Symposium Organizers
Alejandro L. Briseno, University of Massachusetts
Aram Amassian, King Abdullah University of Science and Technology (KAUST)
Iain McCulloch, Imperial College London
Özlem Usluer, Konya Necmettin Erbakan University
Symposium Support
ACS Applied Materials amp
Interfaces | American Chemical Society
Aldrich Materials Science
MD5.1: Theory, Computation and Transport
Session Chairs
Aram Amassian
Iain McCulloch
Tuesday PM, March 29, 2016
PCC West, 100 Level, Room 102 AB
2:30 PM - *MD5.1.01
Impact of Polymer/Fullerene Intermolecular Interactions on the Performance of Organic Solar Cells
Jean-Luc Bredas 1
1 KAUST Thuwal Saudi Arabia,
Show AbstractIn this presentation, we seek to provide a rationalization of the impact that intermolecular arrangements and interactions at the polymer/fullerene interfaces have on the performance of bulk-heterojunction solar cells. We discuss the results of combined electronic-structure calculations and molecular-dynamics simulations both for representative systems reported in the literature and new systems synthesized in our Center. In particular, we examine:
(i) the propensity of the fullerene molecules to dock preferentially on top of the electron-poor moiety or electron-rich moiety of the polymer, as a function of the nature and location of the polymer side chains; and
(ii) the impact that the packing arrangements have on the energetic distribution of the charge-transfer interfacial electronic states and their localization/delocalization characteristics.
This work is supported by King Abdullah University of Science and Technology, in the framework of its Solar & Photovoltaics Engineering Research Center (SPERC) and its Collaborative Research Grant Program (Award CRG3-62140391), and by ONR-Global (Award N62909-15-1-2003).
3:00 PM - MD5.1.02
Developing Chemical Insight as to How Molecular Structure Drives the Solid-State Packing of Organic Semiconductor
Chad Risko 1
1 University of Kentucky Lexington United States,
Show AbstractWhile improved materials, processing protocols, and device designs have ushered organic electronic devices onto the commercial landscape, there remains a need to establish a thorough understanding of the intimate relationships among chemical and molecular structure, processing, solid-state packing, and the underlying physical processes that determine material performance. Through the development and application of multiscale, theoretical materials chemistry approaches, we seek to develop the chemical insight behind these relationships that is necessary to refine and offer novel design pathways for next generation organic semiconducting active layers. In this presentation, we will focus on how such models reveal the influence of seemingly modest changes in chemical structure on the processing and solid-state packing of organic semiconducting active layers.
3:15 PM - MD5.1.03
Ab Initio Investigations on the Donor-Acceptor Interface in Organic Photovoltaics
Hossein Hashemi 1,Michael Waters 1,John Kieffer 1
1 Univ of Michigan Ann Arbor United States,
Show AbstractThe structure and electronic properties of a series of donor-acceptor organic molecules are explored using ab initio calculations to understand the behavior of polaron pairs at the interface of the donor-acceptor junction. Results suggest that one may be able to control polaron pair behavior based on the asymmetry of the donor molecule. This allows for control of thermodynamic losses as well as open circuit voltage. These calculations also revealed the probability distribution of the formation of different types of polaron pairs, especially, as it relates to deposition order (i.e. donor on top of acceptor versus acceptor on top of donor). The energetics of crystalline substrates with different surface terminations are mapped out using a single molecule of the partnering species. Accordingly, the interfacial structure and properties are different depending on whether the substrate is a donor or acceptor due to the incongruency between lattices and the disorder that develops in the contact layers of donor and acceptor.
3:30 PM - MD5.1.04
Energy Level Control in Organic Salts for Efficient, Deep Near-Infrared Organic and Transparent Photovoltaics
John Suddard-Bangsund 1,Margaret Young 1,Tyler Patrick 1,Christopher Traverse 1,Natalia Pajares Chamorro 3,Richard Lunt 2
1 Department of Chemical Engineering and Materials Science Michigan State University East Lansing United States,1 Department of Chemical Engineering and Materials Science Michigan State University East Lansing United States,3 Universidad Politécnica de Madrid Madrid Spain1 Department of Chemical Engineering and Materials Science Michigan State University East Lansing United States,2 Department of Physics and Astronomy Michigan State University East Lansing United States
Show Abstract
Extending photoresponse into the near-infrared (NIR) is one clear route to improving performance of both panchromatic and transparent organic photovoltaics (OPVs). However, current demonstrations of NIR active OPVs have been limited by low open-circuit voltages (VOC) and low external quantum efficiencies (EQE). One reason for this is the challenge of optimizing energy level alignment within the tightened tolerance of a small bandgap. In this work, we show that VOC and EQE can be simultaneously enhanced in organic salt-based OPVs via single-step anion exchanges. We demonstrate that VOC gains are due to improved energy level alignment and that energy levels can be finely tuned by alloying various anions. These effects can be exploited to optimize energy level alignment for abitrary donor-acceptor pairings with novel low bandgap organic salts, and we extend this method to several new molecules with unprecedented VOC (for their spectral range) and photoresponse from 950 nm to as far as 1500 nm. This work bypasses the VOC bottleneck which previously hindered small molecule-based photovoltaics, and presents an exciting path forward for the design of efficient multi- and single-junction transparent photovoltaics.
4:15 PM - MD5.1.05
Organic Donor-Acceptor Charge-Transfer Semiconductors: A Theoretical Characterization of the Microscopic Parameters
Veaceslav Coropceanu 1
1 School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics Georgia Institute of Technology Atlanta United States,
Show AbstractWe used Density Functional Theory (DFT) calculations to study the electronic structure of organic mixed-stack charge-transfer crystals. We investigate the impact that the amount of nonlocal Hartree-Fock exchange (HFE) included in a hybrid density functional has on the geometry, the normal vibrational modes, electronic coupling and electron-vibrational couplings in these systems. The crystal geometry and the frequencies of the phonons are found to be only modestly affected by the amount of HFE. In contrast, the electronic couplings and electron-vibration couplings show a strong dependence on this value. We compare the DFT results with results obtained within the G0W0 approximation as a way of benchmarking the optimal amount of HFE needed in a given functional. The electronic structures of a series of donor-acceptor crystals have been investigated in detail. The charge-transport, ferroelectric and non-linear optical properties of several systems that were investigated will be discussed.
4:45 PM - MD5.1.07
Experimental and Modeling Studies of Charge Transport and Recombination Mechanisms in Fullerene-Based Organic Solar Cells
Liang Xu 1,Jian Wang 1,Yun-Ju Lee 1,Julia Hsu 1
1 Univ of Texas-Dallas Richardson United States,
Show AbstractThe fullerene-based organic solar cells (OSC) with a very minute amount of polymer “donor” have recently attracted intensive research interest due to their simple structure and unique electrical performance especially large Voc. However, charge transport mechanism in such devices with “donor” concentration much lower than the percolation threshold is still unclear. Some studies propose the high Voc is the Schottky barrier height between the anode electrode and the fullerene LUMO. On the other hand, other authors argue that Voc is enhanced because CT states, the major pathways for bimolecular recombination in bulk-heterojunction OSC devices, are significantly reduced in these active layers. Studies based primarily on current density-voltage (J-V) measurements are unable to differentiate different mechanisms. Here we apply impedance spectroscopy (IS), low-energy external quantum efficiency (EQE) spectroscopy, and photoluminescence (PL) dynamics in addition to J-V to study PCBM-based organic solar cells with varying P3HT concentrations. The experimental results are compared to 1D drift-diffusion modeling using SCAPS software.[1] Strong frequency-dependent capacitance behaviors are observed in devices with very low donor concentrations, indicating significant charge accumulation due to space charge limited as well as trap limited transport process. In addition, the role of exciton-polaron annihilation due to poor exciton dissociation and polaron collection will be probed by PL dynamic studies. Finally, impacts of p-type doping of active layer in the fullerene-based OSCs on hole mobility and internal field improvements will be discussed.
Reference:
[1] M. Burgelman, P. Nollet, S. Degrave, Modelling polycrystalline semiconductor solar cells, Thin Solid Films. 361 (2000) 527–532.
This project is sponsored by National Science Foundation DMR-1305893
5:00 PM - MD5.1.08
2-Dimensional Series Resistance Modeling of Thin-Film Solar Cells and Modules: Influence on the Geometry-Dependent Efficiency
Marco Seeland 2,Roland Roesch 1,Harald Hoppe 1,Felix Herrmann-Westendorf 1
2 TU Ilmenau Ilmenau Germany,1 Friedrich-Schiller-University Jena Jena Germany
Show AbstractLimited lateral conductivities of the photo-active materials used in organic thin-film solar cells necessitate the use of semitransparent electrodes for current collection and lateral current transport. Due to the tradeoff between electrical conductivity and optical transmission, which should not underrun 80%, typical values for the sheet resistances of semitransparent electrodes deposited on glass amount to 10–20 Ω/sq. The power loss due to Joule heating and the accompanying voltage drop caused by this sheet resistance increases with cell length in current transport direction and thus defines an upper limit for practical solar cell lengths. In the several approaches existing for calculation of the power loss, the semitransparent electrode layer is either modeled as one lumped resistance in series to the solar cell or as distributed resistance across the whole length of the solar cell in current transport direction. We present a quantitative comparison between these two concepts to investigate the direct influence on the optimal solar cell geometry and to discuss the capabilities as well as limitations of each model. Furthermore the impact of the results on the serial interconnection of a monolithic thin film organic solar cell module in terms of optimal cell lengths and cell interconnection distance is evaluated. The computational study presented here is based on the material system PCDTBT:PC70BM for the photo-active layer material as an example and commonly used semitransparent conductive electrodes: ITO deposited on glass as well as on PET foil and highly doped PEDOT:PSS named PH1000 on PET foil.
5:15 PM - MD5.1.09
How the Energetic Landscape in the Mixed Phase of Organic Bulk Heterojunction Solar Cells Evolves with Fullerene Content
Rohit Prasanna 1,Sean Sweetnam 1,Tim Burke 1,Jonathan Bartelt 1,Michael McGehee 1
1 Stanford Univ Stanford United States,
Show AbstractEnergy levels in the mixed polymer-fullerene phase of bulk heterojunction solar cells are significantly shifted from their values in the pure materials [1]. These shifts are important for solar cell performance: they create energy cascades between the mixed phase and pure donor and acceptor phases, which have been shown to improve geminate splitting and suppress bimolecular recombination. This work investigates the origin of these energy level shifts and explains their effect on the charge transfer (CT) state energy and open circuit voltage (Voc).
We use regiorandom P3HT:PCBM as our model system. Regiorandom P3HT is amorphous, and when blended with PCBM, forms only one amorphous mixed phase, allowing us to study the energetics of the mixed phase without effects of polymer crystallites. We measure the polymer ionization potential (IP) and fullerene electron affinity (EA) as a function of blend composition using cyclic voltammetry (CV). The polymer IP monotonically increases in magnitude by around 400 meV and the fullerene EA by around 200 meV as the fullerene content in the blend is increased from 29% to 71%. The effective band gap measured by CV increases by around 300 meV.
Computational modelling studies using molecular dynamics [2] and classical microelectrostatics [3] have suggested that electrostatic interactions give rise to dipoles at the molecular interface between polymer and fullerene. We hypothesize that the electrostatic potential created by these interfacial dipoles shifts the energy levels of individual molecules in the blend. To test this hypothesis, we design a simplified electrostatic model to compute the effects of interfacial dipoles on polymer and fullerene energy levels. We fit the energy level shifts predicted by this model to experimentally measured values, using the magnitude of the interfacial dipole as the only fitting parameter. With this model, we show that the energy level shifts can be quantitatively accounted for by interfacial dipoles of comparable magnitude to permanent dipole moments in the molecules.
Despite large changes in the effective band gap, the measured CT state energy and Voc shift by only small amounts and show no trend. We show that energetic disorder in the mixed phase results in broadening of all the densities of states. During normal solar cell operation, only the low-energy tail of the CT density of states is filled, and effectively sets Voc. While changes in blend composition produce large changes in the centres of the energy levels, their low-energy tails are only slightly affected, resulting in Voc not varying by much.
In conclusion, this work shows how energy levels in the mixed phase are shifted by dipoles at the polymer-fullerene interfaces. Tuning these energy level shifts is likely to be an important part of future strategies aimed at improving the efficiencies of organic solar cells.
[1] J. Am. Chem. Soc. 2014, 40, 14078
[2] Adv. Mater. 2013, 25, 878
[3] J. Phys. Chem. C 2013, 117, 12981
5:30 PM - *MD5.1.10
Modifying the Optoelectronic Properties of Rubrene by Strain
Sahar Sharifzadeh 1,Ashwin Ramasubramaniam 2
1 Department of Electrical and Computer Engineering Boston University Boston United States,2 Department of Mechanical amp; Industrial Engineering University of Massachusetts Amherst Boston United States
Show AbstractRubrene is a promising material for organic electronics and optoelectronics; it forms crystalline films with high hole mobility and efficient electroluminescence. Recent studies have shown that the electronic properties of rubrene films can be tuned by substrate-induced strain, suggesting a new route towards the design of more efficient devices. Here, we present a first-principles analysis of the strain-induced changes to the mechanical, electronic, and optical properties of rubrene crystals. Density functional theory and many-body perturbation theory studies predict changes in hole motilities in excellent agreement with electrical conductivity measurements when a strain consistent with the experiment is applied. Furthermore, we predict that the optical absorption and nature of low-energy excitons within the crystal can be tuned by an applied strain as low as 1%. This work utilized resources at the Center for Nanoscale Materials, supported by the U.S. Department of Energy under Contract No. DE-AC02-06CH11357.
MD5.2: Poster Session I
Session Chairs
Wednesday AM, March 30, 2016
Sheraton, Third Level, Phoenix Ballroom
9:00 PM - MD5.2.01
Lacunary Polyoxometalates as Effective Electron Conducting Layers for Improving Efficiency in Organic Optoelectronics
Yasemin Topal 2,Marinos Tountas 1,Maria Vasilopoulou 1,Mahmut Kus 2,Mustafa Ersoz 2
2 Selcuk University Advanced Technology Research and Application Center Konya Turkey,1 Institute of Nanoscience and Nanotechnology (INN), National Center for Scientific Athens Greece
Show AbstractOrganicoptoelectronics, such as organicphotovoltaics (OPVs) and organic light emitting diodes (OLEDs), offer the promise of low-cost flexible solar cells, displays, and light sources that have the potential to be manufactured on large-area plastic substrates. One of the key elements for improving efficiencies in organic optoelectronics is finding suitable cathode electrode materials to replace the reactive low work function metals, such as calcium or magnesium, that are typically used to either inject electrons into or extract electrons from the lowest unoccupied molecular orbital (LUMO) of a given organic semiconductor.Polyoxometalates (POMs) are a well-known large group of clusters with frameworks built from transition metal oxo anions linked by shared oxide ions, first reported by Jöns Jacob Berzelius in 1826.One of the most intriguing properties of POM clusters is their high ability to accept a large number of electrons with minimal structural modifications, a property that could enable them to play an important role as excellent electron conductors in electronic devices.
We report here on the preparation of efficient electron conducting layers consisting of lacunaryPOM clusters spin coated from a water/methanol solution between the organic semiconductor and the cathode electrode surfaces for application inorganic optoelectronics. The photoelectron spectroscopy study reveals that these POMsexhibit a high degree of reduction which introduces delocalized electrons that can hop over the metal centers creating thus near Fermi level states that overlap sufficiently with the Fermi level of the metal cathode acting as a highly conductive path for electron conduction through the cathode interface. It is verified also that our method is applicable to a wide range of different organic semiconducting materials and that can be used in state-of-the-art high-efficiency organic electronic devices, including OLEDs and bulk heterojunction (BHJ) OPVs.In particular, OLEDs based on the greenemitting poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1’,3}-thiadiazole)] (F8BT) and a wide range of OPV devices based on photoactive layers composed of mixtures of a polymer donor, such as poly(3-hexylthiophene) (P3HT), poly[(9-(1-octylnonyl)-9H-carbazole-2,7-diyl)-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) or poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2- b :4,5- b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4- b ] thiophenediyl}) (PTB7), and a fullerene acceptor, such as [6,6]-phenyl C71butyric acid methyl ester (PC71BM), exhibited a large enhancement in their efficiencies when used the reduced POMs as electron conduction layers.
9:00 PM - MD5.2.02
Towards Reliable and Fully Automated Estimation of Fundamental Properties of Molecules Suitable for Organic Solar Cells Using State-of-the-Art DFT
Torsten Sachse 1,Benjamin Dietzek 2,Martin Presselt 2
1 Institute of Physical Chemistry Friedrich Schiller University Jena Jena Germany,1 Institute of Physical Chemistry Friedrich Schiller University Jena Jena Germany,2 Department of Spectroscopy/Imaging Leibnitz Institute of Photonic Technology Jena Germany
Show AbstractDespite the many advances, OPV devices still fall behind their inorganic counterparts in terms of power conversion efficiency and longevity. Up to now, disordered blends of donor and acceptor molecules yield the best solar cells. However, every device is a compromise between small and large phases, which favor charge carrier generation and charge carrier transport, respectively. Additionally, optimizing processing conditions is a non-trivial task, which is usually done empirically. Hence, new materials often lag behind expectations obtained from chemical intuition. Furthermore, synthetic chemists open up new routes to synthesize even more molecules applicable to OPV meaning that the sheer number of compounds available renders the experimental selection of suitable compounds difficult. Hence, in-silico pre-screening of such new compounds is desirable.
Although in-silico screening of new compounds is less laborious than the experimental analogue, only a limited number of compounds can be treated due to the manual effort involved. Hence, we employ automation routines in order to minimize manual intervention on runtime-restricted systems that allow for easy screening of a large number of compounds in addition to accelerating calculations by the use of an efficient implementation of electron core potentials [Song et al., J. Chem. Phys., 2015, 143(1)]. Material properties that need to be predicted correctly by the aforementioned screening procedures include ionization potentials, electron affinities and exciton binding energies and absorption spectra both, in vacuum and in solution. This means that correct predictions of absolute frontier orbital energies and optical gaps are necessary in these environments. We could already show the fast reproduction of absorption spectra in some of these cases [Gampe et al., Chem. Eur. J., 2015, 21(20)].
Frontier orbital energies are very important molecular properties that decide whether or not a materials system can produce a photocurrent under irradiation as the difference between these energies is the main driving force behind exciton separation at the interface. However, commonly employed DFT calculations grossly underestimate the fundamental gap. Recently, it was suggested to use the class of range-separated hybrid functionals to overcome this issue by tuning or optimizing the range-separation parameter to the system at hand without the need for empirical data. This is done by minimizing the misfit to Koopman’s theorem and an analogous, inverse variant concerning the LUMO.
We employ this technique to reproduce experimental molecular properties for a variety of dyes applicable to OPV, such as porphyrins. We also compare the results obtained for different functionals. The automation routines we employ render this a feasible approach for research in the field of OPV.
We acknowledge funding from the Bundesministerium für Bildung und Forschung (BMBF FKZ 03EK3507) and the Deutsche Bundesstiftung Umwelt (DBU).
9:00 PM - MD5.2.03
Self-Assembling Amphiphilic Dyes and Tuning of Photonic Properties via Morphology Control
Martin Presselt 2,Saunak Das 1,Felix Herrmann-Westendorf 1,Maximilian Hupfer 1,Benjamin Dietzek 2
1 Friedrich Schiller University Jena Jena Germany,2 Leibniz Institute of Photonic Technology (IPHT) Jena Germany,1 Friedrich Schiller University Jena Jena Germany
Show AbstractThin organic chromophore layers are the essential functional elements in devices for light to electric energy conversion and vice versa, i.e. in organic solar cells (OSCs) and organic light emitting diodes (OLEDs). In OLEDs the morphology of the active layer needs to be engineered for optimal directional characteristics of light emission while in OSCs the morphology of the active layer needs to be elaborately tuned for optimal power conversion efficiency. However, the morphology is generally challenging to target starting from molecular chromophore design. This is due to the fact that predictions of supra-molecular structures are still computationally very demanding at a high accuracy level and the formation of particular morphologies depends on various external physical, chemical and processing parameters. Therefore, various approaches have been developed to control the morphology. One promising approach to design defined supra-molecular structures that are thermodynamically stable is self-assembly. A prominent strategy to induce self-assembly is to introduce amphiphilicity to functional molecules. Beyond self-assembly, amphiphilicity additionally enables utilization of the Langmuir-Blodgett (LB) technique for superior control of molecular ordering and phase formation.
This presentation will focus on self-assembly, morphological and photonic properties of novel donor and novel acceptor molecules that were made amphiphilic. Using the LB-technique we are able to produce thin films of both donor and acceptor molecules with controlled morphology. The surface sensitive photothermal deflection spectroscopy[1-3] is not just applied to determine the UV-vis absorption spectra of even molecular monolayers, but also to monitor self-assembly on surfaces from solutions in situ. On the examples of C60-derivatives, but also merocyanines and novel thiazoles derivatives it is demonstrated how large the influence of the morphology on photonic properties might be and how to control morphologies by molecular assembling.[4]
1. Presselt, M., et al., Influence of Phonon Scattering on Exciton and Charge Diffusion in Polymer-Fullerene Solar Cells. Advanced Energy Materials, 2012. 2(8): p. 999-1003.
2. Presselt, M., et al., Sub-bandgap absorption in polymer-fullerene solar cells studied by temperature-dependent external quantum efficiency and absorption spectroscopy. Chemical Physics Letters, 2012. 542: p. 70-73.
3. Herrmann, F., et al., Influence of Interface Doping on Charge-Carrier Mobilities and Sub-Bandgap Absorption in Organic Solar Cells. The Journal of Physical Chemistry C, 2015. 119(17): p. 9036-9040.
4. Habenicht, S.H., et al., Tuning the Polarity and Surface Activity of Hydroxythiazoles – Extending Applicability of Highly Fluorescent Self-Assembling Chromophores to Supra-Molecular Photonic Structures. to be submitted, 2015.
The authors are grateful for financial support (BMBF FKZ 03EK3507).
9:00 PM - MD5.2.05
Fabrication of Chiral Organic Semiconductor Nanowires and Their Use in Circularly Polarized Light Detection
Xiaobo Shang 1,Inho Song 1,Hiroyoshi Ohtsu 1,Hojeong Yu 1,Tianming Zhao 1,Yoonho Lee 1,Ji Hyung Jung 1,Masaki Kawano 1,Joon Hak Oh 1
1 Pohang University of Science and Technology Pohang Korea (the Republic of),
Show AbstractChirality plays a very important role in scientific and technological research fields with huge potential to be applied in key areas including chemical synthesis, pharmaceutics, catalysis, fundamental physics and biomedicals. However, the separation of chiral molecules is still a challenge for their practical use in chemical and pharmaceutical industries. The most widely used characterizations rely on the traditional methods of measuring the circular dichroism using circularly polarized light. Besides, chirality in nanostructures could potentially be very useful. However, the application of chiral nanostructures, particularly, organic chiral nanomaterials, is still in infancy. In this research, we have successfully synthesized chiral materials, i.e., CPDI-Ph derivatives, and self-assmbled them into nanowires. The prepared 1-D nanomaterials show excellent n-type organic field-effect transistor (OFET) performance and high photo-responsivity under monochromic light irradiations. More importantly, specific photo-responses were obtained from chiral semiconductors upon illuminating circularly polarized light on the active layer of the n-type OFETs, which demonstrates the first example for detecting circularly polarized light from n-type phototransistors based on photoactive 1-D nanomaterials.
9:00 PM - MD5.2.06
UV-Light Mediated Patterning of Conductive Polymers
Jesper Edberg 1,Donata Iandolo 1,Robert Brooke 1,Xianjie Liu 2,Chiara Musumeci 2,Jens Andreasen 4,Daniel Simon 1,Drew Evans 3,Isak Engquist 1,Magnus Berggren 1
1 Department of Science and Technology Linköping University Norrköping Sweden,2 Department of Physics, Chemistry and Biology Linköping University Linköping Sweden4 Department of Energy Conversion and Storage Technical University of Denmark Kgs. Lyngby Denmark3 Future Industries Institute University of South Australia Adelaide Australia
Show AbstractThe development of highly conductive conjugated polymers has been one of the major goals for organic electronics. Recently, conductivities of over 4000 S/cm was demonstrated for vapor phase polymerized poly(3,4-ethylenedioxythiophene):tosylate (PEDOT:Tos), rivaling ITO as the material of choice for transparent flexible electrodes. Substantial research has also been invested in the development of patterning techniques for such conductive polymers. These patterning techniques include laser cutting, ink jet printing, screen printing, transfer printing and photolithography. Each of these methods has its respective advantages and disadvantages. We report a method of patterning vapor phase polymerized PEDOT:Tos using UV-light and show how this new approach has several advantages over existing patterning techniques.
When patterning conductive polymers using photolithography, a mold of the pattern is first made using a photoresist. The polymer is then added to the mold after which the photoresist is dissolved in an organic solvent. This results in a step-like profile of the pattern edges. The procedure involves a number of steps and the chemicals used to strip the photoresist are often environmentally hazardous and can also damage the conductive polymer. Our approach is similar to that of photolithography in that the patterning is mediated by UV-light through a photomask. However, it has fewer processing steps and does not involve any photoresists or hazardous chemicals.
The patterning can either be done with a step-like profile of the pattern or having a continuous polymer film where the conductivity of the film is locally reduced. The reduction in conductivity is controlled by the radiation dose and can span over six orders of magnitude. The exposure also induces a change in the color of the polymer film. Line widths of 100 um can be done effortlessly and work is ongoing towards significantly smaller dimensions.
The simplicity and versatility of this novel patterning procedure show great promise for patterning of organic electronic circuits as well as electrochromic displays.
9:00 PM - MD5.2.07
Control of Pi-Conjugation in Diketopyrrolopyrrole-Based Copolymer Thin Films for High-Performance Organic Transistor
Mi Jang 1,Yun-Hi Kim 2,Hoichang Yang 1,Seungjun Chung 3
1 Inha University Incheon Korea (the Republic of),2 Gyeongsang University Jinju Korea (the Republic of)3 Berkeley Berkeley United States
Show AbstractDonor–acceptor (D–A) semiconducting copolymers, PDPP-TVT-24 and PDPP-TVT-29, were synthesized by alternating diketopyrrolopyrrole (DPP) derivatives with difference alkyl side-chains (2-decylbutadecyl (24) and 7-decylnonadecyl (29)) and thiophene vinylene thiophene (TVT) for organic field-effect transistors (OFETs). PDPP-TVT-24 and PDPP-TVT-29 could form highly π-conjugated structures in as-spun films. In particular, the layer-like conjugated film morphologies could be developed via short-term thermal annealing above 150 °C for 10 min. The strong intermolecular interaction, originating from the fused DPP and D–A interaction, led to the spontaneous self-assembly of polymer chains within close proximity (with π-overlap distance of 3.55–3.70 Å) and formed unexpectedly long-range π-conjugation, which is favorable for both intra- and intermolecular charge transport. Unlike intergranular nanorods in the as-spun film, well-ordered layers in the 200 °C-annealed film could yield more efficient charge-transport pathways. The granular morphologies of the as-spun films produced a field-effect mobility (μFET) of 0.65 cm2 V–1 s–1 (for PDPP-TVT-24) and 1.39 cm2 V–1 s–1 (for PDPP-TVT-29) in OFETs based on a polymer-treated SiO2 dielectrics, while the 200 °C-annealed films showed high μFET values of up to 3.4-3.7 cm2 V–1 s–1. The difference of charge mobilities between PDPP-TVT-24 and PDPP-TVT-29 as-spun films might be originating from polymer chain flexibility with different alkyl side-chain length. The significantly high μFET observed for the thermally annealed copolymers films were mainly related to the extraordinarily layered π-conjugated crystals, owing to the strong intermolecular interactions and the spaced out side-chain arrangement that helped the polymer chains form well-interconnected crystal grains, thus providing efficient pathways for charge transport. The highly π-conjugated nature, excellent solution processability, electrical properties, and simple synthesis of PDPP-TVT-24 and PDPP-TVT-29 suggest that the copolymers can potentially be applied to high-throughput, roll-to-roll solution printing of low-cost OFET circuits and arrays.
9:00 PM - MD5.2.08
Band-Like Transport in Disordered Organic Molecular Semiconductors
Pramod Kumar 1,Akanksha Sharma 1,Varsha Rani 1,Nirat Ray 1,Subhasis Ghosh 1
1 Jawaharlal Nehru University, New Delhi Delhi India,
Show AbstractCharge transport in organic semiconductors is typically described by hopping through localized electronic states in Gaussian density of states (GDOS). The efficiency of charge transport is determined by the position of Fermi level and the corresponding DOS. As the carriers fill the lower-lying states of the organic semiconductor, corresponding to a shift in the Fermi energy, additional charges will occupy states at relatively higher energies. Consequently, these additional charges on average require less activation energy to hop to neighbouring sites, resulting in a higher mobility with increasing carrier density. Carrier mobility in such devices can then be tuned by varying the pushing the Fermi level relative to the maximum of the DOS. However, there are limitations to how much the Fermi level can be shifted, primarily due to device breakdown considerations. As a result, the mobility in three terminal organic semiconductor based devices have been limited to low values.
We have examined the characteristics of Copper Phthalocyanine(CuPc) based organic field effect transistors in both the positive and negative drain-source voltage (VDS) regime. Phthalocyanine based molecular materials are polycrystalline, thermally and chemically stable and are ideally suited for testing any carrier transport model. Moreover, they have the potential for application in organic solar cells, LEDs and field effect transistors. We find that the Fermi level can be moved closer to the maximum of the GDOS by applying a positive VDS. In the positive VDS regime, the IDS-VDS characteristics at zero gate voltage (VG) are similar to the two terminal devices, but an external gate can still be used to modify the carrier density. We observe an increase of almost two orders of magnitude in the mobility in this regime, with mobility values comparable to those observed in single crystals of CuPc. The high mobility can be understood on the basis of a transition from hopping to band like transport, with an applied VDS and VG. Band mobilities calculated using first principle DFT calculations are found to be in good agreement with the observed experimental values.
9:00 PM - MD5.2.09
Selecting Donor Pairs for Parallel-Like Bulkheterojunction Organic Solar Cells
Mary Kelly 1,Qianqian Zhang 1,Wei You 1
1 Univ of North Carolina-Chap Hill Chapel Hill United States,
Show AbstractParallel-like ternary blends provide a possible solution to one of the inherent limitations of organic solar cells: the relatively narrow absorption width of the conjugated polymers used in the active layer. This study focuses on selecting donor polymer pairs for ternary blends which will exhibit the characteristic composition dependent voltage, while extending the absorption and increasing the current, thereby improving the power conversion efficiency. In order to investigate the effect of a common backbone moiety on performance, two donor moieties and two acceptor moieties were mixed and matched to synthesize four polymers: poly(benzodithiophene-dithienyl-benzothiadiazole), poly(benzodithiophene-dithienylbenzotriazole), poly(naphthodithiophene-dithienyl-benzothiadiazole), and poly(naphthodithiophene-dithienybenzotriazole). These polymers were then paired in photovoltaic devices (with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as an electron acceptor) and their performance compared to binary devices made from each individual polymer. It was found that pairings in which the polymers shared a common donor moiety performed well relative to the binary blends, but pairings with either a common acceptor moiety or no common moiety performed poorly, as evidenced by a drop open-circuit voltage. Resonant soft X-ray scattering shows that each blend exhibits markedly different morphology, with P(NDT-DTBT) in particular dominating domain formation when included in a blend. In the most successful pairing, P(BnDT-DTBT) and P(BnDT-HTAZ), each polymer formed independent domains. For this set of polymers, in order to see parallel-like behavior it was essential that the ternary blend include a common donor moiety in the polymer backbone and exhibit independent, constructive domain formation. These insights will serve to guide future selection of polymer pairs for parallel-like ternary blends.
9:00 PM - MD5.2.10
Characterization of n-Type Nitrogen-Doped Single-Walled Carbon Nanotubes Synthesized by Defluorination-Assisted Nanotube-Substitution Reaction
Koji Yokoyama 1,Yoshinori Sato 2,Kazutaka Hirano 2,Shinji Hashiguchi 2,Hiromichi Ohta 3,Kenichi Motomiya 1,Kazuyuki Tohji 1,Yoshinori Sato 4
1 Graduate School of Environmental Studies Tohoku University Sendai Japan,2 Stella Chemifa Corporation Osaka Japan3 Research Institute for Electronic Science Hokkaido University Sapporo Japan1 Graduate School of Environmental Studies Tohoku University Sendai Japan,4 Institute for Biomedical Sciences, Interdisciplinary Cluster for Cutting Edge Research Shinshu University Matsumoto Japan
Show AbstractIt is necessary to develop a method to control the electronic properties of single-walled carbon nanotubes (SWCNTs) for use in a wide range of areas such as electronics and photonics. For example, nitrogen doping into the SWCNTs is one of the methods for tuning their electronic properties, resulting in n-type SWCNTs in air. However, in previously-reported chemical vapor deposition using a nitrogen source, only nitrogen-doped (N-doped) defective SWCNTs can be synthesized and they do not show good conductivity. Here, we employed a defluorination-assisted nanotube substitution reaction, which was performed using ammonia gas, for the synthesis of N-doped crystalline SWCNTs that exhibit good conductivity and n-type conduction.
We synthesized highly crystalline SWCNTs (hc-SWCNTs) by the arc discharge method. The hc-SWCNTs were fluorinated at 250 °C using a gas mixture of F2 (20%) and N2 (80%) for 4 h. The fluorinated SWCNTs (F-SWCNTs) were characterized using X-ray photoelectron spectroscopy (XPS) and their C:F stoichiometry was determined to be CF0.39. Next, The F-SWCNTs were reacted with a gas mixture of NH3 (1%) and N2 (99%) at temperatures of 300-600 °C for 30 min. The resulting samples were characterized using XPS, high-resolution transmission electron microscopy (HRTEM), and Raman scattering spectroscopy. We also measured the volume resistivity of the films of various nanotube samples and investigated their conduction properties in air by thermopower measurements.
The XPS spectra of all the samples subjected to the nitrogen-doping reaction indicated that the nitrogen atoms were introduced into the nanotube framework at concentrations of 1.38-3.04 at.%, which is as high as those reported in previous studies. The N1s XPS spectra could be deconvoluted into several peaks, which corresponded to pyridinic-, pyrrolic-, and graphitic-type nitrogen bonds. Although the HRTEM images of these N-doped SWCNTs showed that their crystalline structure was poor, it was also clear that the nanotube frame of the former remained intact to a certain degree from Raman scattering spectra.
Since the N-doped SWCNTs retained the network of π-electron cloud, their volume resistivities were 1.9-3.7 Ω・cm, which are classified as semiconductors. The thermopower of N-doped SWCNTs synthesized at 400 °C was found to be negative in air, indicating that they exhibited n-type conduction. In contrast, the thermopowers of N-doped SWCNTs synthesized at temperatures greater than 400 °C were positive as with those of hc-SWCNTs. As the reaction temperature with ammonia gas increases, the concentration of graphitic-type nitrogen atoms gradually decreases and the pyridinic- and pyrrolic-type nitrogen atoms become more predominant. Thus, the graphitic-type nitrogen sites act as electron donors.
9:00 PM - MD5.2.11
Electronic Tuning of 4,10-dibromoanthanthrone for n-Type Materials
Robert Johnson 2,Jonathan Tinkham 1,Alan Sellinger 1
2 Chemistry and Chemical Engineering North Carolina State University Raleigh United States,1 Chemistry and Geochemistry Colorado School of Mines Golden United States
Show AbstractOrganic semiconducting materials have a wide-variety of applications, including organic photovoltaics, but while there are many p-type materials in the literature, there are far fewer n-type materials reported. This research aimed to investigate derivatives of the textile dye 4,10-dibromoanthanthrone for use as the basis for n-type materials, due to its large conjugated system, absorptivity, ease of functionalization, and low cost. Conjugated peripheries were attached with the intention of tuning the absorbance, photoluminescence, orbital distributions, and energy levels to achieve optimal n-type character. Ultraviolet-visible spectroscopy and cyclic voltammetry were used to characterize the absorptivity, bandgaps, and orbital energy levels of the derivatives. All the derivatives have a very similar LUMO level, due to the large contribution from the common anthanthrone core, and the electron-donating or withdrawing character of attached units showed control over the HOMO energy levels. The geometry of the attachment point was also found to influence their electronic properties, giving an additional way to tune these systems.
Further tests are in progress to determine carrier mobility and photovoltaic efficiencies of the synthesized derivatives. If the electron mobilities are well matched with PCBM, these materials possess the solubility, synthetic ease, and absorptivity to be more feasible on a larger scale.
9:00 PM - MD5.2.12
Modification of Scaffold Layer Surfaces for Preventing from Hysterisis in Perovskite Solar Cell
Esma Yenel 1,Mahmut Kus 2,Yasemin Topal 3,Mustafa Ersoz 4
1 Department of Chemistry Selcuk University KONYA Turkey,2 Department of Chemical Engineering Selcuk University Konya Turkey3 Department of Chemistry Selcuk University Konya Turkey4 Department of Chemistry Selcuk University Konya Turkey
Show AbstractOrgano-metal halide perovskite has good potential to be used as a light harvester in the solar cell due to its direct band gap, ambipolar transport, large absorption coefficients and high carrier mobility[1]. Both solution-processed planar and mesoporous perovskite solar cells are prone to hysteresis and current instabilities. Here, we firstly report the surface modification of scaffold layer by using metal oxide clusters. The metal oxide clusters decrease PbI3- crystallographic defects by passivating the iodide-rich trap sites on the surfaces during the perovskite self-assembly like PCBM[2,3]. Metal oxide clsuters used in this work can be also introduced to be electron acceptor materials showing tunable structural properties, favorable electron transporting properties, attractive optical properties in terms of high transparency in the visible, nanostructured morphology. Those advantages of metal oxide clusters make them promising materials in such studies. We observed the improvement of current stability and the loss of hysterises depending on the surface modification of scaffold layer.
9:00 PM - MD5.2.13
Visualization of Distributed Charge Density in Polycrystalline Pentacene Thin Films by Gate-Modulation Imaging
Satoshi Matsuoka 2,Jun'ya Tsutsumi 2,Toshikazu Yamada 2,Toshihide Kamata 1,Tatsuo Hasegawa 3
1 University of Tsukuba Tsukuba Japan,2 AIST Tsukuba Japan,2 AIST Tsukuba Japan2 AIST Tsukuba Japan,1 University of Tsukuba Tsukuba Japan2 AIST Tsukuba Japan,3 University of Tokyo Tokyo Japan
Show AbstractPolycrystalline pentacene thin films afford a model semiconductor layer of organic thin-film transistors (OTFTs). Though enormous studies have been dedicated to investigate the charge carrier dynamics in pentacene OTFTs, whole understanding is still challenging, because of the nonuniform multi-domain structures composed of uniaxially-oriented dendritic microcrystal grains with a few μm in size. Here we report spatial distribution of accumulated charge carrier density in polycrystalline pentacene thin films as visualized by high-spatial resolution gate-modulation (GM) imaging technique.
We used polycrystalline pentacene thin films fabricated by vacuum evaporation technique. The prepared pentacene thin films were composed of pieces of microcrystal grains with the dimension of 10 µm. The high-resolution GM imaging measurements were conducted by using objective lens with high magnification power and high numerical apertures. We successfully obtained spatial distribution of charge carrier density at resolution less than 1 mm. The obtained images demonstrated inhomogeneous carrier distribution in the pentacene polycrystalline films: We observed regions showing small GM signal at around the grain boundaries, which indicates that the carrier density should be quite low due to the potential barriers against inter-grain carrier conduction. In contrast, regions showing high GM signals are observed within the microcrystal grains. Surprisingly, these regions can be divided into the two types; one is the region showing positive GM signal, while the other showing negative GM signal. We discuss the origin of these differences in terms of the spectral analyses of the obtained GM images acquired at different wavelengths, which could give a clue to understand the impact of local carrier conduction within and between the microcrystal grains in the device performance of polycrystalline pentacene TFTs.
[1] J. Tsutsumi, S. Matsuoka, T. Yamada, and T. Hasegawa, Org. Electron. 25 (2015).
9:00 PM - MD5.2.14
Three-Dimensionally Stacked Microscale Organic Nonvolatile Memory by Orthogonal Photolithography
Daekyoung Yoo 1,Younggul Song 1,Jingon Jang 1,Youngrok Kim 1,Seok-Heon Jung 2,Jin-Kyun Lee 2,Takhee Lee 1
1 Seoul National University Seoul Korea (the Republic of),2 Inha University Incheon Korea (the Republic of)
Show AbstractDaekyoung Yoo1, Younggul Song
1, Jingon Jang
1, Youngrok Kim
1,
Seok-Heon Jung
2, Jin-Kyun Lee
2, Takhee Lee
11Department of Physics and Astronomy, Seoul National University, Seoul 08826, Korea
2Department of Polymer Science and Engineering, Inha University, Incheon 22212, Korea
*E-mail :
[email protected]Recently organic electronics has attracted great attention due to its material variety, easy and low cost fabrication, flexibility, and spin-coating process. In particular, organic non-volatile memory (ONVM) has been spotlighted for data-storage applications. Most ONVM devices so far have been fabricated with very large cell size of ~100 µm. The reason for this limitation is that the standard chemicals and solvents for conventional photolithography that are necessary to produce microscale patterns damage the existing organic layers during the photolithographic development and lift-off process. In this presentation, I will report microscale three-dimensionally integrated ONVM devices with a 7 µm × 7 µm memory cell size in a 32 × 32 crossbar structure on a silicon substrate by orthogonal photolithography using highly fluorinated photoresist and development solvent. Because highly fluorinated photoresist material has orthogonal property to the most organic polymers, there is no damage for organic memory layers during orthogonal photolithography [1, 2]. Our three-dimensionally integrated microscale ONVM devices showed promising memory performance with good endurance cycle and stability with long retention time over 10
4 s for the vertically stacked layers [3]. And, the operative memory cells in each layer showed stable and uniform electric characteristics. Our research suggests that the realization of three-dimensional integrated microscale ONVM may enable the production of functioning organic electronic devices in practical configurations such as flexible organic devices with high cell density.
[1] J. K. Lee, M. Chatzchristidi, A. A. Zakhidov, P. G. Taylor, J. A. Defranco, H. S. Hwang, H. H. Fong, A. B. Holmes, G. G. Malliaras, C. K. Ober, J. Am. Chem. Soc. 130, 11564 (2008).
[2] Y. Song, J. Jang, D. Yoo, S.-H. Jung, S. Hong, J.-K. Lee, T. Lee, Org. Electron. 17, 192 (2015).
[3] D. Yoo, Y. Song, J. Jang, W.-T. Hwang, S.-H. Jung, S. Hong, J.-K. Lee, and T. Lee, Org. Electron. 21, 198 (2015).
9:00 PM - MD5.2.15
Experimental Analysis on the Effect of Calendering Process on Electrical Characteristics of Printed OTFT
Sang Hoon Lee 1,Jae-Min Kim 1,Sung-Lim Ko 1,Sangyoon Lee 1
1 Konkuk Univ. Seoul Korea (the Republic of),
Show AbstractElectrical characteristics of printed OTFT(Organic Thin Film Transistor) include the on-off ratio and the field effect mobility. They are affected by various factors and one of the major ones is surface roughness of the gate dielectric layer. This paper presents a treatment process called calendering that was applied to the surface of gate dielectric layer for reducing surface roughness. Calendering has been used to improve smoothness of substrates by using a heating roller and a nip roller. During a calendering process, a substrate moving through the rollers becomes subject to the heat and the nip pressure.
Bottom-gate, bottom-contact, TIPS-pentacene OTFT samples were fabricated on PET substrates. Gravure printing (first gate electrode and second gate dielectric layer), inkjet printing (third source/drain electrode layer), and pipetting(fourth active layer) were used for fabrication. As for materials, silver ink(Paru Inc.) was used for printing the gate and source/drain electrode layers and BaTiO3(Paru Inc.) for the gate dielectric layer and TIPS-pentacene for the active layer were used.
In this study, calendering process parameters include three factors(temperature, speed, pressure) with three levels(high, middle, low). The levels of temperature are 100 °C, 80 °C, and 60 °C. For the speed, they are 6 m/min, 4 m/min, and 2 m/min. The levels of pressure are 123 kN/m, 82 kN/m, 41 kN/m. Taguchi method was used to reduce experimental sets from 33 (27 sets) to nine sets and optimize the calendering process parameters.
As a result of Taguchi method, an optimal set of calendering process parameters has been obtained: 100 °C, 4 m/min, 123 kN/m. Both temperature and pressure contribute equally, 35% each, and speed contributes 30% to the electrical characteristics of printed OTFT. The on-off ratio and the field effect mobility of the samples from the optimal parameters are 11000 and 0.008 cm2/Vs, respectively. This shows five times improvement in the on-off ratio and 8% improvement in the field effect mobility, compared to non-calendered samples.
9:00 PM - MD5.2.16
Tuning Doping Type and Concentration in the Active Layer of Organic Photovoltaic Devices Using Contacts
Jian Wang 1,Liang Xu 1,Yun-Ju Lee 1,Julia Hsu 1
1 Univ of Texas-Dallas Richardson United States,
Show AbstractSubstrates can significantly affect the electronic properties of organic semiconductors. Charge transfer to/from the substrate can lead to the formation of an interfacial doped region with different electronic properties from the bulk. Recently, we discovered that high work function MoOx hole transport layer (HTL) could induce p-doping in the active layer and decrease the photocurrent of organic photovoltaic (OPV) devices.1 This finding directly contradicts the current belief that high work function HTL improves the OPV performance by increasing the built-in field. Here we report how electron transport layer (ETL) modify doping in the OPV active layer and the resulting device behaviors. Different doping type and concentration are observed in P3HT:PCBM or P3HT:ICBA devices made on ETLs of ZnO or polyethylenimine (PEI). This phenomenon is ascribed to the interplay between the ETL-induced n-type doping and the p-type nature of P3HT donor. The amount of ETL-induced n-type doping depends on the relative difference between the work function of ETL and the negative integer charge transfer state (EICT-) of the acceptor. Excessive n-type doping from low work function ETL is shown to be deleterious to OPV device performance. The device behaviors are understood by combined experimental and modeling investigations, including capacitance-voltage (C-V) measurement, Hall effect measurement, light-biased external quantum efficiency (EQE) measurement, transfer matrix method (TMM) simulation, and drift-diffusion simulation. These results establish a criterion to select ETL according to the electronic properties of OPV active layer, and shed light on ways to optimize other organic electronic devices.
Reference:
1. Nano Lett. 2015, DOI: 10.1021/acs.nanolett.5b03473.
This project is sponsored by National Science Foundation DMR-1305893.
9:00 PM - MD5.2.17
What Could be the Highest Hopping Mobility in Organic Thin Film Transistor
Varsha Rani 1,Akanksha Sharma 1,Subhasis Ghosh 1
1 Jawaharlal Nehru University Delhi India,
Show AbstractPentacene has emerged as benchmark organic molecular semiconductors because of its relatively high carrier mobility and a typical surface morphology. Charge transport properties of Pentacene have been investigated by a joint experimental and theoretical study. The growth of the Pentacene thin film on the substrate shows mainly two different polymorphic phases, bulk phase and thin film phase. Thin film phase is crucial for the charge transport in two terminal and three terminal devices such organic Schottky diodes and organic thin film transistors (OFETs), respectively. Under optimized growth conditions, Pentacene thin films show typical diffusion limited aggregation (DLA) plus mound growth and devices based on Pentacene thin film grown under these conditions exhibit higher mobility.
However, intermolecular disorders present in thin films limit the charge transport in two terminal and three terminal devices. Intrinsic charge transport properties of Pentacene in thin film phase have been studied using first principle density functional theory (DFT) calculations. In the limit of strong electron–phonon coupling, the charge transport in organic semiconductors can be considered as an intermolecular hopping process. In theoretical framework, the charge transfer in adjacent molecular dimmers can be modelled as a self-exchange reaction process and charge transfer rate is determined by two microscopic parameters, transfer integral and reorganization energy. Reorganization energy describes the strength of hole (electron) vibration coupling of a charge localized on a single molecule and depends on its chemical structure whereas transfer integral depends on the overlapping between the molecular orbitals of two adjacent molecules hence on the molecular packing in the crystal structure. From XRD study, it is known that when Pentacene is deposited on flat substrate, Pentacene molecules stand almost vertically on the substrate, arranged in herringbone manner in ab-plane. Intrinsic mobilities of Pentacene in different planes have been calculated using DFT calculations. In two terminal devices, charge transport occurs along c-axis and mobility calculated in this direction has been found to be 4.45 x 10-5 cm2/Vs. However, charge carrier mobility of ~2cm2/Vs has been obtained along herringbone direction in ab-plane which is relevant for charge transport in three terminal devices. Theoretically calculated mobilities are comparable with experimental values of mobility of 1.3 x 10-5 cm2/Vs and 0.3cm2/Vs in two and three terminal devices, respectively. Mobility in two terminal devices is lower by four orders of magnitude than in three terminal devices due to the strong electronic couplings between molecular dimers located in ab- plane and relatively weak coupling between the layers (c axis).
9:00 PM - MD5.2.18
Characterization of Dye: PCBM Bulk Heterojunction Solar Cells
Anil Kumar 2,Poonam Meena 6,S. Nehra 3,Anshu Sharma 4,M. Singh 2,Y. Vijay 5
1 State Forensic Science Laboratory Jaipur India,2 Department of Physics University of Rajasthan Jaipur India,6 Department of Botany University of Rajasthan Jaipur India2 Department of Physics University of Rajasthan Jaipur India,3 Centre of Excellence for Energy amp; Environmental Studies Deenbandhu Chhotu Ram University of Science amp; Technology Sonipat India2 Department of Physics University of Rajasthan Jaipur India,4 Centre for Non Conventional Energy Resources University of Rajasthan Jaipur India2 Department of Physics University of Rajasthan Jaipur India2 Department of Physics University of Rajasthan Jaipur India,5 Vivekananda Global University Jaipur India
Show AbstractPolymer solar cells are promising alternatives for conventional energy sources. Due to the lower cost of manufacturing, easy processing and flexibility of polymers, solar cells made of organic materials may compete with current inorganic solar cells. In the present article we have characterized Dye:PCBM bulk heterojunction solar cells. A synthesized dye D (2,2'-[1,4-henylenebis(azo)]bis-1Hpyrrole) of low band gap characterized. It was soluble in solvents like tetrahydrofuran THF).Using D and PCBM we prepared blend solution. We have prepared as cast as well blend films on ITO plates using spin coating unit. Blend films were annealed at 373K for 10 minutes. As cast, blend and annealed films were characterized using Keithley electrometer, Uv-Vis spectrophotometer and Atomic force microscopy.
Keywords: Bulk heterojunction solar cells; Dye; I-V characteristics, Uv-Vis; AFM.
9:00 PM - MD5.2.19
Effect of Thermal Annealing on Dopant Site Choice in Conjugated Polymers
Jun Li 1,Chris Rochester 1,Ian Jacobs 1,Erik Aasen 1,Stephan Friedrich 2,Pieter Stroeve 1,Adam Moule 1
1 Univ of California-Davis Davis United States,2 Lawrence Livermore National Laboratory Livermore United States
Show AbstractThis article explores how the widely used molecular dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) affects the doping and morphology of the self-doped organic hole transport layer (HTL) poly(thiophene-3-[2-(2-methoxy-ethoxy)ethoxy]-2,5-diyl) (S-P3MEET). It is found that the molecular dopant additive competes for doping sites with the covalently attached dopants on the polymer. This competition results in large changes in the morphology and doping level of the polymer with little change in the conductivity. Calorimetry measurements were performed to reveal that the F4TCNQ interacts strongly with the side chains of the S-P3MEET, increasing the melting temperature of the side chains by 30 °C with 5 wt% dopant loading. The thermal stability of the doping is also investigated to determine whether doping with F4TCNQ is feasible in a device. Experiments with poly-3-hexylthiophene (P3HT) as the fluorescent reporter layer show that F4TCNQ binds much stronger in S-P3MEET than P3HT and only little F4TCNQ is found in the P3HT layer after annealing. In combination with reflectometry measurements, we show that F4TCNQ binds in S-P3MEET with annealing to 150 °C even though the sublimation temperature for neat F4TCNQ is 80 °C. In contrast, F4TCNQ is shown to slowly diffuse out of P3HT at room temperature. We attribute this difference in binding with the F4TCNQ molecules to the ability of the polar ethyl-oxy side chains of the S-P3MEET to orient around the charged dopant molecule and thereby to stabilize its position. This study suggests that polar side chains are a design element in organic electronic materials that can be used to increase the thermal stability of molecular dopants.
9:00 PM - MD5.2.20
Introducing Solubility Control for Improved Organic P-Type Dopants
Jun Li 1,Guangwu Zhang 2,Daniella Holm 1,Ian Jacobs 1,Pieter Stroeve 1,Mark Mascal 2,Adam Moule 1
1 Department of Chemical Engineering and Materials Science University of California, Davis Davis United States,2 Department of Chemistry University of California, Davis Davis United States
Show AbstractIt is generally understood that the widely used p-type dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) has low solubility in common solvents, which restricts further development of solution-processed and mass-produced organic electronics applications. In this work, we introduce solubility control into molecular dopants based on the F4TCNQ structure by replacing cyano groups with alkyl esters. UV-vis-NIR and cyclic voltammetry measurements show increased solubility of mono- and di-ester substituted dopants with only modest changes to acceptor strength. With the use of UV-vis-NIR, photoluminescence, and conductivity measurements, we demonstrate that the new dopants are able to successfully p-dope poly(3-hexylthiophene-2,5-diyl) (P3HT). Mono-ester substituted dopants show only slightly reduced electron affinity (EA) relative to F4TCNQ, but greater doping effectiveness due to increased miscibility with P3HT. In contrast, di-ester substituted dopants undergo a dimerization reaction before assuming their doped states, which may help anchor dopants into position post deposition, thus decreasing the negative effect of dopant diffusion and drift. We conclude that increased dopant solubility/miscibility not only simplifies film fabrication but also increases the effectiveness of doping in solution-cast polymer films. The ester modification is an effective synthetic route to achieve solubility/miscibility control in TCNQ dopants.
9:00 PM - MD5.2.21
Extended Small Molecules for High-Efficiency BHJ Solar Cells
Philipp Wucher 1,Zhipeng Kan 1,Yuliar Firdaus 1,Joris Laleque 1,Pierre Beaujuge 1
1 KAUST Thuwal Saudi Arabia,
Show AbstractSolution-processable π-conjugated small molecule donors and acceptors have seen their efficiency characteristics improving rapidly in bulk-heterojunction (BHJ) solar cells over the past recent years. One of the most successful approaches relies on the use of π-delocalized sequences of electron-donor and -acceptor motifs that can mimic the wide-range visible absorption spectra of π-conjugated polymers, while adopting particularly high molecular ordering patterns and optimum morphologies in BHJ thin-films. Combining optimizations of (i) π-conjugated sequences,[1] side-chain substituents,[2] functional end groups,[3] and (ii) solution-processing and post-deposition conditions, including the use of processing additives[4] and thermal or solvent annealing steps,[5] the power conversion efficiencies (PCEs) of small molecule donors in BHJs with fullerene acceptors has quickly improved from ca. 4%[6] to >9%[7]. Likewise, some recently reported π-extended small molecule acceptors are showing to be promising systems in combination with polymer donors, with PCEs that have rapidly increased to >8%[8]. At this point in time, the efficiency of small molecules and polymers are comparable in BHJ solar cells, and yet the morphologies, phase composition and the texture of small molecule and polymer thin films are uniquely different – differences that are inherent to the well-defined, finite molecular structure of small molecules. Meanwhile, specific end group modifications[3] and covalent connections[9] between small molecules can direct their self-assembly and induce distinct, improved morphological and efficiency patterns in BHJ solar cells. Our recent developments emphasize some key design principles that can significantly impact the film-forming properties, morphologies and charge transport properties of π-extended small molecules in thin film devices.[10]
[1] P. Bäuerle et al. Angew. Chem. Int. Ed. 2012, 51, 2020; J. Roncali et al. Adv. Mater. 2014, 26, 3821; T.-Q. Nguyen et al. Chem. Mater. 2011, 23, 470.
[2] P.M. Beaujuge et al. 2015, Submitted
[3] P.M. Beaujuge, J.M.J. Fréchet et al. Adv. Mater. 2011, 23, 5359.
[4] A.J. Heeger et al. Nat. Mater. 2012, 11, 44.
[5] Y. Cao et al. Adv. Funct. Mater. 2015, 25, 3514; Y. Chen et al. J. Am. Chem. Soc. 2014, 136, 15529.
[6] T.-Q. Nguyen et al. Adv. Funct. Mater. 2009, 19, 3063.
[7] D.J. Jones et al. Nat. Commun. 2015, 6, 6013; Y. Chen et al., JACS 2015, 137, 3886.
[8] H. Yan et al. Energy Environ. Sci. 2015, 8, 520; Z. Wang et al. J. Am. Chem. Soc. 2015, 137, 11156; C. Nuckolls et al. Nat. Commun. 2015, 6, 8242.
[9] W. Li et al. Chem. Commun. 2014, 50, 7720; K. Sivula Adv. Mater. 2015, 27, 5541
[10] P. Wucher, P.M. Beaujuge et al. 2015, Submitted
9:00 PM - MD5.2.22
The Influence of Bulk Doping on Trap States, Charge Transport and Recombination in Organic Bulk Heterojunction Solar Cells
Zhengrong Shang 1,Rohit Prasanna 1,Thomas Heumueller 1,Michael McGehee 1,Alberto Salleo 1
1 Materials Science and Engineering Stanford University Palo Alto United States,2 Institute of Materials for Electronics and Energy Technology Universität Erlangen-Nürnberg Erlangen Germany,1 Materials Science and Engineering Stanford University Palo Alto United States
Show AbstractMolecular doping has been hypothesized to fill trap states and improve charge carrier mobility in organic bulk heterojunction (BHJ) solar cells [1–3]. However, the addition of dopant ions and excess charges to the film can also create additional trap states and increase recombination. Here, we analyze these effects in two model systems and investigate their impact on device performance. We study both p-doped and n-doped PCDTBT:PC71BM and P3HT:PC61BM solar cells using F4TCNQ and benzimidazoline-radical dimer( (DMBI)2) respectively.
We find that p-type doping slightly increased the power conversion efficiency (PCE) from 5.87% to 6.30% for PCDTBT cells, and from 3.68% to 4.03% for P3HT cells. However, in the majority of doped organic BHJ films, charge carrier mobility and lifetime decrease while the number of trap states increases. We measure the electron and hole mobility separately using metal-insulator-semiconductor charge extraction by linearly increasing voltage (MIS-CELIV) and demonstrate that at low doping ratios the hole mobility initially drops for F4TCNQ-doped polymers in BHJ blend films. At higher doping concentrations, the mobility only increases at the cost of lower shunt resistance in the cell. A similar trend is observed for (DMBI)2 doped fullerenes. In both types of doping, the minority carrier mobility drops at higher dopant concentrations. These findings agree well with models by Arkhipov and Neher [4,5], and Coulombic traps formed by dopant ions are likely responsible for this non-monotonic effect of doping on mobility.
To further examine the changes in trap states, we performed transient photocurrent and charge extraction measurements. PCDTBT:PC71BM and P3HT:PC61BM turn out to behave oppositely. The former has an increased number of trap states while in the latter trap states are filled by dopant-induced charges. We conclude that in order to achieve positive trap-filling outcomes the dopant must dope efficiently and cause minimum morphology change. Caution must be taken when examining the effects of solution-processed doping in thin organic solar cells.
[1] Y. Zhang et al., Adv. Mater. 25, 7038 (2013).
[2] Y. Xiao et al., ACS Appl. Mater. Interfaces 7, 13415 (2015).
[3] F. Deschler et al., Phys. Rev. Lett. 107, 127402 (2011).
[4] V. Arkhipov et al., Phys. Rev. B 72, 235202 (2005).
[5] P. Pingel and D. Neher, Phys. Rev. B 87, 115209 (2013).
9:00 PM - MD5.2.23
The Role of Structural Order in p-Type Doping of P3HT
Lars Mueller 6,Diana Nanova 6,Tobias Glaser 6,Sebastian Beck 6,Annemarie Pucci 6,Anne Kast 6,Rasmus Schroeder 6,Eric Mankel 6,Robert Lovrincic 6,Wolfgang Kowalsky 6
1 Institute for High-Frequency Technology TU Braunschweig Braunschweig Germany,2 Kirchhoff-Institute for Physics Heidelberg University Heidelberg Germany,6 InnovationLab GmbH Heidelberg Germany,2 Kirchhoff-Institute for Physics Heidelberg University Heidelberg Germany,6 InnovationLab GmbH Heidelberg Germany3 CellNetworks, BioQuant Heidelberg University Heidelberg Germany,6 InnovationLab GmbH Heidelberg Germany3 CellNetworks, BioQuant Heidelberg University Heidelberg Germany,4 Centre for Advanced Materials Heidelberg University Heidelberg Germany,6 InnovationLab GmbH Heidelberg Germany5 Materials Science Department, Surface Science Division TU Darmstadt Darmstadt Germany,6 InnovationLab GmbH Heidelberg Germany1 Institute for High-Frequency Technology TU Braunschweig Braunschweig Germany,6 InnovationLab GmbH Heidelberg Germany
Show AbstractMolecular doping of solution processed organic semiconductors is routinely applied in various devices such as organic light emitting diodes (OLEDs). Nevertheless, numerous recent publications reveal new insights on doping mechanisms1 and charge transfer2, emphasizing the need for further investigations. In this work, the impact of structural order in p-type doping of poly(3-hexylthiophene) (P3HT) with 2,3,5,6- tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is studied. We show that for doped films formed from the low-boiling-point solvent chloroform (CF), the conductivity is significantly higher than for layers from high-boiling-point chlorobenzene (CB). To understand this phenomenon, we perform electron diffraction studies with a transmission electron microscope (TEM) to obtain structural information. Infrared (IR) spectroscopy and UV-Vis absorption spectroscopy enable us to connect our findings from TEM to the electronic properties of the doped material.
Surprisingly, P3HT-films from CF exhibit a strong increase in order upon doping, especially in terms of π-π-stacking. On the other hand, this stacking direction does not alter in a comparable manner in doped films from CB. Changes in characteristic stacking distances give evidence that the degree of intercalation of F4TCNQ in P3HT crystallites also varies between films from the two solvents. In addition, our results show that layers from CB in contrast to layers from CF incorporate a noticeable amount of pure P3HT crystallites. Optical spectroscopy reveals stronger and red-shifted polaron absorption for films from CF. This observation matches the higher order compared to films from CB, which is additionally supported by analysis of vibronic features of P3HT from UV-Vis spectra. Furthermore, IR absorption features from the charged dopant molecules also show different signatures. The combination of these methods leads to the conclusion of a varying distribution of the dopant F4TCNQ throughout the film depending on the solvent in use.
Our findings imply that the structural order is already predefined in solution. As literature suggests, dopant concentration3 and temperature1 of the solution affect the molecular order. We extend this view to the effects of solvent characteristics such as polarity, studying the examples of chlorobenzene and chloroform. Finally, we propose a model to link features observed in UV-Vis spectra measured on P3HT:F4TCNQ in solution to our findings for films from both solvents.
References
1Wang et al., Physical Review B, 2015, 91(8), 085205
2Méndez et al., Nature Communications, 2015, 6, 8560
3Duong et al., Organic Electronics, 2013, 14(5), 1330–1336
9:00 PM - MD5.2.24
Excellent Gate Bias Stress Stability of Fluorinated Dielectric-Based Organic Field-Effect Transistors
Minjung Lee 1,Mi Jang 1,Hwanho Shin 1,Mingyuan Pei 1,Ji Ho Youk 1,Hoichang Yang 1,Seungjun Chung 2
1 Inha University Incheon Korea (the Republic of),2 Berkeley Berkeley United States
Show AbstractPolystyrene-random-poly(pentafluorostyrene) pStPFS copolymer series were synthesized using a radical polymerization, and then spun-cast on a SiO2 gate dielectric for pentacene semiconductor. Surface energies (γ) of the fluorinated polymers decreased from 46.9 to 24 mJ m-2 with an increase in the PFS mole%. As the surface energy differences between pentacene (with a (001) crystal plane of 38 mJ m-2) and the polymers became larger, small-sized pentacene grains were dominant on the polymer-treated dielectrics, instead of terrace-like crystals. Organic field-effect transistors (OFETs) based on the resulting pentacene films showed large variations in field-effect mobility (µFET) from 0.82 to 0.28 cm2 V-1 s-1. Additionally, a density optimization of the fluorinated moieties in the copolymer could effectively improve gate bias stress stability of these OFETs on the polymer-coated SiO2 surfaces during a long-term operation, without any severe degradation of μFET. The introduction of 25 mole% PFS-loaded fluorinated polymer (with γ of 34.7 mJ m-2) onto a oxide dielectric could induce charge-transport favorable grains of pentacene, and minimize the affinity (or trapping) of injected charges in operating OFETs, in comparison to the extensively fluorinated polymers.
9:00 PM - MD5.2.25
High Field-Effect Mobility and Bias Stress Stability of a Fused Dithienobenzothiadiazole-tetrathiophene Based Semiconducting Copolymer
Mingyuan Pei 1,Jun Huang 2,Mi Jang 1,Ji-Hoon Kim 2,Minjung Lee 1,Do Hoon Hwang 2,Hoichang Yang 1,Seungjun Chung 3
1 Inha University Incheon Korea (the Republic of),2 Pusan National University Pusan Korea (the Republic of)3 Berkeley Berkeley United States
Show AbstractA high molecular-weight donor-acceptor (D-A) copolymer, pDfTBT-4T, poly(5,8-di[3-octyldodecanthiopen-5-yl]-dithieno[3',2':3,4:2",3":5,6]benzo[1,2-c][1,2,5]-thiadiazole)-alt-2,2'-bithiophene) including alternating dithieno[3',2':3,4;2",3":5,6] benzo[1,2-c][1,2,5]thiadiazole (DTfBT) and 3-octyldodecyl tetrathiophene (4T) was synthesized by a Stille coupling polymerization. We found that pDTfBT-4T had a molecular number average weight of 176 kgmol-1 and formed unexpectedly strong interchain aggregates in dilute solutions at room temperature, which was similar to those in as-spun thin solid films. pDTfBT-4T thin films were spun-cast from a warm dilute chlorobenzene solution on hydrophobic polymer-treated SiO2 dielectrics. Some were further annealed at various temperatures (T) for 10 min to enhance π-overlapped structures as charge-carrier transport paths for use in organic field-effect transistors (OFETs). The resulting ordered domains in the annealed films were tuned from nano-rods to -sheets with an increasing in T, which also provided enhanced crystal orientation. Because of the DTfBT-driven co-planarity of the polymer backbone and strong intermolecular interactions between the D-A copolymer, pDTfBT-4T based OFETs yielded a hole mobility up to 1.45 cm2V-1s-1 as well as negligible hysteresis and excellent negative bias stability.
9:00 PM - MD5.2.26
A Flexible and Transparent Sensor Array Based on Robust Organic Field-Effect Transistors
Cheol Hee Park 1,Eun Kwang Lee 2,Eun Yeob Park 2,A-Reum Han 2,Joon Hak Oh 1
1 Department of Chemical Engineering Pohang University of Science and Technology Pohang Korea (the Republic of),1 Department of Chemical Engineering Pohang University of Science and Technology Pohang Korea (the Republic of),2 School of Energy and Chemical Engineering Ulsan National Institute of Science and Technology Ulsan Korea (the Republic of)2 School of Energy and Chemical Engineering Ulsan National Institute of Science and Technology Ulsan Korea (the Republic of)
Show AbstractPolymeric semiconductors inherently have many advantages for applications in next-generation flexible electronics such as mechanical flexibility, solution processibility, low-cost, large area fabrication. Although many polymeric semiconductors show their high electrical and mechanical properties, their critical drawback is poor stability under various environmental conditions. In general, polymeric semiconductors can be easily damaged by various chemicals, which significantly limits their applications in practical applications. In this work, we utilized a high-performance polymer semiconductor with transparent graphene electrodes for the fabrication of transparent organic field-effect transistors (OFETs) showing ambipolar electrical properties and high robustness against various chemicals. The OFET devices showed both high-performance and superior stability upon exposure to various organic solvents. The prepared OFETs were applied to FET-type sensors for detecting chemical vapors. In addition, a transparent 10 × 10 FET sensor array device was fabricated, which showed uniform electrical properties and high photoresponsivity. Our approach provides a viable way for the fabrication of flexible and transparent FET-type sensors using polymeric semiconductors.
9:00 PM - MD5.2.27
Advanced Computational Techniques to Model Absorption Spectra Including Vibrational Progression
Julia Preiss 1,Torsten Sachse 1,Benjamin Dietzek 2,Todd Martinez 4,Martin Presselt 2
1 Institut of Physical Chemistry Friedrich-Schiller-University Jena Jena Germany,1 Institut of Physical Chemistry Friedrich-Schiller-University Jena Jena Germany,2 Leibniz Institute of Photonic Technology (IPHT) Jena Germany3 Department of Chemistry and PULSE Institute Stanford University Stanford United States,4 SLAC National Accelerator Laboratory Menlo Park United States
Show AbstractUV/vis spectroscopy is an important tool to characterize photoactive molecules for the use in organic light emitting diodes and photovoltaics. The shape and asymmetry of absorption or emission bands assigned to individual electronic transitions are mostly determined by vibrational progression. In cases where just a few vibrational modes are coupled to electronic transitions vibrational satellites might possess considerable intensities and need to be considered for correct assignment of electronic transitions.[1] This discrimination of electronic transitions and vibrational satellites on the basis of experimental data might be very laborious but can be facilitated significantly by ab initio modelling of the vibrational progression.
In this work, we employ different models for the description of vibrational progression to three model systems. For a Ru-polypyridyl complex, we broadened the TDDFT stick spectra based on the Huang-Rhys factors, which were obtained by a fit of experimental data. This rather inexpensive semi-empirical method[2] involves experimental data and can be expanded to treat aggregate effects – the latter is exemplified in a study on a polymer film. In a third example to be discussed in this contribution, we employed the nuclear ensemble method to get a detailed assignment of the spectral features of a tetrapyrrole beyond the Franck-Condon approximation. The three case-studies allow for assessing the applicability of the different approaches to account for vibrational progression or electron phonon coupling in given research questions.
JP and BD want to thank the Nagelschneider Foundation for financial support.
1. Beenken, W., et al., Molecular Structures and Absorption Spectra Assignment of Corrole NH Tautomers. The Journal of Physical Chemistry A, 2014. 118(5): p. 862-871.
2. Preiss, J., et al., How Does Peripheral Functionalization of Ruthenium(II)–Terpyridine Complexes Affect Spatial Charge Redistribution after Photoexcitation at the Franck–Condon Point? ChemPhysChem, 2015. 16(7): p. 1395-1404.
9:00 PM - MD5.2.28
Organic Schottky Barrier Transistor Using Backside−Doped Graphene Electrodes
Jongsu Kim 1,Donghae Ho 1,Young Jin Choi 1,Dain Lee 1,Jaehun Han 1,Beom Joon Kim 1,Jeongho Cho 1
1 SKKU Advanced Institute of Nanotechnology (SAINT) SUWON Korea (the Republic of),
Show AbstractWe fabricated the vertical Schottky barrier (SB) transistors based on the graphene–organic semiconductor (p–type pentacene or n–type N,N′–dioctyl–3,4,9,10–perylenedicarboximide (PTCDI–C8)) heterostructures. The Schottky barrier formed at the graphene–organic semiconductor heterojunction could be modulated by the application of the gate voltage. In order to investigate the graphene doping on the electrical properties of the vertical SB transistors, we introduced a novel backside–doping method. This doping method enabled the effective tuning of the work function of the graphene while maintaining the surface properties of the graphene electrodes, and thus resulted in the enhanced crystalline structures of the overlying organic semiconductors compared with typical topside–doping method. The graphene doping affected strongly the charge injection at the heterojunction and resulting transistor performances including the device on current density and on–off current ratio. The p− and n−type devices based on the optimized backside−doped graphene electrodes yielded a high current density of 20mAcm–2 and a high on–off current ratio of ~103. Furthermore, the complementary inverters were successfully fabricated by assembling p−type and n−type vertical SB transistors. Both proposed backside doping method and systematic investigation of the doping effect open up new opportunities for realizing future organic electronics.
9:00 PM - MD5.2.29
Gate-Modulation Imaging of Organic Thin-Film Transistor Arrays
Jun'ya Tsutsumi 1,Satoshi Matsuoka 2,Toshikazu Yamada 1,Tatsuo Hasegawa 1
1 AIST Tsukuba Japan,2 University of Tsukuba Tsukuba Japan3 University of Tokyo Tokyo Japan,1 AIST Tsukuba Japan
Show AbstractWe report on the application of gate-modulation (GM) imaging technique in rapid and collective inspection of organic thin-film transistor (OTFT) array operations.[1] The method allows visualizing charge carriers accumulated in the OTFT array by time-translational differential image sensing with the use of a charge coupled device (CCD) sensor. The feature makes it possible to visualize the dead pixels, broken channels, or distributed device performance in the OTFT array.
A possible future role of the OTFTs in so-called "ubiquitous electronics" is that any plastic surfaces are decorated electronically by arraying a large number of OTFTs (106-107) to function as active backplanes for displays or sensors. This feature now raises the challenge of developing a rapid and collective inspection technique for OTFT arrays composed of such large numbers of devices, as the inspection may take an extremely long time if conventional electrical measurements are used. The issue is particularly important for solution-processed OTFTs that require arduous process optimization. Charge-modulation spectroscopy (CMS) is known as a technique to optically detect accumulated charge carriers in OTFTs.[2] This method involves observing slight changes in the optical absorption spectrum due to the charge accumulation by the lock-in technique with applying an AC gate bias. The obtained CMS spectra can provide microscopic information on the carrier states. However, the technique has not yet been established for the collective inspection of OTFT arrays to find dead pixels or to investigate distributed device characteristics.
We here developed GM imaging technique in which a CCD area image sensor is introduced into CMS, thus adopting the lock-in technique for each pixel detection.[1] This technique enabled two-dimensional spatial mapping of charge carriers accumulated in an OTFT within a realistic time scale of about 10 min. The method could be a new powerful tool for collective inspection of OTFT arrays to find dead pixels or to investigate distributed device characteristics. We successfully demonstrated the GM imaging of time-developing and equilibrium charge carriers by time-differential image sensing with the use of a CCD area sensor. We show that it is possible to visualize the dead pixels or broken channels by the observation of the equilibrium charges. Additionally, the serial images for the time-developing charge images allow correlating the response time of the OTFTs with the field-effect mobility, thus enabling inspection of the distributed device performance of OTFT arrays. These unique advantages of GM imaging techniques should be quite useful for the rapid and collective inspection of OTFT arrays composed of large numbers of pixels.
References: [1] J. Tsutsumi, S. Matsuoka, T. Yamada, and T. Hasegawa, Org. Electron. 25, 289 (2015). [2] P. J. Brown, H. Sirringhaus, M. Harrison, M. Shkunov, R. H. Friend, Phys. Rev. B 63, 125204 (2001).
9:00 PM - MD5.2.30
Synthesis and Characterization of Donor-Acceptor Type Copolymers Based on Thienylenevinylene for Organic Electronics
Yeong-A Kim 1,Ye-Jin Jeon 1,Minji Kang 1,Seung-Hoon Lee 1,Jueng-Eun Kim 1,Sehyun Lee 1,Yunseul Kim 1,Dong-Yu Kim 2
1 School of Materials Science and Engineering Gwangju Institute of Science and Technology (GIST) Gwangju Korea (the Republic of),1 School of Materials Science and Engineering Gwangju Institute of Science and Technology (GIST) Gwangju Korea (the Republic of),2 Heeger Center for Advanced Materials (HCAM) Gwangju Institute of Science and Technology (GIST) Gwangju Korea (the Republic of)
Show AbstractOver past two decades, interest in organic electronics, organic field effect transistor (OFETs) and organic solar cells (OSCs), has increased due to their cheap, light-weight, and mechanical flexibility compared to inorganic counterparts. To improve organic electronic device performance, research on new materials for semiconducting layers has been required. Among others, conjugated polymers have good film formability and compared to conjugated small molecules and oligomers. In order to improve optical and electronical properties of conjugated polymer, push-pusll structure was employed with alternating structure of electron donating units (D) and accepting units (A) in conjugated backbone. Through this concept, energy level and optical bandgap of conjugated polymers were easily tuned by changing D-A units. In this study, (E)-1,2-bis(3-alkylthiophen-2-yl)ethane, simply thienylene vinylene (TV), was used as donating units which is one of the promising molecular moiety for organic electronics due to its rigid and planar moiety. Vinyl groups between thiophenes make the conjugated backbone highly coplanar as suppressing steric hinderance caused from alkyl substituent on 3-position of thiophene. Polythienylevinylene (PTV) derivatives showed good charge carrier mobilities in OTFTs due to their tendency to form crystalline structures for efficient charge carrier transport. In some cases, despite substitution of alkyl side chains on thiophene, PTV derivatives seem to have limited solubility in common processing solvent. Thus, alkoxy-substitued benzooxadiazole (BO) and benzothiadiazole (BT) moieties were used as acceptor units not only for reducing optical bandgap but also for maintaining good solution processability. In this presentation, four D-A type conjugated polymers, BO8, BT8, TBO12 and TBT12, were designed and synthesized by Stille polymerization. These polymers showed ambipolar charge carrier transporting characterstics, and among others, TBT12 exhibited 0.77 cm2/Vs for hole mobility and 0.01 cm2/Vs for hole mobility. Photovoltaic cells based on TBO12:PC71BM showed an open-circuit voltage of 0.58 V, a power conversion efficiency of 4.7% and a short circuit current of 11.4 mA/cm2.
9:00 PM - MD5.2.31
Highly Efficient Blue Fluorescent Organic Light Emitting Diode Based on Intramolecular Charge-Transfer Emitter Using Azasilin Core
Jin Won Sun 1,Kwon-Hyeon Kim 1,Chang-Ki Moon 1,Jang-Joo Kim 1
1 Seoul National University Seoul Korea (the Republic of),
Show AbstractReplacing phosphorescent materials by efficient Thermally Activated Delayed Fluorescence (TADF) materials will eventually lower cost with the potential of clearing stability issues. Especially, blue dyes among the three primary colors have been considered as the most crucial ones due to their important role in generating white light with good color purity. TADF materials have the donor-connection unit-acceptor structure to have small overlap between the HOMO and the LUMO to minimize the singlet-triplet energy gap. However, it seems quite challenging to search for an appropriate combination of donor and acceptor moieties to achieve good color purity and high EL efficiency for blue fluorescent OLEDs at the same time. Carbazole and dimethyl dihydroacridine have been mainly used as the donor moieties and sulfone oxide and triazine as the acceptor unit in the blue TADF molecules. Even a highly efficient blue fluorescent OLED of 20.6% was reported recently yet the OLED displayed sky-blue light with the CIE coordinates of (0.19, 0.35).
In this article, an efficient blue fluorescent OLED with an unprecedented high EQE of 22.3% was fabricated by doping DTPDDA in a mixed co-host system. The device emitted deep blue with the CIE coordinates of (0.149, 0.197). This is the first report demonstrating the both deep blue emission of y
9:00 PM - MD5.2.33
Synthesis and Characterization of Novel Azaacenes as Chromophores in Active Layers of Organic Opto-Electronic Devices
Martin Kaufmann 2,Dominique Gampe 1,Anja Lorenz 1,Benjamin Dietzek 3,Martin Presselt 2,Rainer Beckert 1,Helmar Gorls 4
1 Institute for Organic Chemistry and Makromolecular Chemistry Jena Germany,2 Institute of Physical Chemistry Jena Germany,1 Institute for Organic Chemistry and Makromolecular Chemistry Jena Germany2 Institute of Physical Chemistry Jena Germany,3 Leibniz Institute of Photonic Technology Jena Germany2 Institute of Physical Chemistry Jena Germany4 Institute of Inorganic and Analytical Chemistry Jena Germany
Show AbstractFast progress is being made in the development of organic electronic and optoelectronic devices[1]. However, many of the recently developed material systems contain scarce or expensive materials, such as ruthenium based metal complexes or fullerenes as acceptor materials, respectively[2]. Hence, there is huge demand for replacing such systems with stable organic dyes, for example, in organic solar cells (OSCs) or organic light emitting diodes (OLEDs). Since the successful stabilisation of pentacene, linear-condensed acenes have found many applications as versatile organic semiconductors[3]. The replacement of CH by other atoms (mainly N or S) results in heteroacenes in which different reactivities and altered photonic properties are observed[4]. One promising candidate for novel materials are polyazaacenes like fluorubines. They exhibit very interesting electronic features like high molar extinction coefficients and fluorescence quantum yields up to 100%, redoxacitivity, and stability in terms of photostability and chemical resistance[5,6].
Here we present a series of new tetraazaanthracenes as precursor for higher acenes[7]. The condensation of aromatic diamines with dichloro-dicyanopyrazine leads to this annulated compounds. Syntheses of N-substituted phenylene diamines as binucleophilic building blocks were developed to enable the introduction of multiple functional groups as ester, amino or nitro groups at the chromophoric system. The optical and electrochemical behavior of the formed heteroacenes was studied to obtain structure-property relationships, which allows the property-tailoring of the novel material.
References:
1: R. Fitzner, E. Mena-Osteritz, A. Mishra, G. Schulz, E. Reinold, M. Weil, C. Korner, H. Ziehlke, C. Elschner, K. Leo, M. Riede, M. Pfeiffer, C. Uhrich, P. Bäuerle, J. Am. Chem. Soc. 2012, 134, 11064—11067.
2: a) M. Grätzel, Acc. Chem. Res. 2009, 42, 1788—1798, c) S. Tschierlei, M. Presselt, C. Kuhnt, A. Yartsev, T. Pascher, V. Sundström, M. Karnahl, M. Schwalbe, B. Schäfer, S. Rau, M. Schmitt, B. Dietzek, J. Popp, Chem. Eur. J. 2009, 15, 7678—7688.
3: F. C. Krebs, Org. Electron. 2009, 10, 761—768.
4:a) J. E. Anthony, Angew. Chem. Int. Ed. 2008, 47, 452--483; Angew. Chem. 2008, 120, 460—492.
5: J. Fleischhauer, S. Zahn, R. Beckert, U.-W. Grummt, E. Birckner, H. Görls, Chem. Eur. J. 2012, 18, 4549 – 4557.
6: J. Fleischhauer, R. Beckert, Y. Jüttke, D. Hornig, W. Günther, E. Birckner, U.-W. Grummt, H. Görls, Chem. Eur. J. 2009, 15, 12799 – 12806.
7: D. M. Gampe, M. Kaufmann, D. Jakobi, T. Sachse, M. Presselt, R. Beckert, H. Görls, Eur. J. 2015, 21, 7571—7581
Acknowledgement:
We acknowledge the Bundesministerium für Bildung und Forschung (FKZ 03EK3507) for financial support.
9:00 PM - MD5.2.34
Effects of Moelcular Weight and Annealing Conditions on P3HT Interlayers for Enahncing the Performance of P3HT:PCBM Solar Cells
Sona Avetian 1,Richard Shallcross 1,Neal Armstrong 1
1 The University of Arizona Tucson United States,
Show AbstractWe explore the viability of pure poly(3-hexylthiophene-2,5-diyl) (P3HT) thin films as hole-selective interlayers in bulk heterojunction (BHJ) organic photovoltaic (OPV) devices. Recent work has shown that insertion of an ultra-thin and semi-crystalline P3HT interlayer between the bottom contact (e.g., ITO/PEDOT:PSS) and the active layer (e.g., P3HT:PCBM) increases charge selectivity at the active layer/bottom contact interface, while simultaneously increasing charge transport in the bulk through formation of more crystalline P3HT domains by means of interfacial templating. These reports suggest that the molecular weight (MW) of the P3HT interlayer material needs to be relatively high (ca. 40 kDa) in order to be sufficiently solvent-resistant so as not to be washed away upon solution deposition of the active layer blend; however, a detailed study does not currently exist that explores the effect of MW on the properties of P3HT interlayers. Herein, we utilize a range of P3HT MWs (between ca. 20 kDa and 100 kDa) and annealing procedures in order to explore their effect on the interlayer crystallinity and resistance to solvation. High-temperature annealing above the melting point of the polymer (ca. 240 °C,) followed by slow cooling (1 °C/min,) produces highly crystalline P3HT layers that are significantly more resistant to dissolution than temperatures typically used to anneal these films (ca. 140 °C.) We use a combination of UV-Vis spectroscopy, atomic force microscopy and electrochemical techniques in order to elucidate the MW-dependence of P3HT crystallinity and retention, surface morphology and charge blocking ability, respectively. Our results indicate that a compromise must be made between crystallinity (higher for lower MW) and retention (greater for higher MW) in order to obtain optimized interlayer properties.
9:00 PM - MD5.2.35
Doped Ambipolar Field-Effect Transistors
Akram Al-Shadeedi 1,Shiyi Liu 1,Scott Bunge 1,Bjoern Luessem 1
1 Kent State Univ Kent United States,
Show AbstractDoping of organic semiconductors has led to tremendous progress in the efficiency of optoelectronic devices such as OLEDs and organic solar cells [1]. Organic doping was as well used to reduce the contact resistance and control the threshold voltage of organic field effect transistors (OFETs) [2, 3], to improve the air-stability of n-channel transistors [4].
Here, we discuss that doping can be used not only to merely improve or tune OFETs, but to enable new functionalities as well. We show that organic ambipolar transistors can be realized by n-doping the contact and channel of OFETs.
Our transistors consist of Pentacene as organic semiconductor, aluminum as source/drain contacts, and Al2O3 as gate oxide. The contact regions and the transistor channel is doped by the molecular n-dopant W2(hpp)4. We show that both, electrons and holes, are injected at the n-doped source contact. Whereas electron injection can be rationalized by the formation of quasi-ohmic contacts, injection of holes can be reasoned by Zener-tunneling at the interface between the n-doped injection layer and the depleted channel region.
Furthermore, doping not only enables a balanced charge injection, but channel doping can be used to balance p- and n-conduction of the transistor. Increasing the thickness of the n-doped transistor channel leads to an increase in the onset voltage for hole transport and thus a suppression of the p-side of the transfer characteristic.
In summary, doping opens new design routes for organic ambipolar transistors. Implications of this research for the design of low-power organic circuits will be discussed and first results on inverter circuits are presented.
References:
[1] K. Walzer, B. Männig, M. Pfeiffer, and K. Leo, Chem. Rev. 107, 1233 (2007).
[2] F. Ante, D. Kälblein, U. Zschieschang, T.W. Canzler, A. Werner, K. Takimiya, M. Ikeda, T. Sekitani, T. Someya, and H. Klauk, Small 7, 1186 (2011).
[3] B. Lüssem, M. L. Tietze, H. Kleemann, C. Hoβbach, J. W. Bartha, A. Zakhidov, K. Leo, Nature Communications 4, 2775 (2013)
[4] P. Wei, J.H. Oh, G. Dong, and Z. Bao, J. Am. Chem. Soc. 132, 8852 (2010).
9:00 PM - MD5.2.36
Performance and Morphology of Novel Naphthalene Diimide (NDI) – Based Small Molecule Acceptors for Organic Photovoltaic Applications
Kira Rundel 1,Christopher R. McNeill 1,Kedar Deshmukh 1,Chao Wang 1,Steven Langford 2,Subashani Maniam 2
1 Materials Science and Engineering Monash University Clayton Australia,2 Chemistry Monash University Clayton Australia
Show AbstractOrganic photovoltaics (OPV) have received much attention due to the promise of low-cost, efficient processing of solar cells onto flexible substrates. [1] Typically, OPVs consist of a polymeric donor and a fullerene acceptor material, however, fullerene shortcomings such as poor light harvesting ability and high synthesis costs have resulted in the exploration of alternative acceptor materials. [2] Small molecule acceptor materials have recently gained attention due to their favourable absorption profile and inexpensive synthesis. While perylene diimide (PDI)-based materials have shown promise as small molecule replacements for fullerenes (showing as high as 6% PCE in devices), naphthalene diimide (NDI)-based materials showed lower performance initially, resulting in their limited exploration, with the highest reported device power conversion efficiency being 2.4%. [3] The lower efficiencies of NDI-based small molecules to date are attributed to weaker absorption in the visible wavelength range as well as susceptibility to forming large domain sizes. Here we report a series of NDI-based small molecule acceptor materials with different architectures (linear and star-shaped) and linkers that have shown promising initial results with a polymeric donor after minimal optimization (PCE ~ 2%, Voc > 1 V). As well as investigating the influence of the molecular architecture of the acceptor on device performance, the influence of acceptor architecture on thin film morphology will also be discussed.
References:
[1] L. Dou, J. You, Z. Hong, Z. Xu, G. Li, R. a. Street, Y. Yang, Adv. Mater. 2013, 25, 6642.
[2] S. M. McAfee, J. M. Topple, I. G. Hill, G. C. Welch, J. Mater. Chem. A 2015, 3, 16393.
[3] Y. Liu, L. Zhang, H. Lee, H.-W. Wang, A. Santala, F. Liu, Y. Diao, A. L. Briseno, T. P. Russell, Adv. Energy Mater. 2015, 5, n/a.
9:00 PM - MD5.2.37
Comparison of Photo-Degradation in P3HT and Deuterium Substituted P3HT-Based Polymer Cells
Satvik Shah 1,Balaji Ganapathy 1,Rana Biswas 2,Vikram Dalal 1
1 Iowa State Univ Ames United States,1 Iowa State Univ Ames United States,2 Ames Laboratory Ames United States
Show Abstract
Stability is a primary problem facing organic solar cells.
Moisture and oxygen are well known to cause extrinsic
degradation which can be reduced by encapsulation. However intrinsic
degradation from light exposure alone cannot be suppressed. We study basic
mechanisms of photo-degradation in organic solar cells by substituting
H by deuterium (D). A common source of degradation in organic
compounds are CH2 defects, which involve as the first step, the breaking of a
αC- H bond where αC is a carbon atom directly connected to the aromatic
backbone, followed by H bonding within the thiophene ring [1]. Ab-initio
simulations[1,2] identified H local motion and rebonding to be a pathway for light induced degradation,
especially in the presence of blue photons[2]. This degradation should
be expected to be considerably slower for the deuterium substituted P3HT
polymer due to the kinetic isotopic effect. This motivated us to study the
photo-degradation of solar cell devices using deuterated P3HT(D) as the donor
material. Shao et al [3] showed that solar cell devices using
P3HT(D):PCBM had markedly different device
characteristics, charge transfer states, and larger electron-phonon-coupling
that affects charge transfer at interfaces, compared to conventional
P3HT:PCBM devices.
We synthesized selectively deuterated P3HT, which has
not been attempted previously. A deuterium substituted P3HT(D) polymer
with a high polydispersity index of 1.15 was synthesized. The structure
of the polymer was confirmed using NMR spectroscopy. Using this polymer
as the donor and PCBM as the acceptor, we fabricated bulk hetero-junction
solar cells. We extensively characterized these devices using current-voltage
curves, capacitance-voltage, capacitance-frequency and quantum efficiency
measurements. We measured an open-circuit voltage of 0.57 V, a short-circuit
current of 9.2 mA/cm2 and a fill-factor of 67% and a power conversion
efficiency (PCE) of 3.5% for such a deuterated polymer solar cell device.
We found very similar electronic density of states as in P3HT.
Measurements of light induced degradation were performed in an
environmental chamber with a solar simulator under a continuous flow of high
purity nitrogen gas, thereby avoiding exposure to moisture
and oxygen. We will show results comparing the degradation
in deuterated P3HT(D) and P3HT devices, and show that the
predicted kinetic slowing down is surprisingly not observed. We will show results of ab-initio
calculations and infrared absorption measurements to investigate the degradation mechanisms, identify bonding changes and interpret these findings.
1) J. Northrup, Appl. Phys. Exp. 6, 121601 (2013).
2) S. Shah, R. Biswas, J. Phys. Chem. C. 119, 20265 (2015).
3) M. Shao et al, Nature Comm. 5, 4180 (2014).
Partially supported by the NSF through grant CBET-133164
9:00 PM - MD5.2.38
Polymer: Fullerene Bulk Heterojunction Solar Cells: Vertical Stratification and Device Performance
Tao Wang 1,Alan Dunbar 2,David Lidzey 2
1 Wuhan Univ of Technology Wuhan China,2 University of Sheffield Sheffield United Kingdom
Show AbstractDuring the fabrication of an organic photovoltaic (OPV) device, a bulk heterojunction layer is usually deposited from a solution containing both electron donors and an electron acceptor. During the film formation process, a complex evolution of film morphology can occur that includes phase separation and vertical stratification. A significant body of work has shown that the nanoscale morphology in both lateral and vertical directions within a BHJ film plays a critical role in determining the efficiency of an organic photovoltaic device.
We have studied the vertical stratification of fullerene acceptor in its blends with a few carbazole- and anthracene- based conjugated polymer donors via neutron scattering. We found that the PCBM vertical stratification in the blends can be affected by the chemical structure of conjugated polymers, the substrates and the processing conditions. We have observed that the PCBM can form depletion regions near either the cathode or anode interface. A PCBM depletion region near the anode region increases the open circuit voltage of an OPV device and consequently improves device efficiency. However, a PCBM depletion region near the cathode interface induces charge recombination and significantly reduces the open circuit voltage and efficiency of an OPV device. If the PCBM content in the depletion region is only 10~20% less than that in the bulk region, limited impacts are observed in device performance.
9:00 PM - MD5.2.39
Multilayer Substituted Polystyrene Dielectrics Used to Decrease Bias Stress and Control Turn-On Voltages in Organic Field Effect Transistors
Olivia Alley 1,Tejaswini Kale 1,Xin Guo 1,Bhupathiraju Manasa 1,Grace McClintock 1,Howard Katz 1
1 Johns Hopkins University Baltimore United States,
Show AbstractHere we present properties of multi-layered polystyrene (PS) based polymer dielectrics. Operating voltages in organic electronics are relatively high due to both the low charge carrier mobility and the low intrinsic free carrier density in organic semiconductors (OSCs). One way to decrease the operating voltage is to increase the carrier density by incorporating static electric fields into the device. This has been done frequently in the literature by trapping charges in the dielectric adjacent to the OSC in a ‘charging’ step.
In this work, we have successfully increased the charge capturing ability of PS by introducing substituents, either C60 or a triaryl amine, to styrene units in each chain. We also controlled the location of the substituted PS by building bilayer or trilayer dielectrics. Underlying PS films were made insoluble by introducing thermally crosslinkable units into the PS chain.
We measured the amount of trapped charges in the PS stack by comparing the threshold voltage (Vth) of a pentacene OFET before and after charging. The threshold voltage was measured, then a bias was held across the dielectric layer for several minutes, followed by a second measurement of threshold voltage.
We also observed that for both dielectrics incorporating substituted PS and those with unsubstituted PS, the typical effect of charging was consistent with charges being injected from the pentacene side into the PS stack. A positive charging voltage increased the threshold voltage, turning the p-channel OFETs more ‘off’. This indicates positive charges are trapped in the dielectric closest to the channel of the OFET. Negative charging decreased the threshold voltage, turning the devices more ‘on’.
While the substituted PS has been seen to trap more charge than unsubstituted PS, by encapsulating substituted PS between two relatively thick layers of unsubstituted PS, we do not see any shift in Vth upon charging—in fact, we believe we are reducing the bias stress susceptibility by using a trilayer dielectric with a substituted PS film in the middle. While the threshold gate voltages for the OFET built on this dielectric is typically between -20 and -30V, the stress gate voltage was +/-100V or greater, and was held from 15 to 60 minutes. Reduced Vth shift was seen compared to a dielectric made from unsubstituted PS. This could be due to a higher polarization resistance in this structure, or due to deep traps in the bulk of the dielectric, which, after filling, are no longer labile to bias stress. The field from these trapped charges could prevent further trapping or polarization in the dielectric or at the OSC/dielectric interface.
By substituting PS with fullerene and triaryl amine substituents, we have made layered dielectrics with the capacity to stabilize more trapped charges than PS alone. Reductions in bias stress are seen when the substituted PS is insulated from the gate and the OSC by thick layers of unsubstituted PS.
9:00 PM - MD5.2.40
Organic Micro-Electro-Mechanical (MEM) Relay Technology for Ultralow-Power Flexible Low-Cost Large-Area Electronics
Yanbiao Pan 1,Jaeseok Jeon 1
1 Rutgers, The State University of New Jersey Piscataway United States,
Show AbstractVarious conjugated polymers and small-molecules have been researched over the last decades as the active semiconductor layer for organic thin film transistors (OTFTs) to enable flexible low-cost large-area electronics. However, due to their low field-effect carrier mobility (below 10 cm2V-1s-1) requiring a relatively large supply voltage (typically above 3 V) for a reasonable on/off current ratio (> 105) and switching frequency (> 1 MHz), and due to rather poor semiconductor/insulator interface quality causing a relatively large off-state leakage current, OTFTs would consume large amounts of static and dynamic power. This would be exacerbated for OTFT-based integrated circuits as p-channel OTFTs usually far outperform n-channel counterparts, rendering it difficult to achieve complementary operations. Here we report a polymer-based electrostatically-actuated micro-electro-mechanical (MEM) relay as a potential alternative (or complement) to conventional OTFTs in order to enable electronics requiring ultralow power, mechanical flexibility, low-cost, low-temperature processing, and large-area. We implement two kinds of well-functioning prototypes: purely-polymeric and partially-polymeric (inorganic-organic hybrid) relays using a low-temperature surface-micromachining process and demonstrate their switching characteristics, showing immeasurably low off-state leakage current (~10 fA), abrupt switching behavior, and high on/off current ratio (> 105 over an input voltage swing of 20 mV), which would provide for zero static power consumption and potentially ultralow dynamic power consumption. The prototypes achieve relatively low contact adhesive forces (< 10 nN/μm2) and hysteresis voltages (< 100 mV) via low-surface-energy contact materials with low Hamaker constant and non-pull-in mode of operation and can operate in a complementary manner owing to the ambipolar nature of electrostatic actuation that allows symmetric pairs of n- and p-channel relays.
9:00 PM - MD5.2.41
Effects of Donor/Acceptor Interfacial Energetics on Carrier Behavior in Organic Photovoltaics Investigated by Impedance Spectroscopy
Tomohiro Mayumi 2,Kyohei Nakano 2,Seiichiro Izawa 2,Kaori Suzuki 2,Kazuhito Hashimoto 1,Keisuke Tajima 3
1 Department of Applied Chemistry, Graduate School of Engineering The University of Tokyo Bunkyo-ku, Tokyo Japan,2 Center for Emergent Matter Science (CEMS) RIKEN Wako-shi, Saitama Japan,2 Center for Emergent Matter Science (CEMS) RIKEN Wako-shi, Saitama Japan1 Department of Applied Chemistry, Graduate School of Engineering The University of Tokyo Bunkyo-ku, Tokyo Japan2 Center for Emergent Matter Science (CEMS) RIKEN Wako-shi, Saitama Japan,3 Precursory Research for Embryonic Science and Technology (PRESTO) Kawaguchi, Saitama Japan
Show AbstractFor further enhancement of efficiency in organic photovoltaics, it is necessary to realize donor/acceptor (D/A) interfacial structures which enable both efficient charge separation and suppressed recombination. However, the relationship between the interfacial structure, device performance and carrier dynamics is still unclear due to the difficulties of controlling and observing the organic interfaces. To study the effect of D/A interfacial energetics precisely, we fabricated planar D/A interfaces without molecular intermixing by contact film transfer (CFT) method and modified interfacial energetics by surface segregated monolayers. We found the device with energetic cascade at D/A interface, in which the first monolayer at the interface of the acceptor has a higher LUMO level than in the bulk, showed higher VOC and FF without loss of JSC compared to the device with normal energetic structure1. In this study, the mechanism of higher performance of the cascade is elucidated by the investigation in carrier behavior with impedance spectroscopy (IS).
Firstly, to interpret impedance response of the bilayer device accurately, the bilayer device (ITO/ZnO/PCBM//P3HT/MoO3/Ag where // donates the interface created by CFT method) was analyzed by IS under light at open circuit condition. As the result, carrier accumulation and recombination at D/A interface were observed separately from other carrier behaviors. The assignment of impedance responses was confirmed by the carrier lifetime measured by transient photovoltage. Next, the bilayer device with energetic cascade was analyzed. The lifetime increased compared to the device with the normal energetic structure, indicating the recombination is suppressed by the energetic cascade. Interestingly, the capacitance was about half of the corresponding normal device at the same carrier density. This lower capacitance suggests that the charges were depleted at D/A interface and capacitance was decreased due to the longer effective distance between charge pairs. To confirm this experimentally, devices with a thin insulating spacer (1~3 nm) inserted between the donor and the acceptor were analyzed. As the result, the capacitance was decreased as the spacer become thicker. This result verifies the charge depletion at D/A interface decreases capacitance. Moreover, the capacitance change was reproduced by calculation using an exponential carrier distribution model.
From these results, we concluded that energetic cascade at D/A interface drives charges away from the D/A interface to the bulk. This makes the distance of the interfacial charge pairs longer and suppresses the recombination. The energetic cascade is proven to be an important structure to enhance the efficiency in organic photovoltaics.
References
1. Izawa, S.; Nakano, K.; Suzuki, K.; Hashimoto, K.; Tajima, K., Dominant Effects of First Monolayer Energetics at Donor/Acceptor Interfaces on Organic Photovoltaics. Adv. Mater. 2015, 27 (19), 3025-3031.
9:00 PM - MD5.2.42
Solution Processable Carbon Nanoelectrodes for Single-Molecule Investigations
Jingyuan Zhu 3,Joseph Mcmorrow 1,Rachel Crespo-Otero 1,Geyou Ao 2,Ming Zheng 2,William Gillin 3,Matteo Palma 1
1 School of Biological and Chemical Sciences Queen Mary, University of London London United Kingdom,3 School of Physics and Astronomy Queen Mary, University of london London United Kingdom,1 School of Biological and Chemical Sciences Queen Mary, University of London London United Kingdom2 National Institute of Standards and Technology Gaithersburg United States3 School of Physics and Astronomy Queen Mary, University of london London United Kingdom
Show AbstractSince the 1970s, researchers have tried to use molecules as building blocks in electronic circuits for the prospect of size reduction offered by molecular-level control. Until recently, different strategies including scanning probe techniques, lithographic approaches, and mechanical/electromigration break junctions have made it possible to investigate molecules in a practical way. Nevertheless, despite the substantial progress in single-molecule electronics from both fundamental and technological standpoints, challenges remain. Principal among these are the time and cost involved in nanogap fabrication, the reliable control of the nanogap size, and the need for a facile (and scalable) technology for the establishment of electrical contact between individual molecules and metal electrodes.
The use of carbon-based nano-electrodes, in particular, has emerged as a promising approach because of the intrinsic nanoscale size of CNTs and graphene, and the reduced electronic mismatch granted by having molecules and electrodes of the same material (Carbon atoms).
Herein we present a facile solution based assembly method for producing molecular transport junctions (MTJs) by covalently linking metallic single-wall carbon nanotubes (SWCNTs) with electrically functional molecules. As a proof of principle, the single-molecule junction conductance of a series of oligophenyles was measured and found to be in line with literature values.
For our studies we employed DNA wrapped SWCNTs separated by length via size exclusion chromatography, and sorted by chirality and electronic structure via ion exchange chromatography [1] (DNA-wrapping further leaves only the terminal ends of the SWCNTs available for direct functionalization). Building on our recent bottom-up assembly strategy for the formation of end-to-end CNT junctions, [2] we linked metallic single-chirality (7, 4) SWCNTs in amidation reactions with three different diamine conjugated molecular linkers. To confirm junction formation we cast low-coverage films on doped silicon wafer substrates coated with a hydrophobic layer shown to induce partial alignment of DNA-wrapped CNTs. The electrical properties of the MTJs fabricated in this study were investigated by measuring their current-voltage (I-V) characteristics as a function of the distance between a metallic AFM tip used as a mobile electrode, and a fixed macroscopic Au electrode Our findings are of general interest for the controlled assembly of CNT junctions as a platform for molecular conductance investigations, towards the fabrication of solution-processable single-molecule devices.
1. Zheng, M., et al., DNA-assisted dispersion and separation of carbon nanotubes. Nature materials, 2003. 2(5): p. 338-42.
2. Palma, M., et al., Controlled formation of carbon nanotube junctions via linker-induced assembly in aqueous solution. Journal of the American Chemical Society, 2013. 135(23): p. 8440-3.
9:00 PM - MD5.2.43
π-Extended Narrow-Bandgap Oligomers for Solution-Processed Organic Solar Cells
Hideaki Komiyama 2,Seiichi Furukawa 2,Yu Hidaka 2,Takahiro To 2,Chihaya Adachi 3,Takuma Yasuda 2
1 International Institute for Carbon Neutral Energy Research (WPI-I2CNER) Kyushu University Fukuoka Japan,2 INAMORI Frontier Research Center Kyushu University Fukuoka Japan,2 INAMORI Frontier Research Center Kyushu University Fukuoka Japan1 International Institute for Carbon Neutral Energy Research (WPI-I2CNER) Kyushu University Fukuoka Japan,3 Center for Organic Photonics and Electronics Research (OPERA) Kyushu University Fukuoka Japan
Show AbstractWith the development of renewable energy sources, solution-processed bulk heterojunction organic solar cells (BHJ-OSCs) have been well recognized as one of the promising green technologies owing to their low cost fabrication, light weight, flexibility, and solution processability. Combining a central electron-donating core with a terminal electron-accepting groups to form acceptor-donor-acceptor (A-D-A)-structured narrow bandgap π-conjugated oligomers should be an effective strategy to tune the optoelectronic properties and thus to achieve high performance on BHJ-OSC devices. The design and implementation of the central donor unit in π-conjugated oligomers open up new possibilities for understanding fundamental organic semiconductor structure-property relationships and further optimizing device parameters. Herein, we introduced fused thiophene derivatives, tetrathienoanthracene and benzodithiophene, as the central donor unit in A-D-A-structured narrow-bandgap oligomers having different terminal electron-accepting units. The effects of changing their central donor unit on the molecular packing, morphology, optoelectronic, and photovoltaic properties were systematically investigated.
9:00 PM - MD5.2.44
Thickness Resolved In Situ Studies of Organic Heterojunction Effect in Pentacene/C60 Heterostructured Organic Thin Film Transistors
Jongboum Kim 1,Kwangseok Ahn 1,Dong Ryeol Lee 1
1 Department of Physics Soongsil University Seoul Korea (the Republic of),
Show AbstractWe present in situ study of organic heterojunction effect in pentacene/C60 heterostructured organic thin film transistors using in-situ electrical transport measurement during the vapor deposition of pentacene and C60 bilayers. The C60 layers were deposited on thin pentacene layers to enhance both film crystallinity and electrical properties of C60 films.[1] During the n-type C60 deposition on the p-type pentacene buffer layers, ambipolar transistor behaviors were observed after the thickness of C60 layer reached 2.5 nm.[2] To investigate organic heterojunction effect that has known to be an accumulation of mobile electrons and holes in the n- and p-type organic semiconductors, respectively, close to the heterojunction interface,[3] we compared C60 film thickness resolved transport measurements for different pentacene layers with a monolayer(ML) and four MLs. For an 1-ML pentacene buffer where the hole transport is mainly determined by the field-effect charges at the insulator/pentacene interface, no enhancement in hole current and hole mobility was observed with increasing C60 film thickness. On the other hand, for a 4-ML pentacene buffer where the insulator/pentacene and pentacene/C60 interfaces are physically separated, a significant enhancement in hole current and hole mobility was observed. This enhancement can be explained by the organic heterojunction effect at the pentacene/C60 interfaces.
[1] K. Ahn et al., Appl. Phys. Lett. 102, 043306 (2013), K. Itaka et al., Adv. Mater. 18, 1713 (2006).
[2] S. J. Noever, S. Fischer, and B. Nickel, Adv. Mater. 25, 2147 (2013).
[3] J. Wang et al., Appl. Phys. Lett. 87, 093507 (2005).
9:00 PM - MD5.2.45
Impact of Aggregate Formation on Spectroscopic Properties of the Interface-Active Amphiphilic Fullerene Derivative PEG-C60: From Solutions to Thin Films
Saunak Das 1,Felix Herrmann-Westendorf 2,Martin Presselt 1,Benjamin Dietzek 1,Felix Schacher 3,Uwe Ritter 4
2 Institute of Physical Chemistry, Friedrich-Schiller-University Jena University of Jena Jena Germany,1 Leibniz Institute of Photonic Technology (IPHT) Jena Jena Germany,2 Institute of Physical Chemistry, Friedrich-Schiller-University Jena University of Jena Jena Germany3 Jena Center for Soft Matter (JCSM), Friedrich-Schiller-University Jena Jena Germany4 Institut für Chemie und Biotechnik Ilmenau University of Technology Illmenau Germany
Show AbstractAlthough the field of bulk heterojunction organic solar cell comprising fullerene based acceptors have seen enormous progress over the last decade[1], controlling and optimizing the morphologies of thin films remains a major challenge for further improving device performances and longevity. Fullerene local environment effects the charge delocalization and dielectric constant, diminishing the activation barrier for exciton splitting [2]. A hierarchical approach for a targeted design of supramolecular ensembles was taken in the past [2]. Our discourse is focused on a fullerene derivative where we adduce the possibility to control electronic transitions by crafting several supramolecular fullerene clusters.This shows how photonic properties change from single molecules to large fullerene based van der Waals clusters based on molecular symmetry. The work of Shubina et. al[3] focused on van der waal’s fullerene clusters inferring the existence of deep electron traps and our research explores their photonic properties intricately.
We designed defined, mechanically stable, ordered assemblies of amphiphilic polyethylene glycol based fullerene derivative (PEGC60) at air-water interfaces using Langmuir-Blodgett technique[4]. We studied the absorption properties of these thin film LB film-aggregates and compared them to spin coated film-aggregates using photothermal deflection spectroscopy (PDS)[5]. Using UV/vis spectroscopy we studied aggregation absorption feautures of PEGC60 solution and used dynamic light scattering (DLS) experiments to delineate the molecular-cluster sizes at different concentrations. For the first time, we demonstrated the Herzberg-Teller intensity stealing phenomena in asymmetric fullerene aggregates using PDS and asserted our experimental findings by theoretical TDDFT simulation. We modelled PEGC60-monomers and aggregates of varied symmetry and inferred the applicability of Laporte’s selection rule to control the allowedness of the HOMO/LUMO transition. These findings will be vital to guide the development of acceptor materials with potentially improved performance and supramolecular design.
References
[1] C.J. Brabec, J.R. Durrant, MRS Bulletin 33 (2008) 670.
[2] S.V. Kesava, Z. Fei, A.D. Rimshaw, C. Wang, A. Hexemer, J.B. Asbury, M. Heeney, E.D. Gomez, Advanced Energy Materials 4 (2014).
[3] T.E. Shubina, D.I. Sharapa, C. Schubert, D. Zahn, M. Halik, P.A. Keller, S.G. Pyne, S. Jennepalli, D.M. Guldi, T. Clark, Journal of the American Chemical Society 136 (2014) 10890.
[4] A. Modlinska, D. Bauman, International Journal of Molecular Sciences 12 (2011) 4923.
[5] F. Herrmann, S. Engmann, M. Presselt, H. Hoppe, S. Shokhovets, G. Gobsch, Applied Physics Letters 100 (2012) 153301.
Acknowledgements
We acknowledge the financial support from the “Bundesministerium für Bildung und Forschung” (FKZ: 03EK3507)
9:00 PM - MD5.2.46
Enhanced Thermoelectric Properties of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) by Binary Secondary Dopants
Chao Yi 1,Xiong Gong 1
1 Univ of Akron Akron United States,
Show AbstractTo simultaneously increase the electrical conductivity and Seebeck coefficient of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) was a challenge for realizing efficient organic thermoelectrics. In this study, for the first time, we report both increased electrical conductivities and Seebeck coefficients, hence, enhanced thermoelectric properties of PEDOT:PSS thin films by doped with binary secondary dopants, dimethyl sulfoxide (DMSO) and polyethylene oxide (PEO). Without modifying film morphology, the molar ratios of PEDOT to PSS are tuned by PEO, resulting in increased proportions of PEDOT in the bipolaron states. Our study provides a facile route to optimizing thermoelectric properties of PEDOT:PSS thin films.
9:00 PM - MD5.2.47
Effect of Energy Gap and Purity in Energetic Cascade Layer at D/A Interface on Organic Photovoltaic Performance
Kyohei Nakano 1,Yujiao Chen 1,Kaori Suzuki 1,Keisuke Tajima 1
1 RIKEN Wako Japan,
Show AbstractWe have investigated the effect of energy gap and purity in energetic cascade layer at D/A interface on device performance of organic solar cells. Investigation of these effects is generally difficult because of uncontrollability of D/A interface in the bulk heterojunction. Here we overcome this difficulty by utilizing a planar heterojunction structure fabricated by contact film transfer method which enables us to realize a well-defined, ideal interface structure. Resulting device structure was ITO/ZnO/PCBM//cascade layer//rr-P3HT/MoOx/Ag, where // denotes the interface fabricated by film transfer method. We selected two polymers for the cascade layer, PTB7 and regiorandom (rra) P3HT. The former has a narrower energy gap compared to the rr-P3HT(donor layer), and the latter has a wider one.
By inserting PTB7 and rra-P3HT at D/A interface, VOC was increased by about 0.3eV. Electroluminescence from charge transfer (CT) state showed that cascade layer generated CT state with PCBM acceptor; thus deeper HOMO level of the cascade layer increased energy of CT state and led to high VOC. On the other hand, the effect of the cascade layers on JSC was different. Cascade layer of PTB7 increased JSC owing to its additional light absorption; however, inserted rra-P3HT layer decreased JSC about 20%. From the measurement of external quantum efficiency, we revealed that the reason of the decreased JSC was blocking of singlet exciton in rr-P3HT by the cascaded rra-P3HT layer owing to its wider energy gap. We also investigated the effect of intermixing in the cascade layer with PCBM. We found JSC could be enhanced by the intermixing regardless of the energy gap of the cascade layer, and facilitation of the VOC was small compared to pure cascade layer owing to the difference of CT state.
These well controlled experiments indicate that the condition of cascade layer, i.e. its energy gap and purity are crucial factors for the device performance of organic photovoltaics.
9:00 PM - MD5.2.48
Thermal Resistances of Thin Films of Small-Molecule Organic Semiconductors
Yulong Yao 1,Maryam Shahi 1,Marcia Payne 1,John Anthony 1,Joseph Brill 1
1 University of Kentucky Lexington United States,
Show AbstractWe are using the 3-omega technique to measure the thermal resistance of films of triethylsilylethynyl anthradithiophene (TES-ADT) on substrates such as sapphire, gold, and silica. Film thicknesses range from 100-300 nm and measurements on both polycrystalline and amorphous films are presented. Although the results would be consistent with thermal conductivities of a few mW/cmK, similar to previous measurements of other organic films, the variation of results for films of similar thicknesses, and lack of dependence on crystallinity and film thickness, suggest that the thermal conduction is dominated by thermal resistance with the interface. Values of the interface thermal resistivities for different substrates will be compared, as will results for both solution-cast and evaporated films of the fluorinated compound, F-TES-ADT.
9:00 PM - MD5.2.49
Electron Injection and Transport in Phosphorescent Organic Light-Emitting Diode: The Role of Dopant Molecule
Jiun-Haw Lee 1,Yi-Hsin Lan 1,Cheng-Pin Chen 1,Bo-Yen Lin 1,Lik-Ka Yun 1,Yuh-Renn Wu 2,Tien-Lung Chiu 2,Chi-Feng Lin 3
1 Graduate Institute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan University, Taiwan R. O. C. Taipei Taiwan,2 Department of Photonics Engineering, Yuan Ze University, Taiwan R. O. C. Chung-Li Taiwan3 Department of Electro-Optical Engineering, National United University, Taiwan R. O. C. Miaoli Taiwan
Show AbstractCarrier distribution in the emitting layer (EML) plays an important role for determining the efficiency and lifetime of an organic light-emitting diode (OLED). In a phosphorescent OLED, dopant concentration is typically high (>5%) for efficient Dexter energy transfer, which significantly affects the carrier injection and transport in the EML. In this report, we demonstrated that the blue phosphor played the role as a trapping center which impeded the electron transport in the matrix below certain dopant concentrations (
Symposium Organizers
Alejandro L. Briseno, University of Massachusetts
Aram Amassian, King Abdullah University of Science and Technology (KAUST)
Iain McCulloch, Imperial College London
Özlem Usluer, Konya Necmettin Erbakan University
Symposium Support
ACS Applied Materials amp
Interfaces | American Chemical Society
Aldrich Materials Science
MD5.3: Morphology and Thin-Film Interfaces I
Session Chairs
Aram Amassian
Stefan Mannsfeld
Lee Richter
Garry Rumbles
Wednesday AM, March 30, 2016
PCC West, 100 Level, Room 102 AB
9:00 AM - MD5.3.01
Solution-Printed High Performance Organic Field Effect Transistors by Controlled Crystallization
Muhammad Niazi 3,Ruipeng Li 1,Er Qiang Li 1,Ahmad Kirmani 1,Maged Abdelsamie 1,Kui Zhao 1,Marcia Payne 2,Detlef Smilgies 3,John Anthony 2,Sigurdur Thoroddsen 1,Aram Amassian 1
3 Cornell High Energy Synchrotron Source Cornell University Ithaca, 14850 United States,1 Division of Physical Science and Engineering King Abdullah University of Science and Technology (KAUST) Thuwal, 23955-6500 Saudi Arabia2 Department of Chemistry University of Kentucky Lexington, 40506 United States
Show AbstractSolution-printed organic field effect transistors (OFETs) have emerged in recent years as promising contenders to be part of electronic and optoelectronic circuits owing to their compatibility with low cost roll-to-roll manufacturing processes. The stringent performance requirements for OTFTs in terms of carrier mobility, switching speed, turn-on voltage and uniformity over large areas require the performance of single crystal-based OTFTs, but these suffer from major scale-up challenges. To achieve device performance approaching that of single crystals by high throughput solution-printing of OTFTs requires extraordinary control over the microstructure and morphology of the organic semiconductor (OSC) in terms of lamellar texture, grain boundary density and interfaces with the gate dielectric and metal contacts. To accomplish this, it requires understanding and an ultimate control of the OSC thin film crystallization during film formation processes and producing very low defect-density films. In this respect, it is important to understand the solution-to-solid phase transformation during film formation and development of microstructure and morphology of the OSC films from solution-printing methods. In-situ monitoring of thin film formation reveals that solution-to-solid phase transformation mechanism is fundamentally different in roll-to-roll compatible process such as blade coating compared to spin coating.
We discover that OSC thin film growth in blade coating proceeds via an intermediate dense phase which is similar to the Two Step Nucleation model proposed previously for proteins, pharmaceuticals and colloids etc. We also find that previous lessons and design rules drawn from spin-coating of OSCs and contact-induced nucleation may have to be revisited in the context of blade coating and in related processes, because the nucleation and growth of OSCs differ in blade and spin coating processes. For instance, we show that blade coating achieves texture purity in case of halogenated OSCs, such as 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TES-ADT), irrespective of whether the contact is chemically treated with a halogenated self-assembled monolayer (SAM) or not which is in contrast to spin coating results. We find that strong OSC-contact interactions due to the presence of halogenated SAMs lead to increased nucleation density both for halogenated and non-halogenated OSCs. The increasing nucleation density disrupts the formation of large domains typically achieved by this method, resulting in significantly reduced carrier mobility. We present a new method based on blade coating of a blend of conjugated small-molecules and amorphous insulating polymers to produce OTFTs with consistently excellent performance characteristics (carrier mobility 1-8 cm2V-1s-1, low threshold voltages of < 1 V and low subthreshold swings < 0.4 V/dec) on par with single single crystal counterparts.
9:15 AM - MD5.3.02
Molecular Basis for Alignment in Solution Coated Conjugated Polymer Thin Films
Ying Diao 1
1 University of Illinois at Urbana-Champaign Urbana United States,
Show AbstractFundamental device physics studies have indicated that charge transport is the fastest along the conjugated backbone direction. This implies that polymer chain alignment between the source-drain electrodes is critical to promoting charge transport. Recent experimental studies provide strong evidence for this inference. Diamond scratched nanogrooved surfaces have been used to align a rigid conjugated polymer of various molecular weights during slow drop casting. Using this method, the long axis of the polymer nanowires lined up along the scratched grooves, resulting in thin film anisotropy of 2-3 times measured by polarized UV-Vis spectroscopy. The hole transport was enhanced by more than an order of magnitude after alignment. However, polymer thin films fabricated using printing or coating methods frequently exhibit poor degree of alignment despite the presence of unidirectional flow in meniscus guided coating techniques.
In this work, we set out to understand the molecular basis for attaining alignment during unidirectional solution coating. We compared several polymer systems of varying degrees of backbone conjugation and persistence length. We found that the extent of alignment is highly sensitive to the molecular structure. We perform detailed structural analysis to characterize the polymer thin film morphology and extent of anisotropy. We link the solution phase structure to the solid-state structure, and eventually to the charge transport characteristics. We also devise methodologies to control polymer self-assembly in solution and therefore promoting alignment and charge transport. Our study sets the stage for establishing the much needed morphology-charge transport properties, and for devising strategies for controlling thin film alignment in conjugated polymer thin films.
9:30 AM - *MD5.3.03
What Happens at a Moving Drying Line: A Hartman-Perdok View of Coating
Detlef Smilgies 1
1 Cornell Univ Ithaca United States,
Show AbstractOne of the fascinating facets of directional coating is that under certain conditions a preferential in-plane orientation of the crystallites can be obtained, without the need of lithographically or chemically patterning the substrate. Comparing different coating techniques, the influence of the various factors is explored that lead to in-plane texture of the solute. At the drying line, where the solute crystallizes from solution, there is a complex variety of effects at work: concentration gradients, Marangoni currents, shear, and the intrinsic crystallization speed of the material. Employing classic crystallization theory, a mechanism for directional growth is proposed.
10:00 AM - *MD5.3.04
High Performance Morphologies in Solution-Sheared Films of Organic Semiconductors and Conductors
Stefan Mannsfeld 1,Zhenan Bao 2,Ying Diao 3,Brian Worfolk 2,Sean Andrews 2,Steve Park 2
1 Center of Advancing Electronics Dresden Dresden Germany,2 Dept. of Chemical Engineering Stanford University Stanford United States3 Chemical amp; Biomolecular Engineering University of Illinois Urbana United States
Show AbstractIn both polycrystalline thin films of small organic semiconductor molecules and polymer thin films, the molecular packing determines the film’s charge transport properties and consequently the electrical performance in thin-film devices such as organic thin film transistors (OTFTs) or solar cells. We have in the past few years demonstrated that the solution-shearing method produces remarkably well-performing film morphologies for small organic molecular materials, semiconducting polymers and most recently also conducting polymers. Here we discuss results for solution-sheared films of the small organic semiconductor 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), benzothieno[3,2-b]benzothiophene (BTBT), and the conducting polymer blend system PEDOT:PSS.
We demonstrate that the nanoconfinement effect combined with the solution-shearing based technique flow-enhanced crystal engineering (FLUENCE) is a powerful and likely material-agnostic method to identify existing polymorphs in small organic semiconductor materials and to prepare the individual pure forms in thin films at ambient conditions. With this method, we prepared high-quality crystal polymorphs and resolved crystal structures of TIPS-pentacene, including a new polymorph discovered via in situ grazing incidence X-ray diffraction and confirmed by molecular mechanic simulations. In addition, we applied this methodology to a BTBT derivative and successfully stabilized its metastable state.
We also used solution shearing to fabricate high performance transparent conducting PEDOT:PSS films. Tunable control of PEDOT backbone orientation, local ordering, and phase separation is demonstrated via precise control of the deposition parameters. Record-high PEDOT:PSS conductivities of 4,600 ±
100 S/cm are obtained and reach a sheet resistance of 17 ± 1 Ω/■ at 97.2 ± 0.4% transmission. A patterning method is also discussed which enables the use of high-conductivity transparent conductive films in capacitive pressure sensors and OPV devices.
11:00 AM - *MD5.3.05
In Situ Studies of Opv Film Formation: Accelerating the Transition from Lab to Synchrotron to Fab
Lee Richter 1
1 NIST Gaithersburg United States,
Show AbstractSolution processing of electrically active layers is a promising route to sustainable manufacturing of functional components on diverse substrates such as flexible foils and textiles. Typically, solution processing does not result in the thermodynamic equilibrium form; instead metastable, kinetically trapped structures dominate. This allows great flexibility in the ability to tailor film structure (and performance) by processing details. The sensitivity of structure and performance to processing is clearly manifest in the fabrication of bulk heterojunction (BHJ) active layers for organic photovoltaics. Organic photovoltaic devices are a promising route to lower costs via roll-to-roll manufacturing. In a BHJ, nano-scale phase separation into nominally bicontinuous donor and acceptor rich regions enables both exciton dissociation and charge extraction. The performance of BHJ based devices is a strong function of the active layer structure and the optimized device structure is, in general, not the equilibrium structure. Recently it has become common to optimize BHJ film formation by introducing small amounts of processing additives. However, the mechanisms by which these additives effect the film formation are not known. We present results from the use of labscale photon based techniques, such as spectroscopic ellipsometry and photoluminescence and synchrotron based grazing incidence x-ray scattering (both wide angle and small angle) on the time evolution of films cast by blade coating, a material conservative model for scalable manufacturing processes. The in-situ measurements provide detailed insights into film thickness, composition, and microstructure. We will discuss highlights from studies of the additive effect in film formation of both polymer and small molecule based BHJs. Multiple mechanisms are revealed with common themes related to control of aggregation through solvent quality and control of growth kinetics via plasticization. We observe that, when elevated solution temperatures are required, significant differences between spin coating and isothermal deposition such as blade coating can arise. We demonstrate that isothermal blade coating is an excellent prototyping tool for slot-die coating by establishing comparable morphologies for small piece blade coating and continuous web slot-die coating.
11:30 AM - MD5.3.06
Optimizing OPV Morphology: Is a Reappraisal of the Mixed Domains Yet Again Needed
Subhrangsu Mukherjee 1,Harald Ade 1
1 Organic and Carbon Electronics Lab NC State University Raleigh United States,
Show AbstractIt is well established that the morphology of the polymer:fullerene BHJ blend is critical to performance and that actual morphologies comprise three phases in many systems, with mixed amorphous regions in addition to relatively pure, aggregated donor and acceptor domains. Understanding its role and controlling the mixed amorphous phase for maximum charge creation and minimum recombination remains a principal challenge. Recent simulation-based studies have suggested that this three phase morphology may be favorable as the electronic structure of both the donor and acceptor depends on the level of aggregation, thus providing an electronic landscape that can help to sweep out charges created in the mixed domains. Simulations have also shown that a high local (terahertz) charge carrier mobility, long CT state decay lifetime and three-phase structure with an energy cascade for either carrier type may increase the geminate pair splitting probability. Whilst charges migrate to the aggregated domains due to a favorable electronic landscape, these charges might be effectively trapped in such aggregates if there is insufficient number of percolating pathways in the amorphous mixed regions. Conversely, if the mixed regions have PCBM concentration much beyond the percolation threshold or if the volume faction of the mixed regions is large, then recombination is enhanced. We will review a number of soft X-ray scattering studies that determined the average composition variations and inferred purities/volume fractions at multiple length scales. This analysis suggests that the mixed regions can be detrimental to good performance, with often a pronounced negative impact on FF. Optimizing performance continues to be a delicate balance of possibly competing factors and the volume fraction of the ideal morphology remains to be determined. We argue that in an ideal morphology the amorphous mixed phase has a small volume fraction and a PCBM concentration near the percolation threshold. We relate this hypothesized morphology to miscibility, chi parameters and phase diagrams.
11:45 AM - MD5.3.07
Structure-Property Relationships in Bulk Hetero Junction Solar Cells: Efficiency Enhancement by Exploring New Electron Transport Interlayers
Dirk Vanderzande 2,Pieter Verstappen 1,Jurgen Kesters 1,Geert Pirotte 1,Sanne Govaerts 1,Jean Manca 1,Laurence Lutsen 2,Wouter Maes 1
1 Univ of Hasselt Diepenbeek Belgium,2 Imomec Imec vzw Leuven Belgium,1 Univ of Hasselt Diepenbeek Belgium2 Imomec Imec vzw Leuven Belgium
Show AbstractThe field of Organic PV has reached recently efficiencies in BHJ solar cells of almost 11% in a single junction. This progress is a result of a combination of new materials, insight in working principles and new architectures. In this respect Low Band Gap (LBG) donor-acceptor alternating copolymers belong to the most successful class of conjugated polymers used in BHJ solar cells. Nowadays a wide variation of such materials are available. They have in common to be able to give in principle power conversion efficiencies (PCE) in combination with PC71BM as acceptor between 6 and 10%.
In this contribution new LBG materials are presented together with their characteristics in devices. As building blocks donor structures [4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl] (BDT) and cyclopenta[2,1-b:3,4-b’]dithiophene (CPDT) were used as being very effective in keeping the HOMO level low enough to ensure acceptable Voc. At the same time high mobilities for charge carriers can be expected. As acceptor structures 5-octyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (TPD) and several 2,3-bis[5’-(2’’-ethylhexyl)thiophen-2’-yl]quinoxaline-5,8-diyl (Qx) derivatives were used. In the latter series the effect of introducing fluor substituents on Voc and PCE was analyzed. All the alternating copolymers could be synthesized with acceptable Mn (>30 kDa) and PDI. All materials were purified with recycling GPC resulting in materials with reduced low molecular weight content and a substantial increase of PCE was observed (from 2.3 % to close to 5.0%). Furthermore introducing fluor substituents gives an increase of Voc from 0.6 V (0F), over 0.7 V (1F) till 0.83 V (2F) yielding increases of the PCE from 3.2% till 5.3%. The highest PCE (7.7%) was obtained for the poly(BDT-alt-TPD) copolymer. For all these materials the nano-morphology of the blend with PC71BM was optimized and clearly shows the intimate relation between the structure (also electronic properties) and the morphology and consequently efficiency.
For the most efficient system (poly(BDT-alt-TPD)) a combination in the device architecture with a recently disclosed type of a hole-blocking top layer was investigated. Earlier work in the group has demonstrated that regio-regular random copolymers of thiophene derivatives with ionic and non-ionic side chains can operate as effective hole-blocking layers and in this way can increase efficiencies of OPV devices with 15 to 20% . Applying such interlayer on the poly(BDT-alt-TPD): PC71BM active layer resulted in a OPC device with a top PCE of more than 9%. This emphasized the importance of identifying and applying different types of hole- and (by extension) electron –blocking layers to control the operation of OPV devices.
12:00 PM - MD5.3.08
Influence of Donor Crystallinity on the Charge Transfer State Energy and the Open Circuit Voltage in Organic Solar Cells
Guy Olivier Ngongang Ndjawa 1,Kenneth Graham 1,Sukumar Dey 1,Aram Amassian 1
1 Physical Sciences and Engineering Division King Abdullah Univ of Samp;T Jeddah-Thuwal Saudi Arabia,2 Department of Chemistry University of Kentucky Lexington United States,1 Physical Sciences and Engineering Division King Abdullah Univ of Samp;T Jeddah-Thuwal Saudi Arabia
Show AbstractTo further improve the performance of organic photovoltaics (OPVs), the ubiquitous ~ 600 meV difference between the open circuit voltage (Voc) and the energy of the charge transfer state (ECT) in single heterojunctions needs to be further decreased while ECT needs to be increased. If Voc loss is significantly minimized, it approaches its upper bound ECT. Equally, realizing a donor/acceptor (D/A) conformation that produces maximum ECT would raise Voc. Several research groups have shown that the efficiency of free carriers generation and Voc would be improved if a mixed phase were present at the D/A interface, providing a downhill energy cascade that reduces recombination. However, interfacial order has also been advocated for organic materials as a means to reduce disorder induced tails states. While order is clearly beneficial in the bulk, it remains unclear whether interfacial order is indeed advantageous for OPVs. We employ a model system based on rubrene/C60 bilayers to investigate how the crystallinity of the donor influences ECT and the Voc in bilayer solar cells. We control the degree of crystallinity of the rubrene layer using thermal annealing, allowing us to compare amorphous to crystalline films, including crystalline films exhibiting differences in the long range order. The same films are used to fabricate bilayer solar cells on which the external quantum efficiency associated to the charge transfer state absorption is measured using sub-gap illumination. We find that the CT absorption observed when a crystalline rubrene layer is used, features in addition to a strong CT band, a secondary but distinct lower lying CT band which causes the absorption below the gap to become effectively more broad and the Voc to drop significantly. This indicates that crystalline phases can introduce new states below gap which are optically accessible and which overall have the adverse effect of lowering and broadening the CT band and decreasing the Voc. This suggests that, in OPVs, D/A interfaces with amorphous phases offer better energetics than ordered phases as has been observed in numerous high performing polymers systems.
12:15 PM - MD5.3.09
Characterizing and Controlling Compositional Gradients in OPV Devices with up to 10.7 % Average Efficiency
Joshua Carpenter 1,Jiang Huang 2,Chang-Zhi Li 2,Jun-Sheng Yu 3,Alex Jen 2,Harald Ade 1
1 Physics North Carolina State University Raleigh United States,2 University of Washington Seattle United States3 UESTC Chengdu China
Show AbstractAs materials with higher quantum efficiencies allow for organic solar cells to approach commercial viability, there is a strong need for film processing conditions to be developed which allow for morphological control. In this work, we focus on characterizing and controlling the vertical composition gradient of the high performing donor polymer PTB7-Th and the acceptor material PC71BM within the active layer with a novel spin-casting technique. Using several characterization techniques, including secondary ion mass spectroscopy (SIMS) and near-edge X-ray absorption spectroscopy (NEXAS), we show that devices fabricated using this method have more PC71BM in the region near the surface of the active layer, which is typically very polymer-rich in polymer:PCBM blends. We show that this improved composition gradient leads to more efficient charge extraction and allows the thickness of the active layer to be increased (> 200 nm), thereby improving Jsc without the typical decrease in FF that is common in thicker films. In our study, 215 nm thick devices fabricated using this technique have an average PCE of 10.71% versus the 8.38% exhibited by devices fabricated using the control spin-coating process. These results show that inverted off-center spinning could be a highly useful technique for solution processed organic solar cells and highlight the importance of morphological control in general for these devices.
12:30 PM - *MD5.3.10
The Principles of Manipulating the Phase Transformations and Solid-State Order of Organic Semiconductors for Manipulation of Their Photophysical Properties
Natalie Stingelin 1
1 Department of Materials and Centre for Plastic Electronics Imperial College London London United Kingdom,
Show Abstract
In the past decade, significant progress has been made in the field of organic functional materials with many new opportunities arising in the field of bioelectronics, energy harvesting and storage, organic electronics and photonics, etc. This progress has predominantly been due to important improvements of existing materials and the creation of a wealth of novel compounds. Many challenges, however, still exist. Intimate knowledge of relevant structure/ processing/performance interrelations are required to further advance this interesting class of materials and to open new application platforms. Here, examples are given of how materials scientists ‘tools’ may be utilized to gain further understanding of organic functional matter and how the physical organization, from the molecular to the macroscale of functional organic matter such as polymer semiconductorscan can be controlled. To this end, we present a survey on the principles of structure development from the liquid phase of this materials family with focus on how to manipulate their phase transformations and solid-state order to tailor and tune the final ‘morphology’ towards technological and practical applications.
MD5.4: Morphology and Thin-Film Interfaces II
Session Chairs
Wednesday PM, March 30, 2016
PCC West, 100 Level, Room 102 AB
2:30 PM - *MD5.4.01
Fullerene Mixtures for FETs and Polymer Solar Cells
Christian Muller 1
1 Chalmers Univ of Technology Goteborg Sweden,
Show AbstractIn this talk the use of fullerene mixtures for field-effect transistors (FETs) and polymer solar cells will be discussed. To realise the often intricate nanostructures that are necessary to optimise device performance, it is critical to tune the solubility of fullerenes in organic solvents as well as their tendency to crystallise. The use of fullerene mixtures, including both unsubstituted fullerenes as well as PCBMs, opens up a toolbox that can be employed to control aspects such as solubility, glass formation and crystal nucleation. Polymer solar cells based on mixtures of pristine C60 and C70 with a highly reproducible power-conversion efficiency of 6 % as well as a thermally stable active layer are reported.
3:00 PM - MD5.4.02
Extraction of Intrinsic Charge Carrier Mobility in Organic Semiconductors by the Gated Van Der Pauw Method
Cedric Rolin 1,Enpu Kang 1,Jan Genoe 2,Paul Heremans 2
1 IMEC Leuven Belgium,1 IMEC Leuven Belgium,2 ESAT Katholieke Universiteit Leuven Leuven Belgium
Show AbstractThe charge carrier mobility and the threshold voltage (VT) are important figures of merit of organic thin film transistors (OTFTs). Thin films of organic semiconductors with high mobility and well-controlled VT are desirable to meet the strict requirements of most applications. These parameters are typically extracted by a linear regression on the transconductance curve measured on an OTFT. This straightforward method presents a major drawback: As the channel is contacted by two probes only, the channel resistance is compounded with the contact resistance (Rc), which lowers the slope of the measured transconductance curve. Furthermore, non-linear effects caused by the dependence of Rc in local potential complicate the shape of the transfer curves. In consequence, the parameters extracted by the transconductance method often diverge from the intrinsic mobility and VT of the thin semiconducting film in the transistor channel. Organic semiconductors, and especially novel high mobility materials with low channel resistances, are often subject to such complications.
In this work, we use the gated van der Pauw (GVDP) method to characterize lateral transport in thin films of organic semiconductors. The original van der Pauw method, first proposed in 1958, extracts the sheet conductivity of the semiconductor film from a simple symmetrical device with four contacts. This measurement is independent from the nature of the contacts and is insensitive to small geometrical variations. Here, in addition, we use a gate to independently control charge accumulation at the semiconductor-dielectric interface in a high density regime similar to OTFT operation. The measurement of sheet conductivity at a controlled charge density enables the extraction of the charge carrier mobility, independently from contacts. We develop a simple formalism that permits the extraction of both the intrinsic mobility and VT from a linear regression on the sheet conductivity curve. The data obtained is very clean, permitting excellent linear fitting over large gate voltage range with low residuals.
We validate this approach by systematically fabricating and comparing GVDP devices and transfer line method (TLM) devices for an array of high-performance semiconductors. Devices with a low Rc measured by TLM show a good correspondence between GVPD and OTFT values (for example, both C10-DNTT devices, with RcW ≈ 200 Ωcm, have a mobility of 7.5 cm2/Vs). In contrast, devices with worse Rc show a discrepancy, with the GVDP values close to the intrinsic value of the film (DNTT devices, with RcW ≈ 4 kΩcm, have a GVDP and OTFT mobility of 3.1±0.1 and 2.0±05 cm2/Vs, respectively).
In conclusion, we demonstrate how, at the cost of a slightly increased complexity, the GVDP method can give access to intrinsic mobility and VT of thin films of organic semiconductors. This method is superior as it provides a direct and easy access to essential film properties, independently from extrinsic contact effects.
3:15 PM - MD5.4.03
Vertical Charge Transport in a High Mobility Polymer Organic Semiconductor with Application in High Speed Rectification
Stuart Higgins 1,David Harkin 1,Mark Nikolka 1,Henning Sirringhaus 1
1 University of Cambridge Cambridge United Kingdom,
Show AbstractStudies of the donor-acceptor copolymer indacenodithiophene-benzothiadiazole (IDT-BT) have yielded the highest yet recorded field-effect mobilities in a polymer semiconductor.[1,2] Coupled with its high solubility, these characteristics make IDT-BT a prime candidate for organic devices, in particular in printed electronics.[3] However for organic diodes it is vertical charge transport through the semiconductor, rather than field-effect mobility, which is of primary concern.
Experiments suggest very high current densities in IDT-BT diodes in comparison to other organic semiconductors. This is of both great interest from a fundamental standpoint (what is the vertical transport behaviour of this system?) as well as for applications, given the longstanding need for easy-to-process, high speed organic rectifiers to satisfy the radio frequency identification (RFID) market.[4]
In this talk I will discuss our current understanding of the vertical behaviour of this system as well as strategies for effective processing and printing. Finally I will present the overall impact this understanding has on developing and fabricating high performance rectifiers.
[1] W. Zhang, J. Smith, S. E. Watkins, R. Gysel, M. McGehee, A. Salleo, J. Kirkpatrick, S. Ashraf, T. Anthopoulos, M. Heeney, I. McCulloch, J. Am. Chem. Soc. 2010, 132, 11437.
[2] D. Venkateshvaran, M. Nikolka, A. Sadhanala, V. Lemaur, M. Zelazny, M. Kepa, M. Hurhangee, A. J. Kronemeijer, V. Pecunia, I. Nasrallah, I. Romanov, K. Broch, I. Mcculloch, D. Emin, Y. Olivier, J. Cornil, D. Beljonne, H. Sirringhaus, Nature 2014, 515, 384.
[3] S. G. Higgins, B. V. O. Muir, M. Heeney, A. J. Campbell, MRS Commun. 2015, 1.
[4] S. Steudel, K. Myny, V. Arkhipov, C. Deibel, S. De Vusser, J. Genoe, P. Heremans, Nat. Mater. 2005, 4, 597.
3:30 PM - MD5.4.04
Edge-Nucleation Driven Hetero-Quasiepitaxial Growth of Organic-Organic Multilayer Films
Pei Chen 1,Richard Lunt 1
1 Michigan State Univ East Lansing United States,
Show AbstractThe performance of a wide range of organic excitonic electronics can be dramatically enhanced with control over crystalline order, orientation and multilayer coupling. Here, we report a new growth mode for organic-organic hetero-quasiepitaxy of two distinct organic crystalline films grown from the bottom up, where the ordering (both in-plane and out-of-plane) is preserved for many multilayer pairs, amazingly reminiscent of quantum well growth in molecular beam epitaxy. Although the energy of the lowest energy surfaces (LES) between the two organic systems are closely matched, we find the emergence of a non-lowest energy plane. To understand this new growth phenomena, we utilize a combination of in situ reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM) and potential energy calculations to reveal that these new dynamics are enabled by a combination of i) organic-organic step-edge nucleation, and ii) high organic-on-organic molecular diffusivities. This work provides routes to tailoring molecular ordering in organic quantum wells through the design of intra- and inter- molecular interactions and therefore can enable new pathways to enhanced excitonic devices.
3:45 PM - MD5.4.05
Energetic Trapping States of Mixed-Isomer Organic Semiconductors within Organic Field-Effect Transistors
Peter Diemer 1,Angela Harper 1,Yaochuan Mei 1,Rawad Hallani 2,John Anthony 2,Oana Jurchescu 1
1 Wake Forest University Winston Salem United States,2 University of Kentucky Lexington United States
Show AbstractOrganic field effect transistor (OFET) performance is dictated by the composition and geometry of the device, as well as the quality of the organic semiconductor (OSC) film, which strongly depends on the purity and structural defects. When present, these give rise to electronic states in the bandgap (traps) of the OSC, as the nature of the pristine crystalline structure is perturbed. The additional density of states (DOS) are manifest as poorer performance of the OFET. In this presentation we focus on the characterization of OSC layers containing a mixture of isomers of the same material. We find that the OFETs fabricated of isomeric mixtures perform worse when compared to those made of the single isomers. In 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TES ADT) devices with Cytop dielectric, the mix, syn- and anti-isomers exhibit mobilities, μ, of 2.0 cm2V-1s-1, 2.2 cm2V-1s-1 and 4.7 cm2V-1s-1 respectively. The same trend appears for the devices on SiO2 dielectric, where lower μ are measured due to the different device geometry and interactions at the OSC/dielectric interface: 0.1 cm2V-1s-1, 0.3 cm2V-1s-1, and 0.4 cm2V-1s-1.
Through temperature dependent current-voltage measurements, we have determined that OFETs containing a mixture of syn- and anti-isomers reveal a discrete trapping state ~0.4 eV above the valence band edge. When the starting material is purified thoroughly, the DOS has an order of magnitude decrease in trap density, however, the discrete trap state persists. When the OSC layer is made of a single isomer, this DOS peak disappears, evidence that one isomer introduces trapping states in the other.
Also, the method of fabrication strongly influences the resulting device performance and trap DOS. Solution drop-cast, also referred to as solvent-assisted crystallization (SAC), produces well-ordered active layers with fewer grain boundaries than spin-coating. However, the weak forces of crystallization in many OSCs cannot prevent the generation of lattice defects, which are relatively stable configurations, but not in the ideal global minimum in energy. In light of this, we have developed a method of device fabrication that introduces gentle mechanical vibrations of less than 100Hz during crystallization. This vibration-assisted crystallization (VAC) promotes greater molecular interactions and assembling in a lower energy configuration by providing the activation energy needed to move a defect out of the local energy minimum. We have seen improved performance on several small-molecule OSCs with this method. For diF-TES ADT, performance improved from a μ of 0.9 cm2V-1s-1 and sub-threshold swing of 1 V dec-1 to 3.0 cm2V-1s-1and 0.43 V dec-1 for SAC and VAC devices, respectively. Interestingly, we observed the lack of the discrete DOS peak in the mixed-isomer diF-TES ADT devices, indicating that the active layer crystal grains contain a single isomer separated from the other through improved molecular configuration.
MD5.5: Device Physics/Charge Transport I
Session Chairs
Alejandro L. Briseno
Alberto Salleo
Wednesday PM, March 30, 2016
PCC West, 100 Level, Room 102 AB
4:15 PM - MD5.5.01
Antioxidants Increase the Photooxidation Stability of Organic Photovoltaic Polymers and Blends
Michael Salvador 1,Andreas Distler 2,Hans-Joachim Egelhaaf 3,Christoph Brabec 1
1 Friedrich-Alexander University Erlangen Nuremberg Erlangen Germany,2 Belectric OPV Nuremberg Germany3 ZAE Bayern Erlangen Germany
Show AbstractOrganic semiconductors have been envisioned for many disruptive technologies but suffer from photooxidation-induced performance loss and need to be carefully encapsulated, compromising both applicability and cost benefits. In the plastic and rubber industry, stabilizaing additives are an integer part of the final formulation to prevent photobleaching induced by photooxidation and radical reactions. A wide catalog of stabiizing additives exists with typically very specific function and compatibility. These antioxidants represent an exciting opportunity for enhancing the life span of organic photovoltaics but remain mostly unexplored in the case of organic semiconducting polymers. Here, we report stabilizing additives that successfully delay the photooxdation kinetics of a wide range of organic photovoltaic polymers. We report a quantitative assessment of the degradation kinetics in the absence and presence of antioxidants for pristine polymers and for polymer-fullerene blends. We draw relationships between the structure, electronic structure, and optical properties of the additive and the semiconducting host polymer. We further present the interference of the antioxidant with respect to the morphology of the active layer and the functionality of full devices. Our results suggest that stabilizing additives could represent a universal toolbox for stabilzing organic electronic devices, thus alliviating the requirement and cost for barrier materials.
4:30 PM - *MD5.5.02
Controlled Nucleation and Growth of Single Crystal Organic Semiconductor Films on Arbitrary Substrates
Paul Heremans 2,Cedric Rolin 1,Pavlo Fesenko 2,Robby Janneck 2,Jan Genoe 2
1 Imec Leuven Belgium,2 University of Leuven Leuven Belgium,1 Imec Leuven Belgium
Show Abstract
We present methods to grow in a controlled way single crystal molecular layers on arbitrary substrates. The goal is to realize high-performance electronic and opto-electronic organic devices such as transistors and light-emitting transistors.
The first method, applicable to small molecules, is based on evaporation through micron-sized shadow-mask openings. By reduction of the shadow-mask aperture size below a threshold, related to the molecular diffusion length on the substrate surface, nucleation in each opening limited to exactly a single nucleus. A monolayer crystal with micron-sized dimensions grows from each single nucleus. We demonstrate the technique with several organic semiconductors. We also show its potential to form thin organic films with a controlled grain localization for the fabrication of thin films with known grain boundary density.
A second method, applicable to soluble molecules, is based on zone casting. We have designed and built an apparatus in which the speed of the linear motion can be matched to the rate of solvent evaporation, between 25 and 200 μm/s. As solvent evaporates from the meniscus formed between the blade and the substrate, dissolved semiconductor molecules condense on a growing crystal at the edge of the meniscus. We demonstrate the applicability of the technique to the growth of C8BTBT films with a thickness of ~20 nm and with single crystalline domain size longer than 1mm. We fabricated bottom gate / top and bottom contact transistors based on these films. We achieve excellent characteristics with high Ion/Ioff ratio, onset and threshold voltages close to 0V, low subthreshold slope, low hysteresis and state of the art carrier mobilities of 8 cm2/Vs.
Acknowledgement: this work is financed by grant # 320680 (EPOS CRYSTALLI) of the European Research Council.
5:00 PM - MD5.5.03
Understanding Electron-Induced Instability of p-Type Field-Effect Transistors fabricated from Low Band-Gap Donor-Acceptor Polymers
Hung Phan 1,Michael Ford 1,Ming Wang 1,Guillermo Bazan 1,Thuc-Quyen Nguyen 1
1 Univ of California-Santa Barbara Santa Barbara United States,
Show AbstractOperational stability of organic field-effect transistors (OFETs) is a crucial aspect needed to be fully understood for their commercialization. Recently, high hole mobility OFETs have commonly achieved with donor-acceptor (D-A) copolymers, which often exhibit a certain degree of ambipolarity due to the copolymers’ low bandgap. It is crucial to know how electron conduction affects the electrical stability of hole conduction in p-type OFETs. One un-resolved observation of several high-performing D-A copolymers in bottom gate FETs with SiO2 gate dielectric is that the slope in the saturated transfer characteristics (Id1/2 vs. Vg) decreases with increasing magnitude of the gate voltage, so called double-slope. In this study, we show the effect of electron trapping on the stability of hole current and the double-slope behavior of the high-mobility p-type OFETs made from D-A copolymers with SiO2 dielectrics. Hole mobility of all the devices with different polymers and processing conditions varies from 0.1 – 7 cm2/Vs. We observed the evidence of electron trapping and formation of -SiO- charges as the devices were bias-stressed at positive gate voltages. It resulted in considerable positive shift of turn on voltage and the occurrence of double-slope behavior. The double-slope can be explained by the trapping and detrapping at the sub-threshold regime of the device transfer curves. Those phenomena are found to be significantly mitigated if hydroxyl-free dielectrics are used instead of SiO2. Our findings provide a new perspective in studying electrical stability of OFETs; and tremendously impact the molecular design and device engineering of OFETs. For example, the strategy of using D-A copolymer motif to increase FET mobility should be modified to minimize ambipolarity. The utilization of hydroxyl-free dielectrics should also be genuinely considered.
5:15 PM - MD5.5.04
Charge Transfer States in Dilute-Donor/Acceptor Blend Organic Heterojunctions
Xiao Liu 1,Kan Ding 1,Anurag Panda 1,Stephen Forrest 1
1 Univ of Michigan Ann Arbor United States,
Show AbstractRecent work on organic photovoltaic (OPV) cells based on dilute-donor and acceptor bulk heterojunctions (HJs) has resulted in a high device efficiency [1]. But the effects of varying the donor/acceptor blend ratio on the energy levels and dynamics of the charge transfer (CT) state are not yet understood. Moreover, the role played by relaxed/non-relaxed CT states in the charge generation process has yet to be investigated [2, 3]. In this work, we study the CT states in small molecule mixed HJs comprised of a nonpolar donor, tetraphenyldibenzoperiflanthene (DBP), and the acceptor, C70, using photoluminescence, electroluminescence, and transient photoluminescence as a function of temperature. We find that the luminescence spectrum, i.e. energy levels of the CT states strongly depend on the blend ratio, and two discrete CT peaks emerge at high fullerene concentration. Density functional theory calculations indicate that this can be attributed to different intermolecular geometries and electronic couplings between DBP and C70. The lifetime of CT states increases as the fullerene ratio increases in the blend. Furthermore, we study the voltage dependence of CT state dissociation by measuring the voltage dependence of the external quantum efficiency (EQE). This work deepens our understanding of how CT properties, such as binding energy and the dissociation and recombination rates change with blend ratio, and impact the charge generation processes in OPVs.
[1] Xin Xiao et al. Adv. Energy Mater. 2014, 4, 1301557
[2] Askat E. Jailaubekov et al. Nature Materials 12, 66-73 (2013)
[3] Koen Vandewal et al. Nature Materials 13, 63-68 (2014)
5:30 PM - *MD5.5.05
Defect Studies of Light Soaked Organic Bulk Heterojunctions
Robert Street 1
1 Palo Alto Research Ctr Palo Alto United States,
Show Abstract
Recombination through deep traps are a significant recombination center in bulk heterojunction organic solar cells. Previous work has shown that prolonged radiation exposure increases the deep trap density and the associated recombination. We present capacitance-frequency studies of a wide range of BHJ cells to obtain the defect density and its energy distribution. All devices show the presence of deep traps in annealed devices and a substantial increase in trap density is observed after a few days of white light illumination. The trap states are also observed by photocurrent spectroscopy. The measurements give additional information about the band tail localized state distribution, also consistent with photocurrent spectroscopy data. These results along with previous studies give more detailed information about recombination mechanisms and trap capture cross-sections. The deep traps arise from defects and their identity is discussed.
MD5.6: Poster Session II
Session Chairs
Thursday AM, March 31, 2016
Sheraton, Third Level, Phoenix Ballroom
9:00 PM - MD5.6.01
Unified Patterning and Annealing of Organic Semiconductor Films with Wet Micro-Grooved Soft Stamps
Kyunghun Kim 1,Yonghwa Baek 1,Tae Kyu An 4,John Anthony 5,Se Hyun Kim 2,Hoichang Yang 3,Chan Eon Park 1
1 POSTECH Pohang Korea (the Republic of),4 Korea National University of Transportation Chungju Korea (the Republic of)5 Kentucky University Kentucky United States2 Chemical Engineering Yeungnam University Kyungsan Korea (the Republic of)3 Inha University Incheon Korea (the Republic of)
Show AbstractUnified patterning and annealing approach was successfully demonstrated for 5,11-Bis(triethylsilylethynyl)-anthradithiophene (TES-ADT) films spun-cast on polymer-treated SiO2 dielectrics. First, rubbery polydimethylsiloxane (m-PDMS) stamps, with microscale periodic grooves, were swollen in 1,2-dichloroethane (DCE), and then softly placed onto amorphous-like TES-ADT films. In treating films, physical contact sides with the wet stamps were quickly absorbed into the PDMS matrix, while the non-contact area formed highly-ordered phases by the solvent-annealing effect. The resulting semiconductor patterns contained discernible crystal phases, where the grain sizes drastically decreased from spherulitic to optically featureless domains with decreasing the line width from 100 to 2.5 mm. Unlike ordinary systems containing irregular spherulitic domains, the 2.5 mm line-confined TES-ADT patterns contained layer-stacked crystallites but optically absent grain boundary (GB), yielding unexpectedly high field-effect mobility of 2.59 cm2 V-1 s-1 in organic field-effect transistor (OFET), with narrow deviation of below 8 % (averaged from 42 devices). The results suggest that regular conjugated grains and their smooth connection are key factors to achieve high performance multi-array OFET applications.
9:00 PM - MD5.6.02
Oxygen-Permeation-Controlled Metal Electrodes for Stable Organic Solar Cells
Hansol Lee 1,Min Kim 1,Eunjoo Song 1,Geunyeol Bae 1,Seokjoo Yang 1,Seon Baek Lee 1,Sung Won Song 1,Kilwon Cho 1
1 POSTECH Pohang Korea (the Republic of),
Show AbstractIn this report, we successfully elucidated the role of oxygen diffusion channels present in the metal top electrode in oxidation-induced degradation of organic solar cells, and proposed a simple and effective strategy for reducing their population. A simple morphological control of the low work function metal electrode effectively regulated the oxygen permeation toward the metal-organic interface by itself, leading to a significant reduction of oxidation-induced degradation. Systematic morphological and electrical investigations revealed that the metal electrode comprising tightly interlocked metal grains could greatly reduce the spontaneous formation of nanoscale pinholes, which were found to be the most prevailing oxygen permeation channels for neat and undamaged metal films. Accordingly, the oxygen permeation toward metal-organic interfaces was effectively reduced and consequently the oxidation of the vulnerable metal-organic interface was relieved, so that the environmental stability of organic solar cells was greatly enhanced. This simple self-passivating approach using metal top electrodes could provide a new strategy in resolving the stability problem of the organic solar cells, which has long been one of the most important issues in the commercialization of organic solar cells.
9:00 PM - MD5.6.03
Charge Formation and Recombination in a Low Disorder, Low Energy Loss Polymer Fullerene Blend
S. Matthew Menke 1,Aditya Sadhanala 1,Hannah Stern 1,Niva Ran 2,Ming Wang 2,Thuc-Quyen Nguyen 2,Guillermo Bazan 2,Richard Friend 1
1 Physics University of Cambridge Cambridge United Kingdom,2 Center for Polymers and Organic Solids University of California - Santa Barbara Santa Barbara United States
Show AbstractOptimal polymer-fullerene blends for organic photovoltaic devices (OPVs) require efficient charge generation, fast charge extraction, with minimal charge recombination. It is not surprising, consequently, that structural and energetic disorder subtend these processes, suppressing OPV performance. Recently, polymer-fullerene blends based on the regioregular electron donating polymer PIPCP have demonstrated remarkably low disorder and reduced energy loss (Eg−eVOC
9:00 PM - MD5.6.04
Development of Solution-Processable Electron-Accepting Indigos for High Performance Fullerene-Free Organic Photodiodes
Xin Li 1,Il Ku Kim 1,Mujeeb Ullah 1,Paul Shaw 1,Robert Wawrzinek 1,Paul Burn 1,Ebinazar Namdas 1,Shih-Chun Lo 1
1 The University of Queensland Centre for Organic Photonics amp; Electronics St Luica Australia,
Show AbstractOrganic photodiodes (OPDs) have attracted increasing attention for application in digital photography, medical sensing, artificial eyesight, and optical communication.1 The active layer of an OPD is generally comprised of an electron donor and an acceptor. At present, fullerenes have been the predominant acceptor utilized in the active layer of the best performing OPDs. However, solution processable fullerene derivatives have a number of disadvantages such as the relatively high cost of synthesis and purification,2 low molar extinction coefficient in the visible region for broadband photodetectors, and conversely a broad absorption wavelength range that makes them difficult to use in narrowband OPDs. Indigo and its derivatives have been important dyes in industry for centuries, but the poor solubility of simple indigos has hindered their development and use in organic optoelectronic applications such as OPDs.
In this presentation, we will describe strategies for the development of new families of solution processable indigos. The photophysical, thermal and electrochemical properties of the new indigos will be discussed, and finally, the OPD properties with an active layer of poly(3-n-hexylthiophene):indigo will be reported. In particular, the OPDs, have an almost spectrally flat response from 350 nm to 750 nm, a high responsivity (0.4 A/W) and detectivity (1×1012 Jones)3 that are comparable to state-of-the-art fullerene based organic and silicon photodiodes.
References
1. A. Armin, R. D. Jansen-van Vuuren, N. Kopidakis, P. L. Burn, P. Meredith, Nature Commun. 2015, 6, 6343.
2. J. T. Bloking, X. Han, A. T. Higgs, J. P. Kastrop, L. Pandey, J. E. Norton, C. Risko, C. E. Chen, J.-L. Bredas, M. D. McGehee, A. Sellinger, Chem. Mater. 2011, 23, 5484.
3. I. K. Kim, X. Li, M. Ullah, P. E. Shaw, R. Wawrzinek, E. B. Namdas, S.-C. Lo, Adv. Mater. 2015, DOI: 10.1002/adma.201502936.
9:00 PM - MD5.6.05
Novel Ferroelectric Buffering Mechanism with Polymer Ultra-Thin Layer to Boost the Transport of Fe-OFET Devices
Yi Shi 2,Yun Li 2,Huabin Sun 2,Qijing Wang 2,Yu Wang 2,Lijia Pan 2,Xinran Wang 2,Youdou Zheng 2,Kazuhito Tsukagoshi 3
1 School of Electronic Science and Engineering Nanjing University Nanjing China,2 Collaborative Center of Advanced Microstructures Nanjing University Nanjing China,3 International Centre for Materials Nanoarchitectonics National Institute for Materials Science Tsukuba Japan
Show AbstractFerroelectric organic field-effect transistors (Fe-OFETs) have been attractive for a variety of non-volatile memory device applications. One of the critical issues of Fe-OFETs is the improvement of carrier mobility in semiconducting channels. Its carrier mobility is always much smaller compared to other kinds of dielectric materials, which indicates other influences that limit the charge transport behaviour at the semiconductor/ferroelectric interface. Here, we report a novel interfacial buffering method that inserts an ultrathin poly(methyl methacrylate) (PMMA) between ferroelectric polymer P(VDF-TrFE) and organic semiconductor layers (Dioctylbenzothienobenzothiophene (C8-BTBT)). Devices with buffered ferroelectric layers exhibited an average and the highest field-effect mobility (μFET) of 3.4 and 4.6 cm2V-1s-1, respectively. Based on the high μFET, the programming process in our Fe-OFETs was mainly dominated by switching between two P(VDF-TrFE) polarizations. This enhanced performance was attributed to suppression of the polarization fluctuation at the semiconductor/insulator interface. In the case of Fe-OFETs using PMMA, it produces a uniform electrical field at the semiconductor/insulator interface, which is favorable for a smooth charge transport at the conducting channel. This study represents a major step in Fe-OFET development. It also reveals that the polarization fluctuation at semiconductor/insulator interfaces, which affect the charge transport in conducting channels, can be buffered effectively using our method.
9:00 PM - MD5.6.06
The Role of Solvent Evaporation Dynamics in Meniscus-Guided Coating Techniques for High-Performance, Single-Crystalline Organic Thin-Film Transistors
Robby Janneck 2,Federico Vercesi 1,Pavlo Fesenko 2,Jan Genoe 2,Paul Heremans 2,Cedric Rolin 1
1 IMEC Leuven Belgium,2 ESAT KU Leuven Leuven Belgium,1 IMEC Leuven Belgium
Show AbstractDriven by the high potential of single-crystalline organic semiconductors for efficient charge transport in high performance devices, a number of techniques have recently been developed to grow highly crystalline films of organic small molecules on large area. To name a few: dip coating, edge casting, solution shearing or zone casting. These techniques have in common the unidirectional displacement of a droplet of solution containing the organic semiconductor across the surface of the substrate. The meniscus along the receding edge of the droplet is a region where the solvent readily evaporates, resulting in a precipitation of the organic molecules on the crystalline front formed in the already dried region. In these meniscus-guided techniques, coating speed and substrate temperature have a profound influence on the complex dynamics of crystal growth, hence on the final morphology and microstructure of the film. So far, the optimization of these parameters to achieve smooth crystalline films has been mainly conducted through trial and error approaches.
In order to increase our understanding of meniscus-guided coating techniques, we conducted systematic measurements of the equilibrium drying speed of the droplet front for many pure solvents at various substrate temperatures. These results, analysed in a framework linking thermodynamic and fluid dynamics models, yield a simple universal formula that gives the equilibrium solvent front drying speed as a function of solvent parameters and substrate temperature. This formula can be used to predict a process window for the growth of large single-crystalline domains at the equilibrium solvent front drying speed. It is generally applicable to many solvents and meniscus-guided techniques.
To verify our findings, we processed two organic semiconductors, C8-BTBT and TIPS-Pentacene, by zone casting at different speeds, temperatures and with different solvents. We also fabricated organic thin-film transistors (OTFTs) in order to assess the electrical quality of our films. There is a clear relationship between film quality and coating speed: Films grown below or above the predicted coating speed display morphological and/or electrical defects. On the other hand, at the equilibrium speed predicted above, smooth cm-long single-crystalline thin films of both semiconductors are obtained. In the case of C8-BTBT, OTFT devices have mobilities up to 7cm2V-1s-1 and threshold voltages up to -10V. Film uniformity is good as 75 transistors distributed across a single sample show an average mobility of 4.4cm2V-1s-1.
In conclusion, we developed a predictive model with a strong basis in thermodynamics which provides a simple and versatile tool for process optimization to every experimentalist in the field.
9:00 PM - MD5.6.07
New Vacuum-Deposited D-A-A Small Molecule Donors for Organic Photovoltaics with High Open Circuit Voltage
Xiaozhou Che 1,Chin-Lung Chung 2,Ken-Tsung Wong 2,Stephen Forrest 4
1 Applied Physics University of Michigan Ann Arbor United States,2 Chemistry National Taiwan University Taipei Taiwan1 Applied Physics University of Michigan Ann Arbor United States,3 Physics University of Michigan Ann Arbor United States,4 Electrical Engineering and Computer Science University of Michigan Ann Arbor United States
Show AbstractSmall molecule organic photovoltaics (OPV) have been widely studied as a means of solar energy solution for its low cost, light weight and flexibility. In recent years, a class of molecules bearing both electron donating (d) and accepting (a,a’) functional groups following the structure of d-a-a’ have been introduced as a route to achieve high efficiency heterojunction OPVs [1,2]. Here we synthesize and characterize a group of vacuum deposited d-a-a’ donor molecules mixed with a C70 acceptor in OPV cells. The optoelectronic and charge transfer properties of the neat donors and their C70 mixtures are studied with X-ray structure analysis, DFT calculations and steady-state / transient photoluminescence. The donor antiBTDC is synthesized with previously reported benzothiadiazole−dicyanovinylene a-a’ unit and a novel unsymmetrical heterotetracene d unit. Its isomer counterpart synBTDC with the same a-a’ unit is designed to investigate the regioisomeric effect on photovoltaic performances. The antiBTDC donor mixed with C70 achieves open circuit voltage (VOC) of 0.91 ± 0.01 V, fill factor (FF) of 0.56 ± 0.01 and power conversion efficiency (PCE) of 7.2 ± 0.3%. In the synBTDC molecule, however, the conjugation is interrupted by the adjacent N element in the d unit. Despite of the electron-donating feature of N atom, such cross-conjugation results in the reduced charge transfer and blue shifted absorption. The highest occupied molecular orbital (HOMO) is stabilized, leading to an increased VOC = 1.01 ± 0.01 V with PCE = 6.1 ± 0.3%. The drop of PCE results from a lower current density due to the lack of near-infrared absorption compared with antiBTDC. Another molecule PYDC possessing the same unsymmetrical heterotetracene ‘d’ unit but with structural modulation of central electron-withdrawing group is also synthesized to further blue-shift the absorption and achieve even higher VOC = 1.06 V. This work offers a way to study the charge transport in d-a-a’ donor molecules and a design approach to improve the device performance, especially VOC.
References
[1] Y. H. Chen et al., Vacuum-Deposited Small-Molecule Organic Solar Cells with High Power Conversion Efficiencies by Judicious Molecular Design and Device Optimization, J. Am. Chem. Soc. 134, 13616 (2012).
[2] O. Griffith et al., Charge transport and exciton dissociation in organic solar cells consisting of dipolar donors mixed with C70, PHYSICAL REVIEW B 92, 085404 (2015).
9:00 PM - MD5.6.08
A Mechanistic Study of the Photodegradation of poly(3-hexylthiophene-2,5-diyl) and poly(3,5-didodecyl-cyclopenta[2,1-b;3,4-b']dithiophen-4-one)
Logan Sanow 1,Jianyuan Sun 1,Cheng Zhang 1
1 South Dakota State University Brookings United States,
Show AbstractOne of the main disadvantages of organic materials being used in photovoltaic applications is their limited lifetime. Most conjugated polymers degrade quickly when exposed to air and light. Our research is focused on studying the mechanisms that lead to degradation and applying that knowledge to design more photostable conjugated polymers. In this study we compared a widely studied conjugated polymer, Poly(3-hexylthiophene-2,5-diyl) (P3HT), with a structurally similar polymer, poly[(3,5-didodecyl-cyclopenta[2,1-b;3,4-b']dithiophen-4-one) ran-(cyclopenta[2,1-b;3,4-b']dithiophen-4-one)] (C6-CPDTO), that we synthesized in our lab. CPDTO contains electron withdrawing ketone groups in the conjugated main chain, this lowers the LUMO level compared to P3HT. CPDTO is more photostable thatn P3HT, CPDTO degrades 2.5 times more slowly than P3HT. We illuminated polymer thin film samples using a xenon arc lamp. Samples were monitored with UV-Vis, GPC and IR spectroscopy. The structural changes in both polymers were similar and indicated that both degraded through a radical mechanism. GPC indicated that extensive cross-linking was occurring in both polymers. Radical degradation is initiated on the side chains and then these generated radicals attack the thiophene rings leading to destruction of conjugation and the loss of optical and electronic properties.
9:00 PM - MD5.6.09
Chemical-Vapor-Deposited Polythiophene Thin Films and Organic Thin Film Transistor Applications
Sunghwan Lee 1,David Borelli 2,Karen Gleason 2
1 Baylor University Waco United States,2 Chemical Engineering M.I.T Cambridge United States
Show AbstractOxidative chemical vapor deposition (oCVD) offers a facile approach to synthesize and deposit conjugated polymers irrespective of polymer solubility or the properties of the substrate material. The oCVD method has the merits of good film uniformity, high conductivity, conformal coating on non-planar and patterned substrates, and relatively low process temperature (As one of conjugated polymers, polythiophene (PT) and its derivativers have attracted much attention in the field of organic electronics as a result of electrical conductivity, chemical and thermal stability, and optical properties. Potential applications include thin film transistors (TFTs), solar cells, and a variety of chemical sensors. However, the insoluble nature of the unsubstituted PT makes the traditional solution process of this material very challenging.
In this presentation, we report on the successful demonstration of vapor-phase synthesis and deposition of PT thin films using oCVD taking advantage of the volatility of the thiophene monomer. Among various deposition parameters, we systematically investigated the effect of working pressure (1-300mTorr) during oCVD process on the chemical, optical, and electrical properties of resulting PT polymer films. The Raman and UN-Vis spectra show that the working pressure during oCVD strongly influences the conjugation length and absorption characteristics of the resulting PT films that are closely correlated with electronic and optoelectronic properties such as conductivity and optical bandgap: PT films deposited at lower pressures exhibit shorter conjugation and higher optical bandgap.
Bottom-gated organic TFTs were fabricated on thermally oxidized Si using oCVD PT as the active channel layer. The PT TFT devices, in this study, operate in p-type enhancement-mode field effect transistors which show the threshold voltages ranging from ~-7 to -1V with the maximum on/off current ratio of >103 depending on the working pressures. The field effect mobility (μFE) measured from PT TFTs is as high as ~0.2-4x10-3 cm2/V s which the highest reported values in PT films. From the TFT device performances and transmission line model (TLM) measurements, the electrical properties such as carrier density and electrical conductivity in oCVD PT films are systematically characterized. In addition, these oCVD PT TFTs are relatively stable in air and thermal stress: no significant degradation in TFT performance is observed when the devices were aged in air over time longer than a month and post-deposition annealing at 60-120°C improves the field effect mobility and on/off ratio by more than an order of magnitude.
As well as providing greater characterization of the oCVD process, this study expands the knowledge of unsubstituted polythiophene, which has traditionally been difficult to incorporate into organic electronics because of its insolubility. Fundamental knowledge of PT is likely to benefit the understanding of its many substituted variants.
9:00 PM - MD5.6.10
Effect of Chiral Ethylhexyl Side Chains on the Chiroptical Properties of PCPDTBT and PCDTPT
Stephanie Fronk 1,Ming Wang 1,Michael Ford 2,Jessica Coughlin 1,Cheng-Kang Mai 3,Guillermo Bazan 2
1 Department of Chemistry and Biochemistry Univ of California-S Barbara Santa Barbara United States,2 Materials Department University of California, Santa Barbara Santa Barbara United States3 Materials Research Laboratory University of California, Santa Barbara Santa Barbara United States1 Department of Chemistry and Biochemistry Univ of California-S Barbara Santa Barbara United States,2 Materials Department University of California, Santa Barbara Santa Barbara United States
Show AbstractNarrow bandgap conjugated polymers have garnered recent interest based on their performance in optoelectronic devices. Specifically, poly[(4,4-bis(2-ethylhexyl)cyclopenta-[2,1-b:3,4-b0]dithiophene)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PCPDTBT) has been studied as a donor material in organic solar cells and regioregular poly[(4,4-bis(2-ethylhexyl)cyclopenta[2,1-b:3,4-b0]dithiophene)-2,6-diyl-alt-[1,2,5]-thiadiazolo[3,4-c]pyridine] (PCDTPT) has been studied in organic field effect transistors. However, the secondary structure of donor-acceptor conjugated polymers remains poorly understood. Optical properties, more specifically chiroptical properties, can provide useful structural information. To study the chiroptical properties of these two polymers, the 2-ethylhexyl side chain was substituted with a 2-(S)-ethylhexyl side chain to produce PCPDTBT* and PCDTPT*. Assuming the asymmetry is translated from the side chains to the optically active backbone, these two polymers can be studied using circular dichroism (CD) spectroscopy.
PCPDTBT* was found to produce a CD signal only in a mixed solvent system where polymer chain aggregation occurs. In good solvents or at higher temperatures, no CD signal is observed. These data suggest PCPDTBT* self-assembles into a helical aggregate. CD signal is also observed for thin films, indicating the asymmetry in the aggregate is translated to the thin film. PCDTPT* was synthesized for comparison and shows a smaller degree of chiral aggregation than PCPDTBT* in solution based on their relative anisotropy factors. The anisotropy factor compares the difference in left and right circularly polarized absorption with linear absorption. However, PCDTPT* does not produce a chiroptical response in the thin film. A possible explanation for the difference in anisotropy factor is the higher calculated rotational barrier of PCDTPT* compared to PCPDTBT*, hindering the formation of a helical structure for PCDTPT*.
9:00 PM - MD5.6.11
Quantitative Evaluation of Inhomogeneous Device Operation in Thin Film Solar Cells by Luminescence Imaging
Marco Seeland 2,Christian Kaestner 2,Roland Roesch 1,Harald Hoppe 1,Felix Herrmann-Westendorf 1
2 TU Ilmenau Ilmenau Germany,1 Friedrich-Schiller-University Jena Jena Germany
Show AbstractApart from other methods, such as Lock-In Thermography1 and Light-Beam Induced Currents, luminescence imaging has evolved to a versatile characterization method for studying the spatially resolved behavior of polymer solar cells. Especially in degradation studies, the use of luminescence imaging is beneficial because it is non-invasive and offers short measurement times. Depending on the excitation mechanism, i.e., either electrical or optical excitation, it allows characterization of the electrical contacts and the active layer separately. However, except for the correction of the active area, the data analysis so far is mainly qualitative, i.e., interpretation of the measured luminescence image by comparison with other techniques. In this work, we present a method for quantitative evaluation of electroluminescence images from thin film solar cells. The method called “quantitative electroluminescence imaging” (QuELI) is based on decoupling local equivalent circuit parameters and allows calculation of the local current-density as well as the local series resistance and saturation current-density. By application of this method to electroluminescence images obtained from polymer-fullerene based solar cells, we show that QuELI allows efficient separation between: properties of the electrodes and their associated interfaces by the local series resistance and properties of the active layer by the saturation current-density. We furthermore reveal large scale lateral phase separation via the strong variation in the saturation current-density, which delivers information on the energetic difference of thermal activation of charge carriers across the effective active band gap.
9:00 PM - MD5.6.12
Growth of Dark Spots by Pinholes in Metal Electrodes of Organic Photovoltaic Cells
Daniel Fluhr 2,Burhan Muhsin 2,Rolf Oettking 2,Roland Roesch 2,Marco Seeland 3,Harald Hoppe 2
1 Center for Energy and Environmental Chemistry (CEEC Jena) Friedrich Schiller University Jena Germany,2 Laboratory of Organic and Macromolecular Chemistry (IOMC) Friedrich Schiller University Jena Germany,3 TU Ilmenau Ilmenau Germany
Show AbstractLifetime is still a major problem of organic photovoltaic (OPV) cells. There are many reasons for solar cell degradation varying from shunts induced by impurities or electromigration over photoinduced oxidation of active layer materials to corrosion and delamination of the metal contact both induced by oxygen or water ingress.
One issue concerns so-called pinholes through the metal back electrode of the device. These pinholes offer pathways for ingress of water and oxygen which may attack the metal-organic interface by introducing delamination through formation of insulating metal oxides or hydrogen evolution. As charge injection and extraction is suppressed at delaminated areas, the active area taking part in power conversion – and hence the overall efficiency – becomes reduced.
We investigated the influence of different environmental conditions on the reduction of the active area of the OPV cell. Spatially resolved measurements give information on location and size of insulated areas induced by pinholes in the metal back contact. Time resolved measurements during degradation of the devices revealed the dynamics and rate of growth of these individual defects. In contrast to previous publications we found nonlinear non active area growth. Our finding is further confirmed by diffusion simulations reproducing a nonlinear growth.
9:00 PM - MD5.6.13
Revealing the Full Charge Transfer State Absorption Spectrum from Optical Polarization Anisotropy of Oriented CT States in Bilayer Organic Solar Cells
Yixin Yan 1,Alyssa Brigeman 1,Michael Fusella 2,Barry Rand 2,Noel Giebink 1
1 Department of Electrical Engineering The Pennsylvania State Univ University Park United States,2 Department of Electrical Engineering Princeton University Princeton United States
Show AbstractCharge transfer (CT) states at the donor acceptor (DA) interface of organic photovoltaic (OPV) cells typically manifest as a weak, low-energy shoulder in absorption that is subsequently fit to a Marcus electron transfer theory lineshape to determine characteristics such as the CT energy. However, because the majority of the CT absorption is buried by excitonic absorption of pure D and A materials at higher energies, independent fittings of CT absorption to the low energy shoulder are weakly constrained, leading to uncertainty in the extracted parameters and preventing an explicit validation of the Marcus lineshape model. Moreover, the excitonic absorption overlap has precluded direct observation of higher energy CT states expected from quantum chemical modeling that are thought to play an important role in charge carrier separation.
Here, we present a simple approach to extract CT absorption from the overlapping excitonic background by exploiting the natural alignment of CT states at a planar heterojunction (PHJ). Because the CT transition dipole is predominantly oriented orthogonal to the DA interface, transverse magnetic (TM) polarized light couples to the CT transition whereas transverse electric (TE) polarized light does not. The difference between TE and TM-polarized external quantum efficiency (EQE) measurements therefore subtracts out the isotropic excitonic absorption (which is the same for both polarizations) to uncover the full underlying CT absorption spectrum.
We use this approach together with polarized electroabsorption (EA) measurements carried out in a multi-bounce attenuated total internal reflection geometry for maximum CT absorption to reconstruct the full CT spectrum of a prototypical pentacene/fullerene C60 bilayer OPV cell. We find that the complete Marcus lineshape is indeed observed in the reconstruction; however, higher energy CT states are also clearly evident. In contrast to the nanocrystalline pentacene/C60 case, planar heterojunction devices based on fully amorphous donors show little to no anisotropy, indicating that molecular-scale roughness and intermixing at amorphous interfaces largely eliminates any CT orientational order. These results support a Marcus description of the lowest energy CT state and provide direct evidence for higher-lying CT states in OPV cells.
9:00 PM - MD5.6.14
Effects of Hole-Transport Layer Homogeneity in Organic Solar Cells – A Multi-Length Scale Study
Huei-Ting Chien 1,Georg Koller 2,Thomas Brenner 3,Marcus Boehm 5,Markus Kratzer 1,Susanna Challinger 4, Iain Baikie 4,Christian Teichert 6,Bettina Friedel 1
1 Institute of Solid State Physics, Graz University of Technology Graz Austria,2 Institute of Experimental Physics, Graz University Graz Austria3 Institute of Physics and Astronomy, University of Potsdam, Germany Potsdam Germany5 Cavendish Laboratory, University of Cambridge, United Kingdom Cambridge United Kingdom4 KP Technology Ltd., Scotland, United Kingdom Scotland United Kingdom6 Institut für Physik, Montanuniversitaet Leoben, Austria Leoben Austria
Show AbstractPoly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) improves the organic photovoltaic (OPV) cells’ performance in its function as hole-extraction layer and interfacial buffer between indium tin oxide (ITO) and photoactive layer. But a drawback is strong degradation upon presence of moisture in acidic PEDOT:PSS causing short life time of the devices. Further, its colloidal nature is suspected to be responsible for spatial inhomogeneities and irreproducibility of device characteristics. While one approach to get rid of the PEDOT:PSS layer are inverted solar cell architectures, an easier way would be replacement by a less problematic solution-processable hole-transport material. Here, molybdenum trioxide, (MoO3), has already been considered and used with quiet good results. In this report, we present novel insights on the effect of nature and homogeneity of the hole-extraction layer on the device functionality. Therefore, two different types of solution-processed MoO3 films for incorporation as hole-extraction layer in OPV are used, one a smooth continuous film, the other a discontinuous nanoparticle layer and directly compared with traditional PEDOT:PSS and blank ITO electrodes. We discuss their electronic properties and morphology from the macro- to the nanoscale and demonstrate their impact on OPV device physics, integrated and also spatially resolved by photocurrent mapping. The unique comparison of morphology, surface potential and device performance on different length scales between the devices with smooth or colloidal hole-extraction layers will provide answers on how such local microscopic inhomogeneities of the cells might affect device output and stability.
9:00 PM - MD5.6.15
Effect of Annealing Temperature on Out-of-Plane Mobility and Structure of MEH-PPV Film
Daisuke Kajiya 1,Tomoyuki Koganezawa 2,Ken-ichi Saitow 1
1 Hiroshima University Higashi-hiroshima Japan,2 Japan Synchrotron Radiation Research Institute Hyogo Japan
Show AbstractCarrier transport property is important for the performance of organic electronics devices such as solar cell, LED, and sensors. We have investigated the hole migration phenomena in the out-of-plane direction of conjugated-polymer thin films. The 8-fold and 50-fold enhancements of the out-of-plane mobility have been observed for P3HT films by rubbing [1] and by the addition of silicon nanocrystals [2], respectively. Here, we show the enhancement of out-of-plane mobility by thermal annealing processing at glass transition temperature (Tg) using the thin film of MEH-PPV that has been a popular polymer used for studies on funcamental optoelectronic properties and the solution-processed organic and hybrid solar cell and LED. The hole mobility was measured using transient photoconductivity method and the 40-fold mobility difference was observed by changing the annealing temperature at around Tg. The maximum mobility was observed at Tg. This increase in mobility is attributed to homogeneous morphology and face-on orientation, which is produces by annealing at Tg, according to the results of atomic force microscope (AFM), and Kelvin probe force microscope (KPFM), and grazing-incidence X-ray diffraction measurement (GIXD).
[1] D. Kajiya, S. Ozawa, T. Koganezawa and K. Saitow. J. Phys. Chem. C, 119, 7987-7995 (2015).
[2] D. Kajiya and K. Saitow, Nanoscale 7, 15780-15788 (2015).
9:00 PM - MD5.6.16
All-atom Molecular Dynamics Studies of the Link between Processing, Structure and Charge Transport in Polymer: Fullerene Based Solar Cells
Sai Vineeth Bobbili 1,Binit Lukose 1,Paulette Clancy 1
1 Cornell University Ithaca United States,
Show AbstractFor over a decade, the prospect of roll-to-roll processing of bulk heterojunction based organic photo-voltaics consisting of polymer donors and fullerene acceptors has inspired research to enable low cost, large area solar cells. As part of a DoE-funded team, our experimental collaborators at Stanford in Bao’s group pioneered the use of shearing organic polymer:-fullerene blends while they are crystallizing, and the solvent is evaporating, in order to increase the mobility of the resulting polymorph by as much as an order of magnitude higher. With such a high mobility, their power conversion efficiency can reach as high as 9%.
We undertook a set of computational studies of the P3HT:PCBM system using Molecular Dynamics to understand the effect of shear on molecular-level conformational details and the subsequent impact on charge transport. We have looked into the structural properties of such blends in vacuum and in two solvents, chloroform and o-dichlorobenzene. We have used all-atom models to represent the components in MD simulations as we have discovered that popular coarse-grained models do not sufficiently capture nuances in conformational preferences that have been overly simplified in the description of the molecule, especially the disposition of the thiophene rings that dominate charge transport in P3HT and similar polymers. We have also studied the effect of varying the crystallinity, blend ratios, evaporation rates and degree of polymerization on structural preference.
Studying details of the morphology are key to understanding the behavior of the polymer-fullerene blend. Accordingly, we have designed algorithms that quantify the morphology in terms of characteristics such as the end-to-end length of the polymer, area-to-volume ratios, phase separation, pi-pi stacking, tacticity and structural motifs. We have observed that shearing leads to increased aggregation of the polymer blend which improves the mobility. Our DFT calculations have determined the barrier heights needed for the monomers to rotate with respect to each other in a given polymer and found it to be above 0.061 eV. DFT calculations have also determined how thiophene rings orient relative to adjacent layers in a crystalline form of P3HT, by sliding or sometimes rotating.
9:00 PM - MD5.6.17
The Role of Counter-Ions in the Processing of poly(3,4-ethylenedioxythiophene) (PEDOT)
Liangqi Ouyang 1,Mohammad Jafari 1,Hassan Abdalla 1,Chiara Musumeci 1,Tino Hoffman 1,Vanya Darakchieva 1,Xianjie Liu 1,Thomas Ederth 1,Martijn Kemerink 1,Olle Inganas 1
1 Linkoping Univ Linkoping Sweden,
Show AbstractIn recent years, the conductivity of PEDOT has exceeded 3000 S/cm,1 making it attractive electrode material for printed solar cells, OLED and bioelectronics. In typically applications, high-boiling-point additives are added into PEDOT:PSS aqueous dispersion prior to film formation and annealing to achieve the high conductivity. Alternatively, the pristine PEDOT:PSS films can be post-treated with acids or certain solvents, which increased the conductivity by several orders of magnitude. However, the mechanism of the conductivity enhancement is still under debate. Here, using a simple drying method,2 we show that these additives serve as good solvents to concentrate free PSS during the drying process. The separation between reorganized PEDOT:PSS complex and PSS due to solubility differences increased the conductivity of the film from 0.1 S/cm to more than 200 S/cm without any thermal annealing. The chemical compositions of the two components were mapped through infrared microscope. The structure and morphology was studied with X-ray diffraction and atomic force microscope. We also show that the dopants in PEDOT films can be exchanged and removed through chemical methods. The influences of doping concentration on the optical properties and the film conductivity were studied with Tera-Hz ellipsometry. The further implication of enhancing thermoelectric properties of PEDOT through dopant removal will also be discussed.
1. Xia, Y.; Sun, K.; Ouyang, J., Solution-Processed Metallic Polymer Films as Transparent Electrode of Optoelectonic Devices. Advanced Materials, 2012, 24, 2436-2440
2. Ouyang,L.; Musumeci, C.; Jafari, M. J.; Ederth, T,; Inganas, O., Imaging the Phase Separation between PEDOT and Polyelectrolyte during Processing of Highly Conductive PEDOT:PSS Films. ACS Applied Materials and Interfaces, 2015, 7, 19764-19773.
9:00 PM - MD5.6.18
The Effect of Molecular Dipole Moment on the Electrical Properties of Molecular Rectifiers
Zach Lamport 1,Angela Broadnax 2,Lee Mendenhall 2,Mark Welker 2,Oana Jurchescu 1
1 Physics Wake Forest University Winston Salem United States,2 Chemistry Wake Forest University Winston Salem United States
Show AbstractThe goal of molecular electronics is to exploit the intrinsic properties of a single molecule, typically a self-assembled monolayer (SAM). The current rectification can be achieved in single molecules, similar to the case of inorganic p-n junction diodes and several strategies for molecular design aimed to control this property have been proposed, many utilizing electron donating or electron withdrawing component. Here, we design and synthesize six new alkylsilane SAMs and explore their rectification behavior in relation to their molecular structure. The molecules are: (E)-1-(4-fluorophenyl)-N-(3-(triethoxysilyl)propyl) methanimine (molecule 1), (E)-1-(perfluorophenyl)-N-(3-(triethoxysilyl)propyl) methanimine (molecule 2), (E)-N-(3-(triethoxysilyl)propyl)-1-(4-(trifluoromethyl)phenyl) methanimine (molecule 3), (E)-N-(11-(triethoxysilyl)undecyl)-1-(4-(trifluoromethyl)phenyl) methanimine (molecule 4), (E)-1-(3,5-bis(trifluoromethyl)phenyl)-N-(11-triethoxysilylundecyl) methanimine (molecule 5), and (E)-1-(4-fluorophenyl)-N-(11-(triethoxysilyl)undecyl) methanimine (molecule 6). These molecules were designed to test the same electron withdrawing group in conjunction with different molecular lengths, and to test the effect of the strength of the electron withdrawing termination on the device properties. The electrical properties of these materials were measured using a eutectic gallium-indium (EGaIn) top contact and highly doped silicon bottom contact in a metal/insulator/metal structure. The devices display good uniformity, high yield (> 90%), and a remarkable resistance to bias stress (over 60 measurements tested with no significant degradation). The monolayer exhibiting the lowest rectification ratio, composed of molecule 4, has a dipole moment of 3.35 D and gives a rectification ratio of 23.1 ± 7.9 whereas the monolayer demonstrating the greatest rectification ratio, molecule 3, has a dipole moment of 4.67 D and resulted in an average rectification ratio of 183.0 ± 41.4. The measurements yield a linear dependence between the rectification ratio and the dipole moment of the particular molecule composing each SAM regardless of molecular length or composition. Our findings provide a unified picture of the effect of molecular structure on the electrical properties of single molecules and may promote a rational design of molecules that can function as molecular rectifiers.
9:00 PM - MD5.6.20
Core-Shell Structured Organic Nanofibers for Flexible Photovoltaic Applications
Min Kim 1,Hansol Lee 1,Sung Won Song 1,Seokjoo Yang 1,Eunjoo Song 1,Geunyeol Bae 1,Seon Baek Lee 1,Kilwon Cho 1
1 Pohang University of Science and Technology Pohang Korea (the Republic of),
Show AbstractOne-dimensional conjugated polymer fibers provide unperturbed percolation pathways for efficient charge transport. This study reports the fabrication of highly photoresponsive one-dimensional (1D) organic semiconductor fibers by using co-axial electrospinning. We produced the core-shell structured 1D organic semiconductor fibers in which the photoactive polymer-fullerene blend are encapsulated with an insulating polymer sheath. After removing the insulating sheath, the photoresponsive properties of the polymer-fullerene blend fibers are characterized thoroughly. It was found that the pristine blend fibers are phase-separated into core and shell phases and contain a uniaxial polymer chain aligned along the fiber direction. These properties are strongly correlated with their highly photoresponsive behavior. The utility of the photoactive fibers in photovoltaic applications was demonstrated by incorporating these fibers into a lateral-architecture photodiode, which was found to exhibit high levels of photocurrent generation. These electrospun conjugated polymer:PCBM blend fibers are promising components for the next generation of organic-fiber-based solar cells.
9:00 PM - MD5.6.21
Stretchable and Transparent Semiconducting Layer for Flexible Organic Transistors
Eunjoo Song 1,Seon Baek Lee 2,Geunyeol Bae 2,Min Kim 2,Hansol Lee 2,Sung Won Song 2,Kilwon Cho 2
1 POSTECH Pohang Korea (the Republic of),2 POSTECH Pohang Korea (the Republic of)
Show AbstractDespite the remarkable progress in the stretchable electronics, few intrinsically stretchable semiconductors have been reported that retain the original electrical characteristics under stretching. In this study, we introduce an intrinsically stretchable and transparent organic semiconducting layer. Blending organic semiconducting poly(3-hexylthiophene) nanowires (P3HT NWs) with poly(dimethylsiloxane) (PDMS) elastomer have resulted in the active material where P3HT NWs are embedded in the transparent and elastomeric PDMS matrix. The vertical distribution of P3HT NWs in the blend films is different depending on the surface characteristics of the underlying substrate. Compared to the blend film spin-cast on a Si substrate showing vertical gradation of P3HT NWs, the NWs are uniformly embedded in the PDMS matrix on a PDMS substrate. Organic transistors prepared with the blend active layers of various P3HT contents show device performances comparable to those of homo-P3HT NW devices, even at 1 wt% P3HT. In comparison with the non-blend system, the blend active layer shows the superior stretching stability under high strains of up to 100%, and does even after 5000 stretching cycles.
9:00 PM - MD5.6.22
Environmentally Processible New Organic Semiconducting Materials
Yun-Hi Kim 1,Dae Sung Chung 2
1 Gyeongsang National University Jinju Korea (the Republic of),2 Chung-Ang University Seoul Korea (the Republic of)
Show AbstractUsing non-halogenated organic solvents for organic semiconductor device fabrication is highly desirable to avoid the negative environmental and health effects of toxic solvents. After the first discovery of semiconducting polymer nanoparticles dispersed in an aqueous medium by means of a mini emulsion process, which was reported by Landfester et al., there have been many research efforts on green processes for organic semiconductors. For the first time, we could demonstrate a high charge carrier mobility of up to 2.5 cm 2 V −1 s −1 from a water-borne polymer colloid. In this study, by utilizing a nonionic surfactant, water-borne stable colloids of DPP-SVS nanoparticles were successfully fabricated by means of the miniemulsion process. In this presentation, it will be introduced the new environmentally processible organic semiconducting materials.
9:00 PM - MD5.6.23
The Interface Morphology Study in the Polymer/Fullerene Based Devices
Yufeng Hu 1,Feng Teng 1,Zhidong Lou 1,Yanbing Hou 1
1 Beijing Jiaotong University BEIJING China,
Show AbstractWe investigated the interface morphology of a poly(3-hexylthiophene) (P3HT)/(6,6)-phenyl C61 butyric acid methyl ester (PCBM) “bilayer” heterojunction film that was fabricated using orthogonal solvents.The results show that even though P3HT is insoluble in dichloromethane (DCM), DCM could decrease the chain packing and the crystallinity of the P3HT film by the swelling effect. DCM also assists in the penetration of the PCBM into the P3HT underlayer during spin-coating. After the DCM evaporates, the PCBM in the P3HT matrix hinders the self-reorganization of P3HT, causing a drop in the absorption intensity of P3HT. The post-annealing effects on the the interface between the aluminum electrode and the active layer in the conventional bulk heterojunction (CBHJ) and the layer-evolved bulk heterojunction (LBHJ) have been compared. Aqueous contact angle and X-ray photoelectron spectroscopy measurements show that P3HT is dominant at the top surface of CBHJ film, while PCBM is dominant at the top surface of LBHJ film. The micronscale morphology evolution of the active layer/Al interface upon post-annealing reveals that the PCBM-rich surface is beneficial for the nucleation and growth of PCBM crystal, which does harm to the contact between the active layer and the electrode and results in the decrease of the fill factor. However, the original P3HT-rich surface prevents the formation of large surface-segregated PCBM clusters upon post-annealing, which is highly desirable for the efficient polymer/fullerene solar cells.
9:00 PM - MD5.6.25
Rethinking Coal: Thin Films of Solution Processed Natural Carbon Nanoparticles for Electronic Devices
Brent Keller 1,Nicola Ferralis 1,Jeffrey Grossman 1
1 MIT Cambridge United States,
Show AbstractDisordered carbon materials, both amorphous and with long-range order, have been used in a variety of applications, from conductive additives and contact materials, to transistors and photovoltaics. Here we show a flexible solution-based method of preparing thin films with tunable electrical properties from suspensions of ball-milled coals following centrifugation. The as prepared films retain the rich carbon chemistry of the starting coals, with conductivities ranging over orders of magnitude, and thermal treatment of the resulting films further tunes the electrical conductivity in excess of 7 orders of magnitude. Optical absorption measurements demonstrate tunable optical gaps from 0 to 1.8 eV. Through low-temperature conductivity measurements and Raman spectroscopy, we demonstrate that variable range hopping controls the electrical properties in as-prepared and thermally treated films and that annealing increases the sp2 content, localization length, and disorder. The measured hopping energies further demonstrate electronic properties similar to amorphous carbon materials and reduced graphene oxide from the coal films.
9:00 PM - MD5.6.26
Investigating Thermally Dependent Morphological Changes in Films Used in Organic Light-Emitting Diodes Using Neutron Reflectometry
Jake McEwan 1,Andrew Clulow 1,Andrew Nelson 2,Paul Burn 1,Ian Gentle 1
1 Centre for Organic Photonics amp; Electronics The University of Queensland St Lucia Australia,2 Bragg Institute Australian Nuclear Science and Technology Organisation Lucas Heights Australia
Show AbstractWhile displays based on organic light-emitting diodes (OLEDs) have reached the commercialisation stage, a better understanding of how these devices can degrade is important for future developments in the field, and in particular for lighting applications. One of these issues for OLEDs is their susceptibility to thermal degradation. It has been reported that exposure to temperatures above 80 °C can lead to a substantial performance drop for devices, with an apparent relationship between device breakdown and the glass transition temperature (Tg) of the materials in the device. [1, 2]
It is thus important to ascertain whether this thermal degradation could be due to morphological changes within the active film of a device. Neutron and X-ray reflectometry have been previously used to non-destructively probe the layered structure of thin film stacks comprised of organic small molecules on silicon, comparable to what you would find in an actual device. [3, 4] Alternating layers of protonated and deuterated materials were used to improve the contrast between the layers and allow resolution of interfacial structure. [5] It was seen that after heating to moderate temperatures the layers diffused into one another, leading to shifts in the position of the interfaces.
In this presentation we will describe the use of neutron reflectometry to probe the structure of both small molecule and dendrimer films, which are prepared by thermal evaporation and solution-processing methods respectively. We will show how the properties of the various materials such as glass transition temperature and molecular size affects the thermally activated diffusion behaviour of the materials in thin films. The changes in film morphology will then be correlated to changes to the luminescence spectra of the films.
References
[1] G. Nenna, G. Flaminio, T. Fasolino, C. Minarini, R. Miscioscia, D. Palumbo, M. Pellegrino, Macromol. Symp. 2007, 247, 326-332
[2] G. Nenna, M. Barra, A. Cassinese, R. Miscioscia, T. Fasolino, P. Tassini, C. Minarini, D. della Sala, J. Appl. Phys. 2009, 105, 123511.
[3] A. R. G. Smith, J. L. Ruggles, H. Cavaye, P. E. Shaw, T. A. Darwish, M. James, I. R. Gentle, P. L. Burn, Adv. Funct. Mater. 2011, 21, 2225
[4] A. R. G. Smith, K. H. Lee, A. Nelson, M. James, P. L. Burn, I. R. Gentle, Adv. Mater. 2012, 24, 822
[5] T. A. Darwish, A. R. G. Smith, I. R. Gentle, P. L. Burn, E. Luks, G. Moraes, M. Gillon, P. J. Holden, M. James, Tetrahedron Lett. 2012, 53, 931
9:00 PM - MD5.6.27
Synthesis of Polybenzoquinolines as Graphene Nanoribbon Precursors
David Dibble 1,Young Park 1,Amir Mazaheripour 1,Mehran Umerani 1,Alon Gorodetsky 2
1 Department of Chemical Engineering and Material Science University of California, Irvine Irvine United States,1 Department of Chemical Engineering and Material Science University of California, Irvine Irvine United States,2 Department of Chemistry University of California, Irvine Irvine United States
Show AbstractThe bottom-up synthesis of all-carbon graphene nanoribbons (narrow strips of sp2 hybridized carbon) has attracted much attention in recent years, with a number of contemporary demonstrations of the preparation of all-carbon systems. However, there are a limited number of studies on the solution-phase synthesis of heteroatom-doped graphene nanoribbons, which remains a significant synthetic challenge. Here, we have developed new synthetic methodologies for oligobenzoquinolines based on the aza-Diels–Alder (Povarov) reaction, controlling the length and sequence of our oligobenzoquinoline precursors via an iterative route. Our straightforward approach also provides access to crowded macromolecular polybenzoquinoline scaffolds with a unique architecture and connectivity, which are key intermediates for the preparation of nitrogen-doped nanoribbons. Our findings hold implications for the bottom-up synthesis of graphene nanoribbons whose edge character, terminal functionalities, doping, and length are precisely controllable.
9:00 PM - MD5.6.28
Electrospinning of Highly Rigid Conjugated Polymer Nanofibers
Yani Chen 1,Jialin Wang 1,Ziqi Liang 1
1 Fudan Univ Shanghai China,
Show AbstractConjugated polymers nanofibers have attracted considerable interest in recent years due to their potential applications in micro to nanoscale electronic and optoelectronic devices. However, most of conjugated polymers with a fairly rigid backbone are hard to form continuous and steady jet during electrospinning process. Here, we have successfully applied direct electrospinning method to rigid copolymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(bithiophene)] (F8T2) without any spinning auxiliaries. By using mixed solvents of xylene and chloroform, ultrafine electrospun F8T2 nanofibers with a mean diameter below 300 nm were obtained, which is significantly thinner than literature reports. Interestingly, this ultrafine nanofibers show remarkably improved electrical conductivity by 3 orders of magnitude compared to spin-coating analogue, due to their decreased fiber diameters and highly anisotropic structures during electrostatic field.
9:00 PM - MD5.6.29
Novel Palladium-Based Excimer for Efficient and Stable Single-Doped White OLEDs
Liang Huang 1,Tyler Fleetham 1,Jian Li 1
1 Materials Science and Enginee Arizona State University Mesa United States,
Show AbstractWhite organic light emitting diodes (WOLEDs) have shown great promise as a potential replacement for existing lighting technologies due to their efficiencies exceeding 100Lm/W, potential for low cost and scalable production, and compatibility with flexible substrates. Through continuous improvements in device designs and by employing phosphorescent Ir or Pt emitters, WOLEDs with high efficiencies and high color quality have been achieved. Nevertheless, major challenges still remain; namely, the deficiency of an efficient and stable phosphorescent blue emitter and the cost prohibitive nature of typical multilayer WOLED structures. Alternatively, phosphorescent excimers have enabled white devices employing a single emissive Pt complex that can achieve emission spanning the visible spectrum while also achieving high efficiencies. However, these devices face the same operational lifetime challenges as blue phosphorescent emitters. Ideally, a WOLED fabricated using a single emissive material with sufficient blue emission, efficient excimer emission, and a molecular design aligned with known stable emitters could be envisioned. However, Ir and Pt complexes have been unable to satisfy this desirable combination of characteristics, necessitating a new avenue of materials development. Pd(II) complexes also have the potential for efficient excimer based white emission due to their square planar geometry.However, Pd(II) complexes have received significantly less attention than their Ir and Pt analogs. This is partially because Pd complexes have typically been non emissive or weakly emissive due to their low radiative decay rates and low lying MC states providing non-radiative decay pathways. Furthermore, none of the previous reports of Pd emitters, to our knowledge, have demonstrated efficient excimer emission.
In this presentation, we develop an efficient excimer emitting Pd(II) complex, Pd3O3 which utilizes a rigid and planar molecular design to acheive efficient blue and white emission while remianing aligned with stable molecular designs. This complex demonstrated efficiencies comparable to its Pt analogs but with higher emission energy and high operational stability. Efficiencies as high as 24.2% were achieved for Pd3O3 devices and one device achieved a peak EQE of 19.9% and a device operational lifetime of nearly 1000hrs at 1000cd/m2. This performance establishes Pd complexes as an emerging class of emissive materials and demonstrates their potential for stable, efficient, and simplified white OLEDs.
9:00 PM - MD5.6.30
A Novel Synthesis Strategy for Dissolving Alternating Conjugated Copolymers in Nonchlorinated Solvents Using an Asymmetric Monomer
Eun Soo Ahn 1,Jang Yeol Baek 1,Seong Jong Park 1,Hyun Ho Choi 2,Kilwon Cho 2,Yun-Hi Kim 1
1 Gyeongsang National Univ Jinju-si Korea (the Republic of),2 Pohang University of Science and Technology Pohang Korea (the Republic of)
Show AbstractWhen used to fabricate organic Field-effect transistors (OFETs), copolymers based on dithienyl-diketopyrrolopyrrole (DPP) have a carrier mobility that exceeds those of a Si-based transistors. Nevertheless, the commercial use of these high-performance copolymers is difficult because the processing solvents for previously reported high carrier mobility DPP-based copolymers have been chlorinated aromatics or alkanes such as chloroform and chlorobenzene. The use of these solvents is problematic because laws often restrict the use of toxic chlorinated sol-vent systems, and alternative methods of processing the polymers using nonchlorinated solvent systems are needed. Here, We introduce a novel synthesis strategy for dissolving DPP-based copolymers in nonchlorinated solvents. We synthesized a new asymmetric thiophene–vinylene–selenophene (TVS) moiety as a donor, chemically coupled with a DPP acceptor moiety. OFET devices fabricated using these novel polymers in non-chlorinated solvents exhibited a high µ up to 8.2 cm2/Vs .
The characteristics of these copolymers and their solubilities in nonchlorinated solvents suggested that solution processes can be used in commercial printing processes such as ink-jet printing, and that the scalable synthesis of OFETs may be feasible.
9:00 PM - MD5.6.31
Optimization of Solution Process Conditions of the Al2O3 Gate Insulators for Organic Thin-Film-Transistor
Park Eung-Kyu 1,Ji-Hwan Kim 1,Hyeong-Jun Cho 1,Dong-Hoon Lee 1,Yong-Sang Kim 1
1 Sungkyunkwan University Suwon Korea (the Republic of),
Show AbstractOrganic semiconductors are considered as a replacement for amorphous silicon in thin-film transistor (TFT) applications, since they show comparable electrical properties and potential advantages for their compatibility with plastic substrates. While a lot of work has been devoted to the study and improvement of the semiconductor materials and of their deposition procedure, the role of the dielectric and of the semiconductor/dielectric interface in the determination of the performances of the OTFT has been realized only recently. Among the dielectric materials, aluminum oxide (Al2O3) appears to be promising due to its high dielectric constant, its robustness and its possibility to be prepared on different type of substrates with easy processing conditions. However, high quality and high dielectric constant films generally require an annealing process at temperature above 400 °C, which is not suitable for the high-throughput and flexible electronic devices.
In this study, we fabricated the bottom-gate and top-contact organic TFTs on glass substrate. Al gate electrode was deposited by thermal evaporation, while the Al2O3 thin films, used as gate insulators, were grown by solution processing using spin coater. The growth of Al2O3 thin films were studied by annealing at various temperatures from 100 ~ 250 °C and solution concentration of 0.1 ~ 1 M. Pentacene was evaporated through a shadow mask at a rate of 0.3 Å/s to form a 70 nm thick active layer. Gold source/drain (S/D) electrodes (100 nm) were patterned using shadow mask on the pentacene layer. The performance of the solution processed Al2O3 based TFT was validated using capacitance-voltage and current-voltage analysis.
9:00 PM - MD5.6.32
Reproducible Performance of Crystallization Controlled Tips-Pentacene Based TFT with Hybrid Insulator
Dong-Hoon Lee 1,Hyeong-Jun Cho 1,Park Eung-Kyu 1,Ji-Hwan Kim 1,Yong-Sang Kim 1
1 School of Electronic and Electrical Engineering Sungkyunkwan University Suwon Korea (the Republic of),
Show AbstractSolution process based tips-pentacene shows non-uniform performance due to random evaporation at the edges. Therefore, it is an important challenge to control the crystallization and use tips-pentacene in the mass production.
Therefore, in this work, we studied and controlled the crystallization of tips-pentacene under nitrogen condition. The tips-pentacene TFT was fabricated with stacking cross-linked poly (vinylpyrrolidone-co-methylmethacrylate) (PVP-co-PMMA) on silicon nitride (SiN) as a hybrid insulator. The source-drain electrode was deposited by evaporating gold through a shadow mask. The tips-pentacene was dropped on the device using a micro-pipette and dried by blowing N2 gas from different directions while curing at 30 oC in a conventional oven. Depending on the direction of N2 blow, the crystallization was successfully controlled. The crystallization direction of tips-pentacene highly influences the hysteresis and mobility of the TFT. When the tips-pentacene crystallizes parallel to the TFT channel, the device shows high hysteresis and low mobility due to large interface trap at the grain boundary. On the other hand, the perpendicularly crystallized tips-pentacene device produced low hysteresis during the characterization. Based on the experimental results, we demonstrate a tips-pentacene based TFT with uniform performance due to controlled crystallization and with a mobility of 0.1 cm2/V-s without hysteresis.
9:00 PM - MD5.6.33
Host Matrix Dependence of Thermally-Activated Delayed Fluorescence Emitter Photophysics
Benjamin Cotts 1,Dannielle McCarthy 1,Naomi Ginsberg 1
1 Univ of California-Berkeley Berkeley United States,
Show AbstractOrganic light-emitting diodes (OLEDs) are a cost-effective and energy efficient alternative to conventional artificial lighting sources. Efficient OLED electroluminescence requires a mechanism in order to interconvert the triplet and singlet populations resulting from the pairing of charges with random spin. Current OLED technology relies on rare-earth heavy metal centers to harvest the two spin states by increasing spin orbit coupling. Elimination of these rare metal centers is crucial in order to realize the full commercial potential of OLED emitters. Thermally activated delayed fluorescence (TADF) OLED emitters offer a potential all-organic realization of high efficiency devices.
TADF molecules are fluorescent emitters designed to have small triplet-singlet energy gaps to enable conversion of triplets into singlets by reverse intersystem crossing at room temperature. Small triplet-singlet energy gaps are achieved by using intramolecular charge transfer (ICT) states to reduce the exchange interaction allowing TADF emitters to reach ~100% internal quantum efficiency. However, such ICT states are highly susceptible to their local environment’s rigidity and polarity. A better understanding of the interplay between host matrix properties and TADF emission will enable superior and more stable device performance.
Time-resolved photoluminescence scans reveal the change in TADF dynamics as a function of host matrix properties to lead to better molecular design of host materials for optimal OLED performance. We have quantified the prompt and delayed fluorescence rates as we linearly vary host matrix properties. This linear control reveals the subtleties in the interactions between host matrix molecules and TADF emitters and helps to identify the ideal range for host matrix parameters.
9:00 PM - MD5.6.34
Doped PEDOT Nanoreceptor for Subnanomolar Fe(III) Detection
Rajen Dutta 1,Victoria Lee 2,Mya Le 1,Crystin Eggers 1,Reginald Penner 1
1 University of California, Irvine Irvine United States,2 Fountain Valley High School Fountain Valley United States
Show AbstractWe present a new device architecture for the detection of Fe(III) cations in solution. This sensor consists of a nanoscopic particle - or "nanoreceptor" - consisting of poly(3,4-ethylenedioxythiophene), or PEDOT, which has been doped with the chelating agent deferoxamine (DFA) during the electropolymerization process. Electrical contact to this doped-PEDOT nano receptor is afforded by two platinum nanowire electrical conduits. The electrical properties of the PEDOT-DFA nanoparticle are altered by the binding of Fe(III), and the measurement of these properties therefore affords a means for electrically transducing the binding of the target species. The utility of two different sensing modes are assessed in this study: 1) Measurement of the electrical impedance of the PEDOT-DFA nanoparticle, and, 2) measure of the transconductance for PEDOT-DFA nanoparticle configured as a transistor as a function of Fe(III) concentration. The objective of this project is understand and identify parameters (particle diameter, detection mode, gate voltage, etc.) that enable rapid (
9:00 PM - MD5.6.35
Correlating Aggregation of Semiconducting Polymers in Solution with Resulting Solid State Morphology
Kathryn O'Hara 1,Christopher Takacs 2,Michael Chabinyc 1
1 Univ of California Santa Barbara Santa Barbara United States,2 Stanford University Stanford United States
Show AbstractThere is increasing interest in understanding how the molecular order in thin films of semiconducting polymers influences charge transport. Recent work suggests that there can be low energetic disorder in films of poorly crystalline semiconducting polymers, particularly indacenodithiophene-co-benzothiadiazole (IDTBT)1. It has been shown that IDTBT can achieve very high field-effect mobilities1, however the specific microstructural features that lead this property need to be explored. Here, the molecular organization of IDTBT in the solution and solid state is examined using grazing incidence wide-angle X-ray scattering (GIWAXS), high resolution transmission electron microscopy (HR-TEM), and solution small-angle scattering (SAXS). GIWAXS provides details on the molecular packing, while HR-TEM enables direct imaging of the lattice planes within polymer crystallites. Although GIWAXS data suggests that the IDTBT films are more amorphous, HR-TEM shows a higher degree of short-range order through the overlap of crystallites on the order of tens of nanometers. The movement of free charges through this material may be facilitated by a high connectivity between smaller regions of order. However, it is unclear how interactions in solution lead to these particular packing geometries. The degree and manner of aggregation of IDTBT in solution was examined through solution-SAXS experiments in which the solvent, concentration, and solution temperature were varied to determine the size, shape, and internal ordering of aggregates or entanglements in solution. We will discus how the solution structure leads to the order observed in these films via HR-TEM.
1. Venkateshvaran, D. et al. Nature 515, 384–388 (2014).
2. Takacs, C. J. et al. Nano Lett. 14, 3096–101 (2014).
3. Schmidt, K. et al. Adv. Mater. 26, 300–5 (2014).
4. Bartelt, J. a. et al. Adv. Energy Mater. 4, (2014).
9:00 PM - MD5.6.36
Employment Y-Doped ZnO Layers as Electron Transport Layer in Organic-Based Solar Cells
Sayantan Das 1,Zhao Zhao 1,Terry Alford 1
1 Arizona State Univ Tempe United States,
Show AbstractHere we propose to interfacial engineer Y-doped zinc-oxide as an electron-transport layer in organic-based solar cell structures. We show that annealing of yttrium doping sol–gel fabricated ZnO layer greatly impacts the device performance in organic-based solar cells. The performance is improved by optimizing the anneal temperature. The optimal temperature is 150 oC and is compatible with temperatures required for processing of flexible substrates. Also, a 30% increase in the photo-conductivity efficiency is obtained by yttrium doping of ZnO and annealing at the optimal temperature. This validates the potential to engineer the efficiency.
9:00 PM - MD5.6.37
Observation of F4TCNQ Diffusion in Patterned P3HT
Shravya Guda 1,Jun Li 1,Daniella Holm 1,Pieter Stroeve 1,Adam Moule 1
1 Chemical Engineering and Materials Science University of California, Davis Davis United States,
Show AbstractThe small molecule 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is an effective p-type dopant for poly(3-hexylthiophene) (P3HT). We previously showed that F4TCNQ doping can be used to additively pattern P3HT films, the doped portion becomes insoluble. In this work, three different casting solvents were used to obtain P3HT films with different initial crystallinity. F4TCNQ was then evaporated through a shadow mask, and undoped parts were washed away using developing solvents. Confocal microscopy was used to determine the position and concentration of F4TCNQ in the P3HT film. After development, we determined residual F4TCNQ position and concentration by observing the fluorescence quenching of the patterned P3HT. Finally, using atomic force microscopy, we observed the patterned film profile. Fitting with a transport model shows that the diffusion rate for the dopants is higher in low crystallinity P3HT films, and results in a poorer rendering of the pattern.
9:00 PM - MD5.6.38
Advanced Multi-Layer Architectures for Organic Solar Cells Fabricated from Organic Nanoparticle Dispersions
Stefan Gaertner 1,Stefan Reich 1,Konstantin Glaser 1,Michael Bruns 2,Alexander Colsmann 1
1 Light Technology Institute Karlsruhe Institute of Technology (KIT) Karlsruhe Germany,2 Institute of Applied Materials (IAM-ESS) Karlsruhe Institute of Technology (KIT) Eggenstein-Leopoldshafen Germany
Show AbstractSurfactant-free organic nanoparticles in aqueous or alcoholic dispersions enable eco-friendly wet-processing of organic solar cells, yielding about the same power conversion efficiencies as devices deposited from common chlorinated solvents. [1,2]
We demonstrate that organic nanoparticle dispersions furthermore allow for multi-layer deposition of equal or similar materials: After deposition and thermal annealing, the nanoparticulate layers become insoluble in the processing agents. This unique property enables, e.g., the fabrication of graded absorber layers by varying the polymer:fullerene concentration within the nanoparticle dispersions. We investigate the vertical material distribution within the photo-active layer by means of kelvin probe force microscopy (KPFM) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). The converse vertical material gradients are beneficial for charge carrier extraction from the solar cell, yielding and enhanced power conversion efficiency due to improved fill factors and open-circuit voltages.
[1] S. Gärtner, M. Christmann, S. Sankaran, H. Röhm, E.-M. Prinz, F. Penth, A. Pütz, E. Türeli, B. Penth, B. Baumstümmler, A. Colsmann, Adv. Mater. 2014, 26, 6653–6657
[2] S. Sankaran, K. Glaser, S. Gärtner, T. Rödlmeier, K. Sudau, G. Hernandez Sosa, A. Colsmann, Org. Electronics, 2015, doi:10.1016/j.orgel.2015.10.011
9:00 PM - MD5.6.39
The Increase in Crystallinity of Low Bandgap Polymers under Nanoconfinement and Their Applications to Organic Solar Cells
Jongkuk Ko 1,Jiyun Song 2,Changhee Lee 2,Ruediger Berger 3,Kookheon Char 1
1 School of Chemical amp; Biological Engineering Seoul National University Seoul Korea (the Republic of),2 School of Electrical Engineering and Computer Science Seoul National University Seoul Korea (the Republic of)3 Max Planck Institute for Polymer Research Mainz Germany
Show AbstractIn organic electronics, molecular structure of conducting polymers (chain orientation, crystallinity, and the size of crystalline phase) critically influences electronic and optical properties which, in turn, determine the device performance. Previous studies have shown that conducting polymers have different structural properties when they are under nanoconfined geometry, particularly 2-dimentionally confinement, because of the anisotropy of polymer chains. However, there have been no systematic studies because it is difficult to realize nanostructures of polymers with complicated chemical structure as well as to analyze their chain structure within a single nanostructure. In this study, we suggest the use of scalable nanoimprint lithography as a method to change the molecular orientation as well as crystallinity by exploiting the nanoconfinement effect. We fabricated nanopillars or nanoholes of conducting polymers with different chemical structure, dimension, and crystallinity based on patterning with soft PFPE templates. With this fabrication method, we were able to realize various sizes (40 nm - 300 nm) of nanopillars of different conducting polymers at low temperature (~ 60 °C) and low pressure (~5 bar) in 1-2 minutes of processing time.
Systematic studies on nanopillars of different p-type and n-type conductive polymers along with single pillar optoelectronic property characterization were performed. GIWAXS measurements showed the changes in both chain orientation and crystallinity depending on the degree of confinement which are quite different from the bulk crystallinity of conducting polymers. PTB7 and PCDTBT, which are well known low bandgap polymers used for organic solar cells, particularly showed dramatic effects by the nanoconfinement. Because of the increased crystallinity by nanoconfinement, those nanopillars showed more than 10 times higher conductivity than a bulk film, as confirmed by cAFM. The conductivity distribution in a single pillar have also shown some difference based on the degree of anisotropy of polymers. Mixtures of PTB7 / PNDIT, representing p-type and n-type mixtures, have also shown the strong nanoconfinement effect with much higher crystallinity when compared with bulk mixed films. As a result, the organic solar cells made with such PTB7 / PNDIT nanopillars have shown higher power conversion characteristics compared with counterpart bulk mixed films.
9:00 PM - MD5.6.40
Doping Effect of PEDOT: PSS Channel Material on Combined Depletion and Enhancement Regime Organic Electrochemical Transistor
Salva Salmani Rezaie 2,Manisha Gupta 2,Carlo Montemagno 2
1 Chemical and Materials Engineering University of Alberta Edmonton Canada,2 Ingenuity Lab Edmonton Canada,3 Electrical and Computer Engineering University of Alberta Edmonton Canada,2 Ingenuity Lab Edmonton Canada
Show AbstractWith the discovery of conductive polymers the field of organic electronics has grown drastically. Organic Electro Chemical Transistors (OECTs) have attracted extensive importance due to the simple fabrication technique, low working voltage, high transconductance and sensitivity, stability in aqueous media and flexibility. Despite many depletion mode OECT reports, yet the field requires more investigation on enhancement mode devices. Enhancement mode device consumes less power as it is normally OFF and if used as sensor, in presence of analyte will turn ON and detect analyte over background signal.
Poly (3, 4-ethylenedioxythiophene) - Poly (styrene sulfonate) (PEDOT:PSS) is widely used material in depletion mode devices. In this work, the reduction level of PEDOT:PSS has been tuned and possibility of the oxidized PEDOT: PSS film as channel material for enhancement mode OECT has been studied. Thin film of PEDOT:PSS layer has been spin coated on quartz sample from solution of PEDOT:PSS, ethylene glycol, dodecyl benzene sulfonic and (3-glycidyloxypropyl)-trimethoxysilane. Film doping level has been modified by treating it with an oxidizing agent. Thickness and optical function (n, k) of doped and undoped films have been studied by spectroscopic ellipsometry analysis. Electronic states and energy levels of film have been studied by UV-Vis and UPS. The absorption of PEDOT:PSS film strongly depends on its oxidation state. Presence of mid bandgap states (polaron /bipolaron) with lower energy than band gap in fully doped sample compared to oxidized sample is obvious in optical spectra of films. The effect of oxidizing agent treatment on PEDOT: PSS chain structure was studied by Raman and XPS spectroscopy which shows the change from quinoid to benzoid structure.
After film characterization, OECT device based on PEDOT: PSS has been fabricated and the effect of channel material’s properties on conversion of device working mode between depletion and accumulation has been investigated. In depletion mode device with no applied gate bias high current passed through the channel and it turns completely OFF by 0.5V gate voltage. This is because of hole depletion of PEDOT: PSS channel in the presence of injected electrolyte’s cations. However, in oxidized and undoped channel of OECT, the device is OFF at zero gate bias and current level increases with positive gate bias. The ON/OFF current ratio is less in enhancement mode OECT as compared to the depletion mode one. Results from these studies will be presented.
9:00 PM - MD5.6.41
Heptamethine-Based Organic Salts for Solar Cells with Near-Infrared Photoresponse up to 1600 nm
Margaret Young 1,John Suddard-Bangsund 2,Tyler Patrick 1,Natalia Pajares Chamorro 3,Richard Lunt 1
1 Michigan State University East Lansing United States,2 Chemical Engineering and Materials Science University of Minnesota Minneapolis United States3 Universidad Politécnica de Madrid Madrid Spain
Show AbstractDeep near-infrared absorbing excitonic molecules have the potential to enhance transparent and opaque multijunction solar cells and provide a pathway for low-cost infrared detectors. However, very few organic molecules with absorption and photoresponse have been demonstrated past 900 nm, which is further compounded with the challenge of aligning energy levels for efficient exciton dissociation in small bandgap molecules. In this work, we synthesize and demonstrate a series of organic salts in solar cells with photoresponse out to 1400 and 1600 nm with a peak external quantum efficiency of 4% in this deep near-infrared window. Additionally, we show that the energy levels in these narrow bandgap donors can be precisely tuned using alloyed blends of different organic anions without modification of the photoresponsive organic cation bandgap, enabling record high open-circuit voltages for this spectral range (near the theoretical excitonic limit). The relationship between photoresponse and donor-acceptor interface gap will be discussed in the context of correlated exciton binding energies and carrier diffusion lengths. The use of organic salts with precisely tunable properties represents a new paradigm for near infrared-absorbing solar cells and detectors, extending the range of spectral photoresponsivity and enhancing the catalog of materials for multijunction cells.
9:00 PM - MD5.6.42
Sequential Deposition for Simple Solution Coating of Doped Semiconducting Polymer Films
Ian Jacobs 1,Erik Aasen 1,Jun Li 1,Adam Moule 1
1 Univ of California-Davis Davis United States,
Show AbstractDoping polymeric semiconductors often drastically reduces the solubility of the polymer, leading to difficulties solution processing doped films and adversely affecting morphology. While this effect on solubility can be exploited to allow for multi-layer deposition and patterning (Jacobs, et. Al., ACS Nano, 2015, 9 (2), pp 1905–1912), these effects present multiple issues. Solution-doped films show a drastic increase in film roughness that can lead to shorting of devices, while aggregation in solution reduces the density of percolation networks between crystallites, leading to reduced carrier mobility. Here, we present a simple method for fabricating doped films with improved morphology. First, we deposit polymer (by spin coating, blade coating, printing, etc.) then swell the film with a solution of the dopant. We demonstrate that by careful choice of solvent and dopant concentration, we can precisely control the doping level, and derive an equilibrium constant which accurately predicts the film doping level as a function of solution concentration. Atomic force microscopy shows that we achieve significantly more uniform films than one- step methods. In addition, sequentially coated films show significantly higher conductivities at a given doping level than solution-doped films, indicating an increase in carrier mobility or a reduction in trapped charge carriers. This method is applicable to a wide range of polymers and molecular dopants, including common families such as TCNQ and Mo(tfd)3 derivatives.
9:00 PM - MD5.6.43
An Approach to Quantitative Dedoping of Conductive Polymer Films
Ian Jacobs 1,Faustine Wang 1,Jun Li 1,Adam Moule 1
1 Univ of California-Davis Davis United States,
Show AbstractThe active layer of most electronic devices based on organic semiconductors, including thin film transistors, photovoltaics, and light emitting diodes, are built from intrinsic (undoped) materials. However, often dopants, either intentional or unintentional, are introduced into these intrinsic films during device processing. These dopants can have adverse effects on device performance or lifetime. In particular, in recent work (Jacobs, et. Al., ACS Nano, 2015, 9 (2), pp 1905–1912), it was demonstrated that dopants can be used to switch the solubility of films off, allowing for solution-based multi-layer deposition of mutually soluble materials, and sub-micron patterning. If such a method is to be applied in device manufacturing, it is necessary to quantitatively remove these dopants. In addition, residual halogens from polymer synthesis are also often present, and can similarly have adverse effects.
Here, we elucidate the mechanism by which the common p-type dopant F4TCNQ can be quantitatively removed from conductive polymers. P-type dopant molecules are by definition strongly electron deficient, and thus vulnerable to attack by nucleophiles. Using UV-vis and fluorescence spectroscopy, multi-dimensional NMR and density functional theory modeling, we compare the reactions of primary, secondary, and tertiary amines on F4TCNQ and doped films of P3HT:F4TCNQ. Tertiary amines act as a competitive donor for P3HT, leading to partial dedoping. However, primary or secondary amines undergo a rapid, irreversible two-fold addition to F4TCNQ, greatly reducing the electronegativity of the dopant molecules, and leading to quantitative recover of as-cast P3HT film fluorescence. In addition, by optimization of dedoping conditions, P3HT films show greater than as-cast fluorescence intensity, indicating removal of intrinsic dopant impurities. Similar results are obtained in other polymers, including highly fluorescent materials such as F8-PFB, which shows full recovery of as-cast fluorescence after doping with F4TCNQ and subsequent dedoping. These results suggest a general strategy for obtaining intrinsic films in other polymer-dopant systems.
9:00 PM - MD5.6.44
Effects of Substituted Alkyl Chain Length on Solution-Processable Organic Semiconductor, Benzothieno-benzothiophene Derivatives
Satoru Inoue 2,Hiromi Minemawari 2,Jun'ya Tsutsumi 2,Takamasa Hamai 3,Shunto Arai 3,Toshikazu Yamada 2,Sachio Horiuchi 2,Mutsuo Tanaka 2,Makoto Yoneya 2,Reiji Kumai 4,Tatsuo Hasegawa 3
1 Nippon Kayaku Co., Ltd. Tokyo Japan,2 AIST Tsukuba Japan,2 AIST Tsukuba Japan3 Univ. Tokyo Tokyo Japan4 KEK-PF/CMRC Tsukuba Japan2 AIST Tsukuba Japan,3 Univ. Tokyo Tokyo Japan
Show AbstractOrganic semiconductors have a unique characteristic among semiconducting materials, in that they can be processed in solution under ambient conditions, which raises considerable recent expectations for their use in the printed electronics technologies. Among them, benzothieno-[3,2-b][1]benzothiophene (BTBT) derivatives are well-known examples of solution-processable small-molecule organic semiconductors. In order to control and improve the solubilities of organic semiconductors, alkyl chain substitution is frequently utilized as an effective chemical modification, whereas the systematic understanding of the effects of substituted alkyl chains on solution processability and semiconducting properties has not yet been achieved.
Here we present a systematic investigation of the alkyl chain length dependence on solution-processable layered crystalline organic semiconductors, namely 7-alkyl-2-phenyl[1]benzothieno[3,2-b][1]benzothiophenes (Ph-BTBT-Cn; 0 ≤ n ≤ 14).[1] We found that the solubility considerably depends on the substituted alkyl chain length; the solubility rapidly increased with increasing chain length from n = 0, and shows a maximum at around n = 3, but decreased gradually at n ≥ 5. From the thermal analyses, we found that the latter compounds at n ≥ 5 exhibit liquid-crystal phase transitions at high temperature. We also found, from the single-crystal x-ray crystal structure analyses, that these properties are strongly correlated with the alkyl length dependence of the layered molecular packing: the compounds with n ≤ 4 do not form independent alkyl chain layers, whereas those with n ≥ 5 form isolated alkyl chain layers and eventually afford isomorphous bilayer-type crystal structures.[2] We conclude that the independent alkyl chain layers contributes to the enhancement of the cohesive energy in the crystals and thereby play a crucial role in the formation of layered crystal structures. We also discuss the role of the high layered crystallinity in the two-dimensional carrier transport, based on the results of some single crystal field-effect transistors with Ph-BTBT-Cn.
[1] Inoue et al. Chem. Mater. 27, 3809 (2015). [2] Minemawari et al. Appl. Phys. Exp. 7, 091601 (2014).
9:00 PM - MD5.6.45
Improved Charge Extraction Using Mixed Solvent Solution Processed ZnO as Electron Transport Layer and Its Effect on Photovoltaic Properties in Inverted Bulk Hetero Junction Solar Cell Processed in Ambient Conditions
Minu Mohan 1,Vikas Nandal 2,Sanish P 1,Ramkumar S 1,Pradeep Nair 2,Manoj Namboothiry 1
1 School of Physics Indian Institute of Science Education and Research Thiruvananthapuram Thiruvananthapuram India,2 Department of Electrical Engineering Indian Institute of Technology Bombay Mumbai India
Show AbstractThe effect of electron transporting layer made of solution processed zinc oxide, processed in different methods, on the photovoltaic properties of inverted bulk heterojunction solar cells consisting of different active layers is studied. Electron transporting buffer layer using ZnO nanoparticle dispersed in mixed solvent (low temperature method) showed improved electrical conductivity and enhanced absorption in the active layer due to nano textured surface morphology compared to that of ZnO buffer layer prepared using high temperature sol gel method. Photovoltaics with nanoparticle based ZnO buffer layer showed enhanced free carrier extraction efficiency and external quantum efficiency. The improved extraction can be attributed to reduced surface recombination and is resulted in increased fill factor. The experimental results are further verified with theoretical models and found to be in good agreement with theory. These factors resulted in power conversion efficiency (PCE) of 9.10 % best device for polythieno[3,4-b]-thiophene/benzodithiophene):[6,6]-phenyl C71 butyric acid methyl ester (PTB7 : PC71BM) blend and a PCE of 5.65 % for poly (3-hexylthiophene-2,5-diyl):[6,6]-phenyl C71 butyric acid methyl ester (P3HT : PC71BM) blend.
9:00 PM - MD5.6.46
C3N3P: Organic or Inorganic Semiconductor
Brian Chaloux 1,Evan Glaser 2,James Yesinowski 3,Albert Epshteyn 3
1 NRC Postdoctoral Associate, Chemistry Division Naval Research Laboratory Washington United States,2 Optical Sciences Division Naval Research Laboratory Washington United States3 Chemistry Division Naval Research Laboratory Washington United States
Show AbstractNeat phosphorus tricyanide [P(CN)3] has been demonstrated to undergo a lossless self-condensation, quantitatively transforming from a crystalline, white compound to a black, amorphous, and pyrolytically stable extended solid with negligible porosity and C3N3P stoichiometry. Spectroscopic analysis by IR, Raman, and NMR showed signatures for structural motifs analogous to other carbon nitrides of (approximately) C3N4 stoichiometry.1 We have subsequently discovered that the black color of C3N3P arises from a well-defined, low energy band gap, typical of inorganic semiconductors. In this presentation, we discuss the optical and electronic properties of C3N3P, as well as how high concentrations of stable free radical species, characteristic of organic semiconductors, result from its chemical structure and contribute to these properties.
Electron paramagnetic resonance (EPR) spectroscopy has been utilized in conjunction with magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy to determine the origin and properties of these stable free radicals. Although typically characteristic of defect structures present in the majority of organic superconductors, these extensively delocalized free radicals are accompanied by a distinctive optical band gap, traditionally indicative of a continuous (instead of discrete) electronic band structure. Optical (e.g. UV-visible-NIR) and impedance spectroscopies have been further utilized to investigate the resulting electronic properties of C3N3P. C3N3P is an intriguing material that defies designation as either a classical organic or classical inorganic semiconductor.
Reference
(1) Chaloux, B. L.; Yonke, B. L.; Purdy, A. P.; Yesinowski, J. P.; Glaser, E. R.; Epshteyn, A. Chem. Mater. 2015, 27, 4507.
9:00 PM - MD5.6.47
Photoinduced Degradation of an Efficient Low Band-Gap Bulk Heterojunction Polymer Solar Cell: EPR and Defect State Formation
Fadzai Fungura 1,William Lindemann 1,Ruth Shinar 2,Joseph Shinar 1
1 Ames Laboratory amp; Iowa State University Ames United States,2 Microelectronics Research Center and Electrical amp; Computer Engineering Dept Iowa State University Ames United States
Show AbstractThe photoinduced degradation of efficient (~9% initial power conversion efficiency) BHJ polymer solar cells based on the low band-gap polymer poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene)-2-carboxylate-2-6-diyl)] (PBDTTT-EFT) and PCBM is monitored and evaluated. The results demonstrate that UV and blue light are largely responsible for the short-term photodegradation. After 24 hours of irradiation, changes in the external quantum efficiency together with largely unchanged absorption in the 400 to 800 nm range point to reduced charge collection at the electrodes. The defect density at the PBDTTT-EFT/PCBM interface increases significantly, as revealed by capacitance-frequency and capacitance-voltage measurements. An increase in defect states in PBDTTT-EFT was revealed by subgap quantum efficiency measurements. Similarly, an increase in the cw EPR signal is observed, which together with light-induced EPR indicates increased light-induced charge trapping, supporting the electronic measurements, and importantly, pointing via analysis of the EPR lineshape and g values to the role of the donor and acceptor in contributing to the photodegradation. The increased defect density results in an irreversible reduction in the cell’s short circuit current and the open circuit voltage, but filtering the UV light during irradiation reduces the short-term degradation significantly. The improved stability is discussed in relation to possible elimination of C-H and/or C-O bond-related defects as well as minor morphological changes, all possibly induced by energetic photons and, more plausibly, by hot polarons generated by their quenching of excitons.
9:00 PM - MD5.6.48
Dramatic Inversion of Charge Polarity in Diketopyrrolopyrrole-Based OFETs via a Simple Nitrile(CN) Group Substitution
Seong Jong Park 1,Hui-Jun Yun 1,Seok-Ju Kang 2,Yong-Young Noh 2,Yun-Hi Kim 1
1 Gyeongsang National University Jinju Korea (the Republic of),2 Dongguk University Seoul Korea (the Republic of)
Show AbstractSolution-processable conjugated polymers have attracted great interest for the last three decades because of the potential to realize various flexible electronic devices based on organic field-effect transistors (OFETs), and organic photovoltaics (OPVs) via cost-effective graphic art printing processes.
Here, we report simple and effective methods to obtain high mobility n-type conjugated polymers from p-type dominant ones by carefully selecting the incorporation point of an EWG (nitrile group) in the vinyl linkage of the alternating copolymer of PDPP-CNTVT. The attachment of only one nitrile group in the polymer backbone can effectively attract electrons through the vinyl linkage without sacrificing the extended coplanar structure of the vinyl backbone or the resulted crystallinity of the semiconducting films. Thus, the newly designed DPP-CNTVT alternating copolymer provides a record-breaking high electron-mobility up to 7.0 cm2 V−1s−1 with a relatively thin semiconducting layer.
Symposium Organizers
Alejandro L. Briseno, University of Massachusetts
Aram Amassian, King Abdullah University of Science and Technology (KAUST)
Iain McCulloch, Imperial College London
Özlem Usluer, Konya Necmettin Erbakan University
Symposium Support
ACS Applied Materials amp
Interfaces | American Chemical Society
Aldrich Materials Science
MD5.7: Device Physics/Charge Transport II
Session Chairs
Mamatimin Abbas
Howard Katz
Vitaly Podzorov
Barry Rand
Thursday AM, March 31, 2016
PCC West, 100 Level, Room 102 AB
9:00 AM - *MD5.7.01
Liquid Crystalline Molecular Donors in High Performance OPV Devices
David Jones 1,Paul Geraghty 1,Jegadesan Subbiah 1
1 Univ of Melbourne Parkville Australia,
Show AbstractMolecular materials have recently proven to be excellent candidates for high performance organic solar cells.1 We recently reported a high performance molecular material (BTR) based on a terthiophene substituted benzodithiophene core with a high temperature nematic liquid crystalline phase.2 Power conversion efficiencies of > 9.5% PCE have been recorded for devices with active layer thickness of over 300 nm and fill factors of > 70%. Structure-property relationship studies are currently underway to better understand the performance profile of molecular donors with liquid crystalline phases, and therefore to allow design of new materials. We report here the first structural modifications, that of the conjugation chain length of the chromophore. In this case we have synthesized the mono-, bis-, ter-, and quaterthiophene (BMR mono-; BBR, bis-; BTR, ter-; & BQR, quaterthiophene respectively). The new materials are all thermally stable with decoposition temperatures > 390 oC. BMR, and BBR have a single phase change at 170-180 oC, while BTR and BQR show three phase changes with the highest temperature change from a nematic liquid crystalline phase to the isotropic phase. BQR has a red shifted UV-Vis spectrum with a prominent π-π stacking peak at 635 nm for BQR compared to 620 nm for BTR. Initial results indicate that BQR results in devices with PCEs up to 9.4% PCE (ITO/PEDOT:PSS/BQR:PC71BM/Ca/Al) with a measured Voc of 0.88 V, marginally lower than the 0.90 V (ave) reported for BTR.
Ternary blend devices combining BQR with the comercially available PTB-7-Th as a mixed donor with PC71BM as the acceptor has resulted in OPV devcies with 10.7% PCE (ITO/ZnO/ PTB7-Th:BQR:PC71BM/MoO3/Ag). Devices are currently being optimised.
1 Kan, B.; Li, M.; Zhang, Q.; Liu, F.; Wan, X.; Wang, Y.; Ni, W.; Long, G.; Yang, X.; Feng, H.; Zuo, Y.; Zhang, M.; Huang, F.; Cao, Y.; Russell, T.; Chen, Y. J. Am. Chem. Soc. 2015, 137, 3886-3893; 2 Sun, K.; Xiao, Z.; Lu, S.; Zajaczkowski, W.; Pisula, W.; Hanssen, E.; White, J. M.; Williamson, R. M.; Subbiah, J.; Ouyang, J.; Holmes, A. B.; Wong, W. W.; Jones, D. J., Nat Commun 2015, 6, 6013. DOI: 10.1038/ncomms7013
9:30 AM - MD5.7.02
Carrier-Selective Contact Limit on the Recombination Dynamics of Organic Photovoltaics Using Ferroelectric Blend Interlayer
Sae Byeok Jo 1,Kilwon Cho 1
1 Pohang University of Science and Technology Pohang Korea (the Republic of),
Show AbstractInterfacial energetics determines the performance of organic photovoltaic cells (OPVs) based on a thin film of organic semiconductor blends. Here, we report a quantitative demonstration of the carrier-selective contact limit on the charge carrier dynamics of organic photovoltaic cells through the rendering of dipole alignment within a ferroelectric blend interfacial layer. The device performance showed a spontaneous adjustment of the charge carrier rectification upon the gradual change of polarization within the blend, and the consequent modulation of the device polarity and photovoltage. Moreover, the transient photovoltaic responses showed a significant variation of the non-geminate recombination rate upon the change of the polarization. These results can be mainly attributed to modulated carrier selectivity in the vicinity of the charge collecting interface, which decides the fate of diffusive charge carrier near open-circuit condition. In addition, the systematic study on the effect of deviation from the Ohmic contact regime by various photoactive materials was also performed. The simultaneous enhancement of the Voc, Jsc and FF by ferroelectric polarization resulted in the significant performance enhancement up to 8.31% of PCE for PTB7 based bulk-heterojunction OPVs. Due to the simplicity and the excellent performance, the introduction of the ferroelectric blend interfacial layer could be a new platform for the tunable interfacial energetics for high performance printed OPV devices.
9:45 AM - *MD5.7.03
Consequences of Extreme Levels of Order on Organic Solar Cell Function
Michael Fusella 1,Yunhui Lin 1,Barry Rand 1
1 Princeton Univ Princeton United States,
Show AbstractUnique to the function of organic PVs are the creation of tightly bound excitons that can only be efficiently separated at a donor/acceptor (D/A) interface capable of providing the necessary energetic driving force for dissociation. At the D/A interface, the presence of charge transfer (CT) states, ground state complexes between the donor and acceptor materials, set the upper bound for the potential that can be extracted from a given D/A pair, but their role in photogeneration is not completely understood. Furthermore, the consequences of extreme levels of order in highly crystalline heterojunctions have not so far been revealed.
We have recently been exploring pinhole-free organic semiconductor-based thin films that feature crystalline grains of up to 1 mm in extent. Because they are pinhole-free, we can apply them in vertical devices such as PV cells to explore various emergent phenomena. For example, we have found that CT states are more easily separated into free charge if they are delocalized; an aspect that becomes most feasible for highly ordered systems. I will discuss our recent efforts to understand, template, and control crystalline film morphology. These films show unprecedented thin film exciton diffusion lengths of 100’s of nm. Also, we are able to directly measure photocurrent from multiple CT states and probe singlet fission-based photocurrent as well. These aspects have important consequences for achieving more efficient photocurrent generation.
10:15 AM - *MD5.7.04
Novel Hall Effect Measurement Technique with a Drastically Improved Sensitivity for OFETs
Vitaly Podzorov 1
1 Rutgers University Piscataway United States,
Show AbstractCharge conduction in organic semiconductors frequently occurs in a regime at the borderline between a band-like coherent motion of delocalazied carriers in extended states and an incoherent hopping through localized states. Many intrinsic factors are competing for defining the dominant transport mechanism, including the strength of intermolecular interactions represented by the transfer integrals, carrier self-localization due to formation of polarons, electron-phonon coupling, scattering and off-diagonal thermal disorder (see, e.g., [1]). Depending on the interplay between these processes, either band-like or hopping charge transport is realized. Besides these intrinsic factors, a significant role in practical devices is played by the static disorder (chemical impurities and structural defects) that leads to carrier trapping at various energies and time scales. In most of these cases, the charge carrier mobility in OFETs is rather small (m = 0.1 - 20 cm2V-1s-1) [1], and in order to carefully and accurately characterize it, Hall effect measurements are necessary. Conventional Hall measurements are extremely challenging in systems with such low mobilities. Here, we present a novel Hall measurement technique that can be carried out in low magnetic fields with an amazing sensitivity, much greater than that attained in conventional Hall effect measurements [2]. We apply this method to mobility measurements in a variety of OFETs with m as low as ~ 0.3 cm2V-1s-1.
References:
[1]. V. Podzorov, “Organic single crystals - addressing the fundamentals of organic electronics”. MRS Bulletin 38, 15-24 (2013).
[2]. Y. Chen, H. T. Yi and V. Podzorov, "Sensitive Hall effect measurements in low-µ materials", submitted (2015).
11:15 AM - *MD5.7.05
3rd Generation Organic Blend Semiconductors for High Mobility Thin-Film Transistor Applications
Thomas Anthopoulos 1
1 Imperial College London London United Kingdom,
Show AbstractSolution-processable organic semiconducting compounds are the subject of great interest due to their potential for application in a wide range of inexpensive, large-area electronics. The interest in this field has led to considerable recent advances in device performance with several laboratory prototype transistors now competing, in terms of charge carrier mobility, with other key emerging technologies such as metal oxide-based transistors. In this presentation I will discuss the development of 3rd generation organic semiconducting blend transistors with field-effect hole mobility values exceeding 10 cm2/Vs. I will show how such high charge mobility values can be achieved routinely through the use of multi-component blend formulations that can be processed from solution at room temperature. Particular emphasis will be placed on the role of material energetics and layer microstructure on overall device performance. Finally, the important role of extrinsic doping will also be discussed.
11:45 AM - MD5.7.06
Towards Highly Stable Polymer Electronics
Mark Nikolka 1,Iyad Nasrallah 1,Katharina Broch 1,Aditya Sadhanala 1,Michael Hurhangee 2,Iain McCulloch 2,Henning Sirringhaus 1
1 Univ of Cambridge Cambridge United Kingdom,2 Imperial College London United Kingdom
Show AbstractDue to their ease of processing, organic semiconductors are promising candidates for applications in high performance flexible displays and fast organic electronic circuitry. Recently, a lot of advances have been made on organic semiconductors with surprisingly high performance and carrier mobilities exceeding those of amorphous silicon and approaching mobilities of polycrystalline silicon. Owing to a high degree of crystallinity and intrinsic grain-boundaries many organic semiconductors however lack the uniformity for large area applications in e.g. displays. Amorphous and semi-crystalline donor-acceptor polymers with high field-effect mobilities > 1cm2/Vs [1] address this issue - however, recent evidence suggest a strong correlation between their crystallinity and stability, posing a major limit for their otherwise, promising application. Additionally, the operational stability as well as device uniformity of donor-acceptor co-polymers so far does not meet the requirements for large scale, high performance applications.
Here, we report a novel technique for dramatically improving the operational stress stability, performance and uniformity of high mobility polymer field-effect transistors by the addition of specific small molecule additives to the polymer semiconductor film. We demonstrate for the first time polymer FETs that exhibit stable threshold voltages with threshold voltage shifts of less than 1V when subjected to a constant current operational stress for 1 day under conditions that are representative for applications in OLED active matrix displays. The approach constitutes in our view a technological breakthrough; it also makes the device characteristics independent of the atmosphere in which it is operated, causes a significant reduction in contact resistance and significantly improves device uniformity. We will discuss in detail the microscopic mechanism by which the molecular additives lead to this significant improvement in device performance and stability.
[1] D. Venkateshvaran*, M. Nikolka* et al., Nature, 515, 384−388 (2014)
12:00 PM - *MD5.7.07
Charge Transport in Conjugated Polymers: Effect of Microstructure from the Nanoscale to the Mesoscale
Alberto Salleo 1
1 Materials Science and Engineering Department Stanford University Stanford United States,
Show Abstract
Efficient charge transport is often needed in electronic devices. For instance, the efficiency of solar cells is degraded when charges cannot leave the device fast enough. In transistors, carrier mobility is directly linked to the transit time across the channel and therefore to switching speed. Understanding of carrier mobility in semiconducting polymers is complicated by the combined effect of phenomena occurring at different length-scales. In this talk I will show how the degree of delocalization of charges can be measured using modulated spectroscopic methods. Further, the effect of molecular weight distribution and tie-chains on transport will be studied using model polymer blends. Finally, the importance of local aggregation vs. long-range order will be highlighted. To conclude, I will show how theoretical models that explicitly take into account the macromolecular nature of the semiconductor can be successfully constructed. Such a multi-scale approach is necessary to understand why polymers exhibit high mobility and evolve design rules to further enhance charge transport.
12:30 PM - *MD5.7.08
Organic Field Effect Transistors for Charge Transport Study in Organic Photovoltaics
Mamatimin Abbas 1,Lionel Hirsch 1
1 CNRS, Université de Bordeaux, Lab. IMS, UMR 5218 Pessac France,
Show AbstractStudying charge transport in bulk heterojunction organic Photovoltaics (OPVs) is essential in achieving high device efficiencies.[1] A number of experimental techniques have been used to study charge transport in this system, showing both advantages and drawbacks. Discrepancies still exist among simulation and experimental results.[2-4] In this talk, an approach to study charge transport of both carriers in OPVs using organic field effect transistors (OFETs) will be presented.
Since one more interface, namely dielectric and active layer, is involved in OFETs, it is crucial to employ a dielectric layer which has similar low trap densities for both types of carriers.[5] When suitable metal contacts were applied to efficiently inject both holes and electrons, balanced carrier mobilities derived from OFETs could be correlated to drastic enhancement in device performance parameters in OPVs in model system poly(3-hexylthiophene) (P3HT) as donor and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as acceptor.[6]
Such a correlation can be further established in low band gap polymer and fullerene systems where the effects of additive in the case of PTB7 and donor-to-acceptor ratio in the case of PCDTBT can be elucidated in terms of charge transport behaviour.[7] Thus, control of carrier mobilities using OFETs has enabled us to optimize OPV device performance in a promising anthracene based polymer (AnE-PV) system, where highest power conversion efficiency has been realized using classical PCBM as acceptor. [8]
These investigations in a number of polymer and fullerene systems have shown that OFET is a reliable technique in studying and controlling the charge transport in OPVs.
Reference:
[1] P. W. M. Blom et al., Adv. Mater. 2007, 19, 1551
[2] J. D. Kotlarski and P. W. M. Blom, Appl. Phys. Lett. 2012, 100, 013306
[3] A. Armin et al., Adv. Energy. Mater. 2014, 4, 1300954
[4] H. Zhou et al., Adv. Mater. 2013, 25, 1646
[5] M. Abbas et al., Organic Electronics 2011, 12, 497
[6] M. Abbas and N. Tekin, Appl. Phys. Lett. 2012, 101, 073302
[7] M. Abbas and L. Hirsch, 2015 submitted
[8] O. Usluer et al., RSC Adv. 2015, 5, 50668
MD5.8: Design and Synthesis of Organic Semiconductors
Session Chairs
Alejandro L. Briseno
Iain McCulloch
Dmitrii Perepichka
Ozlem Usluer
Thursday PM, March 31, 2016
PCC West, 100 Level, Room 102 AB
2:30 PM - *MD5.8.01
Tuning Conjugated Polymer Properties by Post-Polymerization Modification
Martin Heeney 1
1 Department of Chemistry Imperial College London London United Kingdom,
Show AbstractPost-polymerization modification of polymers is an interesting approach to tune material properties and introduce sensitive functionality onto the polymer backbone. In this talk I will discuss various synthetic approaches to post-polymerization modification of conjugated polymers. I will compare the properties of polymers prepared by traditional copolymerization of preformed monomers with those made via post-polymerization methods. In addition I will demonstrate how post-polymerization functionalization can be utilized to introduce sensitive cross-linkable functionality onto donor polymers for organic photovoltaic cells. The influence of cross-linking on solar cell stability and performance will be discussed.
3:00 PM - *MD5.8.02
Electron-Deficient Thienoacenes for Electronic Materials
Kazuo Takimiya 1,Masahiro Nakano 1,Itaru Osaka 1
1 Emergent Molecular Function Research Group RIKEN Center for Emergent Matter Science (CEMS) Wako Japan,
Show Abstract
Thiophene-based fused polycyclic aromatic compounds known as “thienoacens” have been utilized as p-type organic semiconductors and electron rich building blocks for semiconducting polymers and oligomers for the active materials in organic field-effect transistors and organic photovoltaics. This is because of their high-lying HOMO energy levels facilitating hole injection and their rigid and planar molecular structure that enables efficient intermolecular orbital overlap realizing high hole mobility in the condensed phase [1].
On the other hand, we have recently focused new electron-deficient thienoacenes with low-lying LUMO energy level for n-type organic semiconductors and electron-poor building unit for semiconducting polymers. To this end, we have utilized naphthodithiophene cores and modified them with strong electron withdrawing functional groups (Figure 1) [2-5]. By using these electron deficient thienoacens, n-type organic semiconductors and ambipolar/n-type semiconducting polymers have been developed. In the contribution, their synthesis, characterization, and device characteristics will be reported and discussed in terms of their electronic structure and structure-property relationship.
Reference
[1] Takimiya, K. et al. Adv. Mater. 2011, 23, 4347. [2] Fukutomi, Y; Nakano, M. et al. J. Am. Chem. Soc. 2013, 135, 11445. [3] Mori. T. et al. 2014, 16, 1334. [4] Nakano, M. et al. Macromolecules 2015, 48, 576. [5] Nakano, M. et al. Chem. Mater. 2015, 27, 6418.
3:30 PM - MD5.8.03
New Insights into Alkyl Substitution Patterns for Photoactive Polymers
Christian Nielsen 1,James Durrant 1,Iain McCulloch 1
1 Imperial College London United Kingdom,
Show AbstractWe have developed a high-performing photoactive polymer BBTI based on benzo[1,2-b:3,4-b’:5,6-d’]trithiophene (BTT) and 2,1,3-benzothiadiazole-5,6-dicarboxylic imide (BTI). The synthetic design importantly allows for alkyl chains to be introduced on both the electron-rich and electron-deficient components of the polymer, which in turn allows for rapid optimization of the polymer’s alkyl chain substitution pattern. Furthermore, in contrast to most state-of-the-art organic photovoltaic donor polymers, we have found that large branched alkyl chains on the electron-deficient component do not hinder efficient polymer:fullerene interactions. This finding will be elucidated with spectroscopic and structural characterisation techniques underpinned by solar cell device data and important structure-property relations will be discussed and related to other high-performing donor materials.
3:45 PM - MD5.8.04
How Fluorine Substituents Impact Polymer and BHJ Solar Cell Efficiencies
Pierre Beaujuge 1
1 KAUST Thuwal Saudi Arabia,
Show AbstractOn the track to improving polymer and bulk-heterojunction (BHJ) solar cell efficiencies, recent published work has emphasized the use of fluorine (F)-substituted motifs in the design of low-bandgap polymers. With published power conversion efficiencies (PCE) >10% in BHJ solar cells with fullerene acceptors, and promising PCEs >5% in all-polymer BHJ configurations, F-substituted polymers continue to garner much interest across the PV community. In this context, forging a better understanding of the critical material parameters impacted by F substitutions is of paramount importance, and yet the underlying reasons that may justify the importance of F-substituted motifs in polymer donors and acceptors remain a matter of some debate. In many instances, morphological aspects are convoluted with the unique pattern of electronic and self-assembling properties of the polymers, and broader systematic analyses can help improve our understanding of those correlated effects. Our recent work with poly(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b']dithiophene-3,4-difluoro-thiophene) (PBDT[2F]T) and low-bandgap polymer analogues sheds light on the impact of F-substituted motifs on electronic and molecular-scale effects, which directly correlate with the morphological parameters and carrier dynamics in thin films, and ultimately direct material performance and BHJ device efficiencies. In parallel, combining predictive efficiency models and experimental multi-junction device optimizations, we show that F-substituted polymers are some of the most promising systems for use in the subcells of high-PCE tandem and triple-junction solar cells.
4:30 PM - MD5.8.05
Synthesis and Self-Assembly Properties of Alternated Multi-Block Copolymers for Third Generation Organic Photovoltaic
Andrea Gasperini 1,Kevin Sivula 1
1 EPFL SB ISIC LIMNO Ecublens Switzerland,
Show AbstractSolution-processed conjugated polymers are considered as viable semiconductors for the fabrication of cheap electronic devices like transistors and solar cells. Despite their semicrystalline nature, PBTTT, DPP and PDI based copolymers have recently shown an exceptional charge carrier mobility due to their rigid conjugated backbone that allows the formation of a relatively high crystalline structure in the solid state. By precisely selecting the chain lengths (by GPC) and characterizing the resulting microstructure (by AFM, Synchrotron GIXS) and mobility, we widened and characterized new morphologies that are usually hidden in broad polydispersity PBTTT processed films.1 Specific end group functionalization of short, fully conjugated and low polydispersity blocks enabled the synthesis of a prototype PBTTT where fully conjugated moieties are flexibly-linked (FL) to each other by aliphatic chains. Our FL-PBTTT material exhibited improved thin-film formation compared to the low MW starting polymer and unique thermal properties which were exploited to demonstrate the evolution between distinct thin film morphologies without altering the chain length.2 Efficient Stille-coupling of end-group functionalized donor (A-A) and acceptor (B-B) conjugated blocks allowed the synthesis of a new class of conjugated block copolymer based on the alternated sequence of donor-acceptor segments along the same chain (NBlocks = 6-8). High molecular weight fractions, isolated by GPC, were thermally and optically characterized to confirm the presence of the single blocks along the same chain. Thin film microstructure revealed efficient block’s phase separation while maintaining the same degree of crystallinity and charge carrier mobility.3 The potential of our synthetic strategy to both extend the use of the existing library of conjugated polymers in block copolymer synthesis and overcome limitations of traditional chain growth type approaches will be also discussed. 1.Gasperini et al. Macromolecules 2013,46,9349−9358 2.Gasperini et al. Sci. Chem. 2014 3. Gasperini et al. in Press 2015
4:45 PM - *MD5.8.06
Open-Shell Singlet Diradicaloids
Jishan Wu 1
1 Department of Chemistry National University of Singapore Singpore Singapore,
Show AbstractSo far, most reported π-conjugated systems in neutral state have a closed-shell ground state. However, recent research disclosed that a certain type of π-systems could have an open-shell singlet diradical and even polyradical ground state. In this lecture, I will discuss two types of open-shell polycyclic hydrocarbons developed in our group: (a) zethrenes and (b) extended p-quinodimethanes. The discussion will mainly include their challenging synthesis, their characterizations of the ground-state structures, and their unique optical, electronic and magnetic properties. These fundamental studies now allow us to do rational design of stable open-shell systems with tunable physical properties and exploit their applications in molecular electronics, spintronics and photonics.
References
See our recent review/account articles: (a) Sun, Z. et al., Chem. Soc. Rev. 2012, 41, 7857. (b) Sun, Z. et al., Chem. Asian J. 2013, 8, 2894 (feature article). (c) Sun, Z. et al., Acc. Chem. Res. 2014, 47, 2582. (d) Zeng, Z. et al., Chem. Soc. Rev. 2015, 44, 6578 (Theme issue on “Challenges in Aromaticity: 150 Years after Kekulé’s Benzene”).
5:15 PM - MD5.8.08
A Ternary D1-D2-A Structured Conjugated Polymer: High-Performing Green Solvent-Processed Polymer/Neat-C70 Solar Cells
Jianyu Yuan 2,Wanli Ma 1,Guillermo Bazan 2
1 FUNSOM, Soochow University Suzhou China,2 Chemistry and Biochemistry University of California, Santa Barbara Santa Barbara United States,1 FUNSOM, Soochow University Suzhou China2 Chemistry and Biochemistry University of California, Santa Barbara Santa Barbara United States
Show AbstractDespite the tremendous achievements up to date, the research development of OPVs has reached the stage where processing conditions, stability and cost issue need to be further considered, in order to make it suitable for large-scale commercialization threshold. Herein, we focus our attention on an unconventional D-A type conjugated polymer with well-defined ternary building blocks. the D1-D2-A structured polymer PBSF is composed of 2-d alkylthienyl side chain modified benzodithiophene (BDT), dithienosilole (DTS) and difluorobenzothiadiazole (DFBT). Phenomenological, we anticipated the strong donor characteristics of DTS would accentuate intramolecular charge transfer (ICT) characteristics of the excited state as well as the excellent intrinsic hole mobility, while the relatively weaker donor 2-d BDT unit would contribute toward keeping a relatively deep highest occupied molecular orbrital (HOMO) energy level also enhancing intermolecular π-π stacking, the electron acceptor DFBT was reasonably selcected because of the unqiue intra- and intermolecular hydrogen bonding between C-F and H-C in the fluorinated polymers, which can further promote better molecular organization and crystallization to enhance their carrier mobility. Therefore, the D1-D2-A type molecular backbone theoretically can simultaneously balance the optical band gap, energy level, crystallinity, intermolecular packing and charge transport. Indeed, a high PCE of 7.40% were achieved for PBSF/[70]PCBM based device cast from chlorinated solvents o-dichlorobenzene (ODCB) without any additive or annealing. More interestingly, the PBSF:[70]PCBM based device showed comparable performance (6.36%) when processed using none chlorinated solvents 1,2,4-trimethylbenzene (TMB) without any post-treatment. We further adopted neat C70 as the electron acceptor, and quite unexpectedly, high PCEs of 5.30% and 5.00% were achieved for PBSF/C70 blend as cast from ODCB and TMB, respectively, which is the highest reported efficiency for a solution-processed polymer-C70 based BHJ blend. Moreover, systematical morphological characterization, together with charge carrier generation, recombination and transportation investigations have revealed the microcosmic difference between various acceptor and processing condition, which in return provide useful and new opportunities for further materials design and device engineering to achieve higher efficiency for cost-effective and sustainable-processed solar cells.
5:30 PM - *MD5.8.09
Towards Supramolecular Design of Organic Semiconductors
Dmitrii Perepichka 1
1 Department of Chemistry McGill University Montreal Canada,
Show AbstractOrganic molecules with extended π-conjugation can display unusual electronic properties, traditionally associated with solid state inorganic materials. Manipulating a molecular structure, one can design organic metals, semiconductors or superconductor, magnetic, non-linear optical and lasing materials and even combine several of these properties in a single material. However, the optimization of these properties and the performance of corresponding devices (OFET, OPV) rely not only on a fine-tuned molecular structure but also on a more difficult-to-control supramolecular organization in the solid state. Achieving such control, is thus among the key fundamental challenges of organic electronics.
In this lecture I will discuss the current state of art and challenges in using high-fidelity supramolecular interactions in controlling self-organization and properties of organic semiconducting materials. The specific examples will include the (i) application of complementary hydrogen bonding in synthesis of multichannel semiconducting materials and (ii) surface-templated synthesis of semiconducting polymers with high epitaxial order and increased dimensionality (2D conjugated polymers).
Symposium Organizers
Alejandro L. Briseno, University of Massachusetts
Aram Amassian, King Abdullah University of Science and Technology (KAUST)
Iain McCulloch, Imperial College London
Özlem Usluer, Konya Necmettin Erbakan University
Symposium Support
ACS Applied Materials amp
Interfaces | American Chemical Society
Aldrich Materials Science
MD5.9: Thin-Film Devices and Materials I
Session Chairs
Mamatimin Abbas
Michael Chabinyc
Mahmut Kus
Vitaly Podzorov
Friday AM, April 01, 2016
PCC West, 100 Level, Room 102 AB
9:00 AM - MD5.9.01
Synthesis of Photo-Active Inks: Towards Eco-Friendly Fully Printed Organic Solar Cells
Laurie Parrenin 3,Cyril Brochon 3,Eleni Pavlopoulou 3,Georges Hadziioannou 3,Eric Cloutet 3
1 LCPO Pessac France,2 CNRS Pessac France,3 University of Bordeaux Pessac France,1 LCPO Pessac France,3 University of Bordeaux Pessac France
Show AbstractOne of the most important challenges in organic electronics concerns processability of materials that nowadays requires the use of toxic solvents, e.g. halogenated and/or aromatic. These solvents do not fit the industrial requirements for large scale module device production. An alternative would be to synthesize polymeric particles dispersed in an environment-friendly phase (e.g. aqueous media). [1,2,3]
In this contribution, we present the synthesis of low band gap poly(N- 9’-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2’,1’,3’-benzothiadiazole) PCDTBT particles dispersed in alcohol as well as PCDTBT: PC71BM particles dispersed in water. We have selected this low band gap polymer because of its high PCE (7.5% as of 2013[4]). Two methodologies were used in order to prepare these two kinds of nanoparticles:
- p-type PCDTBT particles were synthesized through Suzuki cross-coupling dispersion polymerization in alcoholic media yielding colloidal stable systems with particles size ranging from 330 to 1300 nm, depending on the concentration of the stabilizer. Additionally, we have varied the amount of monomers, base, stabilizer and catalyst, and thus we managed to control the PCDTBT particle size and dispersity. [5]
- Composite particles made of the low band-gap PCDTBT polymer blended with the n-type small molecule PC71BM have been prepared in water via the miniemulsion post-polymerization technique. Dynamic light scattering measurements indicate average hydrodynamic diameters from 50 to 150 nm, depending on the synthesis conditions. Those results were confirmed by Transmission Electron Microscopy and Atomic Force Microscopy which showed a core-shell morphology and a good packing of the particles, perfect for formulating the active layer of the solar cells. The ratio between the p- and n-type materials has been varied as well. Optoelectronic characterizations have been conducted that show the complete quenching of the photo-luminescence upon blending.
Subsequently, both kinds of nanoparticles that we synthesize have been tested for their performance in organic solar cells.
[1] A. J. C. Kuehne, et al, Nat. Commun. 2012, 3, 1088
[2] K. Landfester, et al, Adv. Mater. 2002, 14, 651
[3] J. Pecher, et al, Chem. Rev. 2010, 10, 6260
[4] D. H. Wang, et al, Angew. Chem., Int. Ed., 2013,52, 2874
[5] L. Parrenin, et al, Macromol. Rapid Commun (2015) DOI: 10.1002/marc.201500324
9:15 AM - MD5.9.02
UV Light Activation of the n-Doping of a Low Electron Affinity Organic Semiconductor with Air-Stable Organometallics
Xin Lin 1,Berthold Wegner 2,Karttikay Moudgil 3,Stephen Barlow 3,Seth Marder 3,Norbert Koch 2,Antoine Kahn 1
1 Princeton University Princeton United States,2 Humboldt University of Berlin Berlin Germany3 Georgia Institute of Technology Atlanta United States
Show AbstractVarious approaches have been used to achieve n-doping of organic electronic materials in order to lower charge-injection barrier, increase conductivity and achieve overall better device performance in organic electronics. To satisfy requirements of dopant ambient stability and efficient reduction of low electron affinity (EA) materials, Guo et al. [1] introduced several dimeric organometallic compounds able to n-dope low EA materials like pentacene (EA = 2.8 eV). Here we investigate the use of ultraviolet (UV) light to activate the doping of the very low EA (2.4 eV) electron transport material phenyl-dipyrenylphosphine oxide (POPy2) with the (pentamethylcyclopentadienyl)(1,3,5,-trimethylbenzene)ruthenium dimer ([RuCpMes]2) and the bis(pentamethylcyclopentadienyl)ruthenium cyclophane dimer ([RuCyclo]2). Ultraviolet and X-ray photoemission spectroscopy (UPS, XPS) and inverse photoemission spectroscopy (IPES) are used to determine the electronic structure of undoped POPy2. The work function and conductivity of POPy2 are then measured in the dark and in vacuum (without any pre-measurement illumination of any kind) as a function of doping concentration (molar ratio ranging between 0.1% to 6%) using a Kelvin probe and standard current-voltage (I-V) measurement. For both [RuCpMes]2- and [RuCyclo]2-doped POPy2 before UV activation, the Fermi level is seen to shift toward the lowest unoccupied molecular orbital (LUMO) level by 0.3 – 1.3 eV, while the conductivity remains as low as for the undoped sample, indicating that the dimers only fill deep trap states of POPy2 without activation. Activation of both systems with 375 nm UV light moves the Fermi level very close to the POPy2 LUMO onset, resulting in a 3 to 6 orders of magnitude increase of the conductivity within a few seconds. Following full activation, the sample conductivity remains remarkably stable in the dark and in vacuum, with no significant decay over 100 hours. Some differences are observed between the two dimers. The Fermi level shift in the [RuCpMes]2-doped samples before activation is less pronounced than in the [RuCyclo]2-doped one. However, the former dopant leads to higher conductivities (10-6 – 10-4 S/cm2) after activation than the later (10-8 – 10-7 S/cm2). These results are further combined with optical absorption spectroscopy studies. Potential mechanisms for activation and persistent conductivity are discussed.
[1] Guo, S. et al., Adv. Mater., 24: 699–703 (2012).
9:30 AM - *MD5.9.03
Organic-Inorganic Hybrid Interfaces for New Generation Organic and Perovskite Solar Cells
Mahmut Kus 1,Koray Kara 1,Esma Yenel 1,Cisem Kirbiyik 1,Duygu Akin Kara 2,Yasemin Topal 1,Sumeyra Buyukcelebi 1,Mustafa Ersoz 1
1 Advanced Technology Ramp;A Center Selcuk University Konya Turkey,2 Department of Physics Mugla University Mugla Turkey
Show AbstractRecently, new photovoltaic technologies have gained great attention due to the difficulties in conventional Silicon and related technologies. Hybrid (Organic-Inorganic) and perovskite solar cells are the last generations among those technologies. However, low efficiency of hybrid systems and efficient but reproducibility of perovskite solar cells still the problems must be solved for this generation. On the other hand, scaffold layer is one of the key parameter on solar cell performance. Introducing alternative scaffold layers is still scarce in literature. In addition, the design of novel organic molecules for surface modification leads the increase in efficiency as well as facile production of solar cells.
We investigate the process optimization for reproducibility based on solvent based production, surface modification with some specially synthesized small molecules for better crystallization and introducing novel scaffold layers free from heat treatment at high temperature.
10:00 AM - *MD5.9.04
Doping Semiconducting Polymers for Thermoelectrics
Michael Chabinyc 1
1 Univ of California-S Barbara Santa Barbara United States,
Show AbstractMolecular design of semiconducting polymers has led to carrier mobilities greater than 1 cm2/Vs in thin film transistors. Due to these promising electrical properties and their low lattice thermal conductivities, polymers are a promising class of solution processable thermoelectric materials. To achieve high thermoelectric performance, the electrical conductivity, thermal conductivity and thermopower in doped semiconducting polymers must be optimized in concert. We will present our results on how dopants and processing influence thermopower in both p- and n- type semiconducting polymers. For example, using solution and vapor processing methods, we can strongly modify both the electrical conductivity and thermopower in high performance p-type polymers such as PBTTT. Conductivities above 100 S/cm can be readily achieved using F4TCNQ as a dopant and can be attributed to the alignment of polymer chains observed using polarized resonant soft X-ray scattering. We have also studied how the backbone structure of n-type polymers influences electrical conductivity using various dopants and found that steric effects have a significant influence. Using these studies we will present an outlook for the future performance of polymers as thermoelectric materials.
11:00 AM - *MD5.9.05
Donor-Acceptor Charge Transfer Complexes: From Fundamentals to Applications
Katelyn Goetz 1,Oana Jurchescu 1
1 Department of Physics Wake Forest University Winston Salem United States,
Show AbstractBinary charge-transfer (CT) complexes are combinations of two organic semiconductors, in which one molecule acts as an electron donor (D) and the other as an acceptor (A). The parent compounds are typically n-type or p-type semiconductors, but in the CT complexes novel functionalities often arise due to the interactions and the charge transfer between the two molecular species. Their wider range of properties compared to single-component materials, make the CT compounds attractive for many technologies. Several such materials will be presented, with a sharp focus on their fundamental properties with ramifications on their potential applications. The impact of processing on crystal morphology, stoichiometry and electrical properties, the effect of crystalline structure on electronic and the electrical properties and the mechanism of charge transport in CTs will be discussed by using several model systems. In the perylene: 7,7,8,8-tetracyanoquinodimethane (P-TCNQ) charge-transfer complex, three stoichiometries are obtained, one exhibiting hole-only transport, another electron-only, while the third is ambipolar. In dibenzotetrathiafulvalene-TCNQ (DBTTF-TCNQ) polymorphism impacts the magnitude of the hole and electron mobilities. In the trans-stilbene – 2,3,5,6-tetrafluoro-TCNQ (STB:F4TCNQ), the change in amplitude of the a particular torsional oscillation (libration) results in a crossover from thermally activated to a temperature-independent charge transport by controlling the freezing in of orientational disorder between two conformers present in the system.
11:30 AM - *MD5.9.06
Interfacial Control of Charge Carrier Density and Transport in Organic Electronics and Thermoelectrics
Howard Katz 1
1 Department of Materials Science and Engineering Johns Hopkins University Baltimore United States,
Show AbstractThe transition voltages at which semiconductor devices become conductive are often as important as the conductivity magnitudes in determining device applicability. This is true for organic transistors as well as devices with other functions, such as chemical sensing and energy conversion. This talk describes molecular design, surface chemical, and static charging approaches to the control of gate electrode-independent conductivity in organic semiconductors. For organic transistors, the positions of additives in multilayer dielectrics control bias stress-modulated threshold voltages. In field-effect chemical sensors, static charging of the gate dielectrics ensures that the semiconductor charge density is preset for optimized electronic response to analytes. In thermoelectrics, interfacial potentials along both boundary and particulate surfaces modulate the electronic conductivity and Seebeck coefficient in a way that can be more productive than simple chemical doping. Analytical techniques such as neutron reflectivity and scanning Kelvin probe microscopy are used to analyze device structures and charge distributions. Synthesis and charging of newly designed functional semiconducting polymers, including electron-transporting polymers, will be emphasized.
12:00 PM - *MD5.9.07
Microwave Absorption of Free Carriers in Doped Conjugated Polymer Films
Garry Rumbles 2
1 Chemistry and Nanoscience Center National Renewable Energy Laboratory Golden United States,2 Department of Chemistry and Biochemistry University of Colorado Boulder United States,
Show AbstractFlash photolysis time-resolved microwave conductivity (fp-TRMC)
is a powerful spectroscopic tool for the detection of mobile
charges in organic systems, such as conjugated polymers. We will
report on a study of charge carrier generation in a number of
polymer systems where the solid-state microstructure (SSM) of
the thin films can be controlled using both molecular structure
and processing conditions. By incorporating a low concentration
of molecular acceptors, such as metallo-phthalocyanines, as well
as substituted fullerenes and perylenes, the driving force for
photoinduced electron transfer can be controlled through the
excited state energy and the reduction potential. Our results
indicate the importance of the crystalline phase of the polymer to
stabilise and reduce the rate of recombination of the holes with
the electrons that remain trapped on the acceptor. In addition,
the role that the SSM plays on the stabilization of bound electronhole
pairs, or charge-transfer (CT) states will be examined.
12:30 PM - MD5.9.08
Probing the Molecular Structure of Doped Sites within Crystals by Pyroelectricity and Dispersion Corrected DFT Modeling
Elena Meirzadeh 1,Ido Azuri 1,David Ehre 1,Andrew Rappe 2,Meir Lahav 1,Leeor Kronik 1,Igor Lubomirsky 1
1 Weizmann Inst of Science Rehovot Israel,2 University of Pennsylvania Philadelphia United States
Show AbstractIn this lecture we shall describe the analysis of polar architectures at the nanoscale by pyroelectric measurements and Dispersion-Corrected-DFT modeling. Controlled doping of crystals is a primary tool for the modification of the properties of materials. Doping of non-polar molecular crystals with “tailor-made” auxiliaries often reduces their symmetry, and converts them into polar mixed crystals. Such crystals are pyroelectric, i.e. they display temporary surface charge if subjected to a temperature change[1][2]. When the non-polar crystals of the amino-acids are doped with different L-amino acids of concentrations as low as (
12:45 PM - MD5.9.09
Process-Structure-Property Relationships in Fast-Coated, Nanostructured Polymer Semiconductors for Large-Area Electronics
Sadir G. Bucella 4,Alessandro Luzio 1,Eliot Gann 2,Lars Thomsen 3,Christopher R. McNeill 2,Giuseppina Pace 1,Andrea Perinot 4,Zhihua Chen 5,Antonio Facchetti 5,Mario Caironi 1
1 Istituto Italiano di Tecnologia Milano Italy,4 Politecnico di Milano Milano Italy,1 Istituto Italiano di Tecnologia Milano Italy2 Monash University Clayton Australia3 Australian Synchrotron Clayton Australia5 Polyera Corporation Skokie United States
Show AbstractPolymer semiconductors with steadily improved electronic properties are being synthesized, achieving charge mobility in excess of 5 cm2/Vs for electrons and holes, respectively. Such performances are sufficient for a large range of applications of printed, light-weight and mechanically robust circuits, in diverse fields such as wearable electronics, smart packaging, and bio-electronics. A key to enable these technologies is the possibility of using high-throughput, large-area printing processes to pattern polymer semiconductors with uniform and optimized morphologies. By controlling the self-assembling properties of model donor-acceptor copolymers, in combination with simple, roll-to-roll compatible coatings, it is possible to achieve well-ordered and efficient charge-transport nanostructures over large-areas. Our approach enables directional self-assembling of polymer chains exhibiting large transport anisotropy and an electrons mobility up to 6.4 cm2V-1s-1, allowing very simple n-type device architectures to operate at 3.3 MHz [1]. In particular, such control can be extended from films tens of nanometers thick, down to mono- or sub-monolayers, still retaining high-charge mobility. The mapping of charge-induced features within the channel of working devices is critical to unveil the nexus between film microstructure and electronic properties in such deposited films [2].
The level of control of the deposition process can boost the operational frequencies of printed polymer electronics well into the MHz regime without recurring to extreme downscaling, thus maintaining compatibility with cost-effective manufacturing of large-area circuits.
[1] S.G. Bucella, M. Caironi et al., Nature Communications 2015, 6, 8394
[2] N. Martino, M. Caironi et al., ACS Nano 2014, 8, 5968
MD5.10: Thin-Film Devices and Materials II
Session Chairs
Mahmut Kus
Christian Muller
Jianhui Wu
Friday PM, April 01, 2016
PCC West, 100 Level, Room 102 AB
2:30 PM - MD5.10.01
Synthesis and Characterization of New PEDOT:Polyanion Systems
Anna Hofmann 1,Dimitrios Katsigiannopoulos 1,Cyril Brochon 1,Eric Cloutet 1,Georges Hadziioannou 1
1 Laboratoire de Chimie des Polymères Organiques University Bordeaux Pessac France,
Show AbstractOrganic conducting polymers are of increasing scientific interest and are promising candidates for various applications, such as transparent and flexible electrodes in light emitting diodes, photovoltaics and sensors, as antistatic coatings or as active material in electrochromic devices [1]. The main advantages of conducting polymers are their chemical stability, their flexibility and their biocompatibility as well as their processability on large areas at ambient temperature. One of the most studied conducting polymers is PEDOT:PSS (Poly(3,4-ethylenedioxythiophene):Polystyrene sulfonate) which shows a sufficiently high conductivity and transparency in the doped state and which is therefore used for a large variety of applications. However, its hygroscopy, acidity and poor wetting behavior on organic layers are huge drawbacks for the application and it is obvious that there is still a need for new types of transparent conducting polymers.
We will present a new approach to stabilize PEDOT in aqueous dispersions by replacing the polystyrenesulfonic acid with a non acidic anionic polymer, based on (trifluoromethylsulfonyl)imide (TSFI) side groups attached to a polystyrene backbone. This anionic polymer, which is known for its ion conducting properties [2], renders the design of an innovative PEDOT system possible. The PEDOT:Polyanion system with new chemical and physical properties was characterized regarding its pH, its rheological and swelling behavior, its doping, opto-electronic performance and long term stability. By a slight variation of the structure of the anionic polymer the influence of the nature of the polyelectrolyte on the characteristics of the final PEDOT:polyelectrolyte was investigated. In addition, the influence of the synthesis parameters on the properties named above was studied.
In order to demonstrate the potential of this new polymer complex, the highly transparent PEDOT:polyanion films (238 Ohm.sq-1 at 91% transmittance, σ>260 S.cm-1) were successfully integrated as transparent electrode in OLED and OPV devices [3] and as channel material in organic electrochemical transistors [4].
(1) Chandrasekhar, P. Conducting Polymers, Fundamentals and Applications: A Practical Approach; Springer Science & Business Media, 2013.
(2) Meziane, R.; Bonnet, J.-P.; Courty, M.; Djellab, K.; Armand, M. Electrochimica Acta 2011, 57, 14–19.
(3) Hofmann, A. I.; Smaal, W. T. T.; Mumtaz, M.; Katsigiannopoulos, D.; Brochon, C.; Schütze, F.; Hild, O.R.; Cloutet, E.; Hadziioannou, G. Angew. Chem. Int. Ed. 2015, 54 (29), 8506–8510.
(4) Inal, S.; Rivnay, J.; Hofmann, A.I.; Uguz, I.; Mumtaz, M.; Katsigiannopoulos, D.; Brochon, C.; Cloutet, E.; Hadziioannou, G.; Malliaras, G.G. J. Polym. Sci. B Polym. Phys. 2015, accepted (DOI:10.1002/polb.23938).
2:45 PM - MD5.10.02
Using Alternating Electric Fields to Tune the Molecular Structure and Electrical Properties of Solution-Based Large-Area Organic Semiconductor Thin Films
Francisco Molina-Lopez 1,Xiaodan Gu 1,Hongping Yan 2,Michael Toney 2,Zhenan Bao 1
1 Stanford Univ Stanford United States,2 SLAC National Accelerator Laboratory Stanford United States
Show AbstractSolution-based organic semiconductors (OSC) present a unique opportunity in the manufacture of novel electronic circuits for emerging applications requiring low cost and large area. Among the different OSC processed from solution that have been reported, those based on small-molecule crystalline thin films present the greatest potential to achieve high mobility values, able to compete with alternative inorganic technologies such as amorphous Si or even polysilicon. However, a tight control of the morphology of small-molecule OSC materials is mandatory to optimize their electrical performance. Concretely the crystalline structure and crystal orientation play a main role in the charge carrier transport of small-molecule OSC crystals when employed in thin film field effect transistor (FET) structures. In this work, we propose a method based on the application of alternating electric fields to a drying solution of 2,7-Dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) to tune its final crystalline structure and thus its electrical behavior. C8-BTBT has raised a wide interest among researchers in the last years due to its good solubility in common solvents, potential high mobility and superior stability in ambient conditions. Solution-shearing has been used to cast well oriented crystalline thin films of C8-BTBT of few square cm of surface area, under the influence of an alternating electric field at different frequencies. Such field, applied between the shearing blade and the substrate, is believed to move the C8-BTBT molecules before crystallization, defining packing motifs different from the normal stable ones. Grazing incident x-ray diffraction (GIXD) has been employed to reveal the appearance of different polymorphs as a result of the mentioned electric field. A correlation between these new polymorphs with the employed field frequency and the FET mobility of the material along both, the axis parallel and perpendicular to the shearing direction, was observed. We made use of the theory of dielectrophoresis to model and explain our experimental observations. The proposed method permitted us to find the right excitation frequencies leading to a final improved FET mobility (of several folds in some cases) for every specific in-plane direction. This technique could be expanded to other OSC molecules, opening new roads in the search of polymorphs for high-performing printed organic electronic circuits.
3:00 PM - MD5.10.03
Tuning the Electronic and Structural Properties of Terpolymers for Transistors and Photovoltaic Cells
Kirsty Leong 1,Nicholas Myllenbeck 1,Michael Foster 1,Joseph Cordero 1
1 Sandia National Labs Livermore United States,
Show AbstractThe effects of polymer composition, monomer sequence, and linker identity on the electronic, physical, and transistor properties of donor-acceptor polymers incorporating dithienyldiketopyrrolopyrrole (DPP) and dithienylbenzothiadiazole (BTDT) are investigated. In comparison to the perfectly alternating analog, the stoichiometric equivalent random terpolymer exhibits an increased absorbance on-set wavelength and bandwidth. This trend is supported computationally; moreover, it was identified that a minimum of four consecutive BTDT units are needed to achieve a narrow optical gap than the perfectly alternating analog. In addition, the mobility of these terpolymers in field-effect transistors increases as the ratio of DPP:BTDT increases. Meanwhile, bulk heterojunction solar cells using the terpolymers as the donor and PC61BM as the acceptor show an increase in power conversion efficiency as the ratio of DPP:BTDT decreases. These trends are attributed to variations in molecular packing and thin film morphology. The effect of the coordinating linker (ethylene, thiophene and dithiophene) has also been experimentally and computationally explored. By varying the polymer composition and linker identity, the crystallinity of thin film devices is effectively tuned. Finally, we observe reversible thermochromism that is sensitive to polymer composition and linker identity. This behavior is the subject of future study on the molecular engineering of thermochromic, optoelectronic switches.
3:15 PM - MD5.10.04
Fluxional Pi-Electronics: Attenuated Aromaticities and Spins in Organic Electronic Materials
Benjamin Streifel 1,Justin DeFrancisco 1,Garvin Peters 1,John Tovar 1
1 Johns Hopkins Univ Baltimore United States,
Show AbstractPi-conjugated polymers currently employed for cutting-edge device applications are typically constructed through careful bandgap engineering whereby the electronic properties of the constituent monomers are built into the material target, thus leading to a desired optical bandgap, planarity, etc. This talk will highlight recent studies to transiently engineer high degrees of polarizability and delocalization into organic semiconductors. We take advantage of two specific molecular modifications that enable the realization of photon-triggered electrocyclizations and open-shell diradical characters that are postulated to invoke unusual aromatic topologies beyond the classical Hückel notion. The ability to manipulate local oligomer or polymer electronics could allow for a new direction to bandgap engineering within organic semiconductors.
3:30 PM - MD5.10.05
Mechanical Properties of Organic Semiconductors for Mechanically Stable and Intrinsically Stretchable Devices
Darren Lipomi 1,Suchol Savagatrup 1,Adam Printz 1
1 Univ of California-San Diego La Jolla United States,
Show AbstractThis presentation describes my group’s efforts to understand the molecular and microstructural basis for the mechanical properties of organic semiconductors for organic photovoltaic (OPV) devices. The first goal is to mitigate mechanical forms of degradation of printed modules during roll-to-roll fabrication, installation, and environmental forces—i.e., wind, rain, snow, and thermal expansion and contraction. Mechanical stability is a prerequisite for inexpensive processing on flexible substrates: to encapsulate devices in glass is to surrender this advantage. The second goal is to enable the next generation of ultra-flexible and stretchable solar cells for collapsible, portable, and wearable applications, and as low-cost sources of energy—“solar tarps”—for disaster relief and for the developing world. It may seem that organic semiconductors, due to their carbon framework, are already sufficiently compliant for these applications. We have found, however, that the mechanical properties occupy a wide range of values, and can be difficult to predict from molecular structure alone. We are developing an experimental and theoretical framework for how one can combine favorable charge-transport properties and mechanical compliance in organic semiconductor films. In particular, we have explored the roles of the backbone, alkyl side chain, microstructural order, the glass transition, molecular packing with fullerenes, plasticizing effects of additives, extent of separation of [60]PCBM and [70]PCBM, structural randomness in low-bandgap polymers, and reinforcement by encapsulation, on the mechanical compliance. We are exploring the applicability of semi-empirical “back-of-the-envelope” models, along with multi-scale molecular dynamics simulations, with the ultimate goal of designing electroactive organic materials whose mechanical properties can be dialed-in. We have used the insights we have developed to demonstrate several new applications for OPV that demand extreme compliance, including biaxial stretching and conformal bonding of whole devices to hemispheres, and devices with ultrathin encapsulation mounted on human skin that survive significant cyclic mechanical deformation outdoors
3:45 PM - MD5.10.06
Ultra-Flexible Organic Amplifier System using Pseudo-CMOS Circuits for a Wireless Biosignal Detection
Takafumi Uemura 2,Takafumi Matsumoto 1,Masaya Kondo 2,Teppei Araki 2,Shusuke Yoshimoto 2,Tsuyoshi Sekitani 2
1 ISIR, Osaka University Ibaraki Japan,2 Graduate School of Engineering, Osaka University Suita Japan,1 ISIR, Osaka University Ibaraki Japan
Show AbstractWe fabricated an ultra-flexible voltage amplifier system using pseudo-CMOS inverters based on p-type organic transistors. The organic amplifier system with 36-dB signal gain can be combined with a newly developed wireless 8-channel signal monitoring module, which is driven by a 3.7 V battery. Total size and weight of the module are 7 mm * 5 cm * 10 cm and 30 g, respectively. The whole system is directly attachable to wide variety of surface, which is suitable for monitoring versatile biosignals such as electromyogram, electrocardiogram and electroencephalogram.
Organic thin-film transistors (OTFTs) are promising electrical components for biomedical applications because of their unique properties for lightweight, mechanically flexible and cost effective processability. In order to realize a precise biosignal detecting, flexible and large area electrodes should be tightly placed to the target biological surface. In addition, it is desired to amplify the detected signals nearby the electrodes for minimize background noise. For this reason, ultra-flexible organic amplifier circuits with biocompatible electrodes is suitable for a reliable biosignal sensing [1,2,3].
In this study, we have developed ultra-flexible organic amplifier system, which can be driven by 3.7 V battery. The organic amplifier circuits are realized based on organic pseudo-CMOS inverters. The pseudo-CMOS inverter is fabricated on an ultra-flexible 3-µm-thick parylene film. The Al gate electrodes are deposited by vacuum evaporation and anodized to form an 18-nm-thick AlOx layer. The surface of AlOx is passivated by self-assembled monolayer, and a 30-nm-thick dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene (DNTT) is vacuum deposited on the AlOx/SAM insulator. Top contact Au source/drain electrodes are also deposited by vacuum evaporation. The individual transistors can be operated at 3 V with high yield because of the anodization process. The mobility is 1.7 cm2/Vs and the on/off ratio is 106. The amplifier system has 36-dB signal gain to amplify the 20 mV sine wave at 10 Hz. The whole amplifier system with sensing electrodes can be combined with a newly developed wireless signal processing module. We will demonstrate a biosignal monitoring using the developed wireless system with organic amplifier circuits.
References
[1] M. Kaltenbrunner, T. Sekitani et al., Nature 2013, 499, 458.
[2] H. Fuketa, K. Yoshioka et al., IEEE Int. Solid-State Circuits Conf. 2013, 56, 104.
[3] T. Yokota, T. Sekitani et al., IEEE Trans. Electron Devices 2012, 59, 3434.
4:15 PM - MD5.10.07
Photoactive C3N3P Carbon Phosphonitride Semiconductor
Albert Epshteyn 1,Brian Chaloux 1,James Yesinowski 1,Evan Glaser 1
1 U.S. Naval Research Laboratory Washington United States,
Show AbstractWe recently reported that neat phosphorus tricyanide, P(CN)3, quantitatively condenses on mild heating in the solid state to a black material, providing a direct path for the synthesis of carbon phosphonitrides with a C3N3P stoichiometry. Employing IR, Raman, and NMR spectroscopic analysis, we showed that this new material exhibits structural motifs analogous to other carbon nitrides of the C3N4 type.1 The black color of C3N3P was discovered to arise from a well-defined optical band gap, typical of inorganic semiconductors. In this work, we discuss our approach to producing films of this new carbon phosphonitride, and to the generation of photocurrent in C3N3P under visible light irradiation. Electron paramagnetic resonance (EPR) spectroscopy is utilized to investigate the generation and propagation of stable free radicals through this material. We also employ electron microscopy, electrochemical analysis, and spectroscopic techniques to evaluate this new semiconductor.
References
(1) Chaloux, B. L.; Yonke, B. L.; Purdy, A. P.; Yesinowski, J. P.; Glaser, E. R.; Epshteyn, A. Chem. Mater. 2015, 27, 4507.
4:30 PM - MD5.10.08
The Dependence of the Lifetime of Organic Photovoltaic Cells on Morphology and Molecular Stability
Quinn Burlingame 1,Stephen Forrest 1
1 Univ of Michigan Ann Arbor United States,
Show AbstractUsing Fourier transform infrared spectroscopy, differential scanning calorimetry, and x-ray diffractometry, we explore the morphological and photochemical stability of the individual layers comprising planar-mixed small molecular weight organic solar cells. The cells employ a tetraphenyldibenzoperiflanthene (DBP)/C70 donor/acceptor heterojunction, with a bathophenanthroline (BPhen) exciton blocking layer.[1] While the stabilities of the donor (DBP) and acceptor (C70) are comparable, low glass-transition temperature BPhen readily crystallizes, resulting in a rapid decrease in the open-circuit voltage during aging. Replacing the neat BPhen buffer with an electron-filtering blended buffer[1] improves the morphological stability of the layer, completely eliminating decreases in the open-circuit voltage over time. Replacing BPhen in the blended buffer with a high glass-transition temperature molecule extends the device lifetime even further, such that it operates above 80% its starting efficiency for more than 2500 hr (equivalent to 1.4 yr under normal solar operation conditions) under constant 1 sun illumination, even at high temperatures (130oC). Surprisingly, we find no thermal activation of the device degradation indicating complete stability of the blend layer morphology. The results suggest that performance instabilities can be mitigated with appropriate choices of materials and device architectures, and may provide a path to organic photovoltaic devices whose lifetime meets conventional solar energy harvesting application standards.
[1] X. Xiao, K. J. Bergemann, J. D. Zimmerman, K. Lee, and S. R. Forrest, “Small-Molecule Planar-Mixed Heterojunction Photovoltaic Cells with Fullerene-Based Electron Filtering Buffers,” Adv. Energy Mater., vol. 4, no. 7, p. 1301557, May 2014.
4:45 PM - MD5.10.09
Ultra-Thin Parylene Gate Insulator for Low-Voltage-Operating Organic Transistor Circuits
Masaya Kondo 1,Takafumi Uemura 1,Takafumi Matsumoto 1,Teppei Araki 1,Shusuke Yoshimoto 1,Tsuyoshi Sekitani 1
2 Graduate School of Engineering Osaka University Osaka Japan,1 The Institute of Scientific and Industrial Research, Osaka University Osaka Japan,1 The Institute of Scientific and Industrial Research, Osaka University Osaka Japan
Show AbstractWe have developed a high-yield fabrication process for organic thin-film transistors (OTFTs) with ultra-thin parylene insulators. In a top-contact bottom-gate structure, the proper selection for gate metal is critical for the yield of transistor circuits. The transistor circuits with ~30 nm parylene insulator on Cr gate electrodes are driven within 3 V. In addition, the yield of circuits is over 95 %, which includes 240 transistors inside a few-µm-thick 7*7 cm2 flexible substrate.
OTFTs have advantages in terms of lightweight, flexibility and compatibility with low-temperature and low-cost fabrication processes on flexible substrates. Especially, due to their unique properties, OTFTs are expected to be applied to highly-functional biomedical devices, such as medical catheters covered with ultra-thin functional sensor arrays1. In the use of such a medical application, low-voltage operation of circuits is essential for the safety aspect. Although various efforts have been made to reduce the operating voltages for OTFTs, it has been still challenging to achieve a few voltage operation for OTFTs with ultra-thin gate insulators2-3. Because the OTFTs with thinned gate insulator usually lead to a bad yield.
In this study, we have developed a high-yield process for ultra-thin parylene gate insulator. In the evaluation for the yield of transistors, we have fabricated top-contact bottom-gate transistors with ultra-thin parylene gate insulators. In the device fabrication, each layers were stacked on a few-µm-thick ultra-flexible films on carrier glass substrates. In order to compare the effect of gate metals to the quality of parylene insulator, Au or Cr gate electrodes were vacuum deposited as gate electrodes. As a gate insulator, we deposited ~30 nm parylene film by chemical vapor deposition (CVD). DNTT and Au source/drain electrodes were deposited on the ultra-thin parylene insulators. In the device characterization, high leakage currents were observed on the Au gate devices, on the other hand, the leakage currents were normally less than 1 nA on the Cr gate devices. This result show that the initial growth mode for parylene film is different between the surface of Au and Cr and pinhole-free and ultra-thin parylene films can be formed on the surface of Cr electrode. In addition, we evaluated the yield of circuits quantitatively by measuring ring oscillators which are fabricated on the same substrates by the same processes. As a result, the yield of 5-stage ring oscillators was over 95 %, which includes 240 transistors inside a few-µm-thick 7*7 cm2 flexible substrate. The developed process is promising technology to realize highly-reliable and imperceptible biomedical applications because the parylene deposition by CVD is very simple process, which can be applied to large area surfaces.
1. T. Sekitani, et al. Nature Mater., 9, 1015–1022 (2010).
2. H. Klauk, et al. Nature 445, 745-748 (2007).
3. M. P. Walser, et al. Appl. Phys. Lett., 95, 233301 (2009).
5:00 PM - MD5.10.10
Direct Observation of Bound Multiexciton States in Organic Semiconductor Films and Their >100% Charge Transfer Efficiency
Chaw Keong Yong 1,Andrew Musser 1,John Anthony 3,Richard Friend 1,Jenny Clark 2,Henning Sirringhaus 1
1 Univ of Cambridge Cambridge United Kingdom,3 Department of Chemistry University of Kentucky Lexington United States2 The Department of Physics and Astronomy University of Sheffield Sheffield United Kingdom
Show AbstractThe standard view of singlet exciton fission in organic semiconductor is that one photon creates a singlet exciton which subsequently decays into a correlated triplet pair state (TT) multiexciton states. The triplet pair state then splits to form two free triplets. Although the theoretical description of (TT) is well developed since 1970, it has so far proved difficult to determine the role and nature of the (TT) state in solid films from experiment directly. Here, using a combination of highly sensitive broadband transient absorption and photoluminescence spectroscopies on a range of polyacene films, we demonstrate that the (TT) multiexciton states is bound and energetically stabilised with respect to free triplets in even the most efficient singlet fission materials, such as TIPS-pentacene and pentacene. The (TT) multiexciton state is emissive, and we find that charge-transfer from one (TT) state to the neighboring electron acceptors has a yield of >100%, i.e. more than one charge is transferred per charge-transfer event. Our findings suggest that the formation of spin-correlated (TT) states emits as one particle and generates 2 charges in organic solar cells and thus open a range of fascinating questions regarding the potential to use entanglement to enhance organic photovoltaic efficiency and the application of organic materials in quantum information.
5:15 PM - MD5.10.11
Time-and-Temperature-Independent Local Carrier Mobility and Effects of Regioregularity in Polymer-Fullerene Organic Semiconductors
Meng-Ju Sher 1,Jonathan Bartelt 1,Tim Burke 1,Michael McGehee 1,Alberto Salleo 1,Aaron Lindenberg 3
1 Department of Materials Science and Engineering Stanford University Stanford United States,1 Department of Materials Science and Engineering Stanford University Stanford United States,2 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory Menlo Park United States,3 PULSE Institute, SLAC National Accelerator Laboratory Menlo Park United States
Show AbstractOrganic semiconducting polymers exhibit complex microstructure and disorder over a wide range of length scales, which in turn directly impact the electronic transport properties relevant for the development of organic photovoltaic devices. Typical transport measurements blur together intermolecular and intramolecular conduction processes, making it difficult to develop an understanding of how disorder on different length-scales impacts device performance. This work uses time-resolved terahertz (THz) spectroscopy to selectively study the intramolecular carrier transport in poly(3-hexylthiophene) (P3HT) polymers optimized for solar cell performance. At 2 THz, P3HT polymers with different side chain regularity have fast THz carrier mobility ranging from 0.1 to 0.4 cm2/V/s, and the mobility dependence on the side chain regularity is much smaller than studied by other macroscopic mobility measurements. The fast local carrier mobility allows us to understand how the efficiency of charge-transfer state splitting is high in these polymers and shows that local disorder does not strongly impact intramolecular transport processes. Moreover the local carrier mobility does not exhibit changes with temperature or time within the 1 ns duration probed, indicating that local carrier transport is always fast. To fully understand the differences in time and temperature dependence on carrier transport at different length scales, a transport model that separates local transport from intermolecular carrier hopping is required.
5:30 PM - MD5.10.12
Assessing Stability of High Performance Organic Solar Cells
Rongrong Cheacharoen 1,William Mateker 1,Dylan Sarkisian 1,Bin Kan 2,Qian Zhang 2,John Love 3,Xiaofeng Liu 3,Jiang Kui 4,Henry Yan 4,Yongsheng Chen 2,Thuc-Quyen Nguyen 3,Guillermo Bazan 3,Michael McGehee 1
1 Materials Science and Engineering Stanford University Stanford United States,2 Nankai University Tianjin China3 University of California Santa Barbara Santa Barbara United States4 Chemistry Hong Kong University of Science and Technology Kowloon Hong Kong
Show AbstractSolution-processed small molecules (spSMs) are promising solar cell materials for commercialization because of their narrow polydispersity index, ease of purification, and power conversion efficiencies (PCEs) exceeding 10%. However, knowledge of spSMs:fullerene bulk heterojunction solar cells (SM BHJ) lifetime is limited. We present the first degradation study of six SM BHJ solar cells with high PCEs from 7-10%. We also compare the lifetime of SM BHJ with 10% PCE PffBT4T-2OD polymer: fullerene BHJ solar cells.
We monitor performances of the solar cells held at maximum power point under constant illumination of 1 sun in an inert atmosphere beyond 3000 hours. We observe a similar degradation profile for SM BHJ and the polymer solar cells with an initial, exponential “burn-in” decay followed by a linear degradation. In SM BHJ, most of the degradation is in fill factor (FF) and short-circuit current, while the open circuit voltage is relatively stable. While the magnitude of burn-in loss varies, the burn-in stops around 1000 hours for all SM BHJ and the polymer solar cells. Post burn-in PCEs for the SM BHJ range from 2.6% to 4.1%. The post burn-in PCE for PffBT4T-2OD is 5.8%, which is higher than that of P3HT and the 20-year lifetime PCDTBT. The slopes of the linear degradation of the SM BHJ range from -3.6x10-5 to -5.8x10-5%/ hour while the slope for PffBT4T-2OD is -1x10-5% /hour. Defining lifetime as the time for PCE to decrease 20% after burn-in and assuming 5.5 hours of sun per day, the SM BHJ could last 1.7 to 2.8 years and the PffBT4T-2OD 10 years.
We decouple the dark and light degradation by thermally aging a second set of solar cells in the dark under inert atmosphere at 70°C, the temperature that the solar cells experience under our simulator. For SM BHJ we observe the burn-in loss mostly comes from photo degradation, while for PffBT4T-2OD both photo and thermal degradation contribute equally to burn-in. After about 500 hours, the slopes of the linear degradation portion for the solar cells thermally aged in the dark and under 1 sun are the same. This suggests that the long-term degradation for all high performance materials we have tested is controlled by a thermal process.
We attempt to reduce burn-in degradation in two of the SM BHJ we studied, p-DTS(FBTTh2)2 (T1) and X2. First, we thermally anneal the active layers to induce a more ordered film morphology. Using this strategy, we are able to reduce burn-in by 80% in T1 and 5% in X2. However, doing so reduces the initial PCE of T1 from 7.6% to 5.1% and X2 from 7.0% to 5.1%. Second, we use a more stringent purification protocol with the X2 molecule. Both purified and unpurified solar cells start with similar initial 7% PCE, though the FF of the purified solar cells improves by over 10%. More interestingly, after 1000 hours the purified X2 solar cells are 40% more efficient than the unpurified ones (4.9% vs 3.4% PCE). Further purification of X2 molecules is ongoing.
5:45 PM - MD5.10.13
Novel Functional Conjugated Polymers Derived from a Common Set of Enediyne Building Blocks
Yang Qin 1
1 Univ of New Mexico Albuquerque United States,
Show AbstractOrganic conjugated materials, including small molecule dyes, conjugated polymers and allotropes of carbon, have found widespread applications in modern electronic devices such as biological sensors, transistors, memory devices, lasers, and solar cells. Each application places unique and sometimes stringent requirements on the properties of the materials applied. Discoveries of new materials and properties can enhance performance of existing devices and lead to emerging applications. Thus, new synthetic methodologies for novel materials and basic understanding on the structure-property relationships are greatly sought after.
We have developed facile synthetic methodologies for a set of trans-enediyne (EDY) molecules containing various aromatic moieties directly attached to the central double bonds. These EDYs can serve as versatile building blocks for a variety of functional conjugated polymers and we discuss our efforts in the following areas: (i) the first solution synthesis of polydiacetylenes (PDAs) using a newly developed acyclic enediyne metathesis (AEDMET) polymerization technique;1 (ii) the first examples of polytriacetylenes (PTAs) that contain aromatic groups directly attached to the polymer main-chains, through Glaser-Hay and Pd-catalyzed oxidative coupling reactions; (iii) Pt-segmented PDAs that contain cross-conjugated chromophores, differing from conventional Pt-bisacetylenide polymers;2 and (iv) boron-segmented polyacetylenes (BPAs), representing the first main-chain boron-containing conjugated polymers without aromatic groups. We will present in detail the synthesis and characterization of these materials, analyze their structure-property relationships and discuss their potential applications in optoelectronic devices.
1. Hu, K.; Yang, H.; Zhang, W.*; Qin, Y.* Chem. Sci. 2013, 4, 3649.
2. Hu, K.; Pandres, E.; Qin, Y.* J. Polym. Sci. A Polym. Chem. 2014, 52, 2662.