. As a consequence, while it is well known that charge separation in these structures occurs at the interface between the P3HT donor and PCBM acceptor material, to date, it is unclear at which boundaries (crystalline or amorphous) this process predominantly takes place. We present use of well known clarifiers for the bulk commodity polymer isotactic polypropylene (i-PP), including 1,3:2,4-bis(3,4-dimethyl benzylidene) sorbitol (DMDBS) and an alkyl-substituted 1,3,5-benzenetrisamide, to nucleate P3HT. This nucleation process allows us to modify and control the size of the crystalline regions in P3HT/PCBM blends in an attempt to elucidate the primary region of charge separation. Details of the methods used to manipulate the P3HT/PCBM blend microstructure will be discussed and the correlation between microstructural changes and device characteristics will be presented.
12:15 PM - E6.7
The Effects of Fullerene Intercalation on the Photoconductivity of Polymer:fullerene Bulk Heterojunctions.
William Rance 1 , Andrew Ferguson 2 , Martin Heeney 3 , Iain McCulloch 3 , David Ginley 2 , Garry Rumbles 2 , Nikos Kopidakis 2
1 Physics, Colorado School of Mines, Golden, Colorado, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States, 3 Chemistry, Imperial College of London, London United Kingdom
Show AbstractThe donor:acceptor blend morphology in bulk-heterojunction solar cells plays a major role in determining the overall photovoltaic performance, and understanding the structure-property relationship in blends can be difficult. Recent studies of the polymer poly(2,5-bis(3-alkylthiophen-2yl)thieno[3,2-b]thiophene) (pBTTT) have demonstrated a highly ordered microstructure with large thin film transistor hole mobilities. Additionally it has been shown that a highly ordered bimolecular crystal can be formed in blends of pBTTT and the fullerene derivative phenyl-c71-butyric acid methyl ester (PCBM) with fullerenes intercalating within the lamellar structure of pBTTT [1]. It has also been shown that no intercalation occurs in blends of pBTTT with the bisadduct bisPCBM due to the increased size from the extra side group [2]. In this work, we investigate the dynamics of photogenerated carriers in these various blend morphologies using the contactless Time Resolved Microwave Conductivity (TRMC) technique. As the PCBM content was raised in the intercalated system, an initial increase in photoconductivity was seen due to increased exciton dissociation. A secondary increase in photoconductivity was also seen in heavily loaded PCBM blends due to the formation of a fullerene phase outside the bimolecular polymer:fullerene crystal that exhibited a high electron mobility. In the phase-separated system, a single increase in photoconductivity was seen due to increased exciton dissociation and the formation of a high electron mobility fullerene phase. The highest photoconductivity was seen in the intercalated system that consisted of both a bimolecular crystal phase and fullerene phase, while slightly longer carrier lifetimes were seen in the nonintercalated blends. We compare these results to another prototypical polymer:fullerene blend, P3HT:PCBM, and formulate a model for carrier recombination in these systems. [1] A. C. Mayer et al., Adv. Funct. Mater., vol. 19, 1173 (2009). [2] N. C. Cates et al., Nano Lett., vol. 9, 4153 (2009)
12:30 PM - E6.8
In-situ Monitoring of the Solid-state Microstructure Evolution of Polymer:Fullerene Blend Films Using Field-effect Transistors.
Thomas Anthopoulos 1 , John Labram 1 , Ester Buchaca Domingo 2 , Natalie Stingelin 2 , Donal Bradley 1
1 Physics, Imperial College London, London United Kingdom, 2 Materials, Imperial College London, London United Kingdom
Show AbstractWe present a detailed experimental and theoretical study into the microstructure development of polymer:fullerene blends upon annealing in the solid state. We show that the structural changes that take place throughout a binary polymer:fullerene film can be detected as changes in charge carrier mobility at the semiconductor-dielectric interfaces (bottom and/or top) in OFETs. Known morphological changes observed in the well-studied system of blended poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PC61BM) are reproduced accurately. We show that by carrying out in-situ mobility measurements, the time-dependence of the microstructural evolution in these systems can be probed with great ease. Finally, by implementing a simple percolation-theory-based model we calculate the electron conductivity of a two-dimensional, two-component system. By adjusting the size and distribution of fullerene clusters in this simulation we show that phase-segregation leads to a drop in electron conductivity, in agreement with experimental results.
12:45 PM - E6.9
Photoinduced Degradation of Polymer and Polymer-fullerene Active Layers.
Nikos Kopidakis 1 , Matthew Reese 1 , Alex Nardes 1 , Benjamin Rupert 1 , Ross Larsen 1 , Dana Olson 1 , Matthew Lloyd 1 , Sean Shaheen 2 , David Ginley 1 , Garry Rumbles 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Physics and Astronomy, University of Denver, Denver, Colorado, United States
Show AbstractThe stability of Organic Photovoltaics (OPV) under prolonged illumination has become a subject of great interest in the past few years, as it may determine their commercialization model. In this work we study the stability of the prototypical polymer:fullerene bulk heterojunction P3HT:PCBM without electrical contacts. We subject our samples to light exposure under inert and ambient conditions and use the contactless Time-Resolved Microwave Conductivity (TRMC) technique to monitor the dynamics of photocarrier generation at various stages of degradation. In addition, we probe the absorption spectra and the morphology of the samples in order to determine how the photoinduced structural changes cause the observed degradation of the photoconductivity. We find that under inert atmosphere no changes are observed in photoconductivity and morphology after 1000 hrs of illumination at an intensity of 1 sun. Under ambient conditions we observe bleaching of the polymer film, that is mitigated by the presence of the PCBM in the bulk heterojunction. The changes in photoconductivity of the bulk heterojunction under illumination at ambient conditions can be traced back to the electron mobility in the PCBM domains. Structural analysis of degraded blends shows strong oxidation of the fullerene cage, which has not, to our knowledge, been observed before. Quantum chemical calculations provide insight into the electronic properties of oxidized PCBM and explain the decrease we see in the photoconductivity. We discuss implications on the active layer as a cause for degradation in complete devices and present a comparison of our active layer studies to device results.
E7: Organic Photovoltaics: Materials Design
Session Chairs
Wednesday PM, December 01, 2010
Room 312 (Hynes)
2:30 PM - **E7.1
Manipulation of Band Gap in Conjugated Polymers by Subtle Changes in Structural Design.
Peter Skabara 1 , Filipe Vilela 1 , Alexander Kanibolotsky 1 , Diego Cortizo-Lacalle 1 , Greg McEntee 1 , John Forgie 1
1 Pure and Applied Chemistry, University of Strathclyde, Glasgow United Kingdom
Show AbstractThe use of non-covalent intramolecular interactions is an excellent method for imparting planarity (and good conjugation) within organic semiconducting materials, whilst retaining good solubility. Although planarity can be enforced in ladder type structures, solubility is an issue due to the persistent rigid nature of the polymer structure. Non-covalent contacts between heteroatoms (so-called two-electron, three-centre interactions) are weak enough to allow flexibility of the material in solution, but real enough to encourage a planar conformation in the solid state. As a result, non-covalent intramolecular/intrachain links can be used as a strategy for lowering the band gap of a material. In this work, we focus on several examples of structures where this approach has been used to good effect, whilst maintaining reasonable HOMO levels (essential for ensuring a favourable open circuit voltage in photovoltaic cells). The work presented focuses mainly on design, synthesis and electrochemical characterisation.
3:00 PM - E7.2
Tailor-made Absorber Polymers for Efficient Organic Solar Cells.
Silvia Janietz 1 , Lica Pabel 1 , Eileen Katholing 1 , Hans-Frieder Schleiermacher 2 , Ulrich Wuerfel 2 , Hannah Mangold 2
1 Polymer Electronics, FhG-IAP, Potsdam Germany, 2 , FhG-ISE, Freiburg Germany
Show AbstractConjugated polymers have the possibility to be applied as absorber materials in low cost organic photovoltaic. These polymers offer the possibility to be used by solution-processing like printing, doctor blading or spray deposition. The bulk-heterojunction cell concept is used most widely. It consists of a light absorbing conjugated polymer and a fullerene derivative for the ultra fast charge transfer. Poly(3-hexylthiophene) (P3HT) is often studied as the light absorbing polymer in bulk heterojunction solar cells [1]. P3HT does not present the optimum HOMO and LUMO energy levels with respect to its acceptor counterpart [6,6]-phenylC61-butyric acid methyl ester (PCBM) and with respect to the solar spectrum. A defined lowering of the bandgap, as well as a lowering of the energy offset between the donor-LUMO and the acceptor-LUMO will be desirable. Therefore many research activities were done to design polymers with optimized HOMO and LUMO energy levels to match the solar emission better. Such polymers can be realized through the introduction of alternating electron donating and electron accepting moieties along the polymer backbone to reach bandgaps between 1.8 -1.3 eV. Fluorene based terpolymers with different content of dialkyl substituted diphenyl-benzopyrazine or diphenyl-thienopyrazine and triphenylamine units (MR) were synthesized by Suzuki polymerization. Special attention was spent to increase molecular weights and the solubility of these polymers. Polymers were realized with high molecular weights and with very good solubility in organic solvents. The additional introduction of triarylamine units helps to reach sufficient charge mobilities of such kind of terpolyfluorenes. The alignment of the LUMO energy positions is possible through the introduction of different donor acceptor units in this class of polymers. The introduction of dialkyl substituted diphenyl-benzopyrazine for example in the fluorene main chain leads to LUMO-energy level of 3.1eV and in solar cells to an open circuit voltage of 0.96 V. Different compositions of the terpolymers were studied in solar cells. The relationship between the composition and the achieved performance of the solar cells will be discussed in detail. First results show solar cell power conversion efficiency of 2.8 % under simulated sunlight for example for a composition MR2/8:PCBM 1:2. Further optimization and their application in multi junction devices will lead to even higher efficiencies. Literature[1] A. C. Mayer, S. R. Scully, B. E. Hardin, M. W. Rowell, M. D. McGehee, Materialstoday 2007, 10, 28 -33.
3:15 PM - E7.3
Improved efficiencies of Bulk-heterojunction Hybrid Solar Cells Based on CdSe Quantum Dots and Low-bandgap Conjugated Polymers.
Michael Krueger 1 2 , Yunfei Zhou 1 2 , Michael Eck 1 2
1 Freiburg Materials Research Centre (FMF), University of Freiburg, Freiburg Germany, 2 Institute for Microsystems Technology, University of Freiburg, Freiburg Germany
Show AbstractInorganic semiconductor nanocrystals (NCs) such as CdSe, with tunable bandgaps and high intrinsic charge carrier mobilities can act as good electron acceptors and be incorporated into conjugated polymers to form bulk-heterojunction hybrid solar cells. Recently, we have reported on devices based on poly(3-hexylthiophene) (P3HT) and spherical CdSe quantum dots (QDs) with a simple post-synthetic hexanoic acid treatment, exhibiting the highest reported power conversion efficiencies exceeding 2% under AM1.5G illumination after spectral mismatch correction [1]. Here, we demonstrate our extended investigations on hybrid solar cells by using low band gap polymers (e.g. poly[2,6-(4,4-bis-(2-ethylhexy)-4H-cyclopenta[2,1-b;3,4-b]-dithiophene) -alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT)) and CdSe QDs, resulting in enhanced short-circuit currents and power conversion efficiencies, which can be attributed to the better absorption matching of PCPDTBT to the solar emission spectrum. Details on the optimization of PCPDTBT based devices and the comparison with P3HT based devices will be discussed. [1] Y. Zhou et al. Appl. Phys. Lett. 96, 013304 (2010).
3:30 PM - E7.4
Performance of Silole-containing Low Bandgap Polymer Solar Cells with an Anode Interlayer.
Jegadesan Subbiah 1 , Chad Amb 2 , Pierre Beaujuge 2 , John Reynolds 2 , Huanjun Ding 3 , Yongli Gao 3 , Franky So 1
1 Dept of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 The George and Josephine Butler Polymer Research Laboratory, Department of Chemistry, Center for Macromolecular Science and Engineering, University of Florida, Gainesville, Florida, United States, 3 3Department of Physics and Astronomy, University of Rochester, Rochester, New York, United States
Show AbstractPolymer solar cells (PSCs) have attracted much attention due to their low-cost, light-weight, mechanical flexibility and solution-processability for large-area device manufacturing. High mobility, low bandgap semiconducting polymers were used to realize high efficiency PSCs. Here, we report bulk heterojunction PSCs using a blend of poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PSBTBT) as the donor and {6,6}-phenyl-C71 butyric acid methyl ester (PC71BM) as the acceptor. Under air mass 1.5 illumination conditions, a power efficiency of 4.6% was achieved. We also demonstrated that the performance of these PSBTBT-based solar cells can be improved using interface layers between the ITO electrode and the active layer for efficient charge extraction. Here, ultra-thin double layers of MoO3 and poly(9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine) (TFB) were used as the anode interlayers. With these interlayers, the resulting power conversion efficiency was 5.3%, which is a 15% enhancement compared with the cells using PEDOT:PSS as an interlayer. Based on the results from photoelectron spectroscopy measurements, the enhancements are attributed to the combined effects of efficient electron blocking due to the TFB interlayer and enhanced hole extraction from the photoactive layer to the anode due to the MoO3 interlayer. We also studied the role of morphology and annealing of active layer on the solar cell performance.
3:45 PM - E7: OPV Design
BREAK
4:15 PM - **E7.5
Insight Into the Synthesis, Design and Processing of Narrow Band Gap Organic Semiconducting Polymers for Solar Cell Fabrication.
Guillermo Bazan 1 , James Rogers 1 , Kristin Schmidt 1 , Corey Hoven 1 , Jeff Peet 1 , Robert Coffin 1 , Xuan-Dung Dang 1 , Ed Kramer 1 , Quyen Nguyen 1 , Alan Heeger 1
1 , University of California, Santa Barbara, California, United States
Show AbstractHigh charge separation efficiency combined with the reduced fabrication costs associated with solution processing and the potential for implementation on flexible substrates make “plastic” solar cells a compelling option for tomorrow’s photovoltaics. Control the donor/acceptor morphology in bulk heterojunction (BHJ) materials as required for achieving high power conversion efficiency is therefore of primary concern. Our work has focused primarily on the use of solvent additives as a simple tool to improve the desired BHJ structure. Guidelines have appeared that are specific depending on the solubility characteristics of the narrow bandgap polymer. For example, diiodooctane works under circumstances where polymer aggregation is desired. Conversely, chloronaphthalene may be a better choice when solubility is poor. A combination of structural characterization tools, such as NEXAFS and GIWAXS, have been used to provide insight into how additives influence not only the spatial distribution of donor and acceptor domains, but also the internal structure of the individual components. An examination of how the molecular weight influences the solar cell performance will also be presented, together with methods to gain control over the polymerization reaction.
4:45 PM - E7.6
Unthrottled Push-pull Oligothiophene Dyes as Active Components in Bulk Heterojunction and Dye-sensitized Solar Cells.
Akhil Gupta 1 , Mei Gao 2 , Vanessa Armel 3 , Douglas Macfarlane 3 , Scott Watkins 2 , Gerry Wilson 2 , Udo Bach 3 , Richard Evans 2
1 Chemistry, CSIRO/Monash University, Melbourne, Victoria, Australia, 2 Molecular and health technology, CSIRO, Melbourne, Victoria, Australia, 3 , Monash University, Melbourne, Victoria, Australia
Show AbstractOrganic solar cells have attracted a great deal of attention in view of their potential for the fabrication of low-cost and flexible devices. Many different types of small molecules, polymers and dyes have been used in the fabrication of bulk-heterojunction (BHJ) and dye sensitized (DS) solar cells. One type of molecular framework that has been explored in both types of devices are conjugated donor-acceptor systems. One conventional variety of donor-acceptor (push-pull) systems consists of a substituted nitrogen atom that is attached to a π-spacer (an oligothiophene) through a phenyl ring. In these systems, aromatizable acceptors such as thiobarbituric acid, cyanopyridone and phenyl isoxazolone have then been attached to the opposite end of the oligothiophene. These acceptors have been shown to increase the absorbance of the molecules when compared with the conventional dicyanovinylidene acceptor group. In this paper, we will report a newer model of push-pull small molecules which has been designed, synthesized and tested in BHJ and DS solar cells. In this model the donor fragment is directly attached to the π-spacer as a diarylaminothiophine. Removal of the phenyl linker group between the nitrogen donor and the π-spacer “unthrottles” this connection. We have successfully designed and synthesized materials based on this model, tested them in BHJ and DS solar cells and have demonstrated improved performance compared with their “throttled” analogs. Specifically, we will present results that show that the “unthrottled” connection in our model results in an increase in the extinction coefficient of the molecules. This in turn has led to increases in the performance of devices based on these materials. We will present results showing power conversion efficiencies in excess of 2.3% for BHJ devices based on these materials when used as donor materials with PCBM. Derivatives based on this model have also been prepared with functionalised acceptors for use in DS solar cells. For one example of the “unthrottled” model, derivatives for both BHJ and DS cells were prepared from the same molecular template. The power conversion efficiencies in BHJ solar cells were found to be 1.22%. When used as an absorber in DS solar cells, power conversion efficiencies were found be 6.1%, 4.9% and 5.8% with liquid, ionic and solid state architectures respectively. The latter result is amongst the highest ever reported for solid state DS solar cells.
5:00 PM - E7.7
Studies of Semiconducting Carbon Nanotubes as Near-infrared Absorbers in Organic Semiconductor Blends.
Dominick Bindl 1 , Meng-Yin Wu 1 , Michael Arnold 1
1 Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States
Show AbstractRecent breakthroughs [1] in the post-synthetic isolation of highly pure semiconducting carbon nanotubes (from as-produced mixtures which also contain problematic metallic nanotubes) have made it possible for the first time to exploit semiconducting carbon nanotubes as near-infrared (NIR) absorbers in photovoltaic devices. Semiconducting carbon nanotubes have highly attractive properties for photovoltaics including tunable NIR band gaps, strong optical absorptivity at their band gap > 1E5 1/cm, exceptional charge transport mobility as high as 1E5 cm2/V/s, solution-processability, and stability to photo-oxidation. Here, we present on our recent discovery that photogenerated excitons in semiconducting carbon nanotubes can be harvested by blending the nanotubes with C60 derivatives in bulk heterojunctions.[2] The blends are solution-processed and are in many ways analogous to the blends utilized in polymer solar cells with nanotubes taking on the role of the semiconducting “polymer”. By employing photoluminescence quenching measurements and zero-bias photocurrent spectroscopy, we show that nanotubes (with band gaps in the range of 0.9-1.2 eV) form a type-II heterojunction with C60 derivatives with energy offsets sufficient to result in electron transfer from optically excited nanotubes to the C60 derivatives. The internal quantum efficiency (QE) for exciton dissociation and charge collection in the blends exceeds 75% and is dependent on the thin film microstructure, which we have characterized via transmission electron microscopy. Using the blends, we have realized preliminary photovoltaic devices with peak external QE > 20% in the NIR and photovoltaic power conversion efficiency > 1.4% for broadband NIR (1000-1365 nm) illumination at ~10 mW/cm2. The use of semiconducting carbon nanotubes as next-generation “polymers” is highly advantageous due to their exceptional properties and chemical stability, and the materials and device studies presented here are expected to lead to new classes of high efficiency photovoltaic devices with future optimization of the blend morphology, control over the nanotube band gap (and diameter) distribution, and increased film thickness to improve the absorption efficiency.[1] M. S. Arnold et al., Nature Nanotech. 1, 1 (2006). [2] D. J. Bindl, M. S. Arnold et al. Submitted (2010).
5:15 PM - E7.8
The Photocarrier Dynamics in CdSe Quantum Dot Films as a Function of Interparticle Distance.
Smita Dayal 1 , Nikos Kopidakis 1 , Garry Rumbles 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractColloidal quantum dots (QDs) are widely studied for their applications in optoelectronic devices and a better understanding leading to higher carrier generation yield and long carrier lifetimes is essential for the future development of these devices. Films of QDs can be modified by changing QD size, capping ligand linker chain length etc., making them a simple and ideal system for the study of correlations between structure and photocarrier dynamics.Here, we report on the photoconductivity, charge generation and transfer of alkanedithiol treated CdSe QD films. These highly photoconductive CdSe films were deposited by exchanging the original oleic acid capping ligand with various dithiol ligands by layer-by-layer deposition method. Optical spectroscopy reveals a red-shift for dithiol treated CdSe as compared to as synthesized sample indicating a strong electronic interaction for dithiol treated QD films. The generation, transport and recombination dynamics of carriers is studied using the contactless flash-photolysis time-resolved microwave conductivity (FP-TRMC) technique. A 5-fold increase in the product of free carrier generation yield and charge carrier mobility was observed for 1,2-ethanedithiol (EDT) treated QD films compared to those of 1,8-octanedithiol (ODT) treated QD films. A clear trend of decrease in the signal with increasing chain length is observed. Indeed, with the increase in dithiol chain length a decrease in the electronic coupling is expected, which can account for the changes in the photoconductivity. A model for the effect of interparticle distance on the photoconductivity and charge transfer decay dynamics of QD films is discussed.
5:30 PM - E7.9
Studying Charge Transport and Recombination in Organic Solar Cells Using Photo-induced Carrier Extraction with Linearly Increasing Voltage.
Song Chen 1 , Frederick Steffy 1 , Kaushik Roy Chouldhury 1 , Chad Amb 2 , John Reynolds 2 , Franky So 1
1 Dept of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 The George and Josephine Butler Polymer Research Laboratory, Department of Chemistry, Center for Macromolecular Science and Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractThe performance of solution-processed polymer solar cells is strongly affected by charge transfer, carrier transport and lifetime. Owing to the large anisotropy in carrier transport and a strong dependence of carrier mobility on charge carrier density, transport measurements on these disordered semiconducting polymers yield results that vary widely with the choice of device structure. Among the different techniques currently used to investigate carrier transport in organic devices, CELIV is emerging as a preferred technique. This is due to its unique features such as the capability of characterizing transport of materials with high mobilities and high conductivities without any sample thickness requirements, which is challenging in conventional techniques such as time-of-flight and dark injection. Moreover, the carrier recombination dynamics can be monitored during the measurements. In this work, we fabricated bulk heterojunction (BHJ) photovoltaic (PV) of poly ((4, 4- dioctyldithieno (3,2-b:2',3'-d) silole) -2,6- diyl-alt- (2,1,3- benzothiadiazole) -4,7-diyl) (PSBTBT) and PCBM. Carrier mobility and lifetime are determined from the photo-CELIV measurements.In the photo-CELIV technique, charge carriers generated by short laser pulses are extracted under a reverse bias ramp after an adjustable delay time (tdel). The CELIV mobility of the PSBTBT: PCBM blend at room temperature is found to be ~3x10-4 cm2(Vs)-1 which is consistent with the mobility values determined from space charge limited current. The carrier mobility is strongly related to the value of tdel and the decreased carrier mobility with increasing carrier density is due to a time dependent energy relaxation of charge carriers. Power dependence measurements show that the mobility is independent of the charge carrier concentration. We also studied the carrier mobility and recombination in the P3HT: PCBM systems. While this material system has a similar carrier mobility compared with PSBTBT: PCBM blend, the PSBTBT: PCBM system has a significant longer carrier lifetime compared with the P3HT: PCBM system. This longer carrier lifetime in the PSBTBT: PCBM blend probably contributes a larger short-circuit current in the photovoltaic cells. Additionally, photo-CELIV measurements were done on devices fabricated with an inverted structure. In both PSBTBT and P3HT systems, the results show that the inverted structure gives enhanced extraction of transient current under the same electric-field, which is also reflected by slightly higher solar cell efficiencies in the inverted structure.
5:45 PM - E7.10
Improving Performance of Organic Solar Cell with P3HT-based Diblock Copolymer.
Zhenzhong Sun 1 , Jihua Chen 2 , Xiang Yu 2 , Ilia Ivanov 2 , Dawen Li 1 , Kunlun Hong 2 , Kai Xiao 2 , David Geohegan 2
1 Electrical and Computer Engineering, The University of Alabama, Tuscaloosa, Alabama, United States, 2 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractPolymer solar cells have drawn world-wide attention due to their great potential as a green, flexible and low-cost renewable energy source. Block copolymers are well known to phase separate in highly ordered nanostructures on length scales commensurate with the exciton diffusion length in polymer solar cells. Meanwhile, by incorporating blocks with different functionalities into one block copolymer, both the electronic structures and morphology can be tuned towards achieving high power conversion efficiency or higher open circuit voltage in polymer solar cells. To demonstrate the advantageous applications of block copolymers, a series of diblock copolymer of poly(3-hexylthiophene) with different block compositions were used into polymer solar cells to achieve uniform morphology with nanoscale domain and to enhance the performance of the P3HT/PCBM solar cells. By blending P3HT-b-PS diblock copolymer with various ratio in P3HT/PCBM composites, we found that there is a strong dependence of photovoltaic properties on the adding diblock copolymer, and the maximum power conversion efficiency (PCE) of solar cells can be increased to 3.9 %, comparing with 3.0% of PCE achieved from the P3HT/PCBM devices without adding diblock copolymers. The effect of adding PS-b-P3HT diblock copolymer on the morphology and phase separation of P3HT/PCBM blend is systematically studied through AFM, grazing incidence X-ray diffraction, and TEM. The interfacial crystallinity of P3HT plays a significant role for the device performance in bulk heterojunction devices and we show that it can be significantly increased by the use of block copolymers. Furthermore organic thin film transistors are used to characterize the transport properties in these composites with block copolymer, which are found to exhibit unique properties such as the tunability of electron and hole mobility to balance their charge transport in bulk heterojunction. Our results showed block copolymer approach represents a promising way to enhancing the photovoltaic properties of known homopolymers.
E8: Poster Session: Organic Photovoltaics
Session Chairs
Thursday AM, December 02, 2010
Exhibition Hall D (Hynes)
9:00 PM - E8.1
Synthesis of Low Band Gap Dyad Molecules for High Photocurrent Generation in Organic Solar Cells.
Seiichiro Izawa 1 , Keisuke Tajima 1 , Kazuhito Hashimoto 1 2
1 Department of Applied Chemistry, The University of Tokyo, Tokyo Japan, 2 , HASHIMOTO Light Energy Conversion Project, ERATO, Japan Science and Technology Agency (JST), Tokyo Japan
Show AbstractOrganic solar cells (OSC) have recently drawn much attention as a sustainable energy source due to their possible advantages such as low fabrication cost, lightweight, flexibility, and large device area. In OSC, currently most efficient device structure is so-called mixture bulkheterojunction. In this structure, however, it is difficult to precisely control the film morphology of electron donor and acceptor in nanoscale, which could limit the efficiency of the performance. To solve this problem, control of the nanostructures in organic films by the molecular design is an important topic. Use of a dyad molecule, i.e. covalently attached donor and acceptor, is one of the promising strategies to prevent the large phase separation and therefore to realize efficient charge separation and higher short circuit current (ISC) in OSCs. We have previously reported several molecular dyad systems for OSC application [1-2]. Oligo(p-pheneylenevinylene)-fullerene dyads showed relatively high power conversion efficiency of 1.28 % and ISC of 3.3 mAcm-2 in single-component OSC [2]. For further improvement, introduction of low band gap molecules is promising strategy for further increase of the photocurrent owning to the better matching of the light absorption to the solar spectrum. Recently, it has been reported that diketopyrrolopyrrole (DPP) derivatives achieved high performance in the mixture bulk heterojunction OSC with [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) [3]. In the current contribution, low band gap dyad molecule with DPP core fullerene dyads (LBG-Dyads) were designed and synthesized. For comparison, mixture bulkheterojunction devices with the LBG donor part and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were also fabricated. The single-component OSC devices based on the LBG-Dyads showed the response extended to 800 nm in the external quantum efficiency (EQE) plot. This is the longest photocurrent response among single component OSCs. Atomic force microscopy (AFM) images showed more flat and uniform film morphology in LBG-dyad films compared to the corresponding mixture films where large phase separations of the components were observed. Thanks to this controlled nanostructure of the donor and the acceptor, ISC of OSC drastically increases from 0.48 mAcm-2 with the mixture film to 3.8 mAcm-2 with the LBG-Dyad. From these results, it can be concluded that the low band gap dyad molecule is a promising molecular design for higher ISC. Synthetic details, molecular properties, device performances, and film morphology will be discussed.[1] T. Nishizawa, K. Tajima, K. Hashimoto, J. Mater. Chem., 2007, 17, 2440-2445.[2] T. Nishizawa, K. Tajima, K. Hashimoto, Chem. Commun., 2009, 2469.[3] B. Walker, A. B. Tamayo, X.-D. Dang, P. Zalar, J. H. Seo, A. Garcia, M. Tantiwiwat, T. Q. Nguyen, Adv. Funct. Mater., 2009, 19, 1-7.
9:00 PM - E8.10
Synthesis and Characterization of Novel N-type Materials with LUMO around 4.0eV, Based on Pyridine-borane Complexes.
Tsuyoshi Goya 1 , Yoichi Arimoto 1 , Munehiro Hasegawa 1 , Takeo Akatsuka 1 , Katsuyuki Morii 1 , Naoki Ishida 2 , Taisaku Moriya 2 , Masahiro Murakami 2
1 , NIPPON SHOKUBAI CO.,LTD., Osaka Japan, 2 , Department of Synthetic Chemistry and Biological Chemistry, Kyoto University, Kyoto Japan
Show AbstractElectronic devices based on organic semiconductors have recently attracted enthusiastic attention due to their high potential applications in flexible, lightweight and large area electronic systems and in low-cost solution-based processing. Lots of high-performance p-type organic semiconductors have been investigated, and some materials have been already launched, while n-type organic materials have been less developed. In addition, the stability in the air was focused on in organic electronic devices, de Leeuw et al. have suggested in 1997 that the air-stable n-type materials need LUMO >4.0 eV. So, n-type organic semiconductors with low LUMO have become to be widely required. So far C60 or C70 derivatives have been mainly used for n-type material, but they have not been enough for LUMO level, process and handling. So, in recent years other materials instead of fullerene derivatives have been enthusiastically investigated. One of the candidates is boron compound. They are known as materials having low LUMO level from the vacant p-orbital of the boron atom. However, few organic semiconductors with boron atom are known for the stability. In 2006, Yamaguchi et al. have reported new boron compounds with intramolecular B-N coordination to increase the stability. The possibility of boron compounds as n-type semiconductor was published. Unfortunately, it was difficult to make a library of organic molecules having intramolecular B-N coordination by the reported synthetic method. In this research, we developed novel synthetic method for monomer with intramolecular B-N coordination. The monomers were prepared in high overall yields via new synthetic method. As a result, several new monomers were prepared. Utilizing new synthesized monomers, we produced conjugated novel polymer with low LUMO level via Suzuki-Miyaura coupling reaction. Chemical structures and properties of the polymers were characterized by 1H NMR, FT-IR, mass spectrometry, UV absorption, cyclic voltammetry, photoluminescence, thermogravimetric analysis, differential scanning calorimetry, and by gel permeation chromatography. The synthesis of these new materials including unique physical properties will be presented in detail.
9:00 PM - E8.11
Blue Light Sensitive Organic Sensor.
Yusuke Higashi 2 , Kyu-Sik Kim 1 , Sang-Yon Lee 1 , Musubu Ichikawa 2
2 Functional Polymer Science Course, Shinshu University, Ueda, Nagano, Japan, 1 Display Lab. SAIT, Samsung Electronics Co., Ltd, Yongin-Si Korea (the Republic of)
Show AbstractThe organic photoconductive materials have been intensively studied due to their potential applications in organic photovoltaics (OPVs) from the viewpoint of low-cost large-area fabrications. Among them some organic materials showed wavelength selective photoconductivity, which can be applicable to develop multi-color image sensors such as RGB (red, green, blue) full color organic image sensors. In this study, we report fabrication and performance of a blue light sensitive organic photosensor; we fabricated devices by using BP3T, a thiophene oligomer, as p-type and C60 as n-type by means of vacuum evaporation. Another thiophene derivative, BP1T, layer was inserted between the PEDOT:PSS and BP3T layers as buffer layer. Both BP3T and C60 have strong light absorption for blue light, and thus the device indicate a good sensitivity for blue light. Furthermore, the incorporated BP1T layer provided an enhanced sensitivity for the light. The current-voltage (I-V) characteristics and incident photon-current conversion efficiency (IPCE) shows capability of the device for a blue light sensitive organic photosensor
9:00 PM - E8.12
Enhanced Performance and Air-stability of Polymer Solar Cells with High Work-function Metal Cathode by Self-assembly of Fullerene End-capped Poly(ethylene glycol).
Jae Woong Jung 1 , Jea Woong Jo 1 , Won Ho Jo 1
1 , Seoul National University, Seoul Korea (the Republic of)
Show AbstractAlthough polymer solar cell (PSC) offers special opportunities for low cost, flexible, and portable energy source, still lower efficiency than Si-based solar cell limits the mass-production of PSCs. To increase the efficiency, various approaches such as thermal annealing and other process optimization have been proposed and these methods have been very effective to develop nanoscale phase-separated morphology in horizontal direction (parallel to film surface). However, as considering that charges are transported along perpendicular to the substrate, not only the lateral phase separation but also the vertical distribution of the components in active layer is very critical. If the ideal vertical-morphology of bulk heterojunction (the p-type conjugated polymers are rich near the anode and n-type fullerenes are rich near cathode) could be achieved, the enhanced performance of PSC would be expected. Another fatal problem of PSC is poor stability. Because most conjugated polymers are easily photo-oxidized at room temperature, the performance of PSC is dramatically decreased when the PSC is operated under ambient atmosphere. Therefore, the protection of the active layer from oxidation is essential for high stability of PSC. To enhance both the performance and the air-stability of PSCs, we developed a self-assembled buffer layer by adding fullerene end-capped poly(ethylene glycol) (PEG-PCBM) to the active layer based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Because of lower surface energy of PEG (ca. 40 mJ/m2) than that of the P3HT (26.9 J/m2) and PCBM (37.8 J/m2), the PEG-PCBM molecules are expected to form nano-scale monolayer on the top of active layer. The PEG-PCBM layer is expected to act as a multifunctional buffer layer in PSC device. Since the buffer layer with high dielectric constant induces an interfacial dipole moment between active layer and metal cathode, the energy barrier for electron collection is reduced, which affords exciton dissociation and charge collection, and consequently increases the open circuit voltage and the power conversion efficiency. Another important feature is to develop the ideal vertical distribution (P3HT rich near the anode and PCBM rich near the cathode) in active layer because the fullerenes in PEG-PCBM can effectively induce segregation of PCBM near PEG-PCBM layer. Furthermore, the PEG-PCBM layer protects the active layer from oxygen and Al invasion during the metal deposition, and therefore the stability of the PSC will be increased. To verify the enhanced performance and air-stability, we measured the change of the absorbance and the PCE of PSCs with exposure time to ambient atmosphere. When high work-function metal (Au and Cu) was used as a cathode for more stable PSC, the performance and air-stability of PSC are extremely increased by introducing the PEG-PCBM layer, and therefore the low-cost Cu can be used for future PSC electrode in the presence of PEG-PCBM.
9:00 PM - E8.13
Quantum Chemical Calculations of Reorganization Energy for Self-exchange Electron Transfer of Aromatic Diamines as Electron-donor Molecules.
Qi Wei 1 , Davaasambuu Sarangerel 1 , Khishigjargal Tegshjargal 1 , Chimed Ganzorig 1
1 Center for Nanoscience and Nanotechnology and Department of Chemical Technology, National University of Mongolia, Ulaanbaatar Mongolia
Show AbstractReorganization energy is one of the important factors to decide the rate of electron transfer according to the Marcus theory. Small reorganization energy is highly desirable in molecular design and modeling of organic optoelectronics and nanoelectronics.For this reason, the reorganization energy of aromatic diamine derivatives such as N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) and N,N′-bis(l-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPD) are studied theoretically by self-exchange electron transfer theory.We figure out the optimization points by Gaussian 03 software, which are needed for quantum chemical calculations of the internal reorganization energy of self-exchange electron transfer reaction with Nelson's four-point method. Ionization potentials and electron affinities of these molecules studied will be also calculated.All the calculations are carried out at the density functional theory level with basis set 6-31G (d, p) using Gaussian 03 software.The theoretical results will be compared with experimental absorption and luminescence data and discussed in a more detail presented at the proceedings paper.
9:00 PM - E8.14
Synthesis and Application of Anionic Water-soluble Polyfluorene Derivatives for Inverted Organic Solar Cells.
Rira Kang 1 , Seung-Hwan Oh 3 , Tae-Soo Kim 1 , Dong-Yu Kim 1 2 4
1 Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of), 3 Research Institute for Solar and Sustainable Energies, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of), 2 Nanobio Materials and Electronics, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of), 4 Heeger Center for Advanced Materials, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of)
Show Abstract Conjugated polyelectrolytes(CPEs) have attracted attention due to their semiconducting properties, along with solubility in polar solvents and self-assembly properties of polyelectrolytes. We have shown that CPEs can be used as the interfacial layer in polymer light-emitting diodes (PLED) and organic solar cells (OSCs). Here, anionic water-soluble polyfluorene derivatives with anionic side chains, which were used as the interfacial layer in the inverted organic solar cells, was synthesized by a palladium catalyzed Suzuki coupling. The synthesized anionic polyfluorene can be dissolved in polar solvent, preventing intermixing of the multilayers. The electronic properties of anionic polyfluorene derivatives were characterized by Ultraviolet Photoelectron Spectroscopy (UPS) and other method. The electronic properties of anionic polyfluorene derivatives, were used as the interfacial layer between the active layer and PEDOT:PSS/Ag anode in the inverted organic solar cells. The fill factor was enhanced by providing wettabillity of subsequent PEDOT:PSS layer, comparing with the device without this interfacial layer. Furthermore, the effects of using the interfacial layer in the inverted organic solar cells will be discussed.
9:00 PM - E8.15
Alternating Conductivity of Heterogeneous and Disordered Nanocomposites Based on Non-uniform Energy Barriers Distribution for Hopping Carriers.
Juliana Couto 1 , Mirela Santos 1 2 , Rodrigo Bianchi 1
1 Department of Physics, Federal University of Ouro Preto, Ouro Preto, MG, Brazil, 2 Physics Department, Federal University of Viçosa, Viçosa, Minas Gerais, Brazil
Show AbstractCharge carrier transport and ac conduction in disordered solids has been extensively studied in the past few years both by theoretical modeling or experimental analysis of electrical properties. The importance of the study of these properties goes beyond the analysis of charge transport on bulk and interfacial layers since it interferes directly on the electrical properties of thin films and electronic devices based on organic and inorganic materials. Particularly on this work our interest is related to the universal, frequency-dependent, properties of the alternating conductivity. It is established that charge carrier transport in such materials is mainly dominated by hopping mechanism among localized sites, whose transition rate depends on the spatial distance and on the energy difference between two involved sites. The most acceptable explanation for a conductivity that increases with frequency or temperature is the existence of one or another kind of inhomogeneity in the material, with both microscopic and macroscopic nature. Among the several hopping models, the Random Free Energy Barrier (RFEB) model appears as one of the most used. This model considers a random barrier continuously and uniformly distributed through the material bulk. The carrier should face variable energy barriers between a minimum and a maximum value, where the hopping frequency would be linearly dependent to the energy barriers value. If on the one hand this model fits the ac conductivity universality and is capable of representing the real component of ac conductivity of numerous disordered materials, on the other it is not possible to apply it to heterogeneous systems in which the energy barrier distribution does not show a uniform behavior. Therefore, the main goal pursued in this work is the study of the alternating conductivity in disordered solid systems based on non-uniform energy barriers distribution models and on equal jump distance for charge carries in terms of computer simulation, modeling and analysis of experimental results. In this context, it is proposed a generalization of RFEB model firstly described by J. C. Dyre and R. F. Bianchi for different distributions of energy barriers such as Delta function and Gaussian. Finally, the models developed were applied to heterogeneous disordered nanocomposites systems. Work supported by INEO/CNPq and FAPEMIG.
9:00 PM - E8.16
Layer-by-layer Hybrid Films of Polyaniline and Indium-tin Oxide Nanoparticules Characterized by Impedance Spectroscopy and Atomic Force Microscopy.
Gislayne Goncalves 1 2 , Mirela Santos 3 , Juliana Couto 1 , Sukarno Ferreira 3 , Maximiliano Munford 3 , Rodrigo Bianchi 1
1 Department of Physics, Federal University of Ouro Preto, Ouro Preto, MG, Brazil, 2 , Federal Institute of Minas Gerais - Campus Ouro Preto, Ouro Preto, Minas Gerais, Brazil, 3 Physics Department, Federal University of Viçosa, Ouro Preto, Minas Gerais, Brazil
Show AbstractThere has been considerable interest in the use of nanocomposite systems consisting of inorganic/organic hybrid materials where the composition and particle size can be engineered at our will. The nanocomposite also have immense potential for a wide range of technological applications, for example, in the field of emission displays, gas sensors, etc. In this context polymer light-emitting diodes (PLEDs) are already occupying their niche in the market next to their inorganic counterparts. Electroluminescence (EL) in PLEDs is achieved by carrier injection to the polymer chain from both electrodes proceeding by radiative recombination from self-localized excited states. Therefore these devices the balanced charge injection and collection at the electrodes plays a crucial role in the device performance. In these devices the light emitting polymer is usually deposited onto ITO thin film, which acts as oxygen and metal ion sources. In order to minimize these effects, an intermediate hole transport layer (HTL) between the ITO and the photoactive layer is required. Various studies of utilizing nanocomposites consisting of polymer films as insulating matrix and randomly dispersed fine conductive particles act as an efficient HTL on ITO substrate has been reported. This system facilitates not only the hole injection with higher work functions than ITO, but also improves the efficiency of such devices. In this work, we produced HTL thin films by aqueous solution of polyaniline (PANI) incorporated with ITO nanoparticles (~ 50 nm) using layer-by-layer (LbL) self-assembly. Nanocomposites of PANI/ITO were prepared. ITO nanoparticles were dispersed in poly (ethenesulfonic acid) (PVS) aqueous (pH 2.5) at mass concentration of 0.1, 1 and 10 mg/ml, while PANI was dispersed in aqueous HCl (pH 2.5) at a concentration of 0.01 mg/ml. The films, once prepared, were investigated by means of images of atomic force microscopy (AFM) to determine the morphology, and by alternating conductivity (ac) to determine their electrical properties. Alternating conductivity is examined with varying parameters the filler content, and temperature. The increase in the amount of adsorbed PANI/ITO-PVS films with the number of layers is explained by the increase in adsorption sites when some polymer is already adsorbed. Such an observation was confirmed by AFM. AC conductivity the examined systems exhibit considerable conductivity, which alters by several orders of magnitude with temperature, frequency and filler content gives evidence for the charge carriers transport mechanism via the occurred agreement of results with the employed models in which the organic-inorganic samples is considered to be constituted of semiconductor regions (ITO nanoparticles) sparsely distributed in a disordered PANI material. An effective medium approximation is then used to analyze the conductivity of such nonhomogeneous medium. This work was sponsored by Capes, CNPq, LNLS, INEO/CNPq and Fapemig.
9:00 PM - E8.17
Synthesis of Organic Dye Having Dendrons and Their Interfacial Properties in Dye-sensitized Solar Cells.
Young-Soo Kwon 1 , Ayyanar Siva 1 , Inwoo Song 1 , Inyoung Song 1 , Taiho Park 1
1 chemical engineering, Pohang University of Science and Technology(POSTECH), Pohang, Gyungbuk, Korea (the Republic of)
Show AbstractSince Grätzel first demonstrated dye-sensitized solar cells(DSCs) in 1991, much efforts have been devoted to this research field to improve the energy conversion efficiency. In order to achieve efficient electrochemical cycling in DSCs for better light harvesting by sensitized dyes on nanocrystalline TiO2, in principle, electron injection from photo-excited dyes to the conduction band of TiO2 should be fast then the resulting oxidized dye must be regenerated by redox species in electrolyte or hole-transport materials through electron transfer reaction which in general occurs within a very short distance. These electrochemical reactions, especially the dye regeneration reaction at the interface of TiO2/electrolytes(or hole transport materials) seem to be governed by not only an energy difference between ionization potentials of dyes and redox species but also physical contact between oxidized dyes and electrolyte(or HTM). For example, dyes having hydrophilic end groups could make better physical contact with redox species dissolved in hydrophilic electrolytes such as acetonitrile. On the other hand, dyes having hydrophobic moiety could be useful in solid-state dye-sensitized solar cells(s-DSCs) because s-DSCs in general employs organic or polymeric hole transport materials which are more hydrophobic than acetonitrile. From this viewpoint, we synthesized an organic dye which has dendrons on its wedges. The organic dye has triphenyl amine as a donor part and bithiophene as a bridge between donor and acceptor. We introduced dendrons to investigate a dramatic effect of extremely hydrophobic bulky group, thereby expected to exhibit better contact property in s-DSCs. s-DSCs were fabricated with the synthesized organic dye, exhibiting 1.7% of power conversion efficiency with 0.86V of Voc, 4.0mA/cm2 of Jsc, and 0.50 of fill factor while only 0.1% efficiency in liquid DSCs. The results are well consistent with our expectation and confirm our assumption that physically good contact guarantees efficient dye regeneration. Synthetic description of organic sensitizer and surface properties of the hetero-junction also will be discussed in detail.
9:00 PM - E8.18
Self-assembly Single Crystal Micro/Nanostructures and Their Applications in Organic Optoelectronics.
Chengliang Wang 1 2 , Hongxiang Li 1 , Wenping Hu 1 , Jianbin Xu 2
1 , Institute of Chemistry, Chinese Academy of Sciences, Beijing China, 2 Department of Electronic Engineering, Chinese University of Hong Kong, Hong Kong Hong Kong
Show AbstractSingle crystals of organic semiconductors are supposed to combine the merits of the organic materials and the advantages of single crystals. They can reveal the intrinsic charge transport properties, provide high performance optoelectronic devices, demonstrate the structure-property relationships (among the chemical structures, the arrangements and the properties) and probe the underlying mechanisms of the devices. Under the same self-assembly condition, three compounds displayed micro/nanostructures from one to three dimensions and transistors based on them exhibited mobility decreased successively, suggesting the importance of π-π stacking on morphologies and charge transports properties of the materials. By tuning the self-assembly conditions, crystals of BPEA belong to two phases were obtained, and the micro/nanostructures showed strong phase dependent optoelectronic properties, which would be helpful to probe the structure-property relationships.Acknowledgement:The authors acknowledge the support by the National Natural Science Foundation of China (Grant Nos. 60736004 and 50873105), Ministry of Science and Technology of China (Grant Nos. 2006CB806200 and 2006CB932100), Chinese Academy of Sciences, and the Research Grants Council of Hong Kong SAR, particularly, under Grant Nos. CUHK2/CRF/08 and 418209.References:1.Wang, C.; Liu, Y.; Ji, Z.; Wang, E.; Li, R.; Jiang, H.; Tang, Q.; Li, H.; Hu, W. Chem. Mater. 2009, 21, 2840.2.Wang, C.; Liu, Y.; Wei, Z.; Li, H.; Xu, W.; Hu, W. Appl. Phys. Lett. 2010, 96, 143302.
9:00 PM - E8.19
ZnO:P3HT Heterostructures - The Influence of Processing on Device Performance.
Jon Downing 1 , Mary Ryan 1 , Natalie Stingelin 1 , Martyn McLachlan 1
1 Department of Materials Science and Engineering , Imperial College London, London United Kingdom
Show AbstractThere is increasing interest in the development of low-cost, solution processable photovoltaic devices. Hybrid cells, which feature nanostructured organic-inorganic interfaces, have the potential to achieve improved efficiency whilst minimizing both material and processing costs. The metal oxide, ZnO is of specific importance for its numerous morphologies and high electron mobility. Furthermore, solution processing permits the formation of ZnO structures with feature sizes ranging from a few nanometres to tens of microns over large areas. For organic materials, and in particular poly(3-hexylthiophene)(P3HT), excellent hole transport properties are combined with strong light absorption. Compatibility with solution processing is well documented for this material. We will report studies, which focus on the identification of a reproducible route for the formation of a standard nanostructured ZnO template. Here a well-defined seed layer controls the growth or orientated nanorods directly on ITO coated glass. To study the efficiency of P3HT filling of the ZnO structures we characterised the viscosity of P3HT solutions over the molecular weight (Mw) range 10-440 Kg/mol. The use of a consistent and well-defined template has allowed the role of viscosity on filling efficiency to be studied, whilst the influence of device performance on the Mw of the organic phase has been studied in parallel. We will present SEM, AFM and XRD characterisation of the composite structures and correlate the macroscopic and interfacial structures with the measured device efficiencies.
9:00 PM - E8.2
A Sandwich-type Recombination Layer Structure in Tandem Organic Solar Cells.
Jingchuan Wang 1 , Sanqiang Shi 1 , Chi-wah Leung 2 , Kwok-leung Chan 1
1 Department of Mechanical Engineering, Hong Kong Polytechnic University, Hong Kong Hong Kong, 2 Department of Applied Physics, Hong Kong Polytechnic University, Hong Kong China
Show AbstractIn order to improve photons harvesting efficiency, multiple junction solar cells are designed by stacking two or three unit solar cells in series. This stacked structure needs a recombination layer between two cells to ensure the electrical continuity in the whole device. Until now, metal nano-particles such as silver, gold and aluminum are most frequently used as recombination centers [1, 2]. This scheme has the drawback that the metal nano-particles inevitably quench the excitons in the two adjacent active layers, which will reduce the photocurrent density. If an ineffective recombination layer is used, an “S shape” appears in I-V response, resulting in a reduced fill factor [1-3]. As a result, in despite of obtaining perfect open circuit voltage (Voc) and short circuit density (Jsc) with a tandem structure, the efficiency will be also reduced remarkably. In this work, we inserted a sandwich structure of BCP/ metal nano-particles/MoO3 as the recombination layer into two adjacent unit cells interface instead of using bare metal nano-particles, thus suppressing the chance of exciton quenching by the metal nano-particles. In such a structure, the BCP and MoO3 layers work as exciton blocking layers. We optimized different parameters including the thickness of BCP and MoO3 layers, work function and the size of metal nano-particles, with the purpose of enhancing the recombination of unwanted charges and eliminating the “S-shape” in the I-V profile of the tandem solar cells.References:[1] J. Gilot, M. M. Wienk, and René A. J. Janssen, Appl. Phys. Lett. 90, 143512 (2007).[2] D. Cheyns, H. Gommans, and P. Heremans, Sol. Energy Mater. Sol. Cells, 91 (2007) 399–404.[3] A. Colsmann, J. Junge, and U. Lemmer, Appl. Phys. Lett, 89, 203506 (2006)
9:00 PM - E8.20
Structural Study of Poly(alkylthiophene)-Fullerene Bulk Heterojunctions Using Neutron Scattering.
Margaret Sobkowicz 1 , Dean DeLongchamp 1 , R. Joseph Kline 1 , Ronald Jones 1
1 Polymers Division, NIST, Gaithersburg, Maryland, United States
Show AbstractBulk heterojunctions (BHJs) composed of poly(hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) are promising active layers for organic photovoltaics. The nanoscale morphology of the BHJ active layer is critical to the performance of organic photovoltaic devices because exciton diffusion, charge separation, and charge carrier transport processes require domains that have specific optimal sizes and connectivity. Processing parameters have been shown to have a large influence on the power conversion efficiency of P3HT:PCBM BHJs, but much is still to be learned about how the nanoscale morphology develops during film formation, and how that morphology correlates to device performance. Small angle neutron scattering (SANS) is an ideal measurement technique to study the blend morphology and phase composition in P3HT:PCBM films due to the large intrinsic difference in neutron scattering length density between P3HT and PCBM, the length scale probed, and the excellent sensitivity of SANS to concentration fluctuations. In this work, several strategies are employed to develop the use of SANS as a characterization tool for organic photovoltaic materials. Scattering data from thin films, bulk films, and P3HT:PCBM solutions are evaluated to develop a picture of the nanoscale organization and the influence of processing parameters on morphology.
9:00 PM - E8.21
Intramolecular Charge Transfer and Electro-optical Properties of Highly Twisted Triarylamines for Organic Photovoltaics.
John Chudomel 1 , Paul Lahti 1
1 Chemistry, University of Massachusetts Amherst, Amherst, Massachusetts, United States
Show AbstractWe report the synthesis, optical, and electronic properties of two novel, highly twisted triarylamines with potential as photoconductive additives for organic photovoltaic devices. Each triarylamine incorporates two p-methoxyphenyl units along with a larger chromophore as the third aryl group to induce the twisting of the system. The two large chromophores used were anthracene, attached at the 9 position to the amine (9-DAAA), and an even larger “horseshoe” shaped chromophore (9-DAAH) analogous to anthracene that our group had previously synthesized. The horseshoe chromophore is polycyclic and aromatic but due to steric hindrance, not completely planar. We were interested in the influence of this property on the overall system. The triarylamines were synthesized by first coupling p-anisidine and p-iodoanisole to form a bis-anisyl amine. This bis-anisyl amine was then coupled to each of the large chromophores to give the desired product. Crystallographic data confirm both the expected structure and the twisted nature of 9-DAAA. Computational modeling shows a similar twisted structure for 9-DAAH. Both triarylamines have absorbance peaks of visible light at 450 nm in addition to multiple absorbance peaks for ultraviolet light. Each triarylamine demonstrates a major solvatochromic fluorescence shift of over 100 nm as solvent polarity is varied. They both fluoresce at 505 nm in hexane, at 590 nm in dichloromethane, and at 610 nm in acetonitrile. In addition to the shift of the emission wavelength, the quantum yield for each triarylamine drastically decreases as solvent polarity increases. As each of the two triarylamines are photoexcited, we find evidence for internal charge transfer from the anisyl portion of the molecule to the large chromophore. Solvents of higher polarity interact strongly with the charge transfer excited state and quench fluorescence intensity and lifetime by inducing non-radiative relaxation back to the ground state. Fluorescence lifetime studies indicate a three-fold decrease in fluorescence lifetime as the solvent is switched from hexane to acetonitrile. We are evaluating the viability of these novel triarylamines as additives for P3HT/PCBM organic photovoltaic devices by gathering information via cyclic voltammetry and photoconductivity.
9:00 PM - E8.22
Effect of Vertical Phase Segregation in Inverted and Conventional Solar Cells from a Low-bandgap Donor-acceptor Polymer.
Frederick Steffy 1 , Kaushik Roy Chouldhury 1 , Jegadesan Subbiah 1 , Chad Amb 2 , John Reynolds 2 , Huanjun Ding 3 , Yongli Gao 3 , Franky So 1
1 Dept of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 The George and Josephine Butler Polymer Research Laboratory, Department of Chemistry, Center for Macromolecular Science and Engineering, University of Florida, Gainesville, Florida, United States, 3 Department of Physics and Astronomy, University of Rochester, Rochester, New York, United States
Show AbstractThe vertical phase morphology of solution-processed bulk-heterojunction (BHJ) photovoltaic devices is an important parameter which determines the charge extraction efficiency at the electrode interfaces. In general, it is assumed that there is no composition gradient in the BHJ electroactive layer between the anode and cathode. However, quite a few polymer-based BHJ blends exhibit a vertical phase separation not desirable for optimum performance. We recently observed a non-ideal vertical phase segregation in BHJ blends of a low-bandgap donor-acceptor polymer, poly ((4, 4- dioctyldithieno (3,2-b:2',3'-d) silole) -2,6- diyl-alt- (2,1,3- benzothiadiazole) -4,7-diyl) (PSBTBT) blended with PC70BM, where the electron acceptor preferentially segregates towards the bottom ITO anode. To investigate the effect of this vertical phase segregation on device performance, we fabricated inverted solar cells that exploit the natural vertical phase separation of the active layer. We realized the inverted architecture by deposition a solution-processed layer of ZnO nanoparticles on the bottom ITO electrode, followed by the BHJ layer. By modifying the ITO electrode with ZnO nanoparticles, it becomes an electron-extraction electrode in this inverted structure. A thin layer of MoO3 was used as a hole-extraction layer on top. The performance of this device was compared with one that has a conventional structure of ITO/MoO3/BHJ layer/LiF/Al. The inverted devices exhibited an increase in short circuit current density (Jsc) by 20% over the conventional solar cells (14 mA/cm2) in the inverted devices compared to 11.5 mA/cm2 in the conventional devices). Despite similar carrier mobilities in both the conventional and inverted structures, photoconductivity measurements confirmed improved charge extraction efficiencies in the latter under short circuit conditions. This observation points towards reduced recombination of photo-generated carriers in the inverted solar cells, arising out of more favorable electronic pathways to the electrodes. Additionally, preliminary experiments indicate significant improvement of device lifetime resulting from the inverted devices.
9:00 PM - E8.23
Increased Photocurrent in Ternary Bulk Heterojunction Conjugated Polymer-fullerene-nanoparticle Photovoltaic Devices.
Eric Peterson 1 , Gregory Smith 1 , Robert Coffin 1 , Minglai Fu 2 , Richard Adams 2 , David Carroll 1
1 Physics, Wake Forest University, Winston-Salem, North Carolina, United States, 2 Chemistry, University of South Carolina, Columbia, South Carolina, United States
Show AbstractWe have constructed ternary bulk heterojunction devices composed of conjugated polymer-fullerene-nanoparticle blends. We show how the addition of a nanoparticle-electron acceptor material can influence and change photon collection in these devices. We have shown that inclusion of a certain weight percentage of CdSe/CdS core-shell nanoparticle material can increase short circuit current in P3HT:PCBM based devices. Here, we look at a number of low bandgap polymers in conjunction with CdSe/CdS core shell nanoparticles of different sizes as well as CuInGaAs nanoparticles and PCBM. By investigating a number of these systems, we can start to understand the nature of the photocurrent enhancement as well as gain insight into photon conversion in conjugated polymer based, bulk heterojunction solar cells.
9:00 PM - E8.24
Low Bandgap Dithienophosphole-based Polythiophenes.
Jordan Wilson 1 , Robin Krueger 1 , Thomas Baumgartner 1
1 Chemistry, University of Calgary, Calgary, Alberta, Canada
Show AbstractIn the search for novel polymers intended for organic photovoltaic (OPV) applications, materials with high charge mobilities, low bandgaps, broadened absorption profiles, and a controlled thin film morphology are desirable. The most commonly investigated materials in this field are based on poly-3-hexylthiophene (P3HT) derivatives, due to its ease of synthesis, high charge mobility, and well understood film forming properties. For their high charge mobility, alkyl-thiophene-based polymers seem desirable, but there is a need to further tune the optoelectronic properties, such as broadening or extending the absorption properties further into the NIR region of the solar spectrum. In order to prepare polymers with lower bandgaps, an electron-rich unit (donor) can be polymerized with an electron-poor unit (acceptor), known as the donor-acceptor strategy. To this end, we have prepared a series of polythiophene-based copolymers, containing varying amounts of dithienophosphole as the acceptor material. As part of this study, we investigated the thin film morphology of the dithienophosphole-containing copolymers by comparing materials with head-to-head and head-to-tail coupling arrangements of the alkylated thiophenes. To further tune the optoelectronic properties of these copolymers, we then blended in a second acceptor, benzothiadiazole. These polymers had a random AB-AC arrangement, where the acceptor units (B or C) were linked through an alkylated bithiophene (A). The amount of each acceptor was varied; blending in more dithienophosphole as the amount of benzothiadiazole was decreased, to study the effect on the charge transport properties. The random nature of the polymers lead to broadened absorption profiles, as compared to the parent AB-copolymers.
9:00 PM - E8.25
Improving Organic Solar Cells based on Small Molecules.
Moritz Riede 1 , Christian Uhrich 2 , Johannes Widmer 1 , Ronny Timmreck 1 , David Wynands 1 , Roland Gresser 1 , Toni Mueller 1 , Gregor Schwartz 2 , Wolf-Michael Gnehr 2 , Martin Pfeiffer 2 , Karl Leo 1
1 Institut für Angewandte Photophysik, Technische Universität Dresden, Dresden Germany, 2 , Heliatek GmbH, Dresden Germany
Show AbstractThe development of organic solar cells (OSC) has reached a stage at which the first products have just entered the market. However, OSC efficiencies are still often considered too low for large commercial success despite good progress in recent years and efficiencies close to 8%. OSCs can be made via different approaches and various strategies are followed to push their efficiencies beyond 10%. The main focus lies on new materials, on control of the morphology in the mixed heterojunction and on improved stack designs. Our approach for making OSCs is based on thermal evaporation of small molecules in vacuum to create an organic stack in the p-i-n concept. In this concept, intrinsic absorber layers are sandwiched between p- and n-doped wide gap transport layers, leading to a nearly ideal solar cell structure. This allows for a large freedom in stack design both for investigation of basic physical processes and device optimization.As material system for our investigations we use small molecules like α,ω-bis-(dicyanovinylene)-sexithiophene (DCV6T), phthalocyanines and AZA-bodipy derivatives as donor materials in heterojunctions with C60. By tuning their molecular structures, high open circuit voltages Voc of up to 1V can be achieved in OSC or materials can be made to absorb up to the near infra red. However, using these materials in OSCs requires an individual stack optimization and special focus has to be placed on the properties of the mixed heterojunction. Controlling the morphology of this heterojunction plays a crucial role in charge carrier extraction and there has been much progress in the field of solution processed OSCs, but it only recently received more attention for vacuum deposited OSCs. Our results from AFM and X-ray show that the morphology of pristine layers and mixed heterojunctions strongly depends on the temperature of the substrate on which the organic layers are deposited. This in turn allows for tuning the phase separation and enables for many materials an increase of layer thickness for improved light absorption while retaining high fill factors (FF) at the same time. As result, we have achieved optimized single heterojunction devices with efficiencies above 5%.To go beyond the efficiency limits of single heterojunction OSCs, which are estimated to be just above 10%, tandem concepts are required. Thus, much research has been devoted in recent years to realizing tandem OSCs. We show how the required multilayer structures can efficiently be realized based on the p-i-n concept and describe the strategies for the optical optimization of tandem devices. Combining DCV6T and phthalocyanines to cover the spectral region between 300nm and 750nm leads to a tandem OSC with a certified efficiency of 6.07% on 2cm2 device area, i.e. areas large enough to be of relevance for modules. Furthermore the tandem stack shows ideal behavior under varying illumination angles and a stable performance at elevated temperatures.
9:00 PM - E8.26
Performance Enhancement in Organic Photovoltaic Cells with Nano-spike Structures.
Lian Li 1 , Fadong Yan 2 , Lynne Samuelson 1 , Jayant Kumar 2
1 , US Army Natick Soldier Research, Development & Engineering Center, Natick, Massachusetts, United States, 2 Center for Advanced Materials, University of Massachusetts Lowell, Lowell, Massachusetts, United States
Show AbstractBulk heterojunction organic photovoltaic cells have attracted extensive investigations due to their low-cost, easy-processing, and potential for roll-to-roll manufacturing. Considerable studies have been focusing on increasing the power conversion efficiency of the solar cells. Nano-spikes created on silicon wafers by femto-second laser pulses were used for anti-reflection and light-trapping in silicon based photodiodes. Using soft lithographic technique, the nano-spike structures have been successfully incorporated into the organic photovoltaic cells. The nano-structured organic solar cells exhibited much higher photovoltaic performance as compared with the solar cells without the structures. Larger photocurrent and higher conversion efficiency were measured. These results indicate that the nano-spike structures offer efficient light trapping in the active layer.
9:00 PM - E8.27
Structure-property Relationships in Dithienophosphole-based Dyes for Dye-sensitized Solar Cells.
Terry Gordon 1 , Matthew Henderson 1 , Curtis Berlinguette 1 , Todd Sutherland 1 , Thomas Baumgartner 1
1 Chemistry, University of Calgary, Calgary, Alberta, Canada
Show AbstractThe invention of dye-sensitized solar cells (DSSCs) by Grätzel and co-workers has given rise to a new field of chemistry specializing in the synthesis and study of novel light-harvesting dyes. To this end, many research groups have utilized the donor-acceptor approach whereby molecules are engineered to possess moieties that are electron-rich and electron-poor, often separated by a π-conjugated spacer. Our approach employs a rigid dithienophosphole π-conjugated spacer that serves to connect both the donor and acceptor units of the dye. These units are attached to the dithienophosphole core by standard coupling reactions, i.e., Suzuki, Stille, Sonogashira, etc., and thus, by doing so, we extend the conjugation of the dye and improve the absorption properties of the resulting chromophore. Furthermore, by oxidation of the phosphole center (PIII to PV), we are able to finely tune the electronic properties of the dye via modification of the central P atom. Installation of the titania anchoring moiety, i.e., carboxylic or cyano carboxylic acids, can be accomplished using either functionalization method, yielding either a symmetric or asymmetric dye. The sensitizing dyes are characterized via absorption and photoluminescence spectroscopy and electrochemical techniques such as electrochemical impedance spectroscopy, as well as, employed in DSSCs. Structure-property relationships of the dye adsorbed to the titania surface are also described.
9:00 PM - E8.28
Porphyrin-based Photo-sensitizer for Organic Photovoltaics.
Patchanita Thamyongkit 1 2 , Amorn Petsom 2 , Aritat Luechai 3 , Thanisa Kengthanomma 3 , Helmut Neugebauer 1 , Serdar Sariciftci 1
1 Physical Chemistry, Linz Institute for Organic Solar Cells, Linz Austria, 2 Chemistry, Faculty of Science, Chulalongkorn University, Bangkok Thailand, 3 Program in Petrochemistry and Polymer Science, Faculty of Science, Chulalongkorn University, Bangkok Thailand
Show AbstractPorphyrins as light harvesters in optoelectronic device are useful for their extremely high potential absorption coefficients, excellent charge mobility, and relative ease with which a variety of covalent and non-covalent bonding possibilities. This presentation summarizes syntheses of two series of porphyrinic compounds, investigation of their electrochemical and photophysical properties, and fabrication/evaluation of photovoltaic cells based on these compounds. One series includes two or three porphyrin macrocycles linked via triazine units. When two porphyrin macrocycles are presented, the remaining peripheral position on the triazine ring is taken by a 4-carboxyphenyl group or a 4-(2-carboxy-2-cyanoethenyl)phenyl anchoring group. In the other series, the triazine central unit is replaced by a triphenyl amine one. Introduction of more than one porphyrin macrocycle enhances the light-harvesting efficiency of the compounds. Triazine and triphenylamine groups are known to be good charge transporters. Cyclic voltammograms of these compounds were recorded to obtain HOMO-LUMO energy gaps, and the film morphology was investigated as well. According to findings from these studies, organic photovoltaic cells were prepared and light-to-electrical energy conversion efficiencies of the resulting devices were evaluated. The effect of the increased number of porphyrin macrocycles, the type of central unit and the type of the surface anchoring group will be discussed.
9:00 PM - E8.29
Probing the Orbital Levels of Engineered Fullerenic Molecules from a Nonvolatile Memory Cell.
Sarah Xu 1 , Edwin Kan 1
1 School of Electrical and Computer Engineering, Cornell University, Ithaca, New York, United States
Show AbstractIntroductionThe redox capability [1], electrical conductivity [2] and size monodispersity of the engineered fullerenic molecules (EFM) bring forth scalability and reduced device variations which are essential for nanoelectronics. Large solubility of EFM allows wafer-level fluid process. Chemical functionalization also alters the electronic structure of the molecule, creating programmable HOMO-LUMO levels [3] which are crucial for device design, such as resonant tunneling barrier for Flash memory to overcome the scaling bottleneck [4]. In this study, molecular energy information of C60-PCBM and C70-PCBM was embedded in nonvolatile memory cell structures. The extracted HOMO-LUMO showed good agreement with the literature [5]. Notice that there is much less orbital hybridization than the direct break-junction contacts. ExperimentA Metal-Oxide-Semiconductor (MOS) capacitor structure was fabricated and electrically characterized. The devices were fabricated on top of a p-type Si substrate with a 3nm thermally grown tunnel oxide. Experimental splits included thermally evaporated C60 and C60-PCBM with a thickness of 0.4nm and fluid processed C60-PCBM and C70-PCBM samples spin-coated using 0.5mg/ml toluene solution and a spin-speed of 1000 and 2000 RPM. Toluene residuals were cleaned through evaporation in a nitrogen purge ambient. 2nm electron-beam evaporated oxide was deposited subsequently to protect the EFM. 18nm ALD Al2O3 was chosen as the control dielectric for a smaller trap density and better electrostatics. 100nm Cr and 50nm Al were used as metal gate and patterned. Finally, a 400C annealing was performed in the forming gas for 30min. Results and DiscussionBoth room temperature and low temperature (10K) high frequency CV measurements were performed. Room-temperature Coulomb staircase phenomenon was demonstrated for samples containing both C60-PCBM and C70-PCBM by toluene spin-coating. Low-temperature CV coincided almost exactly with the room-temperature results, confirming the blockade behavior was indeed from C60-PCBM and C70-PCBM molecules instead of from dielectric traps. The band offset between the silicon conduction band edge and the LUMO level (Δχ), as well as the electron self-charging capacitance were extracted with 3D electrostatics solution. Δχ is -3.7eV for the C60-PCBM, matching the published literature data [5]. -4.0eV was obtained for C70-PCBM. Reference[1] U. Ganguly et al., MRS., Boston, MA, Nov. 29 – Dec. 3, (2004)[2] I. I. Mazin et al., Phys. Rev. B 45, 5114-5117, (1992)[3] B. M. Illescas et al., J. Org. Chem., 62 (22), pp 7585–7591, (1997)[4] T-H. Hou et al., Appl. Phys. Lett. 92, 153109, (2008)[5] C-W. Chu et al., Appl. Phys. Lett. 88, 153504, (2006)
9:00 PM - E8.3
Controlling Dye Adsorption Rate to Improve Photo-induced Electron Injection in Dye-sensitized Solar Cells.
JongChul Lim 1 , Young-Soo Kwon 1 , Inyoung Song 1 , Taiho Park 1
1 Chemical engineering, POSTECH, Pohang Korea (the Republic of)
Show AbstractLight harvesting is of importance to obtain higher power conversion efficiency in dye-sensitized solar cells. Therefore, more dyes should be anchored on surface of nanocrystalline TiO2, leading organic chemists to sophistically design new dyes capable of higher molar extinction coefficient and broader absorption window. In addition, light absorption of dyes and electron injection from excited dyes into the conduction band of nanocrystalline TiO2 should be occurred, efficiently and rapidly. However, it is considered that weak anchoring or dye aggregation on surface of TiO2 could diminish light absorption and electron injection, respectively. Especially, the latter is strongly related to decreased power conversion efficiency. To induce well-distributed dye anchoring on the surface of TiO2, co-adsorbents have been, in general, employed but focused on charge recombination reactions of photo-induced electrons with oxidized species in electrolytes and surface dipole moments which change the Fermi-level of TiO2. During the anchoring process, however, the adsorption kinetics could be affected by coadsorbents. Adsorption rate of anchoring group on active sites of TiO2 is able to be affected by reactivity of carboxylic acids and solubility and polarity of dyes. Little attention related to control of the adsorption rate for electron injection from photo-excited dye in the conduction band of TiO2 has been shown so far.From this viewpoint, we have focused on temperature effects which are governing the physical properties such as reactivity, solubility and polarity on adsorption rate for better dye distribution and light harvesting. In this study, alkyl derivatives have been introduced as co-adsorbents onto the surface of TiO2 to investigate the adsorption equilibrium as well as kinetics. The competitive anchoring of dyes and coadsorbents onto the surface of TiO2 is controlled by adsorption temperature, and is focused on mechanism and their behaviors at the interfaces. As the adsorption temperature is decreased below the room temperature, the adsorption rate of sensitizer is decreased more than that of co-adsorbents up to 22%. The relevant results from adsorption kinetics depending on temperature are observed on the dramatic change in molar light harvesting efficiency of sensitizers approximately 5 times.
9:00 PM - E8.30
Efficiency Limits in Organic Photovoltaics: Modeling Exciton Dissociation Constraints Based upon Donor-acceptor Energy Offsets.
Jonathan Servaites 1 , Tobin Marks 2 1 , Mark Ratner 2 1
1 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 2 Chemistry, Northwestern University, Evanston, Illinois, United States
Show AbstractWe present here efficiency limits in organic photovoltaics (OPVs) based upon a comprehensive device model that accounts for exciton dissociation efficiency constraints in these systems. Specifically, we incorporate these constraints based upon the energy offset between the lowest unoccupied molecular orbital (LUMO) levels of the donor (D) and acceptor (A). Recent experimental evidence strongly supports the importance of both the D:A LUMO offset and the electric field strength in overcoming the exciton binding energy (EBE). Here, we propose a model to account for these two factors, as well as materials properties including dielectric constant and exciton radius. Consistent with experimental results, we find a strong dependence on LUMO offset and exciton dissociation efficiencies. The results also show a significant influence from the dielectric constant and coulombic radius. Active layer thickness additionally is important given that the average electric field increases with decreasing thicknesses. Interestingly, even when the LUMO energy offset < EBE, there is some exciton dissociation, only it will be significantly limited. This helps explain previously published experimental results that have exhibited the same behavior. Assuming a dielectric constant = 4.0, a coulombic radius = 1.0 nm, and an active layer thickness = 200 nm, we find the practical power conversion efficiency limit in OPVs to be ~10%. The optimal LUMO offset in this case is ~0.6 eV. This offset is a balance between providing sufficient energy to dissociate most excitons at D:A interfaces while avoiding too much potential energy loss in the dissociation process. The optimal donor bandgap in this case is ~ 1.8 eV – suggesting why lower bandgap materials may not be necessary for optimal OPV efficiencies in today’s state-of-the-art systems. However, note that the optimal LUMO offset and donor bandgap shift when OPV materials properties change. If the binding energy is reduced (e.g., via higher dielectric materials or a larger coulombic radius), we show how both the optimal LUMO offset and donor bandgap shift to lower values. Ultimately, we believe these results will provide an important guide for designing OPV materials and understanding the role of materials properties – including LUMO offset, bandgap, dielectric constant, and coulombic radius – in achieving OPV efficiencies >10%.
9:00 PM - E8.31
Self-assembly of Porphyrin Nanofibers and Single-walled Carbon Nanotubes into Donor-acceptor Nanohybrids.
Albert Wan 1 , Chi-Yuan Chang 1 , Zach Levin 2 , Qingsheng Liu 1 , Zhiping Luo 3 , Jamie Wheeler 1 , Jamie Grunland 2 , James Batteas 1
1 Chemistry, Texas A&M University, College Station, Texas, United States, 2 Mechanical Engineering, Texas A&M University, College Station, Texas, United States, 3 Microscopy and Imaging Center , Texas A&M University, College Station, Texas, United States
Show AbstractNovel nanohybrids based on noncovalently attached porphyrin nanofibers with single-walled carbon nanotubes (SWNTs) have been prepared. Porphyrin nanofibers (NFs) with high aspect ratio up to 400:1 were fabricated by acidifying tetra(p-carboxyphenyl)porphyrin (TCPP) using hydrochloric acid in aqueous solution. SWNTs functionalized with 1-pyrenesulfonic acids were mixed with porphyrin NFs to form donor-acceptor nanohybrids. The energy/charge transfer between porphyrin nanofibers and SWNTs were studied using UV-vis spectroscopy, steady state fluorometry, and time-resolved fluorometry.
9:00 PM - E8.33
Dye-sensitized Solar Cells Based on a Natural Low Cost Halochromic Sensitizer.
Michael Ibrahim 1 2 , Maria Bassil 1 , Umit Demirci 2 , Georges El Haj Moussa 1 , Mario El Tahchi 1 , Philippe Miele 2
1 LPA-GBMI, Department of Physics, Lebanese University - Faculty of Sciences II, Jdeidet Lebanon, 2 LMI, University of Claude Bernard Lyon 1, Lyon France
Show AbstractSince their re-introduction as new generation solar cells in 1991, dye-sensitized solar cells (DSSCs) have been studied extensively to improve their efficiency and their stability. Several types of dyes have been used in the fabrication of DSSCs such as polypyridyl complexes of ruthenium known as N3 dye, black dye, Rose Bengal [1] and cyanine dyes [2]. The highest efficiency reported for a DSSC is about 11% [3]. Low cost dyes such as anthocyanin from black rice have been tested [4] and an efficiency of 3.2 % was recorded. Anthocyanin is present in a multitude of colors ranging from red to yellow to violet according to the pH [5]. Red cabbage is of much interest as a source of anthocyanin being easy to grow, and have few culinary and industrial uses. With these properties, anthocyanin from red cabbage is a good candidate as a sensitizer for DSSC.In this study, two industrial types of TiO2 powders, the Degussa P 25 and the Crystal 128 with different particle sizes 21 and 200 nm respectively, were used to prepare DSSCs. Titania is slot died on a glass substrate with an indium tin oxide (ITO) layer. For this purpose, the titania solution is thickened using either polyvinyl alcohol or sodium silicate in order to select the fittest based on mechanical and optical properties. Afterwards, the as-formed TiO2 layer is annealed at different temperatures ranging from 300 to 500 oC and for different periods. After being annealed, the TiO2 film is immersed for 24 h in an anthocyanin solution with variable color range obtained by changing the solution pH. Finally a redox electrolyte solution containing I-/I3- ions is injected between ITO glass substrates and the device is sealed. The photovoltaic properties of all the samples are tested in order to investigate the variation of the DSSCs efficiencies with the change of dye color and thus the pH. The effect of pH on the light absorption by the anthocyanin dye in the visible range and thus on the efficiency of the DSSC is studied using UV-Vis spectroscopy.[1] M. Roy, P. Balraju, M. Kumar and G. Sharma, Solar Energy Materials & Solar Cells 92, 909 (2008).[2] X. Ma, J. Hua, W. Wu, Y. Jin, F. Meng, W. Zhan and H. Tian, Tetrahedron 64, 345 (2008).[3] B. O’Regan and M. Grätzel, Nature 353, 737 (1991).[4] S. Hao, J. Wu, Y. Huang and J. Lin, Solar Energy 80, 209 (2006).[5] M. Bassil, J. Davenas and M. El Tahchi, Sensors and Actuators B: Chemical 134, 496 (2008).
9:00 PM - E8.34
Determination of Energy Level Alignment at Interfaces of Hybrid and Organic Solar Cells under Ambient Environment.
Robert Davis 1 , Matthew Lloyd 2 , Summer Ferreira 1 , Matthew Bruzek 3 , Scott Watkins 4 , Mats Fahlman 5 , John Anthony 3 , Julia Hsu 1
1 Surface and Interface Sciences, Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States, 3 , University of Kentucky, Lexington, Kentucky, United States, 4 , CSIRO Molecular and Health Technologies, Clayton South, Victoria, Australia, 5 , Linkoping University, Linkoping Sweden
Show AbstractThe field of organic electronics has generated great interests due to the potential of new applications and low-cost fabrication. Device function in organic electronics is critically governed by the transport of charge across interfaces of dissimilar materials. In organic photovoltaics (OPV) and hybrid photovoltaics, charge generation is fundamentally dictated by the energy level alignment at the donor/acceptor interface, while efficient charge extraction depends on proper energy level alignment at the organic active layer/inorganic electrode interfaces. Energy level alignment at metal/organic interfaces also influences the efficiency of charge injection in organic light emitting diodes (OLED). Accurate measurements of energy level positions in organic electronic devices are therefore necessary for assessing the viability of new materials and optimizing device performance. Here we combine Kelvin probe measurements performed in ambient environments to obtain work function values with photoelectron spectroscopy in air to obtain ionization potential, so that a complete energy level diagram for organic semiconductors can be determined. We apply the new approach to study commonly used electron donor and acceptor materials in organic photovoltaics (OPV), including poly(3-hexylthiophene) (P3HT), [6,6]-phenyl C61 butyric acid methyl ester (PCBM), and ZnO, as well as examine new materials. Band alignments across the entire OPV devices are constructed and compared with actual device performance. The ability to determine interfacial electronic properties in the devices enable us to answer the outstanding question why previous attempts to make OPV devices using 6,13-bis triisopropylsilylethynyl) (TIPS)-pentacene as the electron donor was not successful.Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000.
9:00 PM - E8.36
Quality and Stability of Fast Elaborated CdS Quantum Dots.
Mathieu Fregnaux 1 2 , Stephane Dalmasso 1 , Jean-Jacques Gaumet 2 , Jean-Pierre Laurenti 1
1 LPMD, Université Paul Verlaine Metz, Metz France, 2 LSMCL, Université Paul Verlaine Metz, Metz France
Show AbstractRecent advancements in the chemical elaboration of II-VI semiconducting nanoparticles have enabled the huge development of new devices. Due to their size-dependent properties at the nanoscale level, these materials are being used for several applications including biomarkers, solar cells and blue-UV lasers. These emerging technologies require high quality and stable in-time nanocrystals (NCs).In this presentation, we report a systematic study on CdS quantum dots (QDs) generated following two procedures: a single source precursor thermal growth methodology and a microwave synthetic route. Using both protocols, high quality small NCs (some nanometers in size) were produced. Both techniques offer the advantage of being fast and cheap: 2 hours and less than 25 minutes in duration, respectively, using growth temperatures which do not exceed 280°C.For both synthesis procedures, the coupling between physical and chemical characterisation methods provide consistent data on quality and stability of these small size QDs. Powder X ray diffraction analyses are performed to determine their crystalline structure. Soft ionization mass spectrometry methods lead to sizes and size distribution of the NCs. Transmitted electron microscopy confirms these size estimations and provides basic information on the NC shape. Optical spectrometry reveals a good optical quality of QDs in the near UV and blue part of the visible spectral ranges, their spectral response is correlated with their size via quantum confinement effects.
9:00 PM - E8.37
Guided Formation and Interconnection of C60 Crystals on Aligned Carbon Nanotube Scaffolds.
Eric Meshot 1 , Sameh Tawfick 1 , Mostafa Bedewy 1 , Keval Patel 1 , Anne Juggernauth 1 2 , A. John Hart 1
1 Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Macromolecular Science and Engineering Research Center, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractOrganic electronic materials offer certain advantages compared to inorganic systems, including the ability to engineer electronic properties by molecular design, and compatibility with low-cost and low-temperature manufacturing methods such as printing and solution deposition. Specifically, C60 exhibits useful photoelectric and charge transfer properties, can be diversely functionalized, and due to its monodispersity serves as a model building block for study of nanocrystal packing and assembly. However, while C60 can be self-assembled into micro and nano-crystals (e.g., rods, discs) it is challenging to organize and address C60 crystals for use in electronics and energy harvesting. We demonstrate that vertically and laterally oriented carbon nanotubes (CNTs) facilitate oriented in situ formation of C60 crystals into aligned and multi-directional patterns, creating hybrid C60/CNT films and microstructures, wherein CNTs are woven through individual C60 crystals. The hybrids are fabricated by simple drop-casting of C60 dispersed in organic solvents (without surfactants); the solution infiltrates the CNT scaffolds by capillary action, and subsequent evaporation of the solvent induces formation and alignment of C60 crystals along the axis of the CNTs. Cross-sectioning using a focused ion beam reveals that the CNTs penetrate through the C60 crystals. Real-time optical imaging of the evaporation process demonstrates that the C60 crystals nucleate and grow at the liquid interface, at impressive rates of microns per second. Importantly, while the structures crystallize without the presence of the CNT scaffold (i.e., on Si), their resultant orientations are uncorrelated, and C60 rods that are separately synthesized in solution before drop-casting on CNTs do not align nor integrate into the scaffold. Therefore, we suggest that the crystal orientation is influenced by π-π interactions between C60 and CNTs, along with self-guided movement of the solvent meniscus along the CNT direction. Finally, we investigate the anisotropic electrical transport and photoconductivity of hybrid C60/CNT thin films. This elegant method of orienting and interconnecting molecular crystals in situ during self-assembly may enable structural engineering of many other hybrid systems.
9:00 PM - E8.38
Exciton Migration in Electrospun MEH-PPV Nanofibers.
Shufeng Wang 1
1 Physics, Peking University, Beijing China
Show AbstractPoly(phenylene vinylene) (PPV) and its derivatives attract considerable interest for their potential applications. MEH-PPV is one of the most important member of PPV derivatives for their feasibility in wet processing. The photophysical properties of MEH-PPV in bulk films are sensitive to its morphology. Electrospining is one of the important treatments for controlling its morphology. In such MEH-PPV composite, the photoluminescence (PL) spectra are found blue-shifted compared with films due to the weakened interchain aggregation. For the first time, it is reported here that in such electrospun(ES) fibers, the exciton migration in femtosecond time scale is enhance. The polymer composite in this research mixed with fullerene derivative (PCBM) at low concentration, which is used as a ruler for detection of excitation migration.The ultrafast PL dynamics are performed with 400nm femtosecond excitation and recorded by a streak camera. The data can be well de-convoluted into a sum of two exponential decays. For both the ES and SC films, initial fast decay demonstrates a monotonic decrease with increasing PCBM content. At same PCBM concentration, this fluorescence decays of SC sample is slower compare to ES samples. However, the second component which is the excited state lifetime is shorter in SC films. It is noticed that the exciton migration is faster and in larger amplitude in ES samples compare to the SC films. It is first time and interesting to see that the reduced aggregation in this ES fibers is even better for the energy transfer. One of the possible explanation suggest here is the formation of long-range ordering for exciton migration. Under the electric field, the random coil of polymer chains become partially aligned.1 As a result, the orientation for aggregations are partially aligned and therefore form long-range order. Exciton migration is usually modeled with Förster energy transfer through nonradiative dipole-dipole coupling, which depends on the transition dipole orientations.2 Comparing to film sample, ordered orientation in ES nanofibers should benefit the exciton migration between local aggregations due to enhanced dipole moment coupling. High level orientation of the polymer backbones along the fibers axis was demonstrated by Kakade et al.3 Therefore, the exciton transfer depends on the orientation of local orders. Long range order is then expected to enhance the dipole resonance between local regions and help to increase exciton migration. References:1X. T. Hao, N. Y. Chan, D. E. Dunstan and T. A. Smith, J. Phys. Chem. C 113, 11657 (2009).2L. M. Hardison, X. Y. Zhao, H. Jiang, K. S. Schanze and V. D. Kleiman, J. Phys. Chem. C 112, 16140 (2008).3M. V. Kakade, S. Givens, K. Gardner, K. H. Lee, D. B. Chase and J. F. Rabolt, J. Am. Chem. Soc. 129, 2777 (2007).
9:00 PM - E8.39
Surface Potential Measurement of CuPc/C60 Thin Film Fabricated on ITO Electrode by Using FM-KFM Technique.
Shigetaka Katori 1 , Nobuo Satoh 1 , Kei Kobayashi 1 , Shizuo Fujita 1 , Kazumi Matsushige 1 , Hirofumi Yamada 1
1 , Kyoto University, Kyoto Japan
Show AbstractUnderstanding of the carrier dynamics of organic semiconductive devices, such as OLED, solar cell and so on, is one of the most important issues to achieve enhanced performance. The characteristics of the devices greatly depend on the transport properties of the carriers at the heterojunction interface. The interface between an organic layer and a metal electrode and a combination of heterogeneouse materials play an essential role. In this report, we discuss the surface potential of organic semiconductor thin films fabricated on electrode which measured by frequency modulation (FM) Kelvin probe force microscope (KFM).To clarify the interfacial properties of organic solar cell, copper phthalocyanine (CuPc) and fullerene (C60) were deposited on a glass/ITO substrate by vacuum evaporation technique using crossed shadow masks. The film thickness and the deposition rate were 20 nm and 0.1nm/sec, respectively. This deposition technique enables us to make the four different areas in the same substrate. Since the Q-factor and its resultant amplitude response time of the AFM cantilever are drastically increased in vacuum, amplitude modulation (AM) dynamic force microscopy (DFM) is not practically used for vacuum measurements. In this experiment surface potentials of the cross point area of the deposited thin films were investigated by the FM-KFM technique, which is immune from the increase of the cantilever Q-factor.The surface potential of CuPc and C60 against the ITO electrode were +22mV and +107mV, respectively. This result suggests the existence of the hall charges in both films induced by the interfaces between the films and the ITO electrode. However, the surface potential of C60 area which formed on the CuPc area showed negative value (-230mV) in contrast to the other three areas. This result indicates that the carriers generated at the interface between the CuPc and C60 films move into the n-type film (C60). In addition, we found that the surface potential of the C60 film was increased with the increase in the film thickness until the thickness reached about 20 nm. FM-KFM was proven to be a powerful method for studying the carrier behavior at the heterojunction interfaces.The results obtained in this study will greatly contribute to the clarification of carrier dynamics of organic semiconductor devices.
9:00 PM - E8.4
High Performance Polymer Solar Cells Using Solution-processed ZnO Nanoparticles/Polymer Buffer Layer as a Modified Cathode.
Sae-byeok Jo 1 , Ji Hwang Lee 2 , Min Kim 1 , Myungsun Sim 1 , Jong soo Kim 2 , Jong Hwan Park 1 , Kilwon Cho 1 2
1 Chemical Engineering, POSTECH, Pohang Korea (the Republic of), 2 School of Environmental Science and Engineering, POSTECH, Pohang Korea (the Republic of)
Show AbstractThe metal/organic interface plays a critical role in the device performance of organic photovoltaics cells (OPVs) based on p/n-type blend thin-films. So far, the interface has often been modified by the insertion of a functional interfacial layer such as PEDOT:PSS and TiO2 for a better charge collection, light absorption and carrier selection for each electrode. Here, we report on the enhanced device performances of OPVs by incorporating a solution-processed poly(ethylene glycol) (PEG) layer hybridized with ZnO nanoparticles as a cathode interfacial layer. OPVs based on the poly(3-hexylthiophene) (P3HT) : [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) blends with the ZnO-PEG composite interlayer exhibited excellent performance with a power conversion efficiency (PCE) up to 4.4% compared to that of conventional device, 3.23% at the optimum condition. This improved device performance is attributed to an improved polymer/metal contact resulting from the blockage of interfacial dipoles by the insulating PEG, leading to an enhanced carrier injection. In addition, efficient electron extraction and better hole blocking properties also provide lower series resistance across the devices. More importantly, ZnO-PEG composite layer serves as an optical spacer, which enhances light absorption and thereby increases the photocurrent significantly. The simplicity in the deposition of solution-processed ZnO-PEG composite layer along with its excellent characterstics suggests the possibility of practical application to printed electronics for high performance OPVs.
9:00 PM - E8.40
Singlet Exciton Fission in Organic Solar Cells.
Priya Jadhav 1 , Aseema Mohanty 1 , Jason Sussman 2 , Marc Baldo 1
1 Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractThe efficiency of solar cells is controlled by the energy gap of the constituent materials. The “single junction limit” or the “Shockley–Queisser limit”, is imposed because photons with energy higher than the gap lose energy in thermalization and those lower than the gap cannot be absorbed. Tandem cells work around this limit by having more than one photovoltaic structure in parallel absorbing different parts of the solar spectrum. Tandem cells, however, are constrained by the necessity of current matching both cells. Singlet exciton fission, in which a high energy singlet exciton spontaneously decomposes into two low energy triplets, can also potentially surpass the single-junction efficiency limit.Some organic materials like the acenes (anthracene, tetracene and pentacene) exhibit singlet fission. We use tetracene as a donor in the structure: tetracene- copper phthalocyanine -C60. Tetracene absorbs photons in the λ = 450-550-nm region, generating high energy singlets which then split into two lower energy triplets, potentially doubling the photocurrent in this part of the spectrum. C60 is the acceptor, absorbing wavelengths below 500nm and CuPC (copper phthalocyanine) is an additional donor, used to extend the region of absorption to wavelengths higher than 550nm.We find that the addition of the CuPC layer does not affect the flow of excitons from tetracene to the CuPC – C60 junction significantly because of the similarity in the triplet energy levels of tetracene and CuPC. This is proven by the very small difference in external quantum efficiency (EQE) measurements done with and without the CuPC layer. The presence of singlet fission in tetracene is confirmed by the magnetic field dependence of the photocurrent. The application of a .5T magnetic field shows ~-1.5% change in photocurrent, as expected since the rate of singlet fission decreases on application of a magnetic field. Furthermore, this effect is evinced only at excitation wavelengths in the tetracene absorption region and there is no effect on the photocurrent in the red wavelengths where CuPC absorbs.We have demonstrated a novel device structure that overcomes the single junction limit by exploiting singlet fission and show EQEs of ~40% from 375-550nm and ~20% from 550-750nm.
9:00 PM - E8.42
Exciton Antennae and Concentrators from Core-shell and Corrugated Carbon Nanotube Filaments of Homogeneous Composition.
Paulus Geraldine 1 , Jae-Hee Han 1 2 , Ryuichiro Maruyama 3 , Daniel Heller 1 , Woo-Jae Kim 4 , Paul Barone 1 , Chang-Young Lee 1 , Jong Hyun Choi 5 , Moon-Ho Ham 1 , Changsik Song 1 , Cristiano Fantini 6 , Michael Strano 1
1 Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Solid State Ionics Lab, Advanced Material Laboratories, Sony Corporation, Kanagawa Japan, 4 Department of Chemical and Bioengineering, Kyungwon University, Seongnam Korea (the Republic of), 5 School of Mechanical Engineering, Purdue University, West-Lafayette, Indiana, United States, 6 Departemento de Física, Universidade Federal de Minas Gerais, Belo Horizonte Brazil
Show AbstractThere has been renewed interest in solar concentrators and optical antennae for improvements in photovoltaic energy harvesting and new opto-electronic devices. In this work, we dielectrophoretically assemble single-walled carbon nanotubes (SWNTs) of homogeneous composition into aligned filaments that can exchange excitation energy, concentrating and funneling it to the center of core-shell structures with radial gradients in the optical bandgap. We find in such structures an unusually sharp, reversible decay in photoemission that occurs as such filaments are cycled from ambient to only 353 K, attributed to a more strongly activated (37.4 kcal/mol), Auger-like component of a dual, non-radiative pathway in the filament. We show that such filaments tend to form density oscillations of 1 to 3 um spacing along their length that create spatial periodicity in the emission wavelength. Core-shell structures consisting of annular shells of (6,5) (Eg = 1.20 eV ) and (7,6) (Eg = 1.09 eV) and cores with bandgaps smaller than those of the shell (Eg = 1.02 eV (11,3) to 0.98 eV (8,7)) demonstrate the concentration concept: broad band absorption in the ultraviolet (UV)-near-infrared (nIR) wavelength regime with singular photoemission at the (8,7) SWNT (Eg = 0.98 eV). This approach demonstrates the potential of specifically designed collections of nanotubes to manipulate and concentrate excitons in unique ways.
9:00 PM - E8.43
Polarization-resolved Spectroscopy Imaging of Electronic States in Crystalline Organic Thin Films.
Zhenwen Pan 1 , Ishviene Cour 1 , Cody Lamarche 1 , Margaret Sutton 1 , Randall Headrick 1 , Madalina Furis 1
1 Physics and Material Science, University of Vermont, Burlington, Vermont, United States
Show AbstractWe report on results of unprecedented linear dichroism (LD) imaging and time-resolved photoluminescence (PL) microscopy of electronic states in polycrystalline orthorhombic phthalocyanine (H2Pc) films deposited from solution using a novel hollow pen-writing technique [1]. In order to investigate the link between molecular ordering and electronic states in individual grains, we combined photoluminescence and polarization-resolved scanning microscopy techniques originally designed to study spin drift and diffusion in inorganic semiconductors.[2] We were thus able to simultaneously probe the symmetry of optically-allowed electronic states and the excitonic nature of radiative recombination with a spatial resolution of approx. 5µm. Individual grains exhibit large linear dichroism at the HOMO-LUMO gap, that results from “edge-on” stacking of molecules in columns oriented parallel to the substrate. PL microscopy spectra exhibit a large degree of linear polarization that correlates with the observed LD, confirming the excitonic nature of the recombination. Same studies reveal the presence of a highly localized, monomer-like state exclusively associated with 90 degrees grain boundaries. The temperature evolution of the bandgap luminescence and radiative recombination lifetimes indicates the ground state is “dark” (optically forbidden) at 10K. As the temperature increases this state mixes with the lowest “bright” (optically –allowed) state such that at 300K the PL spectrum is resonant to the HOMO-LUMO (Q-band) absorption edge. The measured dark-bright splitting (approx. 70 meV) is significantly smaller than the typical singlet-triplet splitting of individual molecules. These experiments demonstrate that the local structure of the dramatically affects the optical and electronic properties in organic semiconductor films.[1] R.L. Headrick, S. Wo, F. Sansoz, and J.E. Anthony, Appl. Phys. Lett. 92, 063302 (2008).[2] M. Furis, D.L. Smith, S. Kos, E.S. Garlid, K.S.M. Reddy, C.J. Palmstrøm, P.A. Crowell, and S.A. Crooker, New J. Phys. 9, 347 (2007).
9:00 PM - E8.44
Optimization of Synthetic Methodologies in Giant Nanocrystal Quantum Dots Leading to Enhancement of Photophysical Properties and Design of Novel Light-emitting Devices.
Yagnaseni Ghosh 1 , Young-Shin Park 1 , Benjamin Mangum 1 , Florencio Garcia-Santamaria 1 , Sergio Brovelli 1 , Bhola Pal 1 , Joanna Casson 1 , Han Htoon 1 , Victor Klimov 1 , Jennifer Hollingsworth 1
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractNanocrystal quantum dots (NQDs) are increasingly being considered as nearly ideal candidates for light-emission applications due to high quantum efficiencies and narrow-band and particle-size-tunable photoluminescence. However, they suffer from certain disadvantages, including chemical-environment-dependent photo-instability at the ensemble level and intermittency in fluorescence intensity, or “blinking”, at the single NQD level. Prior work in our research team showed for the first time that the growth of ultra-thick shells (number of shell monolayers, n, = 10-20) of the higher bandgap material, CdS, over CdSe NQD cores leads to remarkable photostability and significant suppression of blinking behavior,1,2 which is likely related to the concurrently observed suppression of nonradiative Auger recombination.3,4 The new photophysical behavior afforded by this structurally new class of NQD, the so-called “giant” NQD (g-NQD), promises significant advantages for their application in novel light emitting devices (LEDs). Here, we report our efforts to advance the synthesis of this new NQD structural motif and to demonstrate its utility in light-emission applications. First, to further expand this new approach to other NQD systems and to fully standardize and optimize the CdSe/CdS g-NQD system, we have investigated in detail the underpinning structure-property relationships and the synthetic parameters influencing them. Specifically, we correlate g-NQD structure parameters (e.g., shell thickness, shell uniformity, particle shape, defect structure, crystalline phase) with the photophysical properties of ensemble-level quantum yield, single-NQD non-blinking behavior, and Auger recombination rates. We optimize growth parameters (solvent, ligand, concentration, and growth times) to eliminate non-uniformity in and to optimize structural properties, observing a direct correlation with further enhanced optical properties. In addition to understanding and optimizing physical-structure parameters, we investigate for the first time accurate assessments of electronic structures using cyclic voltammetry and differential pulsed voltammetry on solution-dispersed g-NQDs, and we explore charge-injection efficiencies in g-NQD thin films. Together, this important understanding of conduction and valence-band energy-level alignments allows us to design g-NQD based devices. To this end, we have fabricated preliminary g-NQD-based light-emitting devices as proof-of-principle structures (e.g., ITO/g-NQDs/Al, ITO/NiO/g-NQDs/TiO2/Al). These un-optimized g-NQD LEDs have already shown rectifying electric response and stable, large-area electroluminescence under direct bias, with remarkably low turn-on voltages (~3.5 V) and appreciable quantum efficiencies (0.001 %).1.Chen, Y. et.al., J. Am. Chem. Soc., 2008, 130, 5026.2.Vela, J. et.al., J. Biophotonics, 2010, accepted.3.García-Santamaría, F. et.al., Nano. Lett. , 2009, 9, 3482.4.Htoon, H. et.al., Nano. Lett. , 2010, ASAP
9:00 PM - E8.45
Analytic Framework for Transient Photo-current Experiment – Charge Carrier Extraction by Linearly Increasing Voltage.
Jens Lorrmann 1 , Bekele Homa Badada 2 , Vladimir Dyakonov 1 3 , Olle Inganas 2 , Carsten Deibel 1
1 Experimental physics VI, Julius-Maximilians-University of Würzburg, Würzburg, Bavaria, Germany, 2 Biomolecular and Organic Electronics, IFM, Center of Organic Electronics, Linköping Sweden, 3 , Bavarian Center for Applied Energy Research e.V. (ZAE Bayern), Würzburg Germany
Show AbstractPolymer:fullerene solar cells have reached a considerable performance already, but a detailed analysis of the loss mechanisms is necessary for further guided optimizations. In this context, charge extraction by linearly increasing voltage (CELIV) has attracted much interest for investigating the charge transport in these systems, due to its ability to simultaneously measure the charge carrier mobility and density. Up to now the basic theoretical description of CELIV is solved for a low conductivity approximation only. In this contribution, we present the full analytical solution, thus generalizing the theoretical framework for this method. The comparison of the general solution, numerical simulations and the approximated theory showed that especially for typical organic solar cell materials the latter approach has a limited validity and results in deviations. Therefore, we present parametric equations for the mobility and for the first time for the charge carrier density, which we derived from simulated photo-current transients within the general CELIV framework. These equations can be applied over the entire experimental range.Furthermore, Photo-CELIV measurements on poly(3-hexyl thiophene-2,5-diyl):[6,6]-phenyl-C61 butyric acid methyl ester based solar cells were evaluated by fitting the current transients to the analytical solution. We found that the fit results are in a very good agreement with the experimental observations, if ambipolar transport is taken into account.
9:00 PM - E8.46
Transport Properties and Photovoltaic Efficiency of Colloidal Nanocrystals in Inorganic and Organic Matrices.
Katherine Rice 1 , Aaron Saunders 1
1 Chemical and Biological Engineering, University of Colorado, Boulder, Colorado, United States
Show AbstractWhile the size- and shape-tunable properties of colloidal nanocrystals have made them attractive platforms for sensing, photovoltaic, and electronic devices, such applications rely on the efficient transport of charge carriers through a nanocrystal film. The large surface area of the nanocrystals makes the transport behavior extremely sensitive to the surrounding matrix, whether molecular surfactants, polymeric species, or inorganic materials. Hence, the ability to control the interface between the matrix and nanocrystals, and the electronic properties of the matrix, is essential.In this talk, we discuss the electrical and optoelectrical response of films of colloidal nanoparticles based on surface treatment and the properties of the matrix surrounding the nanocrystals. For nanocrystals in an organic ligand matrix, transport depends heavily on the interparticle spacing, removal or replacement of the capping ligands with more conductive molecules, and the treatment methods. For inorganic matrices, such as those deposited using sputtering techniques or atomic layer deposition, the interface between the nanocrystal and matrix is important, as is the electronic properties of the surrounding metal oxide or semiconductor. Tuning the band offset between the surrounding matrix and the nanocrystals provides a useful method to induce charge separation at the nanocrystal/matrix interface.
9:00 PM - E8.47
Two-photon-induced Fluorescence Study from Soluble Polythiophenes.
Soumitra Satapathi 1 2 , Lian Li 3 , Abhishek Kumar 1 2 , Robinson Anandakathir 2 , Lynne Samuelson 3 , Jayant Kumar 1 2
1 Physics and Applied Physics, University of Massachusetts Lowell, Lowell, Massachusetts, United States, 2 Center for Advanced Materials, University of Massachusetts Lowell, Lowell, Massachusetts, United States, 3 , US Army Natick Soldier Research,Development and Enginnering Center, Natick, Massachusetts, United States
Show AbstractTwo-photon-induced fluorescence from a series of soluble polythiophenes has been investigated using femto-second laser pulses at 800 nm. Strong two-photon fluorescence was observed. The quadratic dependence of the fluorescence on the excitation laser intensity confirmed the two-photon process. The measured two-photon absorption cross-sections are larger as compared to those of other reported polythiophenes. One of these polymers having urethane side chain can be readily hydrolyzed to yield a water soluble polythiophene which could be useful in biological imaging.
9:00 PM - E8.48
P3HT-Silicon Organic-silicon Heterojunction for Photovoltaic Applications.
Sushobhan Avasthi 1 3 , Yabing Qi 1 3 , Grigory Vertelov 2 3 , Jeffrey Schwartz 2 3 , Antoine Kahn 1 3 , James Sturm 1 3
1 Dept of Electrical Engg, Princeton University, Princeton, New Jersey, United States, 3 Princeton Institute for the Science and Technology of Materials (PRISM), Princeton University, Princeton, New Jersey, United States, 2 Department of Chemistry, Princeton University, Princeton, New Jersey, United States
Show AbstractOne approach to low-cost photovoltaics is though a merged organic/crystalline-silicon solar cell in which the p-n junction is replaced by the organic/silicon heterojunction. The devices have a metal/organic/silicon structure which requires simpler processing techniques like spin coating instead of high-temperature diffusion. Furthermore, due to the large offsets (>1eV) possible at organic-silicon interfaces, very low diode J0, and hence high open-circuit voltages (VOC), can be achieved in merged diodes. Here we report such a heterojunction device, using poly(3-hexylthiophene) (P3HT) as the organic semiconductor, with VOC of 0.43V at ~1 sun illumination on a n-type CZ silicon wafer. First we show that the P3HT/silicon interface satisfies the two key band alignment criteria for efficient photovoltaic operation; a) large conduction-band (CB) barrier to block the photo-generated electrons in silicon from recombining at the metal, and b) small valence-band (VB) barrier so that, unlike electrons, the photo-generated holes easily flow across the interface to be collected at the metal. The CB barriers were extracted from the forward-bias dark-current characteristics of the diodes. Diodes made without P3HT, had high J0 of 10-6 A/cm2, which reduced to 10-8 A/cm2 with application of P3HT. Since the currents in n-type silicon are dominated by the majority carriers (electrons), the reduction in J0 indicates a larger barrier for electrons at the P3HT/silicon interface. To measure the barrier at VB, the current was measured under illumination and zero bias (short circuit current (ISC)). The photo-generated holes have to cross the P3HT/silicon interface before being collected at the metal, and any barrier at VB would impede this flow, translating into a reduced ISC. For both with and without P3HT, ISC remained the same (20 mA/cm2), demonstrating absence of a barrier at the VB. Due to reduced J0, P3HT/silicon heterojunction also improved the photovoltaic performance, as the VOC increased from 0.20 V to 0.43 V.Finally, we examine what limits further reduction in J0 in P3HT/silicon devices. In a metal/silicon device, the majority carrier current is the dominant part of J0. However, P3HT blocks the majority carrier (electrons) without affecting the minority carriers, making hole current the dominant component. The high level of hole injection was confirmed by measuring the minority carrier storage times. When applied bias is switched from positive to negative in a diode, there is a time delay before the current settles at the equilibrium value. This delay (storage time) is because of the stored minority carriers. Without P3HT, the storage time was negligible (<50ns), but with P3HT, it increased to 5μs. Clearly, the ‘P3HT’ device has more stored holes than the one ‘without P3HT’. Further reduction in J0 is then realized by reducing the hole injection into silicon, using one of two ways; high-lifetime FZ silicon, and lower work function metals.
9:00 PM - E8.49
Room Temperature Synthesis of Nanoporous Organically Modified Silica Thin Films on Flexible Substrates for Low Cost Devices.
Adem Yildirim 1 , Hulya Budunoglu 1 , Mehmet Bayindir 1 2
1 Material Science and Nanotechnology Institute, Bilkent University, Ankara Turkey, 2 Department of Physics, Bilkent University, Ankara Turkey
Show Abstract Fabrication of electronic and photovoltaic devices on flexible substrates like plastics and paper has attracted considerable interest to achieve flexible and low-cost devices. Flexible nanoporous thin films are desired for such devices as antireflective coatings, dielectric films and supports for catalysts and sensors. Flexible films must deposit in mild conditions (i.e. low temperatures, pH neutral solutions) to these sensitive substrates and films must operate successfully during several deformation cycles. Nanoporous polymer films have been prepared on flexible materials. However, polymers have poordurability in atmospheric conditions and they can easily deform. Organically modified silicas (ORMOSIL) can deliver specific requirements not afforded by organic polymers and inorganics alone like flexibility together with good durability. Thus, ORMOSIL thin films are very promising candidates as durable nanoporous coatings for flexible substrates. However, nanoporous ORMOSIL coatings are prepared by post calcination at high temperatures which makes it impossible to use with these heat sensitive flexible substrates. Here we report a room temperature and template-free sol gel method for preparation of stable ORMOSIL colloidal suspensions to achieve durable nanoporous thin films on flexible substrates. The resulting suspensions are stable for several months and can be deposited on almost every substrate including plastics, paper, glass and silicon by common thin film deposition methods such as; spin, dip and spray coatings. These stable ORMOSIL suspensions offer easy handling and good reproducibility for producing homogeneous porous thin films. Porosity (30 to 70%) and thickness (80 nm to 1 μm) of the films can be controlled by simply adjusting the monomer concentration in starting solution and the ORMOSIL concentration in the sol. Moreover we characterized the anti-reflection properties of the films and we demonstrated stable anti-reflective performance of the films which are on plastic substrates after several cycles of bending.
9:00 PM - E8.5
Organic Photovoltaics Using Mg Doped Fullerene Film as Buffer Layer.
Noboru Ohashi 1 , Tetsuhiko Miyadera 1 , Tetsuya Taima 1 , Yuuji Yoshida 1
1 The Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, Japan
Show Abstract Contact resistance within electrode/semiconductor interface is becoming a critical limiting factor in conventional organic photovoltaics, because series resistance degrades short-circuit current and fill factor. For that reason, the contact resistance must be decreased for further improvement of the efficiency. It is known that carrier doping in the vicinity of the interface decreases the contact resistance. Chikamatsu and co-workers reported that photovoltaics using top Mg:Ag electrode instead of Al electrode reduced the contact resistance.[1] It is indicated that Mg atoms diffused into fullerene layer, and in that case, Mg atoms acted as carrier dopant. In this study, we developed organic photovoltaics intentionally inserted Mg doped fullerene film as buffer layer. Experimental condition is as follows. Glass substrate with ITO electrode was treated by O2 plasma. All organic layers and top Al electrode were prepared by vacuum deposition. Mg doped fullerene layer was obtained by co-evaporation technique. Atomic concentration of the Mg doped fullerene film was examined by X-ray photoelectron spectroscopy, and the results indicated that Mg concentration in the film was controlled well. We note that diffusion of Mg atoms into the fullerene layer was not observed in this system. As a result of processes, Al/C60:Mg/C60/ZnPc/ITO/glass sandwich device was fabricated. We also fabricated the sandwich device without Mg doping for comparison. Current-voltage characteristics of the devices were measured in the dark and under AM1.5G 1Sun illumination. The performance of Mg doped device was much improved compared to non-doped device. Fill factor and short-circuit current in the Mg doped devices are 0.47 and 1.5 mA/cm2, respectively. Conversion efficiency of Mg doped device was 0.22 %, which is 30 times larger than non-doped device. These results clearly indicate that contact resistance within the Al/C60 interface was markedly decreased by the Mg doping. We concluded that Mg doping technique is effective for organic photovoltaics.[1] M. Chikamatsu, T. Taima, Y. Yoshida, K. Saito, and K. Yase, Applied Physics Letters 84, 127 (2004).
9:00 PM - E8.51
Light-harvesting Action Spectroscopy of Single CdSe/CdS Nanocrystals.
Nicholas Borys 1 , Manfred Walter 1 , Jing Huang 2 , Dmitri Talapin 2 , John Lupton 1
1 Physics and Astronomy, The University of Utah, Salt Lake City, Utah, United States, 2 Chemistry, The University of Chicago, Chicago, Illinois, United States
Show AbstractNanocrystal heterostructures offer unique systems to study the migration of excitation energy on nanometer length scales, while enabling accurate control over physical shape and thus electronic structure. Given a semiconductor nanostructure consisting of a wider gap (CdS) and a narrower gap (CdSe) material, we address the question of how relaxation occurs to the CdSe following excitation in CdS. We study three different heterostructure shapes: core-shell particles, nanorods, and tetrapods. These nanostructures are particularly interesting systems since a large amount of absorbing material (CdS) can be grown around a small emitting nucleus (CdSe). We vary the excitation energy and study the luminescence of single particles, at low temperatures. As it is predominantly the CdS which absorbs, these photoluminescence excitation (PLE) experiments illuminate details on light-harvesting action [1]. The PLE spectra reveal a step-like onset of the CdS absorption. In the tetrapod and nanorod structures, the shape of the PLE spectra remarkably fall into one of two classes while the core-shell particles all exhibit one universal functionality. We attribute the general shape of the PLE spectra to the physical shape and quantum confinement of the absorbing CdS in the structure, which we then confirm with correlated SEM and PLE measurements. In addition, spectrally resolving the luminescence as a function of excitation energy in the tetrapods reveals different emission energies of the core depending on whether the excitation occurs in the CdSe core or the CdS arms. The two different ground-state exciton energies for a single nanoparticle can be explained with excitation-dependent carrier localization caused by differences in the band-alignment of the conduction bands between the CdS and CdSe, indicating the presence of a barrier at the heterostructure interface. We show that the band alignment and concomitant carrier localization of these nanostructures can change from one particle to the next due to only slight differences in the shapes of the constituent structures. Excitation spectroscopy reveals information on the overall shape and energetic configuration of the nanoparticle which is not accessible in conventional luminescence experiments. Overcoming disorder in single particle excitation spectroscopy identifies the most efficient light harvesting structures with near perfect coupling. Ultimate control over macroscopic device performance requires such classification of each individual particle in the ensemble, and will necessitate a maximization of the fraction of particles with the most efficient characteristics. [1] Walter et al., Phys. Rev. Lett. 103, 167401 (2009).
9:00 PM - E8.53
Tipping Semicondutor (CdSe, CdS) Nanorods with Metals: Developing A Mechanistic Understanding and a Generalized Synthetic Protocol.
Debraj Ghosh 1 , Angela Lau 1 , A. Paul Alivisatos 1
1 Department of Chemistry & Biochemistry, University of California, Berkeley, Berkeley, California, United States
Show AbstractTo date, a wide variety of metal-semiconductor composite nanostructures have been designed using a diverse set of synthetic protocols. Recently, advances in synthesis have led to the ability for selective placement of gold onto the tips of Cd-chalcogenide nanorods. However, the ability to place other metals onto the tips of such rods has remained a difficult and time consuming task. In fact, only gold, platinum, and cobalt tipped rods have thus far been synthesized. These heterostructures are prime candidates as functional materials for both catalysis and electronic transport. To achieve robust functionality in these applications, a wider range of these heterostructures must be accessible. Herein, we report a simple method by which many transition metals can be selectively placed onto the tips of these nanorod structures. We first demonstrate that the semiconductor rods themselves force the reduction of metal salts onto defect sites all along the body of the rods as well as the tips. This process is a simple redox reaction between the rods and the metallic precursor. The reaction kinetics are governed by the redox potential difference between the given metal and the semiconductor anion. We also demonstrate that a variety of amines and inorganic salts serve as surfactants which control metal nucleation on the tips. Metals with very positive redox potentials are able to reduce onto the tips of the semiconductor rods while reduction is inhibited for metals having more negative redox potential. We establish that the surfactants provide an additional kinetic barrier to metal reduction, yet they are necessary to control nucleation only onto the tips. Our improved mechanistic understanding of the tipping chemistry allows us to controllably provide the necessary driving force by the use of a reducing agent to nucleate a wide array of metals onto the tips of these rods.
9:00 PM - E8.54
Strong Interface P-doping and Band Bending in C60 on MoOx.
Irfan Irfan 1 , Huanjun Ding 1 , Yongli Gao 1 , Minlu Zhang 2 , Ching Tang 2
1 Physics and Astronomy, University of Rochester, Rochester, New York, United States, 2 Chemical Engineering, University of Rochester, Rochester, New York, United States
Show AbstractThe electronic energy level evolution of fullerene (C60) on molybdenum oxide (MoOx)/ conducting indium tin oxide (ITO) interfaces has been investigated with ultra-violet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPES) and atomic force microscopy (AFM). It was found that the thermally evaporated MoOx inter-layer substantially increased the surface workfunction. This increased surface workfunction strongly repel electrons in C60 at the C60/MoOx interface, resulting in inversion. Energy levels of C60 relax gradually as the thickness of C60 increases. An exceptionally long (400 Å) band bending is observed during this relaxation in C60. Such long band bending has not been reported so far, for the organic/ insulator(MoOx) interface. The effect of air exposed MoOx inter-layer between ITO and C60 has also been investigated. After air exposure of MoOx almost no band bending was observed and the electronic energy levels of C60 remained more or less flat.
9:00 PM - E8.55
Conductive Network of Nanostructured Metal Oxides and/or Polymers for Organic Photovoltaics.
Takashi Sagawa 1 , Susumu Yoshikawa 1
1 Institute of Advanced Energy, Kyoto University, Uji, Kyoto, Japan
Show AbstractInspired by developments of bulk heterojunction layer in polymer/fullerene solar cells, Coakley et al[1] and Günes et al[2] had proposed that high-performance bulk heterojunction solar cells possess an interpenetrating network of D-A molecules in the p-n blend film sandwiched between two electrodes bearing different work functions as “ordered bulk hetrojunction.” Various strategies have been suggested to create such a desirable D-A bicontinuous network in p-n blend films, which exhibit efficient charge separation as well as hole and electron transportation through the network. To rectify electron flow to the ITO side, ZnO and TiOx[3] are particularly suitable as electron-transporting layers (viz. hole-blocking layers) considering the large band gaps and good energy level matching. Various types of inverted polymer solar cells with nanostructured ZnO and TiOx have been developed and evaluated. In this context, we coated ZnO nanorod arrays with TiO2 to fabricate a P3HT-[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) devices resulting in the PCE of 0.7% because of the low crystallinity and small surface area of TiO2 nanotubes.[4] We also examined the cell performance of dye-sensitized solar cell (DSSC) by treating the TiO2 nanotube arrays with TiCl4.[5] After the TiCl4–treatment, the TiO2 nanotubes were covered with a small amount of TiO2 crystals. Both the Jsc and the PCE were ca. 2 times larger than those without the TiO2 treatment. On the other hand, utilization of electrospun TiO2 nanofibers as electrode of DSSC exhibited effective light harvesting and improvement of the incident photon-to-current conversion efficiency (IPCE).[6] The IPCE of 85% at 540 nm with the PCE of 8.14% were obtained. Other strategies of the fabrication procedures such as surface modification of the 1D ZnO nanorod or TiO2 nanotube arrays with self assembling low molecular weight organic dyes and origin of the photophysical properties were also studied in terms of device structure and performance. Consequently, conductive network of one-dimensional (1D) nanostructured materials are promising to show characteristic features of the light-harvesting and photocurrent generation.(1) Coakley, K. M.; McGehee, M. Chem. Mater. 2004, 16, 4533-4542.(2) Günes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324-1338.(3) Chen, L.; Hong, Z.; Li, G.; Yang, Y. Adv. Mater. 2009, 21, 1434-1449.(4) Rattanavoravipa, T.; Sagawa, T.; Yoshikawa, S. Sol. Energy Mater. Sol. Cells 2008, 92, 1445-1449.(5) Charoensirithavorn, P.; Ogomi, Y.; Sagawa, T.; Hayase, S.; Yoshikawa, S. J. Electrochem. Soc. 2010, 157, B354-B356.(6) Chuangchote, S.; Sagawa, T.; Yoshikawa, S. Appl. Phys. Lett. 2008, 93, 033310/1-3.
9:00 PM - E8.56
Self-assembly, Structure and Pi-conjugation at the Interface of Poly-3-Hexylthiophene and Carbon Nanotubes for Excitonic Solar Cells.
Marco Bernardi 1 , Michele Giulianini 2 , Jeffrey Grossman 1
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States, 2 Built Environment and Engineering, Queensland University of Technology, Brisbane, Queensland, Australia
Show AbstractClassical molecular dynamics (MD) simulations are employed to study the molecular-scale arrangement and self-assembly at the interface of poly-3-hexylthiophene (P3HT) and single-walled carbon nanotubes (SWNT). Guided by the π-π stacking interactions, our results show that the P3HT wraps around the SWNT, and that the coiling process occurs on a fast timescale (100 ps) and leads to irreversible P3HT adsorption. A number of the conformations recently observed experimentally are found in our simulations, including helices, bundles, and more elongated structures that maximize the stacking area. The templating role of carbon nanotubes in increasing the π-conjugation in the system is demonstrated by analyzing the MD trajectories. We found that SWNT, due to their intrinsic 1-dimensional cylindrical shape and π-conjugation, impart a longer conjugation length to P3HT chains adsorbed at their surface by quenching their torsional disorder, regardless of their conformation and of the nanotube chirality. This effect is more significant for higher SWNT weight fractions in the sample (since it is an interface effect), and we confirmed this trend by acquiring optical absorption spectra of P3HT-SWNT composites with different SWNT concentrations. Using the Bardeen tunneling single particle picture, we estimated that the increase in conjugation length could favor a charge transfer in type-II heterojunctions between semiconducting SWNT and P3HT by a factor linear in the conjugation length increase. Our results validate previous studies of the electronic structure at the interface, and further elucidate the picture of this molecular heterojunction. The impact of these findings on excitonic solar cells (XCS) is discussed. For example, one implication of our results is that XCS made of all-semiconducting SWNT and P3HT with higher SWNT content could be beneficial. With recent advances in carbon nanotube sorting and selective synthesis, as well as in nanoscale characterization of bulk heterojunctions, mixtures of all-semiconducting SWNT and P3HT (or similar conjugated polymers) could continue to push the existing barriers in the use of carbon nanotubes in next-generation XSC.
9:00 PM - E8.57
Synthesis and Opto-electronic Properties of Semiconducting Polymers Containing Benzodithiophene with Phenylethynyl Substituents.
Mihaela Stefan 1 , Prakash Sista 1 , Hien Nguyen 1 , Jing Hao 1 , John Servello 1 , Elizabeth Schmiedel 1 , Bofei Xue 2 , Michael Biewer 1 , Paul Dastoor 2
1 Chemistry, University of Texas at Dallas, Richardson, Texas, United States, 2 Centre for Organic Electronics, The University of New Castle, Callaghan, New South Wales, Australia
Show AbstractSemiconducting polymers containing benzodithiophene with alkyl phenylethynyl substituents have been synthesized by Stille coupling. The fused central ring allows the incorporation of substituents on the central benzene core, while maintaining planarity of the two thiophene units. In addition, the symmetric nature of the benzodithiophene core eliminates the need to control the regioregularity during the polymerization process. Polymers containing pentyl, decyl and hexadecyl solubilizing alkyl substituents have been synthesized and their opto-electronic properties have been investigated. The synthesized polymers were used as donors in bulk heterojunction solar cell devices.
9:00 PM - E8.58
Air Stable and Solution Processable n-Type Perylene Diimide Copolymers.
Ziqi Liang 1 , Russell Cormier 1 , Brian Gregg 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractPerylene diimides (PDIs), known as highly thermo- and photostable electron-deficient molecules that afford high electron mobility, have been pursued as promising n-type semiconductor molecular building blocks. We have successfully synthesized a set of three soluble poly(perylene diimide)s (PPDIs) through the polycondensation of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) with a variety of poly(ethylene glycol) (PEG)- or poly(propylene glycol) (PPG)-based diamine comonomers. The linear PEG or PPG segment is connected with the unsubstituted perylene core through the imide bond in the main chain. The flexible spacer not only offers increased solubility in organic solvents, but also allows the perylene core to readily assume a conformation that promotes favorable cofacial pi-pi interactions. The rigidity and planarity afforded by the unsubstituted perylene ring enables ordered packing and high carrier mobility. The success of polymer synthesis is verified by Fourier transform infrared (FTIR) absorption spectra, and the resulting copolymers have been further characterized structurally. Multiple endo/exothermic transition points among these PPDIs are detected in differential scanning calorimetry (DSC) thermograms. X-ray diffraction (XRD) patterns reveal that the PPDIs tend to self-organize in the solid state under ambient conditions and the ordering behavior varies from PEG to PPG segment in the polymer chain. Significant fluorescence quenching of hole-transporting poly(3-hexylthiophene) (P3HT) by the PPDI within a blend structure was observed owing to efficient electron transfer. Time-of-Flight (TOF) measurement shows electron mobility of a PPDI as ~5 x 10-4 cm2/Vs. Most recent discovery that these PPDIs are soluble in formic acid enables the solution-processing p/n bilayer cells comprising P3HT as the donor layer spun-coated from dichlorobenzene solution. Together, these PPDIs may hold strong promise as a stable electron-conductive layer with high carrier mobility in solution-processing applications of organic photovoltaics and field-effect transistors.
9:00 PM - E8.59
Negative Humidity-dependent Photoconductivity of Solid-state Water Soluble Polymer Bridges.
Jaewu Choi 1
1 , Kyung Hee University, Seoul Korea (the Republic of)
Show AbstractThe humidity-dependent photoresponse of a freestanding polymeric solid-state bridge of sodium poly[2-(3-thienyl)-ethoxy-4-butylsulfonate] (SPTEBS) was studied. Below the water vapor pressure of 12 Torr, there is no significant change in the dark current. However, the dark current dramatically increases with water vapor pressure higher than 12 Torr. Surprisingly, under the light illumination, the short-circuit current of the polymeric solid-state bridge decreases with increasing of water vapor pressure. Particularly above 12 Torr of water vapor pressure, the short-circuit current under the light illumination is even lower than the dark current. This negative photo-response of the polymeric solid-state bridge is attributed to the de-doping by the photo-induced desorption of water molecules.
9:00 PM - E8.6
Novel Solution-processible Small Organic Molecules as Donor for High-performance Organic Photovoltaic Cells Cells.
Zhonglian Wu 1 , Benhu Fan 1 , Feng Xue 2 , Jianyong Ouyang 1
1 Department of Materials Science and Engineering, National University of Singapore, Singapore Singapore, 2 Department of Chemistry, National University of Singapore, Singapore Singapore
Show AbstractOrganic photovoltaic cells, in which the active materials are polymers or small organic molecules as donor material, have been gaining strong attention due to the low fabrication cost, high mechanical flexibility of materials. These devices may be fabricated through solution processing, which is most efficient in terms of the fabrication cost and large-area fabrication. Compared with polymeric donor materials, solution-processible small organic molecules have attracted a great deal of interest due to easy synthesis and purification, monodisperse, and higher charge carrier mobilities. So development of new small organic molecules as donor becomes an efficient avenue to improve the photovoltaic efficiency of the devices. Herein, several new small organic molecules derived from 3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole- 1,4-dione have been synthesized and characterized. Bulk heterojunction (BHJ) devices were fabricated using them as donor and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as acceptor by solution processing. Preliminarily, these devices reach a light-to-electricity conversion efficiency of 1.16 % under simulated AM 1.5 solar (100 mW cm-2).
9:00 PM - E8.61
Enhancing the Performances of Polythiophene/Fullerene Solar Cells by Deposition from Solvent Mixtures.
Antonietta De Sio 1 , Ralph Huber 1 , Felix Deschler 2 , Enrico Da Como 2 , Elizabeth von Hauff 1 , Juergen Parisi 1
1 Institute of Physics, Energy and Semiconductor Research Laboratory, University of Oldenburg, Oldenburg Germany, 2 Photonics and Optoelectronics Group, Department of Physics and CeNS, Ludwig-Maximilians-Universität, Munich Germany
Show AbstractOrganic solar cells fabricated from blends of conjugated polymers and fullerene continue to attract the interest many research groups. While record efficiencies consider blends of these two material systems, the intricate interplay between morphology and solar cell performances is far from being understood. Recently, processing strategies considering solvent additives have opened a new route to improve the solar cells characteristics of blends with non-optimal phase segregated morphologies.In this study we demonstrate how the performance of polymer:fullerene solar cells can be enhanced using solvent mixtures. As a model system we use a bulk-heterojunction (BHJ) solar cell structure, in which the active layer is a blend of poly-3-hexylthiophene (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) dissolved in 1,2-dichlorobenzene (oDCB). It is shown that combining additive solvents with the principal host solvent oDCB, influences the optical and electrical properties of the blend. The effect of using low and high boiling point solvents on the film formation and the optoelectronic properties of the blend is investigated by combining electrical measurements, electron microscopy and a range of optical spectroscopy techniques. We find that by using co-solvents such as tetraidrofuran or chlorobenzene, it is possible to tune the morphology of the BHJ and improve the optical and electrical properties of the blends. This in turn leads to an increase in the short circuit current of the resulting solar cell with respect to blends deposited from oDCB alone. This increase in photocurrent is discussed considering the increased light harvesting of polymer:fullerene blends using solvent additives and the change in the carrier transport and recombination as probed by CELIV and photoinduced absorption spectroscopy. The results show the importance of tuning morphology and solar cell performances by using solvent mixtures in the preparation procedure.
9:00 PM - E8.63
Structural and Electron Transport Investigations of Arylthio-substituted Coronene Monolayers.
Peter Kowalzik 1 , Silke Rathgeber 2 , Marc Gingras 3 , Nicolae Atodiresei 1 , Vasile Caciuc 1 , Stefan Bluegel 1 , Rainer Waser 1 , Silvia Karthaeuser 1
1 , Institut für Festkörperforschung, Forschungszentrum Jülich, Jülich Germany, 2 , Johannes Gutenberg-University Mainz, Institute for Physics, Mainz Germany, 3 , CNRS, Aix-Marseille University, Marseille France
Show AbstractThe properties of π-conjugated molecules on conducting surfaces largely depend on molecule-substrate interfacial interactions and intermolecular interactions. Here, we demonstrate that the self-assembly behavior and the electronic function of tailored conjugated molecules with varying substituents can be tuned to form semiconducting organic nanowires or molecular quantum dot structures. In particular, we present an experimental and theoretical study of the structural and electronic properties of arylthio-substituted coronenes (ATCs). Derivatives of ATC molecules exhibit a large conjugated electron system with an effective delocalization of charges throughout the molecules despite their overall nonplanarity [1,2]. We show that the self-assembly of ATCs on the Au(111) surface and the charge transport through the molecules can specifically be affected by modifying the aryl substituents. Scanning tunneling microscopy, spectroscopy and simulations based on the density functional theory are used to characterize the structural and electronic properties of the ATC monolayers. Self-assembled structures with different orientations of the molecules relative to the substrate and with different intermolecular interactions are realized. The basic molecular building block dodecakis(phenylthio)coronene (DPTC) assembles into stacks of molecules in “edge-on” orientation [3]. Tunneling spectroscopy measurements suggest the formation of delocalized electronic states throughout the columnar stacks and thus a potential of this system as a basis for novel organic semiconductors with anisotropic charge transport properties. Substituents with varying electron-accepting or electron-donating ability are used to modify DPTC and enhance or prevent the molecular stacking. Assemblies of molecules in “face-on” configuration with molecular quantum dot behavior can also be obtained in this way. The tailored functionalization allows a decoupling of the aromatic system of the molecules from the substrate states. This in turn leads to the occurrence of single electron tunneling effects, observable in tunneling spectra at room temperature. [1] J. H. R. Tucker, M. Gingras, H. Brand, J.-M. Lehn, J. Chem. Soc. Perkin Trans. 2 (1997) 1303.[2] M. Gingras, J.-M. Raimundo, Y. M. Chabre, Angew. Chem. Int. Ed. 45 (2006) 1686.[3] P. Kowalzik, S. Rathgeber, M. Gingras, J.-M. Raimundo, N. Atodiresei, V. Caciuc, S. Blügel, R. Waser, S. Karthäuser, (2010) submitted.
9:00 PM - E8.64
The Benefits of Incorporating MoOx and WOx Layers into Small Molecule Organic Photovoltaics.
Ian Hancox 1 , Paul Sullivan 1 , Tim Jones 1
1 Chemistry, University of Warwick, Coventry United Kingdom
Show AbstractThere are recognised problems with indium-tin oxide (ITO) and the hole-extracting material poly(ethyleneoxythiophene): poly(styrenesulfonate) (PEDOT:PSS), particularly the inhomogeneous electrical characteristics across the surfaces and the acidity of PEDOT:PSS. This has resulted into considerable recent research into the development of alternative hole-extracting materials for organic photovoltaic (OPV) cells, notably transition metal oxides.In this work we focus on utilising thin films of molybdenum oxide (MoOx) and tungsten oxide (WOx) as hole-extracting layers in a systematic study of small molecule OPV cells.1 By tailoring the transition metal oxide used for each OPV system depending on energy level positions, large gains in initial cell performances of up to 60% can be obtained over the deposition of donor materials (e.g. ClAlPc and SubPc) directly on to bare ITO, due to improved energy level alignment for hole-extraction. Studies carried out using atomic force microscopy (AFM), X-ray diffraction and UV/vis electronic absorption show similar surface morphology and molecular stacking when donor layers are deposited on the metal oxide and on bare ITO. The cells containing the transition metal oxide hole-extracting layers also show a dramatic improvement in cell operational stability compared to those fabricated on bare ITO. External quantum efficiency (EQE) measurements, taken before and after operational stability testing under constant illumination, show greatly increased stability at the donor/electrode interface with the incorporation of the transition metal oxide layers.The further benefits of inserting MoOx and WOx layers are also described, such as high reproducibility of cell performance and the more homogeneous workfunction of the metal oxide surfaces as measured by Kelvin probe AFM. Devices were found to have greatly reduced performances after exposing the MoOx layers to air before deposition of the active layers, due to poor hole-extraction. However with a simple treatment the performance returns to that of the layers without exposure, showing the stability of the material. In order to further develop the properties of the hole-extracting layers, a combination of electron-blocking layers and metal oxides were used to further enhance device performance. 1 Hancox, I., et al., Energy Environ. Sci., 2010. 3: p. 107
9:00 PM - E8.65
Thermally Stable Amorphous Polymeric Semiconductors Containing Fluorene and Thiophene for Use in Organic Photovoltaic Cells.
Hyojung Cha 1 , Hoyoul Kong 2 , Dae Jung 1 , Yun-Hi Kim 3 , Hong-Ku Shim 2 , Chan Park 1
1 , POSTECH, Pohang Korea (the Republic of), 2 , Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of), 3 , Gyeongsang National University, Jinju Korea (the Republic of)
Show AbstractWe report amorphous polymeric semiconductors containing fluorene and thiophene, specifically poly(2-(5-(9,9-dibutyl-9 H-fluoren-2-yl)-3-hexylthiophen-2-yl)-5-(3-hexylthiophen-2-yl)thieno[3,2-b]thiophene) (P1) and poly(2-(5-(9,9-dibutyl-9 H-fluoren-2-yl)-4-hexylthiophen-2-yl)-5-(4-hexylthiophen-2-yl)thieno[3,2-b]thiophene) (P2), for use as electron donor materials in organic photovoltaic cells. The optical properties of the polymers blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were characterized by UV-Visible absorption and photoluminescence (PL) measurements. A maximum power conversion efficiency (PCE) of 1.8% and a high open circuit voltage (Voc) of 0.97 eV were obtained from the P2:PCBM blend. Morphological and structural analysis of the polymer:PCBM blend films, by atomic force microscopy (AFM) and transmission electron microscopy (TEM), demonstrated that the range of possible side chain arrangements produced distinctly different nano-morphologies and, therefore, different thermal stabilities of the resulting devices. P2 was characterized by long distance between alkyl chains along the polymer backbone that yielded a uniformly amorphous morphology without PCBM clusters to hinder charge transport to electrode and steady photovoltaic performance exhibiting remarkable thermal stability, even at elevated temperatures.
9:00 PM - E8.66
Multilayered Water-based Organic Photovoltaics.
Andrew Stapleton 1 , Ben Vaughan 1 , Elisa Sesa 1 , Bofei Xue 1 2 , Kerry Burke 1 2 , Xiaojing Zhou 1 , Glen Bryant 1 , Oliver Werzer 1 , Warwick Belcher 1 , Erica Wanless 1 , Paul Dastoor 1
1 Centre for Organic Electronics, University of Newcastle, Callaghan, New South Wales, Australia, 2 , CSIRO Energy Technology, Newcastle, New South Wales, Australia
Show AbstractWater-based polymer nanoparticles offer the prospect of addressing two of the main challenges associated with printing large area organic photovoltaic (OPV) devices; namely how to control the nanoscale architecture of the active layer and eliminate the need for hazardous organic solvents during device production. However, to date, the efficiencies of nanoparticulate-based devices have been vastly inferior to that of the corresponding bulk-heterojunction structure. Here we present an approach for producing efficient OPV devices from polymer nanoparticulates through the fabrication of multilayered device architectures. We show that by controlling both morphology and inter-particle interactions it is now possible to build optimized OPV devices from aqueous dispersions of nanoparticles that are more efficient than the corresponding bulk heterojunction structure. This work offers the realistic prospect of the development of printable water-based photovoltaic materials.
9:00 PM - E8.67
A Parallel, Multiple-lattice Kinetic Monte Carlo Method for Elucidating C60 Growth on Pentacene.
Rebecca Cantrell 1 , Paulette Clancy 1
1 , Cornell University, Ithaca, New York, United States
Show AbstractMaking reliable all-organic electronic devices using organic semiconductors remains a challenge. One of the many hurdles to creating better, more reproducible, devices is making highly ordered, crystalline thin films from small organic materials. The focus of this computational study is the poorly understood planar heterojunction involving C60 on pentacene, which is considered to be promising for photovoltaic devices. C60 and pentacene each display relatively high crystallinities and, hence, relatively large charge carrier mobilities, making them, in principle, an ideal combination for organic p-n junction devices. Unfortunately, it has proven to be challenging to grow ordered layers of C60 on pentacene in an ideal layer-by-layer manner, given the tendency of C60 to cluster and dewet the pentacene surface, leading to undesirable three-dimensional growth. However, one study (Itaka et al., Adv. Mater., 2006) found that C60 is capable of wetting a single monolayer of the thin film polymorph of pentacene on Ag2O3. Computational studies of the nature of this interface are also rare. In this study, we have developed a new multiple-lattice (hexagonal fcc for C60 and a polygonal pair of lattices for pentacene) and parallelized kinetic Monte Carlo code to aid in understanding how deposited C60 molecules grow on pentacene as a function of substrate temperature, the pentacene phase (or polymorph), and deposition rate. The goal of these studies is to try to provide guidance for optimized experimental conditions that promote 2D growth. The rate information needed as input to this code has been provided by our extensive molecular-scale modeling of a large library of potential "events" that can occur during deposition and growth. Evidence of the extent and classification of faceted grain shapes during the simulated growth process should provide a new understanding of the C60/pentacene interface. We expect this study to bring unique insight into the properties of this system that would be otherwise difficult or impossible to see experimentally or predict a priori theoretically.
9:00 PM - E8.69
What Photons Tell About Polymer Solar Cells: Characterizations by Spectrally and Spatially-resolved Luminescence.
Marco Seeland 1 , Maik Baerenklau 1 , Roland Roesch 1 , Harald Hoppe 1
1 Institute of Physics, Ilmenau University of Technology, Ilmenau Germany
Show AbstractThe detection of luminescence radiation from polymer solar cells generally allows investigation into the device physics such as the photoinduced charge transfer and the exciton dissociation rate obtained by photoluminescence quenching measurements. We have applied spectrally and spatially-resolved detection of electroluminescence radiation for the characterization of photovoltaic devices. On one hand, spectrally resolved electroluminescence provides useful information regarding the morphology and performance of the devices due to the radiative decay of charge-transfer-excitons at the donor-acceptor interface. On the other hand, the spatially-resolved electroluminescence imaging method allows local investigations on the macroscale. We detect thereby cathode failure caused by degradation processes and relate the electrodes sheet resistances with the spatial current distribution across the device.
9:00 PM - E8.7
Solvent-modulation of Buffer Layers for Efficient Charge Transport Characteristics in Organic Photovoltaic Devices.
Kyung Min Kim 1 , Keun Woo Lee 1 , Abderrafia Moujoud 1 , Sang Hoon Oh 1 , Hyun Jae Kim 1
1 School of Electrical and Electronic Engineering, Yonsei University, Seoul Korea (the Republic of)
Show AbstractIn this research, the effects of solvent-modulated buffer layers on the performance of organic photovoltaic devices (OPVs) based on poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) were investigated. The buffer layers were modulated by two different methods: blending and surface treatment. The blend consisted of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with N,N-dimethylformamide (DMF), and the surface treatment was accomplished by spin-coating DMF onto a PEDOT:PSS film. Both types of buffer layer modulation improved the power conversion efficiency (PCE) by reducing the series and contact resistances of OPVs. The series resistance of OPVs was reduced from 15.6 Ωcm2 to 10.1 Ωcm2 by blending, and 8.7 Ωcm2 by surface treatment. These improvements resulted from the formation of highly conductive pathways of continuous PEDOT-rich domains, leading to reduced resistivity of the buffer layer, with no significant change in the work function. The increase in work function of PEDOT:PSS can form charge extraction barrier. The conductivity of the modulated buffer layer was improved from 1.27 S/cm to one order of magnitude (69.2 S/cm) by blending, and two orders of magnitude (152 S/cm) by surface treatment. The atomic force microscopy of each PEDOT:PSS film showed that when blending is used, excess DMF can induce inhomogeneity and serious phase separation and other defects, which increase leakage current in OPVs. In contrast, surface treatment exhibits homogeneous and sufficient phase separation, leading to superior device performance, by reducing leakage current. Compared with blending which requires a carefully controlled doping level, surface treatment can be processed simply, and exhibits much more obvious improvements after modulation. In short, efficient charge transport was possible after modulation, and high PCE values, 4.17% for blending and 5.14% for surface treatment, were obtained.
9:00 PM - E8.71
Inverted OPV Devices with Laminated Carbon Nanotube Electrodes.
Brian Bailey 1 2 , Brian Larsen 2 , Sean Shaheen 1 , Jeffrey Blackburn 2 , Nikos Kopidakis 2
1 Physics, University of Denver, Denver, Colorado, United States, 2 Physics, National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractWe demonstrate organic photovoltaic (OPV) devices with inverted geometry, fabricated with a single walled carbon nanotubes (SWCNTs) electrode. The SWCNTs can be doped to be either electron collecting, or hole collecting electrodes, thus replacing either the bottom Indium Tin Oxide (ITO) or top Ag electrode. The fabrication technique for the SWCNTs involves a soak in acid or organic solvents, so the SWCNTs cannot be deposited onto an otherwise complete device in the inverted device geometry. To bypass this complication we use lamination, a technique we have reported on earlier, to bring the SWCNT electrode into contact with the active layer of the device. The entire fabrication process is performed in air, and does not require a vacuum step. We present inverted devices with a blend of poly-3-hexylthiophene: [6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) as the active layer. The SWCNT electrode is sprayed onto a flexible PET substrate. We optimize the electrode doping and processing steps as well as the lamination conditions to achieve devices with comparable photovoltaic properties to those of both bottom-up inverted devices with thermally evaporated Ag contacts, and to devices fabricated with laminated Ag contacts.
9:00 PM - E8.73
Inverse Organic Photovoltaic Devices with High Efficiency and Stability Based on Planar and 2D Nanostructured Metal Oxide Charge Extraction Layers.
Stefan Schumann 1 , Raffaello Da Campo 1 , Benoit Illy 2 , Amy Cruickshank 2 , Martyn McLachlan 2 , Mary Ryan 2 , Jason Riley 2 , David McComb 2 , Tim Jones 1
1 Chemistry, University of Warwick, Coventry, West Midlands, United Kingdom, 2 Materials and London Centre for Nanotechnology, Imperial College London, London United Kingdom
Show AbstractThere has been significant recent interest in solution processed inverted device architecture bulk heterojunction (BHJ) organic photovoltaic devices (OPV) due to some distinct advantages over regular OPV device architectures, most notably favourable vertical phase separation of the polymer:fullerene blend, ease of device fabrication, and also the potential for low cost manufacturing.In this study we demonstrate a substantial increase in device performance and operational stability by introducing a zinc oxide (ZnO) interlayer between the electron collecting bottom electrode and the photoactive layer. This interlayer is tuned to perform not only as an efficient electron extracting layer, but also as a hole blocking layer in inverted BHJ OPVs. Selected growth techniques were used to deposit the ZnO films, either by electro-deposition or spray pyrolysis directly onto the bottom indium-tin oxide electrode, leading to dense planar and 2D nanostructured ZnO interlayers, including nanorods, pillars, platelets and honeycomb structures. Detailed thin film characterisation involving atomic force microscopy, scanning electron microscopy and X-ray diffraction were used to compare crystal structure, morphology and surface roughness to gain a deeper understanding of the structure/function relationship when applied in a device.Metal oxide sandwich devices based on the photoactive blend of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) on an electro-deposited ZnO interlayer with a (100) preferential crystal orientation, using a tungsten oxide interlayer at the opposite electrode, resulted in a remarkable increase in power conversion efficiency up to a value of 4.91 %. Additionally, an external quantum efficiency of 74 % and excellent operational stability under both nitrogen and air atmospheres were demonstrated. Further device characterisation included transmission electron microscopy studies on device cross-sections cut by a focused ion beam (FIB-SEM), revealing detailed interface structures.Electro-deposition of the ZnO at low temperature proved to be the most promising technique, allowing the highest control of both film structure and morphology for planar and 2D nanostructured interlayers, as well as leading to significantly improved device efficiency and stability with significant potential for scale-up to larger areas.
9:00 PM - E8.74
Contacting Molecular and Hybrid Materials: Electrical Breakdown Due to nm-sized Features of Macroscopic Contact Surfaces.
David Cahen 1 , Hagay Shpaisman 1 , Hagai Cohen 1 , Rotem Har-Lavan 1 , Daniel Azulai 1 , Nir Stein 1
1 Materials and Interfaces, Weizmann Institute of Science, Rehovot Israel
Show AbstractThe need for low-cost, transparent and flexible electrical contacts for optoelectronics, esp. light–emitting diodes and photovoltaics, has spurred the development of novel methods for preparing such contacts. In several cases the resulting contacts have a high surface aspect ratio (HSAR), e.g., contacts with nm-sized features, such as those involving C nanotubes, metal nanowires or modified nanoparticles. Often, such contacts serve to study low(er) conductance materials. Based on experimental indications from our own and other groups that HSAR contacts influence charge transport through low conductance molecular material,1-3 we prepared and studied a model system of a conducting substrate, with the low conductance layer sandwiched between the substrate and an HSAR contact. We find that nm-features of large area electrical contacts can lead to local electrical breakdown of an underlying dielectric material under bias conditions that are significantly milder than those required for breakdown without nm features. Furthermore, if enhanced breakdown conditions are fulfilled, apparent spontaneous breakdown can occur due to instability of common electrical grids. Chemically resolved electrical measurements support that the breakdown does indeed occur below the nm-sized features. Electrical breakdown can occur in all experiments that involve nm-sized contacts, such as break-junctions, nm-gaps, edge-contacts, C-AFM, STM, especially in molecular electronics. Mostly researchers are aware of this possibility and can even try to take advantage of it, but due to the use of very small contact areas and contact stability issues, extensive data collection and tedious statistical analysis are needed to determine the lowest voltage at which electrical breakdown occurs. Our model system allows determining the minimal voltage of electrical breakdown, so that this phenomenon can be avoided, or put to use, in work with nm-sized contacts.References[1] E. A. Weiss, et al., J. Am. Chem. Soc. 2007, 129, 4336.[2] Y. Jin et al., Angew. Chem. 2006, 118, 6473.[3] Y. Jin et al., Langmuir 2008, 24, 5622.
9:00 PM - E8.75
Single Microsecond Resolution Time-resolved Electrostatic Force Microscopy of Nanostructured Organic Photovoltaic Blends.
Rajiv Giridharagopal 1 , David Ginger 1
1 Chemistry, University of Washington, Seattle, Washington, United States
Show AbstractIn organic photovoltaic blends, device performance is inextricably linked to the film morphology of the active layer. Correlating topographic and local optoelectronic information using high-resolution scanning probe microscopy methods is therefore vital to understanding the nanoscale properties that govern bulk device performance. Time-resolved electrostatic force microscopy (trEFM) provides a robust metrological tool for exploring device performance due to the correlation between the local charging rate measured by trEFM and external quantum efficiency. To date, however, trEFM has been limited to relatively inefficient photovoltaic systems due to the detection limits imposed by conventional microscopy electronics. As a result, trEFM data on high-efficiency systems such as polythiophene:fullerene materials have been elusive, particularly in device-relevant experimental conditions. Here we discuss single microsecond time-resolution trEFM using improved detection methodology and instrumentation, thereby allowing us to explore the fast charging rate behavior of polymer:fullerene blends. By exploiting the physics of cantilever motion, we are able to improve the time resolution by upwards of two orders of magnitude. This also opens the door for further trEFM experiments on a suite of other potential substrates of interest as well as experiments involving the kinetics of charge trapping and detrapping. The data demonstrate that high-speed time-resolved electrostatic force microscopy is a powerful tool for analyzing high-efficiency organic photovoltaic materials and can be used to guide future device work.
9:00 PM - E8.76
Scaling of Transport Properties in Lateral Nanostructured Organic Solar Cells.
Tricia Youngbull 1 2 , Madhusudan Singh 2 3 , Samson Jenekhe 4 5 , Ghassan Jabbour 2 6 , Christine Luscombe 1
1 Department of Materials Science and Engineering, University of Washington, Seattle, Washington, United States, 2 School of Mechanical, Aerospace, Chemical and Materials Engineering, Arizona State University, Tempe, Arizona, United States, 3 , Flexible Display Center at Arizona State University, Tempe, Arizona, United States, 4 Department of Chemistry, University of Washington, Seattle, Washington, United States, 5 Department of Chemical Engineering, University of Washington, Seattle, Washington, United States, 6 Solar and Alternative Energy Research Center, King Abdullah University of Science and Technology, Thuwal Saudi Arabia
Show AbstractIn organic bulk heterojunction (BHJ) solar cells, charge carriers must have a sufficient mobility and a clear charge conduction pathway to drift through the BHJ network structure to the electrodes before they recombine. The effect of lateral device geometries on charge transport in poly(3-hexylthiophene): phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blends is studied using charge extraction by a linearly increasing voltage (CELIV). Nanogap devices ranging from 30 to 200 nm, fabricated using focused ion beam and physical sputter etching, are compared to planar diodes. Relative to planar P3HT:PCBM devices of the same electrode spacing, the charge carrier mobility doubles and the conductivity increases by an order of magnitude in the nanogap devices. Within the nanogap devices, as the gap spacing is decreased from 200 nm to 30 nm, the conductivity increases and the mobility decreases by an order of magnitude, resulting in charge carrier mobilities on the order of 1x10^-8 m^2/Vs and conductivities of about 2x10^-6 S/m at an electric field strength of 2 MV/m. These trends are discussed within the context of a Gaussian disorder model and percolation theory in disordered bulk heterojunction blends. From a disorder perspective, these results suggest that the positional disorder in the 30 nm gaps is greater than in the 200 nm gaps, which is contrary to predictions by certain models. Thus, organic BHJ solar cells with lateral device architectures exhibit enhanced charge transport, leading to greater potential power conversion efficiencies.
9:00 PM - E8.77
Shadowing Effects on Organic Photovoltaic Devices.
Matthew Reese 1 , Ken Steirer 1 3 , Michael Machala 1 , Dana Olson 1 , Sean Shaheen 2
1 National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, Colorado, United States, 3 Physics, Colorado School of Mines, Golden, Colorado, United States, 2 Physics and Astronomy, University of Denver, Denver, Colorado, United States
Show Abstract Shadowing effects (i.e. the partial blocking of incident light) in organic photovoltaic devices have received little attention in the literature. The certification process for device efficiencies requires the use of an aperture for small area devices, which induces shadowing, in order to eliminate collection effects that have been shown to artificially inflate the current density due to collection from a region as much as 5 mm2 outside of the nominal device area.[1] With the impressive number of high power conversion efficiency systems certified in the past two years, it is important to fully understand these effects. This is especially true since the many of these devices are small (~10-25 mm2), meaning it is not unusual for 60% or more of a device to be shadowed during measurement. It has been shown for inorganic photovoltaics that shadowing can have extreme consequences for modules, due to current imbalances between devices connected in series. Shadowing effects, however, can also significantly impact individual devices. There are concerns that in some cases shadowing may be adversely affecting device performance, while in other cases it may artificially inflate it. To address these concerns, we explore the effects of shadowing for multiple organic photovoltaic device architectures, including traditional and inverted geometries, low conductivity and high conductivity transport layers, and multiple active layer materials. We observe strong effects on open circuit voltage (Voc), fill factor (FF), and efficiency (η). For example, with a traditional device architecture, poly(3-hexylthiophene) (P3HT): phenyl-C61-butyric acid methyl ester (PCBM) devices, with 60% shadowing Voc can be decreased ~5-7%, FF can be made either to increase or decrease, and η can decrease from ~2-7%. With more than 80% shadowing, the effects accelerate. We also present a two-diode model based on the light and dark contributions to help predict and understand these effects on organic solar cells. For each case we compare the predicted to experimentally measured results. We then use the model to predict how differently behaving devices respond to shadowing. 1. M-S. Kim, M-G. Kang, L. J. Guo, J. Kim, Appl. Phys. Lett. 92 (2008) 133301.
9:00 PM - E8.79
Charge Generation in Poly(3 hexylthiophene)/Fullerene Bilayers Studied by Time-resolved Microwave Conductivity.
Alexandre Nardes 1 , Alexander Ayzner 2 , Bertrand de Villers 2 , Benjamin Schwartz 2 , Nikos Kopidakis 1
1 Basic Sciences, National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Dept of Chemistry and Biochemistry and California Nanosystem Institute, University of California, Los Angeles, California, United States
Show AbstractState of the art OPV devices comprise a bulk-heterojuntion (BHJ) design where the polymer and the fullerene form an interpenetrating network with the device performance critically dependent on the morphology of that network. The short exciton diffusion lengths in solution-processed conjugated polymers (typically 7-10 nm) makes the BHJ architecture necessary for efficient exciton dissociation and free carrier production.Recently, there has been growing interest into a simpler bilayer structure of polymer and fullerene with devices composed of P3HT/PCBM bilayers reaching 3.5 % efficiency [1]. The remarkable efficiencies reached by the bilayer device are intriguing and warrant further investigation of the charge generation mechanisms in these systems. In this contribution, we have used the time-resolved microwave conductivity (TRMC) technique to study the dynamics of charge generation and loss in P3HT/fullerene bilayers. These bilayers have been fabricated by a) choosing the proper solvent system for each layer: dichlorobenzene (DCB) for P3HT and dichloromethane (DCM) for PCBM or b) by thermally evaporating the fullerene onto the polymer film. TRMC allows us to photoexcite the sample through the polymer layer or through the fullerene in order to examine the relative importance of excitation of the polymer away from the polymer/fullerene interface or at the interface, respectively. The difference between the two types of excitation become larger with increasing polymer thickness. By varying these parameters of the sample and the measurement, and by comparing our results to BHJ samples we elucidate the the mechanisms for free carrier generation and recombination. We perform morphological analysis of the samples by x-ray diffraction (XRD) and XPS depth profiling to understand the correlation of the structure of the bilayer to the photocarrier dynamics. Implications for OPV device architectures will be also discussed.Reference:[1] A. L. Ayzner, C. J. Tassone, S. H. Tolbert, B. J. Schwartz, J. Phys. Chem. C. 2009, 113, 20050 – 20060.
9:00 PM - E8.8
Correlation Among the Ionization Potential, Built-in Potential, and the Open-circuit Voltage of Multi-layered Organic Photovoltaic Devices.
Eiji Itoh 1 , Toshiki Shirotori 1 , Yuji Higuchi 1 , Daisuke Furuhata 1
1 Department of Electrical and Electronic Engineering, Shinshu University, Nagano Japan
Show AbstractWe have investigated the current-voltage characteristics of the multi-layered photovoltaic devices consisting of ITO/oxide /p-type semiconductor /fullerene/ BCP/ Al structures. In this study, we chose various p-type semiconductors (donors) in order to tune the values of ionization potential (IP) of donor molecules, such as poly(3-hexylthiophene) (P3HT), Poly[2-methoxy-5- (2-ethylhexyloxy)- 1,4-phenylenevinylene (MEHPPV), Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-bithiophene] (F8T2), poly (9,9-dioctylfluorene) (F8) for soluble p-type materials, and copper-phthalocyanine (CuPc), 5,10,15,20- zinc- tetraphenylporphyrin (ZnTPP), and 5,10,15,20- tetraphenylporphyrin (H2TPP) for small-molecule materials. The open-circuit voltage (Voc) increases with the increment of IP of donor materials The values of Voc measured at room temperature in a vacuum chamber was, however, limited at approximately 0.6-0.7V for the devices without oxide as a hole collection buffer layer. The upper limit of Voc is probably attributable to the low-built-in potential (Vbi) or so-called work-function difference between ITO and Al electrodes (~0.7eV). On the other hand, the Voc increases up to 0.9V for the devices with NiO as a hole collection layer, and it increases to 1.07V or more for the devices with MoOx layer. The ionization potential and work-function of NiO and MoOx are estimated as 5.0 and 5.3eV for NiO and 5.4 and 5.6eV for MoOx. Here, the work-function of oxide surface was estimated by the Kelvin prove measurement with respect to ITO or Au reference electrode, and the ionization potential was estimated by photoemission spectroscopy in ambient atmosphere (AC-2, Riken). That is, the built-in potential across the organic film increases from 0.7eV to 0.9-1.0 and 1.2-1.3eV for NiO and MoOx, respectively. Therefore, we concluded the upper limit of Voc is attributable to the built-in potential across the organic multi-layers, and both delta E (=energy difference between the LUMO of fullerene and the HOMO of donor) and built-in potential are responsible for the open circuit voltage of organic photovoltaic devices. The temperature dependence of Voc will be also discussed in the presentation.
9:00 PM - E8.80
Energy Transfer in Nanostructured Films Containing Poly(p-phenylene vinylene) (PPV) and Acceptor Species.
Bruna Postacchini 1 , Fernando Dias 3 , Andy Mokman 3 , Valtencir Zucolotto 2 , Osvaldo Oliveira Jr 2
1 Physics Departament, UFOP, Ouro Preto Brazil, 3 Departament of Physics, University of Durham, Durham United Kingdom, 2 IFSC, Universidade de São Paulo, São Carlos Brazil
Show AbstractEnergy transfer processes in the nanometric scale have been widely investigated in molecular-level control of optoelectronic properties in devices, including photovoltaic cells [1,2], light emitting diodes and sensors [3,4]. Conjugated polymers, in particular, are advantageous owing to their tunable electrical and optical properties, allied to easy processability. The first models to deal with resonant energy transfer were proposed by Perrin and Förster [apud 5], who explained excitation transfer mechanisms in terms of dipole interactions between donors and acceptors. The rate of energy transfer (kET) depends on the spectral overlap between the absorption spectrum of the acceptor and the emission spectrum of the donor, in addition to the distance (r) and relative orientation of donors and acceptors. According to the Fermi Golden Rule approximation, kET~FdFa, with interaction between donor Fd and acceptor Fa depending on the geometry.[6] For instance, F~1/r^3 for single dipoles, F~1/r for a 2D dipole array and F~constant for a 3D dipole array. For two single dipoles randomly oriented in solution, the Förster model yields a distance dependence for the energy transfer rate F~(1/r^3)(1/r^3)=1/r^6. For solid films, this point-to-point interaction does not give suitable results and the energy transfer rate depends on the geometry of the structures.[5] The combination of luminescent polymers and suitable energy-accepting materials may lead to a molecular-level control of luminescence in nanostructured films. In this study, the properties of layer-by-layer (LbL) films of poly(p-phenylene vinylene) (PPV) were investigated with steady-state and time-resolved fluorescence spectroscopies, where fluorescence quenching was controlled by interposing inert polyelectrolyte layers between the PPV donor and acceptor layers made with either Congo Red (CR) or nickel tetrasulfonated phthalocyanine (NiTsPc). The dynamics of the excited state of PPV was affected by the energy-accepting layers, thus confirming the presence of resonant energy transfer mechanisms. Owing to the layered structured of both energy donor and acceptor units, energy transfer varied with the distance between layers, r, according to 1/r^n with n = 2 or 3, rather than with 1/r^6 predicted by the Förster theory for interacting point dipoles.
9:00 PM - E8.81
Evidences of Inter Chain Excitation Energy Migration on Conjugated Poly(p-phenylene vinylene) (PPV) Blend.
Bruna Postacchini 1 , Thiago Cazati 1 , Fernado Dias 3 , Andy Monkmam 3 , Osvaldo Oliveira Jr 2
1 Physics Departament, Universidade Federal de Ouro Preto-UFOP, Ouro Preto Brazil, 3 Departament of Physics, University of Durham, Durham United Kingdom, 2 Departament of Physics, Universidade de São Paulo, São Carlos Brazil
Show AbstractUnderstanding the pathway of exciton energy migration in polymers is important for applying these photophysical properties in photovoltaic cells and sensors. This work was focus on energy transfer mecanisms in polymeric thin filmes and inter-chain as well as intra-chain processes could be (tiveran que ser) considered [1-4]. The films were obtained by layer-by-layer (LbL) technique and the properties were investigated with UV-Vis absorption, steady-state and time-resolved fluorescence (PL) spectroscopies. The set of samples were films containing different proportions of poly(p-phenylene vinylene) (PPV) chains hosted in polyelectrolyte poly(dodecyl methylammonium chloride) (PDAC). Increase PDAC means dilutions of the PPV on the blend. Quantum efficiency was evaluated by the ration of the integral of the PL spectra by the optical absorption at the excitation wavelength (390 nm). Excited estate lifetime was performed from decay curves in time resolved fluorescence spectroscopy. The incorporation of PDAC causes the decay dynamics to be slower, the emission quantum efficiency to be higher confirming the existence of a strong in interchain resonant energy transfer (RET) mechanisms. The dilution of PPV in the PDAC matrix hinders the exciton migration owing to a decreased concentration of quenching species and reduced exciton mobility. As a consequence, the non-radiative de-activation is less effective, yielding longer lifetimes. For a less competitive de-activation, the emission of smaller conjugated segments is favored.Keywords: Conjugated polymer PPV. Resonant energy transfer (RET). Steady-State fluorescence. Time-Resolved fluorescence. Layer-by-Layer technique (LbL)Work supported by Instituto de Eletrônica Orgânica (INEO) - Ministério de Ciência e Tecnologia (MCT) and FAPEMIG. [1] I. D. W. Samuel, et al. Synth. Metals. 84, 497, (1997).[2] T-Q. Nguyen, V. Doan, B. J. Schwartz, J. Chem. Phys., 110, 4068 (1999).[3] M. Yan, et al. Phys. Rev. Lett. 73, 744 (1994).[4] Greemham, N. C. et al. Chem. Phys. Lett. 241, 89 (1995).
9:00 PM - E8.82
Morphology Control of P3HT:Fullerene Bulk-heterojunction Solar Cells.
Hans Lademann 1 , Markus Kaiser 1 , Heike Klesper 1 , Klaus Meerholz 1
1 Department of Chemistry, University of Cologne, Cologne, NRW, Germany
Show AbstractOrganic bulk-heterojunction solar cells based on the polymer poly(3-hexylthiophene) (P3HT) and the fullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) constitute a recent benchmark system.[1] Several studies concentrate on influencing and optimizing the polymer:fullerene blend morphology [2, 3] as well as determining the hole mobilities and conductivities in films made with different P3HT molecular weights via organic field effect transistor (OFET) measurements.[4] However, the exact impact of polymer aggregation and fullerene agglomeration, respectively, on the resulting local energy levels in the thin-film blend and the performance of solar cells is still not fully understood.We investigated the influence of the degree of polymer aggregation (UV/Vis, AFM, REM) on the energy levels (Kelvin Probe) in blends of P3HT and PCBM and the resulting solar cell performance (especially the open-circuit voltage VOC and built-in voltage V*, respectively). To achieve different degrees of P3HT aggregation in the thin films (i.e. different ratios of amorphous to aggregated P3HT) several preparation methods were investigated:- Adding a secondary solvent to the primary solvent used for spin coating- Changing the P3HT content in the polymer:fullerene blend- Preparation of as-cast as well as annealed films- Variation of the fullereneWith knowledge about the variation of the polymer HOMO-level regarding its aggregation degree, we are able to make statements about the relative LUMO-level position of the fullerene.Furthermore, the dark current behaviour of the solar cells was studied, and we found that it correlated with the above mentioned results.References:[1] F. Padinger, R. S. Rittberger, N. S. Sariciftci, Adv. Funct. Mater. 2003, 13, 85.[2] A. J. Moulé, K. Meerholz, Adv. Mater. 2008, 2, 240.[3] J. K. Lee, C. J. Brabec, J. Yuen, J. S. Moon, J. Y. Kim, K. Lee, G. Z. Bazan, A. J. Heeger, J. Am. Chem. Soc. 2008, 130, 3619.[4] R. J. Kline, M. D. McGehee, E. N. Kadnikova, J. Liu, J. M. F. Fréchet, M. F. Toney, Macromolecules 2005, 38, 3312.
9:00 PM - E8.9
Photogenerated Charge Recombination Process in Poly3-octylthiophene/CdS Based Solar Cells.
Hugo Cortina 1 , Edwin Pineda 1 , Monica Castillo 2 , Maria Elena Nicho 3 , Hailin Hu 1
1 Solar Materials, Centro de Investigacion en Energia, UNAM, Temixco, Morelos, Mexico, 2 Departamento de Investigacion en Polimeros y Materiales, Universidad de Sonora, Hermosillo, Sonora, Mexico, 3 Centro de Investigacion en Ingenieria y Ciencias aplicadas, Universidad Autonoma del Estado de Morelos, Cuernavaca, Morelos, Mexico
Show AbstractOrganic solar cells have been an increasing research area in worldwide laboratories during the last two decades. Although the conversion efficiency has been reached to about 7% in small samples under inert ambient, topics related to the chemical stability, photon absorption and charge generation mechanisms, etc., need to be widely approached from different disciplines for a better understanding about these emerging cells. In this work charge transport and recombination phenomena in cadmium sulfide (CdS) /poly3-octylthiophene (P3OT) hybrid photovoltaic (PV) cells have been studied by using transient photovoltage and photocurrent techniques. Two configurations of CdS/P3OT junctions have been prepared: (1) planar and (2) mixed (bulk). In the planar structure CdS thin films were deposited on transparent conducting glass slides (ITO) by chemical bath deposition at different bath temperature. P3OT solution was drop cast on CdS surface, and gold contact was evaporated on top of P3OT film to form PV cells. In the bulk structure CdS was deposited on acetate cellulose (AC) fibers by chemical bath deposition. The CdS coated AC fibers were dispersed in methyl sulfoxide (MSO), and P3OT dissolved in 1,2-dichlorobenzene (DCB). The mixture of two solutions was deposited on PEDOT coated ITO glass. Both gold and aluminum were used as top contact for the resulting PV cells. Transient photovoltage and photocurrent measurements were taken out on these two types of PV cells at open circuit conditions under illumination of a tungsten-halogen lamp and light perturbation introduced by a pulse laser. Charge recombination and transport phenomena in CdS/P3OT junctions were analyzed as functions of CdS and P3OT preparation conditions. In the case of planar structure the results indicate a clear correlation between CdS morphology and charge recombination rate in the cells. For bulk configuration, the CdS:P3OT and MSO:DCB proportions as well as film drying conditions play an important role in PV performance.
Symposium Organizers
R. Joseph Kline National Institute of Standards and Technology
Iain McCulloch Imperial College London
Garry Rumbles National Renewable Energy Laboratory
Alberto Salleo Stanford University
E11: Poster Session: Thin Film Transistors
Session Chairs
Thursday PM, December 02, 2010
Exhibition Hall D (Hynes)
E9: Thin Film Transistors: Materials Design
Session Chairs
Michael Chabinyc
Martin Heeney
Thursday PM, December 02, 2010
Room 312 (Hynes)
9:30 AM - E9.1
Organic Semiconductor/Graphene Hybrid Field-effect Transistors.
Jia Huang 1 2 , John Cumings 1 3 , Daniel Hines 2 , Vince Ballarotto 2 , Michael Fuhrer 1 , Ellen Williams 1 2
1 Center for Nanophysics and Advanced Materials, University of Maryland, College Park, Maryland, United States, 2 , Laboratory for Physical Sciences, College Park, Maryland, United States, 3 Department of Materials Science and Engineering, University of Maryland, College Park, Maryland, United States
Show AbstractOrganic semiconductor materials offer the potential for low-cost, high volume manufacturing of flexible electronics. However, these materials typically exhibit low mobilities that restrict the range of possible applications. Carbon-based materials such as graphene exhibit higher mobility but suffer from low on/off ratio. With the aim of enhancing the effective mobility of organic field-effect transistors (OFET) while keeping the on/off ratio sufficiently high, we have developed composite thin-films containing both organic semiconductor materials and graphene flakes. Organic field-effect transistors have been fabricated using these organic/graphene hybrid semiconducting films. In this work, graphene exfoliated from graphite in solution has been mixed with PQT12 semiconductor. Without any substrate surface treatment or annealing process, PQT12/graphene hybrid FETs exhibited average mobility of 0.2 cm2/Vs, while control devices fabricated from PQT12 with no added graphene had average mobility of 0.008 cm2/Vs. Typical on/off ratios for PQT12/graphene hybrid FETs were 10,000 and were unchanged from the pure PQT12 devices. We expect that device performance could be improved further by optimizing our device fabrication procedures. We expect that adding graphene flakes to an organic semiconductor thin-film material is a general route to enhance the performance of organic electronics, providing a low-cost avenue for enhancing OFET performance.
9:45 AM - E9.2
High Mobility Organic Field-effect Transistors: Controlling Performance by Molecular Design and Device Processing.
Oana Jurchescu 1 2 , Natalia Azarova 1 2 , Daniel David 1 2 , Katelyn Goetz 1 2 , Claire McLellan 1 2 , Jack Owen 1 2 , Eric Chapman 1 2 , Zhong Li 3 , Balaji Purushothaman 3 , Brad Conrad 4 , John Anthony 3
1 Physics, Wake Forest University, Winston-Salem, North Carolina, United States, 2 Center for Nanotechnology and Molecular materials, Wake Forest University, Winston-Salem, North Carolina, United States, 3 Chemistry, University of Kentucky, Lexington, Kentucky, United States, 4 Physics and Astronomy, Appalachian State University, Boone, North Carolina, United States
Show AbstractOrganic semiconductors offer an attractive balance between cost and performance, complemented by compatibility with room temperature processing and functionality accomplished by means of molecular engineering. We report on our effort to increase solubility and improve the electronic properties of small molecule organic semiconductors by tuning the solid-state order to induce strong pi-stacking interactions. We present the growth, structure and properties of functionalized pentacenes and anthradithiophenes deposited by low cost solution deposition methods, such as spin-coat, drop-cast and air-brush. We demonstrate that chemical structure, as well as differences in surface treatment and drying conditions, greatly affects the formation of crystals and device properties, allowing us to measure mobilities from 10^(-5) cm2/Vs to 1 cm2/Vs. We discuss the effect of trialkylsilyl substituents, as well as the effect of the number of fluorine atoms on electronic properties of fluorinated pentacene derivatives. With anthradithiophenes, we explore three materials differing only by number of benzene rings in the backbone, which yields very different crystal packing. Difluoro- (tri-sec-butylsilyl ethynyl) anthradithiophene (ADT) derivative exhibits one-dimensional pi-pi stacking, with low thin-film transistors mobilities (on the order of 10^(-3) cm2/Vs). Difluoro- (tri-sec-butylsilyl ethynyl) tetracenedithiophene (TDT) derivative exhibits increased, two-dimensional pi-pi stacking, but significant disorder in the crystal, resulting in field effect mobilities in the order of 10^(-2) cm2/Vs, and difluoro- (tri-sec-butylsilyl ethynyl) pentacenedithiophene (PDT) allows for the most two-dimensional pi-pi interactions with minimal disorder and exhibits mobilities of 1 cm2/VsOur results demonstrate the importance of fine tuning the crystal packing and prove the viability of combining molecular design with device processing in achieving high performance organic electronic devices.
10:00 AM - **E9.3
Tuning Electronic Properties and Assembly of Organic Semiconductors for Thin Film Electronics.
Zhenan Bao 1
1 , Stanford University, Stanford, California, United States
Show AbstractOrganic semiconductor materials are interesting alternatives to inorganic semiconductors in applications where low cost, flexible or transparent substrates, and large area format is required. Currently they have been incorporated into organic thin-film transistors (OTFT), integrated display driver circuits, photovoltaics and radio frequency identification tags. In this talk, I will present recent results on material design, surface and interface control for achieving efficient charge carrier transport and large area patterning of organic semiconductors.
10:45 AM - E9.5
Manipulating Island Density and Shape in Organic Thin Film Growth: The Nucleation of Perfluoropentacene on Self-assembled Monolayers.
Tushar Desai 1 , Arthur Woll 2 , James Engstrom 1
1 School of Chemical & Biomolecular Engineering, Cornell University, Ithaca, New York, United States, 2 Cornell High Energy Synchrotron Source, Cornell University, Ithaca, New York, United States
Show AbstractA significant challenge in fabricating organic thin film transistors is that of controlling and understanding the properties of the interface between the organic semiconducting layer and the dielectric. It has been observed that charge transport can be affected significantly by chemically altering the dielectric surface with self-assembled monolayers (SAMs). However, the effects of the molecular scale interactions between the organic molecule and the substrate remain unclear. Here, we have examined the nucleation and growth of perfluoropentacene (PFP) on silicon dioxide and on a variety of surfaces possessing different terminating SAMs using in situ real time synchrotron x-ray scattering and ex situ atomic force microscopy (AFM). The SAMs ranged from very low surface energy hydrophobic surfaces (perfluorooctyltrichlorosilane, FOTS), to higher surface energy hydrophilic surfaces (3-methacryloxypropyltrichlorosilane, MAOPTS). From real time x-ray scattering we find that the growth of PFP, while crystalline, becomes 3D after completion of the first 1-2 monolayers, independent of the substrate surface termination. Concerning growth in the submonolayer regime, we find that nucleation is homogeneous, and that the absolute density of islands depends strongly on the surface termination, while the relative change of the island density with increasing growth rate is essentially independent of the underlying SAM. From the latter we find that a critical island size of ~ 3 molecules can describe all the data. On the other hand, the dependence of the island density on surface termination implicates a significant change in the diffusivity of PFP with the identity of the SAM, with values differing by over two orders of magnitude. The shape of the islands also depends on the surface termination, but somewhat unexpectedly—the islands are most compact and facetted on surfaces where the diffusivity of isolated PFP molecules is the smallest. The shapes of the islands on these surfaces can be interpreted by arguments based on equilibrium, where polygonal islands expose low energy facets. These results demonstrate the sensitivity of the initial stages of nucleation in organic thin film growth to the nature and identity of the underlying substrate.
11:00 AM - E9: TFT Design
BREAK
11:30 AM - **E9.6
Nanoscopic Materials for Organic Photovoltaic and Transistors Applications.
Antonio Facchetti 1 2 , Hakan Usta 2
1 Chemistry, Northwestern University, Evanston, Illinois, United States, 2 , Polyera Corporation, Skokie, Illinois, United States
Show AbstractIn this presentation we will report the design, synthesis, and characterization of new molecular precursors for the fabrication of self-assembled (inter)layers on SiOx, metal oxide, metals, and organic films. These building blocks are designed to strongly adhere to the surfaces of these conductors altering charge injection, charge trapping, and light absorption. Several transistors and photovoltaic cell device architectures based on these interlayers are fabricated and shows that these materials enable improved performance or new functions.
12:00 PM - E9.7
Organic Electronic Ratchets as Power Sources.
Erik Roeling 1 , Wijnand Germs 1 , Barry Smalbrugge 2 , Erik Jan Geluk 2 , Tjibbe de Vries 2 , Rene Janssen 1 , Martijn Kemerink 1
1 Applied Physics, Eindhoven University of Technology, Eindhoven, NB, Netherlands, 2 COBRA Research Institute, Eindhoven University of Technology, Eindhoven, NB, Netherlands
Show AbstractThe possibility to extract work from periodic but undirected forces has intrigued scientists for over a century. In particular, the random motion of particles can be rectified by a ratchet potential. Such potentials are periodic but lack symmetry. These ratchet systems, introduced by Smoluchowski1 and Feynman2, operate in a non-equilibrium regime where the second law of thermodynamics no longer prescribes zero net motion or current. Transport of charges with ratchets has been studied in tunnelling ratchets and single electron ratchets. However, these electronic ratchets typically operated at cryogenic temperatures and generated currents and voltages in the sub-nA and sub-mV regimes, respectively. Here, we present organic electronic ratchets that operate up to RF frequencies at room temperature and generate currents and voltages that are orders of magnitude larger. This enables their use as DC power source. As a first example, the ratchets are integrated into logic circuits, in which they act as the DC equivalent of the AC transformer and generate enough power to drive the circuitry. While these results, for the first time, demonstrate that electronic ratchets may be of actual use, the device characteristics have maintained the intriguing, highly non-linear behaviour that is typical of ratchets. The latter we relate to the interplay between asymmetry in the ratchet potential and breaking of spatial periodicity by the necessary contacts.The presented ratchets are based on organic field effect transistors (OFETs) in which pentacene, an organic semiconductor, is in contact with source and drain electrodes and separated by a dielectric (SiO2) from the gate contact. In the gate dielectric of the OFET asymmetrically spaced interdigitated fingers are embedded. Sinusoidal potentials, applied on the two sets of fingers create the typical time dependent asymmetric potential. The gate electrode of the OFET is used to accumulate charges at the organic-dielectric interface. As a result, a net, ratchet propelled current can flow between the source and drain contacts at zero source-drain bias. 1 Smoluchowski, M. v. Physikalische Zeitschrift 13, 12 (1912).2 Feynman, R. P., Sands, M. L. & Leighton, R. B. The Feynman lectures on physics. (Addison-Wesley, 1989).
12:15 PM - E9.8
Charge Carrier Mobility Control in Organic Single Crystal.
Keke Zhang 1 , Ke Jie Tan 1 , Changji Zou 1 , Laurie McNeil 2 , Subodh Mhaisalkar 1 , Christian Kloc 1
1 School of Materials Science and Engineering, Nanyang Technological University, Singapore Singapore, 2 Department of Physics and Astronomy, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States
Show AbstractRubrene (5,6,11,12-tetraphenylnaphthacene), a typical p-type semiconductor material, has the highest field-effect charge mobility among organic semiconductors. It has been used oft for the fundamental research of intrinsic charge transport properties. Whether the high mobility is attributed to the pristine rubrene or to the oxidation process of rubrene surface, it remains an open question. The rubrene single crystals were grown by physical vapor transport (PVT) technique with flow of Ar or H2 gas at ambient pressure. Crystal purities were analyzed by fourier transform infra-red (FT-IR), high-performance liquid chromatography (HPLC) and liquid chromatography mass spectroscopy (LCMS). The photoluminescence spectroscopy (PL) was used to investigate the surface of rubrene. In these studies, the charge transport of rubrene single crystal was studied. Rubrene single crystal air-gap field-effect transistors (FET) were made by polydimethylsiloxane (PDMS) stamp technique. The PL data of pristine rubrene shows two distinct peaks at 570 nm and 600 nm. A new peak at 650 nm appears when the crystals were exposed to oxygen for several hours. To modulate the rubrene surface oxidation of the transistor conducting channel, the reducing gas (H2) or oxidizing gas (O2) was introduced into the channel. The mobility changed dramatically depending on the gas environment. Under hydrogen ambient, the mobility decreased from 10 cm2/Vs to as low as 10-2 cm2/Vs. Subsequently, when switched to oxygen ambient, the mobility returned to 10 cm2/Vs. The highest mobility achieved so far was 12 cm2/Vs. The oxidation and reduction were reversible and the rubrene single crystal mobility was modulated repeatedly in the range of 10-2 to 10 cm2/Vs by introducing oxygen or hydrogen into the channel. Mobility of tetracyanoquinodimethane (TCNQ), which is an n-type organic semiconductor, is observed being affected by the gas environment as well. Hydrogen reduces the mobility; on the other hand, oxygen enhances the mobility. These experiments suggested that the oxidation of organic surface was responsible for the high charge mobility.
12:30 PM - E9.9
Understanding the Charge Injection and Charge Transport Effects in Organic Thin-film Transistors Toward Universal Device Modeling.
Zihong Liu 1 2 , Alberto Salleo 3 , Zhenan Bao 4 , Yoshio Nishi 2
1 , IBM T.J. Watson Research Center, Yorktown Heights, New York, United States, 2 Department of Electrical Engineering, Stanford University, Stanford, California, United States, 3 Department of Materials Science and Engineering, Stanford University, Stanford, California, United States, 4 Department of Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractOrganic semiconductor thin-film transistors (OTFT) offer huge opportunities for the emerging large-area flexible electronics. Despite the tremendous progress that has been achieved for OTFTs in the past decade, many fundamental device physics remain elusive. Here we propose and develop a universal physical model for organic transistors by incorporating both charge injection effects at the metal-organic interface and charge transport properties in the organic semiconductor film. An extended polaronic mobility-edge (EPME) model taking into account the electric field dependence of the carrier mobility is introduced to describe the charge transport. Contact effects are described in a system of diffusion-limited charge injection into a disordered organic film with Gaussian density of states (DOS) at the metal-organic interface. The universal device model has been successfully implemented in a device simulator and applied to resolve many elusive physical phenomena observed so far for OTFTs, such as the contact resistance effect, the mobility scaling behavior, and the mysterious surface potential profiles.
E10: Thin Film Transistors: Morphology and Interfaces
Session Chairs
Joseph Kline
Rachel Segalman
Thursday PM, December 02, 2010
Room 312 (Hynes)
2:30 PM - **E10.1
Interfaces of Semiconducting Polymers in Organic Transistors.
Michael Chabinyc 1
1 Materials Department, University of California, Santa Barbara, California, United States
Show AbstractMolecular interfaces are intrinsic features of organic electronic devices. The charge carriers in polymer thin film transistors (TFTs) accumulate in the first few molecular layers near the interface with the gate dielectric. Despite significant advances in development of high performance semiconducting polymers, relatively little is understood about the design of dielectric polymers and their impact on charge transport. We will describe recent efforts at rational methods to form controlled interfaces in polymer TFTs that allow a detailed understanding of their impact on charge transport. Dry thin film transfer methods allow us to fabricate TFTs where the semiconducting layer has the equivalent microstructure on any dielectric layer. This method allows us to disaggregate the contribution of microstructure, dielectric constant, and molecular structure to the observed field-effect mobility of the semiconducting polymer. The microstructure of semicrystalline semiconducting polymers transferred to a variety of dielectric polymers has been characterized by x-ray scattering, atomic force microsopy and other techniques. We find that the interfacial microstructure is the dominant factor affecting field effect mobility rather than simple characteristics such as the dielectric constant.
3:00 PM - E10.2
Correlations Between the Mechanical and Electrical Properties of Polythiophenes.
Brendan O'Connor 1 , Edwin Chan 1 , Calvin Chan 2 , Brad Conrad 2 , Lee Richter 3 , R. Kline 1 , Martin Heeney 4 , Iain McCulloch 4 , Christopher Soles 1 , Dean DeLongchamp 1
1 Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 2 Semiconductor Electronics Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 3 Surface and Microanalysis Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 4 Chemistry, Imperial College, London United Kingdom
Show AbstractAn advantage often attributed to organic semiconductors is the potential for flexible electronic devices. Under flexure, these devices, which are typically composed of multiple layers having unique physical properties, will develop a complex stress distribution that may lead to device failure. In addition, the electrical properties of semicrystalline organic semiconductors may be dependent on their mechanical properties. Thus, accurately characterizing the mechanical properties of these films, and correlating their mechanical and electrical properties is critical in guiding the development of candidate materials for flexible applications.Here, we compare the elastic modulus of polythiophenes, regioregular poly(3-hexylthiophene) (P3HT) and poly-(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) (pBTTT), to their field effect mobility showing a proportional trend. The elastic moduli of the films are measured using a buckling based metrology, and the mobility is determined from the electrical characteristics of bottom contact thin film transistors. In addition, the crack onset strain of these films is measured showing that the increased elastic modulus correlates to the onset of brittle behavior. For pBTTT, the crack onset strain is less than 2.5 %, whereas that of P3HT is greater than 150 %. These results show that increased long-range order in polythiophene semiconductors, which is generally thought to be essential for improved charge mobility, can also stiffen and enbrittle the film. This work highlights the critical role of quantitative mechanical property measurements in guiding the development of flexible organic semiconductors.
3:15 PM - E10.3
Surface and Bulk Structural Characterization of a High-mobility Electron Transporting Polymer.
Torben Schuettfort 1 , Samuele Lilliu 2 , Emyr Macdonald 2 , Chris McNeill 1
1 Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom, 2 School of Physics and Astronomy, Cardiff University, Cardiff United Kingdom
Show AbstractWe investigate the interplay between structural order and device performance in high-electron mobility polymer field-effect transistors based on the naphthalene-bis(dicarboximide)/bithiophene copolymer P(NDI2OD-T2). In particular we examine P(NDI2OD-T2) films which have been annealed at different temperatures using optical absorption spectroscopy, Raman spectroscopy, Grazing-Incidence X-ray Diffraction (GIXD), Atomic Force Microscopy (AFM), and Near-Edge X-ray Absorption Fine Structure (NEXAFS) Spectroscopy. With annealing we see a blue-shift in the optical absorption spectrum, suggestive of a reduction in electronic relaxation. GIXD shows in-plane lamellae stacking, with annealing to 210 °C producing enhanced ordering with a splitting of the π-π and lamellae peaks suggesting the existence of two different packing motifs. One of these motifs indicates ordered π-π stacking along the substrate normal, while the other motif is polycrystalline with a slightly larger stacking distance. The NEXAFS results, on the other hand, show a lack of pronounced molecular orientation, with both the conjugated backbone and side chains exhibiting a predominantly random interfacial orientation. This structural information is correlated with transistor performance which point to a complicated interplay between molecular conformation, chain packing and charge transport.
3:30 PM - E10.4
New Insights into Solution-processing of Molecules, Polymers and Their Blends.
Ruipeng Li 1 , Kui Zhao 1 , Mingjie Zhang 1 , Debora Marques 1 , Kang Chou 1 , Detlef-M. Smilgies 2 , John Anthony 3 , Aram Amassian 1
1 Materials Science and Engineering, Division of Physical Science and Engineering, KAUST, Thuwal Saudi Arabia, 2 , Cornell High Energy Synchrotron Source, Ithaca, New York, United States, 3 Chemistry, University of kentucky, Lexington, Kentucky, United States
Show AbstractLow-cost solution processes are deemed to be crucial to the future commercial success of organic electronics and photovoltaics. As such, solution processing of small-molecules, polymers and polymer-molecule blend thin films deserves special attention. The liquid environment, in contrast to vacuum and low-pressure environments, does not lend itself well to in situ probing via traditional surface science tools. We have therefore developed alternative strategies to investigate solution processes (drop- and spin-casting) by simultaneously monitoring the formation of the thin film (i.e., heterogenous nucleation, deposition rate, solvation, crystallization, mosaicity, texture and phase separation) in relation to the state of the solution (i.e., evaporation rate, concentration, aggregation/nucleation) and processing conditions. To do so, we have combined powerful techniques such as fast, in situ optical reflectometry, with quartz crystal microbalance, and/or grazing incidence X-ray scattering, thus gaining unprecedented insight into mechanisms and kinetics of self-assembly, crystallization, and thin film formation. Our results provide new insight into the formation of model solution-cast thin film systems of relevance to organic thin film transistors (e.g., TIPS-pentacene, TES-F-ADT, P3HT) and organic solar cells (e.g., P3HT/PCBM blends) prepared via drop- and spin-casting. Our results point to important differences between the growth behavior of polymers and small-molecules. The benefits of this insight are demonstrated through concrete examples, including carrier transport in OTFTs and efficiency of solar cells.
3:45 PM - E10.5
Structural and Electronic Properties of Poly(3-hexylthiophene) (rr-P3HT) and Poly(3-hexylselenophene) (rr-P3HS) Crystals from First-principles.
Takao Tsumuraya 1 , Jung-Hwan Song 1 , Arthur Freeman 1 2
1 Department of Physics and Astronomy, Northwestern University, Evanston, Illinois, United States, 2 Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States
Show AbstractFunctional conducting polymers, like regio-regular-poly(3-hexylthiophene) (rr-P3HT), are of growing interest in a variety of applications for organic electronic devices, such as electron-donors in organic bulk heterojunction (BHJ) photovoltaic cells. [1] To improve power conversion efficiencies of the BHJ solar cells, it is necessary to understand and develop electron-donor polymers with suitable band gaps that can capture more photon flux from the solar spectrum. More recently, some experimental groups have extensively studied regio-regular-poly(3-hexylselenophene) (rr-P3HS) as a promising alternative to the rr-P3HT. [2] The bonding nature and crystal structure are of broad interest and fundamental importance for understanding the electronic properties of the polymers; however, structural parameters of the polymers, including space group and atomic positions, have yet to be unambiguously characterized. In the present paper, we propose two possible crystal structures of rr-P3HT using periodic boundary conditions analogous with that of poly(3-butylthiophene) (P3BT). [3] One of these unit cells is a base-centered monoclinic structure (50 atoms/cell, space group C2 (No.4)), and the other is a base-centered orthorhombic structure (100 atoms/cell, C2221 (No.20)). We investigated the stability of the two structures from first-principles density functional calculations using the all-electron full-potential linearized augmented plane wave (FLAPW) method. [4] We found that the monoclinic structure is more stable than the orthorhombic one, and performed full structural optimization for the monoclinic structure of both rr-P3HT and rr-P3HS including the monoclinic angle. The electronic and transport properties of the optimized crystal structures are discussed by calculating their electronic band structures, conduction/valence band topologies and anisotropic electron/hole effective mass. We found that the top of the valence band and bottom of the conduction band are mainly composed of extended p-orbitals of carbon and sulfur atoms for thiophenes and selenium atoms for selenophenes, which may affect the mobility of electrons and holes and photo-excitations. Lastly, we report on the relation between the electronic structure and structural differences of rr-P3HT and rr-P3HS. Acknowledgements: This work is supported by Argonne-Northwestern Solar Energy Research (ANSER) Center (an Energy Frontier Research Center sponsored by the Office of Basic Energy Sciences, U.S. Dept. of Energy).[1] G. Li, et al. , Nature Mater. 4, 864 (2005).[2] A. Patra and M. Bendikov, J. Mater. Chem. 20, 422 (2010).[3] P. Arosio et al., Chem. Mater 21, 78 (2009). [4] E. Wimmer, H. Krakauer, M. Weinert, and A. J. Freeman, Phys. Rev. B 24, 864 (1981).
4:00 PM - E10: TFTs
BREAK
4:30 PM - E10.6
Surface Potential Mapping of SAM-functionalized Organic Semiconductors by Kelvin Probe Force Microscopy.
David Ellison 1 , Bumsu Lee 2 , V. Podzorov 2 , C. Daniel Frisbie 1
1 CEMS, University of Minnesota, Minneapolis, Minnesota, United States, 2 Physics, Rutgers University, Piscataway, New Jersey, United States
Show AbstractPreviously, it has been shown that formation of self assembled monolayers (SAMs) of alkyl-silanes on the surfaces of organic molecular crystals results in an orders-of-magnitude increase in the surface conductivity of these semiconducting materials. The magnitude of this effect depends on the chemical structure of the alkyl tail of the SAM: where alkyls functionalized with more electron withdrawing groups induce a more pronounced surface conductivity change in p-type organic semiconductors. This effect can be understood as an interfacial, chemical doping where electrons are withdrawn from the organic semiconductor molecule (rubrene for the current study) by the silane derivative. The efficacy of this process depends on how electron withdrawing the silane is and can be related to the magnitude of the net dipole moment of the SAM-rubrene molecule. We show that the surface dipole of the SAM can also be viewed qualitatively as an effective electric field at the interface that induces a dense sheet of holes in the underlying semiconductor and directly influences the surface conductivity. We can quantify the direction and magnitude of this field for a set of fluorinated and un-fluorinated alkyl-silanes by Kelvin Probe Force Microscopy (KFM). KFM is a surface sensitive technique that provides surface potential information in relation to the sample morphology and is ideal for probing structure-property relationships for layered systems at the nanometer scale. The combination of surface topographic and surface potential information provided by KFM is a powerful approach to understanding the impact of microstructure and adsorbates on the electrical properties of organic semiconductors.
4:45 PM - E10.7
Morphology of Conjugated Polymers at the Single-molecule Level.
Johanna Brazard 1 , Jan Vogelsang 1 , Takuji Adachi 1 , Joshua Bolinger 1 , Paul Barbara 1
1 NST, University of Texas at Austin, Austin, Texas, United States
Show AbstractConjugated polymers are potential organic materials for low-cost devices such as OLEDs, PVs, etc. Since the morphology of conjugated polymers is highly correlated with device functionality, the characterization and control of their morphology are key parameters to their utilization in devices. Polarization modulation excitation spectroscopy allows characterization of the efficiency of folding for single conjugated polymer chains of poly[2-methoxy-5-(2’-ethylhexyl)oxy-1,4-phenylene-vinylene] (MEH-PPV) embedded in PMMA host matrix. We observed that MEH-PPV chains are highly organized in some nano-domains. Real time measurements of solvent annealing were provided with a gas flow system coupled with a fluorescence microscope, which offers simultaneous solvent annealing and single molecule detection. Most MEH-PPV single chains demonstrate higher absorption anisotropy following solvent annealing, suggesting a higher degree of conformational order. Further, during solvent annealing, the chains swell and an increase of the fluorescence intensity is observed. This new experiment opens the door to understanding the process of solvent annealing at the single molecule level and a fundamental insight to the relation between photophysical/electronical parameters and morphology can be obtained.
5:00 PM - E10.8
Grain Nucleation and Growth Effects on Charge Transport in Solution-processed Organic Semiconductor Thin Films.
Songtao Wo 1 , Lan Zhou 1 , John Anthony 2 , Randall Headrick 1
1 Department of Physics, University of Vermont, Burlington, Vermont, United States, 2 Department of Chemistry, University of Kentucky, Lexington, Kentucky, United States
Show AbstractThe hollow rectangular capillary method for film deposition from solution is used to controllably vary the grain size, grain orientation, and thickness of films over a wide range. We will report results for thin films of 6,13-bis(triisopropyl-silylethynyl) pentacene with grain sizes from a few microns to mm-scale in which the thin film transistor field effect mobility is strongly correlated with grain size and changes by 4 orders of magnitude, up to as high as ~1 cm2/V-s. This striking dependence on grain morphology is explained by several effects, including carrier scattering at grain boundaries, dislocations or other defects within the grains, and grain orientation due to the anisotropic mobility which is intrinsic to the material. In order to understand the variation in film structure and carrier mobility at a fundamental level, we have also carried out an extensive study of the deposition process itself, which is related to widely used processes such as blade coating, using real-time video microscopy to monitor film deposition over 4 orders of magnitude of writing speed. The results reveal two distinct regimes governing the film thickness: In the slow regime, convective effects dominate and the films become thinner as the writing speed increases up to a crossover to the fast regime. In the fast regime, the results are in agreement with the Landau-Levich-Derjaguin (LLD) theory where viscous forces dominate and the film thickness increases with speed. Significantly, the video microscopy results reveal a profound change in film nucleation between the two regimes, which ultimately determines the physical properties of the films mentioned above, i.e. grain size, grain orientation, crystallinity, and TFT mobility.
5:15 PM - E10.9
Towards Understanding the Structure-property Relationships of Dibenzo[b,def]chrysene Organic Semiconductor Derivatives for Application in Organic Electronics.
Gavin Collis 1 , Mark Bown 1 , Christopher Dunn 1 , Tino Ehlig 1 , Giovanni Fanchini 1 , Craig Forsyth 2 , Craig Francis 1 , Manuela Jorg 1 , Peter Kemppinen 1 , Katalin Hegedus 1 , Melissa Skidmore 1 , Th. Birendra Singh 1 , Scott Watkins 1 , Gerard Wilson 1 , Kevin Winzenberg 1
1 Molecular and Health Technologies, CSIRO, Melbourne, Victoria, Australia, 2 School of Chemistry, Monash University, Melbourne, Victoria, Australia
Show AbstractThe development of new p- and n-type organic semiconductor materials that are high performance, yet amenable to commercial uptake is a challenging task. Understanding the structure-property relationships in organic semiconductor materials could provide an opportunity to design and develop new materials for use in OFETs, OPVs and OLEDs. The integration of our organic electronics projects enables us to make and evaluate materials in varied device applications and to probe some fundamental challenges. The presentation will describe the synthesis and characterisation of a class of small molecule polyaromatic hydrocarbons, the dibenzo[b,def]chrysenes (DBC). These materials were initially evaluated in BHJ devices where varying the functional group on the DBC led to a variation in solar cell performance, producing efficiencies ranging from 0.16-2.-25% PCEs. The results also indicated that solvent choice and DBC:PCBM blend ratio were important to achieve satisfactory performance with two DBC compounds. Further studies were then undertaken with Photo-CELIV and OFET devices. Photo-CELIV showed that the DBC derivatives behave as p-type material with bulk mobilities in the order of 10-4 to 10-3 cm2/Vs, while OFET devices afforded moderate mobilities from 10-5 to 10-3 cm2/Vs. These results suggest a subtle structure-property relationship. The most significant results were obtained from Photo-CELIV data of DCB derivatives:PCBM blends where the hole mobility was observed to drop by two orders of magnitude. This decrease in mobility was attributed to the disruption of the crystal packing on the pure DBC caused by the introduction of PCBM. These findings prompted us to investigate bilayer devices where the morphology and mobility could be retained. Preliminary results have produced PCEs in the range of ~2%. Our current understanding of structure-property relationship in DBCs and how this relates to Phot-oCELIV, OFET and OPV (BHJ and Bilayer devices) will be presented.
5:30 PM - **E10.10
Mechanism of Improved Electrical Properties of Small Molecule - Polymer Blend Semiconductors .
Do Yoon 1 , Yeon Sook Chung 1 , Nayool Shin 1 , Young Eun Jo 1 , Jihoon Kang 1 , Vivek Prabhu 2 , Dean Delongchamp 2 , Joe Kline 2 , Lee Richter 2 , David Gundlach 2 , John Anthony 3
1 Department of Chemistry, Seoul National University, Seoul Korea (the Republic of), 2 , National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 3 Department of Chemistry, University of Kentucky, Lexington, Kentucky, United States
Show AbstractThe blend films of small molecule semiconductors with insulating polymers exhibit not only an excellent solution processability, but also superior performance characteristics (field-effect mobility, on/off ratio, threshold voltage and stability) over those of neat small molecule semiconductors. We have investigated the origin of such improved electrical properties by measuring the vertical phase segregation profile and also monitoring the effects of organic impurities in blend semiconductor films. The neutron reflectivity and variable-angle spectroscopic ellipsometry reveal the presence of vertically segregated zones of crystalline pure organic semiconductor layer and mixed amorphous layer of small molecule semiconductor and insulating polymer in blend films. The results of DSC, WAXD, GIXD, and OTFT measurements indicate that the formation of such vertically separated zones adjacent to the gate dielectric surface allows the impurity species to remain preferentially in the mixed polymer-rich amorphous layer, thereby removing the impurities from the active channel layer. Such “zone-refinement” effect would remove both organic impurities and ionic charges, which are detrimental to the OTFT performance but are very difficult to remove completely from the sample solutions.
E11: Poster Session: Thin Film Transistors
Session Chairs
Friday AM, December 03, 2010
Exhibition Hall D (Hynes)
9:00 PM - E11.1
Electrical Conductance of Single Oligothiophene Molecular Wires.
S. Lee 1 , Ryo Yamada 1 , Hirokazu Tada 1 , Shoji Tanaka 2
1 Graduate School of Engineering Science, Osaka University, Toyonaka-Shi, Osaka, Japan, 2 Research Center for Molecular Scale Nanoscience, Institute for Molecular Science, Okazaki, Aichi, Japan
Show AbstractSince the first proposal for the concept of single molecule devices by Aviram and Ratner [1], great efforts have been made to understand the mechanism of charge transport of single molecules bonded between metal electrodes. Theoretical studies have predicted the crossover of coherent tunneling and decoherent transport mechanism [2]. It is important to clarify the existence of the crossover. Our group has previously shown that scaling of conductance behaviour of oligothiophene molecular wires with the lengths ranging from 2 nm to 9 nm (m=1~7) changed from exponential to linear at the molecular length of approximately 5.6nm [3,4]. In the present study, we have measured the temperature dependence of conductance of the wires to clarify the charge carrier transport mechanism.In our experiment, break junction measurements at various temperatures were carried out by using a home-built scanning tunneling microscope (STM). We have analyzed the results on the temperature dependence of the conductance of 5T-di-SCN, 14T-di-SCN and 17T-di-SCN molecules and have concluded that the conductance value of 5T-di-SCN and 14T-di-SCN does not show temperature dependence which indicates that the carrier transport is dominated by tunneling while conductance of 17T-di-SCN shows an exponential dependence on the temperature. The activation energy was estimated to be 0.3eV, which indicates that the carrier transport is hopping based on the conformational gating.1. A. Aviram and M. Ratner, Chem. Phys. Lett. 29, 277–283 (1974).2. D. Segal, A. Nitzan, and W. B. Davis, J. Phys. Chem. B 104, 2790 (2000).3. R. Yamada, H. Kumazawa, T. Noutoshi, S. Tanaka, and H. Tada, Nano. Lett., 8, 1237 (2008). 4. R. Yamada, H. Kumazawa, S. Tanaka, and H. Tada, Appl. Phys. Express. 2, 025002 (2009)
9:00 PM - E11.10
Flexible Organic Transistors Functioned at Low Voltage with Low-temperature Cross-linked High-k polymer Cyanoethyled Pullulan as a Gate Insulator on Stainless Steel Substrates.
Wentao Xu 1 , Chang Guo 1 , Shi-Woo Rhee 1
1 Chemical Engineering, Pohang University of Science and Technology, Pohang Korea (the Republic of)
Show AbstractHigh-dielectric constant (high-k) gate insulators have been proposed in recent years to achieve low-voltage operable organic thin-film transistors (OTFT). However, most of the high-k components were inorganic based materials, which always require high temperature or vacuum processes. On the other hand, candidates of high-k polymer gate insulators were rarely reported, which can be easily solution-processed at low temperatures and have good flexibility. In this presentation, we demonstrate a series of polymeric high-k gate insulators based on blends of a high-k polymer cyanoethylated pullulan (k~16.5) and various cross-linking agents, and the fabrication of flexible pentacene TFTs on chemical mechanical polished (CMP)stainless steel substrates (STS). The gate insulators were spin-casted and cross-linked at low temperatures (70~200 °C), as monitored by thin film FT-IR in a transmittion mode. With low drain voltage (1~3 V), the TFTs showed high mobility (μ) 1.3~2.2 cm<2>/Vs, on/off current ratio (Ion/Ioff) ~10<5>, and subthreshold slope (SS) of (0.076~0.098 V/dec), which might be the lowest SS with a polymer gate insulator so far, approaching the theoretical minimum of 0.057 V/dec. The bending test was conducted by measuring electrical performance of bended device, whose substrates were attached to out-surface of glass arcs with different diameters. Only very slight degradation in μ and Ion/Ioff was observed during the bending test, which proved a good mechanical robustness of the cross-linked polymeric gate insulators. STS is one of the flexible substrates with high corrosion resistance, high conductivity and thermal stability, and it can be smooth enough to serve as a gate electrode through chemical mechanical polishing process. Its excellent thermal conductivity as a metal is helpful for the thermal management of the organic device.
9:00 PM - E11.11
New Solution Processible Polycyclic Aromatic Hydrocarbon Semiconducting Material: Synthesis and Device Characteristics.
Melissa Skidmore 1 , Christopher Dunn 1 , Peter Kemppinen 1 , Birendra Singh 1 , Kevin Winzenberg 1
1 Molecular and Health Technologies, CSIRO, Clayton South, Victoria, Australia
Show AbstractThe cost-effective production of flexible electronic components and fabrication of devices over large areas or on lightweight flexible substrates is reliant on the development of new organic semiconducting materials. To realise the potential of organic electronic devices new high purity semiconducting material that can be synthesized in simple, industrially scalable processes from cheap, widely available precursors or simple starting material are required. Towards this aim we have identified a novel polycyclic aromatic hydrocarbon based on a rubicene core as a possible candidate for use in electronic devices. The synthesis of these solution processible rubicene derivatives is readily achieved in four moderate to high yielding steps, starting from commercially available materials.This paper will discuss the synthesis and device characteristics of these materials.
9:00 PM - E11.12
Surface Morphology and Crystallization of Poly(3-hexylthiophene) in Polymer Blend Films.
Yasunari Tamai 1 , Hiroyuki Aoki 1 , Hideo Ohkita 1 2 , Hiroaki Benten 1 , Shinzaburo Ito 1
1 Department of Polymer Chemistry, Kyoto University, Kyoto Japan, 2 , PRESTO, JST, Saitama Japan
Show AbstractRegioregular poly(3-hexylthiophene) (P3HT) is the most widely studied conjugated polymer for organic electronic devices, such as organic photovoltaic devices and field-effect transistors, in which the blend morphology and the crystallinity of P3HT are key factors to improve the device performance. In this study, we investigated the surface morphology of P3HT in blend films with poly(alkyl methacrylate)s (PAMAs) by atomic force microscopy. For P3HT/PMMA blend films, the film surface was almost covered with P3HT even at only 1wt% of P3HT. Moreover, highly crystalline fibrillar structures were observed for blend films containing 1wt% P3HT. UV-Vis absorption spectra indicated that the crystallinity of P3HT increased with decreasing P3HT contents in P3HT/PMMA blend films. On the other hand, the crystallinity of P3HT/PBMA blend films containing 1wt% P3HT was as low as that of pure P3HT film. The difference in the crystallinity came from the different phase segregation as follows: for P3HT/PMMA blend films, P3HT is likely to segregate to the polymer/air interface because the surface free energy of P3HT is lower than that of PMMA, while the surface segregation of P3HT is not remarkable for P3HT/PBMA blend films because surface free energy of P3HT is comparable to that of PBMA. These results suggest that the crystallization of P3HT was induced by the surface segregation.
9:00 PM - E11.13
Synthesis of a New Thiophene-based Conjugated Polymer for Electronic Device Applications.
Juhwan Kim 1 , Kang-Jun Baeg 4 , Byung-Kwan Yu 1 , Hyung-Gu Jeong 1 , Dong-Yu Kim 1 2 3
1 Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju Korea (the Republic of), 4 Convergence Components and Materials Research Laboratory, Electronics and Telecommunications Research Institute (ETRI), Daejeon Korea (the Republic of), 2 Nanobio Materials and Electronics, Gwangju Institute of Science and Technology (GIST), Gwangju Korea (the Republic of), 3 Heeger Center for Advanced Materials, Gwangju Institute of Science and Technology (GIST), Gwangju Korea (the Republic of)
Show Abstract In recent years, conjugated polymers have attracted attention for use in organic light-emitting diodes (OLEDs), organic photovoltaic cells (OPVs), and organic thin-film transistors (OTFTs) because of their suitability in a wide range of promising electronic, optical, and electrochemical applications. Therefore, a great number of research and development efforts have been devoted to the field of conjugated polymer materials due to this high potential probabilities and broad application areas. The thiophene-based polymers such as P3HT, PQT, pBTTT, and PBDTTT-CF represent the most important conjugated polymers showing a high mobility and low band gap for the applications to OPVs and OTFTs due to their excellent electrical and optical properties. This class of polymers was considered on the basic of following design rules. i) long alkyl side-chain for processability; ii) structural regularity; iii) proper control of extended π-conjugation to achieve a band gap tuning. Therefore, we synthesized a novel thiophene-based conjugated polymer containing long alkyl side-chain part for solution processabiliy and coplanar unit to achieve an extent of π-conjugation and control of band gap for electronic device applications. This polymer was synthesized via Grignard, Kumada, Mcmurry and stille coupling reactions. We investigated the basic material characterization using NMR, DSC, TGA and GPC. In addition, optical and morphological properties were analyzed using UV/Vis spectroscopy, AFM and XRD. In this presentation, we will discuss on the synthesis of a novel thiophene-based conjugated polymer and performance of electronic devices such as OPVs and OTFTs.
9:00 PM - E11.14
Dielectric Response of Complex Networks Models.
McGlennon Regis 2 1 , Carlos Pinheiro 1 , Rodrigo Bianchi 1 , Americo Bernardes 1
2 , Federal Insitute of Minas Gerais - Campus Congonhas, Congonhas, Minas Gerais, Brazil, 1 Department of Physics, Federal University of Ouro Preto, Ouro Preto, MG, Brazil
Show AbstractMany systems around us display rather complex topologies that often seem random and unpredictable. Complex networks are currently being studied across many fields of science. Undoubtedly, many systems in nature can be described by models of complex networks, which are structures consisting of nodes or vertices connected by links or edges. This work presents an investigation into the frequency dependent properties of complex networks as the Erdös-Rényi and Scale-free networks. The conduction processes are simulated on a (random and scale-free) network composed by electrical circuits. The circuits are formed by resistors and capacitors become related with the displacement current or only resistors associated with the conduction current. Capacitances and resistances are randomly attributed following some statistical distribution. The source terminals in the complex network are based on the intuitive notion of peripheral and central regions proposed as two different load modes. In the first load mode the source terminal is applied between the central and the periphery terminal while in the second the source terminal is applied between P1 and P2 (the two halves of the peripheral region). We also investigated the some parameters (as nodes and links number) on the simulation of the electrical properties from disordered materials yet. The results indicated that the electrical properties can be simulated by a statistical model based on complex networks. The theoretical results are similar to experimental results obtained from semiconducting polymers, such as polyaniline. Both results indicated that interchain processes govern the resistivity behavior in the low-frequency region while, for higher frequencies, intrachain mechanisms are dominant. The model intends to take the inherent inhomogeneity of scale-free networks to takes into account the structure aspects of the real polymer, and simulate the transport mechanisms in disordered materials. Work supported by CNPq, FAPEMIG and CAPES. [1] A.-L. Barabási and R. Albert. Science, 286 (1999) p. 509512.[2] C. F. S. Pinheiro and A. T. Bernardes, Phys. Rev. E, 72 (2005), p. 046709.
9:00 PM - E11.15
Synthesis and Characterization of New Heteroacene Polymers with Intermolecular Interdigitation for High Crystalline Phase.
Hyung-Gu Jeong 1 , Dong-Yoon Khim 1 , Byung-Kwan Yu 1 , Jin-Mun Yun 1 , Juhwan Kim 1 , Dong-Yu Kim 1 2 3
1 Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of), 2 Nanobio Materials and Electronics, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of), 3 Heeger Center for Advanced Materials, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of)
Show Abstract Heteroacenes are potentially attractive as electron-transporting materials in organic electronics. In particular, thiophene-based heteroacenes would be more attractive materials in thin-film transistor due to their coplanar structure and pronounced molecular assembly through the intermolecular π-interactions and sulfur-sulfur (S-S) interactions. To achieve high charge mobility, the chain packing is clearly an important factor, but the introduction of suitable alkyl chains for solubility could preclude molecular ordering and polymerization. Therefore, we synthesized thiophene-based heteroacenes with pendant alkyl chains along the conjugated backbone to facilitate molecular ordering through intermolecular interdigitation. Synthesized heteroacenes were characterized by NMR, FT-IR spectroscopy, GPC, DSC, and electrochemical and optical properties were studied by cyclic voltammetry, UV-Vis absorption and photoluminescence, and FET performance will be discussed.
9:00 PM - E11.16
Improved Thermoelectric Performance of Organic Thin-film Elements Utilizing a Bilayer Structure of Pentacene and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane.
Mao Sumino 1 2 , Kentaro Harada 1 2 , Koji Miyazaki 3 2 , Chihaya Adachi 1 2
1 Center for Organic Photonics and Electronics Research, Kyushu University, Fukuoka Japan, 2 , Bio Electromechanical Autonomous Nano Systems (BEANS) Project, Fukuoka Japan, 3 Department of Biological Functions and Engineering, Kyushu Institute of Technolohy, Kitakyushu Japan
Show AbstractThermoelectric properties of organic materials have occasionally been addressed to elucidate the fundamental carrier transport mechanism. While potentially high thermoelectric performances have been reported for some polymeric systems, challenges to small molecule-based thermoelectric devices have so far rarely been addressed. In this study, we demonstrate an improved thermoelectric performance of small molecular thin films fabricated by thermal deposition of pentacene as the p-type conduction layer. In order to enhance the performance, a bilayer structure composed of an intrinsic pentacene layer and an acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane layer is utilized as the prototype thermoelectric element. With the bilayer structure, the electrical conductivity reaches 0.43 S/cm, exhibiting a positive Seebeck coefficient of about 200 microV/K. We thus obtain a high power factor of 2.0 microW/mK with an optimized layer thickness.
9:00 PM - E11.17
Synthesis and Characterization of Solution-processible Pentacene Based Organic Semiconductors.
Heung Gyu Kim 1 , Hyun Ho Choi 1 , Kilwon Cho 1 , E-Joon Choi 2
1 Department of Chemical Engineering, Pohang University of Science and Technology, Pohang Korea (the Republic of), 2 Department of Polymer Science and Engineering, Kumoh National Institute of Technology, Gumi Korea (the Republic of)
Show AbstractIn order to obtain enhanced oxidative stability and increased solubility of pentacene, 6,13-disubstituted pentacene derivatives were synthesized and the structures of the compounds were characterized by FT-IR, NMR spectroscopy. The synthesized pentacene derivatives were soluble to organic solvents such as THF, dichloromethane and chloroform. Oxidative stability of the synthesized pentacene derivatives was investigated in solution by using UV-vis spectroscopy. The oxidation of synthesized pentacene derivatives was very slow compared to pentacene and the color could be even observed after 3-5 days. Charge carrier mobility and on/off current ratios of the fabricated device based on synthesized pentacene derivatives were studied systematically.
9:00 PM - E11.19
Low Voltage Organic Transistor with Minimal Gate Dielectric Oxide and Bi-functional Organic Semiconductor.
Byung Jun Jung 1 , Bal Mukund Dhar 1 , Josue Martinez 1 , Jia Sun 1 , Thomas Dawidczyk 1 , Kevin See 1 , Howard Katz 1
1 Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland, United States
Show AbstractWe designed new n-channel naphthalenetetracarboxylic diimide semiconductor molecules with long fluoroalkyl and semifluorinated side chains. These side chains not only aid in self-assembly and kinetically stabilize injected electrons, but also act as part of the gate dielectric. On Si substrates with only the native oxide (2 nm thick), NTCDI semiconductor films were deposited with thicknesses from 17 nm to 60 nm. Top contact Au electrodes were deposited as sources and drains. The devices showed good transistor characteristics in air with 0.1-1 μA of drain current at 0.5V of VG and VDS, and W/L from 12 to 20, even though channel width (250 μm) is over 1000 times the distance (20 nm) between gate and drain electrodes. The gate leakage was as little as 20 nA. The extracted capacitance times mobility product, defined as the sheet transconductance, can exceed 100 nS/V, two orders of magnitude higher than typical organic transistors. The thickness dependence of the sheet transconductance provides information about the mobilities of the individual semiconductor layers. In addition, the response of the devices to explosive vapors will be presented as an approach to electronic vapor sensing at 0.5 V.
9:00 PM - E11.2
Field Effect Transistors of Octathio[8]circulene with an Ionic Liquid Gate Insurator.
Takuya Fujimoto 1 , Michio Matsushita 1 , Kunio Awaga 2
1 Department of Chemistry, Nagoya University, Nagoya Japan, 2 Research Center for Material Science, Nagoya University, Nagoya Japan
Show AbstractCarbon-sulfur compounds have been studied extensively as key materials in organic/molecular electronics, to develop field effect transistor (FET) and light-emitting diodes. Octathio[8]circulene, called “sulflower,” is a new molecule that is attracting much attention from various viewpoints such as orbital degeneracy and the intermolecular packing caused by the sulfur atoms exposed to the outside of the molecular rings. In the present work, we carried out transistor fabrications of the thin films of sulflower. Thin films were obtained by vacuum vapor deposition, and thin-film XRD indicated a lamellar structure in which the (011) planes are parallel to substrates. We fabricated thin film FETs of sulflower using ionic liquid as a gate dielectric layer. While this technique has not yet been applied to organic thin films, due to their solubility in ionic liquids, the persistency of the thin films of sulflower enabled us to carry out this experiment. We obtained stable FET performance with a relatively high mobility and a very low-power operation. We also fabricated the dual-gate thin film FETs of sulflower using ionic liquid as a top and SiO2 as a bottom gate dielectric layer, respectively. This operation led to better transistor performance with high mobility, a high on/off ratio, and low power operation, thus surpassing the performance of the single-gate FETs of sulflower.
9:00 PM - E11.20
Effect of Metal Nanocrystal Thickness on Small-molecule Nonvolatile Memory Embedded with Ni Nanocrystals Surrounded by Ni Oxide.
Kwang Hee Park 1 , Sung-Ho Seo 1 , Woo-Sik Nam 1 , Jong-Sun Lee 1 , Jea-Gun Park 1
1 , Hanyang University, Seoul Korea (the Republic of)
Show AbstractCurrently, an organic nonvolatile memory has attracted much interest as one of candidate devices for next generation nonvolatile memory because of its simple process, small device size, and high speed. This study shows a process to form Ni nanocrystals layer having regular shaped and uniformly distributed nanocrystals using in-situ O2 plasma oxidation. In addition, we investigate the effect of the metal nanocrystals thickness for electrical characteristics. We developed small-molecule nonvolatile memory-cells fabricated with the sandwiched structure of Al / Alq3 (Aluminum tris(8-hydroxyquinoline)) / Ni nanocrystals surrounded by NiO / Alq3 / Al. The metal nanocrystals layer was deposited by using a high-vacuum thermal evaporation method and the amorphous oxide layer was formed by O2 plasma oxidation method. To confirming the effect of metal nanocrystals thickness, Ni nanocrystals layer formed with various thicknesses, i.e., about 3, 8, and 10 nm.As a result, increasing Ni nanocrystals thickness, average size of Ni nanocrystals is increased. However, Ni nanocrystals for 8 nm thickness show the smallest deviation of Ni nanocrytals size and the highest density of Ni nanocrystals. The small-molecule nonvolatile memory embedded with Ni nanocrystals layer of 8 nm thickness represents symmetric memory characteristics, stable I-V curve and good endurance cycles. On the other hands, the small-molecule nonvolatile memories embedded with Ni nanocrystals layer of 3 and 10 nm thickness show unstable memory characteristics. In conclusion, we can fabricate the well-isolated and uniformly distributed Ni nanocrystals by the optimization of metal nanocrystals thickness and it does affect significantly to the small-molecule nonvolatile memory characteristics.*AcknowledgementThis research was supported by “The National Research Program for Terabit Nonvolatile Memory Development” sponsored by the Korean Ministry of Knowledge Economy.
9:00 PM - E11.21
Nonvolatile Hybrid Memory-cell Fabricated with Ni Nanocrystals Embedded in Conductive Polymer.
Hyun-Min Seung 1 , Jong-Dae Lee 2 , Kyoung-Cheol Kwon 1 , Jae-Sung Lim 2 , Jung-Nam Lee 2 , Jea-Gun Park 2
1 Division of Nanoscale Semiconductor Engineering and Tera-bit Nonvolatile Memory Development, Hanyang University, Seoul Korea (the Republic of), 2 Department of Electrical & Computer Engineering, Hanyang University, Seoul Korea (the Republic of)
Show AbstractWe researched nonvolatile hybrid polymer memory cells embedded with Ni-nanocrystals. The 4F2 hybrid polymer memory-cells were fabricated with the device structure of conductive polymer (P3HT, Poly(3-hexylthiophene)) embedded with Ni-nanocrystals surrounded by nickel oxide tunneling barrier between Al upper and bottom electrodes. The nickel oxide tunneling barriers were produced by low rate thermal evaporation of Ni layer and following in-situ O2 plasma oxidation process after deposition of bottom electrode and bottom P3HT layer. The 4F2 hybrid polymer memory-cells showed non-volatile memory characteristics such as memory margin(Ion/off) of 5 × 101 , retention time more than 105 sec and more than 103 endurance cycles of program and erase. The hybrid polymer memory cells with P3HT indicate improved nonvolatile memory characteristics than hybrid polymer memory cell with PS(Poly Styrene) that we reported before. In our study, we will suggest the current conduction mechanism by fitting the current-voltage curve.Acknowledgement* This research was supported by "The National Research Program for Terabit Nonvolatile Memory Development” sponsored by the Korean Ministry of Knowledge Economy.
9:00 PM - E11.22
Crystal Structure Analysis of Small Molecule Aromatic Organic Semiconductor Materials for Application in Organic Electronics.
Rachel Williamson 1 , Mark Bown 2 , Gavin Collis 2 , Craig Forsyth 3 , Craig Francis 2 , Manuela Jorg 2 , Peter Kemppinen 2 , Melissa Skidmore 2 , Anthony Wilson 2 , Kevin Winzenberg 2
1 , Australian Synchrotron, Clayton, Victoria, Australia, 2 Molecular and Health Technologies, CSIRO, Clayton South, Victoria, Australia, 3 School of Chemistry, Monash University, Clayton, Victoria, Australia
Show AbstractThe Australian Synchrotron was officially opened in 2007 and currently consists of 9 beamlines, from infrared to hard X-rays, used for a wide-range of analytical techniques. The Macromolecular Crystallography and Micro Crystallography beamlines at the Australian Synchrotron are capable of rapid data collection on samples ranging from proteins to inorganic complexes and organic drug molecules. The finely focused X-ray beam of the Micro Crystallography beamline is ideal for high-resolution structure determination of extremely small crystals. The intense beam allows for the determination of single crystal structures of small molecule samples from crystals less than 1 micron in size, enabling structure determination for samples that are otherwise not suitable for single crystal analysis such as powder grains, micro needles and other micro-crystals. A collaboration has recently been established between the Australian Synchrotron and the Australian government laboratory, CSIRO, to undertake research in the area of organic electronics. The charge-carrying properties, or mobilities, of organic semi-conductor materials are highly reliant on many parameters, such as their thin-film morphology. Thin-film morphology is dependent on the deposition method, e.g. drop-casting, spin-coating, or evaporation. The functional groups and structural framework of the organic semi-conductor will also determine how these materials will pack. A series of compounds will be presented showing the structure and crystal packing of a range of novel materials for organic electronic applications.
9:00 PM - E11.23
Modeling and Extraction of Material Properties of Organic Thin Films in Devices.
Mark Barycza 1 , Norman Gunther 1 , Darrell Niemann 1 , Mahmudur Rahman 1
1 Electrical Engineering, Santa Clara University, Santa Clara, California, United States
Show AbstractThere has been widespread interest in using organic polymers in electronic devices due to ease of manufacturability and inexpensive fabrication. Typically, material properties of thin films are characterized before incorporation into devices. This can be problematic because of higher order effects, which may alter these properties during and after fabrication of the devices. The Capacitance-Voltage (CV) characteristic is an established analytic technique for investigating properties of electronic devices. Here, we present an application of our variational thermodynamic model to systematically characterize the CV of Metal-Insulator-Silicon (MIS) devices to extract properties of their constituent materials. Our approach allows us to extract the free charge density and distribution as well as effective dielectric constants and film thicknesses through the measurement and modeling of the CV characteristic of the device.Of particular interest are the processes under which the organic electronic devices degrade over time. Our model provides insights into the changes of the organic polymer as the device ages. We believe that the primary degradation phenomenon is oxidation of the organic materials. The film degradation manifests as a decrease in the total free charge in the polymer and causes a decrease in the effective dielectric constant. While it currently does not explain the origin of the free charges, our model importantly extracts quantitative limits on the free charge density magnitude and distribution in the polymer as a function of film material, applied voltage, frequency, and film thickness. We present an analysis of experimental devices exhibiting such degradation over the course of a month. In summary, we have applied our variational thermodynamic approach to model MIS devices with varying polymer thickness. Using our model, we extract properties of the devices as well as their constituent materials. We also model the frequency response of the free charges in the device as a function of the frequency of the applied voltage. Our model has been verified with empirically collected data on a number of thin film organic semiconductor devices and with further refinement should be able to provide insights into the charge interaction at the polymer interfaces.
9:00 PM - E11.24
Identifying Charge-trap Precursors in Polycrystalline Pentacene Films Using Electric Force Microscopy.
Justin Luria 1 , Louisa Brown 1 , Katie Schwarz 2 , Richard Hennig 2 , John Marohn 1
1 Chemistry and Chemical Biology, Cornell University, Ithaca, New York, United States, 2 Materials Science and Engineering, Cornell University, Ithaca, New York, United States
Show AbstractCharge trapping in organic devices causes a wide array of functional problems, but remains poorly understood. It has recently been proposed that pentacene shares a common trapping mechanism with other organics [1]. In order to characterize the nature of charge trapping, we studied polycrystalline pentacene transistors using time-resolved electric force microscopy to measure trap-clearing rates during irradiation as a function of the wavelength of the irradiating light. We report spatially-resolved action spectra for trap clearing in polycrystalline pentacene. By comparing the observed trap-clearing spectra to results obtained from time-dependent density functional theory, we draw conclusions about likely trapping mechanisms at play in pentacene. To confirm the identity of the trapping species in pentacene, we synthesize and co-evaporate trap precursors and compare with observed de-trapping spectra. [2,3][1] Mathijssen, S. et al. Adv. Mater. 19, 2785 (2007).[2] J.E Northrup and M.L. Chabinyc, Phys. Rev. B 68, 041202 (2003).[3] M.J. Jaquith, E.M. Muller, and J.A. Marohn, J. Phys. Chem. B 111, 7711 (2007).
9:00 PM - E11.25
Charge Transport in Ambipolar Pentacene Thin Film Transistors.
Ronak Rahimi 1 , Dimitris Korakakis 1 2
1 Lane Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, West Virginia, United States, 2 , National Energy Technology Laboratory, Morgantown, West Virginia, United States
Show AbstractAmbipolar organic transistors are technologically interesting because of their potential applications in light-emitting field-effect transistors [1] and complementary-metal-oxide-semiconductor (CMOS) devices by providing ease of design, low cost of fabrication, and flexibility [2]. Although common organic semiconductors show either n- or p-type charge transport characteristic, organic transistors with ambipolar characteristics have been reported recently. Ambipolar transport in organic transistors can be achieved by several methods: utilizing blends or bilayers of n-type or p-type organic materials [2,3]; employing a high or a low work function metal, respectively, for hole or electron injection [4,5]; or using different dielectric layers and manipulating semiconductor-dielectric interfacial properties by adding an additional layer between organic and dielectric layers [6]. In this work, we show that ambipolar transport can be achieved within a single transistor channel using LiF gate dielectric in the transistors with pentacene active layer. This ambipolar behavior can be controlled by the applied source-drain and gate biases. It was found that at low source-drain biases multistep hopping is the dominant conduction mechanism, while in high voltage regimes I-V data fits in Fowler-Nordheim (FN) tunneling model. From the slop of the FN plots, the dependency between field enhancement factor and the transition point in conduction mechanism upon gate bias has been extracted. The transition points show more dependency on gate voltage for negative biases compared to the positive biases. While sweeping negative gate voltages from -5 to -20 V, the source-drain voltages change from about 27 to 17 V. On the other hand, for positive gate voltages from 5 to 20 V, the value of the transition point stays at approximately 36 V. In order to further understand the transport mechanisms, new structures with an interface layer between dielectric and active layer have been fabricated and characterized. As expected, a significant decrease in the amount of the source-drain current has been observed after introducing the interface layer.1.C. Rost, S. Karg, W. Riess, M. A. Loi, M. Murgia, and M. Muccini, Applied Physics Letters, v 85, n 9, p 1613-15 (2004).2.H. Yan, T. Kagata, and H. Okuzaki, Applied Physics Letters, v 94, p 23305 (2009).3.A. Opitz, M. Bronner, W. Brutting, Applied Physics Letters, v 101, p 063709 (2007).4.T. Yasuda, T. Goto, K. Fujita, and T. Tsutsui, Applied Physics Letters, v 85, p 2098 (2004).5.S. Noro, T. Takenobu, Y. Iwasa, H. C. Chang, S. Kitagawa, T. Akutagawa, and T. Nakamura, Advanced Materials (Weinheim, Ger.) v, 20, p 3399 (2008).6.S. R. Saudari, P. R. Frail, and C. R. Kagan, Applied Physics Letters, v 95, p 23301 (2009).
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Fabrication of Organic Devices Using Almost Damage Free Neutral Beam Etching.
Junji Adachi 1 2 , Tomohiro Kubota 3 2 , Masayuki Yahiro 4 2 , Seiji Samukawa 5 2 , Chihaya Adachi 1 2
1 Center for Organic Photonics and Electronics Research, Kyushu University, Fukuoka, Fukuoka, Japan, 2 , BEANS Laboratory, Chiyoda, Tokyo, Japan, 3 Institute of Industrial Science, The University of Tokyo, Meguro, Tokyo, Japan, 4 , Institute of Systems, Information Technologies and Nanotechnologies, Fukuoka, Fukuoka, Japan, 5 Institute of Fluid Science, Tohoku University, Sendai, Miyagi, Japan
Show AbstractNeutral beam etching technique (NBET) is an atomic layer level damage free etching technology1 ) which has been vigorously developed in silicon semiconductor devices. NBET is free from charge damage and UV irradiation damage because a holey carbon plate named aperture, which is set between a plasma chamber and a substrate, cut UV light and radicals, and exchange minus ion beams to neutral beams.Ordinary plasma beams consist of charged ion particles, radicals and UV light with high energy and cause severe damage to organic layers of organic devices. We irradiated nitrogen neutral beams on to organic light emitting diodes (OLEDs) which have cylindrical metal electrodes (MgAg(150nm)/Ag(10nm)). Organic layers of α-NPD(50nm) and Alq3(50nm) were sharply etched and cylindrical OLEDs were obtained. Metal electrodes acted as masks of organic layers. Diameters of metal electrodes are 200 micrometer, 500 micrometer and 1,000 micrometer. Organic layers were sharply and vertically etched along with the edges of electrodes. EL of etched and non-etched cylindrical OLEDs were observed. Optical and electrical characteristics were measured and confirmed that both etched and non-etched OLEDs showed similar characteristics. The results shows that NBET is a promissing dry process of organic integrated circuits with nano-scale fabrication.1) S. Samukawa, Jpn. J. Appl. Phys., 45, 2395 (2006)
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Investigation of the Charge Transport and Trapping Mechanism of Organic Field Effect Transistors During Vapor Sensing.
Davianne Duarte 1 , Brian Cobb 1 , Ananth Dodabalapur 1
1 Electrical Engineering, University of Texas Austin, Austin, Texas, United States
Show AbstractOrganic thin film transistors (TFTs) based on the field effect transistor architecture provide a methodology for sensing by exhibiting a change in the transport properties such as shifts in mobility, threshold voltage and conductivity. Chemical recognition is achievable by various methods including the two processes which we are studying, direct analyte interactions with the semiconductor and specific receptor molecules on the semiconducting surface. Previous work demonstrates the effects of carrier concentration, grain size (surface morphology), and channel length on the sensing response to analytes such as alcohols which exhibit a moderate dipole moment. When the alcohol interacts with the organic channel the addition of a trap and a positive charge occurs at the grain boundaries. At low carrier concentrations the added charge has the effect of producing an increase in current for the sensing response. At higher carrier concentrations the occurrence of trapping overwhelms the effect of the positive charge and you see and reduction in current. Typically the mobility shifts which occur during sensing are correlated with trapping for polar analytes. Another aspect of organic materials is the fine tuning of the chemical sensitivity by modifying the surface with receptor sites for specific interactions affecting the sensing behavior. Our current studies of pentacene TFT devices have been done to clarify device behavior during sensing with and without incorporating receptor molecules on the semiconducting surface. These studies include dynamic and current voltage temperature dependent measurements under various ethanol concentrations (100-2000ppm at temperatures between 300 and 350K) and probing the surface during device operation to elucidate on the physical nature which contributes to the surface potential while sensing such as the surface dipole and charge trapping. Additionally, a four-terminal hybrid silicon-organic field-effect sensing device has been studied. In the chemical memory mode this sensor depends on trapped charge in the organic semiconductor producing threshold shifts in the silicon channel that is capacitively coupled to the organic semiconductor channel. We will correlate the extent of trap charge with transport properties and sensor response. The work with this unique device will include an assessment of the incorporation of receptor molecules on the semiconducting layer for enhanced sensitivity and their affect on the surface during vapor sensing.
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Flexible Nonvolatile Memory Devices Based on Organic Thin Film Transistors and Self-assembled Metallic Nanoparticles.
Soo-Jin Kim 1 , Young-Su Park 1 , Si-Hoon Lyu 1 , Ji-Min Song 1 , Jang-Sik Lee 1
1 , Kookmin University, Seoul Korea (the Republic of)
Show AbstractWe fabricated nonvolatile memory devices based on organic thin-film transistors (OTFTs) and self-assembled gold nanoparticles. Organic nano-floating gate memory (NFGM) devices were developed based on solution-processed dielectric layers and organic pentacene TFTs on the flexible plastic substrates. Spin-coated poly(vinylphenol) (PVP) layers were used as the blocking and tunneling dielectric layers. Controlled gold nanoparticles (AuNP), which acted as charge trapping elements, were adsorbed onto the PVP blocking organic dielectric layer by a dipping process. Pentacene layer, which was used as the p-type active layer, was deposited by thermal evaporation. Patterned gold thin films were used as the gate and source/drain electrodes. The programming/erasing operations showed that the organic memory devices exhibited good programmable memory characteristics that resulted in a gate voltage controlled reliable threshold voltage shift of the programmed/erased state. In organic memory devices, the reliable operations are very important. We measured the data retention, endurance, and mechanical bending stability using the fabricated organic memory devices. The endurance, data retention, and bending cycling measurements confirmed that the flexible memory devices exhibited good electrical reliability as well as mechanical stability. Therefore, this approach could potentially be used in integrated plastic electronic circuits as well as advanced flexible/plastic electronic devices. In this presentation detailed devices fabrication and electrical/mechanical characterization will be discussed with an emphasis on the charge carrier trapping in gold nanoparticles.
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Micellar Poly(styrene-b-4-vinylpyridine)-nanoparticle Hybrid System for Non-volatile Organic Transistor Memory.
Wei Lin Leong 1 , Nripan Mathews 1 , Pooi See Lee 1 , Subodh Mhaisalkar 1
1 Division of Materials Technology, Nanyang Technological University, Singapore Singapore
Show AbstractThe implementation of plastic electronic solutions to large area displays, disposable sensor arrays, radio-frequency identification tags (RFIDs), and various smart packaging devices necessitate the development of organic memories which are solution-processable and readily integrated with the transistors for digital logic. We present organic field-effect transistor (OFET) memories where discrete gold nanoparticles in-situ synthesized in self-assembled polystyrene-block-poly-4-vinylpyridine (PS-b-P4VP) block copolymer nanodomains were used to store charges and modulate the working ranges of the transistor. This was demonstrated in both p-type pentacene and n-type perfluorinated copper phthalocyanine (F16CuPc) based OFET memory, in which the gold nanoparticles trap holes or electrons in the p- or n- channel respectively. We will also be featuring our recent work on integrating semiconducting polymers with the memory element and developing a completely solution processable memory transistor on flexible substrate, where an operating voltage below 20V, on-off ratios close to three orders and programming speeds of 1ms were attained. The results clearly demonstrate the general applicability of the floating gate element in transistor configuration. These devices exhibit the characteristics needed to satisfy the new demands for memory application and indicate the usefulness of the utilization of nanoparticle-polymer composite for organic memory device fabrication.
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Molecular Conductance Switching Processes of a Ru(II) Alkyl-tailed Polypyridine Complex.
Sohyeon Seo 1 , Alexander Konchenko 1 , Junghyun Lee 1 , Gyeong Sook Bang 1 , Hyoyoung Lee 1
1 , Sungkyunkwan University, Suwon Korea (the Republic of)
Show AbstractIn thiolate monolayers on the gold electrode, the chemical bonding of sulfur to gold and the van der Waals interactions between the alkyl chains of neighboring molecules are important factors in the formation of well-defined monolayers and in the control of the electron transport rate. Many functionalized organic thiol molecules with a long carbon backbone are widely used in molecular electronics. Self-assembled monolayers (SAMs) with alkyl chains result in better molecular device yields by reducing the number of defect sites on the metal electrode and avoiding electrical short circuits by increasing the film thickness. Therefore, to implement a high-density molecular electronic device, device fabrication with a crossbar system consisting of a vertical structure (i.e., the metal/molecule/metal junction) is not only favoured over a horizontal nanogap junction, but is considered crucial for the real applications. It is important to understand the interfacial electron transport in accordance with the increased film thickness of alkyl chains that are known as an insulating layer, but are required for molecular device fabrication. It is also necessary to investigate how the intrinsic properties of alkyl-tailed molecules correlate with molecular electronic device characteristics in terms of electron transport processes.In this study, we demonstrate that electron transport through the alkyl-tailed Ru(II) terpyridyl complex proceeds rapidly enough via a Ru-centered redox orbital using electrochemistry. Electrochemical voltammetry and current-voltage (I-V) characteristics are measured to elucidate electron transport processes in the bistable conducting states of single molecular junctions of a molecular switch, alkyl-tailed Ru(II) terpyridine complexes. (1) On the basis of the Ru-centered electrochemical reaction data, the electron transport rate increases in the mixed self-assembled monolayer of Ru(II) terpyridine complexes, indicating strong electronic coupling between the redox center and the substrate, along the molecules. (2) In a low-conducting state before switch-on, I-V characteristics are fitted to a direct tunneling model, and the estimated tunneling decay constant across the Ru(II) terpyridine complex is found to be smaller than that of alkanethiol. (3) The threshold voltages for the switch-on from low- to high-conducting states are identical, corresponding to the electron affinity of the molecules. (4) A high-conducting state after switch-on remains in the reverse voltage sweep, and a linear relationship of the current to the voltage is obtained. These results reveal electron transport paths via the redox centers of the alkyl-tailed Ru(II) terpyridine complexes, a molecular switch.
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A Study on the Reliability Improvement of Gold Nanoparticle-based Non-volatile Memory Devices.
Young-Su Park 1 , Soo-Jin Kim 1 , Si-Hoon Lyu 1 , Ji-Min Song 1 , Jang-Sik Lee 1
1 , Kookmin University, Seoul Korea (the Republic of)
Show AbstractActive research has been performed on the fabrication of metallic nanocrystal (NC)-based non-volatile memory devices for future high-performance memory applications.[1-3] In this type of memory devices controlled synthesis of metallic NCs is very important since the programmable memory characteristics is highly dependent on the geometry, species, and density of metallic NCs. Normally, the metallic or semiconducting NCs have been used as the charge trapping elements. In this case the tunneling oxide layer is essential to ensure the data retention capability.In this study, the surface of gold nanoparticles, which was functionalized by polyvinylpyrrolidone (PVP), was coated with silica to form a core/shell structure through the solution process. The coated silica shell can acted as the tunneling dielectric layer, so tunneling barrier was constructed without using deposition of tunneling dielectric layers. The thickness of silica shell was exactly controlled according to the synthesis time and the solution concentrations. Synthesized nanoparticles were deposited as a monolayer by a spin-coating method for memory applications. The core/shell structure of nanoparticles and the thickness of silica shell were characterized using high-resolution transmission electron microscopy. The fabricated memory devices showed excellent data retention properties due to the potential-well structure of NCs. In this presentation, detailed device fabrication and electrical characterization will be discussed with an emphasis on the controlled synthesis of gold/silica core/shell structures as the charge trapping elements. [1] J.-S. Lee et al., Appl. Phys. Lett. 91, 153506 (2007).[2] J.-S. Lee et al., Nature Nanotechnology 2, 790 (2007).[3] J.-S. Lee et al., Adv. Mater. 21, 178 (2009).
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Nucleation Process - Leads to Vertically Aligned Columnar Structures with Remarkable Increase in Carrier Mobility in Discotic Materials.
Ishviene Cour 1 , Zhenwen Pan 1 , Madalina Furis 1 , Randall Headrick 1
1 Physics, University of Vermont, Burlington, Vermont, United States
Show AbstractSubstituted phthalocyanines have emerged as a promising class of soluble organic semiconductors that have gained increased attention due to their stability and discotic nature. The discotic (disc-like) molecules can self assemble into long columnar structures possessing significant long range ordering and anisotropic properties. We have produced highly ordered crystalline surface layers of discotics by the solution processable hollow capillary technique. The as-deposited films exhibit macroscopic uniaxial orientation of columns with an edge-on arrangement of molecules on ITO substrates as confirmed by Polarized Optical microscopy, Linear Dichroism and X-ray Diffraction. Thermal treatment of highly pure crystalline films through its liquid crystalline phase results in super heating effect due to the lack of nucleation sites available in the film. This also results in an increase in the clearing temperature of the discotic material. Super cooling effect is prevalent while cooling it back to the room temperature. Although in previous studies many discotic materials have been aligned homeotropically, with the materials mostly sandwiched between two ITO glass slides, but we have successfully been able to produce vertically aligned columnar structures with high mobility direction perpendicular on to a single substrate. Further, a remarkable increase in carrier mobility of three orders of magnitude in vertical direction due to the homeotropic alignment of discotic molecules has been verified by space charge limited current measurements before and after the thermal treatment.
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Electrically Interfacing Proteins to Conducting Polymers.
Millicent Firestone 1 , Gregory Becht 1 , Sungwon Lee 1
1 Materials Science, Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractIn this work, we describe the synthesis of a novel multifunctional polymer that incorporates both electronic and ionic conductivity. The polymers are prepared from an imidazolium IL-derived bifunctional monomer possessing both thienyl and vinyl moieties, 3-(10-vinylimidazole-1-oxydecane)-4-methylthiophene bromide, [3-VImC10O-4-Thienyl+][Br-]. The prepared polymers can be further modified to incorporate a group that permit selective coupling to a surface reactive cysteine residue on the exterior surface of a protein. In brief, the bromination of the PVPT was carried out by the nucleophilic addition of the bromine by N-bromosuccinimide. The electrochemical properties of the protein-polymer conjugates are evaluated and the electronic transport properties are determined using scanning tunneling spectroscopy and conductive atomic force microscopy. The polymers serve to promote efficent electrical communication between the protein and an external circuit.
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Report on Anthracenetetracarboxydiimides.
Matthew Morantz 1 , Dmitrii Perepichka 1
1 Department of Chemistry, McGill University, Montreal, Quebec, Canada
Show AbstractSemiconducting materials must transport charge by carrying either holes (in which case they are termed p-channel carriers) or electrons (n-channel carriers). The use of organic materials as semiconductors in thin film transistors (TFTs) is slowly transforming an industry heretofore dominated by difficultly processable and inflexible inorganic components. Despite a host of suitable p-channel candidates, n-channel materials are -with a few notable exceptions- rare and usually suffer from electron-trapping defects, particularly when exposed to air. One strategy to circumvent this trapping has been to lower LUMO energy by incorporation of strongly electron-withdrawing groups.Anthracenetetracarboxydiimides (ATCDIs) embody this idea and are a new and relatively unreported class of n-type semiconducting material. We have synthesized a number of ATCDI derivatives, focussing on substituent variation at both the N,N'- and 9,10- positions of the molecule. We have examined the electrochemical, fluorescence and absorption properties of these materials as well as their performance in TFTs. We complement the results obtained with DFT calculations of the electronic levels and reorganization energies of the compounds. Furthermore, we report the first observation of ATCDI self-assembled molecular networks, imaged using Scanning Tunnelling Microscopy (STM). Finally, we relate the data derived from these STM images to the specific goal of this study, i.e. probing the effects of core and alkyl chain variation on molecular packing and charge mobility in tetracarboxydiimide moiety-containing acenes.
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Very High Carrier Mobilities in Polyhedral Oligomeric Silsesquioxane (POSS) -acene Nanocomposites: A Theoretical Study.
Changgua Zhen 1 , Feng Qi 1 , John Kieffer 1
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractPolyhedral oligomeric silsesquioxane (POSS) based nanocomposites have been widely investigated in fire retardant, protective coatings, catalysts, and biomedical applications due to the enhanced chemical, thermal, mechanical properties by covalently bonded to or dispersed into their nonhybrid counterparts. However, the applications of POSS in electronic devices as semiconductors or conductors are limited by high bandgap and low conductivity because of the inorganic silica-like core. In order to take advantages of the chemical, thermal, and mechanical properties of POSS, we designed hybrid molecules by functionalizing POSS cage with acenes for application in organic electronics. We used quantum mechanical calculations to determine the electronic structure of these hybrid molecules and found that the organic acene segments dominated their electronic properties. We further studied the possible crystal structures of the hybrid materials by combining molecular dynamics and density functional theory. For the example of POSS-pentacene (POSS cage with 2 pentacene molecules attached along the body diagonal), the stacking of POSS cages induce the pentacene segments to layer with their faces parallel to each other. The hopping probability in the layers and between the layers are more evenly distributed than that in pure pentacene, indicating more isotropic charge transport in the POSS-pentacene crystal. The calculated hole and electron charge mobility in POSS-pentacene are about one order of magnitude larger than those in pure pentacene, reaching 96.7 cm2V-1s-1 and 18.1 cm2V-1s-1 at 300K, respectively. Thus we show that the POSS-organic hybrid materials are interesting candidates for application in organic electronic devices.
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Fabrication of Freestanding Polymer Nanowire Waveguides by Near-field Dry Spinning.
Jaeyeon Pyo 1 , Ji Tae Kim 1 , Jung Ho Je 1
1 Materials Science and Engineering, X-ray Imaging Center, Pohang University of Science and Technology, Pohang Korea (the Republic of)
Show Abstract Polymer nanowires have attracted increasing interests as photonic waveguide (1-3) owing to their advantages of rapid processing, incorporation possibility, and refractive index controllability, comparing with inorganic ones (4). A universal issue on nanowire waveguide is the optical loss induced by the contact between the waveguide and the substrate (1-3, 5-8). The loss is inevitable unless the waveguide is fully isolated from the substrate, floated in the air (7). The idea of suspending the waveguide over trench, even though fulfilling the isolation (3, 8-9), encounters unavoidable strong scattering at the end facet of the trench (3). We propose a Near-Field Dry Spinning (NFDS) method to fabricate freestanding structure of polymer nanowire (down to ~ 200 nm) for the isolation. Dry spinning is a common method to fabricate polymer wires by spinning polymer solution through a nozzle using an external pumping. In NFDS, the nozzle is placed very close to the substrate, forming a near-field region where coulomb interaction dominates over other forces, to induce the spinning of charged solution. For instance, an exceedingly enhanced coulomb interaction at a very small nozzle-substrate distance of 2μm enables to spin out the solution through a tiny aperture of one micrometer. A unique strength of NFDS is the foot formation of well-defined dimension (~1 μm), which is a critical factor especially for photonic integration.We demonstrate the successful fabrication of freestanding polystyrene nanowires with controlled diameters (down to ~200 nm) with perfect optical isolation from the substrate. In addition, the enhancement of the guiding ability is discussedEmail:
[email protected](1) F. D. Benedetto, et al. Nature Nanotech. 3, 614 (2008)(2) D. O’Carroll, et al. Nature Nanotech. 2, 180 (2007)(3) H. Liu, et al. Small, 2, 495 (2006)(4) H. Ma, et al. Adv. Mater. 14, 1339 (2002)(5) M. Law, et al. Science 305, 1269 (2004)(6) L. Tong, et al. Nature 426, 816 (2003)(7) P. Domachuk, B. J. Eggleton, Nature Mater. 3, 85 (2004)(8) F. Gu, L. Zhang, X. Yin, and L. tong, Nano Lett. 8, 2757 (2008)(9) R. Yan, et al. Proc. Natl. Acad. Sci.USA 106, 21045 (2009)
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Controlled Growth of Organic-inorganic Hybrid Nanowires.
Yuliang Li 1 , Huibiao Liu 1 , Yongjun Li 1
1 CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing China
Show AbstractLow dimension organic/inorganic hybrids of nanostructures are materials that combination of functional organic molecules and inorganic molecules to produce new class organic/inorganic solid materials which have distinct properties that were not observed in the individual component on nanosize and their bulk materials. This may include either new or improved chemical and physical properties that can be exploited for fabricating novel nanoscale devices. As it is known that the P-N junctions are of great importance both in modern electronic applications and in understanding other semiconductor devices. We reported that a series of organic/inorganic semiconductor hybrid nanowires were fabricated and investigated their properties.CdS-PPY (polypyrrole) heterojunction nanowires were prepared using ordered porous AAO templates. The CdS-PPY heterojunction nanowires are well-defined, with a smooth surface and with diameters of 200-400 nm. TEM and HRTEM images show a clear interface between organic and inorganic materials. The segmented structure of CdS-PPY hybrid nanowire can also be confirmed by optical image and fluorescence image. The unique phenomenon of light controlled diode on the single nanowire was observed. The typical I-V curves under illumination of different light intensity, the CdS-PPY heterojunction nanowire acts as a diode and exhibits rectifying property at room temperature. With the forward bias, current increases as the increment of applied bias. In reverse bias, the current transporting through the junction nanowire is close to zero. With the increase of illumination intensity, the conductivity of the CdS-PPY heterojunction nanowire increased. With an applied bias of 5.0V, the rectification ratio of the diode increased from 8.0 to 13 along with the enhancement of light intensity from 1.12 mW/cm2 to 5.76 mW/cm2. The rectification ratio of the diode at 10.0V is the same as the ratio at 5.0V (about 13), under 5.76 mW/cm2 illuminations. 1)Guo, Y.; Tang, Q.; Liu, H.; Zhang, Y.; Li, Y.; Hu, W. P.; Wang, S.; Zhu, D. J. Am. Chem. Soc. 2008, 130, 9298-9299.2)Gan, H.; Liu, H.; Li, Y.; Zhao, Q.; Wang, S.; Jiu, T.; Wang, N.; He, X.; Yu, D.; Zhu, D. J. Am. Chem. Soc. 2005, 127, 12452–12453. 3)Li, Y.; Li, X.; Li, Y.; Liu, H.; Wang, S.; Gan, H.; Li, J.; Wang, N.; He,X.; Zhu, D. Angew. Chem., Int. Ed. 2006, 45, 3639–3643. 4) Huang, S.; Wen, L.; Liu, H.; Li, Y.; Liu, L.; Yuan, Y.; Zhai, J.; Jiang L.; Zhu, D. Adv. Mater. 2009, 21, 1721-1725.5)Cui, S.; Li, Y.; Guo, Y.; Liu, H.; Song, Y.; Xu, J.; Lv, J.; Zhu, M.; Zhu, D. Adv. Mater. 2008, 20, 309-313.6)Cui, S.; Liu, H.; Gan, L.; Li, Y.; Zhu, D. Adv. Mater. 2008, 20, 2918-2925.7)Guo, Y.; Li, Y.; Xu, J.; Liu, X.; Xu, J.; Lv, J.; Huang, S.; Zhu, M.; Cui, S.; Jiang, L.; Liu, H.; Wang, S. J. Phys. Chem. C 2008,112, 8223–8228.8)Guo, Y.; Liu, H.; Li, Y.; Li, G.; Zhao, Y.; Song, Y.; Li, Y. J. Phys. Chem. C, 2009, 113, 12669–12673
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H-aggregation Strategy to Design Molecular Semiconductors for High Performance Organic Thin Film Transistors.
TaeKyu An 1 , Soon-Ki Kwon 2 , Chan Park 1
1 Chem. Eng., Pohang university of science and technology, Pohang Korea (the Republic of), 2 , School of Nano & Advanced Materials Science and Engineering and ERI, Jinju Korea (the Republic of)
Show Abstract We have designed four kinds of quaterthiophene derivatives with end-groups of dicyclohexyl ethyl, (DCE4T) dicyclohexyl butyl, (DCB4T) cyclohexyl ethyl, and (CE4T) cyclohexyl butyl (CB4T). All of the materials have appreciable solubility in nonpolar solvents which may allow better solution processability than reported other thiophene-derivatived oligomers. From UV-vis absorption measurement and its quantitative analyses, we found out that quaterthiophene derivatives with asymmetrical end-group substitution, CE4T and CB4T, prefer to aggregate by H-type while those with symmetric end-group substitution, DCE4T and DCB4T, prefer to aggregate by J-type. We could successfully support the origin of molecular structure-dependent packing types (H or J) of those derivatives by analyzing the molecular packing structure within a thin film on a substrate through Grazing-Incidence Small-Angle X-ray Scattering (GISAXS). We have fabricated a thin-film transistor (TFT) using the semiconducting materials and found out obvious correlations between molecular packing type and charge carrier mobility. The field-effect mobilities of the device using asymmetrical molecules, CE4T and CB4T, were quite high around 10-2 cm2/(Vs), while those of symmetrical molecules, DCE4T and DCB4T, were very poor below 10-4 cm2/(Vs). These findings show that the smart molecular design, in this case, substituting end groups asymmetrically, can introduce effective H-type packing to the solution processed molecular semiconductors-based thin films. Considering that almost all of the previously reported high performance molecular semiconductors have J-type packing structures, we believe that our findings take us one step closer towards the development of high performance organic TFTs.
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The First Molecular Wheel: A Theoretical Investigation.
Gustavo Brunetto 1 , Fernando Sato 2 , Xavier Bouju 3 , Douglas Galvao 1
1 Apply Physics Department, Unicamp, Campinas, São Paulo, Brazil, 2 Departamento de Fisica, Universidade Federal de Juiz de Fora, Juiz de Fora, Minas Gerais, Brazil, 3 Nanosciences Group, Centre d'Elaboration de Materiaux et d'Etudes Structurales, Toulouse France
Show AbstractRecently [1], the first molecular nanowheel was synthesized and characterized from STM experiments. It was demonstrated that a specifically designed hydrocarbon molecule (C44H24) could roll over a copper substrate along the [110] surface direction. The molecule consists of two wheels based on the triptycene groups which are connected by a carbon axle [1]. In this work we report a detailed theoretical analysis of the isolated molecules and of its rolling process over different Cu surfaces.. To investigate the molecule over Cu surface we carried out ab initio (DMol3) and classical molecular dynamics (UFF) calculations. We have considered different crystallographic orientations ([111], [110], and [100]) for the copper surface, in order to determine how these different orientations affect the molecular rolling processes. Our results are in good agreement with the available experimentally data. The simulations showed that the rolling mechanism is only possible for the [110] direction. In this case the spatial separation among rows of copper atoms is enough to trap the molecule and to create the necessary torque to roll it. Also, the relative position of the molecule on the surface is very important. The molecule should be with its main axis in the parallel direction to [110]. This commensurability effect, between the molecule and the surface, is similar to the surface selective diffusion recently observed for other classes of organic molecules [2]. For the other directions ([111] and [100]) the surfaces are too smooth and can not provide the necessary torque to the rolling process. For these cases the molecule just slides (translational movement), without rolling when interact with the simulated STM tip. The experimentally observed pushing, pulling, and rolling profiles with the STM signals can also be explain in terms of these geometrical features between the molecule and the different Cu crystallographic directions.[1] L. Grill, K. –H. Rieder, F. Moresco, G. Rapenne, S. Stojkovic, X. Bouju, and C. Joachim, Nature Nanotechnology 2, 95 (2007).[2] R. Otero, F. Hummelink, F. Sato, S. B. Legoas, P. Thostrup, E. Laesgsgaard, I. Stensgaard, D. S. Galvão, and F. Besenbacher, Nature Materials 3, 779 (2004)
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Statistically-determined Molecular I-V Curves Measured by STM-break Junction.
Jonathan Widawsky 1 , Masha Kamenetska 1 , Mark Hybertsen 2 , Latha Venkataraman 1
1 Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York, United States, 2 Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York, United States
Show AbstractUnderstanding current-voltage characteristics of single metal-molecule-metal junctions is of fundamental importance to the development of functional nanoscale, organic-based devices. Here, we present a comparison between the current-voltage (I-V) characteristics of a series of amine-linked/HOMO-conducting [1] and pyridine-linked/LUMO-conducting [2] single-molecule junctions. The junctions are created using the STM-based break-junction technique where an Au point contact is broken in a solution of the target molecules in ambient conditions. After the Au point contact breaks, the tip-substrate distance is held temporarily constant in order to achieve a stable metal-molecule-metal junction. The bias across the junction is ramped while current is measured to generate an I-V curve [3]. Thousands of such I-V curves are compiled into 2D histograms to determine a statistically significant I-V curve for each molecule. For amine-terminated molecules, the statistically determined I-V curves show that, on average, junctions which can sustain a higher bias have a lower conductance. In contrast, pyridine-terminated molecules have I-V curves that are independent of the applied voltage ramp. Furthermore, with the pyridine-linked molecules, we are able to achieve a bias sweep of +/-1.3 V and observe very non-linear current-voltage characteristics, suggesting that we open a bias window large enough to approach a molecular resonance. Past theoretical works have shown that both amines [1] and pyridines [4] bind through the formation of a donor-acceptor bond between the nitrogen lone pair and an under-coordinated gold atom. However, the alignment of the molecular orbitals to the metal Fermi level is different for amines and pyridines, with amine conductance dominated by tunneling mediated by the HOMO versus the LUMO for pyrdines. Furthermore, in pyridines, the lone pair is in the plane of the ring, while the dominant LUMO for conductance is in the pi system. When a bias is applied across a molecular junction some charge redistribution occurs at the bonds. To explain the voltage dependent stability of the junction, we hypothesize that for the amine case, where bonding and charge transport involve the same orbital, charge transfer between the metal and the molecule weakens the Au-N bond. In pyridines, with distinct orbitals involved in bonding and conduction, the bond is relatively unaffected by applied bias, even in the regime where a resonance is being partially probed.[1] Venkataraman et al., Nano Letters 6, 458 (2006).[2] Kamenetska et al, J.Am. Chem. Soc., 132, 6817 (2010) [3] Widawsky et al, Nanotechnology 20, 434009 (2009)[4] Quek et al, Nature Nano. 4, 230, (2009)
9:00 PM - E11.46
Meyer-Neldel Rule and Disorder in Fullerene Based Devices.
Mujeeb Ullah 1 , Ivan Fishchuk 2 , Andrey Kadashchuk 3 4 , Almantas Pivrikas 5 , Philipp Stadler 5 , Alexander Kharchenko 6 , Clemens Simbrunner 1 , Niyazi Sariciftci 5 , Helmut Sitter 1
1 Institute of Semiconductor and Solid State Physics, Johannes Kepler University Linz, Linz Austria, 2 Institute for Nuclear Research, National Academy of Sciences of Ukraine, Prospect Nauky 47,03680, Kyiv Ukraine, 3 , IMEC, Kapeldreef 75, B-3001 , Heverlee Belgium, 4 Institute of Physics, National Academy of Sciences of Ukraine, Prospect Nauky 46, 030280, Kyiv Belgium, 5 Linz Institute of Organic solar cells (LIOS), Johannes Kepler University Linz, Linz Austria, 6 , PANalytical B.V, 7600 AA Almelo Netherlands
Show AbstractThe performance of organic material based field effect transistors (OFETs) depends on the field effect mobility which, now-a-days, approaches to 20 cm2 V−1 s−1 with short molecules, and 0.2 cm2 V−1 s−1 with polymers. Fullerene “C60” is known as n-type organic semiconductor materials with its symmetric structure, low ionization potential and high mobilities depending on different morphologies of C60 films (0.6-6cm2V-1s-1) [1-2]. In the present work we fabricated the OFETs using different substrate temperature to grow different morphologies of C60 films by Hot Wall Epitaxy system. Atomic Force Microscopy images and XRD results showed increasing grain size with increasing substrate temperature. A shift in field effect mobility was observed for different OFETs with different morphologies of C60 films with increasing grain size. The temperature dependence of the field effect mobility was determined in the accumulation regime of the device characteristics in the temperature range from 300K to 80K. A change from an activated charge carrier transport to a non-activated transport at lower temperatures was observed. The temperature dependence of these devices showed different Arrhenius activation energies. More over these C60 based OFETs follow the empirical relation between the Arrhenius activation energy and the mobility prefactor named as Meyer-Neldel Rule [2-3]. A shift in characteristic Meyer-Neldel energy, which can be described as energetic disorder parameter of the material, was observed with changing C60 film morphology [3].Mobility measurements in diode configuration by the technique Charge Extraction by Linearly Increasing Voltage (CELIV) suggested that the Meyer-Neldel energy in C60 films is independent of the device, although the mobility and charge carrier concentration are different in FETs and diodes [4].As a consequence, the Meyer-Neldel energy can be considered as material quality criterium, characterizing the width of energetic disorder [5]while the carrier mobility depends on many other parameters not only on the degree of disorder in the material.Reference:1.Mujeeb Ullah, D. M. Taylor, R. Schwödiauer, H. Sitter, S. Bauer, N. S. Sariciftci and Th. B. Singh, Journal of Applied Physics, 106, 114505 (2009).2.Mujeeb Ullah, T.B. Singh, H. Sitter, N.S. Sariciftci, Applied Physics A, Materials Science & Processing 97 (2009), 521.3.Mujeeb Ullah, I. I. Fishchuk, A. Kadashchuk, P. Stadler, A. Pivrikas, C. Simbrunner, V. N. Poroshin, N. S. Sariciftci, and H. Sitter, Appl. Phy. Lett. 96,213306 (2010).4.I. I. Fishchuk, A. K. Kadashchuk, J. Genoe, Mujeeb Ullah, H. Sitter, Th. B. Singh, N. S. Sariciftci, and H. Bässler, Phys. Rev. B 81, 045202 (2010).5.A. Pivrikas,1,a) Mujeeb Ullah,2 Th. B. Singh,1 C. Simbruner,2 G. Matt,1 H. Sitter,2 and N. S. Sariciftci1, Organic Elecrtronics submitted.
9:00 PM - E11.47
Reliable Switching Characteristics of Organic Non-volatile Memory on a Flexible Substrate.
Yongsung Ji 1 , Byungjin Cho 1 , Sunghoon Song 1 , Minhyeok Choe 1 , Yung Ho Kahng 1 , Takhee Lee 1
1 Department of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of)
Show AbstractOrganic-based electronics have received great attention due to their material variety and advantageous properties such as flexibility, printability, and light-weightness [1,2]. Especially, flexibility is important for future electronic applications such as foldable and wearable electronics. Much research has been done to apply flexible electronics technology to various devices such as organic solar cells, thin-film transistors, light-emitting diodes, and displays [1-3]. The research on flexible memory was also initiated for these future flexible electronics applications. In particular, organic-based flexible memories have merits such as a simple, low-temperature, and low-cost manufacturing process. Several fabrication results of organic resistive memory devices on flexible substrates have been reported [4,5]. However, earlier studies have focused mainly on manifesting the resistive switching characteristics on flexible substrates without the detailed investigations of the switching characteristics under bending conditions.Here, we report on the organic non-volatile memory devices on a bent flexible substrate. We fabricated PI:PCBM organic memory devices in a 8 × 8 cross-bar array structure on poly(ethylene terephthalate) (PET) flexible substrates. The fabricated organic memory devices showed a typical unipolar-type switching behaviour. In particular, the memory devices showed successful rewritable switching properties under the bending circumstances. The switching characteristics did not change significantly after the bending, showing the resilience of the electrical property of our memory device with the bending stress. The ON/OFF current ratios were about 104 regardless of the degree of bending or the number of bending cycles. Furthermore, our flexible organic memory devices exhibited a long retention time more than 104 seconds and stable endurance under the bending conditions, demonstrating promising characteristics for application to future flexible memory devices [6].Reference[1]T. Sekitani et al, Science 2009, 326, 1516.[2]J. Y. Kim et al, Science 2007, 317, 222.[3]Q.-D. Ling et al, Progress in Polymer Science 2008, 33, 917.[4]L. Li et al, Org. Electron. 2007, 8, 401.[5]D.-I. Son et al, Nanotechnology 2008, 19, 055204.[6]Y. Ji et al, Adv. Mater. DOI: 10.1002/adma.200904441.Acknowledgements: National Research Laboratory program from the Korean Ministry of Education, Science and Technology and the Program for Integrated Molecular System at GIST.
9:00 PM - E11.49
Molecular Self-assembly on Calcite (10-14).
Philipp Rahe 1 , Markus Nimmrich 1 , Jens Schuette 1 , Sebastian Rode 1 , Angelika Kuehnle 1
1 Institut für Physikalische Chemie, Fachbereich Chemie, Johannes Gutenberg-Universität Mainz, Mainz Germany
Show AbstractBottom-up fabrication of future devices, beginning at the single molecule level, provides utmost control of structure and functionality. Exploiting molecular self-assembly on surfaces offers a promising potential for cost-effective production and improved efficiency. Consequently, molecular self-assembly has been studied extensively using scanning tunneling microscopy on metallic and semi-conducting surfaces, resulting in well-controlled structure formation with two-, one-, or quasi zero-dimensionality [1,2]. For many applications such as novel (opto) electronic devices, however, insulating surfaces are mandatory in order to reduce electronic coupling to the substrate surface. So far, employing molecular self-assembly on insulating surfaces such as KBr and NaCl has been hampered by the comparatively high molecular mobility at room temperature.Here, we present a non-contact atomic force microscopy (NC-AFM) study of molecular self-assembly on the (10-14) cleavage plane of calcite. Calcite, the most stable polymorph of CaCO3, is one of the most abundant simple salts in nature. Calcite is of high importance in a broad variety of different fields including biomineralization, enantiospecific biomolecule adsorption as well as environmental geochemistry.Experiments were performed using NC-AFM under ultra-high vacuum conditions. Imaging of the calcite (10-14) surface is possible down to the atomic level in-vacuo [3], as well as under liquid conditions [4]. The properties of the (10-14) surface have been widely discussed in literature, and recently, we were able to report a clear signature of the (2x1) reconstruction [3].Deposition of different classes of organic molecules results in a rich variety of patterns that are stable at room temperature. One-dimensional nanowires of well-defined width will be presented, which are based on a subtle interplay between π-π interactions and hydrogen bonds [5]. Two-dimensional islands with a highly ordered inner structure will be shown that are stabilized by hydrogen bonds solely. The presented systems demonstrate how to gain precise control of structure formation on an insulating surface by adjusting the relative influence of molecule-molecule and molecule-substrate interactions.[1]J. Barth et al. Annu Rev Phys Chem 58, 375 (2007)[2]A. Kühnle. Curr Op Coll Interf Sci 14, 157 (2009)[3]J. Schütte et al. Langmuir 26, 8295 (2010)[4]S. Rode et al. Langmuir 25, 2850 (2009)[5]P. Rahe et al. J Phys Chem C 114, 1547 (2010)
9:00 PM - E11.5
Utilizing Orthogonal Processing to Explore Organic Electronics.
Carol Newby 1 , Jin Kyun Lee 1 , Christopher Ober 1
1 MS&E, Cornell University, Ithaca, New York, United States
Show AbstractAs the first products containing organic light emitting diodes (OLEDs) and thin-film transistors (OTFTs) come to market there remains a need for continued development in organic electronics before further commercialization can be realized. For the field to mature a greater wealth of knowledge is needed about the fundamental science on which organic electronic devices are founded. Utilizing orthogonal processing we fabricate previously unattainable device architectures to allow the investigation of device physics.Orthogonal processing involves the use of fluorinated chemistry to allow photolithographic patterning of materials for organic electronics. Conventional photolithography uses organic resists which are incompatible with fabrication of organic electronics because organic active materials are soluble in the same solvents as the resists. Fluorinated materials are only soluble in fluorinated solvents which are immiscible with aqueous and organic solvents allowing organic electronic materials to be patterned using fluorinated photoresist without being damaged.Orthogonal processing therefore allows us to reliably pattern organic electronic devices at high resolution and in architectures analogous to conventional electronics. These architectures, unattainable with alternative processing strategies such as shadow-masking or inkjet printing, make it possible to better understand device operation and investigate materials properties. For example, transport properties in various active materials can be probed through fabrication of short-channel top-contact TFTs.
9:00 PM - E11.50
Fine Tuning of the Electronic Structure of π-conjugated Molecules for Molecular Electronics.
Silvia Karthaeuser 1 , Maria Lennartz 1 , Vasile Caciuc 1 , Nicolae Atodiresei 1 , Stefan Bluegel 1
1 Institut für Festkörperforschung, Forschungszentrum Jülich, Jülich Germany
Show AbstractWe present an experimental work combined with density functional theory (DFT) calculations aimed to systematically investigate the relationship between the theoretical electronic structure of carboxylic acids chemisorbed on the Cu(110) surface and the corresponding transport properties as revealed by scanning tunnelling spectroscopy (STS) experiments [1-5]. The tuning process is realized by inserting nitrogen atoms in a six-membered aromatic ring or a carboxylic functional group at the aromatic ring or both. Depending on the specific nature of the substituent the relative position of the occupied or unoccupied electronic states with respect to the Fermi level can be specifically controlled. This in turn modifies the transport properties of the studied molecular systems as proven by our scanning tunnelling spectroscopy measurements. On the basis of the insight gained by experiment and first-principles calculations we are able to predict the specific molecular character (σ or π) of the orbitals involved in the transport process of the carboxylate-Cu(110) system when both functionalization paths mentioned above are simultaneously employed. As a key result we show how to intentionally tune the HOMO-LUMO gap of the molecule-surface system by a systematic chemical functionalization process. Furthermore combining DFT calculations and distance dependent STS enables an electronic mapping of molecules connected to a metal surface. Thus the LDOS calculated for a specific aromatic carboxylic acid bonded to a metallic surface is used as a fingerprint for analyzing the respective adsorption geometry.[1] M.C. Lennartz et al., Langmuir 25 (2009) 856.[2] N. Atodiresei et al., Physical Review B 76 (2007) 115433.[3] N. Atodiresei et al., Physical Review B 78 (2008) 045411.[4] N. Atodiresei et al., Phys. Rev. Lett. 102, (2009) 136809.[5] M.C. Lennartz et al. (2010) submitted.
9:00 PM - E11.51
Orbital-modulated Carrier Transport in Molecular Transistors.
Hyunwook Song 1 , Youngsang Kim 2 , Yun Hee Jang 1 , Heejun Jeong 2 , Mark Reed 3 , Takhee Lee 1
1 Department of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of), 2 Department of Applied Physics, Hanyang University, Ansan Korea (the Republic of), 3 Departments of Electrical Engineering and Applied Physics, Yale University, New Haven, Connecticut, United States
Show AbstractTo overcome the increasing difficulties and soon fundamental limitations that current complementary metal-oxide semiconductor (CMOS) technology faces upon further downscaling in the quest for higher performance, single molecules have been considered as potential building blocks for future nanoelectronic systems. Until now, a variety of characteristic functions illustrated by single molecules, including rectifiers, switches, and transistors have been accordingly designed and reported [1]. All these aspects render single molecules as a promising candidate for the next generation of electronics. Here, we report the observation of direct gate modulation of the orbital energy in molecular transistors where transport current is controlled by an external gate [2]. We show this using a multiprobe approach combining a variety of transport techniques that elucidates the transport mechanisms and electronic structure of molecular transistors. To make the transistors, individual molecules are connected to tiny gaps between source and drain electrodes with a bottom-gate control electrode using an electromigration-induced break junction technique. We have examined two prototype molecules: (a) the control, octanedithiol with an alkyl σ-backbone as a saturated aliphatic molecule, and (b) the active device, benzenedithiol with a delocalized π-electron aromatic ring as a conjugated molecule. We also demonstrate that a change of the molecule attachment endgroups (specifically, from thiol, -SH to cyanide, -CN) can dramatically change the orbital alignment with respect to the contact Fermi level, transforming the behavior from HOMO-dominated (i.e., p-type) to LUMO-dominated transport (i.e., n-type). These results show that a complementary set of molecular transistors can be created with appropriate chemical design of the metal-molecule contact. Resonant-enhanced coupling to the frontier molecular orbitals is revealed by electron tunneling spectroscopy to verify the identity of the molecule in the transistors and to determine the amount of orbital coupling, demonstrating direct molecular orbital gating in an electronic device. In particular, we contrast molecular structures that have near-resonant coupling to ones that have far-from-resonant coupling. Our findings provide direct, clear evidence of an orbital-modulated molecular transistor and so validate the concept of such an electronic device. [1] M. Galperin et al., Nuclear coupling and polarization in molecular transport junctions: Beyond tunneling to function. Science 319, 1056 (2008). [2] H. Song et al., Observation of molecular orbital gating. Nature 462, 1039 (2009). Acknowledgements: the National Research Laboratory program from the Korean Ministry of Education, Science, and Technology and the Program of Integrated Molecular Systems at GIST.
9:00 PM - E11.53
Design and Characterization of a Novel Dosimeter for Applications in Radiotherapy.
Tiago Schimitberger 1 , Giovana Ferreira 1 , Eduardo deAzevedo 2 , Rodrigo Bianchi 1
1 Department of Physics, Federal University of Ouro Preto, Ouro Preto, MG, Brazil, 2 , Physics Institute of São Carlos, São Carlos, São Carlos, Brazil
Show AbstractConjugated polymers have been appeared as very promising materials for use in next generations of displays, not only because of their good processability in solutions and lightweight, but also due to their higher luminance with low power consumption. However, even though they are good candidates for lighting applications, they still have major problems in terms of stability. Because of their conjugated nature, they are highly susceptible to degradation processes which dramatically change the emission color and reduce the efficiency and durability of their devices. This result reveals, on the one hand, the low reliability of polymer devices, and on the other reflects the possibility to design and develop radiation dosimeters [1]. Moreover, high-energy X ray is desired specially in radiation therapy for cancer treatment where overdose and error in software and operation is not infrequent in literature. In this work we investigated the photoluminescence behavior of poly[2-metoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) and Tris(8-hydroxyquinolinato)aluminium (Alq3) solutions under the effect of low-dose radiation (6 MeV) delivered by linear accelerators for use in radiation therapy. The changes in optical properties were studied by absorption and fluorescence spectroscopy and the results show that the MEH-PPV/Alq3 solution presents a gradation of color from orange to yellow clearly, while its peak position emission shifts from orange-red (571 nm) to green (540 nm) with an estimated radiation dose from 0 to 100 Gy. Finally, the optical response of MEH-PPV/Alq3 system to radiation was investigated to design and develop an electronic device to easily represent the radiation dosage generally used in radiation therapy. The basic idea behind this concept considers the sensor as a light device where red represents underdose and green the prescribed dose or overdose. Finally, changes on the structural properties of MEH-PPV/Alq3 systems were followed by gel permeation chromatography, as well as solid state NMR e IR spectroscopes and mass spectrometry in order to evaluate the role of the radiation on the chemical structure of the organic materials. This work was sponsored by Fapemig, Capes, CNPq and INEO/CNPq.
9:00 PM - E11.6
QSPR Study for Dielectric Constants of Chromophore-like Organic Compounds.
Areum Lee 1 , Kyung-Hyun Kim 1 , Daejin Kim 1 , Seung-Hoon Choi 1 , Dong Hyun Jung 1
1 , Insilicotech Co.,Ltd., Seongnam, Gyeonggi-Do, Korea (the Republic of)
Show AbstractThe dielectric constant of chromophore-like organic compounds is one of the most important properties when they are used as materials for electronic devices. Although quantitative structure property relationship (QSPR) models have been proposed in order to predict the dielectric constant of general organic compounds, the previous models are inapplicable for the chromophore-like organic compounds. In this study, QSPR models were generated from a data set of 152 organic compounds which have more than one double-single-double bond linkage in their structure. A subset of 127 compounds and the other subset of 25 compounds were regarded as the training set for model generation and the test set for model validation, respectively. For the training set, the statistical regression model between the experimental dielectric constants and a total of 422 molecular descriptors was generated by using the genetic function approximation (GFA) algorithm. After eliminating 9 compounds from the training set based on the outlier analysis, the final QSPR model generation was performed. The best model shows R2 value of 0.95 for a training set of 118 compounds and also shows R2 value of 0.95 for a test set of 25 compounds.
9:00 PM - E11.7
Poly(4-hexyloxythiazole): A New Low Band Gap Semiconductor for Polymer Electronics.
Qingshuo Wei 1 , Shoji Miyanishi 2 , Erjun Zhou 1 , Keisuke Tajima 1 2 , Kazuhito Hashimoto 1 2
1 , Exploratory Research for Advanced Technology (ERATO), Japan Science Technology Agency (JST), Tokyo Japan, 2 , The University of Tokyo, Tokyo Japan
Show AbstractVarious low band gap polymers have recently been extensively studied for application to polymer solar cells (PSCs) and ambipolar field effect transistors (FETs). For these devices, it is highly desirable to combine high charge carrier mobility with long-wavelength absorption. One promising strategy for designing such polymers would be to start from bench mark regioregular poly(3-hexyl thiophene) (P3HT) and modifying the energy levels of the polymers while maintaining the highly ordered structure. To reduce the band gap of the poly(3-alkylthiophene) analogs, the incorporation of electron-donating groups such as alkoxy groups at the 3-position of the thiophene ring has been reported. Although the band gap of the polymer was reduced substantially, the electron-donating effect of the alkoxy group raised the level of both the polymer`s highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). This makes the resulting polymer easily doped with oxygen in the air, and therefore it is difficult to use the polymer in actual electronic devices. In the present study, we use a building block of thiazole, in which one carbon of a thiophene ring is replaced with nitrogen. It is known that the thiazole unit is electron-deficient compared with thiophene; therefore, the homopolymer of thiazole has much deeper HOMO and LUMO levels compared with polythiophene. Introduction of electron-donating side chains to polythiazole could lead to a low band gap polymer with deep HOMO and LUMO levels. In this presentation, we report the synthesis of a novel low band gap polymer poly(4-hexyloxythiazole) (P4HOTz) and its use in organic electronic devices. P4HOTz shows an optical band gap of 1.4 eV from the onset of the absorption at 900 nm, which corresponds to a reduction of 0.5 eV from the band gap of P3HT. X-ray diffraction (XRD) measurements reveal a high degree of crystallinity of P4HOTz in the film and that the side chains of P4HOTz are oriented vertical to the substrate to form an edge-on orientation, while the intermolecular p-p stacking between the thiazole rings is parallel to the substrate. Owing to a high degree of crystallinity and lamellar packing, P4HOTz shows a high FET mobility of 0.02 cm2/Vs, which is comparable to those of P3HT devices fabricated in a similar manner. In a bulk heterojunction type PSC, P4HOTz was used as the electron donor material in combination with a fullerene derivative. The cell shows a photoresponse up to about 900 nm, which corresponds to the absorption spectrum of P4HOTz.
9:00 PM - E11.8
The Direct Measurement of Energy Barrier Height at Metal/ Polyfluorene Derivatives Interface by Internal Photoemission Spectroscopy.
Eiji Itoh 1 , Shinya Takaishi 1
1 Department of Electrical and Electronic Engineering, Shinshu University, Nagano Japan
Show AbstractIn general, the barrier height for electrons is assumed as the difference of the lowest unoccupied molecular orbital (LUMO) of organic material, and the work functions of the cathode. However, the reported values of work functions and LUMO levels have strong variation depending on the measuring method, and the surface states and the interfa-cial dipole moment also modify the barrier height seriously. Therefore, the direct measurement method for the barrier height at the electrode/ organic interface is required under the device operation. Internal photoemission (IPE) spectroscopy provides the most straightforward way to characterize the relative energies of electron states at interfaces of insulators (or wide-gap semiconductor) from the Fermi level of metals by measuring the spectral onset of electron/hole photoemission. However, it is difficult for organic thin film device to reduce the leakage current less than 1nA/cm2 under the electric field with the order of 100kV/cm, while the typical measured IPE current is the order of 0.1-10nA/cm2. Moreover, IPE of electrons are emitted from the cathode while IPE of holes is possible from the anode. In order to suppress the counter charge injection, we recently developed the efficient “electron only devices” with three layered structure consisting of TiO2 layer, thin film electron transporting inter-layer and the target organic films. In this study, we have investigated the metal/ polyfluorene derivatives interface by IPE techniques and correlated the results with current analysis based on the Shottky ther-mal emission current model, and interfacial electronics states. Here, we employed the three layered structure consisting of TiO2 deposited on indium tin oxide (ITO), very thin electron transporting polyimide inter layer (IL), and fluorine based conducting polymer which is covered by various types of cathode. The work-functions of Au and ITO electrodes were estimated to be 4.75eV and 4.8eV from low energy photo-electron emission measurement under ambient at-mosphere by AC-2 (Riken Keiki Co. Ltd.). The work func-tion of MgAg, Al, and Ag was measured from the contact potential difference between Au electrode and these metal electrodes as 4.0, 3.7, 4.3eV. From the IPE spectra, the barrier heights, at metal/F8 interface for CsF/Al, MgAg, Al, and Au interface are estimated as 0.9, 1.1, 1.45, and 1.76eV, respectively. Almost same electron injection barrier heights are estimated from the temperature dependence of thermal emission current (Schottky current) with CsF/Al and MgAg cathode. Similar discussions were also done for F8BT/ cathode interface. And the relationships between the work-function or electronegativity of cathodes and the barrier height will be discussed by taken into account the induced interfacial gap-states.
9:00 PM - E11.9
6,6-Dicyanofulvenes as Effective Electron-accepting Moieties: Towards the Synthesis of N-type Conducting Polymers.
Trisha Andrew 1 , Timothy Swager 1
1 Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractThe synthesis and characterization of a series of 6,6-dicyanofulvenes is presented. 6,6-Dicyanofulvenes (CNFs) can be accessed in high yield starting from substituted acetone derivatives and a vicinal diketone. The ambient stability of CNFs can be tuned by the steric bulk of the substituents. CNFs were found to be powerful electron-acceptors and undergo two sequential one-electron reductions to form stable dianions. Incorporation of CNF moieties into the repeat unit of common conjugated organic polymers results in a new class of low band-gap, donor-acceptor polymers. Furthermore, the controlled homopolymerization of CNFs to yield n-type conducting polymers is currently under investigation.
Symposium Organizers
R. Joseph Kline National Institute of Standards and Technology
Iain McCulloch Imperial College London
Garry Rumbles National Renewable Energy Laboratory
Alberto Salleo Stanford University
E12: Supramolecular Structure and Self-assembly
Session Chairs
Christopher McNeill
Alberto Salleo
Friday AM, December 03, 2010
Constitution B (Sheraton)
9:30 AM - **E12.1
Architecture vs. Function Relationship in Supramolecular Nanomaterials for Organic Electronics.
Paolo Samori 1
1 Institut de Science et d'Ingénierie Supramoléculaires, Université de Strasbourg, Strasbourg France
Show AbstractThe processing, manipulation and quantitative study of the physico-chemical properties of multifunctional materials across multiple length scales are crucial for their technological applications in (opto)electronics. My lecture will review recent results obtained in our laboratory in four different research themes: (i) Development of novel processing and post processing methods to produces highly ordered supramolecular electroactive architectures. The emphasis has been given to solvent vapour annealing post-treatments as a route to form millimetre long crystalline fibers of the semiconducting perylene-bis-dicarboximide (PDI). The mechanism of this process complies with an Avrami-type nucleation governed growth.[1a] By modulating the temperature of the set-up it has been possible to increase the molecular reorganization on the microscopic scale.[1b](ii) Supramolecular scaffolding to control the position of functional units at surfaces and interfaces. This has been accomplished both using metallo-ligand interaction in anthracene incorporating molecular tectons,[2] and employing H-bonding between guan(os)ine derivatives exposing oligothiophene moieties.[3](iii) Responsive interfaces which have been visualized on the sub-nm scale by Scanning Tunneling Microscopy (STM). Prototypes of light-powered mechano-chemical switches operating at surfaces can be developed: Significantly, the photochemical isomerization of a new terminally thiolated azobiphenyl rigid rod, forming a single component and tightly packed SAM on metallic surfaces, was found by STM to depend on intermolecular interactions and to be complete over a molecular 2D crystal.[4a] Optically modulable nano-junctions were for the first time explored by conducting AFM.[4b] When incorporated into a Hg drop based junction the azobenzene SAM could operate as a current photoswitch; interestingly the light induced vertical displacement of the Hg drop revealed that our SAMs could also act as light-powered cargo lifter.[4c] (iv) Scanning Probe Microscopies beyond imaging to gain direct and quantitative insight into electronics processes in multicomponent architectures including the Kelvin Probe Force Microscopy (KPFM)[5] quantitative mapping of the photovoltaic activity in electron acceptor/donor blends, on the hundreds of nanometers[6] and on the few nanometers scale.[7]References[1] (a) Adv. Funct. Mater., 2007, 17, 3791-3798 & 3687. (b) Small 2009, 5, 112.[2] (a) Angew. Chem. Int. Ed. 2007, 46, 245. (b) Adv. Mater. 2009, 21, 1131[3] Adv. Mater. 2008, 20, 2433[4] (a) PNAS 2007, 104, 9937. (b) J. Am. Chem. Soc. 2008, 130, 9192. (c) Angew. Chem. Int. Ed. 2008, 47, 3407.[4] Adv. Mater. 2006 18, 1276-1280.[5] (a) Adv. Mater 2006, 18, 145. (b) Acc. Chem. Res. 2010, 43, 4, 541-550[6] Adv. Funct Mater. 2007, 17, 472[7] (a) J. Am. Chem. Soc. 2008, 130, 780. (b) J. Am. Chem. Soc. 2008, 130, 14605
10:00 AM - E12.2
Electric Field Assisted Self-organization of Polymer-fullerene Hybrids on the Photovoltaic Performance.
Chih-Cheng Lin 1 , Yun-Yue Lin 1 , Hsuen-Li Chen 1 , Chun-Wei Chen 1
1 Department of Materials Science and Engineering, National Taiwan University, Taipei Taiwan
Show AbstractWe would like to demonstrate electric field annealing to assist the self-organization of conjugated polymer and to improve the photovoltaic performance of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) hybrid solar cells. By applying a D.C electric field perpendicular to the substrate during the solvent drying process, the formation of fibrous P3HT crystalline domains and bicontinuous phase separation of bulk heterojunctions can be achieved. A power conversion efficiency over 4 % can be achieved by applying this simple process to fabricate the photovoltaic device in air. The anisotropic optical and electrical measurements for devices perpendicular and parallel to the substrates indicate that electric field can assist the self-organization of polymer in both directional orientations. This result can be further supported from the structural and morphological analyses of films by GIXRD, AFM and TEM. Compared to the conventional thermal annealing process, this method has the advantage of non-contact and low-temperature during film formation, which can be easily implemented in the fabrication of large area devices and compatible with the commonly used plastic substrates for flexible solar cell applications.
10:15 AM - E12.3
Supramolecular Origin and Coupling of Exciton Transitions in Highly Uniform Double-walled Nanotubular Dye J-aggregates.
Dorthe Eisele 1 2 , Jasper Knoester 3 , Craig Cone 4 , Kirstein Stefan 1 , David Vanden Bout 4 , Jurgen Rabe 1
1 Department of Physics, Humboldt-Universität zu Berlin, Berlin Germany, 2 IRIS Adlershof, Humboldt-Universität zu Berlin, Berlin Germany, 3 Institute for Theoretical Physics and Zernike Institute for Advanced Materials, University of Groningen, Groningen Netherlands, 4 Department of Biochemistry and Chemistry and Centre for Nano and Molecular Science and Technology, University of Texas at Austin, Austin, Texas, United States
Show AbstractWell-defined and highly uniform artificial light-harvesting model systems are desired to examine the fundamental details of exciton transport and localization in quasi one-dimensional (1D) molecular dye aggregates, where the first key-step is to obtain a basic understanding of the supramolecular origins and coupling of their exciton transitions. Double-walled nanotubular J-aggregates of amphiphilic cyanine dyes are a fascinating quasi 1D excitonic model system [1], displayed in the schematic below. To investigate the uniformity of the aggregate structure we directly probed individual aggregates: the aggregates are deposited from solution onto solid substrates while maintaining their optical and morphological properties. We demonstrate by means of polarization resolved near-field scanning optical microscopy (NSOM) that the supramolecular structure of these aggregates is not only extremely uniform along the individual aggregate but also between all the aggregates in the ensemble [1]. Moreover, we address the supramolecular origin and coupling of the exciton transitions. We show that the nanotubular aggregate consists of two weakly coupled aggregate sub-systems with two distinct spectra. This could be accomplished by chemical oxidation of the chomophores in preferentially one of the nanotubes using an external oxidation agent [2] allowing to isolate specifically the spectrum of the inner wall aggregate system. The coupling between the excitons in the aggregate systems within the inner and outer wall (see schematic above) can be directly observed through a 25 cm-1 shift of the main exciton band in the spectrum of the full double-walled system upon the oxidation. The fact that the nanotubes are electronically coupled even at a separation of 4 nm offers the exciting possibility of coupling the system to other systems such as inorganic nanostructures to form inorganic/organic hybrid systems to study light/matter interaction on the nanoscale. In addition, with this isolated absorption spectrum of an inner wall cylinder it is now possible to make advances in theoretical modeling of cylindrical aggregates to better address how their supramolecular structure affects their electronic and optical properties.[1] D.M. Eisele, J. Knoester, S. Kirstein, J.P. Rabe and D.A. Vanden Bout, Nature Nanotech. 4 (2009) 658; [2] D.M. Eisele, H. v. Berlepsch, C. Böttcher, K.J. Stevenson, D.A. Vanden Bout, S. Kirstein, and J.P. Rabe, JACS 132 (2010) 2104.This research is supported by the Deutsche Forschungsgemeinschaft (Sfb 448) and University of Texas at Austin (R.A. Welch Foundation).
10:30 AM - E12.4
High Efficiency Organic Solar Cells Using Semiconducting Nanowires.
Jong Soo Kim 1 , Joo-Hyun Kim 1 , Min Kim 1 , Kilwon Cho 1
1 Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang Korea (the Republic of)
Show AbstractIn the solution processing of bulk heterojunction organic solar cells based on crystalline organic semiconductors, it is still difficult to obtain maximum device efficiency using a single step process because of the poor carrier mobility and light absorption efficiency of randomly mixed organic semiconductors in as cast photoactive layers. Therefore, post-treatments such as annealing processes using thermal energy or solvent vapor, which increase the complexity of device processing, are necessary to maximize the efficiency of device. Here we are going to present facile methods to fabricate high efficiency devices using preformed organic semiconducting nanowires which have high carrier mobility and light absorption efficiency. P3HT nanowire formation was promoted through the controlled cooling of a saturated P3HT solution in a marginal solvent or mixing of non-solvents. Without any post-treatment or with only applying low temperature thermal annealing to as cast active layers, we achieved optimal active layer morphology of self assembled P3HT nanowires and PCBM molecules. Furthermore, we fabricated highly efficient solar cells with advanced device architectures by utilizing advantages of P3HT nanowires. The highly efficient multilayered solar cells were fabricated by inserting a hole transport/electron blocking pure P3HT layers between the hole collecting electrode and P3HT photoactive active layers. From this device structure, enhanced optical and electrical properties in the blend film of P3HT nanowires/PCBM were achieved and finally high efficiency was obtained. Additionally, we have suggested relatively thick photoactive layers favorable for the printed electronics such as role-to-role and spray printing. An optimized morphology of the blend film was achieved via two separate steps of P3HT crystallization with nanowire formation from mixed solvents and subsequent phase separation on low temperature annealing. With the help of optimized morphology and percolation pathway, carrier mobility was dramatically increased and thickness of the most efficient active layers was shifted up to 300nm which is enough for the whole absorption of incident light with minimum recombination loss of the carriers.Acknowledgement. This work was supported by a Manpower Development Program for Energy and Resource (MKE, 20094020100050-11-1-000), New and Renewable Energy Program (20093020010040-11-1-000) and the National Research Foundation of Korea Grant (MEST, No. 313-2008-2-D00254).
10:45 AM - E12.5
Air-stable Operation of High-mobility C60 TFTs with Organic/Inorganic Hybrid Encapsulations.
Takafumi Uemura 1 , K. Nakayama 2 , M. Yoshizumi 2 , Y. Nakazawa 2 , J. Takeya 1 2
1 ISIR, Osaka University, Osaka Japan, 2 Graduate School of Science, Osaka University, Osaka Japan
Show AbstractThe recent developments of device performances on both p- and n-type OFETs are remarkable. In p-type OFETs, there have been some reports about high-mobility devices with excellent air stability. On the other hand, in n-type OFETs which is indispensable to develop high-performance complementary circuits, many of high-mobility devices have been demonstrated only in inert atmosphere. For example, it is reported that C60 OFETs shows the highest mobility of 6 cm2/Vs among the reported n-type OTFTs; however, the performance readily degrades in air. Since the degradation is caused by oxidative species such as oxygen or moisture, encapsulation techniques are useful to improve the air-stability of n-type OFETs. Here, we tested availability of organic/inorganic hybrid encapsulations on the C60 OFETs. As a result, the encapsulated devices operate over 15 days in air without degradations.The C60 devices with LiF/Al source and drain electrodes are fabricated in glove-box and the initial mobilities were estimated to be ~1.5 cm2/Vs. In the first encapsulation process, parylene insulating layers were deposited on these devices by chemical vapor deposition. On the top of the parylene layer, Al layer was deposited by vacuum evaporation to complete the hybrid encapsulation. In whole processes of the encapsulation, the devices were not exposed to air. In addition, because of the moderate process of the parylene coating, the encapsulated devices remained in initial performances. To evaluate the effect of the hybrid encapsulations, in this study, we compared the air stability of three kinds of devices without any encapsulation, with only parylene encapsulation and with complete hybrid encapsulations. As a result of the air-stability test, the mobility of the device without any encapsulation degraded to 1/10 within only 1 hour. The mobility of the device with only parylene encapsulation also degraded to 1/10 within only 4 hours. On the other hand, the performances of the device with complete hybrid encapsulations remained almost constant over 1 day. Moreover the effect of hybrid encapsulation lasted over days and therefore the mobility remained over 1.5 cm2/Vs during 15 days.In conclusion, we demonstrated the hybrid encapsulation dramatically improve the air-stability of n-type OFETs. As a result, we realized the air-stable operation of the C60 OFETs. In addition, the mobility was estimated to be ~1.5 cm2/Vs which is the highest value among reported air-stable n-type OFETs.
11:00 AM - E12: Assembly
BREAK
11:30 AM - **E12.6
Controlling Morphology and Self-assembly in Conjugated Polymers through Molecular Design.
Rachel Segalman 1 2 , Bryan Boudouris 1 2 , Victor Ho 1 2
1 Chemical Engineering, UC Berkeley, Berkeley, California, United States, 2 Materials Science Division, Lawrence Berkeley National Laboratories, Berkeley, California, United States
Show AbstractControl over structures on a molecular through nanoscopic lengthscales is vital to optimize polymeric devices. For example, while molecular structure affects the electronic properties of semiconducting polymers, the crystal and grain structure greatly affect bulk conductivity, and nanometer lengthscale is vital to charge separation and recombination in photovoltaic and light emitting devices. Careful control over the organic/inorganic interface appears to play a vital role in thermoelectric effects. Poly(3-hexylthiophene) is commonly used because of its relatively high charge mobility, low band gap, and solution processiblity. However, strong intermolecular interactions lead to the formation of nanofibers during crystallization, which prevents long-range microstructural control. We show rod-rod interactions, parameterized by the Maier-Saupe parameter, can be controlled by rational polythiophene side chain design. Effects of side chain passivation are evidenced by a depressed melting temperature and the presence of a liquid crystalline region. Additionally, the Maier-Saupe parameters are estimated for poly(3-dodecylthiophene) and poly(3-ethylhexylthiophene); the relative magnitudes of each are related to the interchain spacings obtained by x-ray diffraction experiments. Due to the relatively low melting temperature of poly(3-ethylhexylthiophene), we are able to controllably undercool during processing to control domain size and orientation.
12:00 PM - E12.7
Nanoscale Co-assembly of Zinc Oxide and Conjugated Organic Molecules in Oriented Macroscopic Films via Electrodeposition.
David Herman 1 , Joshua Goldberger 2 , Stephen Chao 1 , Daniel Martin 1 , Samuel Stupp 1 2 3
1 Materials Science, Northwestern University, Evanston, Illinois, United States, 2 Chemistry, Northwestern University, Evanston, Illinois, United States, 3 Feinberg School of Medicine, Northwestern University, Chicago, Illinois, United States
Show AbstractOne challenge in controlling the assembly of hybrid materials is to precisely direct the growth of the material over many length scales simultaneously. For example, to create a morphology ideal for photovoltaics, a nanoscale periodicity of 5-10 nm between donor and acceptor materials, as well as a perpendicular orientation of the lamellar structure with respect to the substrate surface is important. Using a one-step electrodeposition process, we can create ordered hybrid lamellar assemblies between ZnO and electron donor organic molecules, such as pyrene and thiophene surfactants. In an attempt to control the macroscale growth and orientation of the lamellar structures on indium tin oxide (ITO), factors such as surfactant concentration and solvent composition are shown to have the greatest effect in altering the top-down morphology. Modifying the ITO surface with the conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) changes the initial nucleation and growth processes, and perpendicular orientations of the lamellar structures extending through the macroscale are achieved, as characterized by 2D-GISAXS. This lamellar structure with nanoscale ordering and perpendicular orientation extending throughout the macroscopic film represents an ideal photovoltaic architecture, obtained through a one-step electrodeposition process.
12:15 PM - **E12.8
Towards High Mobility Ambipolar Polymers.
Martin Heeney 1
1 Dept. of Chemistry, Imperial College London, London United Kingdom
Show AbstractAmbipolar organic FETs that are capable of both hole and electron transport under appropriate biasing conditions are of interest as an approach to mimic the performance of CMOS. CMOS based logic has many potential advantages over unipolar logic including lower power consumption, improved robustness and faster switching speeds. Polymer materials that are capable of balanced, high mobility hole and electron transport from common source/drain electrodes are thus highly desirable. In this talk, I will discuss our strategies to design such polymers. We demonstrate how we chemically modify the polymer backbone structure, leading to changes in both the backbone configuration and microstructure, as well as the electronic energy levels of the molecular orbitals such that hole and electron injection are facilitated. The effects of changes in molecular structure on thin film morphology and ambipolar transistor performance are discussed.
E13: Thin Film Transistors: Devices
Session Chairs
David Gundlach
Brendan O'Connor
Friday PM, December 03, 2010
Constitution B (Sheraton)
2:30 PM - **E13.1
Contacts and Organic Semiconductor Devices.
David Gundlach 1 , Yuanyuan Li 2 3 , Dalong Zhou 2 3 , Devin Mourey 2 4 , Thomas Jackson 2 3
1 Semiconductor Electronics Division, NIST, Gaithersburg, Maryland, United States, 2 Center for Thin Film Devices and Materials Research Institute, Penn State University, University Park, Pennsylvania, United States, 3 Dept. of Electrical Engineering, Penn State University, University Park, Pennsylvania, United States, 4 Dept. of Materials Science and Engineering, Penn State University, University Park, Pennsylvania, United States
Show AbstractOrganic semiconductors are attractive for use in a range of thin film electronic applications because their electronic, optoelectronic, and material properties can be finely tuned, and they offer potentially significant manufacturing cost savings because they can be processed by using web compatible deposition methods. Bridging the gap between molecular design and macroscopic device performance has proven challenging since materials processing, film microstructure, and device architecture each impact macroscale device performance and their combined influences are not easily disentangled. Despite significant advances in materials characterization techniques, the electronic device itself, e.g. light emitting diode, field effect transistor, or photovoltaic cell, remains the most sensitive and convenient test vehicle to characterize the electronic properties of materials. However, extracting the “intrinsic” electronic properties from the current-voltage characteristics of imperfect devices is fraught with difficulty. Electrical contacts are common to all electronic devices, as they are required for applying an external bias, and injecting and extracting charge. In general, characterizing contacts and accounting for their effect on device operation is challenging. Forming contacts which provide efficient charge injection is notably problematic for organic semiconductor devices. Since “conventional” doping schemes remain less than effective, metal-semiconductor contacts are formed; giving rise to the potential for deleterious Schottky barrier formation. Depending on the device structure (OLED, TFT, or OPV) and current density requirements, contact effects will dramatically affect device operation and distort and confuse extracted device parameters. In this presentation we discuss charge injection demands placed on contacts for different devices and technologies. We highlight challenges in extracting fundamental electronic properties and key device parameters when a Schottky barrier is formed at the injecting electrode. We model contact effects that may provide insight into anomalously large OTFT mobility values reported in the literature and examine the limitations of methods used to account for parasitic contact effects in current-voltage measurements. We conclude by examining approaches taken to improve the contact characteristics and potential limitations of “conventional” contact region doping.
3:00 PM - E13.2
Electronic Anisotropic Transport in Organic Single Crystals Investigated by Means of Synchrotron-based IR Spectroscopy.
Alessandro Fraleoni Morgera 1 , Beatrice Fraboni 2 , Marta Tessarolo 2 , Leonetta Baldassarre 1 , Andrea Perucchi 1 , Stefano Lupi 1 3
1 , Sincrotrone Trieste SCpA, Basovizza (TS) Italy, 2 Dept. of Physics, University of Bologna, Bologna Italy, 3 Dept. of Physics, University of Roma ``La Sapienza'', Roma Italy
Show AbstractOrganic single crystals (OSCs) may be viewed as model materials for a thorough understanding of the charge transport in organic semiconductors, because of i) the absence of defects related to grain boundaries, ii) well defined geometrical disposition of molecules, iii) intrinsic high degree of order. In particular, the anisotropic electronic properties of OSCs are the subject of intense research in the field [1,2].In this view, we recently presented clear and well reproducible evidences of three-dimensional electronic anisotropy in solution-grown single crystals of 4-hydroxycyanobenezene (4HCB) [3-5].Here we present recent investigations over 4HCB single crystals carried out using synchrotron light Fourier Transform Infra-Red (FT-IR) spectroscopy measurements [6]. The crystals have been used as the active semiconducting layer in organic field-effect transistors. In this configuration, anisotropic polaronic signatures along the two main planar axes of the crystals have been observed, under gate voltages ranging from 0 to 250 V. In addition, polarized FT-IR measurements show anisotropic changes in both the shape and the intensity of the absorption peaks along the two main crystal axes, for varying drain-source voltages. The features showing the most prominent anisotropy along the two planar directions have been assigned to well defined molecular functional groups. Upon these attributions, and considering the 3D structure of the crystals, a tentative correlation between the structure of the 4HCB molecule, as arranged in the lattice, and the observed electronic properties of the crystal, is presented, together with some hypotheses over the transport mechanisms occurring in the material.References[1] D. Braga, G. Horowitz, Adv. Mater., 2009, 21, 1473-1486[2] R. W. de Boer, M. E. Gershenson, A. F. Morpurgo, V. Podzorov, Phys. StatusSolidi A 2004, 201, 1302.[3] B. Fraboni, R. DiPietro, A. Castaldini, A. Cavallini, A. Fraleoni Morgera, L. Setti, I. Mencarelli, C. Femoni, Org. Electron. 2008, 9, 974.[4] B. Fraboni , C. Femoni, I. Mencarelli, L. Setti, R. Di Pietro, A. Cavallini, A. Fraleoni-Morgera, Adv. Mater., 2009, 21, 1835-1839[5] B. Fraboni, A. Fraleoni-Morgera, A. Cavallini, Org. Electron., 2010, 11, 10-15[6] A. Fraleoni-Morgera et al., submitted
3:15 PM - E13.3
Inkjet Printed Circuits Based on Small Molecule P- and N-type Organic Semiconductors.
W.t.t. Smaal 1 , Ashutosh Tripathi 1 , Charlotte Kjellander 1 , Bas van der Putten 1 , Xiaoran Li 1 , Kris Myny 3 , Henry Yan 2 , Antonio Facchetti 2 , Gerwin Gelinck 1
1 , TNO / Holst Centre, Eindhoven Netherlands, 3 , imec, Leuven Belgium, 2 , Polyera Corporation, Skokie, Illinois, United States
Show AbstractMany applications of flexible organic electronics, such as RFID tags and line drivers for OLED displays, require digital circuits with high performance transistors, manufactured using low temperature processes compatible with a flexible substrate. Inkjet printing of soluble small molecule organic semiconductors (smOSC) has high potential for flexible electronics applications due to the high performance of these crystalline molecules and the direct patterning nature of the inkjet technique. Here we present inkjet printed p-type only- and complementary circuitry based on organic thin-film transistors (OTFTs) on flexible substrates. We chose TIPS-PEN as p-type and a Polyera material as n-type smOSC, due to their excellent TFT performance, environmental stability and good solubility. For both p- and n-type smOSC we inkjet printed bottom contact bottom gate OTFTs having charge carrier mobilities larger than 1 cm2/Vs.Integration of p- and n-type semiconductors on a common substrate is non-trivial. In this study we optimized ink formulations and surface treatment to achieve a balanced OTFT performance for both p- and n-type OTFTs. Bottom gate mobilities larger than 0.1 cm2/Vs on a substrate with common dielectric and metal contacts are obtained resulting in CMOS ring oscillators operating at frequencies above 3 kHz.
3:30 PM - E13.4
Molecular-scale Properties of Molybdenum Trioxide Dopants in Pentacene.
Sieu Ha 1 , Jens Meyer 1 , Antoine Kahn 1
1 Electrical Engineering, Princeton University, Princeton, New Jersey, United States
Show AbstractElectrical doping in organic electronics is becoming more widespread as researchers explore techniques to enhance device performance. However, much of the physics of doping in organic materials and the intimate interactions between dopant and host molecules are not well understood. To elucidate these local properties, we previously examined pentacene p-doped with tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) using scanning tunneling microscopy (STM) and found that donated holes are localized by the counter potential of ionized dopants. Here, we extend our former studies with the molybdenum trioxide (MoO3)-doped pentacene system. Ultraviolet photoelectron spectroscopy (UPS) measurements clearly show that MoO3 incorporation into pentacene films shifts the Fermi level closer to the pentacene HOMO, indicative of p-doping. MoO3 molecules aggregate in the gas phase when sublimated. All species of MoO3 (i.e. monomers, polymers, etc.) are expected to donate holes to the pentacene lattice, but they have varying effects on STM images. For low molecular weight species, a depression feature is observed in occupied states imaging, implying that the donated hole is localized by the ionized dopant potential, similar to as seen in F4-TCNQ-doped pentacene. For larger MoO3-related features, no such localized hole effects are observed because the anion counter potential fully masks the Coulomb potential of the localized hole. The observation of charge localization in MoO3-doped pentacene agrees well with results from F4-TCNQ-doped pentacene and suggests that donated carrier localization in doped organic films may be a general phenomenon. We speculate that MoO3 monomers and dimers enhance film conductivity, but larger polymers and clusters, which make up the majority of sublimated MoO3, do not, leading to low doping efficiencies in organic devices. MoO3 species do not diffuse on the pentacene surface and do not perturb the host lattice structure. Scanning tunneling spectroscopy measurements show that the electron conductivity of MoO3–pentacene is diminished by MoO3 doping. This is due to electron trapping by dopants, as expected in p-doping, and should scale with doping concentration.
3:45 PM - E13.5
A Comparative Study of Charge Transport and Meyer-neldel Rule in Fullerene Devices.
Mujeeb Ullah 1 , Almantas Pivrikas 2 , Clemens Simbrunner 1 , Gebhard Matt 1 , Niyazi Sariciftci 2 , Helmut Sitter 1
1 Institute of Semiconductor and Solid State Physics, Johannes Kepler University Linz, Linz Austria, 2 Linz Institute of Organic solar cells (LIOS), Johannes Kepler University Linz, Linz Austria
Show AbstractWe report the charge carrier mobility using two techniques, in the bulk of fullerene films using Charge Extraction by Linearly Increasing Voltage (CELIV) technique [1] and at the interface of insulators using Organic Field Effect Transistors (OFET) [2]. We observed that the electron mobility is at least two orders of magnitude higher than hole transport in the C60 films prepared by thermal evaporation. Electric field, carrier concentration and temperature dependences of the electron mobility was measured using both methods. More than one order of magnitude higher charge carrier mobility values are measured in OFET configuration due to high charge carrier concentrations at the quasi 2D transport near the dielectric interface. We comparatively present the carrier concentration and electric field dependence of the charge carrier mobility in both devices. Meyer-Neldel Rule (MNR) is observed in both fullerene devices [3,4]. Meyer-Neldel energy, EMN = 35 meV, which is interpreted as disorder parameter [5], is the same in both device geometries, which suggest that the level of disorder is similar in the bulk of fullerene films and at the interface with insulators. Reference:1.A. Pivrikas,1,a) Mujeeb Ullah,2 Th. B. Singh,1 C. Simbruner,2 G. Matt,1 H. Sitter,2 and N. S. Sariciftci1, Organic Elecrtronics submitted.2.Mujeeb Ullah, D. M. Taylor, R. Schwödiauer, H. Sitter, S. Bauer, N. S. Sariciftci and Th. B. Singh, Journal of Applied Physics, 106, 114505 (2009).3.Mujeeb Ullah, T.B. Singh, H. Sitter, N.S. Sariciftci, Applied Physics A, Materials Science & Processing 97 (2009), 521.4.Mujeeb Ullah, I. I. Fishchuk, A. Kadashchuk, P. Stadler, A. Pivrikas, C. Simbrunner, V. N. Poroshin, N. S. Sariciftci, and H. Sitter, Appl. Phy. Lett. 96,213306 (2010).5.I. I. Fishchuk, A. K. Kadashchuk, J. Genoe, Mujeeb Ullah, H. Sitter, Th. B. Singh, N. S. Sariciftci, and H. Bässler, Phys. Rev. B 81, 045202 (2010).
4:00 PM - E13: TFT Devices
BREAK
4:30 PM - E13.6
Hall Effect and Charge Transport Mechanism in High-mobility Organic Transistors.
Jun Takeya 1 2 , T. Uemura 1 , M. Yamagishi 2 , Y. Okada 2 , J. Soeda 2 , Y. Nakazawa 1
1 , Osaka University, Ibaraki Japan, 2 , Grad. School of Science, Toyonaka Japan
Show AbstractDevelopment of functional materials and understanding of the microscopic mechanisms mutually benefit through their close interaction. To accelerate development of organic semiconductor films for industrial application to flexible electronics devices, it is essential to understand mechanisms of charge transport in conjunction with molecular-scale charge transfer. Here, we examine universality of the idea that practically attractive high-mobility charge transport in organic transistors is caused by band-like carrier dynamics using several different molecular systems as the active semiconductor layers. We employ Hall-effect measurement which differentiates the diffusive band transport from site-to-site hopping. We prepared both single-crystal and polycrystalline samples with rubrene, dinaphtho[2,3-b:2',3'-f]thieno[3,2-b] thiophene (DNTT), 2, 7-dialkyl derivatives of benzothienobenzothiophene (BTBT), pentacene, and C60, all of which have mobility exceeding 1 cm2/Vs. The result of the measurement shows Hall coefficient identical to inverse charge density for the devices of rubrene, DNTT, and BTBT, indicating well extended nature of the carriers accumulated at the surface of these semiconductors. On the contrary, the devices of pentacene and C60 have exhibited obvious discrepancy between the Hall coefficient and inverse charge density, meaning the free-electron like picture is violated. Measuring temperature dependence of the devices, we argue about fundamental difference in the electronic states between the organic molecular systems of the above two categories.Furthermore, the devices gated by ionic liquid are also studied to investigate the charge transport at the solid-to-liquid interfaces of organic semiconductors and the ionic liquid. It turned out that rubrene single-crystal transistors show apparent coincidence between the Hall coefficient and inverse charge density, meaning that the diffusive band-like transport holds even when coupled to the molecular motion of the liquid surfaces. The results provides ingredients to fully understand the complex charge dynamics in organic semiconductors with the variation of molecular species and interfaces constructed with varied gate insulators.
4:45 PM - E13.7
Carbon Nanotubes Arrays as Contacts for P- and N-Type Organic Field Effect Transistors.
Fabio Cicoira 1 2 , Richard Martel 2
1 Istituto di Fotonica e Nanotecnologie, Consiglio Nazionale delle Ricerche, Trento Italy, 2 Departement de Chimie, Universite de Montreal, Montreal, Quebec, Canada
Show AbstractThe outstanding electrical and mechanical properties of carbon nanotubes, combined to chemical inertness, thermal stability and high aspect ratio, make them ideal materials for electronics. Here we explore the performance of carbon nanotube array electrodes in organic thin-film transistors (OTFT). We demonstrated that these array electrodes provide effective hole and electron injection with pentacene and Phenyl-C61-butyric acid methyl ester (PCBM) organic semiconductors, respectively. In comparison to metal contacts, OTFT with nanotube electrodes display a close to ideal injection behavior, high field-effect mobility and fast switching characteristics. The exceptional injection efficiency of nanotube electrodes represents a breakthrough in organic electronics and opens exciting scenarios for OTFT.
5:00 PM - E13.8
Universal Properties of Linear Magnetoresistance in Strongly Disordered Semiconductors.
Hannah Johnson 1 , Steve Bennett 2 , Thomas Nummy 1 , Radhika Barua 3 , Laura Lewis 3 , Don Heiman 1
1 Physics, Northeastern University, Boston, Massachusetts, United States, 2 Mechanical Engineering, Northeastern University, Boston, Massachusetts, United States, 3 Chemical Engineering, Northeastern University, Boston, Massachusetts, United States
Show AbstractImperfections in materials, such as impurities and defects, alter the electronic structure of materials to provide the backbone of many advanced technological applications. Of major interest is the role played by large-scale disorder on the electronic and especially magnetoelectronic properties of materials. Materials and devices with predictable magnetoresistance (MR) – the change in resistance in an applied magnetic field – are important for applications such as sensors for magnetic memories. In homogeneous semiconductors having limited electrical disorder, the MR increases quadratically with increasing magnetic field and eventually saturates at relatively small fields. On the other hand, the introduction of strong electrical disorder in semiconductors provided by compositional inhomogenities generates anomalously large MR that increases linearly with increasing magnetic field,[1] and does not saturate even at very high fields[2]. Materials with positive, nonsaturating linear magnetoresistance (LMR) comprise a distinct class of disordered semiconductors. At the root of this behavior is the presence of insulating or metallic particles embedded in the semiconductor matrix.We describe the correlation of linear magnetoresistance to the fundamental electrical properties of a strongly disordered semiconductor system. Transverse MR and Hall measurements were carried out on thin composite films containing self-assembled MnAs metallic nanoparticles embedded in a GaAs matrix. We find large MR in this quasi-2D system which is linear up to H=14 T. The magnitude of the LMR is found to be numerically equal to the magnitude of the macroscopic carrier mobility over a wide range of temperature (T=50-300 K) and mobility. This equality, MR(T)=μ(T)H, is obeyed despite the fact that the carrier concentration changes by several orders of magnitude as a function of temperature. Since LMR is governed by carrier mobility and independent of carrier density, it is complementary to the classical Hall effect that is governed by the carrier concentration and is independent of carrier mobility. In addition, by examining different samples with a variety of disorder, the link between LMR and mobility does not appear to be affected by the specific details of the disorder, such as the size and concentration of the disordered regions. These results point to a universal model of MR in semiconductors with strong local disorder, offering insights for designing new MR systems.Acknowledgements: Work supported by the NSF grant DMR-0907007. H.J. and S.B were supported by an NSF-REU supplement. We thank W. Fowle for expertise with SEM imaging.[1] M.M. Parish and P.B. Littlewood, Nature (London) 426, 162 (2003).[2] A. Husmann, J.B. Betts, G.S. Boebinger, A. Migliori, T.F. Rosenbaum, and M.-L. Saboungi, Nature (London) ,417, 421 (2002).
5:15 PM - E13.9
Ferroelectricity in Supramolecular Networks of Charge Transfer Complexes.
Alok Tayi 1 , Alex Shveyd 2 , Chi-Hau Sue 2 4 , Kang Wang 4 , J. Fraser Stoddart 2 , Samuel Stupp 1 2 3
1 Materials Science & Engineering, Northwestern University, Evanston, Illinois, United States, 2 Chemistry, Northwestern University, Evanston, Illinois, United States, 4 Electrical Engineering, UCLA, Los Angeles, California, United States, 3 Feinberg School of Medicine, Northwestern University, Evanston, Illinois, United States
Show AbstractFerroelectrics are novel materials with spontaneous, bistable electric polarization that can be used in non-volatile electronics, sensors, and nonlinear optical systems. Organic ferroelectric materials are of particular interest as they are solution-processable, lightweight, and inexpensive. Crystalline organic charge-transfer complexes containing mixed stacks of donor and acceptor molecules are interesting candidates for ferroelectric behavior. In these systems, collective transfer of electrons from the donor to the acceptor molecule results in the formation of dipoles oriented along a single crystal axis. Some of these crystals are known to undergo a phase transition to a noncentrosymmetric lattice at low temperature that results in spontaneous polarization. Though extensively studied, only a small number of ferroelectric candidates have been developed and polarization switching by an external electric field has yet to be successfully demonstrated. In this work we demonstrate polarization bistability and D-E hysteresis loops in novel hydrogen-bonded supramolecular charge-transfer crystals. These systems have potential for the development of non-volatile information storage based on organic ferroelectrics.
5:30 PM - E13.10
Device Configurations for Ambipolar Transport in Flexible, Pentacene Transistors.
Sangameshwar Rao Saudari 1 , Yu Jen Lin 1 , Yuming Lai 2 , Cherie Kagan 1 2 3
1 Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 2 Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 3 Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractAmbipolar organic transistors have recently attracted considerable attention for applications in CMOS like inverters and light emitting transistors. But reports of ambipolar transport in transistors based on high mobility semiconductors such as pentacene and rubrene are still very few and their integration onto flexible plastic substrates has not been explored. Here we report the fabrication of solution-deposited pentacene transistors on flexible plastic substrates and the effects of device configuration [bottom contact-bottom gate (BC-BG), top contact-bottom gate (TC-BG) and bottom contact-top gate (BC-TG)] on unipolar/ambipolar transport. By engineering the device interfaces and device scaling, ambipolar transport was achieved in BC-BG and TC-BG devices. Flexible pentacene transistors are fabricated on DupontTM Kapton® substrates by depositing evaporated metal source, drain, and gate electrodes, hydrophobic solution-deposited benzocylobutene or sublimed parylene gate dielectrics, and a solution-deposited precursor that is thermally converted to semiconducting thin films of pentacene. In BC-BG, the Au source-drain electrodes are derivatized with thiolate and carbodithiolate self-assembled monolayers to improve electron and hole injection. Hole and electron mobilities of 0.05–0.1 and 0.01–0.05 cm2/Vs are achieved in these devices. In the same aspect ratio (width/length of channel) devices, we observed an increase in electron and hole currents with increasing channel length due to a lowering of the contact resistance. As compared to devices in BC-BG geometry, those in TC-BG geometry showed better balanced hole and electron currents and lower contact resistances for both type of carriers, which is anticipated to arise from the increased electrode-pentacene contact area. We also studied device stability in ambient and nitrogen environments and under bias. While the hole current is stable is ambient air, the electron current is not air stable. The devices showed little hysteresis in transfer characteristics when the gate voltage is swept only in the hole or electron accumulation regime as compared to when the gate voltage is swept widely in gate voltage across hole and electron accumulation regimes. This hysteresis is consistent with electron traps in pentacene or at the pentacene-dielectric interface.
5:45 PM - E13.11
Characterization of Charge Transport in Organic Thin Films by Thermoelectric Measurement.
Gun-Ho Kim 1 , Max Shtein 2 , Kevin Pipe 1
1 Mechanical Eng., University of Michigan, Ann Arbor, Michigan, United States, 2 Material Science, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractA thorough understanding of charge transport mechanisms in organic semiconductors is critical for improving the performance of organic transistors, light emitting devices, and solar cells. In particular it is important to relate physical properties such as crystallinity, impurity concentration, and molecular structure to key transport parameters such as carrier mobility and density of states. While several techniques exist for measuring field effect mobility in organic semiconductor inversion layers, measuring charge mobility in the bulk (which has important implications for numerous device applications) is considerably more difficult, in large part due to a high electrical resistance. Measurement of the thermoelectric Seebeck coefficient shows promise as a convenient means to distinguish the contributions of carrier concentration and mobility to the electrical conductivity, since the Seebeck coefficient primarily depends only on the carrier concentration and density of states. Here we perform thermoelectric measurements on iodine-doped pentacene thin films to demonstrate the benefits of this technique. Using a low-noise setup that includes an instrumentation amplifier and liquid-cooled heat sink, we achieve highly precise measurement of the Seebeck coefficient over a range of hole concentrations during iodine dedoping. From this data we derive the hole density of states, the position of the Fermi energy with respect to the energy of conducting holes, and the bulk hole mobility as a function of hole concentration. For the range of concentrations studied, the bulk hole mobility is shown to be constant at high concentrations and strongly increase as the concentration falls below a threshold value.