Symposium Organizers
Jiangeng Xue University of Florida
Chihaya Adachi Kyushu University
Russell J. Holmes University of Minnesota
Barry P. Rand IMEC vzw
S1: Organic Light-emitting Devices I
Session Chairs
Monday PM, November 30, 2009
Room 210 (Hynes)
9:30 AM - **S1.1
White OLEDs as General Lighting.
Junji Kido 1 2
1 Organic Device Engineering, Yamagata University, Yonezawa, Yamagata, Japan, 2 , Research Institute for Organic Electronics, Yonezawa, Yamagata, Japan
Show AbstractThe performance of white-light-emitting OLEDs have been steadily improved and, today, they are considered to be the light source of the next generation. In this talk, the recent development of high performance OLEDs will be discussed. High quantum efficiencies can be obtained by using phosphorescent emitters such as iridium complexes. It is important to use wide-energy-gap materials as the host and carrier transport materials to confine the triplet excited energy of the phosphorescent emitter. We synthesized a variety of wide gap materials and succeeded to fabricate extremely high efficiency OLEDs. External quantum efficiency (QE) of 25—30% was achieved for blue, green and red OLEDs, which correspond to the internal QE of nearly 100%. Device lifetime has been significantly improved by using the multiphoton structure. For a white multiphoton OLED, lifetime of over 30,000 hours with the initial luminance of 5000 cd/m2 was achieved, which is long enough for the general lighting applications. Regarding the long-life OLEDs, we recently developed a simple method to fabricate OLEDs having graded organic/organic interfaces to improve lifetime. The newly developed method employs the in-line vacuum evaporator. Operating lifetime for the device with the graded structure appears to be longer than that of the conventional hetero-junction structure. By combining the above techniques, OLEDs can be extremely efficient and possess extremely long lifetime. Luminaires using high performance white OLEDs will be intriduced.
10:00 AM - S1.2
Thermally Activated Delayed Fluorescence From Sn4+-Porphyrin Complexes and Their Application to Organic Light Emitting Diodes – A Novel Mechanism for Electroluminescence.
Ayataka Endo 1 , Mai Ogasawara 2 , Takahashi Atsushi 3 , Yokoyama Daisuke 1 , Kato Yoshimine 2 , Adachi Chihaya 1
1 Center for Future Chemistry, Kyushu University, Fukuoka Japan, 2 Department of Materials Science and Engineering, Kyushu University, Fukuoka Japan, 3 , Sogo Pharmaceutical Co., Ltd., Fukushima Japan
Show AbstractWe propose a novel electroluminescence mechanism of thermally activated delayed fluorescence (TADF) to achieve compatibility of harvesting both singlet and triplet excitons and to avoid of triplet annihilation. In TADF materials, heat accelerates reverse intersystem-crossing (ISC) from a triplet excited state (T1) to a singlet excited state (S1), leading to an increase of the fluorescence intensity. We searched for highly efficient TADF materials and found tin (IV) fluoride-octaethylporphine (SnF2-OEP) showing rather intense TADF. We fabricated polymer dispersed films which contain SnF2-OEP as a dopant in the emitting layer, 2wt%-SnF2-OEP: poly(vinylcarbazole) (PVCz). Intense fluorescence and TADF were observed at 570nm while the weak room temperature phosphorescence was also observed at 703nm. The quantum efficiencies of each process were carefully estimated using an integrated sphere photoluminescence measurement system. A prompt fluorescence efficiency (ηF) of 0.6% was observed between T=5 K and 300 K and then decreased slightly over T=300 K, and resulted in ηF=0.4% at T=400 K. In a similar manner, the phosphorescence quantum efficiency (ηPHOS) displayed a gradual decrease over T=300 K. On the other hand, the quantum efficiency of delayed fluorescence (ηTADF) increased steeply from 0.6% at T=300 K to 2.4% at T=400 K. Thus, the overall photoluminescence quantum efficiency was significantly enhanced over T=300 K.TADF was also confirmed in the ITO/PEDOT/2 wt%-SnF2–OEP:PVCz/MgAg/Ag device under electrical excitation. Under application of short electrical pulse excitation, prompt and delayed electroluminescence components were clearly observed. The delayed component was composed of both TADF and phosphorescence, and the TADF component significantly increased with an increase of the temperature.Finally, we discuss the rate constant of reverse ISC (kRISC) in SnF2-OEP. It was found that kISC(intersystem crossing from S1 to T1), kR(radiative decay rate from S1) and kP (phosphorescence decay rate) are independent of temperature, while kRISC(reverse ISC from T1 to S1) exhibited strong temperature dependence. Although kRISC has a very low value of 5×101 at T=300 K, the phosphorescence decay rate of kP=0.14 is significantly lower than kRISC, which resulted in an appreciable TADF, even at T=300 K. At T=400 K, kRISC was further enhanced up to 5.5×102, resulting in intense TADF. However, the kRISC value is still too small to induce efficient up-conversion. Since kRISC should be proportional to exp(–ΔEST/kBT), the Arrhenius plot of kRISC versus 1/T was examined. The activation energy was estimated to be ΔEST=0.24 eV, which corresponds to the difference in the energy levels between T1 and S1. Thus, in order to increase kRISC, it is requisite to obtain a smaller energy gap.
10:15 AM - S1.3
Synthesis and Characterization of Cyclometalated Platinum Complexes for Displays and Lighting Applications.
Jian Li 1 , Eric Turner 1 , Nathan Bakken 1 , Zixing Wang 1
1 School of Materials and Advanced Photovoltaics Center, Arizona State University, Tempe, Arizona, United States
Show AbstractThe development of new light-emitting materials has attracted great attention over the past decade. The square planar platinum complexes have attracted a great deal of interest as phosphorescent emitters for potential application in OLEDs for their potential to harvest both electrogenerated singlet and triplet excitons and achieve 100% internal quantum efficiency. Such examples include platinum(II)[2-(4’,6’-difluorophenyl)pyridinato-N, C2’)](2,4-pentanedionato) (FPt) and 1,3-difluoro-4,6-di(pyrid-2-yl)-benzene platinum chloride (Pt-4).In this presentation, we will discuss our continuing efforts on the design, synthesis and characterization of novel platinum complexes for displays and lighting applications. The emission color of platinum complexes can be tuned over a large range from deep blue to red, by varying the functional groups of platinum complexes, thus giving prospects to full-color display. The photo-physics, electrochemistry and electroluminescent properties of these novel platinum complexes, including fluorine-free Pt-based deep blue emitters, will be discussed.
10:30 AM - S1.4
Hybrid Organic-inorganic Light Emitting Diodes.
Michele Sessolo 1 , Hicham Brine 1 , Henk Bolink 1
1 Instituto de Ciencia Molecular, University of Valencia, Valencia Spain
Show AbstractRecently, transition metal oxides (TMO) have been employed as charge transport and particularly as electron injection layers in hybrid organic-inorganic light emitting diodes (HyLEDs), demonstrating the possibility to prepare air-stable electroluminescent devices.[1, 2] The use of an unreactive metal oxide as the cathode is appealing as this would allow the preparation of OLEDs having no or only a simple encapsulation, hence significantly reducing the cost. Such a cost reduction greatly enables the use of OLEDs in display and especially lighting applications.Promising luminance values, >30000 cd/m2, at low voltages have been reported for F8BT based HyLEDs.[1-3] A simple model was described that showed that the efficiency is determined by the device ability to block holes at the metal oxide-organic light emitting interface.[4] The use of thin interlayers of Cs2CO3 that are deposited in between the metal oxide and the organic luminescence layer, effectively block the holes and increases the device efficiencies.[5, 6] Additionally, with this interlayer wider bandgap light emitting materials can be used in allowing for the achievement of white light emitting HyLEDs.[7] We will report on further advances in HyLEDs obtained using solution processed interlayers and phosphorescent emitters. The possibility of processing from solutions together with an increased efficiency improves the versatility of the devices making this class of devices a real alternative to the established OLEDs.[1]K. Morii, M. Ishida, T. Takashima, T. Shimoda, Q. Wang, M. K. Nazeeruddin, M. Graetzel, Appl. Phys. Lett. 2006, 89, 183510.[2]H. J. Bolink, E. Coronado, D. Repetto, M. Sessolo, Appl. Phys. Lett. 2007, 91, 223501.[3]D. Kabra, M. H. Song, B. Wenger, R. H. Friend, H. J. Snaith, Adv. Mater. 2008, 20, 3447.[4]H. J. Bolink, E. Coronado, D. Repetto, M. Sessolo, E. Barea, J. Bisquert, G. Garcia-Belmonte, J. Prochazka, L. Kavan, Adv. Funct. Mater. 2008, 18, 145.[5]K. Morii, T. Kawase, S. Inoue, Appl. Phys. Lett. 2008, 92, 213304.[6]H. J. Bolink, E. Coronado, J. Orozco, M. Sessolo, Adv. Mat. 2009, 21, 79.[7]H. J. Bolink, E. Coronado, M. Sessolo, Chem. Mater. 2009, 21, 439.
10:45 AM - S1.5
High Efficiency and Low Roll Off Blue Phosphorescent OLEDs using Mixed Host Architecture.
Neetu Chopra 1 2 , James Swensen 1 , Evgueni Polikarpov 1 , Lelia Cosimbescu 1 , Daniel Gaspar 1 , Franky So 2 , Asanga Padmaperuma 1
1 Energy and and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington, United States, 2 Material Science & Engineering, University of florida, Gainesville, Florida, United States
Show AbstractOrganic light-emitting diodes (OLEDs) are a very promising candidate for solid state lighting and display applications. Besides the obvious advantages associated with this technology such as ease of processing and low manufacturing cost, recent advances in efficiency and lifetime of these devices have ensured a bright future for this technology. For commercializing white OLEDs for lighting or components of a full color display, it is very important to achieve high efficiency red, green and blue OLEDs. Hereby, we demonstrate a highly efficient blue phosphorescent organic light emitting diode (PHOLED) with very low efficiency roll off based on a mixed host layer approach. The devices were fabricated using a mixed layer of di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC), a hole transport material and 2,8-bis(diphenylphosphoryl)dibenzothiophene (PO15), an electron transport material, as the host layer doped with blue phosphor iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C2’]picolinate (FIrpic). Using a mixed layer as host allowed us to achieve better charge balance in the device. Charge balance is a very important factor in device operation of OLEDs; not only does it influence the peak efficiency in the device but also greatly affects the EQE roll off. Usually in an OLED device, the hole mobility of the hole transport layer (HTL) is much higher than the electron mobility of the electron transport layer (ETL). Hence, most devices are not charge balanced. This leads to lower efficiency and lifetime. Apart from the HTL and ETL, host also plays a very important role in determining the overall charge balance in the device. Most commercially available host materials such as 3,5'-N,N'-dicarbazole-benzene (mCP) are not ambipolar and have poor transport properties which makes it difficult to achieve charge balance in the emissive zone. In this study we used a mixed host approach to achieve charge balance in the blue PHOLED devices with FIrpic. This mixed host layer exhibits bipolar transport characteristics which help in improving the charge balance and lower operation voltage in the device. The ratio of TAPC and PO15 was varied in the EML and optimized to achieve charge balance. Hence using this approach we were able to achieve a high peak efficiency of 52 lm/W at 60 cd/m2. The details of optimization and understanding of the underlying device physics for these mixed host systems will be discussed.
11:30 AM - **S1.6
Printing Nanostructures for LEDs and MEMS.
Vladimir Bulovic 1 , Corinne Packard 1 , Jennifer Yu 1 , Apoorva Murarka 1
1 , MIT, Cambridge, Massachusetts, United States
Show AbstractWe demonstrate new methods for solvent-free deposition and patterning of thin film of semiconducting molecular organic materials. The solvent-free deposition ensures the quality of the printed material, minimizing morphological changes due to solvent presence and eliminating solvation-induced energetic chances in excitonic and electronic levels. The printing techniques are applied to micron-scale patterning of organic LED active layers and contacts, and are the basis for a new technique for fabricating large area MEMS structures.
12:00 PM - S1.7
Analysis of Metal-oxide-based Charge Generation Layers Employed in Stacked Organic Light-emitting Diodes.
Xiangfei Qi 1 , Ning Li 1 , Stephen Forrest 1 2 3
1 Physics, University of Michigan, Ann Arbor, Michigan, United States, 2 Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan, United States, 3 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractWe report on a thermally assisted tunneling model that accurately describes carrier generation in metal-oxide-based compound charge generation layer (CGL)(1) widely used in efficient stacked organic light-emitting diodes (SOLEDs)(2). Temperature-dependent current-voltage as well as capacitance-voltage measurements are performed in a series of hole-only and electron-only devices. The data suggest that hole injection occurs from a combination of tunneling injection through a barrier of (1.9±0.2)eV, assisted by a trap level of (0.07±0.01)eV above the MoO3 valence band maximum. Factors leading to improved carrier injection in the CGLs are analyzed. The CGL with 10nm-thick MoO3 and Li-doped 4,7-diphenyl-1,10-phenanthroline with 1:1 molar ratio, are demonstrated to have the highest generation efficiency of the various compound CGLs investigated. When engineered as a functional unit in SOLEDs, charge balance in each OLED component is the key to achieve a SOLED with high efficiency and brightness. Peak external quantum efficiencies of 10.5%, 5.3%, and 8.9% are observed for green electrophosphorescent OLEDs with ITO anode/CGL cathode, CGL anode/Al cathode, and conventional ITO anode/Al cathode, respectively. These results indicate that carrier injection from the CGL does not limit SOLED efficiency, yet the presence of the CGL strongly influences charge balance in the stacked elements. Optimized SOLED design and performance based on this analysis will be presented. (1)H. Kanno, R. J. Holmes, Y. Sun, S. Kena-Cohen, and S. R. Forrest, Adv. Mater. (Weinheim. Ger.) 18, 339 (2006).(2)X. Qi, M. Slootsky, and S. R. Forrest, Appl. Phys. Lett. 93, 193306 (2008).
12:15 PM - S1.8
Computational Design of Extremely High-efficient Blue Organic Light-emitting Diodes.
Changgua Zhen 1 , Yanfeng Dai 2 , Zhikuan Chen 2 , John Kieffer 1
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 , Institute of Materials Research and Engineering, Singapore Singapore
Show AbstractAlthough organic light-emitting diodes (OLEDs) using phosphorescent emitters (PhOLEDs) have shown ~20% external quantum efficiency (EQE), the efficiency roll-off at high current density is severe, which limits their application when high brightness is required, such as for outdoor lighting. While OLEDs using fluorescent emitters (FOLEDs) exhibit better efficiency roll-off due to shorter exciton lifetime and less bimolecular quenching, they can only use singlet excitons to convert electrical energy to light, which limits the EQE to maximally 5%. However, recent theoretical studies indicate that it is possible to achieve highly efficient FOLEDs because the singlet generation rate is faster than that for triplets, which leads to a higher singlet fraction, and thus conversion efficiencies, than indicated by the simple statistical limit. To realize such high efficiencies, we designed a series organic molecules bearing electron-donating and/or electron-withdrawing fragments and predicted their frontier orbital structures, energy levels, reorganization energies, exciton binding energies, singlet generation fractions using density functional theory (DFT) calculations. Accordingly, designed molecules possessing both electron-donating and electron withdrawing fragments have lower carrier injection barriers and more balanced hole-electron transport properties. Furthermore, the singlet generation fraction of the emitters was calculated to be about 50%, twice that predicted from classical statistics. Our simulation-based predictions have already been translated into experiment. By optimizing molecular structures via incorporating both electron-donating and electron-withdrawing groups, we attain a remarkable improvement in the maximum EQE of undoped device from 2.0% to 4.99%. Optimizing the thickness of the emitting layer and thermal annealing treatments raise the EQE to 7.40% for the undoped sky-blue device. Doping the emitters in a suitable host material, 4,4'-bis(carbazol-9-yl)biphenyl (CBP) at optimized concentrations, we achieve deep blue emission with a maximum EQE of 10.4% (at 47 mA/cm2, 3650 cd/m2) and CIE coordinates of (0.15, 0.09). Even at very high injection current density of 132 mA/cm2 (9170 cd/m2), the EQE remains at 9.3%, which is superior to any reported PhOLEDs.
12:30 PM - S1.9
Coherent Spin Dynamics of Carrier Generation and Recombination in Organic Semiconductor Devices.
Dane McCamey 1 , William Baker 1 , Kipp van Schooten 1 , Seo Young Paik 1 , Sangyun Lee 1 , Nicholas Borys 1 , John Lupton 1 , Christoph Boehme 1
1 Department of Physics, University of Utah, Salt Lake City, Utah, United States
Show AbstractOrganic semiconductors are materials of low atomic order number and consequently generally display weak spin orbit coupling. Charge carriers, either formed by dissociation of optical excitations or injection from electrodes, constitute well-defined spin-1/2 excitations, which are readily probed using electron paramagnetic resonance techniques. Carrier dissociation and recombination are inherently spin dependent, limiting, for example, the maximum electroluminescence quantum yield of a fluorescent OLED [1]. A complete understanding and ultimately control over these processes in organic devices necessitates suitably sensitive spectroscopic tools, which relate the inherently quantum mechanical nature of spin excitations to macroscopic observables such as a current.Spin mixing in organic semiconductors can be both irreversible (spin-lattice relaxation [1]) or reversible, given a coherent coupling to a driving field. A microwave field pulse can shuttle spin excitations from one state to another [2]: a bound carrier pair (charge transfer state or polaron pair) changes its spin multiplicity from singlet to triplet and back again [3]. Following this approach we have succeeded in demonstrating coherent control of the photocurrent in an organic device [3], yielding pronounced Rabi oscillations in the current.A more sophisticated analysis allows differentiation between electron and hole spin resonances. Both carrier species can precess in the resonance experiment. Provided that electron and hole have slightly different g-factors, a quantum mechanical beating of the spin precession occurs, which is revealed by anharmonic Rabi oscillations in an OLED. This effect, unobserved in other mesoscopic spin systems, arises from the substantial difference (~1 mT) in local hyperfine field which split the electron and hole resonance, but which is still experimentally accessible. Coherent spin spectroscopy of organic semiconductors therefore provides unique access to hyperfine fields, which are crucial in controlling magnetoresistive effects [4].[1] Reufer, M. et al. Nature Mater. 4, 340-346 (2005).[2] Boehme, C. & Lips, K. Phys. Rev. Lett. 91, 246603 (2003). [3] McCamey, D. R. et al. Nature Mater. 7, 723-728 (2008).[4] Lupton, J. M. & Boehme, C. Nature Mater. 8, 598 (2008).
12:45 PM - S1.10
Adamantane Derivatives as Host Materials for Efficient Deep-Blue Phosphorescent Organic Light Emitting Diodes.
Hirohiko Fukagawa 1 , Norimasa Yokoyama 2 , Shiro Irisa 2 , Shizuo Tokito 1
1 , NHK Science and Technology Research Laboratories, Tokyo Japan, 2 , Hodogaya Chemical Co. Ltd., Tsukuba Japan
Show AbstractBecause of their high emission efficiencies in comparison with conventional fluorescent organic light-emitting diodes (OLEDs), the OLEDs using phosphorescent dyes will be applied to future full-color displays and other illumination applications.[1] However, the realization of highly efficient deep-blue and white phosphorescent OLEDs has been difficult due to the lack of appropriate host material suitable for a deep-blue phosphorescent guest. There is a few host materials which have the high triplet energy needed to confine triplets in the guest.[2] Although p–bis(triphenylsilyly)benzene (UGH2) is widely used as a host material for the deep-blue and white phosphorescent OLEDs[3,4], the carrier transportability of UGH2 is poor. In addition, UGH2 is not suitable for solution process because of its low solubility. In this study, we have synthesized new host materials, which are adamantane derivatives, with a high triplet energy, high carrier transportability, high solubility and a high glass transition temperature. The carrier transportability of the hosts can be controlled by changing the substituent, i.e. both a hole transporting host and an electron transporting host have been successfully synthesized. Previously, we reported (i) a high efficient, deep-blue phosphorescent OLED by evaporation process and (ii) an efficient white phosphorescent OLED by solution process, using the conventional hole transporting host.[5,6] However, the driving voltage of these two devices were relatively high since UGH2 was used as a hole blocking and electron transporting layer. On the other hand, we have found that the driving voltage of the deep-blue OLED, consisting of two adamantane derivative hosts, is lower than that of previously reported one. The lowering of the driving voltage in this OLED is ascribed to the better electron transportability of the new electron transporting host than that of UGH2. [1] M. A. Baldo, et al., Nature 395, 151 (1998). [2] S. Tokito, et al., Appl. Phys. Lett. 83, 2459 (2003). [3] R. J. Holmes, et al., Appl. Phys. Lett. 83, 3818 (2003). [4] B. W. D’Andrade, et al., Adv. Mater. 16, 624 (2004). [5] H. Fukagawa et al., Appl. Phys. Lett. 93, 133312 (2008). [6] H. Fukagawa et al., Org. Electron., In press.
S2: Organic Light-emitting Devices II
Session Chairs
Monday PM, November 30, 2009
Room 210 (Hynes)
2:30 PM - **S2.1
Phosphorescent White OLEDs for Solid-state Lighting.
Sean Xia 1 , Peter Levermore 1 , Vadim Adamovich 1 , Chun Lin 1 , Raymond Kwong 1 , Michael Weaver 1 , Julie Brown 1
1 , Universal Display Corporation, Ewing, New Jersey, United States
Show AbstractWhite OLEDs (WOLEDs) fabricated using energy efficient Phosphorescent OLED (PHOLED) technology open up exciting new ways to develop efficient white lighting. WOLEDs have the potential to transform the lighting industry. In this presentation, phosphorescent WOLEDs with high conductivity transport layers will be discussed. White light can be generated by partial energy transfer from blue to green and red. Luminaire Correlated Color Temperature (CCT) and chromaticity can be tuned by altering emitter concentrations and layer thicknesses. A single materials set can be used to match the Energy Star color criteria for 2700K, 3000K, 3500 K and 4000K lighting. Different techniques to improve optical outcoupling will also be discussed.
3:00 PM - S2.2
Highly Efficient Excimer-based White Phosphorescent Devices with Improved Power Efficiency and Color Rendering Index.
Xiaohui Yang 1 , Zixing Wang 1 , Sijesh Madakuni 1 , Jian Li 1 , Ghassan Jabbour 1 2
1 School of Materials and Photovoltaic Center, Arizona State University, Tempe, Arizona, United States, 2 Felxible Display Center, Arizona State University, Tempe, Arizona, United States
Show AbstractUtilization of excimer and exciplex emission to achieve white emission is interesting due to its simple device geometry, decreased number of emissive dopants and effective reduction in differential color aging. We reported highly efficient phosphorescent excimer devices with internal quantum efficiency approaching 100%, however, emission spectrum of the devices showed dominant excimer emission, resulting in unsatisfactory Commission Internationale d'Enclairage (CIE) coordinates of (0.46, 0.47) and color rendering index (CRI) of 69.1 In this contribution, CIE coordinates, and CRI of white phosphorescent excimer devices were improved by utilizing two platinum complexes as emissive dopants, and exploring different device structures. Compared with devices containing two emissive layers each having a different dopant, devices having two dopants incorporated in a single host layer showed better color stability with change in operating voltage. White phosphorescent excimer devices having the double-doped emissive layer showed an external quantum efficiency and a power efficiency of 14.5% and 17 lm/W, respectively, at 500 cd/m2. The CIE coordinates and CRI of the devices were (0.382, 0.401) and 81, which were nearly independent of the drive voltage.1. E. L. Williams, K. Haavisto, J. Li, G. E. Jabbour, Adv. Mater. 19, 197 (2007).
3:15 PM - S2.3
Ultra High Intensity Stacked Organic Light Emitting Diodes under Pulsed Condition.
Yifan Zhang 1 , Stephane Kena-Cohen 2 , Stephen Forrest 1 3
1 Department of Physics, University of Michigan, Ann Arbor, Michigan, United States, 2 Department of Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 3 Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractOrganic light emitting diodes (OLEDs) have demonstrated nearly 100% internal quantum efficiency at current densities below 100mA/cm2. However, due to efficiency roll-off and device breakdown at high current density, OLEDs fail to reach very high emission intensity, generally making them unsuitable for bright illumination source applications. By making a Stacked OLED (SOLED) with 1mm2 emitting area and driving the device under pulsed conditions, we demonstrate injection at current densities as high as 32A/cm2 (corresponding to a power density of 2.5 kW/cm2), which is to our knowledge, the highest injection current in an OLED reported to date. The SOLED has a spectrum centered at 490nm and reaches steady state external quantum efficiency of 0.5% at the highest current density. We further investigate the transient emission of the device and find there is a turn-on overshoot that results from trapped charge introduced by doping in the emitting layer. The highest temporal light intensity of 1W/cm2 is achieved during the overshoot with duration of 100ns, with approximately twice the intensity of steady state. We describe a theory used to analyze the dynamic response of high intensity OLEDs, and draw conclusions about the fundamental limitations to OLED performance under extremely high current drive conditions.
3:30 PM - S2.4
Reduction and Recovery of Surface Plasmon Losses in Organic Light Emitting Diodes.
Joerg Frischeisen 1 , Bert Scholz 1 , Bettina Reisner 1 , Benedikt Arndt 1 , Stefan Nowy 1 , Wolfgang Bruetting 1
1 Institute of Physics, University of Augsburg, Augsburg Germany
Show AbstractIn recent years, great efforts have been devoted to the improvement of the luminous efficiency of organic light emitting diodes (OLEDs). In particular, the low light extraction efficiency due to the coupling to waveguide (WG) modes and surface plasmons (SPs) remains a challenge. In typical OLEDs, these loss channels dissipate around 50% of the available energy. Consequently, it is inevitable to either reduce coupling to them or to recover energy lost to WG modes and especially SPs in order to increase the efficiency of OLEDs.The most obvious concept to reduce plasmonic losses is to simply increase the distance between the emitting dye and the cathode. It is confirmed by simulations that this method results in no overall benefit due to stronger coupling to other loss channels, such as WG modes or absorption in the organic layers. As a second approach, the influence of the dipole orientation is investigated. A dipole oriented perpendicular to the interface between the organic layers and the cathode couples predominantly to surface plasmons. Therefore, by using dipoles oriented solely in the film plane the light extraction can be increased by up to a factor of 1.5 as compared to an isotropic dipole orientation.In addition to the mentioned techniques to reduce coupling to plasmons, three methods are presented in order to recover energy from SPs. First, the plasmon can be utilized to excite an additional dye with a red-shifted emission spectrum via near-field coupling. In this approach the dye is either positioned close to the metal or even outside the OLED stack if the cathode has an adequate thickness. Secondly, the application of a periodic surface grating to scatter WGs and SPs into visible radiation is examined. Particularly, the influence of the period and amplitude of the grating is studied. Finally, a new and promising method to recover plasmonic losses without affecting the OLED itself is introduced. All presented techniques are compared concerning their effectiveness.
3:45 PM - S2.5
Achieving Ultra-low Turn-on Voltages in Polymer Light-emitting Devices with a Nanoparticle Charge Injection Layer.
Ying Zheng 1 , Lei Qian 1 , Kaushik Choudhury 1 , Franky So 1 , Paul Holloway 1 , Jiangeng Xue 1
1 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractThe research on organic light-emitting devices (OLEDs) based on small molecules and conjugated polymers has gained significant interests since the first demonstration of efficient electroluminescence from a small molecule bilayer structure and the first observation of light emission from poly(p-phenylene vinylene) (PPV) based conjugated polymers. Drive voltage is a key device operating parameter that influences the power consumption of the OLEDs as well as lifetime, and typically it is greater than or close to the emitted photon energy divided by the electron charge. Here we report the observation of very low drive voltages in polymer-based light-emitting devices by incorporating a solution-processed electron injection/transport layer composed of ZnO nanoparticles. The ZnO nanoparticles are spin-coated on top of the polymer light-emitting layer from an aqueous solution to avoid dissolving the underlying organic layer. Using poly[2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) as the emissive layer, a very low turn-on voltage of (1.3 ± 0.1) V was observed in the device at room temperature, compared to the emitted photon energy of approximately 2.1 eV. Temperature study of the device shows that the turn-on voltage decreases from 2.9 V at 177 K to 1.2 V at 357K. The turn-on voltage in PLEDs based on poly(dioctylfluorene-alt-benzothiadiazole) (F8BT) or poly(N-vinylcarbazole) (PVK) doped with a phosphorescent emitter is also significantly lower than that in control devices with a LiF/Al cathode directly on top of the polymer layer, although it matches closer to the emitted photon energy than in MEH-PPV devices. Possible mechanisms leading to the low drive voltages in these devices will be discussed.
4:30 PM - **S2.6
White Organic Light-Emitting Diodes with Fluorescent Tube Efficiency.
Sebastian Reineke 1 , Frank Lindner 1 , Gregor Schwartz 1 , Nico Seidler 1 , Karsten Walzer 1 , Bjorn Luessem 1 , Karl Leo 1
1 Institut fuer Angewandte Photophysik, TU Dresden, Dresden Germany
Show AbstractWhite organic LEDs are seen as one of the next generation light-sources, with their potential to reach internal efficiencies of unity and their unique appearance as large-area and ultrathin devices. However, to replace existing lighting technologies, they have to be at least on par with the state-of-the-art. In terms of efficiency, the fluorescent tube with 60-70 lumen per Watt (lm W-1) in a fixture is the current benchmark. In the scientific literature, so far only values of 44 lm W-1 have been published for white OLEDs.Here, we present results (Reineke et al., Nature 459, 234 (2009)) of white OLEDs with 90 lm W-1 at an illumination relevant brightness of 1,000 candela per square meter (cd m-2). Extracting all light from the glass substrate using a 3D light extraction system, we even obtain 124 lm W-1. In order to achieve such high efficacy values, we reduced the energetic losses prior to photon emission that include ohmic and thermal relaxation losses, leading to very low operating voltages. This is accomplished by the use of doped transport layers and a novel, very energy efficient emission layer concept. Equally important, we addressed the optics of the OLED architecture, because about 80% of the generated light remains trapped in conventional devices. Therefore, we used high refractive index substrates to couple out more light and placed the emission to the second field antinode to avoid plasmonic losses. Our devices are also characterized by an outstandingly high efficiency at high brightness, reaching 74 lm W-1 at 5,000 cd m-2.
5:00 PM - S2.7
Effect of Substrate Topography on the Optoelectronic Properties of OLEDs.
Yiying Zhao 1 , Kwang-Hyup An 2 , Kevin Pipe 2 , Max Shtein 1
1 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Mechanical Engnieering, the University of Michigan, Ann Arbor, Michigan, United States
Show AbstractOLEDs deposited on nanostructured substrates have attracted considerable attention due to enhanced light extraction efficiency and the potential applications of nanoscale light sources in microscopy.[1,2] Previous studies have shown that light extraction improvements can arise from the scattering of waveguided modes by a corrugated sample surface.[3] However, few investigations have studied the effects of substrate shape on an OLED’s current-voltage characteristics. Of particular interest are substrates whose surfaces have features comparable in size to the typical device thickness (i.e. ~100 nm). Such nanoscale substrate features are easily realizable in organic devices, which do not require lattice-matching to a substrate. The electric field concentration associated with these features can potentially modify drift currents and energy barriers at device interfaces, leading to enhancements in current injection that have important implications for device performance and reliability.In this work, we study charge injection, transport, and recombination in model OLEDs deposited on textured substrates to understand in detail the influence of meso-scale substrate features on device electrical properties. Simulation results show that a spherically-symmetric electric field (which models an organic heterostructure atop a substrate asperity) strongly modifies field-dependent processes such as carrier injection and transport, leading to substantial predicted variations in the current-voltage-luminescence characteristics of OLEDs. These effects are experimentally demonstrated by means of optical and electrical characterization of sub-micron OLED emitters fabricated on pyramidal arrays. This study provides guidance for the design of device architectures on nano-structured substrates, and also furnishes basic understanding for the effects of surface roughness on OLED degradation.[4,5]References:[1] Yoon-Chang Kim, Young Rag Do, OPTICS EXPRESS, 2005 13(5) 1598[2] Yiying Zhao, Kwang-Hyup An, Shuo Chen, Brendan O’cornor, Kevin Pipe, Max Shtein, Nano Letters, 2007(12) pp. 3645-3649.[3] Soon Moon J EONG, Fumito ARAOKA, Yoshimi MACHIDA, Yoichi TAKANISHI, Ken I SHIKAWA, Hideo TAKEZOE, Suzushi NISHIMURA1, and Goro SUZAKI, Japanese Journal of Applied Physics, 47(6) 2008, pp. 4566–4571[4] Ki-Beom KIM, Yoon-Heung TAK, Yoon- Soo HAN, Kwang- Heum BAIK , Myung- Hee YOON and Moon- Ho LEE, Jpn. J. Appl. Phys. 2003, 42 pp. L 438–L 440[5] Aziz, H; Popovic, ZD, CHEMISTRY OF MATERIALS 2004,16(23), p4522-4532
5:15 PM - S2.8
Independently Controllable Stacked OLEDs with High Efficiency by using Semitransparent Al/oxide/Ag(Au) Intermediate Connecting Layer.
Wallace Choy 1 , H. Zhang 1 , Y. Dai 1
1 Department of Electrical & Electronic Engineering, The University of Hong Kong, Hong Kong China
Show AbstractIn stacked OLEDs, the intermediate connection layer plays critical role in modifying the device performance. For the independently controllable stacked OLEDs, the intermediate connecting structure should simultaneously function as an anode for one unit and as a cathode for another unit. To realize the simultaneous function, at present a semitransparent metal layer or a transparent indium-tin oxide (ITO), such as, Mg:Ag/ indium zinc oxide [1], CuPc /indium tin oxide (ITO) [2], and LiF/Ca/Ag [3], is generally used. However, the potential thin film damage due to sputtering of ITO and the transmission problem of the thin metal electrodes may limit the improvement of the device performance. Therefore, it is desirable to develop the intermediate controllable metal electrodes with the targets of not only improving the transmission but also enhancing the efficiency of the stacked OLEDs.A high efficiency OLED has been fabricated by using a vertical stack of two OLED units connected by Al/WO3/Ag. In the stacked OLEDs, each unit can be independently biased. It is found that the introduction of WO3 between Al and Ag (or Au) enhances the electroluminescent (EL) efficiency of top emissive unit. Meanwhile, the EL efficiency of the stacked OLED with Al/WO3/Ag and Al/WO3/Au interconnecting electrode is improved by 6 Cd/A and 8 Cd/A with respect to the case of using Al/Ag and Al/Au as the connecting electrode respectively. Our results indicate that Al/WO3/Ag(Au) is one of excellent connecting structures for fabrication of independently controlled high efficiency stacked OLEDs.[1] Z. Shen, P. E. Burrows, V. Bulovic, S. R. Forrest, and M. E. Thompson, Science 276, (1997) 2009.[2]G. Gu, G. Parthasarathy, P. E. Burrows, P. Tian, I. G. Hill, A. Kahn, and S. R. Forrest J. Appl. Phys. 86 (1999) 4067.[3] J. X. Sun, X. L. Zhu, H. J. Peng, M. Wong, H. S. Kwok, Organic Electron., 8 (2007) 305.
5:30 PM - S2.9
Integration of a Rib Waveguide Distributed Feedback Structure into a Light-Emitting Polymer Field-Effect Transistor.
Michael Gwinner 1 2 , Saghar Khodabakhsh 1 , Myoung Hoon Song 1 , Heinz Schweizer 2 , Harald Giessen 2 , Henning Sirringhaus 1
1 Physics, University of Cambridge, Cambridge United Kingdom, 2 Physics, University of Stuttgart, Stuttgart Germany
Show AbstractThe realization of electrically pumped organic lasers is considered the holy grail of organic electronics as such devices would be highly attractive for a wide range of applications. Additional optical losses in electrically pumped devices such as electrode absorption, charge induced absorption, and the presence of triplets, have prevented the realization yet, and thus create the necessity to develop architectures that minimize these effects. Organic ambipolar light-emitting field-effect transistors (LEFETs) efficiently emit light due to charge recombination in the channel. Since emission can be chosen to occur far away from metal electrodes, LEFETs have been proposed as architecture for electrically pumped lasers. A promising feedback structure is based on a high refractive index material such as tantalum pentoxide (Ta2O5) comprising the distributed feedback (DFB) grating. We investigate the possibility of combining a top gate/bottom contact LEFET based on poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) with an efficient light confinement and feedback structure provided by a Ta2O5 rib waveguide architecture with integrated DFB grating [Gwinner et al., Adv. Funct. Mater. 19, 1360 (2009)]. Moreover, F8BT annealing is crucial for both transistor current and light gain [Gwinner et al., submitted]. As annealing beyond the glass transition is incompatible with lasing, we show that modifying the injecting electrodes with 1-decanethiol allows for lower temperatures.In the integrated devices, the emitted light is coupled into the resonant mode of the DFB waveguide when the recombination zone of the LEFET is placed above the ridge. The resonance can be shifted by varying grating period and duty cycle, as well as ridge height. We show that electrode absorption is eliminated due to the strong 2D confinement. The lasing threshold for optical pumping of the LEFET structure with all electrodes (4.5 μJ cm-2) is as low as that of devices without electrodes, which is in excellent agreement with mode simulations optimizing the dielectric thickness and gate material.These results enable us to quantitatively judge the prospects for electrically pumped organic lasers based on ambipolar LEFETs by comparing the observed lasing threshold for optical pumping with the achievable current densities. Moreover, we investigate the influence of field-effect induced charge carriers with their associated induced absorption on the threshold of optically pumped structures. Despite the fact that the required singlet exciton density cannot be reached yet, the proposed device provides a powerful, low-loss architecture for integrating new high-performance organic semiconductors to yield electrically pumped lasing.
5:45 PM - S2.10
High Performance and Tuning of Threshold Voltage by Interfacial Carrier Doping in Ambipolar Field-effect Transistors Based on Organic Single Crystal Oligo (p-phenylenevinylene) Derivatives with Intense Electroluminescence.
Hajime Nakanotani 1 , Chihaya Adachi 1 , Hiroaki Nakamura 2 , Masatoshi Saito 2
1 , Center for Future Chemistry, Kyushu University, Fukuoka Japan, 2 , Central Research Laboratories, Idemitsu Kosan Co., Ltd., Chiba Japan
Show AbstractSingle crystal organic field-effect transistors (SC-OFET) have been extensively investigated to achieve high performance FETs and to clarify the basic properties of the carrier transport mechanism. In our previous work, we demonstrated that single crystals of oligo(p-phenylenevinylene) (OPV) derivatives have excellent photoluminescence (PL) and ambipolar OFET characteristics. In particular, a 1,4-Bis(4-methylstyryl)benzene (CH3-P3V2) single crystal showed a high PL quantum efficiency of ΦPL = 89±2%, a low amplified spontaneous emission threshold of Eth = 25 μJ/cm2, relatively high hole (μh > 0.1 cm2/Vs) and electron (μe ∼ 0.01 cm2/Vs) mobilities with intense blue EL in the SC-OFET. However, the electron mobility is still an order of magnitude lower than that of the hole mobility. Although the OPV derivatives, which are model compounds of the poly-p-phenylenevinylene (PPV) based polymers, have recently been extensively investigated, an investigation into the conjugation length dependence of the electron mobilities in OPV based FETs has, so far, not been performed. In this study, to clarify the relationship between molecular structures and the effect of the substituent effect of OPVs on the electron mobility, we fabricated SC-OFETs based on highly-luminescent OPV derivatives with gold-calcium asymmetric source-drain electrode, and investigated their conjugation length and substituent dependency of the field-effect mobility. We discovered that an increase in conjugation length from three phenylene rings (P3V2) to four phenylene rings (P4V3) and five phenylene rings (P5V4) in the OPV derivatives can enhance the electron mobility by up to one order of magnitude, resulting in μe > 10–1 cm2/Vs while keeping a high hole mobility of μh > 10–1 cm2/Vs due to the reduction of the electron injection barrier between Ca (WF = 2.9 – 3.0 eV) and the LUMO level of a OPV single crystals. The highest values of μh and μe of 0.38 and 0.28 cm2/Vs were observed in the P5V4 single crystal based SC-OFET. Furthermore, we demonstrated the drastic shift of threshold voltage (Vth) for hole accumulation from Vth = -80 ± 3 V to 2 ± 3 V by inserting of a MoOx layer between a P5V4 single crystal and a Au electrode in a single crystal based OFET, and revealed that the origin of the large Vth shift is due to the electron transfer from P5V4 molecules to a MoOx layer. The significant reduction of the hole injection provided the highly balanced ambipolar mobilities of μh = μe ∼ 0.2 cm2/Vs, leading to dual injection and accumulation of very high current density of J > 100 Acm-2 with intense edge electrolminescence from the middle of the channel in the P5V4 single crystal based OFET.
S3: Poster Session
Session Chairs
Chihaya Adachi
Russell Holmes
Tuesday AM, December 01, 2009
Exhibit Hall D (Hynes)
9:00 PM - S3.1
Degradation in Alq3-Based OLEDs Studied by Deep-Level Optical Spectroscopy.
Yoshitaka Nakano 1
1 Institute of Science & Technology Research, Chubu University, Kasugai, Aichi, Japan
Show AbstractOrganic light-emitting diodes (OLEDs) have attracted a great deal of attention because they can be used full-color flat-panel displays. In general, OLEDs are easily subject to the degradation through a long-term intrinsic decrease in electroluminescence (EL) efficiency during operation. Although the lifetime and the EL efficiency of OLEDs have been recently reported to be extended by doping Alq3 with quinacridone (Qd), the intrinsic degradation of OLEDs is still an open issue [1]. So, it is important to understand the origin of this degradation from the viewpoint of band gap states.In our previous study, we have clarified electronic band gap states at the emissive interface between Alq3 and α-NPD layers in the most well-known Alq3/α-NPD-based OLEDs by using modified deep-level optical spectroscopy (DLOS) [2]. In this study, we have first applied this modified DLOS to the degraded OLEDs, and have investigated emissive interface states before and after the degradation. We have used α-NPD/Alq3/LiF/Al OLED samples fabricated on ITO coated glass substrate. The active area is 3.0x3.0 mm2. The OLED samples were degraded through the constant-current operation of 44.4 mA/cm2. The final luminance decreased down to 30 % of the initial value (L0 : 1830 cd/m2).Before the degradation, a discrete trap level (I) is revealed to be located at ~1.77 eV below the conduction band of Alq3 in the emissive interface region, in addition to near-band-edge (NBE) transitions of Alq3 at 2.5 - 3.6 eV and band-to-band transitions (T1 and T2) of charge carriers from α-NPD to Alq3. On the other hand, after the degradation, the interface trap I is found to significantly shift from ~1.77 eV to ~1.39 eV, being the same level as an Alq3 single layer [3]. Similarly, the peak position of the NBE transitions also shifts to the lower photon energy side. These variations in band gap states are probably induced by the degradation and indicate that initial structural orientations peculiar to emissive interface are significantly transformed into bulk-like relaxed ones through the degradation. Furthermore, the T1 and T2 transitions are largely diminished by the degradation, which is likely associated with a significant decrease in hole injection rate revealed from conductance-voltage measurements. That is, these results support that the α-NPD layer is damaged near the emissive interface. Thus, modified DLOS has been proven to be a powerful tool for understanding the intrinsic degradation in the OLEDs from the viewpoint of electronic states.[1] H. Aziz et al., Science 283, 1900 (1999).[2] Y. Nakano et al., Jpn. J. Appl. Phys. 47(1), 464 (2008).[3] Y. Nakano et al., Appl. Phys. Lett. 88, 252104 (2006).
9:00 PM - S3.10
Enhancing Waveguided Light Extraction in Organic Light-emitting Devices via an Ultra-low-index Grid Fabricated by Oblique Deposition.
Michael Slootsky 1 , Stephen Forrest 1 2 3
1 Physics, University of Michigan, Ann Arbor, Michigan, United States, 2 Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan, United States, 3 Material Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractImproved outcoupling of light into substrate modes of an organic light-emitting device (OLED) by an ultra-low-index dielectric grid (UltraLIG) fabricated using glancing-angle deposition and standard lithographic techniques is demonstrated. Previous work1,2 has demonstrated that a dielectric grid embedded in the organic is a wavelength-independent method for efficiently extracting light trapped in the high-index regions of the device (the organic layers and ITO anode) to the substrate. The outcoupling of these waveguided modes (which are >45% of the emitted light) by a LIG in conjunction with microlenses or a similar method used to extract the light from the substrate can significantly increase the external quantum (ηEQE) and power (ηp) efficiencies of the OLED1. By decreasing the refractive index of the dielectric grid, light extraction can be further improved. In this work, a porous silica (SiO2) grid was obliquely deposited and patterned on the ITO-coated substrate; the increased porosity reduces the refractive index to n ≈ 1.15 (UltraLIG) from n = 1.45 (LIG) as produced by conventional methods. Outcoupling into the substrate for green tris(2-phenylpyridine) iridium [Ir(ppy)3]-based OLEDs grown on substrates with the UltraLIG was increased by a factor of ~1.28 over a conventional device at the peak external quantum efficiency, compared with an increase of ~1.10 for LIG devices. The enhancement factor increases to ~1.5 and ~1.11 for UltraLIG and LIG devices, respectively, at a luminance of 100cd/m2 due to efficiency roll-off at higher current densities. Provided efficient light outcoupling at the substrate-air interface, the UltraLIG devices can attain ηEQE ≈ 19% and ηp ≈ 46 lm/W at their peak efficiencies, falling off to ηEQE ≈ 12% and ηp ≈ 14 lm/W at 100 cd/m2. In contrast, the combined air and substrate modes in a conventional OLED yield peak efficiencies of ηEQE <15% and ηp < 39 lm/W, and ηEQE < 11% and ηp < 9 lm/W at 100 cd/m2. These results are consistent with simulations2 indicating that lowering the index of the dielectric grid is an effective approach to achieving high-external-efficiency OLEDs while retaining the advantages of the LIG for white lighting and full-color display applications.References:1. Y. Sun, and S. R. Forrest, "Enhanced light out-coupling of organic light-emitting devices using embedded low-index grids," Nature Photon. 2, 483 (2008).2. M. Slootsky, and S. R. Forrest, "Full-wave simulation of enhanced outcoupling of organic light-emitting devices with an embedded low-index grid," App. Phys. Lett. 94, 163302 (2009).
9:00 PM - S3.12
Efficient White OLED Using Pure Blue Phosphorescent Emitter.
Junichi Takamatsu 1 , Takayuki Chiba 1 , Takao Motoyama 1 , Yong-Jin Pu 1 , Ken-ichi Nakayama 1 , Junji Kido 1
1 Organic Device Engineering, Yamagata University, Yonezawa, Yamagata, Japan
Show AbstractWhite organic light-emitting devices using phosphorescent emitters were developed. Phosphorescent blue emitter, BP-1, has a peak wavelength of 445 nm in poly(methylmethacrylate) and an OLED having a structure of ITO / arylamine derivative (TAPC)/BP-1 doped into CzBPO (carbazole derivative with phosphine oxide)/CzBPO/BmPYPB (wide-gap ETM with pyridine moieties)/LiF/Al showed blue emission with a CIE coordinate of x=0.15 and y=0.20. A power efficiency of 20 lm/W and an EQE of 13% were obtained. Based on this blue OLED, an orange emitter, (BT)2Ir(acac), was combined with the blue emitter to generate white light. An OLED with a structure of ITO/TAPC/carbazole derivative (TCTA)/(BT)2Ir(acac) doped into wide-gap host (26DCzPPy)/BP-1 doped into CzBPO/BmPYPB/LiF/Al provided stable white color of CIE x= 0.33 and y=0.35 and CRI of 72. The power efficiency of 45 lm/W and an EQE of 19 % were observed at 100 cd/m2. The relationship between the device structure and the improvement of the device performance will be discussed.
9:00 PM - S3.13
Ultra Highly Efficient Green Phosphorescent OLED Having Multiphoton Structure.
Ryoichi Miyazaki 1 , Takayuki Chiba 1 , Yong-Jin Pu 1 , Ken-ichi Nakayama 1 , Junji Kido 1
1 Organic Device Engineering, Yamagata University, Yonezawa, Yamagata, Japan
Show AbstractIn the multiphoton OLED, positive and negative charge carriers are generated at the charge generation layers (CGLs) and injected into the adjacent emissive units. As a result, external efficiency, number of emitted photons / injected electrons can be extremely high when the multiple CGLs and emissive units are used. In this study, a green phosphorescent material (Ir(ppy)3) and three emissive units were used. The conventional one unit device, ITO/TAPC/double layer green emitter/B4PYMPM/LiF/Al, gave a power efficency of 113 lm/W and an EQE of 25%. When the two emitter units were used in the multiphoton OLED structure, 50% EQE was observed at 100 cd/m2. The three units OLED provided 67 % EQE and 256 cd/A current efficiency. The importance of the optical design of the layer thickness and the use of appropriate CGL materials are discussed.
9:00 PM - S3.14
Soft X-ray Spectroscopic Study of the Electronic Structure of the Organic Semiconductor Lithium Quinolate (Liq).
Alexander DeMasi 1 , Sang Wan Cho 1 , Louis F. J. Piper 1 , Andrew R. H. Preston 1 , Kevin E. Smith 1
1 Department of Physics, Boston University, Boston, Massachusetts, United States
Show AbstractThe electronic structure of the metal-organic complex lithium quinolate (Liq), which can be used as an electron injection layer in OLEDs, has been measured using synchrotron radiation-excited resonant x-ray emission spectroscopy (RXES), x-ray absorption spectroscopy (XAS) and x-ray photoelectron spectroscopy (XPS). Samples were in the form of thin films, grown in-situ in an organic molecular beam deposition chamber attached to the spectrometer system. Our measurements are compared to the results of density functional theory (DFT) calculations. The data will be discussed in the context of similar soft x-ray spectroscopic studies of Alq3, with attention paid to differences in the occupied and unoccupied partial densities of states near the Fermi level for the quinoline ligand’s nitrogen and oxygen atoms.This work was supported in part by the NSF under CHE-0807368. Experiments were undertaken at the NSLS, which is supported by the U.S. DOE.
9:00 PM - S3.15
Study of Interface Interactions Between an Electron-withdrawing OLED Hole Injection Molecule and Various Metal Surfaces.
Hyeseung Kang 1 , Ji Hoon Kim 1 , Jeoung Kyu Kim 1 , Young Mi Lee 1 2 , Jouhahn Lee 3 , Jeong Won Kim 2 , Young-Kyun Kwon 1 , Yongsup Park 1
1 Dept.of Physics, Kyung Hee University, Seoul Korea (the Republic of), 2 , Korea Research Institute of Standards and Science, Daejeon Korea (the Republic of), 3 , Korea Basic Science Institute, Jeonju Korea (the Republic of)
Show Abstract1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) has recently received much attention as it is an efficient hole injection layer in organic light emitting diodes (OLEDs). We investigated the interface electronic structures of HAT-CN adsorbed on Cu(111) and other metallic surfaces using photoemission spectroscopy and near-edge x-ray absorption fine structure. Initial reduction and later dramatic increase of work function up to over 5.6 eV was observed as a function of HAT-CN thickness on Cu(111). Although the specific values are different, similar trend was observed for all the metallic surfaces we investigated including polycrystalline Ca, Au, Al, and ITO. At the same time the highest occupied molecular orbital of 100 nm thick HAT-CN on Cu(111) was located 4.1 eV below the Fermi level, which would put its lowest unoccupied molecular orbital very close to the Fermi level. This will make this molecular layer very interesting as it looks like it will behave almost like a metal. To gain a further insight into what is happening at these interface we conducted a first-principles density functional theory study of the interface. We discuss the implications of the experimental and theoretical results in the context of OEDs and other organic device applications.
9:00 PM - S3.16
Surface Plasmon-mediated Transfer of Electrically Pumped Excitons.
Kwang Hyup An 1 , Max Shtein 2 , Kevin Pipe 1
1 Mechanical Engineering, University of Michigan, Ann Arbor, Ann Arbor, Michigan, United States, 2 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractWe observed strong plasmon-mediated transfer of exciton energy from electrically-pumped dipoles through a smooth metal film, in an organic heterostructure comprising organic thin films and a metal cathode capped by a dye-doped organic layer. This phenomenon is closely related to the previously observed plasmon-mediated resonant energy transfer in optically-pumped organic-metal-organic layers.[1] We calculate the dipole coupling power using a classical electromagnetic model, allowing the thicknesses of the active layers, cathode, and capping layer to be optimized such that most of the electrically-pumped dipoles at the heterojunction couple to surface-plasmon polaritons (SPPs) in the metal cathode. Molecular excitons at the organic heterojuction resonantly excite both symmetric and anti-symmetric surface-plasmon modes in the metal electrode, which evanescently couple to dye molecules near the electrode’s exterior surface. Dye fluorescence in the capping layer decays rapidly with distance from the electrode. A six-fold enhacement of light emission through an optically thick metal electrode is observed when the coupled surface plasmon modes are excited. This mechanism appears to be useful for improving top-emission in OLEDs. Furthermore, we propose a scheme for an integrated electrically-pumped evanescent wave generator with applications in very high resolution optical microscopy and lab-on-a-chip.
9:00 PM - S3.17
Analytical Evaluation of Temperature-dependent Optoelectrical Phenomena in the Disordered ITO/PEDOT/PF/Ca/Al Polymer Light-emitting Diodes.
Yu-Ting Chen 1 , Jen-Wei Teng 1 , Jen-Cheng Wang 1 , Tzer-En Nee 1
1 Department of Electronic Engineering, Chang Gung University, Kwei-Shan, Tao-Yuan, Taiwan
Show AbstractSince the first report of electroluminescence in polymers, the study and understanding of optical and electrical properties of organic light-emitting diodes (OLEDs) have attracted much attention to understand the emission mechanisms in novel organic materials. In order to verify the organic semiconductor devices of disorder materials, it is necessary to examine systematically the electrical characteristics of PF-based polymer light-emitting diode (PLEDs). In this study, we studied the unique correlations between the electrical and optical characteristics of ITO/PEDOT/PF/Ca/Al PLEDs structures at a temperature range of 20 to 300 K.Further, the Berthelot-type microscopic model has successfully described the linear and the nonlinear temperature dependent behaviors of many disordered systems. Based on the Berthelot-type model, it has been found the sample with high driving voltage exhibits a higher Berthelot temperature TB, indicating that carrier transports in disordered system manifest Berthelot-type behaviors. As strong electron-electron scattering arising from a high excitation and a microcrystalline randomization, a shorter static barrier width led to that the Berthelot-type process correspondingly dominated with a large TB. To elucidate the carrier transport of the PF-based PLEDs, we performed current–voltage (I–V) measurements and calculate the slope of the logJ-logV plot with trap charge limited current (TCLC) power-law factor are 16.1, 20.3, and 20.7 for PLED at 200 K, 260 K and 300 K, respectively. The power-law factors increased as the temperature increased, it has corroborated that the Berthelot-type behavior were response to the microstructure disordering arising from the organic nonstoichiometry.In order to inspect unique correlations between the electrical and optical characteristics of the PLED, it is of interest to examine the radiative recombination of the confined electrons and holes at low temperature. When temperature is slightly decreased from 300 K, the luminescent intensities for the PLEDs with 0.1 mA and 0.5 mA efficiently increase and reach the maximum at 260 and 200 K, respectively. With carriers drifting coherently across the crystallite microbarriers, it was essential to observe a high TB due to the predominance of the Berthelot-type process. When temperature is further decreased from turning point temperature to 80 K, the reduction of EL intensity is much faster and steeper for both current. It also has found that the device with 0.1 mA exhibits a smaller reduction of intensity, the device with 0.5 mA exhibits larger one. That is, electrically injected carriers are not efficiently captured and recombined in PLEDs diode when low temperature range. Therefore, it is observed the anomalous temperature behavior of EL spectra from the sample are investigated by the localized carrier hopping and thermalization process and exhibited to be in a good agreement with the experimental results.
9:00 PM - S3.18
High Efficiency Polymer Solar Cells Based on P3HT/Dye/PCBM Ternary Blends.
Satoshi Honda 1 , Seiichirou Yokoya 1 , Hideo Ohkita 1 , Hiroaki Benten 1 , Shinzaburo Ito 1
1 Polymer Chemistry, Kyoto University, Kyoto, kyoto, Japan
Show AbstractIn order to improve the efficiency of bulk heterojunction solar cells based on blends of poly(3-hexylthiopene) (P3HT) and a fullerene derivative (PCBM), we employed a silicon phthalocyanine dye (SiPc) that has an intense absorption band in the near-IR region. The introduction of SiPc increased the short-circuit current (Jsc) from 9.7 to 10.8 mA cm−2, the open-circuit voltage (Voc) from 0.58 to 0.61 V, and hence raised the power conversion efficiency from 4.0 to 4.5 %, compared to the control P3HT/PCBM solar cell without SiPc. The increase in Jsc is ascribed to the photocurrent from SiPc, because it can be quantitatively explained by the external quantum efficiency at wavelengths for the near-IR absorption band of SiPc. Comparing the increase in Jsc with the dye concentration in the ternary blend, it can be said that most of SiPcs contribute to the photocurrent generation at the interface of P3HT/PCBM. The increase in Voc is also due to the localization of SiPcs at the interface, which probably suppress the charge recombination effectively. These findings suggest that SiPc dye molecules loaded at the interface of bulk heterojunction solar cells can substantially increase both of Jsc and Voc, consequently improve the efficiency of energy conversion.
9:00 PM - S3.19
Enhanced Efficiency in Triple-Tandem Organic Solar Cells.
Dewei Zhao 1 3 2 , Xiaowei Sun 1 , Lin Ke 3 , Swee Tiam Tan 2
1 , Nanoelectronics Laboratory, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore Singapore, 3 , Institute of Materials Research and Engineering, Singapore Singapore, 2 , Institute of Microelectronics, Singapore Singapore
Show AbstractWe present an enhanced efficient polymer-small molecule triple-tandem organic solar cell (OSC), consisting of poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) bulk heterojunction as the first and second bottom cells, and small molecules copper phthalocyanine (CuPc) and fullerene (C60) as the third top cell, which is the first report for the triple-tandem OSC based on the combination of polymer and small molecules. These sub-cells are connected by an intermediate layer of Al(1 nm)/MoO3(15 nm), which appears to be highly transparent, structurally smooth, and electrically connected. Compared to our previous triple-tandem polymer solar cells (2.03%), this polymer-small molecule triple-tandem organic solar cell achieves an improved power conversion efficiency (PCE) of 2.18% with short-circuit current density (Jsc) = 3.02 mA/cm2, open-circuit voltage (Voc) = 1.51 V, and fill factor (FF) = 47.7% under simulated solar irradiation of 100 mW/cm2 (AM1.5). This is attributed to the increased photocurrent generation in the third cell since the third cell has the complementary absorption with two bottom cells despite slightly reduced Voc. Therefore, it is demonstrated that organic materials with complementary absorption spectra play a crucial role in the performance of tandem organic solar cells. In addition, our developed intermediate layer shows the most potential application to multiple-tandem organic solar cells and light-emitting diodes.
9:00 PM - S3.2
Efficient Emission Properties and Electroluminescent Devices of F8BT:MDDOPPV Composite Thin Film.
Naoyuki Yamasaki 1 , Masahiko Watanabe 1 , Yasuo Miyake 1 , Hitoshi Kubo 1 , Akihiko Fujii 1 , Masanori Ozaki 1
1 Erectrical, Electronic and Information Engineering, Osaka University, Osaka Japan
Show AbstractOptical properties and electrical characteristics of conducting polymer composite thin films have been studied. We focus on a composite material mixed a PPV derivative, poly(2-methoxy-5-dodecyloxy-p-phenylenevinylrne) (MDDOPPV), which shows excellent laser emission properties, and a PDAF based copolymer, poly(9,9-dioctylfluorene-co-benzothiadiazole)(F8BT), which shows high electron carrier mobility in field effect transistors.Absorption and PL spectra of the F8BT:MDDOPPV composite films were measured. The absorption spectra of the composite films are intermediate shape between those of F8BT film and MDDOPPV film, furthermore, the absorption peak wavelength gradually shifts to shorter wavelength as the ratio of F8BT increases, The PL spectra of the composite films are almost the same as that of MDDOPPV film. We have investigated further optical properties such as ASE, lasing threshold and time-resolved PL spectra, of the composite films. We demonstrated the ASE of F8BT:MDDOPPV composite films with low pumping power and a low-threshold photoexcited-lasing by using a micro-capillary cavity[1]. Moreover, using a time-resolved PL measurement technique, we clarified the rapid intermolecular energy transfer from F8BT to MDDOPPV with high efficiency in the composite films. Besides, we investigated the EL characteristics of OLED, the emission layer of which is the F8BT:MDDOPPV composite film. the emission from MDDOPPV was predominant in the OLEDs with composite films, which means the similar emission process with PL. Utilizing the composite films as an emitting layer, we obtained the high performance OLEDs, achieving 23-fold and 6-fold improvement on luminance and current efficiency, respectively. These improvements in performance of OLEDs are attributed to the intermolecular energy transfer in the composite films[2].[1] M.Watanabe et al. Synth. Met. ,159, 935,(2009)[2] N.Yamasaki et al. submitted to Jpn. J. Appl. Phys.
9:00 PM - S3.20
A Morphology Controller for Bulk-Heterojunction Polymer Solar Cells.
Bogyu Lim 1 , Jang Jo 1 , Dong-Yu Kim 1
1 Dept. of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju Korea (the Republic of)
Show AbstractBulk-heterojunction (BHJ) polymer solar cells based on a composite film using an electron donor polymer and an electron acceptor have been attracting tremendous attention due to their low cost and easy of fabrication. The efficiency of the BHJ solar cells can be improved by the contlloring the morphology of the active layer. Well-organized morphology can form a maximized interfacial area between donor and acceptor for maximum exciton dissociation as well as continuous pathways for charge transport to both of electrods. Recently, power conversion efficiency (PCE) of BHJ solar cells has been increased to near 5 % using regioregular poly(3-hexylthiophene) (rr-P3HT) as an electron donor and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor through the annealing treatments. These treatments should generate a nanosclae interpenetrating network with highly crystalline morphology. In this study, we synthesized a hydrophobic end-functionalized rr-P3HT (F-P3HT) via Grignard metathesis (McCullough) method. We fabricated BHJ solar cells using P3HT:F-P3HT:PCBM blending solutions. When the F-P3HT was mixed with P3HT:PCBM, the fill factor (FF) was improved maintaining the comparable open-circuit voltage and short-circuit current. We characterized the morphology of blend film using TEM, AFM, and XRD. The hydrophobic F-P3HT may induce phase separation with relatively hydrophilic PCBM preserving the crystallinity of P3HT, which could improve the device performance.
9:00 PM - S3.22
P3HT:PCBM Bulk-heterojunction Solar Cells: Processing Variables and Device Analysis.
Bakhtyar Ali 1 , Steven Hegedus 2 , Syed Shah 1
1 Materials Science and Engineering, University of Delaware, Newark, Delaware, United States, 2 Institute of Energy Conversion (IEC), University of Delaware, Newark, Delaware, United States
Show AbstractOrganic photovoltaics (OPV) based on the conjugated polymers are an option for the alternatives to conventional photovoltaic technologies. Solution-processed organic solar cells have gained significant attention, due to the increasing demands for the low cost power production. The growing performance of the OPV suggests a promising future of the solution based photovoltaic technology. However, the efficiencies of these devices are still low for commercialization and need to be improved to at least reach the magic number of 10%. Moreover, from the device perspective, controlling the carriers’ generation, transport in the active layer and collection at the electrodes need to be fully understood. In this paper we report the performance optimization of regioregular poly (3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) based solar devices. The PV parameters are shown to improve with annealing the devices. In particular, the effects of applying a magnetic field during annealing are considered. Besides the improved efficiency, the effects of the magnetic field annealing are studied through X-ray diffraction, photo absorption spectroscopy and photoluminescence. Furthermore, the study of the solubility of P3HT and PCBM on the morphology and hence the device efficiency is performed. The stirring duration of the solution and the metallization steps are observed to have strong influence on the device performance. We have investigated the effects of evaporation rate during metallization as well as thickness of the cathode (Al). The optimum thickness in our study turns out to be 80nm at 0.2nm/sec evaporation rate of Al. The conduction behavior of the samples is studied as a function of temperature and PCBM dose. The data are analyzed by adopting the variable range hopping (VRH) of small polarons. The activation energies for the carrier conduction are 195 meV and 240 meV for the pristine P3HT and P3HT/ PCBM blend (1:2). Devices of area 0.1 cm2 studied here have delivered reproducible efficiency ~4.8% under AM1.5 and 100 mW/cm2 conditions at room temperature. Analysis of current voltage characteristics will be presented providing insight into the recombination and field dependent carrier collection mechanisms. Similarity and differences with other low mobility, low dimensional semiconductor solar cells will be discussed.
9:00 PM - S3.23
Organic Heterojunctions of Layered Perylene and Phthalocyanine Dyes: Characterization with UV Photoelectron Spectroscopy and Luminescence Quenching.
Dana Alloway 1 2 , Neal Armstrong 2
1 , Concord University, Athens, West Virginia, United States, 2 Chemistry, University of Arizona, Tucson, Arizona, United States
Show AbstractOrganic heterojunctions play a critical role in organic solar cells -- the offset between the HOMO energy in the donor material, and the LUMO energy in the acceptor material (EHOMO,D – ELUMO,A) correlates with the photopotential (VOC) obtainable in such devices. Heterojunctions created from two different base perylene dye layers – perylenetetracarboxylicdianhydride (PTCDA) or N,N'-di-n-butylperylene bis(dicarboximide) (C4-PTCDI), and two trivalent metal phthalocyanines – chloroaluminum phthalocyanine (ClAlPc) and chloroindium phthalocyanine (ClInPc) have been studied. UPS studies of 12 nm thickness PTCDA and C4-PTCDI films, with 0-20 nm of added Pc show quite different interface dipole effects as either Pc is added to form the organic heterojunction. Interface dipoles of at least 0.3 eV are seen for the PTCDA/Pc heterojunctions, whereas no significant interface dipoles were developed at the C4-PTCDI/Pc interfaces. Luminescence quenching studies of similar 12 monolayer films of either PTCDA or C4-PTCDI on KCl (001) single crystal substrates showed significant quenching owing to a combination of energy transfer to the Pc, and charge exchange between the Pc donor and the perylene acceptor. Addition of the bis-(tri-arylamine) TPD to either PTCDA or C4-PTCDI layers, instead of the Pc, in which energy transfer quenching is not possible, provides a means of estimation of the degree of quenching through charge transfer. Charge transfer quenching is clearly favored for PTCDA films, regardless of thickness, versus C4-PTCDI films, suggesting that the geometry of the donor/acceptor contact is important for exciton dissociation processes.
9:00 PM - S3.24
Lifetime Test of Polymer-based Organic Solar Cells Under Light Irradiation and Dark Conditions.
Toshihiro Yamanari 1 , Hiroyuki Ogo 1 , Testuya Taima 1 , Jun Sakai 2 , Jun Tsukamoto 3 , Yuji Yoshida 1
1 , National institute of advanced industrial science and technology (AIST), Tsukuba Japan, 2 , Panasonic Electric Works, Ltd., Kadoma Japan, 3 , Toray Industries, Inc., Sonoyama Japan
Show AbstractOrganic solar cells (OSCs) have potential advantages in their mechanical flexibility, portability, and low manufacturing cost. OSCs based on conjugated polymers and soluble fullerenes have been improved those power conversion efficiencies (PCEs) markedly in the recent decade. For practical use, long-term stability of the cell performance is essential, as well as high efficiency. In contrast to considerable efforts on improving the PCE of OSCs, researches on the stability of OSCs has been performed only since the last few years so that the degradation mechanism and key technologies for the long-term stability are still unclear. In this study, we investigated the stabilities of OSCs under Air Mass 1.5 Global (AM 1.5 G) solar simulated light irradiation for 50 h and dark conditions. We fabricated 3 types of cells containing different buffer layers (PEDOT:PSS, Molybdenum oxide (MoOx), no buffer layer). For the photovoltaic layer, we used regioregular poly(3-hexylthiophene) (P3HT) as a p-type semiconducting polymer and [6,6]-phenyl-C61-buteric acid methyl ester (PCBM) as an n-type semiconductor molecule. The device structure were [Glass/ITO/buffer layer/P3HT:PCBM/Al].After the light irradiation, PCEs of the OSCs were decreased to a half of initial values. Interestingly, by thermal annealing, the PCE was almost fully recovered to the initial values. Although the incident photon-to-electron conversion efficiency (IPCE) was reduced entirely, its spectral shape was unchanged. This result indicates that degradation of organic molecules had not occurred. The deterioration of cell performance under light irradiation may be due to charge accumulations on trap sites, such as oxygen, water or isolated p-type or n-type materials in carrier transport networks. Under the dark condition in ambient air, the OSC with PEDOT:PSS layer was degraded very quickly. On the other hand, the degradations of the OSCs with MoOx layer or no buffer layer were relatively slow. In addition, we observed the XPS depth profile of the aluminum oxide (Al2p-O signal) of the cells. The Al2p-O signal was markedly increased in the OSC with PEDOT:PSS layer. These results suggest that PEDOT:PSS accelerates the oxidation reaction so that the oxidation of aluminum electrode of the OSCs causes the reduction of PCE. We conclude that the introduction of PEDOT:PSS layer, which brings about a high initial PCE and is used generally though, is not the best way in view of the stability of the OSCs.
9:00 PM - S3.25
Synthesis of New Pyridazine-based Monomers and Related Polymers for Photovoltaic Organic Cell Application.
David Gendron 1 , Pierre-Olivier Morin 1 , Mario Leclerc 1
1 Chemistry, Université Laval, Quebec, Quebec, Canada
Show AbstractHarvesting energy from sunlight using photovoltaic cells is a very efficient technique to fight against the global warming and the greenhouse effect. Based on this problematic, many conjugated polymers have been developed to make bulk heterojunction solar cell. In fact, copolymers derivatives incorporating 2,7-carbazole unit and a 4,7-dithienyl-2,1,3-benzothiadiazole or 4,7-dithienyl[1,2,5]thiadiazolo[3,4-c]pyridine moiety have led to a solar conversion efficiency up to 6%. To further improve this value, a lower LUMO energy level (near -4.0 eV) and a bandgap between 1.2 to 1.9 eV must be obtained. To solve this problem, four new aromatic compounds with a pyridazine core have been synthesized. The synthesis involved simple condensation reaction and ring closure procedure to afford the 4,7-bis(5-bromo-2-thienyl)[1,2,5]thiadiazole[3,4-d]pyridazine derivatives. With the four comonomers in hand, they were reacted with the 2,7-carbazole unit to obtain the corresponding copolymers by Suzuki coupling polymerization. Three polymers (poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl)[1,2,5]oxadiazolo[3,4-d]pyridazine] (P1), poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl)-2,3-dioctylpyrazino[2,3-d]pyridazine] (P2) and poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl)pyrazino[2,3-d]pyridazine] (P3)) were then characterized.The number average molecular weight (Mn) of the polymer (P1, P2, and P3) are between of 4 and 5 kDa. Cyclovoltametric mesurements were carried out and show a LUMO energy levels between -4.15 and -3.80 eV. All polymers have their HOMO energy levels below -5.27 eV (-5.53 to -5.76 eV), so they are considered air-stable. The bandgaps are between 1.55 to 1.79 eV. In fact, the incorporation of the pyridazine core is a good way to modulate the electronic properties by lowering the LUMO energy levels compare to the pyridine or the benzene core. Thus, the resulting conjugated polymers show a great potential in organic solar cells due to their optimized HOMO, LUMO and bandgap energy levels. Photovoltaic devices are currently investigated.
9:00 PM - S3.26
Investigating the Effects of Thermal Annealing Upon the Morphology of Polymer-Fullerene Blends with In-situ Grazing Incidence X-ray Scattering.
Eric Verploegen 1 2 , Rajib Mondal 1 , Chris Bettinger 1 , Michael Toney 2 , Zhenan Bao 1
1 Chemical Engineering, Stanford University, Stanford, California, United States, 2 Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, Menlo Park, California, United States
Show AbstractThe optimization of bulk heterojunction blend morphologies is critical in order to develop high efficiency organic photovoltaic devices. In order to achieve increased performance in these devices the morphology and molecular packing of both the acceptor and donor within their respective domains should be optimized for maximum electron and hole mobilities. In addition, the nanophase segregated heterojunction domain size must be about the same as the exciton diffusion length in order for efficient conversion of adsorbed photons into electrical current. It is desirable to optimize the morphology/packing – e.g. increase the crystallinity – of these domains without allowing for the size of the domains to grow. In this work we characterize polymer – fullerene blends with in-situ grazing incidence X-ray scattering (GIXS) to reveal key transition temperatures where mobility is introduced into the system, resulting in morphological rearrangements. We use GIXS to investigate the effects of thermal annealing upon the morphology of poly(3-hexylthiophene) (P3HT), phenyl-C61-butyric acid methyl ester (PCBM), and a series of P3HT – PCBM blends. We developed a heating chamber enabling in-situ GIXS studies, allowing for the morphological characterization of thin films at elevated temperatures. We also investigate the morphology of samples that have been annealed over a wide range of temperatures that corroborate the results of the in-situ experiments. Using these techniques we observe a reorientation of the P3HT crystallites upon annealing above the polymer’s melting point and the cold crystallization of PCBM. We detail how the incorporation of these materials into bulk heterojunction blends effects the nature of these transitions as a function of blend ratio. These in-situ measurements provide critical insights into the dynamics of the morphological evolution during the thermal annealing process.
9:00 PM - S3.27
Significant Conductivity Enhancement of PEDOT:PSS through Salt or Surfactant Treatment and Its Application as the Transparent Electrode of Polymer Photovoltaic Cells.
Yijie Xia 1 , Benhu Fan 1 , Jianyong Ouyang 1
1 Materials Science and Engineering, National University of Singapore, Singapore Singapore
Show AbstractTo develop novel and low-cost materials with good conductivity and high transparency is an urgent task for optoelectronic devices. PEDOT:PSS has high transparency but the conductivity is relatively low. Here, we report the conductivity enhancement of PEDOT:PSS by more than two orders in magnitude through a treatment with salt or surfactant. The conductivity enhancement is related to the soft parameter of the ions and the structure of the surfactant. The mechanisms for the conductivity enhancement will be presented as well. Polymer photovoltaic cells with the highly conductive PEDOT:PSS as the transparent electrode were demonstrated.
9:00 PM - S3.29
Free Carrier Generation in Molecular Photovoltaic Bilayers Studied by Time-Resolved Microwave Conductivity.
David Coffey 1 , Garry Rumbles 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractIn the standard model of organic photovoltaics cells, excitons diffuse to type-II heterjunction interfaces and dissociate into free charge. However, this simple model fails to predict the relative importance of energy levels, morphology, energy transfer, and charge extraction pathways to the efficiency of charge creation. To help isolate the importance of energy levels, we have fabricated a series of molecular donor/acceptor bilayers, with and without intermediate charge and energy blocking layers, and measured performance with Time-Resolved Microwave Conductivity (TRMC) and in full devices. TMRC allows us to assess type-I and type-II heterojunctions in isolation that cannot easily be probed in full devices. Our results show that charge transfer, energy transfer, and the local energy landscape all play important roles in the charge creation process.
9:00 PM - S3.3
Solution-processable Molecules and Their Optoelectronic Properties.
Wenjing Tian 1 , Lili Xue 1 , Zaifang Li 1 , Jiating He 1
1 , State Key Lab of Supramolecular Structure and Materials, Jilin University, Changchun China
Show AbstractSolution-processable molecules have attractive features for optoelectronic applications. They offer well-defined molecular structures, mono-dispersibility, relatively simple and reproducible synthesis and purification, and typically low-cost processing technology. Triphenylamine (TPA) derivatives have been widely investigated for almost two decades because of the unique 3D geometry, glass-forming property, relatively high oxidation potential as well as excellent hole-transporting property of TPA. On the other hand, dicyanomethylene pyran (PM) group has attracted much attention due to its relatively strong electron-withdrawing ability which can lower the lowest unoccupied molecular orbital energy level and reduce the optical band gap. And the strong intermolecular dipole-dipole interaction or intermolecular stacking of PM-type organic molecules may be beneficial to the charge carrier transportation. Here, we reported the synthesis and photophysical properties of TPA-based dendrimers with truxene cores (Tr-TPA3,Tr-TPA9) and PM-based D-A small molecules (DADP, BTPM, TTPM, HTPM), and their applications in organic light-emitting diodes (OLED) and photovoltaic cells.The two dendrimers used as hole-transport layers exhibited excellent solubility, good film-forming property, high thermal stability with high Tg and proper HOMO energy level which facilitated hole injection. The OLED with the structure of ITO/dendrimer/Alq/LiF/Al shows the turn-on voltage of 2.5 V, the maximum luminance of about 11058 cd m-2 and the maximum current efficiency of 4.01 cd A-1. The four PM-based D-A small molecules exhibited excellent solubility, good film-forming property, extended absorption at long wavelengths due to intramolecular charge transfer, and enhanced charge carrier mobilities. Bulk heterojunction photovoltaic cells with a structure of ITO/PEDOT-PSS/ DADP:PCBM/LiF/Al exhibits Voc of 0.98 V, Isc of 4.16 mA/cm2, FF of 0.37 and power conversion efficiency (PCE) of 1.50% under the illumination of AM 1.5 simulated solar light (100 mW/cm2).
9:00 PM - S3.30
Star-Shaped Donor-Acceptor Molecules Based on Triphenylamines with Tunable Energy Levels for Organic Photovoltaics.
Scott Hammond 1 , Muhammet Kose 1 , Nikos Kopidakis 1 , Dana Olson 1 , Zbyslaw Owczarczyk 1 , David Ginley 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractStar-shaped molecules based upon triphenylamine and containing electron-poor acceptor moieties have been designed and synthesized for organic photovoltaic (OPV) applications. Adjunct electron-withdrawing moieties allow tuning of the lowest unoccupied molecular orbital (LUMO) energy levels around that of the LUMO level of the fullerene electron acceptor used in bulk heterojunction OPV devices. Theoretical calculations indicate three derivatives have LUMO levels just above, just equal to, and just below that of phenyl C61 butyric acid methyl ester (PC61BM), which allows for empirical testing of the LUMO offset that is required to ensure efficient charge transfer to the PC61BM and successful separation of the charges. Synthesis, characterization, and bulk heterojunction device results for the new materials will be presented.
9:00 PM - S3.31
The Open-circuit Voltage of Inverted Small Molecule Organic Photovoltaic Cells.
Xiaoran Tong 1 , Stephen Forrest 2
1 Materials Science and Engineering, University of Michigan--Ann Arbor, Ann Arbor, Michigan, United States, 2 Electrical Engineering and Computer Science, Physics, and Materials Science and Engineering, University of Michigan--Ann Arbor, Ann Arbor, Michigan, United States
Show AbstractInverted small molecule organic solar cells are promising for their possible use as an inexpensive future energy source on flexible surfaces. By employing top-sputtered indium tin oxide, the inverted solar cells can be grown on opaque substrates, allowing for more flexibility in device structure design. Under simulated one sun AM 1.5G illumination, the inverted planar solar cells with the structure Aluminum/C60/Subphthalocyanine/MoO3/ITO showed a fill factor of 0.52, an open-circuit voltage of 0.60V and a responsivity of 0.045A/W, resulting in a power conversion efficiency of 1.4%. The efficiency of the inverted solar cell is approximately 40% as a conventional subPc/C60 planar cell, primarily due to the low open-circuit voltage, which is 0.60V as opposed to 0.98V for the latter cells. The open-circuit voltage dependence on the ITO sputtering conditions and surface treatment of the bottom metal contacts with different work functions are investigated using ultraviolet photoelectron spectroscopy and other techniques.
9:00 PM - S3.32
Nanostructure of the Codeposited i-Layer of ZnPc:C60 p-i-n Solar Cells.
Kai Iketaki 1 , Toshihiko Kaji 1 , Satoru Nakao 1 , Masahiro Hiramoto 1
1 , Institute for Molecular Science, Okazaki Japan
Show Abstract Organic solar cells have attracted much attention because of their potential for low-cost solar energy conversion. For organic p-i-n solar cells, it is important to control the nanostructure of the i-interlayer to increase the power conversion efficiency. We report the nanostructure of the codeposited i-layer of ZnPc:C60 p-i-n solar cells, including the molecular orientation of ZnPc. The nanostructure of ZnPc:C60 p-i-n cells was controlled by changing the substrate temperature in the range from 23 to 120 °C during codeposition. At a substrate temperature of 60 °C, the short-circuit current density of 11.8 mA/cm2 and the power conversion efficiency of 2.7% have been obtained under AM 1.5 simulated solar illumination of 100 mW/cm2. X-ray diffraction reveals that ZnPc molecules form a regular π-stacked structure in the i-layer regardless of the substrate temperature. The π-stacking direction is parallel to the substrate, because only the (200) reflection of ZnPc was observed. In addition, the crystallite size along [200] is estimated to be 20−30 nm. Scanning electron microscope images of the cross-section of the i-layer show a granular structure with a grain size of 20−30 nm. On the basis of these results, a crystallite of ZnPc, including the molecular orientation, can be depicted. It appears that the π-stacking direction is not necessarily suitable for hole transport to the electrode. Thus, higher power conversion efficiencies may be achieved by changing the π-stacking direction from parallel to perpendicular to the substrate.
9:00 PM - S3.33
Enhanced Photovoltaic Properties of Interpenetrating Heterojunction Type Organic Solar Cells by Improvement of Crystallinity.
Tetsuro Hori 1 , Varutt Kittichungchit 1 , Hiroki Moritou 1 , Hitoshi Kubo 1 , Akihiko Fujii 1 , Masanori Ozaki 1
1 Division of Electrical, Electronic and Information Engineering, Graduate School of Engineering, Osaka University, Suita, Osaka Japan
Show AbstractIn this study, we report on the enhanced photovoltaic properties of interpenetrating heterojunction type organic solar cells by improvement of crystallinity. In the organic thin-film solar cells, it is ideal state that donor/acceptor has three dimensional contact and the carrier path is formed. In fact, it is very suitable for the organic thin-film solar cells to make the interpenetrative interface of fullerene (C60) and conducting polymer, that is, the donor/acceptor interpenetrating interface. We use poly(3-hexylthiophene) (PAT6) as a conducting polymer and the typical structure of solar cell is indium-tin-oxide(ITO)/C60/PAT6/Au.The enhancement of the photovoltaic properties of interpenetrating heterojunction type organic solar cells by heating substrates during the C60 vapor deposition are reported. The substrate heating during the C60 deposition improves the crystallinity of C60 film, and the photovoltaic properties of interpenetrating heterojunction type organic solar cell were enhanced. From atomic force microscopy (AFM) and X-Ray Diffraction (XRD) of C60 film the crystallinity was confirmed, and C60 film fabricated without heating substrates was amorphous. The improved crystallinity of C60 film by heating substrates, results in increasing the electron mobility. Therefore, it is considered that the photovoltaic properties of interpenetrating heterojunction type organic thin-film solar cell are enhanced by theating substrate in C60 film fabrication process. [Varutt Kittichungchit, Takeshi Shibata, Hideki Noda, Hiroshi Tanaka, Akihiko Fujii, Noriaki Oyabu, Masayuki Abe, Seizo Morita and Masanori Ozaki: Jpn. J. Appl. Phys., 47, 1094, (2008).]We also report the improved photovoltaic properties of interpenetrating heterojunction type organic solar cells by solvent vapor treatment. In the interpenetrating heterojunction type organic thin-film solar cells, the improvement of the crystallinity of the PAT6 layer itself is also essential. The crystallinity of PAT6 film fabricated by spin-coating is low, therefore we tried to improve the crystallinity of PAT6 layer by solvent vapor treatment. The crystallinity of the PAT6 films could be confirmed by XRD. As a result, the solvent vapor treatment enhances the crystallinity and electrical characteristics (the hole mobility and the conductivity) of the PAT6, and the photovoltaic properties of interpenetrating heterojunction type organic solar cell were improved. [Varutt Kittichungchit, Tetsuro Hori, Hiroki Moritou, Hitoshi Kubo, Akihiko Fujii and Masanori Ozaki: Thin Solid Films, in press, (2009).]
9:00 PM - S3.35
Electrical Effect of Layers Adjacent to Photoactive Region in Bulk Heterojunction Organic Solar Cells.
John Tumbleston 1 , Doo-Hyun Ko 2 , Edward Samulski 2 , Rene Lopez 1
1 Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States, 2 Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States
Show AbstractVarious materials are frequently incorporated between the photoactive layer and either electrode in bulk heterojunction organic solar cells to boost optical performance (e.g. optical spacers) and/or electrical function (e.g. electron/hole transporting layers). We show that these layers can influence electrical performance and affect the interpretation of physical processes restricted to the photoactive layer. For example, the experimental exciton dissociation probability changes by two orders of magnitude depending on the mobility of an electron transporting layer which can be tuned via treatment with ultraviolet light. This work sets the stage for a generalized metal-insulator-metal model that includes layers adjacent to the photoactive region which are disregarded in the traditional electrical description. Conditions are also given in terms of the electrical properties of these layers that describe when it is appropriate to use the traditional model and when the more general multi-layer model must be invoked.
9:00 PM - S3.36
Highly Conductive PEDOT:PSS Films and Their Application to ITO-Free Organic Solar Cells.
Seok-In Na 1 , Gunuk Wang 1 , Seok-Soon Kim 2 , Takhee Lee 1 , Dong-Yu Kim 1
1 , GIST, Gwang-Ju Korea (the Republic of), 2 , KNU, Kunsan Korea (the Republic of)
Show AbstractAmong the available polymer-based BHJ systems, poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) networks produced by spin-coating the blends have shown the highest efficiency (4-5 %); however, the current state-of-the-art polymer solar cells (PSCs) are typically fabricated on rigid glass substrates coated with indium tin oxide (ITO), and, as a result, do make a limitation for low-cost photovoltaic energy conversion due to the ever-increasing cost of indium. The realization of organic solar cells (OSCs) with flexible, low-cost, and high-efficiency will be the obvious goal in this field. For this reason, use of high-conductivity PEDOT:PSS films as anodes has received significant attention. However, compared with ITO-based cells, ITO-free OSCs using PEDOT:PSS films normally showed poor performance due to their low and anisotropic conductivity. More importantly, in use of PEDOT:PSS films as electrodes, key research areas remain to be addressed: the evolution of electronic and morphological properties with processing; the origin of the enhanced conductivity in solvent-modified PEDOT:PSS films; and, the optimal parameters for films to be used as anodes in ITO-free cells. In this respect, highly conductive PEDOT:PSS films, obtained by addition of a polar solvent to an aqueous solution of PEDOT:PSS, were thoroughly investigated to gain a deeper understanding of the fundamental characteristics of the solvent-modified PEDOT:PSS film. Use of the solvent-modified PEDOT:PSS film as a transparent anode to achieve low-cost and high-efficiency ITO-free organic solar cells (IFOSCs) based on P3HT and PCBM was also examined.
9:00 PM - S3.37
Novel Organic Photovoltaic Devices Using Electrochemically Grown Polymers.
He Gao 1 , Yixuan Chen 1 , Yi Luo 1
1 Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Show AbstractIn most current polymer photovoltaic devices, solution based poly-thiophene derivatives (e.g. P3HT) play a critical role as active photon absorption materials, and ITO is the dominant material for transparent electrodes. On the other hand, electrochemically polymerized conducting polymers exhibit distinct advantages that can potentially lead to alternative solutions for polymer photovoltaic applications. The electrochemically grown conducting polymers can provide adequate optical and electrical properties, and excellent solvent resistance. Here we report a new approach of fabricating conducting polymer hetero-junction photovoltaic devices using electrochemically grown polymer layers for both photon-charge carrier conversion and transparent electrode.In our study, polymer layer(s) are grown on the surface of a metal electrode using electrochemical method. The grown polymers are further transferred onto a transparent substrate (either rigid or flexible) with the help of a thin layer of transparent adhesive. Photovoltaic devices that contain single or multiple layers of different conducting polymers can be fabricated with this method.Our investigation involves three aspects: (1) Electrochemically grown poly-thiophene (PT) as active photon absorption material: A PT layer is grown on a platinum thin film on a transparent substrate. A PCBM layer is then spin-cast on top of PT followed by aluminum deposition. Significant photovoltaic effects have been observed from this device, indicating excellent light absorption and exciton transport properties for the electrochemically grown PT. (2) Electrochemically grown poly(3,4-ethylenedioxythiophene) (PEDOT) as transparent electrode: A highly doped PEDOT layer is grown electrochemically on a platinum electrode, and is subsequently transferred to a transparent substrate without ITO. A mixture of P3HT and PCBM is then spin-cast on top of the PEDOT layer, followed by the deposition of aluminum electrode. This device also exhibits photovoltaic effects which demonstrates the feasibility of using the electrochemically grown PEDOT in a solar cell without ITO. (3) Finally, device with both PT and PEDOT grown electrochemically: With a double-layer scheme, PT layer is first grown on a platinum electrode followed by the growth of PEDOT layer. The double-layer is then transferred in a single step onto a transparent substrate without ITO, and is subsequently covered by PCBM and aluminum. Such devices yield robust photovoltaic effects, suggesting that this new material/fabrication approach can potentially lead to novel multi-layer solar cells that do not contain ITO substrates.
9:00 PM - S3.4
Degradation of Single Layer MEH-PPV Light Emitting Diodes by Nucleated Spiral Blisters.
Wali Akande 1 3 , Onobu Akogwu 2 3 , Tiffany Tong 1 3 , W. Soboyejo 2 3
1 Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 3 Princeton Institute for the Science and Technology of Materials (PRISM), Princeton University, Princeton, New Jersey, United States, 2 Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey, United States
Show AbstractThis paper presents the results of a study into adhesion-related degradation mechanisms in single layer polymer light emitting diodes with Poly [2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene] (MEH-PPV) as the active layer. The study reveals spiral-shaped blister patterns that are associated with rapid degradation in the electrical performance of the devices. Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) images show these blister patterns to be spiral telephone cord failures that propagate as a result of compressive thermal stresses in the device. A theory based on thin film mechanics is used to explain the formation and growth of these defects.
9:00 PM - S3.5
Improving Operating Lifetime by Enhancing Hole Injection in Organic Light.
Song Chen 1 , Jaewon Lee 1 , Mikail Shaikh 1 , Franky So 1
1 Dept of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractThe effect of hole injection layer on the operating lifetime in organic light emitting diodes (OLEDs) was studied by using two different hole injection polymer layers {Polyethylene dioxythiophene polystyrene sulfonate (PEDOT:PSS) and polythienothiophene poly(perfluoroethylene-perfluoroethersulfonic acid) (PTT-PFFSA)}. We found that hole injection polymers with large work function significantly increases the OLED operating lifetime. Capacitance-voltage (CV) characteristics were measured to investigate the degradation mechanisms. Our data showed that hole accumulation at the indium tin oxide (ITO) /hole transporting layer (HTL) interface generates fixed charge resulting in device degradation. The OLEDs used in this study has the following structure: ITO /no hole injection layer (HIL) or PEDOT:PSS (30nm) or PTT-PFFSA (40nm)/α-NPD (60nm)/Alq3(60nm)/LiF/aluminum. While the devices without the HIL has the shortest lifetime, the operating lifetime of the devices with PTT-PFFSA as a HIL is significantly longer than that of the device with PEDOT:PSS. The lifetime of OLED with PEDOT:PSS is longer than that of device without HIL. The transition voltage (V0) obtained in the CV measurements shifts to larger voltage as the devices degrade. The shift in transition voltage observed here is attributed to the formation of fixed charge at the ITO/HTL interface and the degradation rate is related to the hole injection barrier height. By inserting a large working function HIL between the ITO anode and the HTL, the lifetime is substantially enhanced.
9:00 PM - S3.7
Organic Light Emitting Device with Reduced Efficiency Roll-off Behavior by Managing Charge Carriers and Excitons.
Tianhang Zheng 1 , Wallace C.H. Choy 1 , Wai-Yeung Wong 2
1 Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong China, 2 Department of Chemistry, Hong Kong Baptist University,, Hong Kong China
Show AbstractRecently, Organic light emitting device (OLED) with 100% internal quantum efficiency in red, green and white color emission can be realized by using phosphorescent-type emitters, which move OLED forward for become a practical candidate for solid state lighting applications. However, one issue existing related to phosphorescent-type OLED is that the device efficiency drops significantly with the increase of driving current, which is called efficiency roll-off behavior. Previous studies have shown that such behavior is mainly induced from (a) triplet-triplet annihilation due to the long decay time of triplet excitons in phosphorescent dopants, (b) triplet-polaron annihilation and (c) the electric field caused dissociation of excitons. Although some methods have been proposed presently, for instance, the adoption of mixed hosts, doubled emission layer (EML), new electron transport layer with high mobility, and adding exciton blocking layer, it is desirable for further improvement and alternative strategies to reduce efficiency roll-off behavior in OLED.Here, we propose a new approach to solve this issue by doping fluorescent- and phosphorescent-type emitters respectively into two different layers to form a fluorescence-interlayer-phosphorescence (FIP) structure. The structure will function as the emission region of the OLED. Our green OLEDs with FIP EML exhibit a much smaller efficiency roll-off value of 26% compared with that of 51% for conventional phosphorescent OLED with single EML in 5-150 mA/cm2 range. The mechanism of such improvement has been discussed based on characteristics of the electrical and optical properties of the materials and device structures. It shows that such enhancement should be attributed to the improved balance of charge carriers, the redistribution of excitons in recombination zone, the suppression of non-radiative exciton quenching processes and the elimination of energy transfer loss with the help of FIP EML. Our method may provide a route to develop efficient OLED for high luminance applications.Keywords: Organic Light Emitting Devices; Fluorescence; Phosphorescence; Energy Transferring; Efficiency Roll-off
9:00 PM - S3.8
Hybrid White Organic Light Emitting Diodes Processed by Organic Vapor Phase Deposition.
Manuel Boesing 1 , Florian Lindla 1 , Christoph Zimmermann 1 , Frank Jessen 1 , Philipp van Gemmern 2 , Dietrich Bertram 2 , Nico Meyer 3 , Dietmar Keiper 3 , Michael Heuken 1 3 , Holger Kalisch 1 , Rolf Jansen 1
1 Chair of Electromagnetic Theory (ITHE), RWTH Aachen University, Aachen Germany, 2 , PHILIPS Technologie GmbH, Aachen Germany, 3 , AIXTRON AG, Herzogenrath Germany
Show AbstractPhosphorescent white organic light emitting diodes (OLED) are about to outperform fluorescent tubes in terms of luminous efficiency. Unfortunately, the lifetime of most phosphorescent white OLED is strongly limited by the poor lifetime of today’s phosphorescent blue emitters. The concept of hybrid white OLED combines the high luminous efficiency of the robust phosphorescent red and green emitting materials and the long lifetime of fluorescent blue emitting materials and therefore offers the possibility to realize efficient pure-white OLED with a long lifetime. The hybrid white OLED discussed have been processed by organic vapor phase deposition (OVPD) on 6" x 6" ITO-coated glass plates. Parasitic triplet exciton diffusion from the phosphorescent emissive layers into the blue fluorescent emissive layer has been identified as a major process of efficiency loss also hampering the control of the color point. By employing dicarbazolylbiphenyl (CBP) as an interlayer with a high triplet energy level and with reasonable hole and electron mobilities, triplet exciton blocking and charge carrier recombination at both sides of the interlayer could be obtained.Different concepts to tune the color coordinates of the hybrid white devices are discussed with respect to the luminous efficiency. When tuned to cold-white color coordinates (CIE 0.36/0.38), the device reached a luminous efficiency of 16.0 cd/A and 10.2 lm/W whereas, when tuned to warm-white color coordinates (CIE 0.42/0.42), an efficiency of 20.0 cd/A and 13.3 lm/W was achieved (without additional light outcoupling enhancement). Lifetime values were around 500 h and will be discussed in comparison to those of the corresponding monochrome OLED stacks.
9:00 PM - S3.9
Effects of Horizontal Molecular Orientation on Charge Transport Characteristics in Organic Amorphous Films.
Daisuke Yokoyama 1 , Akio Sakaguchi 2 , Michio Suzuki 2 , Chihaya Adachi 1
1 , Center for Future Chemistry, Kyushu Univ., Fukuoka Japan, 2 , J. A. Woollam Japan Co., Tokyo Japan
Show Abstract Organic semiconductor amorphous films fabricated by vacuum deposition have played an important role in the development of organic light-emitting diodes (OLEDs). The advantages of organic amorphous films, such as smoothness of the surface and easy controllability of the thickness, have in turn significantly increased the rate of development of organic thin film devices in the past 20 years. However, little attention has been paid to the molecular orientation in vacuum-deposited organic amorphous films because it has generally been believed to be random. In this study, to demonstrate the importance of the molecular orientation in organic amorphous films, we discuss horizontal molecular orientation in vacuum-deposited organic amorphous films and its effect on device characteristics. First, we focused on the optical properties of organic amorphous films to obtain the information on the molecular orientation in the films. The horizontal orientation of linear-shaped styrylbenzene derivatives and many kinds of charge transport materials in vacuum-deposited amorphous films was investigated using wide-range variable angle spectroscopic ellipsometry and cutoff emission measurement. The dependence of the orientation on molecular structure shows that the anisotropy in the molecular structure causes the anisotropy in the molecular orientation in the films. The longer the molecular length is, the larger the anisotropy in the molecular orientation becomes, leading to the large anisotropy in the optical properties of the film. This horizontal orientation could be observed on any underlying layers, which means that it can be utilize in devices. In addition, molecular orientation can be controlled by the substrate temperature during deposition. Next, we investigate the effects of the horizontal molecular orientation on charge transport characteristics using electron transport materials of oxadiazole derivatives. Although the main conformers of the single molecules of 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD7) and 1,3-bis[2-(2,2’-bipyridin-6-yl)-1,3,4-oxadiazo-5-yl]benzene (Bpy-OXD) have similar geometric structures and LUMO, the electron mobilities in their films were quite different from each other. We determined the optical anisotropies of the OXD7 and Bpy-OXD films using variable angle spectroscopic ellipsometry and found that the Bpy-OXD film has significantly large anisotropy in the molecular orientation, whereas the OXD7 film is isotropic. We conclude that this orientation of Bpy-OXD is due to the highly planar structure of the conformers and it makes the intermolecular overlaps of LUMOs larger, leading to the high electron mobility. We stress that the molecular orientation is a crucial factor which affects charge transport characteristics even in vacuum-deposited amorphous films.
Symposium Organizers
Jiangeng Xue University of Florida
Chihaya Adachi Kyushu University
Russell J. Holmes University of Minnesota
Barry P. Rand IMEC vzw
S4: Device Fundamentals for Organic Photovoltaics
Session Chairs
Tuesday AM, December 01, 2009
Room 210 (Hynes)
9:30 AM - **S4.1
Excitons, Charges, and Crystal Structure: Their Fundamental Influence on Organic Solar Cell Efficiency.
Stephen Forrest 1
1 EECS and Physics, Univesity of Michigan, Ann Arbor, Michigan, United States
Show AbstractIn this talk, we discuss several strategies toward achieving high power conversion efficiency in organic solar cells. Growth and crystalline order is one such pathways. We will discuss methods including organic vapor phase deposition and growth from the liquid phase, to achieve long range crystalline order in materials that are important candidates for use in organic solar cells. The relationship between crystalline order, charge mobility, chemical composition and exciton diffusivity has been explored in detail. For example, we show an unambiguous dependence of exciton diffusion length on the extent of crystalline order. Indeed, in some cases, we are able to extend the crystalline domain size to that of the substrate. We demonstrate several solar cell architectures and new donor/acceptor materials systems that result in increased power conversion efficiencies and relate their performance to a fundamental understanding of charge and energy transport dynamics developed in this study.
10:00 AM - **S4.2
Electronic Processes at Heterojunctions in Organic Solar Cells.
Jean-Luc Bredas 1
1 Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractIn general, our work seeks to provide a detailed theoretical characterization of the electronic and optical processes taking place in organic solar cells. In this presentation, we report our recent results on: (i) the description of the electronic structure of low bandgap polymers and the relationship between localization of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO levels) and the oscillator strength of the lowest-energy optical transition; and (ii) the description of the donor-acceptor (D/A) interface in terms of: the electronic structure of the ground and excited electronic states; the polarization effects due to the inhomogeneous nature of the surrounding medium at the heterojunctions; and the role of interfacial molecular packing and of the intra- and inter-molecular geometry relaxations upon excitation. Our goal here is to provide a coherent quantum-mechanical characterization of the ultrafast charge-transfer (CT) and charge-separation (CS) processes occurring at D/A interfaces. This requires the determination of the electronic couplings between the molecular (exciton) states and CT/CS states, which we calculate by using a computational approach recently developed in our group.
10:30 AM - S4.3
Interfacial Excited States in Organic Donor-acceptor Blends: Limits to Performance in Organic Photovoltaic Devices.
Clare Dyer-Smith 1 2 3 , Saif Haque 3 , Donal Bradley 1 , Jenny Nelson 1
1 Department of Physics, Imperial College, London, London, United Kingdom, 2 Grantham Institute for Climate Change, Imperial College, London, London, United Kingdom, 3 Department of Chemistry, Imperial College, London, London, United Kingdom
Show AbstractThe efficiency of photoinduced charge separation is an important factor in determining the power conversion efficiency of organic donor-acceptor blend solar cells. Several studies have highlighted the importance of the energy level offset at the donor-acceptor interface, which may influence both charge separation efficiency and Voc. Increasing the offset between the donor HOMO and acceptor LUMO (identified as the energy of the charge separated state ΔEcs) is expected to improve Voc (Scharber, Advanced Materials 2006), but may decrease photocurrent through a number of mechanisms, including a lowering of the driving force for charge separation or through the formation of other excited states which compete with charge separation and provide pathways for exciton energy loss. We have explored this competition between charge separation and other excited states in polymer:fullerene blends using a series of polyfluorene polymers in combination with PCBM and related fullerene acceptors, for which the level of the LUMO is changed by altering the side groups. This allows us to study the effect of changing the blend energy levels upon the probability of forming charge pairs relative to other excited states in the system. Changing the acceptor LUMO is expected to increase the open circuit voltage of the solar cell, as has been recently demonstrated using P3HT as the donor polymer (Lenes, Advanced Materials 2008). However, we have identified a threshold for the HOMO-LUMO offset, above which triplet formation competes with charge separation. Using transient absorption spectroscopy and device measurements we show that the formation of triplets is associated with a loss in photocurrent. In addition, time-resolved PL decay measurements are used to probe the mechanism of this alternative decay pathway, which is believed to result from energy transfer from donor to acceptor, followed by intersystem crossing within the PCBM derivative. These results demonstrate a limitation to the open circuit voltage obtainable using PCBM and related acceptors with high-IP polymers, and will therefore be of importance for the development of new material combinations.
10:45 AM - S4.4
Enhanced Exciton Diffusion in an Organic Photovoltaic Cell by Energy Transfer using a Phosphorescent Sensitizer.
Wade Luhman 1 , Russell Holmes 1
1 Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractA key challenge for realizing organic photovoltaic cells (OPVs) with high power conversion efficiency is the characteristically short exciton diffusion length (LD) of many potential active materials. Typically the LD of these materials is much less than the optical absorption length, forcing the use of thin active layers which limit OPV absorption efficiency. Several approaches have been investigated to overcome this “exciton bottleneck” including the use of bulk heterojunctions and uniform layers of phosphorescent materials. In this work, energy transfer via a phosphorescent sensitizer is used to populate the long-lived triplet state of a fluorescent donor material, permitting an increase in LD and active layer thickness. We demonstrate enhanced exciton diffusion in an organic photovoltaic cell using a composite electron donor layer consisting of a N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPD) host doped with the phosphorescent guest fac-tris(2-phenylpyridine) iridium (Ir(ppy)3). The enhancement in LD relies on a multistep process beginning with the absorption of light and formation of singlet excitons on the NPD host. The generated excitons are transferred to the singlet state of the Ir(ppy)3 guest by Förster transfer, followed by rapid intersystem crossing to the triplet state. A final transfer occurs from the triplet level of Ir(ppy)3 to that of NPD. With the photogenerated excited state occupying the NPD triplet, a larger LD is possible due to the long lifetime of the triplet relative to the singlet. An increase in the NPD LD from (6.5±0.3) nm to (11.8±0.6) nm is extracted from measurements of the external quantum efficiency for donor layers containing 5 wt.% Ir(ppy)3. This enhancement in LD leads to a ~80% improvement in the power conversion efficiency relative to devices containing an undoped donor layer. This approach allows for a decoupling of the functions of optical absorption and exciton diffusion, potentially broadening the scope of materials suitable for use in OPVs.
11:30 AM - **S4.5
Charge Density Measurements, Charge Mobility and Recombination Dynamics in Polymer/Fullerene Solar Cells.
James Durrant 1
1 Chemistry, Imperial College London, London United Kingdom
Show AbstractThe function of organic solar cells is based upon the photogeneration of dissociated charge carriers at a donor / acceptor interace, and the subsequent collection of this charges by the device electrodes [1]. In my talk I will consider the efficiency of these processes, and in particular the importance of geminate (or monomolecular) and bimolecular recombination processes in limiting the steps of, respectively, charge dissociation and collection in these devices.My talk will focus on P3HT/PCBM bulk heterojunction devices, and analogues thereof employing alternative electron donating polymers and small molecule electron acceptors. Experimentally, my talk will focus on the use of transient optical and optoelectronic techniques to monitor charge carrier densities, yields and lifetimes both in thin films and functioning devices. These measurements will be correlated with materials and film structure and morpholology, and furthermore with device current / voltage performance. The main part of my talk will focus upon the current / voltage performance of complete devices, and in particular upon the quantification of bimolecular recombination losses in limiting device performance. Transient absorption and photovoltage measurements will be employed to determine charge carrier lifetimes on the micro- to milli-second time scales in devices operating under cw bias illumination [3]. Charge carrier densities will be determined by differential charging and charge extraction. From these data, we determine the charge carrier losses in the device under operation. We furthermore employ these data to analysis the charge density dependence of carrier mobility. We show, for P3HT/PCBM devices, that these losses are dominated by bimolecular recombination processes. Employing a simple device model based upon these bimolecular recombination losses, we show that we are able to obtain an excellent simulation of the device current / voltage performance. [1] Brabec CJ & Durrant JR, MRS Bulletin, July 2008[2] Ohkita et al. J. Am Chem Soc 2008, 130, 3030 – 3042[3] Shuttle et al., App. Phys. Lett 2008 92, ISSN: 0003-6951
12:00 PM - **S4.6
Engineering Charge Transfer State Recombination in Organic Photovoltaic Cells.
Jiye Lee 1 , Timothy Heidel 1 , Marc Baldo 1
1 EECS, MIT, Cambridge, Massachusetts, United States
Show AbstractThe open circuit voltage in a solar cell is a measure of the internal electric field required to force recombination of the photogenerated charges. The recombination rate is therefore a crucial determinant of both the external quantum efficiency and open circuit voltage of a solar cell. This presentation will summarize our work on charge recombination in small-molecular weight and polymeric organic cells, including the spin dependence of charge recombination, direct spectroscopy of charge transfer states, and engineering interfacial layers to block charge recombination. We present spectroscopic data demonstrating that photoexcitation of excitons is more likely to yield photocurrent than direct photoexcitation of bound charge transfer states. This suggests that the dissociation of excitons does not necessarily involve intermediate bound charge transfer states. To test this further, we have also investigated the spin dependence of charge recombination by selectively creating either singlet or triplet excitons. Comparisons of the open circuit voltage at constant short circuit current exhibit only a minimal dependence on spin. The longer lifetime expected from the triplet charge transfer state does not substantially reduce recombination losses, suggesting again that bound charge transfer states are not necessarily involved in photocurrent generation but that the formation of a bound charge transfer state will result in charge recombination. Finally, we demonstrate small molecular weight devices with a multiple step charge dissociation process at the donor-acceptor interface. We insert a material with energy levels positioned between those of the acceptor and donor. This destabilizes the charge transfer states at the donor-acceptor interface and we observe that about one monolayer of the intermediate material can substantially enhance both the short circuit current and the open circuit voltage by reducing recombination rates.
12:30 PM - S4.7
Understanding Organic Solar Cells Near Open Circuit: Direct Evidence for the Roles of Geminate and Bimolecular Recombination.
Noel Giebink 1 2 , Brian Lassiter 2 , Stephen Forrest 2
1 Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 2 Electrical Engineering, Materials Science, and Physics, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractIt is generally accepted that the open circuit voltage (Voc) of an organic photovoltaic cell is limited by the energy difference between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor.1 Unfortunately, most cells exhibit Voc significantly below this maximum under standard operating conditions. This discrepancy has been attributed to both geminate2 and non-geminate (bimolecular)3 recombination losses, however, the extent and role of each in determining Voc remain unclear. Here, we directly monitor geminate recombination at the interface of a planar boron subphthalocyanine chloride /C60 solar cell by recording the intensity of its associated exciplex emission, whereas non-geminate recombination is probed with intensity modulated photocurrent spectroscopy (IMPS).4 We find that the rates of both geminate and non-geminate recombination increase with forward bias, and that non-geminate recombination becomes dominant approaching Voc. Extending this approach to a copper phthalocyanine /C60 solar cell, we show that its comparatively lower Voc is a consequence of significantly increased non-geminate recombination. The increase in Voc observed upon decreasing temperature directly corresponds to a decrease in the rate of non-geminate recombination. We comment on the physical basis underlying the difference in recombination rates between these two cells, and their implications for designing organic solar cells with Voc closer to that of the theoretical limit.1B. P. Rand, D. P. Burk, and S. R. Forrest, Phys. Rev. B 75, 115327 (2007).2R. A. Marsh, C. R. McNeill, A. Abrusci, A. R. Campbell, and R. H. Friend, Nano Lett. 8, 1393 (2008).3C. G. Shuttle, B. O'Regan, A. M. Ballantyne, J. Nelson, D. D. C. Bradley, and J. R. Durrant, Phys. Rev. B 78, 113201 (2008).4L. Dloczik, O. Ileperuma, I. Lauermann, L. M. Peter, E. A. Ponomarev, G. Redmond, N. J. Shaw, and I. Uhlendorf, J. Phys. Chem. B 101, 10281 (1997).
12:45 PM - S4.8
On the Origin of the Open-circuit Voltage of Polymer:Fullerene Composite Solar Cells.
Koen Vandewal 1 , Kristofer Tvingstedt 2 , Abay Gadisa 1 , Olle Inganas 2 , Jean Manca 1
1 Institute for Materials Research (IMO), Hasselt University, Diepenbeek Belgium, 2 Department of Physics, Chemistry and Biology, Linköping University, Linköping Sweden
Show AbstractIn order to increase the efficiency of polymer:fullerene solar cells, a better understanding of the origin and of the fundamental limits of the open-circuit voltage (Voc) is crucial. In this work we demonstrate that Voc in this type of photovoltaic devices is limited by the formation of weak ground-state charge transfer complexes between polymer and fullerene. Charge transfer (CT) optical transitions are shown to dominate the electroluminescence spectrum1 and the low energy region of the photovoltaic external quantum efficiency spectrum2. In order to illustrate the universal validity of these findings, devices comprising five conjugated polymers from different material families blended with C60 and C70 fullerenes are studied. We show that these CT transitions are related to Voc as predicted by the assumptions of quasi-equilibrium and detailed balance 3, 4, without making use of any fitting parameter. This work underlines the importance of ground-state interaction between polymer and fullerene and suggests pathways to improve the Voc of donor/acceptor based devices.References1 K. Tvingstedt, K. Vandewal, J. V. Manca, O. Inganäs, J. Am. Chem. Soc. (2009), submitted.2 K. Vandewal, A. Gadisa, W. D. Oosterbaan, S. Bertho, F. Banishoeib, I. Van Severen, L. Lutsen, T. J. Cleij, D. Vanderzande, J. V. Manca, Adv. Funct. Mater. 18 (2008) 2064.3 W. Shockley, H. J. Queisser, J. Appl. Phys. 32 (1961) 510.4 U. Rau, Phys. Rev. B 76 (2007) 085303.
S5: Interfaces and Electrodes
Session Chairs
Tuesday PM, December 01, 2009
Room 210 (Hynes)
2:30 PM - **S5.1
Interface Engineering and the Performance of Organic Photovoltaic Systems.
Tobin Marks 1
1 Chemistry, Northwestern U., Evanston, Illinois, United States
Show AbstractThe ability to fabricate molecularly tailored interfaces with nanoscale precision for enhancing charge transport through hard matter-soft matter interfaces, and thus enhancing photonic efficiency in organic optoelectronic structures, presents great challenges and opportunities, both scientific and technological. In this lecture, these challenges and opportunities are illustrated for charge transport across hard matter-soft matter interfaces in organic photovoltaic cells. For the latter, power conversion efficiencies as high as 5.6% are achieved in organic bulk-heterojunction cells.
3:00 PM - **S5.2
Interface Effect and Bulk Carrier Transport in Organic Photovoltaic Cells.
Franky So 1
1 Dept of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractOrganic photovoltaic cells are attractive for the next generation photovoltaics because of their compatibility with flexible substrates and low manufacturing costs. In this presentation, we will show our recent results on interface effect and bulk carrier transport in organic bulk heterojunction small molecule and polymer bulk heterojunction solar cells. For interface effect study, we have studied the effect of molybdenum oxide interlayer at the ITO electrode on solar cell performance. The power conversion efficiencies of small molecule cells with the molybdenum oxide interlayer were enhanced by 38% due to a significant enhancement in the fill factor. The improved fill factor is attributed to the reduction of series resistance. Our ultraviolet photoemission spectroscopy data indicate the strong band bending and the built-in field at the oxide interface leads to enhancement in hole extraction towards the anode. For carrier transport study, we have carried out CELIV and photoconductivity measurements on different polymeric bulk heterojunction cells to determine the factors limiting the cell efficiency. The results of our measurements indicate that the low carrier mobility, imbalance of carrier transport and low charge separation efficiency are responsible for the low cell efficiency.
3:30 PM - S5.3
Electronic Structure of C60/Subphthalocyanine/ITO Interfaces Studied Using Soft X-ray Spectroscopies.
Sang Wan Cho 1 , L. Piper 1 , A. DeMasi 1 , A. Preston 1 , Kevin Smith 1 , K. Chauhan 2 , T. Jones 2
1 Department of Physics, Boston University, Boston, Massachusetts, United States, 2 Department of Chemistry, University of Warwick, Coventry United Kingdom
Show AbstractThe interface electronic structure of a bilayer of C60 and boron subphthalocyanine chloride (SubPc) grown on indium tin oxide (ITO) has been studied using synchrotron radiation-excited photoelectron spectroscopy. The energy difference between the highest occupied molecular orbital (HOMO) level of the SubPc layer and the lowest unoccupied molecular orbital (LUMO) level of the C60 layer (EDHOMO – EALUMO) was determined and compared with those of copper phthalocyanine (CuPc)/C60 bilayers and chloro-aluminum phthalocyanine (ClAlPc)/C60 bilayers. The EDHOMO – EALUMO of a heterojunction with SubPc was found to be 1.75 eV, while that with CuPc and ClAlPc were 1.00 eV and 1.15 eV respectively. This difference is discussed in terms of the difference of the ionization energy of each donor materials. Additionally, we have studied the molecular orientation of SubPc on ITO using angle-dependent x-ray absorption spectroscopy. The SubPc film showed significant disorder compared to the CuPc and ClAlPc films.This work was supported in part by the NSF under Grant No. CHE-0807368. Experiments were performed at the NSLS, which is supported by the U.S. DOE. Financial support from the EPSRC, UK is also acknowledged.
3:45 PM - S5.4
Thickness-tunable Metal Doped Molybdenum Oxide Spacer Layers in Organic Photovoltaic Devices.
David Cheyns 1 , Benjamin Kam 1 2 , Barry Rand 1 , Claudio Girotto 1 2 , Paul Heremans 1 2
1 OPV, imec vzw, Leuven Belgium, 2 esat, KULeuven, Leuven Belgium
Show AbstractThe high absorption coefficients and low charge carrier mobilities of organic semiconductors produce significant restraints on the layer thicknesses used in organic solar cells. Typically, the total device thickness is approximately that of the wavelength of incident light in the organic layers. Thus, reflection and transmission of photons at each interface generates a complex energy distribution inside the device that depends on the refractive index and layer thickness, and which needs to be taken into account when optimizing photocurrent.One technique that has been successful for photocurrent optimization is to introduce transparent layers that can be used as optical spacers. A promising material class for use as optical spacers is the class of metal oxides. These materials tend to be transparent, and have electrical characteristics ranging from insulating to (semi-)conducting. In this work, we focus on molybdenum oxide (MoO3) as a hole conducting optical spacer. This material is highly transparent in the wavelength range beyond 350 nm and yields smooth films that are solvent-resistant, making them ideal for further processing.Besides transparency, good film formation, and solvent resistance, spacer layers should possess sufficient conductivity such that layer thickness can be tuned without consequences to the electrical performance of the cell. However, we find that increasing the thickness of an undoped MoO3 spacer layer significantly increases diode series resistance, leading to a decreasing fill-factor (FF), with values below 30% for 100 nm thick MoO3 layers. By doping the MoO3 layer with small amounts of metal, the conductivity is improved by more than one order of magnitude, and cell series resistances are substantially reduced. The metal of choice turns out to be unimportant, as the effect is reproduced with Ag, Au, Al and Yb doping. Using the doped spacer, the FF remains nearly constant with spacer thickness, while the effect of the absorption of the MoO3 layer on the photocurrent is small. The optimal volume ratio of doping atoms is found to depend on the required thickness of the spacer. In order to maintain FF >55%, the usable layer thickness is limited to 10 nm for undoped spacers. This increases to 50 nm for 0.5% Ag doping, and above 100 nm for 2% Ag doping. The loss in short-circuit current for a 100 nm thick spacer starts from 12% without doping, and increases to 25% for 2% doping. This loss is reproduced in optical simulations, and is attributed to the increasing absorption of the metal atoms in the MoO3 matrix. The resulting optical spacers are therefore ideal candidates in complex structures (inverted or tandem devices), where the location of the interference pattern has important implications on the device performance.
4:30 PM - **S5.5
Transparent and Processible Polymer Anodes for Flexible Solar Cells.
Olle Inganas 1 2 , Kristofer Tvingstedt 1 2 , Tang Qun 1 2
1 Biomolecular and organic electronics, IFM, Linköpings Universitet, Linköping Sweden, 2 Center of Organic Electronics (COE) IFM, , Linkopings Universitet, Linkoping Sweden
Show AbstractSystems design of modules of flexible polymer solar cells requires connections of individual solar cells in parallel and/or serial configuration, to fully cover a surface with the photovoltaic converters. Printing such modules on very large surfaces with reel-to-reel methods at rapid rates requires patterned deposition of transparent anodes (or cathodes). Such materials do not have sufficient conductivity for making very large devices; current collectors made of metals are necessary to bring currents to the outer world without excessive ohmic losses in the transparent electrodes. The balance between loss of current due to shadowing of solar cells due to such current collectors and the loss of power due to ohmic losses in electrodes can be found. We have used silver metal current collectors, patterned on the 100 µm scale, together with the transparent polymer electrode PEDOT(PSS), to solve this problem. In another approach, we use silver nanowires in the transparent electrode to improve conductivity for enhanced power conversion efficiency.
5:00 PM - **S5.6
Efficient, ITO-free Organic Photovoltaic Device Architectures.
Max Shtein 1 , Brendan O'Connor 2 , Yiying Zhao 1 , Kwang-Hyup An 2 , Abhishek Yadav 2 , Chelsea Haughn 1 , Kevin Pipe 2
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractVan Der Waals-bonded molecular organic compounds can be deposited as high quality thin films onto a variety of non-crystalline substrates, potentially enabling cost-effective lighting and solar energy harvesting devices. In this context, for which low-cost device fabrication over large areas is a necessity, ITO-free devices deposited on opaque and non-planar substrates are of particular interest.
While smooth metal films typically have increased parasitic optical absorption compared to ITO and related oxides, their high reflectivity enables the trapping of light both within the active layers of a device, and between adjacent devices. Such light trapping can be used effectively to increase optical absorption in OPVs. Furthermore, the use of noble metal electrodes enables the engineering of plasmon-assisted energy transfer processes to efficiently extract light in OLEDs that would otherwise be lost to waveguiding or absorption.
This talk will focus mainly on ITO-free organic solar cells that use smooth metal electrodes yet achieve comparable efficiency to conventional cells. Both planar and non-planar device architectures will be discussed, including metal-organic-metal device structures deposited onto fiber substrates that enable novel optical designs with very efficient light capture. Experiments and simulations are used to identify device geometries that can surpass planar device efficiency.
5:30 PM - S5.7
Impact of Contact Evolution on the Shelf Life of Organic Solar Cells.
Matthew Lloyd 1 , Dana Olson 2 , Ping Lu 1 , Erica Fang 1 , Diana Moore 1 , Matthew Reese 2 , David Ginley 2 , Julia Hsu 1
1 , Sandia National Labs, Albuquerque, New Mexico, United States, 2 , National Renewable Energy Lab, Golden, Colorado, United States
Show AbstractChanges in the active organic component are often cited as the leading degradation pathway in organic photovoltaic devices. In this shelf life study, we monitor device performance after storage in the dark at ambient conditions to show that decay behavior is dominated by changes at the top metal electrode and/or the metal-organic interface. Devices investigated in this study consist of conventional and inverted polymer-fullerene bulk heterojunctions as well as polymer-metal oxide hybrid solar cells. Cross-sectional TEM reveals extensive void formation to be the primary degradation mechanism for Ca/Al contacts. Kelvin probe measurements show the work function of Ag contacts increases over time, consistent with silver oxide formation detected by TOF-SIMS depth profiles. The evolution of the work function is found to be advantageous for Ag as a hole-extracting contact, but unsuitable for electron-extraction. The combination of Ag positive contacts (to collect holes) and ITO/ZnO negative contacts (to collect electrons) yields devices that are stable up to one year when stored in ambient conditions.
5:45 PM - S5.8
Modifying the Dielectric Properties of Conjugated Polymers for Improved Charge Separation.
Claire Woo 1 , Xiaoyong Zhao 2 , David Kauvlak 2 , Thomas Holcombe 2 , Erik Westling 1 , Jean Frechet 1 2
1 Chemical Engineering, University of California, Berkeley, Berkeley, California, United States, 2 Chemistry, University of California, Berkeley, Berkeley, California, United States
Show AbstractOne of the major distinctions between organic and inorganic semiconductors is their dielectric constant. The low dielectric constant (ε ~ 3) of organic polymers leads to high exciton binding energies and the formation of strongly bound geminate pairs upon photoexcitation. Here, we present the synthesis and solar cell performance of new conjugated polymer derivatives with increased dielectric constants, and we examine the correlation between the active layer dielectric constant and charge separation efficiency. Upon optimization, improved overall efficiency and higher photocurrent is demonstrated in devices using higher dielectric polymers. Approaches designed to increase the dielectric constant at the donor/acceptor interface also result in higher charge separation efficiency. The higher dielectric constant is believed to increase the geminate pair radius, thus facilitating the subsequent separation into free charges. The stabilization of free charges in a higher dielectric medium can also reduce losses from bimolecular recombination. Our work is one of the first examples of increasing the polymer dielectric constant for use in solar cells and demonstrates a key material design criterion that can be exploited to achieve higher efficiency organic photovoltaics.
Symposium Organizers
Jiangeng Xue University of Florida
Chihaya Adachi Kyushu University
Russell J. Holmes University of Minnesota
Barry P. Rand IMEC vzw
S6/C7: Joint Session: Large-area Organic Optoelectronic Devices
Session Chairs
Wednesday AM, December 02, 2009
Room 210 (Hynes)
9:30 AM - **S6.1/C7.1
Strategies for Achieving Highly Efficient Polymer Solar Cells.
Jianhui Hou 1 2 , Ziruo Hong 2 , Yue Wu 1 , Hsiang-Yu Chen 1 2 , Srinivas Sista 2 , Mi-Hyae Park 2 , Gang Li 1 , Yang Yang 1 2
1 , Solarmer Energy Inc., El Monte, California, United States, 2 Materials Science and Engineering, University of California, Los Angeles, Los Angeles, California, United States
Show AbstractSolar cells (or photovoltaic devices) transfer photonic energy into electricity. Optical absorption, open circuit voltage, short circuit current, and fill factor are four important parameters to determine the over-all efficiency of solar cells. In this presentation, we will present the strategies of enhancing these parameters. These strategies include (a) using tandem cell structures for increasing of the band width of optical absorption and Voc; (b) novel polymer energy level tuning to enhance Voc; Jsc and the combination of both; (c) improving the interface for enhancing the fill factor; (d) manipulation of polymer morphology for improving charge transport, hence the short circuit current. Finally, we will also present our recent breakthrough in polymer solar cell with NREL certified power conversion efficiency of 6.77%.
10:00 AM - **S6.2/C7.2
Future Challenges for the Industry of Organic Photovoltaics.
C. Lungenschmied 1 , G. Dennler 2
1 , Konarka Austria GmbH, Linz Austria, 2 , Konarka Technologies Inc., Lowell, Massachusetts, United States
Show AbstractSince the discovery of the ultrafast photoinduced electron transfer from a conjugated polymer to a C60 molecule, the scientific field of Organic Photovoltaics (OPV) has been growing exponentially. In 2009, the first products developed by Konarka have entered the market place. These photovoltaic modules, produced with a high speed, low temperature, roll-to-roll machine, onto a flexible, light weight polymeric substrate, offer completely new ways of utilizing the sun energy.Efficiencies higher than 6 % and lifetimes longer than 5 years have been recently shown. But the OPV technology has to increase further its performances to enter new applications. During this talk, we will present some of the important scientific challenges, and discuss them within an industrial perspective.
10:30 AM - S6.3/C7.3
The Influence of Processing Parameters on the Performance of Gravure printed Organic Solar Cells.
Monika Voigt 1 , Justin Dane 1 , Xuhua Wang 1 , Peter Levermore 1 , Jenny Nelson 1
1 Physics, Imperial College, London United Kingdom
Show AbstractSolution processable organic semiconductor materials are of great importance for low-cost solar energy conversion due to the potential for high-throughput manufacturing with conven¬tional printing and coating techniques. They are compatible with flexible semitransparent modules, easy to process and can be manufactured on different substrates and with low environ¬mental impact. Power conversion efficiencies of ~5% have been reported for small area polymer based solar cells deposited by spin coating [1]. However, achieving the same performance with large volume production techniques remains a challenge.In this work, the effect of gravure printing process parameters on the quality of organic layers and of the subsequent impact on organic solar cell devices is investigated. Gravure printing is a fast technique to achieve thin layers of organic material. Recently, it was used to fabricate multiple layer organic transistors with performance comparable to the best spin coated devices [2]. Printed bulk heterojunction solar cells require uniform thin films, high quality interfaces and optimized phase separation. Standard devices consist of a flexible PET/ITO substrate coated with a layer of poly(styrene sulphonate) doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) followed by a blend film of poly-3-hexylthiophene (P3HT) and [6,6] phenyl C61 butyric acid methyl ester (PCBM), and a top metal contact. Film quality, phase separation and wetting behaviour are influenced by process parameters including the choice of solvent, polymer molecular weight, solution viscosity, surface energy and the resolution/pattern sizes of the gravure cliché used. The influence of substrate surface energy on the quality of the printed layer is examined. It is found that the surface energy is a very important factor in the printability of materials. Commercially available PEDOT:PSS has a high surface energy, however the addition of small amounts of fluorosurfactant reduces the surface energy considerably, leading to improved wetting and printability. It is found that the next layer (P3HT:PCBM) lies within the surface energy parameter window of the anode material, i.e. PEDOT:PSS, and prints well. We shall also report on the use of gravure printing to deposit a novel organic conducting anode, vapour phase polymerized (VPP) PEDOT, which is an effective alternative to ITO [3]. Using these approaches multiple layer, ITO-free, printed solar cell devices can be realized. [1] J. Peet et al., Nature Mater. 2007, DOI 10.1038/nmat1928[2] M. M. Voigt et al., ’Polymer field effect transistors fabricated by sequentially gravure printing the polymer semiconductor, two insulator layers and metal ink gate’, submitted to Adv. Funct. Mater., 2009, regarding the EU-project CONTACT.[3] P. Levermore et al., Adv. Mater. 19, 2379 (2007)AcknowledgementsWe wish to thank EPSRC and UKERC for financial support and members of the CONTACT project.
10:45 AM - S6.4/C7.4
Area-scaling of Organic Solar Cells and Integrated Modules.
Seungkeun Choi 1 , William Potscavage 1 , Bernard Kippelen 1
1 School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractOrganic photovoltaics have received great interest in recent years due to the potential for the development of large-area, low-cost, light-weight solar modules with highly flexible form factors. While much emphasis was put on the optimization of the performance of small-area devices, less attention has been given to the modeling, fabrication, and testing of large-area devices and modules. In most practical applications, organic solar cell will be used in modules and large-area single cell will be preferred to comprise modules to reduce interconnection cost between cells.In this talk, we will report a detailed study of the area-scaling of the performance of organic solar cells based on the model compounds pentacene and C60. In addition, metal grid integration with ITO is proposed to reduce resistive power losses. We will report on the modeling, fabrication, and characterization of single solar cells with an area ranging between 0.1 cm2 up to 36 cm2. Our results show that the performance of large-area cells can be improved by integrating metal grids on the indium tin oxide (ITO) electrodes. Metal grids were fabricated by electroplating and had a thickness of up to 4 micro meter. Furthermore, metal grids are passivated with an insulator to prevent electrical shorts during the deposition of the subsequent organic layers and the top Al electrode. An analysis of the different sources of power loss and their relative magnitude will be discussed for organic solar cells in general. Solar cell modules were fabricated and tested to form either series or parallel connections of metal grid integrated individual cells. Individual cell has an active area of 7 cm2. For series connected module, four individual cells are connected in series. Linear increment of open circuit voltage (Voc) was exhibited. For parallel connected module, two individual cells are connected in parallel exhibiting doubling of short circuit current density (Jsc). The developed metal grid integration platform will be useful not only for large area single solar cell, but also for solar cell modules by minimizing series resistance over large area.
11:30 AM - **S6.5/C7.5
Multilayer, Polymer OLED Devices.
Joseph Shiang 1 , James Cella 1 , Kelly Chichak 1 , Christian Heller 1 , Kevin Janora 1 , Gautam Parthasarathy 1 , Jeffrey Youmans 1 , Anil Duggal 1
1 , GE Global Research, Niskayuna, New York, United States
Show AbstractOptical electronic polymer materials have a number of features that make them quite attractive as candidate materials for incorporation into OLEDs fabricated using high speed coating methods. A major limitation of these materials is the difficulty of forming multilayer structures, both over large areas and in smaller devices that are used to develop and test new design concepts. This can be a bottleneck in the development of polymer materials since testing a new device idea using polymer materials can often involve significant additional chemical development effort. In this talk, we will present two strategies that can be applied to a wide range of materials to make multilayer OLEDs and discuss some of the test OLED designs that we were able to fabricate using these techniques.Notice: Some of this work was conducted under Department of Energy contract # DE-FC26-05NT42343. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
12:00 PM - **S6.6/C7.6
Enabling High Throughput, Low Cost Manufacturing of OLED Display and Lighting Panels.
Michael Long 1 , Bruce Koppe 1 , Neil Redden 1 , Michael Boroson 1
1 OLED Business Unit, Eastman Kodak Company, Rochester, New York, United States
Show AbstractLarge scale manufacturing of OLED panels for lighting and display has been slower to materialize than first expected. The high cost and scarcity of fine metal masks beyond the Gen 4 scale has been a limiting factor in the scale-up of large format OLED display manufacturing. In the interest of reducing manufacturing costs and sidestepping the mask limitation, Kodak pioneered a white emitting architecture in combination with a red, green, blue color filter array similar to that used in LCD displays. This approach eliminated the cost and availability limitations of large precision masks, improved cycle time by eliminating three precision mask alignment steps as well as the successive depositions of red, green and blue emitting layers. In addition, the problem of differential aging of the individual red, green and blue emitting pixels was substantially eliminated. As a result, all of the OLED layers can be deposited through a common, coarse mask. This white emitter with color filter architecture has losses through the color filters that would have increased power consumption but Kodak realized most scenes have substantial white content that can be efficiently rendered with white emitting pixels supplementing the red, green and blue pixels. The addition of white pixels was found to offer a substantial power savings that makes the white with color filter approach viable from a power consumption perspective and advantaged from a manufacturing cost perspective. In addition, there are strong synergies between the white display and white lighting efforts that benefit both programs.Simplifying the patterning process opened the door to the potential for much higher throughput and reduced manufacturing costs on Gen 5 and larger substrates. However, scaling from the four minute cycle time, Gen 2 equipment in use today to the Gen 5 and larger equipment necessary to achieve $100/m2 manufacturing costs for lighting panels requires a thirty-fold increase in the vapor generation rate. Simply increasing the temperature of the vapor generating crucibles to attain higher deposition rates falls far short of the necessary, thirty-fold increase because of the limitation of material decomposition that even today is a limiting factor with several popular materials. This presentation introduces Kodak’s Vapor Injection Source Technology, a flash vaporization process that can deliver 20 nm/s static deposition rates for small-molecule OLED materials without increasing material decomposition. This flash vaporization process meters the organic material to a heating element on an as-needed basis, limiting the thermal exposure time to a few seconds and enabling vaporization at a controlled and steady rate to be initiated and interrupted in seconds.
12:30 PM - S6.7/C7.7
Fully Spray Coated Efficient Polymer Solar Cells.
Claudio Girotto 1 2 , Barry Rand 1 , David Cheyns 1 , Afshin Hadipour 1 , Jan Genoe 1 , Paul Heremans 1 2
1 Organic Photovoltaics, IMEC vzw, Leuven Belgium, 2 ESAT, Katholieke Universiteit Leuven, Leuven Belgium
Show AbstractThe promise of solution processed organic solar cells lies in their low-cost high-throughput manufacturability. However, this low cost aspect can only be fully realized if all of the layers are deposited by solution based, in-line compatible methods. Spray coating is a high-rate deposition technique characterized by the ability to deposit thin films over large areas by the superposition of femtoliter-size droplets.Here, with an automated ultrasonic spray coater we replaced each of the spin coated layers of a polymeric solar cell. We alternatively replaced the active layer, based on a poly(3-hexyl thiophene) (P3HT):(6,6)-phenyl C61-butyric acid methyl ester (PCBM) mixture, and the hole transport layer, a thin film of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), in a standard spin coated device with evaporated top contact yielding efficiencies above 3%, a performance which is comparable to that of the spin coated corresponding device. The film quality, deduced from atomic force microscopy and absorption measurements, confirms that polymers can be deposited in smooth and uniform layers with characteristics comparable to spin coated films.Furthermore, a Ag nanoparticle (NP) based solution can be applied as the patterned metal top contact, replacing vacuum evaporation, on top of spin coated layers. After sintering the Ag nanoparticle film at 150°C, a temperature which is compatible with processes on flexible substrates, the metal contacts and the resulting solar cells show conductivities and performances of 2.5%, comparable to those of reference devices with evaporated top-contacts.Spray coating of both polymers and Ag NP solutions can then be combined for the production of a fully spray coated organic solar cell with an overall efficiency of ~2%. We will overview the strategies for achieving these cells as well as the challenges for making further improvements.
12:45 PM - S6.8/C7.8
Scaling of Organic Vapor Phase Deposition for Large-Area Organic Optoelectronics.
Brian Lassiter 1 , Richard Lunt 2 1 , Stephen Forrest 1
1 Departments of Electrical Engineering and Computer Science, Materials Science and Engineering, and Physics, University of Michigan, Ann Arbor, Michigan, United States, 2 Department of Chemical Engineering, Princeton University, Princeton, New Jersey, United States
Show AbstractOrganic vapor phase deposition (OVPD) is a promising technology for deposition of organic thin films for optoelectronic devices.1,2,3 OVPD can grow films over large areas with very high material usage efficiency while providing control over film morphology. Growth occurs by transporting molecules in an inert carrier gas to a cooled substrate through a hot walled reactor. Morphological control of the resulting film is achieved by varying gas flow rate, pressure, and substrate temperature. In this work, we demonstrate the scalability of OVPD through a combination of theoretical analysis and experiment. Organic films are grown in a fully-automated OVPD reactor consisting of 12 source cells mounted radially on a 250 mm stainless steel gas transport tube. Film thickness uniformity of 3% across 200 mm wafers and material utilization efficiency above 25% have been achieved. Phosphorescent devices with 9.5% peak external quantum efficiency (EQE) and 33 lm/W have been fabricated, compared to 8.9% EQE and 24 lm/W from vacuum thermal evaporation-grown control devices. Large-area (~cm^2) OLEDs have also been fabricated and show improvement over small-area devices (mm^2). Scaling laws and the effect of OVPD growth parameters on the material yield and performance of OLEDs are discussed. We show the extension of this approach to 2m x 2m (1.6m)^2 substrates through scaling relationships and continuum-dynamics modeling. The authors thank Angstrom Engineering (Kitchener, Ont.) for assistance in design and the construction of the automated OVPD reactor.1M. A. Baldo, V. G. Kozlov, P. E. Burrows, S. R. Forrest, V. S. Ban, B. Koene, and M. E. Thompson, Appl. Phys. Lett. 71, 3033 (1997)2Theodore X. Zhou, Tan Ngo, Julie J. Brown, Max Shtein, and Stephen R. Forrest, Appl. Phys. Lett. 86, 021107 (2005)3F. Yang, M. Shtein, S. R. Forrest, Nature Mater. 4, 1476 (2005)
S7: Materials and Processing for Organic Photovoltaics
Session Chairs
Wednesday PM, December 02, 2009
Room 210 (Hynes)
2:30 PM - **S7.1
Improving Materials, Structure, and Morphology in Organic Photovoltaic Devices.
Richard McCullough 1
1 , Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Show AbstractIn organic photovoltaics, very little is understood on how to control and optimize the large number of critical parameters in these devices. Plextronics has produced world-class devices and progress through a systematic program of optimization that is helping to drive the commercialization of organic solar cells. Here, we will discuss important parameters in the creation of high performance OPV devices.
3:00 PM - S7.2
Solution-processable Triphenylamine-containing Organic Molecule Photovoltaic Materials.
Yongfang Li 1
1 , Institute of Chemistry, Chinese Academy of Sciences, Beijing China
Show AbstractOrganic photovoltaic devices (OPVs) have attracted much attention recently because of its easy fabrication, light weight, low cost and possibility to make flexible devices. The OPVs are commonly composed of a blend film of conjugated polymer or organic molecule as donor and a soluble C60 derivative PCBM as acceptor sandwiched between an ITO positive electrode and a low workfunction metal negative electrode.1 In comparison with conjugated polymer materials, solution-processable conjugated organic molecule materials possess the advantages of high purity and definite molecular weight. In this presentation, our recent progress on the studies of solution-processible organic molecules will be reported. The organic materials we studied are triphenylamine (TPA)-containing molecules with D-A-D structure.2-6 A star-shaped molecule with benzothiadiazole as acceptor unit and TPA as core and end groups shows strong absorption in visible region, and the power conversion efficiency of the solution-processed device based on the molecule as donor and PCBM as acceptor (1:3, w/w) reached 1.33% under the illumination of AM1.5, 100 mW/cm2.4 By using PC70BM as acceptor, the efficiency was further improved to 1.73%. Recently, our new star-shaped molecules with thiophene unit as the end group show the power conversion efficiency over 2%.References[1] G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science 1995, 270, 1789.[2] C. He, Q. He, Y. He, Y. Li, F. Bai, C. Yang, Y. Ding, L. Wang, J. Ye, Sol. Energy Mater. Sol. Cells 2006, 90, 1815.[3] C. He , Q. He, X. Yang, G. Wu, C. Yang, F. Bai, Z. Shuai, L. Wang, Y. Li, J. Phys. Chem. C, 2007, 111, 8661.[4] C. He, Q. He, Y. Yi, G. Wu, F. Bai, Z. Shuai, Y. Li, J. Mater. Chem. 2008, 18, 4085.[5] G. Wu, C. He, J. Zhang, Q. He, X. Chen, Y. Li, Sol. Energy Mater. Sol. Cells 2009, 93, 108.[6] G. Zhao, G. Wu, C. He, F. Bai, H. Xi, H. Zhang, Y. Li, J Phys. Chem. C 2009, 113, 2636.* This work was supported by NSFC (Nos. 50633050, 20721061, 20821120293, 20874106, 50803071)
3:15 PM - S7.3
Process Control Towards 6% Efficiency in PCDTBT-based Organic Solar Cells.
Salima Alem 1 , Ta-Ya Chu 1 , Salem Wakim 1 , Shing-Chi Tse 1 , Jianping Lu 1 , Serge Beaupre 2 , Mario Leclerc 2 , Francis Belanger 3 , Denis Desilets 3 , Russell Gaudiana 4 , David Waller 4 , Sheila Rodman 4
1 , National Research Council Canada, Ottawa, Ontario, Canada, 2 , Université Laval , Quebec City, Quebec, Canada, 3 , St-Jean Photochemicals, St-Jean-sur-Richelieu, Quebec, Canada, 4 , Konarka Technologies Inc., Lowell, Massachusetts, United States
Show AbstractThe organic solar cells represent a highly appealing solution for low cost and clean energy due to their flexibility and easy processibility. Nevertheless, despite their potential advantages, the efficiency of organic cells still needs to be improved for commercial use. Among various structures used in organic solar cells, the bulk heterojunction (BHJ) structure has received much attention since the discovery of ultra-fast photoinduced electron transfer from a conjugated polymer to buckminsterfullerene C60 [1] and the significant enhancement in solar cell efficiency by using a blend of PPV-derivatives or P3HT and a fullerene derivative [2]. We have developed a single-cell BHJ device with a power efficiency of 6% and an active area of 1cm2. The active layer employs poly (N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole) (PCDTBT) as electron donor and [6,6]-phenyl C70-butyric acid methyl ester (PC70BM) as electron acceptor. The relative low highest-occupied-molecular-orbital of PCDTBT (~ 5.45 eV) ensures a better air-stability [3] and leads to a large open circuit voltage (0.9V) when mixed with the fullerene derivative in solar cells [4]. We have investigated in detail the effect of solvents, the donor-acceptor proportion, layer structures and the thermal annealing on the photovoltaic parameters and charge transport properties. The best device shows a short circuit current density of 10.7 mA/cm2, a fill factor of 62 % and an open circuit voltage of 0.9V, under air mass 1.5 global (AM 1.5 G) irradiation of 100 mW/cm2. Our 1cm2-devices exhibit as well a very low series resistance about 5 Ωcm2. [1] N. S. Sariciftci, L. Smilowitz, A. J. Heeger, and F. Wudl, Science 258, 1474 (1992).[2] G. Yu, J. Gao, J. C. Hummelen, F. Wudi, A. J. Heeger, Science 270, 1789(1995); H. Hoppe, N.S. Sariciftci, Journal of Materials Research 19, 1924 (2004).[3] D. M. de Leeuw, M.M.J. Simenon, A.R. Brown, R.E. F. Einerhand, Synthetic Metals 87, 53 (1997).[4] N. Blouin, A. Michaud, and M. Leclerc, Advanced Materials 19, 2295 (2007).
3:30 PM - S7.4
The Role of Side-chain Length on Photo-voltaic and Charge Transport Properties of Poly(3-Alkylthiophene)/PCBM Bulk Heterojunctions.
Abay Dinku 1 , Wibren Oosterbaan 1 , Koen Vandewal 1 , Jean-Christophe Bolsee 1 , Sabine Bertho 1 , Jan D'Haen 1 , Laurence Lutsen 1 2 , Dirk Vanderzande 1 2 , Jean Manca 1 2
1 Institute for Materials Research, Hasselt University, Hasselt Belgium, 2 IMEC-IMOMEC, vzw, Hasselt University, Hasselt Belgium
Show AbstractAmong the family of poly(3-alkylthiophene) (P3AT) polymers, poly(3-hexylthiophene) (P3HT) is the most popular because of its superior performance in solar cells. Here we show that by controlling processing conditions, bulk heterojunction films of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) with poly(3-butylthiophene) (P3BT) or poly(3-pentylthiophene) (P3PT) give solar cells with efficiencies exceeding 3% and 4%, respectively. Reference solar cells with active layers of P3HT:PCBM blends give efficiencies reaching 4.6%, while all types of blends deliver similar order of photo-current yield (exceeding 10 mA/cm2) irrespective of side-chain length. Morphological studies reveal an increase in the degree of phase separation with increasing alkyl-chain length. Moreover, while P3PT:PCBM and P3HT:PCBM films have similar hole mobility, measured by hole only diodes, the hole mobility in P3BT:PCBM lowers by nearly a factor of four. Bipolar measurements made by field-effect transistor (FET) showed a decrease in the hole mobility and an increase in the electron mobility with increasing alkyl-chain length. Balanced FET charge transport is only achieved in the P3HT:PCBM blend. As compared to P3HT, P3PT is proved to be a potentially competitive material.
3:45 PM - S7.5
All Air-Processed High Performance Polythiophene-Fullerene Blend Solar Cells
Chang-Yong Nam 1 , Dong Su 1 , Charles Black 1
1 Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton , New York, United States
Show AbstractIn this report, we demonstrate high photovoltaic device performance in the ambient-air processed bulk heterojunction solar cells having an active blend layer of organic poly(3-hexylthiophene) (P3HT): [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). By applying a suitable thermal annealing scheme, we achieved power conversion efficiencies as high as 4.1% - comparable to state-of-the-art bulk heterojunction devices fabricated in air-free environments. We combine high resolution cross-sectional transmission electron microscopy with detailed analysis of electronic carrier transport in order to quantitatively understand the effects of oxygen exposure and different thermal treatments on electronic conduction through the highly nanostructured active blend network. We find that the improvement in photovoltaic device performance by post-fabrication thermal processing results from the reduced oxygen charge trap density in the active blend layer and is consistent with a corresponding slight thickness increase in a ~4 nm aluminum oxide hole-blocking layer present at the electron-collecting contact interface. This research was supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences, under Contract No. DE-AC02-98CH10886
4:30 PM - **S7.6
New Materials for Organic Photovoltaics.
Mark Thompson 1 , Stephen Forrest 2 , M. Dolores Perez 1 , Siyi Wang 1 , Guodan Wei 2 , Cody Schlenker 1
1 Chemistry, University of Southern California, Los Angeles, California, United States, 2 Electrical Engineering and Physics, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractWe have developed a range of metal complexes and organic materials for use as donor and acceptor materials in organic solar cells and M(dikenotate)3 complexes as buffer layers in OPVs. Devices prepared with a wide range of new and established materials were used to investigate the connection between the OPV open circuit voltage (Voc) and the energy difference between the donor HOMO and acceptor LUMO energies [E(DA)]. Within closely related families of materials there is a clear correlation between Voc and E(DA), however, we have found marked differences between the Voc values for materials systems with very similar E(DA) values. A model will be presented that accurately couples Voc and E(DA) values with a D/A coupling term. This approach gives us a new path to develop high Voc materials. We have also explored the use of a range of metal porphyrin complexes as donor materials in OPVs. The complexes we have chosen have high nonplanar structures in the ground state and excited state. I will discuss the use of both Pt and Pd complexes, illustrating how these materials can be used to enhance both the efficiency and Voc.
5:00 PM - S7.7
Cyclometalated Metal Complexes as Absorbers for Photovoltaic Applications.
Jian Li 1 , Zixing Wang 1 , Cameron Adler 1 , Nathan Bakken 1
1 School of Materials and Advanced Photovoltaics Center, Arizona State University, Tempe, Arizona, United States
Show AbstractThe cyclometalated metal complexes have been intensively studied as phosphorescent emitters for organic light emitting diodes. However, there are few reports on exploring their use as absorbers for photovoltaic applications, even though they offer several advantages over their pure organic counterparts such as thermal and electrochemical stability, versatility of modifying molecular properties, extended absorption band and longer exciton lifetime, making them excellent candidates for photovoltaic materials. In this presentation, we will discuss our preliminary results on the use of metal complex-based broad-band absorbers for photovoltaic applications. The synthesis, photo-physics and electrochemistry of these novel metal complexes will be reported. Some of absorbers have demonstrated a strong absorption in the red and near infra-red region. These broadband absorbers will also be characterized in the device settings. Moreover, the influence of heavy metal ions on the exciton diffusion length of materials will also be discussed.
5:15 PM - S7.8
Molecular Design for Improved Photovoltaic Efficiency by Band Gap and Absorption Coefficient Engineering.
Rajib Mondal 1 , Sangwon Ko 1 , Eric Verploegen 1 4 , Hector Becerril 1 , Joseph Norton 3 , Michael Toney 4 , Jean-Luc Bredas 3 , Michael McGehee 2 , Zhenan Bao 1
1 Chemical Engineering, Stanford University, Stanford, California, United States, 4 , Stanford Synchrotron Radiation Laboratory, Menlo Park, California, United States, 3 School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Materials Science And Engineering, Stanford University, Stanford, California, United States
Show AbstractThe optimization of polymer bulk heterojunction solar cells requires balancing at best a series of materials parameters. A wide range of polymer materials properties, such as band gap (Eg) and frontier molecular orbital energies are usually achieved by various molecular designs. In addition, maintaining high light absorption strength or absorption coefficients (ca. 105 cm-1) of the polymers is equally important in generating higher photocurrent in solar cells, because it is typically difficult to extract charge from devices thicker than 200 nm. However, there is limited understanding about enhancing the absorption coefficients. We have shown that the absence of the adjacent thiophene groups around the acceptor core in low band gap polymers markedly increases the optical absorption by enhancing the HOMO/LUMO overlap and raises the ionization potential of the polymer. As a consequence, significant enhancement in PCE is observed in solar cell devices using the fused thienopyrazine-based polymer without adjacent thiophenes, due to increased open-circuit voltage and short-circuit current.
5:30 PM - S7.9
The Use of Cyanine Dyes in Solid State Organic Heterojunction Solar Cells.
Bin Fan 1 , Jakob Heier 1 , Hadjar Benmansour 1 , Fernando Castro 1 , Thomas Geiger 1 , Matthias Nagel 1 , Roland Hany 1 , Frank Nueesch 1
1 , EMPA (Swiss Federal Laboratories for Materials Testing and Research), Duebendorf Switzerland
Show AbstractToday a plethora of soluble cyanine dyes absorbing from the ultra-violet to the near-infrared domain are available owing to more than a century of research and development, mostly in photographic industry. Numerous properties of cyanine dyes suggest that this material class would be interesting for organic solar cell applications. Most importantly the unparalleled absorption coefficients allow using very thin films for harvesting the solar photons. Cyanines also own favourable redox potentials making it possible to use them as electron donors and acceptors in organic heterojunction solar cells. With respect to crystallinity, these polymethine dyes tend to form aggregates where charge and excited states are delocalized over hundreds of molecules. Furthermore, cyanines are cationic polymethine dyes, offering the possibility to tune the materials by defining the counter-anion. Under applied bias, counter ions can be moved across the organic heterointerface and induce substantial photocurrent increase. Optimized bilayer cyanine devices present power conversion efficiencies reaching up to 4%.
5:45 PM - S7.10
Solution and Vapor Processable Squaraine/PC70BM Bulk Heterojunction Photovoltaic Cells.
Guodan Wei 1 , Siyi Wang 4 , Ning Li 2 , Jeramy Zimmerman 2 , Kai Sun 1 , Mark Thompson 4 , Stephen Forrest 1 2 3
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 4 Department of Chemistry, University of Southern California, Los Angeles, California, United States, 2 Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan, United States, 3 Department of Physics, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractPreviously squaraine donor based devices have been demonstrated based on both solution processing and vacuum deposition, with the vacuum deposited devices having performance superior to those based on solution1,2. In this work, we introduce small molecule based photovoltaic cells fabricated by inserting a thin, transparent MoO3 layer thermally evaporated between the anode (indium tin oxide) and the solution processed blended film of the squaraine (SQ) /[6,6]-phenyl C71 butyric acid methyl ester (PC70BM) active layer. The MoO3 reduces dark current, hence enhancing the open circuit voltage3. The nanoscale distribution of the two donor/acceptor materials have been studied by atomic force microscopy and transmission electron microscopy . By optimizing the component ratio of SQ and PC70BM to 1:6, we have demonstrated a bulk solar cell with 2.8% power conversion efficiency obtained under simulated 100 mW/cm2 AM1.5 illumination with a 9.0 mA/cm2 short-circuit current density and an open-circuit voltage of 0.90 V. We will discuss strategies to further enhance device performance, such as optimizing spatial phase separation between SQ and PC70BM materials to improve the fill factor and charge carrier collection efficiency. 1.S. Y. Wang, E. I. Mayo, M. D. Perez, L. Griffe, G.D. Wei, P. I. Djurovich, S. R. Forrest, M. E. Thompson, App. Phys. Lett. 94,233304(2009), 2.F. Silverstri, M. D. Irwin, L. Beverina, A. Facchetti, G. A. Pagani, T. J. Marks, J. Am. Chem. Soc. 130, 17640(2008). 3.N. Li, B. E. Lassiter, R. R. Lunt, G. D. Wei, S. R. Forrest, Appl. Phys. Lett. 94, 023307(2009).
S8: Poster Session
Session Chairs
Thursday AM, December 03, 2009
Exhibit Hall D (Hynes)
9:00 PM - S8.1
Charge Transport Study in Organic Semiconductors via Carrier Extraction with Linearly Increasing Voltage.
Song Chen 1 , Kaushik Roy Choudhury 1 , Jegadesan Subbiah 1 , Pierre Beaujuge 2 , John Reynolds 2 , Franky So 1
1 Dept of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 The George and Josephine Butler Polymer Research Laboratory, Dept of Chemistry, Center for Macromolecular Science and Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractSolution-processible semiconducting polymer-based electronic and optoelectronic devices rely on charge transport. Owing to the large anisotropy in carrier transport and a strong dependence of carrier mobility on charge carrier density, transport measurements on these disordered semiconducting polymers yield results that vary widely with the choice of device geometries and operating conditions. Among the different techniques currently used to investigate carrier transport in vertically-stacked organic devices, CELIV is emerging as the preferred one. This is due to the capability of characterizing transport in devices with high mobility and high bulk conductivity without thickness requirements, which is challenging in traditional methods including time of flight (TOF) and dark injection (DI). In this work, we investigate carrier transport in two different polymer systems using CELIV: a) bulk heterojunction (BHJ) photovoltaic (PV) cells from blends of P3HT and different fullerene acceptors, and b) a series of dithienosilole-benzothiadiazole donor-acceptor copolymers with tailored optical properties and hole-mobilities.BHJ PV cells fabricated from blends of P3HT and different acceptors including PC60BM, PC70BM and bis-PC60BM were studied. The carrier mobility in devices with P3HT:PC70BM blends was found to be about two times as high as that in the devices containing PC60BM, at similar electric fields. This surprising observation, along with the enhanced optical absorption of PC70BM, can account for the increased photocurrent under short-circuit conditions as well as at the maximum power point in the P3HT:PC70BM devices. On the other hand, PV cells containing bis-PC60BM as the acceptor exhibited carrier mobilities similar to that in P3HT:PC60BM devices. This demonstrates that charge transport is not the factor limiting the short circuit current (relatively lower) in these devices. We also investigated carrier transport in a series of dithienosilole-benzothiadiazole donor-acceptor copolymers (P1, P2, P3 and P4) having varying concentration of electron-donating and -withdrawing segments. Hole mobilities determined using CELIV and space charge limited (SCL) conduction showed close correspondence. A maximum hole mobility of 7.5X10^(-5) cm^2/Vs was obtained from CELIV measurements for P4. This, along with the homogeneous and broad absorption of P4 across the visible solar spectrum, makes it a promising candidate for PV applications. Interestingly, the hole mobility for all copolymers extracted through CELIV are more than two orders lower than the corresponding FET mobilities recently reported by Beaujuge et al., emphasizing the relevance of this technique in studying carrier transport in vertically-stacked devices.
9:00 PM - S8.10
Synthesis and Characterization of Novel Fullerene-Based n-Type Materials for Polymer Solar Cells Application.
Jean-Francois Morin 1 , Simon Rondeau-Gagne 1
1 Chemistry, Universite Laval, Quebec, Quebec, Canada
Show AbstractThe ease of functionalization and astonishing electrochemical properties of C60 has led it to become one of the most used n-type materials for the development of efficient polymer-based BHJ solar cells. PCBM, a soluble derivative of C60, is by far the best candidate for BHJ solar cells since it provides good film morphology with most of the polymers and exhibit very good electron transport properties. However, PCBM is not the best electronic match for all the conjugated polymers tested so far (ex. P3HT and poly(2,7-carbazole) derivatives). On one hand, the LUMO level of the n-type material should lie at least 0.3-0.4 eV below the LUMO level of the conjugated polymer to allow efficient charge separation and to minimize the occurrence of back-transfer event. On the other hand, the LUMO level of the acceptor must lie as far as possible from the HOMO level of the conjugated polymer in order to obtain the higher Voc value and, consequently, the best device efficiency possible. As exemplified in recent reports, the difficulty to modulate the LUMO energy level of C60 comes from the introduction of two or more sp3 carbons between the C60 cage and the functional group, thus reducing the electronic conjugation between both species.Ethynylation reaction has proven to be very efficient to bridge the C60 with different substituents in moderate to good yields. For the purpose of the LUMO energy level modulation, this reaction is particularly interesting since the electronic density can be tuned through the substituent bearing the terminal alkyne (R) but also through the electrophilic species (E) used to quench the reaction (scheme 1). Moreover, an ethynyl moiety directly attached to C60 participates to a through-space p-orbital overlapping, called “periconjugation”, thus indirectly increasing the electronic communication between C60 and the R group. Therefore, we hypothesize that the insulating barrier between both moieties can be significantly reduced by using a substitution reaction that leads to a single sp3 carbon bridge. In this presentation, we will report on the synthesis, electrochemical properties and preliminary device testing of a series of six C60 derivatives prepared by ethynylation reaction using different electron-donor groups. We will show how these groups can be used to modulate the LUMO level of C60 using an ethynyl bridge. DFT calculations on these derivatives will also be reported and compared to the experimental results.
9:00 PM - S8.11
Water-soluble and Self-doped Conducting Polyaniline Graft Copolymer as Hole Transport Layer for High Efficiency Organic Photovoltaics.
Jae Woong Jung 1 , Jea Uk Lee 1 , Won Ho Jo 1
1 , Seoul National University, Seoul Korea (the Republic of)
Show AbstractThe conventional OPV device is composed of four layers, indium tin oxide/poly(3,4-ethylenedioxythiophene) (PEDOT):poly(styrenesulfonate) (PSS)/active layer /Al. In this structure, PEDOT:PSS has been most widely used as hole transport layer (HTL). Although the performance of OPV which use PEDOT:PSS as HTL has remarkably been progressed by smoothing the ITO surface, lowering the work function of anode and promoting the efficient hole transport, various problems of PEDOT:PSS in the OPV device have been reported. First, since PEDOT:PSS is dispersed in water with large particle size with ca. 60-80 nm, the aggregate of particles may provide defects of the device, and thus induce degradation of OPV. Second, because of the strong acidic nature of PSS, it may degrade the ITO surface and as a result can lead deterioration of solar cell performance. Third, the cost of PEDOT:PSS (CLEVIOS P VP AI 4083) is very expensive. Fourth, it has low electrical conductivity (σ~0.001 S/cm). Therefore, it is highly demanded to develop a new hole transport material, which is completely soluble in water and has higher electrical conductivity than PEDOT:PSS, to fabricate highly efficient and stable OPV.In this work, we synthesized water soluble conducting copolymer based on polyaniline (poly(styrene sulfonic acid)-graft-polyaniline: (PSSA-g-PANI)), and compared optical and electrochemical properties of PSSA-g-PANI with those of PEDOT:PSS, to apply the PSSA-g-PANI to OPV as hole transport material. It was observed that the OPV device fabricated with PSSA-g-PANI showed higher power conversion efficiency than the device fabricated with PEDOT:PSS, because PSSA-g-PANI film shows higher transmittance than PEDOT:PSS film in 450-650 nm wavelength which is main absorption range of P3HT and because PSSA-g-PANI has higher electrical conductivity than PEDOT:PSS. Furthermore, in order to examine the effect of electrical conductivity of HTL material on the performance of OPV, a series of PSSA-g-PANIs which have different molar ratio of aniline to styrene sulfonic acid were also prepared and applied to OPV devices. When higher conductive PSSA-g-PANI copolymer than PEDOT:PSS is used as HTL, JSC and VOC are significantly increased while FF is almost not changed, resulting in higher power conversion efficiency (η~4.0 %) which is an enhanced value over 10% than the device with PEDOT:PSS (η~3.3 %). Particularly, the enhancement of JSC is the result of increased light absorption of P3HT and efficient transport of holes from P3HT to anode.
9:00 PM - S8.12
n-Type Conjugated Polymers for Photovoltaic Applications.
Li Jia 1
1 , University of akron, Akron, Ohio, United States
Show AbstractDesign and synthesis of π-conjugated organic molecules and polymers with high electron affinity continue to be a research challenge because of the demand of n-type semiconducting materials for photovoltaic, microelectronic, and optoelectronic applications . For heterojunction organic solar cells, the lowest unoccupied orbital (LUMO) of the n-type material must be lower than that of the p-type material by a finite margin so that the energy offset between the LUMOs [ΔE(LUMO)] is large enough to overcome the exciton binding energy [E(c-p)] in the p-type material [1], i.e., ΔE(LUMO) > E(c-p). This proves not an easy task when regio-regular poly(3-hexylthiophene) (rr-P3HT), which has the desirable optical absorption window and high electron mobility, is used as the p-type material in a heterojunction cell [2]. In fact, only fullerenes and their derivatives, of which the exceptionally high electron affinities arise from the topological characteristics of their π-orbitals and their surface curvatures [3,4], have been shown to be able to quench the photoluminescence of rr-P3HT. As the result, the all-conjugated-polymer photovoltaic [5,6,7]. A polymeric material with adequate electron affinity to separate the electron-hole pair in rr-P3HT would be therefore very desirable from the viewpoint of photovoltaic application as well as in the general context of organic optoelectronic devices. We report here the use of the inductive effect and the empty p-orbital of boron to low the LUMO energy of π-conjugated polymers. Synthesis and the electrochemical and photophysical properties of the new polymers will be discussed. Those that can quench the photoluminescence of rr-P3HT will be highlighted. [1] B. A. Gregg, J. Phys. Chem. B 107 (2003) 4688.[2] S. Günes, H. Neugebauer, N. S. Sariciftci, Chem. Rev. 107 (2007) 1324.[3] R. C. Haddon, Phil. Trans.R. Soc. Lond. A 343 (1993) 53.[4] P.-M. Allemand, A. Koch, F. Wudl, Y. Rubin, M. M. Alvarez, S. J. Anz, R. L. Whetten, J. Am. Chem. Soc. 113 (1991) 1050. [5] S.-S. Sun, C. Zhang, A. Ledbetter, S. Choi, K. Seo, C. E. Bonner, Jr., M. Drees, N. S. Sariciftci, Appl. Phys. Lett. 90 (2007) 043117.[6] M. M. Mando, W. Veurman, L. J. Koster, M. M. Koetse, J. Sweelssen, B. de Boer, P. W. M. Blom, J. Appl. Phys. 101 (2007) 104512.[7] Granström, M.; Petritsch, K.; Arias, A. C.; Lux, A.; Andersson, M. R.; Friend, R. H. Nature 1998, 395, 257-260.
9:00 PM - S8.14
Molecular Origin of Charge Transfer Exciton Recombination in Polymer Fullerene Blends.
Markus Hallermann 1 , Josef Berger 1 , Ilka Kriegel 1 , Enrico Da Como 1 , Jochen Feldmann 1
1 Photonics and Optoelectronics Group, CeNS, LMU München, München Germany
Show AbstractBlends between conjugated polymers and fullerene derivatives constitute a promising material combination for organic photovoltaics. Currently efficiencies up to 5% have been reached, with the potential for better performances. To further advancing this technology and make it competitive, a thorough understanding of loss mechanisms in carrier photogeneration is mandatory. Recombination appears to be a major limit, although a clear picture on the electronic states and molecular parameter controlling this process is still lacking.In this communication we unravel the origin of charge transfer excitons (CTE) [1] forming in such blends and we discuss their importance for recombination processes after the initial photoinduced charge transfer. We show that CTE photoluminescence can be observed in several material combinations. For example, in conjugated polymers such as poly3-hexylthiophene and poly(phenylene-vinylene) when blended with the fullerene acceptor PCBM. By combining electron microscopy images and photoluminescence spectroscopy, we show that CTE recombination is only slightly influenced by the mesoscopic morphology. Surprisingly, large differences in CTE recombination have origin in the polymer chain conformation. This study provides new insights on limiting recombination in polymer/fullerene solar cells.[1] M. Hallermann, S. Haneder, E. Da Como, Applied Physics Letters 93, 053307 (2008)
9:00 PM - S8.15
The Role of Triplet Excitons in Enhancing Polymer Solar Cell Efficiency: A Photo-induced Absorption Study.
Suchi Guha 1 , K. Yang 1 , U. Scherf 2
1 Physics, University of Missouri-Columbia, Columbia, Missouri, United States, 2 Chemistry, Universitat Wuppertal, Wuppertal Germany
Show AbstractInclusion of heavy metal atoms in a polymer backbone allows transitions between the singlet and triplet manifolds. Interfacial dissociation of triplet excitons constitutes a viable mechanism for enhancing photovoltaic (PV) efficiencies in polymer heterojunction-based solar cells. We have recently shown that a ladder-type polymer (PhLPPP) with less than ~100 ppm palladium blended with a fullerene-derivative (PCBM) yields 10 times greater solar power conversion efficiency compared to the pristine ladder-type polymer (MeLPPP) with no Pd atoms. [1] Such a low concentration of metal atoms does not significantly modify the formation of triplet excitons but enables strong localized spin-orbit coupling. Evidence is presented for the formation of a weak ground-state charge-transfer complex (CTC) in the blended films of the polymer and PCBM, using photo-induced absorption spectroscopy. The CTC state in MeLPPP: PCBM is not entirely below the acceptor singlet/triplet excitonic energies resulting in a radiative recombination. In contrast the CTC state in PhLPPP:PCBM lies entirely below the gap as a localized state, contributing to the enhancement of PV efficiency. [1] M. Arif, K. Yang, L. Li, P. Yu, S. Gangopadhyay, M. Foerster, U. Scherf, and S. Guha, Appl. Phys. Lett. 94, 063307 (2009).
9:00 PM - S8.16
Real-time Observation of Morphological Changes in Organic Bulk-heterojunction Solar Cells.
Christoph Lungenschmied 1 , Reinhard Schwoediauer 2 , Siegfried Bauer 2 , Sheila Rodman 3 , Darcy Fournier 3 , Christoph Brabec 1
1 , Konarka Technologies Austria, Linz Austria, 2 Soft Matter Physics, Johannes Kepler University, Linz Austria, 3 , Konarka Technologies Inc, Lowell, Massachusetts, United States
Show AbstractMorphology control and thermal stability are most crucial parameters for two decisive factors determining the commercial success of printed organic bulk heterojunction solar cells: power conversion efficiency and lifetime.For instance, in the material combination which is probably studied in most detail (poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM)) the correlation between morphology and performance is striking. The optimum efficiency can be achieved by establishing the ideal degree of phase separation upon thermally annealing the thin film device.Material parameters of the components such as the glass transition temperature (Tg) or the melting point of the polymer are crucial for optimizing the annealing conditions. Even more, these parameters decide over the temperature stability of the morphology and therefore the device lifetime under solar irradiation at elevated temperatures.We herein present a study on the thermal properties of state-of-the-art photovoltaic donor/acceptor systems, using a novel method to directly study temperature induced transitions in thin films of organic semiconductors. This technique determines the Tg of semiconducting polymers in actual thin film devices and also enables the real-time observation of temperature induced morphological changes in donor/acceptor blends. It is based on the determination of the thermal expansion of the active layer in thin film devices, hence being a dilatometric method. The impedance of the resulting parallel-plate capacitor is determined as a function of temperature and analyzed in terms of thermal expansion of the dielectric.In P3HT:PCBM, the high accuracy of the dilatometry reveals two distinct steps in the annealing process. First, the P3HT softens at lower temperature followed by the morphological rearrangement of the material when heated further.The results are compared to differential scanning calorimetry (DSC), which has been the method of choice so far.We claim the following advantages over DSC: Higher accuracy and more details on temperature induced transitions. Furthermore, standard DSC cannot be performed on thin films, because considerable amounts of material are required for sufficient sensitivity. Only the bulk properties, not those of a thin, confined film are accessible. In addition, in multi-component blends the degree of intermixing can differ significantly for thin films and bulk material: Donor/acceptor blends cannot be studied by DSC in the morphology which defines the photovoltaic performance. However, this is possible with the presented dilatometric method.
9:00 PM - S8.17
All-aromatic Liquid Crystal Triphenylamine-based Poly(azomethine)s as Hole Transport Materials for Opto-electronic Applications.
James Hindson 1 , Burak Ulgut 1 , Richard Friend 1 , Ben Norder 2 , Arek Kotlewski 2 , Theo Dingemans 3 , Neil Greenham 1
1 Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom, 2 NSM, Delft University of Technology, Delft Netherlands, 3 AsE, Delft University of Technology, Delft Netherlands
Show AbstractWe have explored the opto-electronic properties of a new series hole transport materials based on main-chain triphenylamine-based poly(azomethine)s. 4,4’-Diamino-triphenylamine (TPA) was polymerized under benign conditions with either Terephthalaldehyde (TPA-14Ta), 2,5-Thiophene-dicarboxaldehyde (TPA-25Th) or 1,3-Isophthalaldehyde (TPA-13Iso) to yield polymers with an Mn of 5700 – 16000 g/mol. Despite the non-linear, or ‘kinked’, backbone geometry, all polymers form lyotropic solutions in Chloroform and this liquid crystal (nematic) ordering could be maintained in the solid film after spin casting. Differential Scanning Calorimetry (DSC) indicated that the polymers are completely amorphous and exhibit high Tg’s (> 250 oC) while Thermo Gravimetric Analysis (TGA) confirmed the outstanding thermal stability of this class of polymers, i.e. 5% wt loss in excess of 400 oC under nitrogen. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of these polymers, as determined by Cyclic Voltammetry (CV), were in the range of 5.01–5.26 and 2.41–3.06 eV, respectively. Introduction of a 2,5-substituted thiophene heterocycle (TPA-25Th) resulted in low optical band gap material of approximately 2.0 eV, where as TPA-14Ta and TPA-13Iso showed optical band gaps of 2.3 and 2.6 eV. A photovoltaic diode based on a TPA-25Th/PCBM blend (1:3) showed and External Quantum Efficiency (EQE) of 20% at 500 nm. Despite a poor fill factor (0.24) this device performs reasonably well under simulated sunlight (AM1.5 conditions), i.e. Voc = 0.41 V, Jsc = 1.23 mA.cm-2 and has an efficiency of 0.12%. Since this class of polymers are cheap and easy to synthesize we believe this approach could lead to novel hole-transport materials for opto-electronic applications.
9:00 PM - S8.18
Simulation of Copper Phthalocyanine (CuPc)/Fullerene (C60) Hheterojunction. photovoltaic cell with and without electron transport layer (ETL)
Nikhil Satyala 1 , Ron Pieper 1 , Wudyalew Wondmagegn 1
1 Electrical Engineering, University of Texas at Tyler, Tyler, Texas, United States
Show Abstract Organic polymer photovoltaics (PV) are being pursued extensively in the recent decades for harvesting solar energy due to their prominent features such as low cost of fabrication and ease of processing. Among the most favorable designs of organic PV cells are the combinations of donor/acceptor heterojunction cells, introduced by Tang [1]. These cells make use of the donor/acceptor interface for the dissociation of excitons generated in the active layers by photon absorption. Since then numerous techniques introduced to improve the photovoltaic cells have helped the efficiencies reach as high as 5.7% [2]. Puemans and Forrest introduced an electron transport layer (ETL) in to the bilayer CuPc/C60 PV cell using bathocuproine (BCP), which also efficiently blocks the diffusion of excitons to the cathode [3]. This technique consequently lowered the exciton quenching at the cathode/donor interface and in turn improved the quantum efficiency. In this paper, we present a simulation model for a bi-layer organic PV cell, based on copper phthalocyanine (CuPc) and fullerene (C60). This comprehensive device model, which was developed for the bi-layer organic PV cell, is based on the experimental PV cell design reported with and without the electron transport layer (ETL) [4]. The dark current and photo-current densities vs. voltage (J–V) characteristics were modeled using 2D finite element device simulations and a detailed comparison with the reported characteristics is provided. The conduction mechanisms in the active layers were modeled using Frenkel-Poole mobility. A selective combination of singlet and triplet states were invoked in the simulations for the exciton generation and dissociation processes. The simulation results clearly demonstrated that the performance of the reported device can be well described by the above mentioned models. The absorption, generation and recombination processes are analyzed in detail with relevance to the photo conversion efficiencies. Also presented from this study is the effect of various ETLs such as bathocuproine (BCP), CuPc, AlQ3 and C60 on the performance of the CuPc/C60 bi-layer PV cell. The efficiency of devices was primarily related to the short circuit current and the absorption capability of the ETL. The short circuit current is found to be dependent on the barrier height between the C60 and the ETL. The performance of the device also showed a considerable dependence on the ETL thickness. The model predicted a gradual decrease in the power conversion efficiency, consistent with other reported works, with increase in the thicknesses of the ETL. References:[1] C. W. Tang, Applied Physics Letters, Vol. 48, Issue 2, 1986.[2] J. G. Xue, S. Uchida, B. P. Rand, and S. R. Forrest, Applied Physics Letters, Vol: 85, pp. 5757, (2004).[3] P. Puemans and S. R. Forrest, Applied Physics Letters, Vol. 79, No. 1, 2001 [4] M. Y. Chan et. al., Journal of Applied Physics, Vol. 100, 094506, 2006
9:00 PM - S8.19
The Impact of Oxygen on Trap States in P3HT:PCBM Solar Cells.
Julia Schafferhans 1 , Andreas Baumann 1 , Alexander Foertig 2 , Carsten Deibel 1 , Vladimir Dyakonov 1 2
1 Experimental Physics VI, Julius-Maximilians University of Würzburg, Würzburg Germany, 2 , ZAE Bayern, Div. Functional Materials for Energy Technology, Würzburg Germany
Show AbstractThere is a growing interest in using conjugated polymers in organic solar cells due to their low cost processability from solution. Power conversion efficiencies (PCE) of 6% for organic solar cells have already been achieved [1] with growing tendency. A limiting factor, so far, is the lifetime of these devices. To gain a detailed understanding of the device stability, the underlying degradation mechanisms and their impact on the solar cell performance is an important prerequisite for lifetime enhancement. Accordingly, the presence of defect states can be critical, as they reduce the charge carrier mobility, disturb the internal field distribution and can act as recombination centers. The trap distribution in P3HT:PCBM blends, one of the most promising candidates for organic solar cells, was investigated by applying a fractional thermally stimulated current (TSC) technique. Thereby a broad trap distribution with activation energies between 10 meV and 400 meV and trap densities of 6×1016 cm-3 could be revealed. Systematic exposure of the state of the art solar cells (PCE > 3%) to dry air, to investigate the influence of oxygen on the absorber blend, yields a lowering of the TSC signal with exposure time. This is in contrast to pristine P3HT samples, which exhibited a strong increase of the TSC signal and thus the trap density with oxygen exposure time [2]. Nevertheless, additional charge extraction by linearly increasing voltage (CELIV) measurements showed a decrease of the charge carrier mobility with oxygen exposure time for the blends, as well as the pristine P3HT based devices, indicating an increase of the trap density in both systems. The lowering of the TSC signal in the blends can be explained by a decrease of the charge carrier lifetime due to an enhanced recombination rate with oxygen exposure, as revealed by transient photocurrent and photovoltage measurements. Since the TSC is proportional to the lifetime of the detrapped charge carriers, the decrease of the lifetime results in a lowering of the TSC signal, although the trap concentration increases. In conclusion, oxygen exposure of P3HT:PCBM solar cells results in a decrease of the mobility as well as the charge carrier lifetime and an increase of the trap density.[1] S. H. Park et al., Nature Photonics, 3, 297 (2009)[2] J. Schafferhans et al., Applied Physics Letters, 93, 093303 (2008)
9:00 PM - S8.2
Ball and Socket Joints for Organic Photovoltaics.
Noah Tremblay 1 2 , Alon Gorodetsky 1 2 , Marshall Cox 3 , Theanne Schiros 2 , Bumjung Kim 1 2 , Aaron Sattler 1 2 , Woo-Young So 4 , Yoshimitsu Itoh 1 2 , Michael Toney 5 , Arthur Ramirez 6 , Ioannis Kymissis 3 , Michael Steigerwald 1 2 , Colin Nuckolls 1 2
1 Department of Chemistry, Columbia University, New York, New York, United States, 2 The Center for Electron Transport in Molecular Nanostructures, Columbia University, New York, New York, United States, 3 Department of Electrical Engineering, Columbia University, New York, New York, United States, 4 Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York, United States, 5 , Stanford Synchrotron Radiation Lightsource, Menlo Park, New York, United States, 6 The Jack Baskin School of Engineering, University of California - Santa Cruz, Santa Cruz, California, United States
Show AbstractWe have made a new class of organic photovoltaic devices (OPVs) in which the electronic character of the p-n junction is controlled by using molecular shape complementarity to self-assemble the interface. In ambient atmosphere, these devices exhibit exceptionally high conversion efficiencies under UV irradiation and open circuit voltages that approach within 10% of the theoretical maximum. This exploitation of host-guest chemistry demonstrates a promising direction for OPV device design. We use synchrotron-based surface sensitive techniques, including GIXD and NEXAFS, to study the microstructure, morphology and interfacial assembly of these highly tunable materials and and gain information critical to the realization of high efficiency OPV devices based on controlled nanostructures.
9:00 PM - S8.20
Effects of N2, O2, and Moisture on the Hole Transport in P3HT:PCBM.
Shing C. Tse 1 , Nicolas Drolet 1 , Ye Tao 1
1 Institute for Microstructural Science, National Research Council Canada, Ottawa, Ontario, Canada
Show AbstractThe hole transport properties of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blended films have been examined by time-of-flight (TOF) technique under a wide range of electric-field. In TOF measurement, P3HT:PCBM exhibited a typical non-dispersive characteristics after thermal annealing at 336K (63°C) for 24hrs. The annealing effect can be reversed by exposing the blended film in air. The TOF signal of the P3HT:PCBM film degraded gradually as the air dosage increases. Further examination was also attempted with nitrogen, oxygen, and moisture. Our results suggest that only O2 interacts with P3HT:PCBM at certain dosages and results in highly dispersive TOF signals. Meanwhile, the attached O2 molecule can be eliminated by thermal annealing. With non-dispersive TOF signals, the hole mobilities of P3HT:PCBM blends with different phase segregation are also addressed.
9:00 PM - S8.21
Modified Thiophene Based Dendrimers for Improved Photovoltaic Performance.
William Rance 1 , Benjamin Rupert 2 , Andrew Ferguson 2 , Muhammet Kose 2 , David Ginley 2 , Nikos Kopidakis 2
1 Physics, Colorado School of Mines, Golden, Colorado, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractConjugated dendrimers are a promising class of organic materials that allow for a high degree of control over their properties with relatively simple modifications and substitutions of their molecular structure. Our group has previously reported on the synthesis of a series of phenyl-cored thiophene dendrimers and their performance in bulk-heterojunction solar cells with the fullerene derivative PCBM. In this work we extend our studies to dendrimers with electron withdrawing cyano groups on the phenyl core and electron donating dibutylaniline moieties located at the periphery of the thiophene dendrons. The effects of these functional groups on the dynamics of photogenerated carriers in both neat and blended films with PCBM were investigated using the contactless Time Resolved Microwave Conductivity (TRMC) technique and are correlated with device performance measurements where an efficiency of 1.2% was seen in a bulk-heterojunction with the cyano modified dendrimer and PCBM. Carrier lifetimes were found to be longer lived in cyano dendrimer:PCBM blends compared to previous dendrimer:PCBM blends. We discuss the photoinduced charge separation for each substituted dendrimer blended with PCBM with respect to the type of substitution (electron donating or electron withdrawing), the position of the substituent on the dendrimer and the resulting overall structure and morphology of the bulk-heterojunction film. From these studies we show that carrier recombination in some dendrimer:PCBM blends can limit device performance.
9:00 PM - S8.23
Spectroelectrochemical Study of Electron Trapping Behaviors in Organic Photovoltaic Bilayers.
Yunfeng Li 1 , Vince Cammarata 1 , Chin-Che Tin 2
1 Department of Chemistry and Biochemistry, Auburn University, Auburn, Alabama, United States, 2 Department of Physics, Auburn University, Auburn, Alabama, United States
Show AbstractSpectroelectrochemistry (SEC), which is a coupling of electrochemical and spectroscopic techniques, is widely used in the characterization of electroactive films. SEC is a powerful technique to determine energy levels and monitor the spectral change of conducting polymer films during electrochemical growth and doping/dedoping processes. In this paper, we present SEC studies on electron trapping behaviors in several diphenylamine end-group electroactive polymers bilayers, which were electropolymerized from their monomer solution, FD (1,1’-bis[[p-phenylamino- (phenyl)]amido]-Ferrocene), DNTD (N, N’-Di[p-phenylamino(phenyl)]-1,4,5,8-Naphthalene Tetracarboxylic Diimide) and DPTD (N,N’-di[p-phenylamino(phenyl)]- perylene-3,4,9,10-tetracarboxylic diimides) and Cl4DPTD (N,N’-di[p-phenylamino(phenyl)]- 1,6,7,12-tetrachloro-3,4,9,10-perylenetetracarboxylic diimides). Also, the LUMO and HOMO difference between poly (DNTD) and poly (Cl4DPTD) or poly(DPTD) and poly(Cl4DPTD) allows electron trapping between these n-type materials. UV-Vis spectroscopy results show that these polymer bilayers,poly(FD)|poly(DPTD) or poly (FD)|poly(Cl4DPTD) have absorption spectra that cover a wide range of the solar spectrum and can therefore be used in photovoltaic cells.
9:00 PM - S8.24
Design and Synthesis of Novel Donor-Acceptor Polymers for Organic Photovoltaics and Thin Film Transistor.
Rajib Mondal 1 , Sangwon Ko 1 , Ying Jiang 1 , Eric Verploegen 1 , Hector Becerril 1 , Zhenan Bao 1
1 Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractDevelopment and tuning the fundamental properties of polymeric semiconducting materials have become an active area of research in recent years due to their potential uses in light weight and flexible optoelectronic devices, such as organic photovoltaics (OPVs) and organic thin film transistors (OTFTs). In this regard, several fused aromatic thienopyrazine (TP) copolymers were synthesized and investigated. The fused aromatic TP unit provides a planar and rich π-face, which promotes the π-π stacking between the polymeric chains and enhances the device efficiencies. The electronic properties, such as band gap and frontier molecular orbital energies of these polymers were tuned using different aromatic groups in the TP unit. Power conversion efficiency up to ~2% in OPV devices and hole carrier mobility of ~0.2 cm2/Vs in OTFT devices have been achieved using some of these polymers. Another family of studied polymers is based on a vinylene linked donor-acceptor design motif. Introduction of vinylene linkage in π-conjugated systems can make the polymer backbone planar by eliminating torsional strain between rigid donor-acceptor units.
9:00 PM - S8.26
Enhancement of Device Performance of Organic Solar Cells by Modification of Exciton Blocking Layer Growth.
Inho Kim 1 , Hanna Haverinen 2 , Jian Li 1 , Ghassan Jabbour 1 2 3
1 School of Materials, Arizona State University, Tempe, Arizona, United States, 2 , Flexible Display Center, Tempe, Arizona, United States, 3 , Advanced Photovoltaics Center, Tempe, Arizona, United States
Show AbstractMost of high efficiency organic solar cells based on small molecular weight materials have adopted an optically transparent exciton blocking layer (EBL) between cathode and acceptor. Bathocuproine (BCP) is the most widely used EBL material in small molecular weight organic solar cells. In this study, growth of exciton blocking layer was modified by insertion of thin perylene diimide derivative with hexyl chain (PTCDI-C6) on top of fullerene. Modified growth of BCP by insertion of 5 nm PTCDI-C6 expedites diffusion of Ag element into BCP, resulting in high quality ohmic junction between the cathode and fullerene. Insertion of PTCDI-C6 between fullerene and BCP improves power conversion efficiency of ZnPc/fullerene solar cells, compared to cells without PTCDI-C6, which is mostly due to enhanced charge collection through electrodes. 2.5 % of power conversion efficiency was achieved with an optimized device structure of ZnPc (25 nm)/fullerene (30 nm)/PTCDI-C6 5 nm/BCP (15 nm)/Ag at a light intensity of 100 mW/cm2 (AM 1.5G), while only 1.9 % was obtained without PTCDI-C6 having the same device structure.
9:00 PM - S8.29
Fabrication and Sustainability of Multi-junction Polymer Solar Cells.
Annick Anctil 1 , Paul Jarosz 1 , Jason Staub 1 , Adam Podell 1 , Matt Scafetta 1 , Michael Brindak 1 , Amber Monfette 1 , Brian Landi 1 , Ryne Raffaelle 1
1 , RIT - NanoPower Research Labs, Rochester, New York, United States
Show AbstractThere are several challenges restricting the commercial viability of polymer solar cells involving derivatized fullerene-polymer blends. Most notably are the device efficiencies and environmental stability of active materials. Currently, the theoretical efficiency in an optimized device is limited by the poor mismatch to the solar spectrum. To overcome this issue, near-infrared small molecules including Zinc phthalocyanine (ZnPc) and Aluminum 1,8,15,22-tetrakis (phenylthio)-29H,31H-phthalocyanine chloride (PhS)4PhAl), were investigated for use in a multi-junction polymer solar cell. Co-evaporation of both donors and acceptor molecules to form a planar mixed heterojunction instead of planar heterojunction was shown to be a promising option to overcome the poor charge mobility of the small dye molecules. The development of such devices absorbing below the bandgap of the polymer can result in a dramatic improvement in polymer solar cells efficiencies by producing polymeric analog to a multi-junction III-V solar cell. The potential for tandem fabrication using a NIR small molecule diffuse bulk heterojunction in concert with a conventional PCBM-poly(3-hexyl thiophene-P3HT) active layer has also been demonstrated. In particular, the role of the donor: acceptor ratio on the morphology of the active layer has shown to be of critical importance for improved efficiencies. The use of a diffuse heterojunction rather than a planar heterojunction allows increased light absorption due to the formation of an interpenetrating network, similar to a polymer bulk heterojunction structure, which is necessary for charge transport over the thickness of the layer. We will present results on tandem devices made of ZnPc and the derivatized AlPc combined with C60 and discuss the prospects for the sustainable manufacturing of polymer solar cells. In addition, we evaluate the use of ultrasonic spray deposition as an alternative strategy to depositing the bulk P3HT composite layer compared to conventional spin-coating or ink-jet deposition. Overall, the multi-junction polymer solar cell approach will be discussed from a lifecycle perspective to provide a context on the sustainability issues with polymer solar cell materials and processing.
9:00 PM - S8.3
Laser Annealing of P3HT/PCBM Organic Solar Cells.
Emmanuel Okraku 1 , Mool Gupta 1 , Kenneth Wright 2
1 Electrical & Computer Engr., University of Virginia, Charolttesville, Virginia, United States, 2 Optical Sensors & Measurements Branch, NASA-Langley Research Center, Charolttesville, Virginia, United States
Show AbstractThermal annealing plays a significant role in the improvement of organic solar cell efficiency through increased molecular order. Organic solar cell efficiency improvements, by more than a factor of 100, have been achieved by thermal annealing at 130 0C for 2 minutes for P3HT/PCBM material systems. Thus, thermal annealing is an integral part of the organic solar cell fabrication process. Currently, thermal annealing is carried out by placing the sample in a furnace or on hot plate. There is an eminent need for thermal annealing methods that can be applied through front surface irradiation without the need for placing the object in a limited thermal furnace environment. Laser annealing is a non-contact process of generating heat and is more energy efficient as heat is only generated in the needed area of the thin film. Laser annealing methods are applicable to organic films coated on sensitive substrates and large coated structures can be annealed through front surface irradiation.In this work, we investigate the effect of pulsed laser annealing on the performance of P3HT/PCBM organic solar cells fabricated on indium tin oxide (ITO) coated glass substrates with aluminum as the top electrode. An Nd: YAG pulsed fiber laser, with a wavelength of 1064 nm, was used for laser annealing studies. We have measured the dependence of the solar cell efficiency, open circuit voltage, short circuit current density and fill factor on the laser energy density and have shown that performance improvements comparable to furnace or hot-plate annealing can be achieved using laser annealing. In addition, using AFM studies, we have shown that similar morphology changes induced by furnace annealing occur under the fast heating and cooling profiles of laser processing. This presentation will illustrate the results of a detailed study on the laser annealing of P3HT/PCBM organic solar cells.
9:00 PM - S8.30
Adhesion and Degradation Mechanisms in Organic Photovoltaic Structures.
Onobu Akogwu 1 2 , Tiffany Tong 1 2 , Winston Soboyejo 2
1 Electrical Engineering, Princeton University, Princeton , New Jersey, United States, 2 Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey, United States
Show AbstractThis poster presents the results of an experimental study of the effects of adhesion and active layer thickness on the performance of organic photovoltaic cells (OPV) containing P3HT/PCBM and MEHPPV/PCBM active layers. The adhesion between the layers is quantified using atomic force microscopy (AFM) techniques, while the current-voltage characteristics are studied as functions of active layer thickness. The damage mechanisms associated with the degradation of the organic PV structures are elucidated using a combination of focused ion beam (FIB) microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The implications of the results are discussed for organic electronic structures
9:00 PM - S8.31
Monte Carlo Modeling of the Spatially Dispersive Carrier Transport in Organic Semiconductors and Bulk Heterojunctions.
Xin Jiang 1 , Alexandre Nardes 1 , Peter Graf 2 , Sean Shaheen 1
1 Department of Physics and Astronomy, University of Denver, Denver, Colorado, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractThe presence of traps, arising from morphological or chemical defects, can be critical to the performance of organic semiconductor devices. Traps can reduce the charge carrier mobility, disturb the internal electrical field, drive recombination, and reduce the overall device efficiency as well as operational stability. In this work, we investigate the role of traps in determining charge transport properties of organic semiconductors and blends such as P3HT and P3HT:PCBM through Monte-Carlo (MC) simulations in conjunction with time-of-flight (TOF) mobility measurements. We employ a Marcus theory description of individual hopping events based on the molecular reorganization energy (λ) for the MC simulations. Trap states are modeled as diffuse bands that reside at some energy away from the main transport band. This model is used to simulate TOF transients, and the results are compared to experimental data. As is expected from the Marcus theory equation, the mobility is seen to be maximum for an optimal value of lambda. This optimal value is strongly field dependent, but is found to be independent of the trap density. In comparing MC simulations with TOF data, it is found that inclusion of traps results in a much better fit to the data and provides for a mechanism to simulate dispersive transport with a long tail resulting from trapping and detrapping of carriers before they exit the device. We present results for a range of trap densities and statistical distributions and discuss the implications on the operation of bulk heterojunction organic photovoltaic devices.
9:00 PM - S8.32
Luminescent Solar Concentrators Employing Dyes Aligned by Polymerizable Liquid Crystals.
Carlijn Mulder 1 , Heekyung Kim 1 , Phil Reusswig 1 , Carmel Rotschild 1 , Marc Baldo 1
1 Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractLuminescent Solar Concentrators (LSCs) use emissive dyes to absorb diffuse light and concentrate it within a transparent waveguide. Here, we investigate LSCs with oriented dye molecules. Using polymerizable liquid crystals we fabricate LSCs with dyes aligned both in-plane and perpendicular to the plane of the waveguide.When dye molecules are aligned in-plane, the LSC absorption can be linearly polarized. This allows LSCs to replace the linear polarizers presently employed in displays to enhance the contrast ratio. We will describe a new energy-harvesting strategy for portable and mobile electronics in which the purely absorptive polarizers in conventional displays are replaced with two linearly polarized luminescent concentrators (LSCs) that channel the energy of absorbed photons to photovoltaics at the edge of the display. As a proof of concept, we align the rod shaped dye molecule, Coumarin 6, within a polymerizable nematic liquid crystal. Photons absorbed and re-emitted by C6 are guided to the edges of the LSC by total internal reflection, where they are collected by solar cells. This concept allows the photovoltaics to be located in the frame of the display, which minimizes their area, while leaving the entire front surface available for the display. We observe optical quantum efficiencies of 40% and polarization selection ratios of 3. Finally, we will present results for LSCs with vertically-aligned dye molecules to improve the coupling between the waveguide and dye re-emission. Typically, less than 75% of light is trapped within the waveguide for non-aligned dyes. Here, we demonstrate more than 75% coupling into the waveguide. This approach can improve the performance of LSCs at high optical concentrations.
9:00 PM - S8.33
Exciton Formation in Metal-Organic Mixed Layers.
Abhishek Yadav 1 , Yansha Jin 2 , Max Shtein 2 , Kevin Pipe 1
1 Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractMixed layers of organic semiconductors and metals are interesting for OPV applications due to the plasmon enhancement of optical absorption. However, device efficiency potentially suffers from exciton quenching at metal atoms, as well as from transport and dissociation limitations in the presence of molecular disorder. Characterizing how the exciton population evolves with film growth parameters is therefore important for improving device performance. Modulation spectroscopy, utilizing electric field as modulation parameter (or Stark effect spectroscopy), has been widely used to study excitons in organic semiconductors. However, its application to conductive samples such as metal-organic mixed layers is complicated by a number of detrimental effects such as: 1) built-in electric fields, 2) the formation of interface dipoles, and 3) Thomas Fermi screening. In addition, the electric field in such a film can be very non-uniform, if the metal atoms are aggregated or otherwise dispersed in a non-uniform manner through the composite film. In this work, we show that temperature modulation spectroscopy can circumvent these difficulties to provide a great deal of information about excitonic transitions in organic semiconductor-metal mixtures. In particular, we study the shifts in exciton energies upon metal (Ag, Al, Au) incorporation in copper phthalocyanine (CuPc) thin films. First, we assign the nature of each excitonic energy level to Frenkel or charge-transfer (CT) exciton types in pristine CuPc films. We also identify the effect of temperature variation on the lifetime broadening of each excitonic transition. Then, we demonstrate the range of growth parameters over which the increased disorder in the CuPc lattice structure due to incorporation of the metal leads to an optical spectrum that transitions between that of a CuPc thin film and a single CuPc molecule. Finally, we quantitatively investigate the reduction in CT exciton formation rates due to quenching at incorporated metal atoms.
9:00 PM - S8.34
Controlled Interface for Efficient Hole Extraction in Organic Bulk Heterojunction Solar Cells.
Kyung-Geun Lim 1 , Mi-Ri Choi 1 , Dong-Hun Kim 1 , Nam Su Kang 2 , Byung Doo Chin 3 , Tae-Woo Lee 1
1 Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Gyeongbuk Korea (the Republic of), 2 Center for Energy Materials, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 3 Department of Polymer Science and Engineering, Dankook University, Yongin, Gyeonggi-do Korea (the Republic of)
Show AbstractThe interface between the photoactive layer and the hole extraction anode in organic bulk heterojunction solar cells is critically important to enhance the power conversion efficiency. Although significant progress has been made in organic bulk heterojunction solar cells, the detailed fundamental study on the interface control of the hole extraction layer (HEL) has not been reported so far to the best of our knowledge. Here we have performed systematic experiments to tune the surface work function and the surface energy of the polymeric HEL, which has been done by treating various self-assembled monolayers (SAMs) on the layer and modifying the compositions of polymeric HEL. We have tuned the surface work function to efficiently extract holes from poly(3-hexyl-thiophene):1-(3-methoxy-carbonyl)propyl-1-phenyl-(6,6)C61 (P3HT:PCBM) and the surface energy to control the self-organized film morphology of the photoactive layer, which has been confirmed by ultraviolet photoelectron spectroscopy and wide angle x-ray scattering, respectively. We have analyzed the relationship of the power conversion efficiency of the solar cells with the HEL surface work function and the self-organized P3HT:PCBM film morphology.
9:00 PM - S8.35
Relationships Between the Chemical Structure and Photophysical Properties of Thiophene-based Model Compounds.
Andrew Ferguson 1 , Justin Johnson 1 , Allison Kanarr 2 , Benjamin Rupert 1 , Scott Hammond 1 , Muhammet Kose 1
1 , NREL, Golden, Colorado, United States, 2 Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, United States
Show AbstractModel compounds, based on the fundamental building blocks present in light-harvesting materials used in the best-performing organic photovoltaic (OPV) devices, are an invaluable tool to investigate and understand relationships between chemical structure and the solid-state photophysical properties.Our group has previously reported on the basic photophysics (UV/Vis absorption, photoluminescence) of compounds consisting of oligomeric thiophene arms bound in different patterns around a central phenyl core.We extend these studies to probe the precise fate of the photogenerated singlet excited-state in these materials, with particular emphasis on the relaxation pathways available to the excitation. We show that both internal conversion to the ground state and intersystem crossing to the first triplet state compete effectively with radiative decay. In addition we will demonstrate the effect that blending these compounds with the fullerene derivative PCBM has on the intrinsic processes that take place after photoexcitation. In particular, we explore the competition between energy transfer to PCBM and exciton dissociation at the donor-acceptor interface to form charge-carriers, and discuss the impact these observations may have on OPV device performance.
9:00 PM - S8.36
Atmospheric Effect on the Photovoltaic Properties of Very High Purity Organic Solar Cells.
Toshihiko Kaji 1 , Kai Iketaki 1 , Masahiro Hiramoto 1
1 Research center for Molecular Scale Nanoscience, Institute for Molecular Science, Okazaki Japan
Show AbstractOrigin of carrier types, p/n, in organic semiconductors has long been a question in the studies of organic devices. In most cases, the origin of p-type is thought to be oxygen molecules. Inherent origin of n-type, however, has still not been clarified despite the importance of its carrier number for organic devices. For example, the numbers of the carriers can strongly affect the performance of organic solar cells since only built-in potential works as driving force to transfer photo-carriers. This point makes organic solar cells different from other organic devices whose carrier transfer can be easily accelerated by external voltage. Thus in future, finely controlled doping should become more essential for organic solar cells. To realize this fine doping, removal of unintentional impurities from organic semiconductors is one of the most integral issues.In this study, purification of organic semiconductor crystals and assessment of atmospheric effect on the photovoltaic properties of small molecular weight organic thin film solar cells are demonstrated. For the purification, we have succeeded in centimeter-scale crystal growth of organic semiconductors such as fullerenes (C60), phthalocyanines (H2Pc and ZnPc), and perylenes (PV), all of which are important materials for organic solar cells. Especially in the case of C60, purity was confirmed to reach seven-nine (7N) by SIMS measurements. For the atmospheric control, we have improved our fabrication and characterization system for organic solar cells to keep the atmosphere inert.To assess the condition of the system, we compared two C60/ITO Schottky cells transferred under different conditions. The first cell was exposed to the air during transfer from a vacuum deposition chamber to a measurement system equipped with vacuum pumps, and the other cell was transferred through an atmospheric control type of a glovebox directly connected to the vacuum chamber. During this transfer, concentrations of oxygen and water were kept under 10 ppm, and open circuit voltage was revealed drastically smaller than the voltage of the air-exposed case. This difference seemed as if the Schottky barrier had disappeared. Importantly, these results suggest that the C60 thin film became nearly intrinsic. Using this condition as a reference, we would discuss about the effect of atmospheric gases such as oxygen and hydrogen, and other unintentional impurities on small molecular weight organic solar cells.
9:00 PM - S8.37
New Benzobisoxazole Based Conjugated Polymers: Synthesis and Photovoltaic Activity.
Kanwar Nalwa 1 , Jared Mike 1 , Daniel Putnam 1 , Molly Reida 1 , Malika Jeffries-El 1 , Sumit Chaudhary 1
1 , Iowa State Univ, Ames, Iowa, United States
Show AbstractWe report the synthesis of two new red/near-infra-red band-gap conjugated polymers and efficient photovoltaic devices based on their blend with methanofullerenes. These two benzobisoxazole containing polymers are: poly[(3,4-bisdodecylthiophene vinylene)-2,6-diyl benzo[1,2-d;5,4-d’]bisoxazole] (cis-PTVBBO) and poly[(3,4-bisdodecylthiophene vinylene)-2,6-diyl benzo[1,2-d;4,5-d’]bisoxazole] (trans-PTVBBO) was accomplished by the Horner-Wadsworth-Emmons polymerization between 3,4-bisdodecylthiophene dicarboxalaldehyde and the corresponding 2,6-dimethyl benzobisoxazole-diethylphosphonate ester. The polymers had good molecular weights and are soluble in non-protic organic solvents such as THF, chloroform and dochlorobenzene. Their blends with methanofullerenes were prepared in dichlorobenzene and efficient photovoltaic behavior was observed with significant spectral responsivity in red and near-infrared region.
9:00 PM - S8.38
Practical Efficiency Limits in Organic Photovoltaic Cells: Dependencies of Fill Factor and External Quantum Efficiency.
Jonathan Servaites 1 3 , Mark Ratner 1 2 3 , Tobin Marks 1 2 3
1 Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 3 Materials Research Center, Northwestern University, Evanston, Illinois, United States, 2 Department of Chemistry, Northwestern University, Evanston, Illinois, United States
Show AbstractWe report on the strong correlation between practical cell power conversion efficiency limits and the LUMO level offset of the electron donor and acceptor materials in organic bulk-heterojunction photovoltaic (BHJ OPV) cells, considering the dependencies of fill factor and external quantum efficiency (EQE). Our approach is to treat fill factor as a function of LUMO offset, demonstrating how fill factor can increase with decreasing LUMO offset. We also evaluate the EQE as a function of wavelength, basing this function on transmittance and internal quantum efficiency limitations. For a given LUMO offset energy, we numerically optimize donor bandgap and cell efficiency. We show that, if OPV systems could be prepared to generate free charge carriers efficiently at a LUMO level offset approaching ~0.3 eV, power conversion efficiencies well over 10% are possible.
9:00 PM - S8.4
Evaluating Carrier Accumulation in Degraded Bulk Heterojunction Organic Solar Cells by a Thermally Stimulated Current Technique.
Kenji Kawano 1 2 , Chihaya Adachi 1
1 Center for Future Chemistry, Kyushu university, Fukuoka Japan, 2 Advanced Technologies Development Laboratory, Panasonic Electric Works., Ltd., Kadoma Japan
Show AbstractWe investigate the initial photo-degradation of encapsulated P3HT:PCBM bulk heterojunction organic solar cells. The degraded device is recovered by thermal annealing treatment. Thermally stimulated current (TSC) measurement reveals that the cause of photo-degradation is carrier accumulation and that the degraded organic solar cell has two broad trap levels of 0.71 eV and 0.81 eV, respectively. These traps are independent of the thickness of the photoactive layers, the mixing ratio of the photoactive materials, and the cathode materials. In addition, we confirm that there is a close relationship between the degree of degradation and the amount of accumulated charge carriers.
9:00 PM - S8.5
Energy Level Evolution of Varying Molybdenum Trioxide Inter-layer Between Conducting Indium Tin Oxide (ITO) and Chloro Aluminum Pthalocyanine (AlPc-Cl).
Irfan Irfan 1 , Huanjun Ding 1 , Yongli Gao 1 , Do Young Kim 2 , Jegadesan Subbiah 2 , Galileo Sarasqueta 2 , Franky So 2
1 Physics and Astronomy, University of Rochester, Rochester, New York, United States, 2 Material Science & Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractWe investigated 0 to 300 Å thick stepped molybdenum trioxide (MoO3) inter-layer between in-situ oxygen plasma treated conducting indium tin oxide (ITO) and layer by layer evaporated chloro-aluminum pthalocyanine (AlPc-Cl) up to 228 Å, with ultra-violet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). In the presence of MoO3 inter-layer, occupied levels of AlPc-Cl were observed to shift towards the higher binding energy (BE). This shift trend was more pronounced with increasing thickness of MoO3 inter-layer. Vacuum level shift due to the deposition of MoO3 on ITO was measured up to 1.6 eV. Highest occupied molecular orbital (HOMO) energy level shift from 10 Å to 300 Å MoO3 was found to be less than 0.1 eV. Based on Fermi level and vacuum level alignment possible device properties can be qualitatively assessed. A comparison of in-situ oxygen plasma treatment vs conventional ex-situ oxygen plasma treatment will also be discussed.
9:00 PM - S8.6
Effects of UV Light-irradiated Buffer Layer on the Performance of Polymer Solar Cells.
Hang Ken Lee 1 , Jai-Kyeong Kim 2 , O Ok Park 1
1 Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of), 2 Center for Energy Materials Research, Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractRecently, polymer solar cells(PSCs) have attracted great interest as potential alternatives for inorganic-based solar cells due to thier possibility of low-cost fabrication, low-weight, and mechanical flexibility[1]. Typical structure of PSCs is P3HT/PCBM based BHJ active layer sandwiched between an indium tin oxide (ITO) anode and a low work function metal cathode. Here, one of the general problems is photogenerated carrier loss at the active layer/electrode interface. In the case of holes, the band offset between the work function of ITO and the highest occupied molecular orbital(HOMO) of the P3HT plays a role as a transport barrier so the overall external photocurrent is reduced. To solve this problem, a buffer layer, poly(3,4- ethylenedioxythiophene): poly(styrenesulfonate)(PEDOT:PSS) is commonly inserted between the ITO anode and the active layer[2,3]. However, some of the holes nonetheless cannot be transported to the anode owing to the high bulk resistance and/or improper contact condition of the PEDOT:PSS. For this issue, several attempts have been made to modify the PEDOT:PSS. Ko et al. and Zhang et al.[4,5] reported that hole extraction can be improved by doping of polyalcohols (alcohols with more than two OH groups on each molecules) such as mannitol and sorbitol into PEDOT:PSS. Polyalcohol-doped PEDOT:PSS showed lower resistivity, however, careful control of the doping level was required because excess doping caused phase separation and defects in PEDOT:PSS. On the other hand, Yoon et al.[6] inserted an additional buffer layer between ITO and PEDOT:PSS to modify the contact condition. Facilitating hole collection by adjusting interface energy step resulted in an improved PCE in PSCs but an extra evaporation step in a vacuum is not a recommended method in the cost point of view. We investigate the effect of a UV-irradiated PEDOT:PSS buffer layer on the performance of polymer photovoltaic cells based on P3HT and PCBM blends. It was found that UV irradiation can reduce the bulk and contact resistance of PEDOT:PSS films, improving the power conversion efficiency by as much as 3.47% due to the lower device series resistance under an illumination of AM1.5G, 100mW/cm2. The work function change after UV irradiation has no significant effect on hole transport. In addition, negligible surface morphology change was noticed. [1] S. Gunes, H. Neugebauer, and N.S.Sariciftci, Chem. Rev. 107 (2007) 1324.[2] S.J. Yoon, H.K. Lee, J.H. Park, and OO Park, Appl. Phys. Lett., 92, (2008)143504[3] F. Zhang, M. Johansson, M.R. Andersson, J.C. Hummenlen, and O. Inganäs, Adv. Mater. 14 (2002) 662.[4] C. J. Ko, Y. K. Lin, F. C. Chen, and C. W. Chu, Appl. Phys. Lett. 90 (2007) 063509.[5] F. L. Zhang, A. Gadisa, O. Inganas, M. Svensson, and M. R. Andersson, Appl. Phys. Lett. 84 (2004) 3906.[6] W-J Yoon, and P. R. Berger, Appl. Phys. Lett. 92 (2008) 013306.
9:00 PM - S8.7
Energy Level Alignments at the Interfaces of ITO/Pentacene/C60, ITO/CuPc/C60 and ITO/SubPc/C60 for Organic Photovoltaic Devices.
Hyunbok Lee 1 , Pyung Eun Jeon 1 , Hyun Sung Kim 1 , Kwangho Jeong 1 , Yeonjin Yi 2
1 Insititute of Physics and Applied Physics, Yonsei university, Seoul Korea (the Republic of), 2 Division of Industrial Metrology, Korea Reseach Institute of Standards and Science, Daejeon Korea (the Republic of)
Show AbstractThe structure of a donor-acceptor heterojunction has been applied in small molecular organic photovoltaic cells. Generally, acceptor materials have been employed fullerene (C60) or its derivatives, while donor materials have been adopted planar phthalocyanine (Pc) materials (typically CuPc) or pentacene. Recently, remarkably enhanced organic photovoltaic cell with donor material as boron subphthalocyanine chloride (SubPc) was reported because it showed the larger open circuit voltage (Voc) than CuPc though the short-circuit current (Jsc) was not reduced seriously. However, the detailed interfacial electronic structures which determine the electrical properties are not well known. In this study, we investigated the interfacial electronic structures of ITO/pentacene/C60, ITO/CuPc/C60 and ITO/SubPc/C60 using in situ ultraviolet and x-ray photoelectron spectroscopy. All spectra were obtained with stepwise deposition sequence and complete energy level diagrams were obtained. Comparing energy levels of each interface gives a deep insight into the highly efficient photovoltaic cells.
9:00 PM - S8.8
N-type Perylenediimide Copolymers as Acceptor for All-Polymeric Solar Cells.
Erika Kozma 1 , Filomena Munno 1 , Darek Kotowski 1 , Silvia Luzzati 1 , Marinella Catellani 1
1 , Istituto per lo Studio delle Macromolecole, CNR, Milano Italy
Show AbstractOrganic solar cells based on polymer semiconductors are of great interest as low cost wide area approach to photovoltaic solar energy conversion. Currently, the most widely studied bulk heterojunction organic solar cells have active layers composed of an electron-donating polymer poly-3-hexylthiophene (P3HT) and an electron accepting molecule (soluble fullerenes). In order to increase the photovoltaic performances towards large area devices a great efforts have been dedicated to optimize the blend morphology and the processability of photoactive layers. Although various conjugated polymers have been explored as electron donor in bulk heterojunction, not so many polymers have been used as n-type electron acceptor materials.Perylenediimide derivatives exhibit good electron mobilities and suitable electronic levels to act as acceptor in bulk heterojunctions with conjugated polymers. The introduction of different substituents either in the imide position or in the bay position will definitely influence the packing and the properties of the perylene derivatives. Moreover, the substitution in the core of the perylene bisimide system, offers the most flexible possibilities for modulating the electronic properties. Indeed, photophysical and redox properties are modified by this type of substitution, which demonstrate the chemical versatility of these molecules and also, the possibility to use them as building blocks for the preparation of new materials.With the aim to obtain stable photoactive materials with a good processability, we have prepared a series of n-type alternating polymers containing N,N’-bis(10-nonadecyl)perylene-3,4,9,10-tetracarboxylic diimide and oligothiophenes (single-bond and triple bond linked) or other substituents with donor properties, by Stille or Sonogashira coupling reactions.These conjugated systems shown interesting ambipolar electrochemical properties, high electronic affinities when compared with the most common conjugated polymers, wide electronic absorption in the visible spectrum, and they can be processed easily from solution. Also, all these polymers have good thermal stability with high onset decomposition temperatures.In this work, the optical and electronic properties of these donor/acceptor series will be presented, along with their photovoltaic characteristics in solar cells prepared with P3HT.
9:00 PM - S8.9
Polymer Photovoltaic Cell Fabricated with Double Donors and Acceptor Organic Layers.
Dal-Ho Kim 1 , Su-Hwan Lee 1 , Ji-Heon Kim 1 , Keun-Lim Ku 1 , Tae-Hun Shim 1 , Jea-Gun Park 1
1 Nano-SOI Process Laboratory, Hanyang university, Seoul Korea (the Republic of)
Show AbstractIn current organic photovoltaic cell, the power conversion efficiency (PCE) of organic photovoltaic cells using small-molecular or polymer layer is low due to the narrow absorption spectra of small-molecular and polymer materials and it remains a major limitation in achieving high PCE. Thus, we investigated the improvement of light absorption with the copper phthalocyanine (CuPc):poly(3-hexylthiophene-2,5-diyl) (P3HT):[6,6]-phenyl-C61 butyric acid methyl ester (PCBM). The CuPc and P3HT were used for the small-molecular (long wavelength light absorption, ~660 nm) and polymer (middle wavelength light absorption, ~520 nm) donating material, and the PCBM was used for electron acceptor material in organic photovoltaic cells. The light absorbance of the CuPc:P3HT:PCBM blended layer was improved ~30% in the 300-500 nm as well as 550-800 nm wavelength in the visible light range compare with the P3HT:PCBM blended layer. The CuPc, P3HT, and PCBM were dissolved in chlorobenzene at a weight ratio of 1:2:1 and stirred at 50oC on a hot plate for more than 72 hours in a nitrogen (N2) glove box before spin casting to form the blend layer. The bar-shape ITO anode was patterned and the active area for light absorption (1.5 mm x 1.5 mm) was opened using a hard photoresist patterning. And then, the organic photovoltaic cells with a device structure of ITO / PEDOT:PSS / CuPc:P3HT:PCBM / BCP / Al were fabricated with small-molecular and polymer donating materials blended layer. We observed a high power conversion efficiency (PCE) of 7.4% with high short-circuit current (Jsc) of 17.9 mA/cm2, open-circuit voltage (Voc) of 0.655 V, and fill factor (FF) of 63.9% of organic photovoltaic cell. Acknowledgement* This research was supported by "The National Research Program for Terabit Nonvolatile Memory Development” sponsored by the Korean Ministry of Knowledge Economy.
Symposium Organizers
Jiangeng Xue University of Florida
Chihaya Adachi Kyushu University
Russell J. Holmes University of Minnesota
Barry P. Rand IMEC vzw
S9: Device Architectures for Organic Photovoltaics
Session Chairs
Thursday AM, December 03, 2009
Room 210 (Hynes)
9:30 AM - **S9.1
Polymer Tandem Solar Cells.
Rene Janssen 1 , Jan Gilot 1 , Martijn Wienk 1
1 Molecular Materials and Nanosystems, Eindhoven University of Technology, Eindhoven Netherlands
Show AbstractBy conserving a larger fraction of the incident photon energy, multijunction solar cells form a promising strategy to further increase the power conversion efficiency of organic photovoltaics. Owing to the large number of different layers in such devices, optimization of their performance is challenging. Here we demonstrate that the combined analysis of the optical absorption and electrical characteristics of the individual single junction subcells is essential to identify the optimum device layout of the corresponding tandem cell. It will be shown that the performance of tandem cells can be accurately predicted by combining the characteristics of representative single junction cells. This strategy provides a universal method to establish the optimal device layout and further enhance the efficiency of future polymer tandem solar cells. Using the methodology, a solution processed polymer tandem cell with an efficiency of 4.1% under AM1.5G conditions has been obtained. In this tandem cell the recombination layer, that connects the two subcells, does not impose important losses.
10:00 AM - **S9.2
Efficient and Long-term Stable Organic Vacuum Deposited Tandem Solar Cells.
Martin Pfeiffer 1
1 , Heliatek, Dresden Germany
Show AbstractWe report on latest progress in the field of p-i-n type tandem solar cells. An efficiency of 5.9% on an active area of 2cm^2 has been confirmed by NREL for a cell that combines bulk-heterojunctions with two complementary absorber systems in a tandem cell. We discuss the remaining loss factors in the present system and the way towards further optimization. Moreover, we show that p-i-n type tandem solar cells can be extremely stable: Extrapolated lifetimes corresponding to more than twenty years of illumination have been achieved. Finally we will report on efficient modules with integrated series interconnection and discuss further upscaling issues.
10:30 AM - S9.3
Enhanced Efficiency of Multi-fiber Organic Photovoltaic Systems.
Brendan O'Connor 1 , Denis Nothern 2 , Kevin Pipe 1 , Max Shtein 2
1 Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractMolecular organic-based solar cells deposited concentrically around long fibers have been demonstrated and characterized in detail.[1] Here, a combination of experiments, ray tracing, thin-film optical modeling, and transport simulations are used to examine several optical management schemes afforded by the fiber geometry. The individual fiber OPV cells use a metal-organic-metal structure, obviating the need for the undesirable (eg. brittle, expensive) indium tin oxide (ITO). When placed in an optimized bundled configuration, the resulting fiber architecture shows large performance gains and can significantly outperform the planar ITO-based OPV cell counterpart. Specifically, we show that multi-fiber architectures that employ external dielectric coatings and place fibers adjacent to one another as would occur in a woven pattern show increased performance by over 30% when compared to a single fiber OPV cell, and can outperform planar glass-ITO OPV counterparts. In addition, a volume-filling arrangement of fibers housed within a transparent composite matrix is predicted to trap light very efficiently (>90%) over the visible spectrum. For OPV devices with very high internal quantum efficiency (such as have been demonstrated recently [2]), this architecture shows very high predicted external quantum efficiency and large gains in short circuit current. Finally, when using a set of “color-tuned” fiber OPV cells that are separately designed to have high efficiency over a narrow bandwidth, this multi-fiber composite architecture is predicted to have high external quantum efficiency conversion over a broad range of wavelengths. This system of fibers is similar to a tandem OPV cell, but with much more relaxed current-voltage matching requirements. Such multi-fiber systems have important implications for realizing efficient energy harvesting fabrics and structural composites, providing a novel approach to efficient and low-cost structural and building-integrated solar energy technologies.[1] O'Connor, B., Pipe, K. P., and Shtein, M. Fiber based organic photovoltaic devices. Applied Physics Letters, 92(19), 193306. (2008).[2] Park, S.H., Roy A., Beaupre S., Cho S., Coates N., Moon J.S., Moses D., Leclerc M., Lee K., and Heeger A.J., Bulk Heterojunction solar cells with internal quantum efficiency approaching 100%. Nature Photonics, 3, 297-302 (2009).
10:45 AM - S9.4
Organic p-n Heterostructures and Superlattices of Pentacene and Perfluoro-pentacene.
Stefan Kowarik 1 , Alexander Hinderhofer 2 , Katharina Broch 2 , Alexander Gerlach 2 , Oriol Osso 3 , Cheng Wang 4 , Alexander Hexemer 4 , Frank Schreiber 2 , Stephen Leone 1
1 Department of Chemistry, UC Berkeley, Berkeley, California, United States, 2 Institut für Angewandte Physik, Universität Tübingen, Tübingen Germany, 3 MATGAS 2000 A.I.E., Esfera UAB, Barcelona Spain, 4 Advanced Light Source, Berkeley National Laboratory, Berkeley, California, United States
Show AbstractThe morphology and molecular structure at organic – organic interfaces are crucial for the performance of organic photovoltaic applications. Here we investigate organic semiconductor heterostructures of pentacene (PEN) and perfluoro-pentacene (PFP), which are promising p-type and n-type organic semiconductors respectively. We use soft and hard x-ray reflectivity measurements, scanning transmission x-ray microscopy (STXM) and atomic force microscopy for structural investigations of PFP-PEN heterostructures. Chemical contrast in STXM allows us to determine the lateral length scales of PEN and PFP islands in pure films and bilayer p-n structures. Interestingly, the crystalline orientation of PEN islands can be determined from in-plane dichroism, which gives strong contrast in STXM. For a superlattice of alternating PFP and PEN layers grown by organic molecular beam deposition, X-ray reflectivity measurements demonstrate a high degree of structural order and long-term stability. We find superlattice reflections that vary strongly when tuning the X-ray energy around the fluorine edge, clearly demonstrating alternating PFP and PEN layers.
11:30 AM - **S9.5
Organic Solar Cells: Multi-Terminal Multijunction Cells, Recombination Modeling and Roll-to-Roll Fabrication.
Peter Peumans 1
1 Electrical Engineering, Stanford University, Stanford, California, United States
Show AbstractIn this talk, I will describe a multijunction cell architecture that overcomes many of the limitations of conventional series-connected multijunction cells. The architecture uses multiple instances of a low-cost laminatable transparent electrode with sufficient in-plane conductivity to allow for current extraction between adjacent cells. This removes the current-matching requirement and allows for more efficient cells to be constructed with a given set of materials. By correctly arranging cells in a module, the overall module is still a simple two-terminal device. I will show demonstration devices that use this architecture. I will also review temperature dependent measurements of the performance of small-molecular weight organic solar cells and a recombination model that is fully consistent with all recorded data, both in the dark and under illumination. The model considers two types of recombination reactions, i.e. exciplex and exciton recombination, each with their characteristic activation energy and rate constant. This model allows us to accurately predict the dependence of open-circuit voltage on temperature and light-intensity. It also explains the temperature-dependent non-ideality factor observed when fitting the dark current-voltage characteristics of organic solar cells to the Shockley diode equation. Finally, I will discuss our efforts to modify a vacuum roll-to-roll system for organic solar cell deposition and will show how small-molecular weight organic solar cells made using this process are equivalent in performance to those deposited in a conventional deposition system. At the deposition rates achieved, the capital expense associated with the roll-to-roll vacuum coater is adequately small.
12:00 PM - S9.6
Bulk Heterojunction Organic Photovoltaic Cells Grown by Oblique Angle Deposition.
Ning Li 1 , Stephen Forrest 1
1 Electrical Engineering and Computer Science, Material Science and Engineering, and Physics, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractOrganic Photovotaic (PV) cell efficiency is continuously improving through the optimization of the bulk heterojunction (BHJ), where the distance an exciton must diffuse from its generation to dissociation sites is reduced in an interpenetrating network of the donor and acceptor materials. Various methods of making BHJ organic PV cells have been demonstrated. In this work, we make small molecule BHJ PV cells with oblique angle deposition, which has been shown to be capable of growing nano-structured porous films.[1] SEM images of oblique angle deposited chloroaluminium phthalocyanine (ClAlPc) film on Indium Tin Oxide (ITO) substrate show surface feature sizes of ~30nm, which is close to the exciton diffusion length of many organic materials. By introducing oblique angle deposited ClAlPc in ITO/ClAlPc/C60/bathocuproine (BCP)/Al PV cells grown by vacuum thermal evaporation, the external quantum efficiency is increased by more than 60% at the wavelength of 700nm, resulting in a power conversion efficiency of 3% at one sun. The porosity of the film leads to a higher dark current and lower open circuit voltage, which is suppressed by inserting a current blocking layer such as MoOx. Similar surface morphology and efficiency improvement is achieved for copper phthalocyanine /C60 PV cells as well. The oblique angle growth conditions leading to optimized BHJ PV cells will be described. [1] M. M. Hawkeye and M.J. Brett, “Glancing angle deposition: Fabrication, properties, and applications off micro- and nanostructured thin films,” J. Vac. Sci. Technol. A 25(5) Sep/Oct 2007, pp1317-1335
12:15 PM - S9.7
Organic Photovoltaic Cells Based on Graded Donor-acceptor Heterojunctions.
Richa Pandey 1 , Russell Holmes 1
1 Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractOrganic photovoltaics cells (OPVs) have drawn significant interest as a source of renewable energy due to their compatibility with high throughput, roll-to-roll fabrication processes. Optical absorption in an organic semiconductor leads to the creation of a tightly bound exciton that must be dissociated in order to generate photocurrent. Exciton dissociation is typically realized using a donor-acceptor (D-A) heterojunction, where the energy level offset exceeds the exciton binding energy. Excitons diffuse to the D-A heterojunction and are dissociated into their component charge carriers. In many organic semiconductors, the optical absorption length is much larger than the exciton diffusion length. Consequently, not all the photogenerated excitons can reach the D-A interface, limiting the overall cell efficiency. This work presents a new approach to the design of OPVs that employs graded film compositions to increase the D-A interface area for improving the exciton diffusion efficiency, while preserving the charge collection efficiency. Using a donor layer of copper phthalocyanine and an acceptor layer of C60, a maximum power conversion efficiency of ηP=(2.2±0.1)% has been realized using a graded heterojunction. This represents an improvement in ηP of ~30% compared to a uniformly mixed heterojunction and ~70% relative to a planar heterojunction OPV. Overall, this device architecture provides the ability to tune the exciton diffusion and charge collection efficiencies based on the D-A composition profile, permitting greater control over device performance.
12:30 PM - S9.8
Hot Press Lamination of Inverted Bulk Heterojunction Organic Photovoltaic Devices.
Brian Bailey 1 , Dana Olson 2 , Sean Shaheen 1 , Nikos Kopidakis 2
1 , University of Denver, Englewood, Colorado, United States, 2 , National Renewable Energy Labrotory, Golden, Colorado, United States
Show AbstractWe demonstrate lamination as a technique for fabrication of organic photovoltaic (OPV) devices. Immediate advantages of lamination are that the devices are encapsulated and protected against shear stress in the same step. Furthermore, there is no need for a high-quality (and expensive) metal evaporation chamber as the metal is not evaporated onto the organic active layer. Lamination also allows control of interface properties that are not accessible in the conventional, bottom-up OPV device fabrication methods. In this work, inverted devices from a blend of poly-3-hexylthiophene: [6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) are fabricated by spin-coating the electron transport and active layers onto one electrode substrate and the hole transport layer onto another followed by laminating the two together. The active layer half of the lamination is fabricated on ITO-coated glass with zinc oxide as the electron transport layer. A poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) hole transport layer is spin-coated onto a flexible PET substrate that has a thermally evaporated silver electrode as the top contact. As per a report in recent literature, the PEDOT:PSS hole transport layer is doped with d-sorbitol to improve its adhesive properties and to enhance the electrical contact and the mechanical robustness of the laminated structure. Our lamination process uses a hot press in which two heated plates are compressed using a hydraulic pump. We show that efficiencies of laminated devices are approaching those of bottom-up inverted devices with thermally evaporated Ag contacts. The laminated devices exhibit higher fill factors but lower currents than those employing evaporated Ag electrodes. These results confirm that lamination is a viable alternative to direct vacuum thermal deposition for contacting OPV devices and it opens the way to further work on interface and active layer optimization.
12:45 PM - S9.9
Interfaces Between C60 and Pentacene: Cluster Formation and Electronic Structure.
S. Robey 1 , G. Dutton 1 , D. Dougherty 4 , W. Jin 2 , J. Reutt-Robey 2 , B. Conrad 3 , W. Cullen 3 , E. Williams 3
1 , NIST, Gaithersburg, Maryland, United States, 4 Physics, N.C. State University, Raleigh, North Carolina, United States, 2 Chemistry, University of Maryland, College Park, Maryland, United States, 3 Physics, University of Maryland, College Park, Maryland, United States
Show AbstractNew technologies to exploit organic materials, such as organic photovoltaics, depend markedly on the molecular and electronic structure of the organic interfaces. Understanding the impact of the molecular morphology and interface structure on electronic level offset and charge transfer processes is thus vital to continued improvements of these devices. The combination of pentacene (Pn) with fullerene (C60) has been used as a donor-acceptor pair in bilayer photovoltaic structures. The morphology of the C60 and Pn films, molecular structure at the interface, and corresponding electronic structure, depend on a number of factors. By controlling the deposition sequence and growth conditions, we have been able to engineer C60:Pn interfaces with vastly different relative molecular orientations and morphologies and study these structures with STM, AFM and electronic spectroscopies. “Co-facial” interfaces between C60 and Pn can be formed by deposition of C60 on “flat lying” Pn grown on metallic substrates such as Ag(111). In contrast, Pn deposited on a hexagonal close-packed C60 layer adopts an "upright" orientation, forming large dendritic islands as on SiO2. Subsequent deposition of C60 on this surface leads to an interface dominated by C60 cluster formation. Electronic level alignments critical to photovoltaic operation are impacted by these structural changes. For the formation of an upright Pn phase on hexagonal close packed C60, the HOMO offset that plays a role in determining the energy available for charge separation is 1.26±0.1eV. Formation of C60 clusters at the interface leads to an increase in this energy difference by about 0.3 eV to 1.46±0.1eV. However, this probably also leads to a correspondingly reduced potential Voc. Details of STM and AFM investigation of interface molecular structure and photoemission studies of electronic structure will be discussed along with connections to theoretical results for interface formation.
S10: Hybrid Materials for Photovoltaics and LED
Session Chairs
Rene Janssen
Peter Peumans
Thursday PM, December 03, 2009
Room 210 (Hynes)
2:30 PM - **S10.1
Recent Progresses in Polymer-Oxide Hybrid Solar Cells.
Julia Hsu 1
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractIn this talk, I will discuss the pros and cons of using metal oxide, primarily ZnO, to replace PCBM as the electron acceptor in organic photovoltaics (OPVs). From a shelf life study, we show that device degradation behavior in OPVs is dominated by changes at the top metal electrode and/or the metal-organic interface, in contrast with common belief that changes in the active organic component are the leading degradation cause. Judicious choice of device architecture with suitable electrode materials leads to longer shelf life. In our laboratory, using Ag as positive contacts (to collect holes) and ITO/ZnO as negative contacts (to collect electrons) yields devices that are stable up to one year when stored in ambient conditions. In addition, I will show the adaptation of nanostructures and roughened surfaces to mitigate poor short circuit current (Jsc) in OPVs with metal oxide acceptor. Finally, controlling the electronic property of ZnO through processing, alloying, or interfacial modification enables us to obtain higher open circuit voltage (Voc). The combination of high Voc while maintaining Jsc is the key to improve device performance.
3:00 PM - S10.2
Applications of Metal Oxides in Organic Photovoltaics for Enhanced Performance and Lifetime.
Dana Olson 1 , Matthew White 1 2 , N. Widjonarko 1 2 , Brian Bailey 1 3 , Michael Machala 1 , Jordan Chesin 1 , Alisha Humphries 1 , David Ginley 1 , Joseph Berry 1
1 NCPV, National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Physics, University of Colorado, Boulder, Colorado, United States, 3 Physics and Astronomy, University of Denver, Denver, Colorado, United States
Show AbstractImprovements in the performance and lifetime of organic photovoltaics (OPV) are critical to the development of OPV as a viable technology for large scale power generation. Metal oxides can be leveraged to improve both of these factors through their use in many different applications in OPV devices. The most common use of metal oxides in OPV devices is in the transparent conducting oxide (TCO). Indium tin oxide (ITO) is the most commonly used TCO for OPV applications. Our group is actively researching ITO alternatives that could help to enhance device performance and lifetime as well as reduce the materials and processing costs of future OPV technologies. The second application is the use of metal oxides as selective hole or electron transport layers (HTL/ETL) in OPV devices to enhance device performance and stability. These allow for ohmic contact to one phase of the active layer while acting as a electron or hole blocking layer to reduce recombination and dependance on unstable electrode materials. Finally, metal oxides have been successfully used as electron acceptors in hybrid-OPV devices through the use of polymer/metal oxide nanoparticle blends or nanostructured metal oxide acceptors.Here we will present on our recent efforts on metal oxide systems for OPV devices in terms of the applications mentioned above. We will present results on the effects of work function, carrier concentration, and induced electric fields on charge extraction and device performance. Additionally, we will present initial studies on the resulting effects of such engineered contacts on device lifetime.
3:15 PM - S10.3
Incorporation of TiO2 Nanorods in Organic Photovoltaic Devices.
Pinyi Yang 1 , Guozhong Cao 1 , Christine Luscombe 1
1 Materials Science and Engineering Department, University of Washington, Seattle, Washington, United States
Show AbstractSemiconducting polymers are actively under development for use in light-weight, flexible, disposable organic light-emitting diodes, and thin-film transistors. A key application which is currently attracting a lot of interest for semiconducting polymers is their use in organic photovoltaic devices (OPVs). The main drive for developing OPVs is the lower cost associated with their manufacturing, because of the fact that organic semiconducting polymers can be solution processed. Additionally, because of the potential of being able to produce an all-organic device, one can foresee the production of flexible solar cells for portable applications. In order for the organic materials to become competitive with existing inorganic semiconductors, there is a great need to improve their efficiency to >10% (the greatest efficiency achieved for organic devices is currently only 5-6%), and to improve their device lifetimes. The use of organic-inorganic hybrid devices could potentially offer the best of both worlds in that the robust inorganic material can provide stability and improved charge transport while the organic material can maintain the low cost nature of the devices. Here we present our results obtained by incorporating TiO2 nanorods into P3HT/PCBM devices. Annealing conditions were altered to optimize the composite morphology. The incorporation of the nanorods results in a 20-30% increase in power conversion efficiency compared to our control group (P3HT/PCBM) when devices are tested in air.
3:30 PM - S10.4
Improving the Efficiency and Air Stability of Hybrid P3HT/CdSe Solar Cells with a ZnO Buffer Layer.
Jihua Yang 1 , Lei Qian 1 , Renjia Zhou 1 , Aiwei Tang 1 2 , Paul Holloway 1 , Jiangeng Xue 1
1 Materials Sceince & Engineering, University of Florida, Gainesville, Florida, United States, 2 Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing China
Show AbstractHybrid solar cells based on conjugated polymers and colloidal-synthesized inorganic semiconductor nanoparticles have been recognized as an alternative to all-organic solar cells. The inorganic material may complement the absorption of the organic phase and provide better charge transport properties due to the intrinsically higher carrier mobility in inorganic semiconductors, while maintaining the processibility of organic materials. Here we report the use of a ZnO nanoparticle buffer layer in hybrid solar cells based on poly(3-hexyl thiophene) (P3HT) and colloidal CdSe nanospheres. The ZnO nanoparticles were synthesized using a sol-gel method, and spin-coated onto the hybrid P3HT/CdSe layer from an aqueous solution before the deposition of an Al cathode. Compared to control devices without the ZnO layer, the devices with the ZnO layer showed 40-70% higher short-circuit current density, depending on the size of CdSe nanospheres. With slight changes in open-circuit voltage and fill factor, the ZnO-containing devices show a maximum power conversion efficiency of 2.5-2.8%, compared to approximately 1.6% in the best P3HT/CdSe nanosphere devices without the ZnO layer. In addition to the efficiency enhancement, the ZnO layer also drastically improves the air stability of the hybrid P3HT/CdSe solar cells. While the devices without the ZnO layer degrade completely after one to three days’ air exposure, the ZnO-containing devices exhibited only a modest 35% efficiency decrease after >70 days of storage in air.
3:45 PM - S10.5
Insulator-less MIS Inversion-layer Solar Cells, WhereInorganic-, Molecular- and Organic- Electronics Meet.
David Cahen 1 , Rotem HarLavan 1 , Izhar Ron 1
1 Materials and Interfaces, Weizmann Institute of Science, Rehovoth Israel
Show AbstractUse of a monolayer of organic molecules that self-assemble onto n-Si provides a near-ambient temperature, simple and potentially low-cost approach to fabricate solar cells. The molecules passivate and buffer the Si surface via direct binding to the oxide-free Si surface. In this way a sufficiently strong interface dipole can form to effectively reduce the Si electron affinity by as much as 0.7 eV and, thus, with the appropriate metal, yield an inverted surface. A thin layer of conducting polymer with high work function, that is deposited onto the molecular layer plays a threefold role:1. its high work function induces Si inversion, yielding an n-p+ Si homojunction;2. with n~1.5 refractive index the polymer acts as an anti-reflective coating to the Si.3. because it protects the molecular monolayer, subsequent metal deposition and encapsulation is now possible.Good and stable interface passivation along with strong inversion allow minority carriers, generated by absorbed sun light, to move laterally within the inverted Si top layer and to be collected by a minimal area metal grid, deposited on the conducting polymer. The same lateral conductance minimizes photo-current losses, due to polymer sheet resistance. Not only are cells fabricated without high temperature steps, the use of small organic molecules appears to convey here a unique advantage over inorganic passivation or buffer layers. The approach, which includes no or minimal (for metal grid) high vacuum steps, should be applicable to other inorganic absorbers, amorphous materials and thin films to improve photovoltaic solar energy conversion.
4:30 PM - S10.6
Charge Dissociation Engineering in Oxide/Polymer Hybrid Heterojunctions.
Alexandre Nardes 1 , Matthew White 2 , Nikos Kopidakis 2 , Dana Olson 2 , Joseph Berry 2 , Sean Shaheen 1 , David Ginley 2
1 Department of Physics and Astronomy, University of Denver, Denver, Colorado, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractPulse laser deposition (PLD) has been used to obtain thin films of zinc oxide (ZnO) and amorphous titanium oxide (a-TiOx) to be employed as acceptor materials in hybrid oxide/polymer organic photovoltaic (h-OPV). The PLD technique has a relatively simple setup, using a laser beam for vaporizing target material to be deposited as films on a substrate. The use of a short laser pulses offers the advantage of excellent control during deposition, which allows PLD to preserve stoichiometry during mass transfer from the target to the substrate. It also allows us to grow films with varying electrical and morphological properties offering a great advantage on optimizing materials and interfaces for h-OPV. For example, substrate temperature, target-substrate distance, laser pulse energy and oxygen partial pressure are easily controlled and affect the physical properties of the layers grown by PLD. The polymeric layer of poly(3-hexylthiophene) (P3HT) is spin coated on top of the oxides to serve as donor material. A detailed insight of the charge generation mechanism at the interface oxide/polymer is provided by Time-Resolved Microwave Conductivity (TRMC) measurements that are presented here and correlated to device performance. Single, bi- and tri-layer configuration of these materials have been studied. We found that by varying the carrier concentration of the oxide acceptor layer in the h-OPV oxide/polymer bi-layer devices, one can control the electric field at the planar donor-acceptor interface thereby enhancing charge separation at the interface. The effects of the interfacial electric field are reflected by an increase in the TRMC photoconductivity signal and consistent with the short-circuit current and fill factor improvements observed in devices. Moreover, we find that an interfacial layer of a-TiOx between the ZnO and the polymer reduces recombination with corresponding benefits to device performance.
4:45 PM - S10.7
Hybrid Solar Cells from Vertically Oriented TiO2 Nanotube/Nanowire Arrays and Soluble Functionalized Linear Acenes.
Karthik Shankar 1 , Zhong Li 4 , Oomman Varghese 2 , Xinjian Feng 2 , Chih-Min Lin 3 2 , John Anthony 4 , Craig Grimes 3 2
1 Electrical & Computer Engineering, University of Alberta, Edmonton, Alberta, Canada, 4 Chemistry, University of Kentucky, Lexington, Kentucky, United States, 2 The Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania, United States, 3 Electrical Engineering, The Pennsylvania State University, University Park, Pennsylvania, United States
Show AbstractNanostructured hybrid photovoltaics require the deposition of light-absorbing organic donors with a high charge carrier mobility and wide absorption cross-section into the nanometer-scale pores of an inorganic scaffold. Small molecules are well-suited to this application, since they tend to possess high hole mobility, strong absorption across the visible spectrum, and can be modified to interact with various surfaces. Yet, research into solution processable heterojunction solar cells has focused mainly on polymer-fullerene bulk heterojunctions. This is because the poor solubility and low air-stability of highly absorbing small-molecule organic semiconductors coupled with the need for extensive interface engineering has impeded progress. Using silylethyne-functionalized linear acene derivatives and vertically oriented one dimensional titania nano-structures, we demonstrate that all the above problems can be overcome. In particular, pentacene is a crystalline molecular organic semiconductor with a HOMO-LUMO gap of 1.9 eV and a hole mobility larger than 1 cm2/V-s, whose use in organic thin film field effect transistors has been extensively studied. Also, pentacene is more environmentally stable and reliable than most other organic semiconductors. For the purposes of comparison, consider that the exciton diffusion length of highly pure regioregular polythiophene is <15 nm and the hole mobility is only 0.01 cm2/V-s whereas the exciton diffusion length in crystalline pentacene films has been estimated to be as large as 70 nm. The carboxylic acid residue is common to nearly all dyes used in sensitized solar cells, and is critical to forming a strong mechanical and electronic interface to the oxide surface. Therefore, we used linear acenes with carboxylate and cyanoacrylate endgroups at the hetero-interface, which resulted in formation of the interfacial layer by self-assembly and very good electron injection across the interface into the n-type TiO2. For the hole transport material, we used the functionalized linear acenes, both in isolation and in blends with triarylamines. The best photovoltaic device performance was obtained when carboxylated and fluorinated derivatives of 5,11-bis(triethylsilylethynyl) anthradithiophene (TES-ADT) were used as the photoabsorber. Heterojunction solar cells using solution processed TES-ADT derivatives showed Voc of 0.65-0.75 V, Jsc of 2-8 mA cm-2 and fill-factors of 0.5-0.6 under AM 1.5 one sun illumination. An additional benefit to using derivatives with carboxylate and cyanoacrylate linkers was that these endgroups redshifted the absorption of the linear acene to longer wavelengths due to the longer conjugation length in the derivatized molecule. For the inorganic scaffold, we used both polycrystalline anatase nanotube arrays and monocrystalline rutile nanowire arrays and compared their performance.
5:00 PM - S10.8
Interfaces in Organic Photovoltaics Discussed for Titanium Oxide with and without S-shape Current-voltage Characteristics.
Roland Steim 1 2 , Pavel Schilinsky 1 , F. Rene Kogler 1 , Christoph Brabec 1
1 , Konarka Technologies GmbH, D-90443 Nürnberg Germany, 2 , Light Technology Institute, Universität Karlsruhe (TH), D-76131 Karlsruhe Germany
Show AbstractThe most significant improvements in achieving recent record efficiencies of organic photovoltaic (OPV) devices are mainly based on the design and synthesis of novel semiconducting polymers. Maximum performance, however, is only guaranteed by the use of proper interface materials which can have several tasks in a device, the most important of which are first the formation of selective contacts for electrons and holes at the electrode materials, and second the adjustment of the polarity in the device. The right choice of interfacial materials guarantees high fill factors and low leakage currents, two parameters which are a must for high and stable power conversion efficiencies for both outdoor and indoor applications. For instance, a reduced degradation under reverse bias voltage, an important characteristic for module shading effects, was shown for low leakage current devices [1]. Current limitations of interface materials will be discussed using the example of TiOx which has been successfully used as electron conducting layer in both regular and inverted device geometry. TiOx is especially interesting as it changes properties upon ultraviolet light illumination, turning it into an excellent interface material for OPV devices [2]. The resulting differences in current-voltage characteristics will be discussed and experimental data are correlated with results from current-voltage simulations. The findings contribute to a deeper understanding of the function of interface materials, and design rules for improved materials can be deduced.[1] R. Steim, S. A. Choulis, P. Schilinsky, U. Lemmer, and C. J.Brabec, Formation and impact of hot spots on the performance of organic photovoltaic cells, Appl. Phys. Lett. 94, 043304 (2009)[2] R. Steim, S.A. Choulis, P. Schilinsky and C.J. Brabec, Interface modification for highly efficient organic photovoltaics, Appl. Phys. Lett. 92 (2008), p. 093303-1-3
5:15 PM - S10.9
Organic-inorganic Composites with Improved Photoconductivity Based on Si Nanoparticles.
Hongzheng Chen 1 , Nan Liu 1 , Mang Wang 1
1 Department of Polymer Science & Engineering, Zhejiang University, Hangzhou China
Show AbstractOrganic-inorganic nanoparticle composites have attracted great attention due to their potential applications in photovoltaic and photoconductive devices. Silicon is regarded as the basic material of modern microelectronic industry due to its superior chemical and electrical properties. Great interest in silicon nanoparticles (NPs) has been aroused because of the visible luminescence observed from them as a result of quantum confinement effect. Many preparation methods for silicon NPs have been developed, such as laser ablation, silicon wafer anodization, nonthermal plasma synthesis, and solution-phase synthesis. To date, silicon NPs have been demonstrated to have potential applications in solar cells, light-emitting diodes, and bioprobes, etc.. In this work, Si nanoparticles are hybrid with organic semiconductors to produce new organic-Si NPs composites with improved photoconductivity. Si NPs are prepared by using a solution-phase method and are adequately characterized. The composites of Si NPs with organics are prepared by a simple solution method. By changing organic component, energy transfer or electron transfer can be realized in the Si NPs-based composites. The energy transfer from polyvinylcarbazole (PVK) to silicon NPs is observed in PVK-Si NPs composite. The electron transfer is demonstrated in N-dodecyl-N’-phenyl-3,4,9,10-perylenetetracarboxylic diimide (DOPP)-Si NPs composite, whose photosensitivity increases with increasing amount of DOPP until the mixture reaches a maximum and then additional DOPP results in a reduction of the photoconductivity. This work is expected to stimulate further progress in the application of silicon nanoparticles.
5:30 PM - S10.10
Air-Stable ZnO Precursor for Hybrid Bulk Heterojunction Photovoltaic Device.
Yun-Ju Lee 1 , David Wheeler 1 , Robert Davis 1 , Ping Lu 1 , James Voigt 1 , Julia Hsu 1
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractDue to a combination good electron mobility, suitable band positions, environmental stability, and low cost and toxicity, ZnO is a promising material as electron acceptor in hybrid heterojunction photovoltaic devices with a semiconducting polymer, e.g. poly(3-hexylthiphene) (P3HT), as the electron donor. To increase the device interfacial area, several research groups have spin coated blends of diethylzinc (a ZnO precursor) with semiconducting polymers, which upon exposure to air and/or water vapor forms a ZnO/semiconducting polymer hybrid bulk heterojunction with intimate mixing of the two phases, resulting in power conversion efficiency of > 1% at AM 1.5. However, because diethylzinc reacts instantaneously and vigorously with air and water vapor, the initial steps of device processing must be performed inside controlled atmosphere such as a glove box, which increases fabrication complexity. Here, we report our progress toward an air-processed hybrid bulk heterojunction photovoltaic device using air-stable ZnO precursors. The relationship between Zn coordinating group substitution and the air and water stability of the precursor, as well as the miscibility with P3HT in common organic solvents, are examined. Solution and thermal processing conditions are varied to optimize blended film morphology and ZnO conversion. Finally, the PV performance of an air-processed ZnO/P3HT hybrid bulk heterojunction device is compared to a diethylzinc device. The observed differences in current density-voltage response and external quantum efficiency spectra will be examined in term of bulk heterojunction morphology. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
S11: Poster Session
Session Chairs
Russell Holmes
Barry Rand
Friday AM, December 04, 2009
Exhibit Hall D (Hynes)
9:00 PM - S11.1
Comparison of Binary and Ternary Metal Sulfides as Acceptor Phase in Inorganic/Organic Hybrid Photovoltaics prepared by an In-situ Formation Process.
Michael Edler 1 2 , Achim Fischereder 1 2 , Eugen Maier 1 , Thomas Rath 1 2 , Wernfried Haas 2 3 , Ferdinand Hofer 3 , Gregor Trimmel 1 2
1 Institute for Chemistry and Technology of Materials, Graz University of Technology , Graz Austria, 2 Christian Doppler-Pilotlaboratory for Nanocomposite Solar Cells, Graz University of Technology & NanoTech Center Weiz Forschungsgesellschaft mbH, Graz Austria, 3 Institute for Electron Microscopy and Fine Structure Research, Graz University of Technology, Graz Austria
Show AbstractIn recent years hybrid photovoltaic cells based on blends of nanoparticles and organic semiconducting polymers have achieved promising power conversion efficiencies.In this contribution, we focus on the preparation of metal sulfide/polymer nanocomposite bulk heterojunction solar cells. The active layer was prepared by first spin coating of a solution containing the proper inorganic metal salts (e.g. zinc acetate), a sulfur source (e. g. thioacetamide) and a conjugated organic polymer (e.g. poly-para-phenylene-vinylenes or different polythiophene derivatives) followed by a mild thermal temperature step (below 200 °C). By a proper choice of processing conditions a nano-structured network of the acceptor and donor phase is formed. We show that various inorganic acceptor phases can be obtained by this method. We have prepared CdS, PbS, ZnS, CuInS2 directly in the polymer matrix.The nanocomposite layers were analyzed by X-ray diffraction, transmission electron microscopy as well as optical spectroscopy. These analyses show that in most cases the particle size is in the range of 3 to 8 nm with the exception of PbS, which formed much larger crystallites. Bulk heterojunction solar cells were assembled by preparing the nanocomposite layers directly on indium-tin oxide coated glass substrates followed by evaporation of aluminium electrodes. The solar cells were characterized by measuring current density/voltage curves and incident photon to current efficiency spectra.
9:00 PM - S11.10
Energy Transfer Distance-Dependence in Nanostructured Films Containing Poly(P-Phenylene Vinylene) (PPV) and Acceptor Species.
Bruna Postacchini 1 2 , Valtencir Zucolotto 2 , Fernando Dias 3 , Andy Monkman 3 , Osvaldo de Oliveira 2
1 Physics Departament, UFOP, Ouro Preto Brazil, 2 Polymer Group Bernhard Gross, USP, São Carlos Brazil, 3 Organic Electroactive Materials Research Group, University of Durham, Durham United Kingdom
Show AbstractEnergy transfer processes in the nanometric scale have been widely investigated in molecular-level control of optoelectronic properties in devices, including photovoltaic cells [1,2], light emitting diodes and sensors [3,4]. Conjugated polymers, in particular, are advantageous owing to their tunable electrical and optical properties, allied to easy processability. The first models to deal with resonant energy transfer were proposed by Perrin and Förster [apud 5], who explained excitation transfer mechanisms in terms of dipole interactions between donors and acceptors. The rate of energy transfer (kET) depends on the spectral overlap between the absorption spectrum of the acceptor and the emission spectrum of the donor, in addition to the distance (r) and relative orientation of donors and acceptors. According to the Fermi Golden Rule approximation, kET≈FDFA, with interaction between donor (FD) and acceptor (FA) depending on the geometry.[6] For instance, F≈1/r^3 for single dipoles, F≈1/r for a 2D dipole array and F≈constant for a 3D dipole array. For two single dipoles randomly oriented in solution, the Förster model yields a distance dependence for the energy transfer rate kET≈(1/r^3)(1/r^3)≈1/r^6. For solid films, this point-to-point interaction does not give suitable results and the energy transfer rate depends on the geometry of the structures.[5] The combination of luminescent polymers and suitable energy-accepting materials may lead to a molecular-level control of luminescence in nanostructured films. In this study, the properties of layer-by-layer (LbL) films of poly(p-phenylene vinylene) (PPV) were investigated with steady-state and time-resolved fluorescence spectroscopies, where fluorescence quenching was controlled by interposing inert polyelectrolyte layers between the PPV donor and acceptor layers made with either Congo Red (CR) or nickel tetrasulfonated phthalocyanine (NiTsPc). The dynamics of the excited state (lifetime) of PPV was affected by the energy-accepting layers, thus confirming the presence of resonant energy transfer mechanisms. Owing to the layered structured of both energy donor and acceptor units, energy transfer varied with the distance between layers, r, according to 1/r^n with n = 2 or 3, rather than with 1/r^6 predicted by the Förster theory for interacting point dipoles.[1] Scully, S.R.,et al, M. D. Adv. Mat. 2007, 19, 2961.[2] Liu, Y.-X.; et al, Appl. Phys. 2006, 99, 093521.[3] Parkinson, P., et al, Phys. Rev. B 2007, 75, 165206.[4] Posson, D. J.; et al, Nature 2005, 436, 848.R.E.Ference, Macromol. 39, 3400 (2006) (fonte Arial 10)[5] Kuhn, H. J. Chem. Phys. 1970, 53, 101.[6] Yun, C. S., et al, J. Am. Chem. Soc. 2005, 127, 3115.
9:00 PM - S11.11
Stability of Electrical Properties of Silicon (100) Surfaces Passivated with 9,10-Phenanthrenequinone.
Sushobhan Avasthi 1 3 , Yabing Qi 1 3 , Grigory Vertelov 2 3 , Antoine Kahn 1 3 , Jeffrey Schwartz 2 3 , James Sturm 1 3
1 Department of Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 3 Princeton Institute for the Science and Technology of Materials (PRISM), Princeton University, Princeton, New Jersey, United States, 2 Department of Chemistry, Princeton University, Princeton, New Jersey, United States
Show AbstractThe electrical passivation of the dangling bonds on the silicon surface is critical for organic/Si heterojunction devices, since organic semiconductors usually do not covalently bond to silicon. The residual Si dangling bonds at the surface serve as recombination sites which limit device operation. Such devices are of interest for organic/Si hybrid solar cells and wide bandgap heterojunction contact to silicon [3]. We recently demonstrated excellent silicon (100) surface passivation using an organic semiconductor, 9,10-phenanthrenequinone (PQ) [3], with surface recombination velocities improved over two orders of magnitude compared to unpassivated surfaces. In this abstract, we report that the electrical quality of such PQ-passivated silicon surfaces improves over time, rather than degrading, if the surface is covered with another organic layer. According to the mechanism proposed by Fang et al., PQ bonds to the Si surface dimers through a heteroatomic Diels-Alder reaction [4]. This reaction is slow and we attribute the improvement in passivation to the long time required for the reaction between PQ and Si to reach completion. A test sample, with estimated bulk lifetime of 71 μs, was passivated by PQ. Due to the reduction in surface defects, the effective minority carrier lifetime of the n-type Si wafer increased from 17 μs to 43 μs compared to the bare (native oxide covered) surface. This translates to a reduction in the extracted surface recombination velocity (SRV), which is a direct measure of interface states from ~5x105 cm/s to 250 cm/s. To protect the PQ/Si layer from ambient contaminants while allowing the reaction to reach completion, we encapsulate the wafer with an organic polymer layer; a novolac-resin-based photoresist (AZ5214 E). The encapsulated test sample, shows a further increase in the effective lifetime over 6 days to reach 71 μs. This value is close to the estimated bulk lifetime of the wafer which yields an extracted SRV of less then 10 cm/s, comparable to very high quality Si/SiO2 interfaces.To confirm the high quality of the PQ/Si interface, we also fabricated metal-insulator-semiconductor (MIS) capacitors using the same structure; PQ passivation followed by an AZ5214 insulator and then a gate metal. The structures exhibit classic capacitance-voltage curves, with the expected accumulation and depletion capacitances, showing that surface defects do not pin the Fermi level. Without PQ, the capacitance is pinned and does not vary with voltage. Over time (days), the PQ-passivated devices improve, showing a sharper transition between accumulation and inversion. This further reduction in the interface state density is consistent with the minority carrier lifetime data. [1]F Garnier, J. Opt. A: Pure Appl. Opt., 4, 2002. [2]J. Ackermann, et al., Thin Solid Films, 403-404, 2002.[3]S. Avasthi, et al. to appear in Proc. of 34th IEEE Photovoltaic Specialists Conf., 2009.[4]L. Fang, et al., Surface Science, 514, 2002.
9:00 PM - S11.13
Influence of the Film Thickness on the Optical and Electrical Properties of ITO.
Aleksandra B. Djurisic 1 , Man Kin Fung 1 , Kai Yin Cheung 1
1 Physics, The University of Hong Kong, Hong Kong China
Show AbstractIndium tin oxide (ITO) is widely used for opto-electronic products such as organic light-emitting diodes, organic photovoltaic devices and liquid crystal displays due to its high transparency and electrical conductivity. Since there is a trade-off between the conductivity and transparency of ITO, it is necessary to optimize performances of opto-electronic products by balancing the sheet resistance and transmittance. Both sheet resistance and transmittance are affected by a number of factors such as working temperature, working pressure and oxygen-to-argon ratio during the fabricating process. In our study, ITO thin films were deposited on glass substrates by dc sputtering. Effects of ITO with different thicknesses, sheet resistances, and transmission spectra on the performance of bulk heterojunction photovoltaic devices were investigated. Film thickness was determined by a field emission scanning electron microscopy. Film properties were determined by optical transmittance measurements and sheet resistance measurements using a four-point probe technique.Since the annealed Poly(3-hexylthiophene): [6,6]-phenyl-C61-butyric acid methyl ester (P3HT: PCBM) bulk heterojunction photovoltaic devices have achieved the highest efficiencies of 5 % among all organic photovoltaic devices and different research groups also reported efficiencies in the range of 2.5 – 4.8 % based on different fabricating conditions and treatments, this material combination has been chosen for our study. In this work, the influence of different parameters of ITO on the performance P3HT: PCBM bulk heterojunction photovoltaic devices has been studied by the J-V characteristics measurements in the dark and under simulated solar illumination (AM 1.5, 100 mW/cm2). It has been found that thicker ITO films result in lower sheet resistance but also lower transmittance. The device performance was more significantly affected by changes in the transmittance of ITO compared to the changes in sheet resistance.
9:00 PM - S11.15
Investigation of Transport Properties in Polymer/Fullerene Blends using Transient Electroluminescence Technique.
Hyunkoo Lee 1 , Changhee Lee 1
1 Electrical Engineering and Computer Science, Seoul National Univ., Seoul Korea (the Republic of)
Show AbstractThe hole transport properties of regioregular poly(3-hexylthophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blends with various blend ratio are investigated using transient electroluminescence measurement. For electroluminescence, organic light emitting diodes (OLEDs) are fabricated with P3HT/PCBM blends as a hole transport layer. To reduce absorption, a red phosphorescent emitter with bis(1-phenylisoquinolinato-N,C2’)iridium(acetylacetonate) ((piq)2Ir(acac)) is used. The hole mobility of P3HT/PCBM (1:0.8 by weight) is about 5.4×10-5 cm2/Vs at room temperature and it increases as about 7.3×10-5 cm2/Vs when annealed at 150°C for 30minutes. The hole mobility is field dependent at low field and saturates at high field. The field and temperature dependences of the hole mobility are compared with the result obtained with the time-of-flight photoconductivity measurement. Furthermore, we analyze the result through the Gaussian disorder model.
9:00 PM - S11.16
Synthesis and Characterization of Star-shape Imide Compounds Containing Fluorine and Triphenylamine Moieties.
Guntae Kim 1 , Tae-Ho Yoon 1 , Eun-Ju Jeon 1
1 Materials Science and Engineering Department, Gwangju Institute of Science and Technology (GIST), Gwangju Korea (the Republic of)
Show AbstractNovel star-shape imide compounds, containing electron-withdrawing fluorine moieties as well as electron-donating triphenylamine groups, were synthesized via 3-step process. First, 3,6-dibromo-1,2,4,5-tetramethylbenzene (2B4MB) was oxidized to afford 3,6-dibromopyromellitic acid (2B4BA) which was reacted with aminophenyl diphenylamine) or 3,5-ditrifluoromethylaniline, providing N,N’-bis(triphenylamine)-3,6-dibromo-N,N’-pyromellitimide (TPPMI) or N,N’-bis(3,5-(ditrifluoromethyl)phenyl)-3,6-dibromo-N,N’-pyromellitimide (6FPMI), respectively. Next, the TPPI and 6FPMI were subjected to Suzuki coupling reaction with 3,5-bis(trifluoromethyl)phenylboronic acid and 4-(diphenylamine)phenylborolane, resulting in N,N’-bis(triphenylamine)-3,6-bis(3,5-(ditrifluromethyl)phenyl)-N,N’-pyromellitimide (TP6FPI) and N,N’-bis(3,5-(ditrifluromethyl)phenyl)-3,6-bis(triphenylamine)-N,N’-pyromellitimide (6FTPPI), respectively. In addition, the star-shape imide compounds containing all electron-withdrawing fluorine moieties or electron-donating triphenylamine groups such as 6F6FPI and TPTPPI were also prepared for comparison. The imide compounds were characterized by NMR, FT-IR, melting point analyzer and solubility measurement. Finally, optical and electrical properties were evaluated by fluorescence spectroscopy, UV-Vis spectroscopy and cyclic voltammetry(CV).
9:00 PM - S11.17
Possible Formation of H-type Aggregates in H2TPyP/PMMA Guest Host Films.
Wesley Mota 1 , Wanessa Melo 2 , Luis Nunes 3 , Antonio Eduardo Machado 4 , Alzir Batista 5 , Ilde Guedes 2 , Newton Barbosa 1
1 Instituto de Fisica, Universidade Federal de Uberlandia, Uberlandia - MG, Minas Gerais, Brazil, 2 Departamento de Física, Universidade Federal do Ceará, Fortaleza, Ceará, Brazil, 3 Instituto de Física de São Carlos, Universidade de São Paulo, São Carlos, São Paulo, Brazil, 4 Instituto de Química, Universidade Federal de Uberlândia, Uberlândia, Minas Gerais, Brazil, 5 Departamento de Química, Universidade Federal de São Carlos, São Carlos, São Paulo, Brazil
Show AbstractSince molecular aggregates allow one the possibility of studying and understanding intermolecular interactions, increasing interest in its structure and formation dynamics has been verified in the past few decades. Basically, there are two types of molecular aggregates: H- and J- type. H-type aggregates are formed by the face-to-face stacking of the molecules, while J-type aggregates are formed in an edge-to-edge configuration. The formation of aggregates in water soluble porphyrins has extensively been investigated mainly because porphyrins are present in several fundamental natural processes. Understanding how the aggregate formation modifies the photophysics properties of the monomer is extremely important for applications in drug delivery systems and photodynamic therapy against cancer. Many authors have shown that the porphyrin aggregates formation follows a route where first occurs the protonation of the imino-nitrogens of the ring.[1] Then, the aggregation results from an electrostatic interaction between the positively charged sites in the macrocycle and the negatively charged substituents of peripheral sites. According to this route, the aggregate formation for porphyrins with zero charged aryl groups in solution, is not possible. For instance, Qian et al[2] employed photothermal and fluorescence spectroscopy to verify that no aggregate formation takes place for free-base tetrapyridyl porphyrin (H2TPyP), even for highly concentrated solutions. In this work we have employed UV-Vis and excitation spectroscopy in addition to quantum chemical calculations to investigate the probable formation of H-type aggregates of zero charged aryl substituted porphyrin in a guest- host solid state structure. Films with different porphyrin concentrations were factored by mixing H2TPyP and polymethilmethacrilate polymer chlorophorm solutions. The absorbance spectrum shows that the Soret band shifts upwards and broadens when the porphyrin concentration ranges from 0.08 up to 1.5x1016 cm-3. For concentrations higher than 2x1016 cm-3, we observed from the excitation spectrum the appearance of a band at around 398 nm, which becomes more intense as the concentration increases. Quantum chemical calculations show that the shift results from the distortion of the porphyrin ring probably caused by the increase of porphyrin concentration. The appearance of a band at 398 nm is tentatively assigned to the formation of H-type aggregates, which can be caused by the enhancement of the dipolar interaction due to the distortion of the porphyrin ring. A pathway for the possible formation of H-type aggregate is suggested.[1] J. M. Ribó, J. Crusats, J-A Farrera, M. L. Valero, J. Chem. Soc. Chem. Commun. 6, 681 (1994).[2] D-J Qian, A. Planner, J. Miyake, D. Frackowiak, J. Photochem. Photobio. 144, 93 (2001).Acknowledgement: the authors are grateful to CNPq (INF-MCT), CAPES, FAPEMIG, FAPESP and FUNCAP.
9:00 PM - S11.19
Interfacial Electronic Structure of Si-C and Ge-C Linked Organic Monolayers on Crystalline Semiconductor Surfaces.
Antti Makinen 1 , Gary Kushto 1 , Sang Ho Lee 1
1 Optical Sciences, Naval Research Laboratory, Washington, District of Columbia, United States
Show AbstractSi-C and Ge-C linked monolayers bear potential for a number of applications, including passivation layers in solar cells, high-k gate dielectric materials, organic electronics, and biosensors. In this paper, we report our study of the interfacial electronic structure resulting from grafting organic monolayers directly on undoped Si(111) and Ge(111) substrates. These monolayers include alkanes, attached via hydrosilylation and hydrogermylation of alkenes on Si and Ge surfaces, respectively, and functionalized aryl groups, attached by diazonium activation on Si surfaces. A combination of ultraviolet and x-ray photoelectron spectroscopy (UPS and XPS) measurements indicate that there is a minimum amount of band bending associated with assembling various organic monolayers on the semiconductor substrates. The absence of band bending implies that the semiconductor band structure is not influenced significantly by the dipole moment of individual molecular moieties. This result points to the critical role of the Si-C and Ge-C bond formation in defining the interfacial electronic structure of the passivated Si(111) and Ge(111) surfaces.
9:00 PM - S11.2
Surface Modification of Zinc Oxide Nanorods.
Darick Baker 1 , Christian Weigand 2 , Jamie Adamson 1 , Cary Allen 3 , Matthew Bergren 1 , Dana Olson 4 , Matthew White 5 , Cecile Ladam 6 , David Ginley 4 , Reuben Collins 1 , Thomas Furtak 1
1 Physics, Colorado School of Mines, Golden, Colorado, United States, 2 Electronics and Telecommunications, Norwegian University of Science and Technology, Trondheim Norway, 3 Materials Science and Engineering, University of Arizona, Tucson, Arizona, United States, 4 , National Renewable Energy Laboratory, Golden, Colorado, United States, 5 , University of Colorado, Boulder, Colorado, United States, 6 , SINTEF Materials and Chemistry, Trondheim Norway
Show AbstractZinc oxide (ZnO) nanorod arrays have a number of advantages over both planar ZnO/P3HT and ZnO/P3HT:PCBM blend solar cell devices. The morphology of ZnO nanorod composite solar cells can be tuned through control of the nanorod growth process. In addition, electron mobility in the oxide is larger than it is in the organic phases of a conventional polymer blend solar cell. However, the performance of ZnO nanorod devices still lags that which can be achieved in typical OPV blend devices. In this study, molecular surface modifications of the ZnO have been explored as a strategy for improving both charge transfer and polymer morphology at the ZnO surface. Surface molecular layers were formed on planar ZnO and ZnO nanorod arrays using octadecyltriethoxysilane (OTES), phenyltriethoxysilane (PTES), octadecanethiol (ODT), and thiophenol (TP). FTIR, SEM, UV-Vis, XPS and contact angle data were used to characterize the resulting layers. The effects of these surface treatments on both bilayer planar ZnO/P3HT devices and nanorod ZnO /P3HT cells are reported. Well-characterized monolayers were identified on the ZnO surface in each system. Improved polymer ordering at the ZnO surface and improved intercalation of the polymer into nanorod arrays was also demonstrated. Device performance, however, was not improved by these treatments. Molecular layers with different attachments to the ZnO but the same terminal group showed different behavior, confirming that both the terminal and attachment group play important roles in interface structure, energetics, and charge transfer. This research is aimed at improving organic solar cell performance, yet is applicable to a broad range of hybrid organic/inorganic systems. Support from NSF Awards DMR-0606054 and DMR-0820518 is gratefully acknowledged.
9:00 PM - S11.21
Temperature-dependent of Picoseconds Conformal Relaxation in Polyfluorene Co-polymers.
Thiago Cazati 1 , Fernando Dias 2 , Andy Monkman 2
1 , University of Ouro Preto, Ouro Preto Brazil, 2 University of Durham, Physic Department, Durham United Kingdom
Show AbstractThe conjugated polymers have become the focus of intense research in recent years not only because of their promising applications in organic devices , but also because they can be regarded as a disordered molecular material, where charge and excitons are localized and their transport, described by hopping between localized estates, has implication for the relaxation of the excited states. In conjugated polymer the excited state is in general spatially delocalized and it leads the exciton mobility inside the polymeric chain over a few monomeric units. Furthermore, the exciton is still capable for migrating along different energy polymer sites over several nanometers during its lifetime. The exciton migration process from higher to lower energy sites, which is known as excitation energy transfer (EET), is liable for energetic relaxation in conjugated polymer. Excitation energy transfer takes place in two different time scales: downhill energy transfer normally occur during the first few picoseconds after photoexcitation and on picosecond time scale where isoenergetic EET is observed during hundreds of picoseconds. However, relaxation times could be different for these two modes of energy transfer, which depends on both the excitation wavelength and conformational disorder of the chains. Hence, flexibility of the main chains is an essential condition to elucidate the relaxation process in conjugated polymer. Extremely rigid backbone of main chain, for example in the poly (p-phenylenevinylene), PPV, and poly (phenylene), LPPP, prevents conformational changes after photoexcitation. However, PPV and LPPP derivatives as MeLPPP in solution and MH-PPV films with flexible main chain have shown a significantly relaxation process observed in time-resolved emission spectra. Although incorporation of different monomers units into oligo- or polyfluorene as dibenzothiophene-S,S-dioxide units (S) has provided stability and highly fluorescent with bright blue emission with improved electron affinity in both solution and the solid state, theses units affect conformational changes within the backbone due torsional motion of the monomers unit. In this present work, we show the dependence-temperature of relaxation process for (9,9-dioctylfluorene)-(dibenzothiophene-S,S-dioxide) co-polymer, PFS0.3, with 0.3 molar fraction of dibenzothiophene-S,S-dioxide units and for (9,9-dioctylfluorene)-(dibenzothiophene), PFDBT0.3, co-polymer with 0.3 molar fraction of dibenzothiophene units, both in solid state. Time-resolved fluorescence measurements using Streak Camera have indeed revealed a time-dependent red shift of fluorescence spectrum in accordance with a dispersive relaxation process observed in molecular system that show twisting or planarization of the excited state by intramolecular charge transfer (CT).
9:00 PM - S11.22
Novel Organic Reversible Saturable Absorber Activated by Weak Optical Input.
Shuzo Hirata 1 , Chihaya Adachi 1
1 Center for Future Chemistry, Kyushu Univeristy, Fukuoka Japan
Show AbstractReversible saturable absorber (RSA) is currently playing an important role as materials for optical limiting application such as the protection of eyes and sensitive optical components from intense light pulse. RSA shows strong absorbing characteristics when the population of excited state is larger than that of ground states due to effective storage under photo irradiation is important for threshold of optical input power. Therefore, increasing excited state lifetime at RT under the atmosphere is important for decreasing the threshold of RSA. However, excited lifetime of conventional organic chromophores is within ms order at RT under the atmosphere. Thus, the optical limiting phenomena is observed just when intense light pulse such as YAG laser is irradiated to RSA. We have already found that some aromatic deuterated compounds dispersed into some steroid compounds show excited triplet lifetime more than 1 s at RT under the atmosphere, which is a promising candidate for drastic low threshold RSA. We used the steroid compound doped with metal-complex and deuterated aromatic compound for novel RSA. This material showed effective formation of excited triplet lifetime, long triplet excited lifetime more than 1 s, and 14 times larger transient absorption more than absorption in ground state at 405 nm. Threshold of RSA was 10-2 W cm-2, which is 107 times smaller than excellent RSA such as phothalocyanines and Pt bis(acetylide) complexes. Consequently, this novel RSA realized very high sensitivity to rather weak excitation power by commercial laser pointer and strong LED lights. Further, novel RSA can be useful for compact optical component system.
9:00 PM - S11.23
Interchain Coupling Effects on the Charge Photogeneration Yield and Dynamics of Photoexcited Conjugated Polymers.
Rafael Miranda 1 , Lorenzo Stella 1 , Andrew Horsfield 2 , Andrew Fisher 1
1 Department of Physics and Astronomy, University College London, London United Kingdom, 2 Department of Materials, Imperial College London, London United Kingdom
Show AbstractConjugated polymers have attracted considerable attention in the last few decades because their unique properties make them suitable candidates for a wide range of applications in optoelectronics. In particular, the nature of the states created by photoexcitation is of much current interest, and the mechanism and efficiency of charge carrier photogeneration remain open questions (at least theoretically). In this work, we investigate the formation and dynamical properties of localised photoexcitations in coupled conjugated polymer chains, using a nonadiabatic molecular dynamics method which allows for the coupled evolution of the nuclear degrees of freedom and of multiconfigurational electronic wavefunctions. We discuss how the relaxation of an electron-hole pair, initially localised on a single polymer strand, leads to exciton hopping and to the formation of interchain polaron pairs, and show how the branching ratio of such products critically depends on the interchain coupling strength.
9:00 PM - S11.24
Time Resolved Simulations of Non-Radiative Decay of Excitons in Conjugated Polymers.
Lorenzo Stella 1 2 , Rafael Miranda 1 2 , Andrew Horsfield 3 , Andrew Fisher 1 2
1 , London Centre for Nanotechnology, London United Kingdom, 2 Department of Physics and Astronomy, University College London, London United Kingdom, 3 Department of Materials, Imperial College London, London United Kingdom
Show AbstractConjugated polymers are often characterized by strong coupling between electron motion and ion displacements. This enables non-adiabatic exchange of energy between the two subsystems, and thus provides a non-radiative decay mechanism for excitons. In particular, it is possible to show that neglecting the quantum behavior of ions can lead to qualitative wrong numerical predictions. Indeed, quantum fluctuations of the molecular degrees of freedom can trigger the decay of an excited electronic state through a sort of spontaneous emission (of phonons) process. Here we present a robust and systematically convergent extension of Molecular Dynamics (PolyCEID) for computing the time evolution of excitons in the presence of strong coupling between electronic and ionic degrees of freedom. The abilities of this methodology are demonstrated by using results obtained from simulations of oligomer strands described by the Su-Schrieffer-Heeger Hamiltonian.
9:00 PM - S11.25
Formation and Control of LiF Nanoparticles Islands at the Organic/Electrode Interface in Organic Devices.
Ayse Turak 1 , Felix Maye 1 , Carmen Munera 1 , Esther Barrena 1 2 , Helmut Dosch 1 3
1 , Max Planck Institute for Metals Research, Stuttgart Germany, 2 , Universitaet Stuttgart, Stuttgart Germany, 3 , DESY, Hamburg Germany
Show AbstractAt thicknesses typical for organic devices, electron injection controlling interlayers, such as LiF, are believed to not completely cover the organic surface. Therefore the size and dispersion of the islands on the organic surface are expected to have a significant effect on the device properties. Using atomic force microscopy, Kelvin probe force microscopy, x-ray diffraction and transmission electron microscopy, we have examined the growth and dispersion of LiF on highly ordered organic thin films. On both electron acceptors and donors, we were able to confirm that LiF forms nano-sized (~3nm) single crystalline islands, with a strong (100) texture. This texture is in contrast to previous reports on amorphous surfaces, where the exact nature of the LiF overlayer has generally been difficult to quantify due to the influence of the substrate roughness. The dispersion and organization of the nano-islands can be controlled during growth, ranging from a random distribution to nanochain-like structures segregated at domain boundaries within the organic layer. The control of the LiF morphology has significant implications for the performance of both single layer and bulk heterojunctions diodes.
9:00 PM - S11.26
Investigation of Single Conjugated Polymer Molecules with Sub-Diffraction Limit Fluorescence Microscopy.
Matthew Traub 1 , Joshua Bolinger 1 , Takuji Adachi 1 , Paul Barbara 1
1 Chemistry, University of Texas at Austin, Austin, Texas, United States
Show AbstractThe performance of conjugated polymers in photovoltaic, LED, and transistor devices is strongly dependent on their nanoscale morphology, environment and charge trapping behavior. Over the past 20 years, fluorescence spectroscopy and microscopy have emerged as powerful methods to study these materials. However, conventional optical microscopy cannot directly resolve effects well below the diffraction limit of light, such as the location of trapped charges on single polymer chains. In this work, sub-diffraction limited fluorescence microscopy has been used to interrogate the injection of positive charges into single molecules of poly(2-methoxy-5-(2’-ethylhexyl-oxy)-p-phenylene vinylene (MEH-PPV) in a PMMA host matrix. Emission centroids of single molecules can be determined with precision of better than 1 nm. By applying positive bias to MEH-PPV molecules embedded in capacitive devices, we observed shifts in centroid position between neutral molecules and positively charged, partially quenched molecules. Centroid jumps were repeatable over multiple bias cycles, consistent with trapping of charges at specific sites in the MEH-PPV chain. Implications of these results for the morphology and electronic properties of single conjugated polymer molecules will be discussed.
9:00 PM - S11.27
Novel Cathodic Electrodeposition Methods for Conducting Polymer Films.
Nikhilendra Singh 1 , Yongju Jung 1 , Kyoung-Shin Choi 1
1 Chemistry, Purdue University, West Lafayette, Indiana, United States
Show Abstract Conducting polymers combining the advantages of organic polymers and the electronic properties of semiconductors are attractive materials for use in energy conversion/storage, optoelectronics, coatings, and sensing applications. Conducting polymers have also been used as a matrix to disperse and embed various metal particles. Such metal-polymer hybrid materials are widely used in sensors and electrocatalysts. Conducting polymers have mostly been synthesized via chemical or electrochemical oxidative polymerization. The electrochemical polymerization is achieved by the application of an anodic bias to a conducting substrate immersed in a monomer solution. In this presentation, we report an electrochemical method that allows for the cathodic deposition of conducting polymers for the very first time. The method utilizes the in-situ generation of an oxidizing agent, NO+ ion, capable of initializing polymerization, through electrochemical nitrate reduction at the working electrode surface. Since the cathodic deposition offers a different deposition mechanism than anodic polymerization, new micro- and nano-scale morphologies previously inaccessible by anodic polymerization were stabilized via this method. The detailed deposition mechanism and optimum deposition conditions for various conducting polymers (e.g. polypyrrole, polyaniline and poly(3,4-ethylenedioxythiophene)) as well as spectroscopic detection of NO+ ions produced in the plating solution will be discussed in detail. The cathodic deposition method also offers the possibility of producing metal-polymer hybrid materials in a one-step deposition because both the polymerization and metal reduction can occur under the same cathodic condition. A few examples of metal-conducting polymer composite electrodes prepared via our cathodic deposition method will also be presented. The method described here will significantly broaden our ability to fabricate conducting polymer and conducting polymer-based composite films for use in various applications.
9:00 PM - S11.28
Photoinduced Charge Transfer at Organic/Inorganic Interfaces in FETs.
Josef Spalenka 1 , Peerasak Paoprasert 2 , Ryan Franking 2 , Robert Hamers 2 , Padma Gopalan 3 , Paul Evans 3
1 Materials Science Program, University of Wisconsin, Madison, Wisconsin, United States, 2 Department of Chemistry, University of Wisconsin, Madison, Wisconsin, United States, 3 Department of Materials Science and Engineering, University of Wisconsin, Madison, Wisconsin, United States
Show AbstractFET device-like structures may be employed as probes of hybrid organic-inorganic electronic interfaces because the charge accumulation layer resides within a few molecular layers of the gate dielectric interface and is sensitive to the presence of dipoles and trapped charge. We present bottom contact FET structures fabricated with a layer of ZnO nanoparticles at the interface of pentacene and the gate dielectric. Under illumination, the ZnO nanoparticles trap electrons arising from excitons generated in the pentacene. This process results in a threshold voltage shift of more than 30 V that persists for many minutes after the light has turned off. Functionalizing the surfaces of the ZnO nanoparticles with polythiophenes or long alkane molecules is found to have large effects on the charge transfer process that depend on the chemical structure of the attached molecule. This technique allows us to understand the relationship between the structure at the organic/inorganic interface and the number and rate of charges transferred to the ZnO.
9:00 PM - S11.29
Conductive Polymer (PEDOT:PSS) Contacts for Silicon Micro-rod Array Photoconversion Applications.
Michael Walter 1 , Xueliang Liu 1 , Leslie OLeary 1 , Nathan Lewis 1
1 Chemistry, California Institute of Technology, Pasadena, California, United States
Show AbstractConductive polymeric materials are needed to form flexible, ohmic contacts for p-Si micro-rod photocathodes for use in a dual-rod water splitting membrane. In addition, a conformal, rectifying contact to an n-Si micro-rod array is desired to create a lightweight, Si rod array photovoltaic film. Solution processable poly(3,4-ethylenedioxy thiophene) poly(styrenesulfonate) (PEDOT:PSS) was evaluated with both p-Si and n-Si planar electrodes to determine its contact behavior. Si/conductive polymer film junctions were created using drop-cast or spin-coated suspensions of PEDOT:PSS on chemically etched Si (111) surfaces followed by Au evaporated top contacts. Improved PEDOT:PSS film uniformity was observed when conductive polymer solutions were applied to methylated p-Si and n-Si (111) planar surfaces. PEDOT:PSS/methylated p-Si junctions exhibited ohmic behavior while rectifying contacts were produced with n-Si methyl surfaces. Photovoltaic properties of the junctions prepared using methylated n-Si (111)/PEDOT:PSS under 100 mW/cm2 light illumination produced 15 mA/cm2 with an open-circuit voltage (Voc) between 0.5 - 0.57 V. Utilizing methods developed for planar Si surfaces, conductive polymer contacts to vapor-liquid-solid (VLS) grown, high-aspect ratio n-Si micro-rod arrays were evaluated by drop casting solutions of PEDOT:PSS followed by heating under vacuum. Preliminary data is presented demonstrating a conformal, mildly rectifying conductive polymer junction with n-Si micro-rod arrays.
9:00 PM - S11.3
Modifying Substrate Work Function to Improve ZnO-P3HT Hybrid Photovoltaics.
Robert Davis 1 , Matthew Lloyd 1 , Yun-Ju Lee 1 , Julia Hsu 1
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractDevice performance in hybrid photovoltaics, utilizing inorganic semiconductors such as ZnO as the electron acceptor and polymers such as P3HT as the electron donor, is influenced greatly by the band alignment at the donor-acceptor interface. Hybrid cells based on ZnO/P3HT heterojunctions have the advantage of being fabricated using solution processing but suffer poor photovoltaic performance compared to all-organic cells which use PCBM as the electron acceptor. It has been shown that the work function of the organic materials depends on the substrate work function and can be pinned at the polaron energy level. In order to improve the performance of hybrid ZnO/polymer devices, a greater understanding of electronic properties of ZnO in the hybrid solar cells is needed. In this work, the work function of ZnO sol gel films, as a function of the work function of the underlying substrate, is measured by Kelvin probe in ambient. We argue that the observed dependence of ZnO film work function on the substrate may be described by the integer charge transfer model, a model commonly used to explain interaction between conjugated organics and metal substrates. Use of the integer charge transfer model allows for the estimation of the position of the conduction band of ZnO in ambient, which in conjunction with the optically-measured band gap allows for the estimation of the complete ZnO band alignment. Finally, the relationship between the work function of ZnO, as tuned by influence from the underlying substrate material, is compared to performance of hybrid photovoltaic devices.Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
9:00 PM - S11.30
Efficient Light Emitting Field Effect Transistors Based on Tetracene Thin Films on Organic Dielectric Substrates.
Clara Santato 1 , Simone Bertolazzi 1 , Fabio Cicoira 2 , Charles Brosseau 1
1 , Ecole Polytechnique Montreal, Montreal, Quebec, Canada, 2 , Cornell University, Ithaca, New York, United States
Show AbstractA class of Organic Electronic devices is attracting tremendous attention: Organic Light Emitting Field Effect Transistors. (1) The planar geometry of the device (compared to the vertically stacked geometry of OLEDs), offers direct access to optical probes to image electroluminescence. OLEFETs based on a number of molecular and polymeric semiconductors, processed by thermal evaporation or from solution, and with unipolar or ambipolar electrical characteristics have been realized. At present, a few ambipolar OLEFETs have been demonstrated, where both types of charge carriers, h+ and electrons, e-, are transported. The movement of the location of the light emission region within the transistor channel by varying the gate bias (Vg) between the h+ injecting and the e- injecting electrodes’ bias has been demonstrated for ambipolar OLEFETs. This is beneficial for EL efficiency as excitons are generated far from the metal electrodes, which quench light emission, thus indicating the advantage of the three- versus the two-electrodes configuration of OLEDs.Based on our experience on the fabrication and characterization of Tetracene unipolar OLEFETs, we extended our research activity to the investigation of the possibility to achieve ambipolarity in tetracene films deposited on organic dielectric substrate surfaces. (2) We carefully investigated charge injection, transport, and trapping in Tetracene OLEFET making use of polystirene (PS), polyethylene (PE), and polymethylmetacrylate (PMMA)-passivated SiO2/n-Si substrates surfaces. In these devices, we imaged the light recombination region within the transistor channel.The electrical and optical characteristics of the devices have been paralleled by a series of morphological characterization of the tetracene films by Atomic Force Microscopy to shed light onto the effect of the films' microstructure on their optoelectronic characteristics. F. Cicoira, C. Santato, Adv. Funct. Mater. 17, 3421 (2007).L.-L. Chua et al. Nature, 434, 194 (2005).
9:00 PM - S11.31
Ultraviolet Photoelectron Spectroscopy of Pure Sodium Poly[2-(3-thienyl)-ethoxy-4-butylsulfonate] (PTEBS)and Doped with Perylene Tetracarboxylicdiimide (PTCDI) Nanobelts.
Leon Pinto 1 , Sandra Zivanovic Selmic 1 , Yashdeep Khopkar 1 , Mark Koorie 1 , David Chambers 1 , Orhan Kizilkaya 2 , Yaroslav Losovyj 3 , Hai-Feng Ji 3
1 Institute for Micromanufacturing and Electrical Engineering Program, Louisiana Tech Universtity, Ruston, Louisiana, United States, 2 Center for Advanced Microstructures & Devices, Louisiana State University, Baton Rouge, Louisiana, United States, 3 Department of Chemistry, Drexel University, Philadelphia, Pennsylvania, United States
Show AbstractConjugated polymers comprise of a backbone of sp2-hybridized carbon atoms attached in a linear fashion. These polymers exhibit higher efficiency in optoelectronic devices when doped. The sodium poly[2-(3-thienyl)-ethoxy-4-butylsulfonate] (PTEBS) is an interesting polymer for solar cell application because of the easy tuning of the absorption spectrum by changing the pH value of the polymer solution. PTEBS is a water soluble semiconducting polymer, therefore it has the advantage of being environmental friendly. However, the PTEBS solar cells have shown a relatively low power conversion efficiency. Angle resolved photoemission spectra were acquired using a 3 m toroidal grating monochromator at the Center for Advanced Microstructures and Devices in Baton Rouge, Louisiana. The ultraviolet photoelectron spectroscopy (UPS) data for PTEBS display a shift in the sigma bond energy peaks with respect to the vacuum level when Na atom in PTEBS was substituted with H+ or OH- . The UPS data also show an increase in the density of states near the Fermi level and shifts to lower binding energies of the occupied molecular orbitals with PH level decrease, which is in agreement with the published optical absorption characteristics of PTEBS. We investigated the possibility of doping PTEBS with 2-(3-thienyl)-ethoxy-4-butylsulfonate (PTCDI) nanobelts through UPS measurements. For our experiment, PTEBS was tuned to absorb maximum light in the range of 450nm to 550nm which corresponds to the maximum solar irradiance of the Earth’s atmosphere. Nanobelts of PTCDI were synthesized by gas phase self assembly process. Doping PTEBS with PTCDI nanobelts causes a shift in the Fermi level of the composite material with respect to the vacuum level as observed in the photoemission spectrum. The peaks corresponding to the sigma bonds shift towards the vacuum level with higher concentrations of the dopant. With increased PTCDI doping, PTCDI does not act much like an electron donor, but more like an electron acceptor. Since UPS data confirms that PTCDI nanobelts dope the PTEBS, this composite might be a promising material for optoelectronic application.
9:00 PM - S11.5
A Simple Model to Estimate the Photocurrent and Efficiency in Hybrid BHJ Solar Cells.
Mingdong Wang 1 , Jianbin Xu 1
1 Electronic Engineering, The Chinese University of Hong Kong, Hong Kong China
Show AbstractIn organic solar cells (OPV), the photocurrent is usually smaller than inorganic solar cells such as crystalline Si and GaAs and so on. Firstly, the material band gap, conduction band width, valence band width, and effective density of states, involving into photon absorption process, determine the most important factor of photon absorption coefficient spectra. Secondly, the mobility in organic material is usually low in order 10-7~10-1cm2/v*s and the recombination time is usually in ~10-4 s or lower. So film thickness is trade off between the light absorption and the effective carrier output before recombination. Finally, photons are absorbed, then to be excitons, not directly to form free carriers in OPV. The excitons are dissociated at interface between D/A interface in BHJ solar cells, and the diffusion length of excitons is usually very small (usual 3~10 nm), so the morphology is critical to high efficiency in OPV. Here, a simple method is given to estimate photocurrent in hybrid OPV, considering materials parameters of absorption coefficient, mobility, recombination time, diffusion length, thus to optimize the film thickness and the shape of inorganic nanorods or nanoparticles and theirs density to get higher efficiency. For P3HT and TiO2 or ZnO nanorods system, the possible efficiencies are estimated to be around 5% with diameter of 20 nm, and around 2.5% with diameter of 100 nm, thickness around 200 nm, and under parameter conditions of absorption coefficient 1.5*105 cm-1, mobility 4*10-3 cm2/v*s, recombination time 10-4s. And if considering the possible Voc loss about 0.2V, the PEC will drop down to around 3.5% and 1.7% respectively.
9:00 PM - S11.6
Hybrid Organic/Inorganic Nanocomposites Synthesized by Thermolytic and Photolytic Processes.
Vincenzo Resta 1 , Anna Laera 1 , Monica Schioppa 1 , Emanuela Piscopiello 1 , Leander Tapfer 1
1 , ENEA, Department of Advanced Physical Technologies and New Materials, Brindisi Italy
Show AbstractHybrid organic/inorganic materials containing semiconductor nanocrystals (NCs - e.g., CdS, CdTe quantum dots) are very suitable and promising for the design of new devices such as efficient organic light emitters (OLED) or organic-based solar cells (OPV).A very convenient and versatile route to synthesize semiconductor NCs in a polymeric matrix is based on the in-situ thermolysis or the laser irradiation induced decomposition of unimolecular precursors containing cadmium and sulfur or tellurium. The main advantages in using such unimolecular precursors, containing both the metal and the non-metal part, are: i) the ease in the technological processing; ii) the stoichiometrical control of the thermolytic or photolytic process; and the homogeneous dispersion in the polymer matrix. Here, we present the data and results on the synthesis, by using thermolysis and/or laser irradiation, of polymer nanocomposites made of CdS and CdTe NCs dispersed in conjugated polymers (MEHPPV and P3DOT). We investigated the influence and role of the key process parameters such as annealing temperature and time during the thermolysis process and the laser energy, fluence, and number of shots during the laser irradiation process. In particular, detailed studies of the thermolysis and laser irradiation process of 1-metylimidazole (MI) adducts of Cd-bis(benzyl)thiolates Cd(SBz)2 in MEH-PPV to synthesize CdS NCs is presented. We found that, the Cd(SBz)2(MI)n complex is highly soluble in most of the common organic solvents and can be well dispersed in polymers. On one hand, the semiconductor NCs can be obtained at annealing temperatures below 200C and in relatively short time (~15 min.); on the other hand, they can be produced with fluences smaller than 100mJ/cm2. These results demonstrate that both the devised synthetic strategies are highly suitable for the fabrication of inorganic-organic nanocomposites.The semiconductor NC growth and the nanocomposite formation have been monitored by thermo-gravimetric analysis measurements. The optical, optoelectronic and microstructural properties of the nanocomposite materials have been characterized in detail by absorption and photoluminescence spectroscopy, X-ray diffraction measurements and transmission electron microscopy.
9:00 PM - S11.7
Hybrid P3HT/CdSe Photovoltaic Cells: Effects of Nanocrystal Size and Device Aging.
Jihua Yang 1 , Aiwei Tang 1 2 , Renjia Zhou 1 , Jiangeng Xue 1
1 Materials Sceince & Engineering, University of Florida, Gainesville, Florida, United States, 2 Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing China
Show AbstractHybrid solar cells based on conjugated polymers and colloidal-synthesized inorganic semiconductor nanoparticles have been recognized as an alternative to all-organic solar cells. The inorganic material may complement the absorption of the organic phase and provide better charge transport properties due to the intrinsically higher carrier mobility in inorganic semiconductors, while maintaining the processibility of organic materials. We have studied hybrid solar cells based on the polymer poly(3-hexylthiophene) (P3HT) and CdSe nanocrystals, using the device structure of indium-tin oxide/PEDOT:PSS/P3HT:CdSe/Al. Using CdSe nanospheres with varying size, we have found that the power conversion efficiency of these devices increases monotonically with the CdSe nanocrystal size, from ~0.5% for 4.0 nm nanospheres to ~1.9% for 6.5 nm nanospheres. The efficiency increase with nanocrystal size is mostly due to the increase of the short-circuit current (Jsc), whereas the open-circuit voltage (Voc) and fill factor are mostly unaffected. Note that the highest efficiency achieved here is approximately the same as previous reported results based on CdSe nanorods, which should in principle provide better charge transport along the nanorod length. The devices also exhibit abnormal initial aging behavior when exposed to air, as an increase of both Jsc and Voc during the first 30 min or so leads to an increase in the power conversion efficiency.
9:00 PM - S11.8
Effect of ZnO Nanowire Doping on the Properties of Poly(3-hexylthiophene) Schottky Diodes.
Rachel Aga 1 , Roberto Aga 2 , Richard Mu 2
1 Department of Chemistry, Wright State University, Dayton, Ohio, United States, 2 Department of Physics, Fisk University, Nashville, Tennessee, United States
Show AbstractBlending of inorganic nanomaterials with an organic matrix to form a hybrid nanocomposite is an attractive approach to develop new lightweight optoelectronic materials with unique or improved properties. In this work, poly(3-hexylthiophene) (P3HT) is doped with ZnO nanowires at different P3HT-to-ZnO concentrations and used to build Schottky diodes. ZnO nanowires are prepared via a physical vapor method with Zn as a source. The nanowires are dispersed in chlorobenzene, and P3HT powder is then added. Properties of the nanocomposite diodes are investigated using capacitance-voltage and current-voltage measurements. The possible role of ZnO in the bulk electrical transport as well as in the charge transfer between the metal and semiconductor will be discussed.
9:00 PM - S11.9
Advanced CdSe QDs/SWNTs Polymer System for Solar Cells.
Tingying Zeng 1 , Tzu-Fan Chen 1 , Yan-Fen Li 1 , Jordan Norris 1
1 Laboratory for Nanostructures, Department of Chmeistry, Western Kentucky University, Bowling Green, Kentucky, United States
Show Abstract*Corresponding author‘s current address: Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307, Email:
[email protected] semiconducting polymers offers the high possibility of processing low cost, large area, light weight, and mechanically flexible, organic photovoltaics (OPVs). Buckminster-fullerene C60 and its derivatives as electron acceptors in combination with P3HT or other p-type semiconducting polymers have been dominated as the highest performance nanopattern materials. However, compared with inorganic solar cells, the corresponding single device has only achieved low power conversion efficiency within the range of 4 to 6.5 % based on the reports of current state of the art. The main factor for the low efficiency of OPVs lies in its low intrinsic carrier mobilities, and the photoinduced excitons in the active polymer layer have to dissociate into free charges in order to be transported to the electrodes. A motivation to replace the fullerene family with single-wall carbon nanotubes (SWCNTs) to improve the PV performance was generated. SWCNT has high electron affinity, mechanical strength and Young’s modules, as well as good thermal conductivities. The high metallic property of SWCNTs will lead to a strong electrical field formed to dissociate excitons, which results from the interaction of SWCNTs through its graphitic layer with conjugated polymer according to our early research. The high electrical conductivity increases the polymer thin film conductivity by increasing the charge carrier mobility. All these advantages would significantly improve the power conversion efficiency and mechanical properties for polymer solar cells, In this report, a novel P3HT/single walled carbon nanotubes (SWNTs) donor-acceptor nanohybrid organic photovoltaic system was constructed based on the bulk heterojunction concept. CdSe quantum dots modification to this system has been performed, and the single device photovoltaic (PV) performance was studied. Comparing with that of the pristine P3HT/SWNTs device, the PV performance of the modified hybrid system is significantly improved. A dramatically quenching P3HT fluorescent emission via the nanostructures of QDs on SWNTs was observed, indicating an efficient charge/energy transfer from excited P3HT to SWNTs. CdSe QDs may functional as both a photo-sensitizer and an electron collector as well in this system. This work was partially supported by the U.S. National Science Foundation of contract #0520789, and by the KY NSF EPSCoR REG Program of contract #260501, respectively.
Symposium Organizers
Jiangeng Xue University of Florida
Chihaya Adachi Kyushu University
Russell J. Holmes University of Minnesota
Barry P. Rand IMEC vzw
S12: Electronic Processes and Other Devices
Session Chairs
Hongzeng Chen
Jiangeng Xue
Friday AM, December 04, 2009
Room 210 (Hynes)
9:30 AM - **S12.1
Defects, Doping and Transport in Excitonic Semiconductors.
Brian Gregg 1
1 , NREL, Golden, Colorado, United States
Show AbstractMost organic semiconductors have a surprisingly high charge density, often more than 10 orders of magnitude higher than expected for an intrinsic material. In essence they are doped by their defects. The electrostatic influence of these charges extends over 100 Å or more in these low dielectric materials, thus they have a powerful influence on the transport of both excitons and charge carriers, as well as being responsible for the high conductivity. The same factors that cause exciton formation upon light absorption also cause most charge carriers to remain electrostatically bound near their counterions. The free carrier density is thus just a small fraction of the total charge density. This is also observed in low-defect, purposely doped organic semiconductors in which only 1 free carrier is created per every ~100 added dopants. The Poole-Frenkel (PF) mechanism, although simplistic, accounts naturally for the interaction between a charge bound in a Coulomb well and an applied electric field. Together with a field-dependent mobility (not included in the original PF model), it provides a semiquantitative description of the doping efficiency and charge transport in excitonic semiconductors (XSCs).The sp2-hybridized carbons comprising the backbone of a pi-conjugated polymer would ideally form a planar wire. But in solid films the backbone inevitably has a number of kinks and bends in it that perturb the planarity and optimal bond angle of the sp2 carbons, thus deforming high energy π-bonds and creating defects (states in the bandgap) that may be charged. Chemical impurities in the polymer may also be charged. Conductivity measurements on thin films of π-conjugated polymers such as poly(3-hexylthiophene), P3HT, together with mobility measurements provide an estimate of the free hole density, pf ≈ 10^15 – 10^17 cm^-3, with the lower range approachable only in extensively purified materials. The charged defects from which the free carriers arise almost certainly control much of the observed (photo)electrical behavior of π-conjugated polymers. We introduce chemical treatments that beneficially modify the defects in these polymers resulting in substantially greater carrier mobility, exciton diffusion length and stability against photodegradation. Most transport models do not yet take into account the high concentration of bulk charges in pi-conjugated polymers, but we note that the existence of these charges provides an immediate explanation for the otherwise puzzling Poole-Frenkel mobility of carriers even at low fields as well as the observed correlated energetic disorder.
10:00 AM - S12.2
Relationship Between the Crystalline Order and the Exciton Diffusion Length in Organic Semiconductors.
Richard Lunt 1 2 , Jay Benziger 1 , Stephen Forrest 2
1 Dept. of Chemical Engineering, Princeton University, Princeton, New Jersey, United States, 2 Dept. of Electrical Engineering and Physics, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractThe ability to enhance the exciton migration of organic semiconductors has been of considerable interest in the field of thin-film organic electronics for over two decades. However, there has been little effort to quantitatively connect the exciton diffusion length to changes in crystalline morphology even though other key electrical properties such as mobility have been convincingly linked to crystalline order. In this work, we show that highly-localized singlet exciton diffusion in the archetypal organic material, 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA) increases with increasing crystalline order. Using spectrally resolved photoluminescence quenching (SR-PLQ) we find that the diffusion length increases from 6.5(±1.0)nm in the amorphous limit, to 21.5(±2.5)nm for large grain crystalline films, to 25(±2.5)nm for single crystals. This change in the exciton diffusion length is found to correspond to a concomitant increase in the fluorescence yield that results from a reduction in the non-radiative exciton recombination rate. The variation in the diffusion length is quantitatively described through a combined model of intermolecular Förster-assisted migration and non-radiative quenching at grain boundaries. The application of this observation to other crystalline organic systems, and in particular, photovoltaic cells and OLEDs is crucial for the management of energy transport in excitonic organic electronic device and materials.
10:15 AM - S12.3
Efficient and Stable p-type Dopant for Organic Semiconductors.
Yabing Qi 1 , Tissa Sajoto 2 , Stephen Barlow 2 , Eung-Gun Kim 2 , Leszek Wielunski 3 , Leonard Feldman 3 , Robert Bartynski 3 , Jean-Luc Bredas 2 , Seth Marder 2 , Antoine Kahn 1
1 Department of Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 2 School of Chemistry and Biochemistry and Center for Organic Electronics and Photonics, Georgia Institute of Technology, Atlanta, Georgia, United States, 3 Physics and Astronomy Department, Rutgers University, Piscataway, New Jersey, United States
Show AbstractControlling the electronic properties of organic molecular semiconductors by chemical doping has recently become a very active research topic with numerous applications in organic photovoltaic (OPV) and organic light-emitting diode (OLED) devices. In this study we present experimental and theoretical results on molybdenum tris-[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd)3) as a p-dopant for organic semiconductors. Thin films of pure Mo(tfd)3 were studied by ultra-violet photoemission spectroscopy and inverse photoemission spectroscopy (UPS, IPES) to determine the compound electronic structure. Theoretical support, with excellent agreement with experiments, was provided by simulations based on the DFT calculations. The electron affinity of the organometallic compound, determined via IPES, is 5.6 eV, i.e., 0.4 eV larger than that of the commonly used p-dopant F4-TCNQ. Efficient p-doping of a standard hole transport material (α-NPD) is demonstrated via measurements of Fermi level shifts and orders-of-magnitude enhancement of hole-conductivity in α-NPD thin films doped with 0.5, 1, 2 and 5% Mo(tfd)3. Variable temperature current-voltage measurements reveal that the ohmic conduction in α-NPD:Mo(tfd)3 (2%) can be well described by the polaron trap-and-release model with an activation energy of 0.35 eV. Rutherford backscattering measurements show good stability of the three-dimensional Mo(tfd)3 molecule in the host matrix with respect to diffusion at temperatures up to 110 °C. The use of Mo(tfd)3 to p-dope hole-transport organic semiconductors with HOMO level as deep as 5.5-5.8 eV is therefore a promising method to improve device performance in OPV and OLED applications.
10:30 AM - S12.4
Electronic Processes at Interfaces in Organic Electronic Devices.
Slawomir Braun 1 , Xianjie Liu 1 , Parisa Sehati 1 , William Salaneck 1 , Mats Fahlman 1
1 IFM, Linköping University, Linköping Sweden
Show AbstractOrganic electronic devices as solar cells, light-emitting diodes and field-effect transistors are multi-layered devices where their ultimate performance is to a large extend dominated by the electronic processes at interfaces. The relative position of energy levels across the stack of thin organic layers is of high importance for device engineering and optimization. In spite of a great progress that has been made on this topic a detailed and unified understanding of the electronic processes occurring at these interfaces is not yet achieved. Experimental technique: Ultraviolet Photoelectron Spectroscopy (UPS), has been a valuable tool in the exploration of interfacial properties. Photoelectron Spectroscopy results obtained on various metal-organic and organic-organic hetero-junctions led to the formulation of integer charge transfer (ICT) model. ICT model describes and predicts interfacial interactions in the class of interfaces found in organic electronic devices. The behavior of specific organic hetero-junctions can be predicted by the ICT model using values of the integer charge transfer states (EICT+, EICT-)and electrode work functions as measured by UPS. It will be shown that due to equilibration of the chemical potential throughout the multi-layered organic structure, the relative position between the molecular (polymer) pinning levels (EICT+, EICT-) and the electrode Fermi level plays a key role for determining the energetic alignment. Hence, not only the relative position between charge transfer states of the given materials forming the interface is important, but also their energetic position with respect to the electrode Fermi level. Finally, the latest results on materials relevant for photovoltaic applications (P3HT, PCBM60, PCBM70, etc.) and their impact on device characteristics will be reviewed.
10:45 AM - S12.5
Low-lying Unoccupied States in Molybdenum Trioxide Thin Films: Impact on Hole Injection.
Michael Kroeger 1 , Antoine Kahn 1
1 Department of Electrical Engineering, Princeton University, Princeton, New Jersey, United States
Show AbstractDue to low intrinsic carrier densities in organic molecular and polymeric thin films, efficient charge injection at the contacts is of major concern for the performance of organic devices. Transition metal oxides WO3 and MoO3 are often used as anode buffer layers to enhance the hole injection and have been demonstrated to significantly improve the device performance.[1-3] When a 5 nm thin MoO3 film is inserted at the anode of a single-layer device, we observe an increase of the hole current by several orders of magnitude as compared to a control device. Due to a mistaken but widely shared concept of the transition metal oxides energy levels,[3,4] the origins of this improvement are still somewhat unclear. We examine here the electronic structure of MoO3 deposited on a substrate (here Au) via ultraviolet and inverse photoelectron spectroscopies (UPS, IPES). We find that the oxide ionization energy (IE) and the electron affinity (EA) are 9.7 eV and 6.7 eV, respectively. The Fermi level is pinned close to the oxide conduction band minimum (CBM), resulting in a large work function of 6.86 eV. Electrons are the dominating carrier species in the oxide layer, which has major implications for the application of MO3 and other transition metal oxides in organic devices. To exemplify these implications, we study an archetypal anode interface ITO/ α-NPD and compare it to an ITO / MO3 / α-NPD anode structure in a series of UPS experiments. Derived from these experiments we can draw an energy level alignment. It shows that the transfer of excess electrons from ITO to the lower-lying unoccupied states in the MoO3 leads to an interface dipole of 2.5 eV and only a negligible electron injection barrier between the ITO Fermi level and the MoO3 CBM is present. Another dipole of -2.1 eV is found at the MoO3 / α-NPD interface and is due to the transfer of electrons from the highest occupied molecular orbital (HOMO) of α-NPD (~5.4 eV) to the CB of the oxide. With the outlined energy level alignment, the hole injection mechanism can only be described as a result of electron injection from α-NPD into the conduction band of MoO3 and not as a result of hole transit through the valence band of the oxide layer. [1] T. Matsushima, Y. Kinoshita and H. Murata, Applied Physics Letters 91, (2007).[2] J. Meyer, S. Hamwi, T. Bulow, H. H. Johannes, T. Riedl and W. Kowalsky, Applied Physics Letters 91, (2007).[3] K. J. Reynolds, J. A. Barker, N. C. Greenham, R. H. Friend and G. L. Frey, Journal of Applied Physics 92, (2002).[4] C. W. Chu, S. H. Li, C. W. Chen, V. Shrotriya and Y. Yang, Applied Physics Letters 87, (2005).
11:30 AM - S12.6
Simultaneous Increase in Seebeck Coefficient and Electrical Conductivity in a Semiconducting Polymer.
Howard Katz 1 , Jia Sun 1 , Mingling Yeh 1 , Joseph Feser 2 , Arunava Majumdar 2
1 Materials Science and Engineering, Johns Hopkins University, Baltimore, MD, Maryland, United States, 2 Department of Mechanical Engineering, University of California, Berkeley, California, United States
Show AbstractThermoelectric materials are of interest for a wide range of applications such as deep space energy production, automotive waste-heat recovery, and on-chip cooling modules. The performance of thermoelectric materials is determined by a dimensionless quantity called the figure-of-merit, ZT, proportional to the square of the Seebeck coefficient S. Organic and polymeric semiconductors (OSCs) have been considered as potentially high-ZT materials because of their acknowledged low value of the thermal conductivity κ, 2-3 orders of magnitude below that of inorganic semiconductors and metals. The quantity S in doped OSCs is on the order of tens of μV/K, well below inorganics, though pure (and low-conductivity) organics can have S as high as inorganics. Low values of S in doped OSCs may be due to unfavorable energy and spatial distribution of the density of states relative to the ground state charge carrier energy. In various theoretical models for S, the increase in number of states with respect to energy at or just above the Fermi level is the major determining factor. This work represents the first attempt to create a large gradient in the density of states just above the Fermi level in OSCs. Small quantities of additives that set the carrier energies are introduced to semiconducting polymers in which charge transport would be expected to occur in the bulk of the polymer at higher energy levels, thus contributing a degree of thermal excitation to the transport that would not otherwise occur. Specifically, a mixture of a poly(alkylthiophene) and a poly(alkylthiothiophene) additive, doped with tetrafluorotetracyanoquinodimethane, had S on the order of 500 μV/K, but more importantly, for the first time, displayed a regime in which S and electrical conductivity increased simultaneously as the dopant concentration increased. This unprecedented observation opens a new pathway for optimizing ZT in organic and polymeric semiconductors, in the pursuit of flexible thermoelectric films.
11:45 AM - S12.7
High Efficiency Organic Multilayer Photodetectors based on Singlet Exciton Fission.
Jiye Lee 1 , Priya Jadhav 1 , Marc Baldo 1
1 Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractWe employ an exciton fission process that converts one singlet exciton into two triplet excitons to increase the quantum efficiency of an organic multilayer photodetector beyond 100%. The photodetector incorporates ultrathin alternating donor-acceptor layers of pentacene and C60, respectively. By comparing the quantum efficiency after separate pentacene and C60 photoexcitation we find that singlet exciton fission in pentacene enhances the quantum efficiency by (45±7)%. In quantitative agreement with this result, we also observe that the photocurrent generated from pentacene excitons is decreased by (2.7±0.2)% under an applied magnetic field of H = 0.4T, while the C60 photocurrent is relatively unchanged. These results suggest that singlet exciton fission can be employed to improve the quantum efficiency of various organic photodiodes including photodetectors, photovoltaics and dye-sensitized solar cells.
12:00 PM - S12.8
The Origin of Electro- and Photoluminescence in Strongly Coupled Organic Microcavities.
Grant Lodden 1 , Russell Holmes 1
1 Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractThe confinement of a semiconductor material to an optical microcavity leads to an inherent coupling between light and matter. Depending on the lifetime of the excited state of the semiconductor (the exciton) and the cavity photon, two distinct regimes of interaction are possible. The system is said to be weakly coupled if either the exciton or the cavity photon decay before the two species are able to interact. Weak exciton-photon coupling results in a modification of the exciton lifetime, the spectral shape, and the angular dispersion of emission from the microcavity. Conversely, when the lifetimes of the exciton and cavity photon are long enough so that an interaction occurs prior to either state decaying, the regime of strong exciton-photon coupling is realized. In this case, the cavity photon and exciton reversibly and periodically exchange energy with one another. The timescale for coupling is the Rabi period, which depends on exciton and cavity parameters including the exciton oscillator strength and transition linewidths. The eigenstates for the strongly coupled system are known as microcavity polaritons. Microcavity polaritons have unique properties arising from their mixed exciton-photon character, permitting the realization of novel optoelectronic devices. Organic semiconductors are attractive for application in strongly coupled systems due to their large exciton binding energy (~1 eV), which permits a robust coupled state that is stable at room temperature and under electrical excitation. In addition, organic semiconductors exhibit large exciton oscillator strengths (~1015 cm-2) resulting in a strong interaction between the cavity photon and the exciton. In order to apply organic semiconductor microcavities in novel strongly coupled devices, the mechanism for the formation and luminescence of microcavity polaritons must be well understood. We present results probing these processes, and offer a theoretical model to explain the formation and luminescence of microcavity polaritons in organic microcavities containing tetraphenylporphyrin (TPP). Based on these findings, we propose a novel device architecture aimed at circumventing challenges associated with microcavity polariton excitation in organic microcavities.
12:15 PM - S12.9
Multilayer Barrier Films Comprising Nitrogen Spacers Between Free-standing Barrier Layers.
Jimmy Granstrom 1 2 , Michael Villet 3 , Tirtha Chatterjee 4 , Jeffrey Gerbec 5 , Anshuman Roy 6 , Ji Sun Moon 2 , Griffin Rowell 7 , Evan Jerkunica 7 , Mikael Hedenqvist 8 , Jonathan Yuen 2 , Sung Heum Park 6 , Jae Hyun Lee 6 , Yunhua Xu 9 10 , Shinuk Cho 6 , Craig Hawker 1 4 9 , Guillermo Bazan 1 2 9 , Alan Heeger 1 2 7
1 Materials Sciences, Mitsubishi Chemical Center for Advanced Materials, University of California - Santa Barbara, Santa Barbara, California, United States, 2 Materials Sciences, Center For Polymers And Organic Solids, University of California - Santa Barbara, Santa Barbara, California, United States, 3 Chemical Engineering, University of California - Santa Barbara, Santa Barbara, California, United States, 4 Materials Research Laboratory, University of California - Santa Barbara, Santa Barbara, California, United States, 5 , MC Research and Innovation Center, Goleta, California, United States, 6 Center For Polymers And Organic Solids, University of California - Santa Barbara, Santa Barbara, California, United States, 7 Physics, Center For Polymers And Organic Solids, University of California - Santa Barbara, Santa Barbara, California, United States, 8 Fibre and Polymer Technology, Royal Institute of Technology, Stockholm Sweden, 9 Chemistry, Mitsubishi Chemical Center For Advanced Materials, University of California - Santa Barbara, Santa Barbara, California, United States, 10 Chemistry, Center For Polymers And Organic Solids, University of California - Santa Barbara, Santa Barbara, California, United States
Show AbstractPermeation of gases through polymer-based barriers is often described by the “solution-diffusion” model,[1] in which gas molecules dissolve in the barrier and then diffuse down a concentration gradient through the barrier. The total permeation rate is influenced by both the diffusion and the sorption properties of the permeant in the barrier. The rate of diffusion within the barrier is described by the diffusion coefficient. The permeant concentration at the surface (inside the barrier film) is assumed to be determined by its concentration (pressure) in the surrounding gas phase and its solubility within the film. [1], [2]When a barrier shielding a downstream sink is exposed to an upstream permeant source, there is initially a transient process during which permeant begins to accumulate in the barrier; eventually, a steady-state concentration profile and permeation rate is reached. Limiting permeation during either of these regimes will increase device lifetime. At constant temperature and pressure, sorption and diffusion properties are fixed by the barrier material; use of low-sorption materials, such as the hydrophobic perfluorinated polymer Cytop™, has been shown to significantly improve OLED device lifetime.[3], [4] The barrier thickness also controls permeation rate during both transient and steady regimes; at steady-state, the permeation rate is inversely proportional to the barrier thickness, whereas the time until steady state increases with the square of the thickness. However, increasing barrier thickness necessarily increases encapsulation cost; an alternative encapsulation architecture that increases device lifetime without additional cost is highly desirable.One way to reach this goal is the insertion of contaminant-free (e.g. pure N2) gas-phase spacers between free-standing barrier films in a multilayer structure.[5] The spacers themselves do not exhibit any barrier properties (diffusion of gas permeants in a gas phase is orders of magnitude faster than in a solid), but they delay the attainment of steady state. The spacer also reduces the chemical potential gradient across downstream barrier layers during the transient regime, reducing permeation rate to the device. Furthermore, if sorption is not fully equilibrated and introduces a kinetic barrier to transport, the additional sorption and desorption steps needed for permeant to reach the device may also slow the steady-state permeation rateThe performance of different encapsulation designs, using nitrogen spacers as described above, was evaluated with the calcium thin film optical transmission test. A low-cost polyethylene terephthalate (PET) film increases the calcium lifetime of a Cytop-Kureha structure from 7000 to 12000 min. A single PET film yields only a calcium lifetime of 200 min in a conventional design, i.e. with the same calcium thickness and substrate size, but without nitrogen spacers. [1] J. G. Wijmans, R. W. Baker, Journal of Membrane Science, 1995, 107, 1[2] M. S. Hedenqvist, U. W. Gedde, Packag. Technol. Sci., 1999, 12, 131 [3] J. Granstrom, J. S. Swensen, J. S. Moon, G. Rowell, J. Yuen, A. J. Heeger, Appl. Phys. Lett., 2008, 93, 193304 [4] J. Granstrom, A. Roy, G. Rowell, J. S. Moon, E. Jerkunica, A. J. Heeger, Thin Solid Films, 2009 (under review) [5] J. Granstrom, M. Villet, T. Chatterjee, J. A. Gerbec, E. Jerkunica, A. Roy, Appl. Phys. Lett., 2009, 95, 093306
12:30 PM - S12.10
Plasmon-Enhanced Absorption and Polaron Yields in Polymer Thin Films Using Self-Assembled Silver Nanoprism Monolayers.
Abhishek Kulkarni 1 , Kevin Noone 1 , Keiko Munechika 1 , David Ginger 1
1 Chemistry, University of Washington, Seattle, Washington, United States
Show AbstractPlasmon resonant metal nanoparticles are being explored as optical antennas and scattering centers to improve light harvesting in thin-film organic solar cells to enable the use of thinner active layers leading to improved photocurrents and lower recombination loss. Approaches attempted to date have had limited success because of the use of small (< 20 nm diameter) spherical metal particles that have high optical loss/scattering ratio and plasmon resonances below 450 nm precluding spectral optimization of the device. Here, we report on enhanced absorption in neat semiconducting polymer thin films using optically dense monolayers of large, anisotropic silver nanoprisms (~50-100 nm edge length) that have larger scattering/loss ratios and plasmon resonances that are tunable across the visible spectrum. Up to three times larger polaron yields were measured in polymer:fullerene blend thin films on top of the nanoprism monolayers using photoinduced absorption. We investigated the excitation enhancement by varying the spectral overlap between the nanoparticle plasmon resonance and the polymer absorption and the polymer-nanoparticle separation distance using dielectric spacers. UV-visible extinction, steady-state and time-resolved photoluminescence spectroscopies and absolute thin-film quantum yields were used to correlate the changes in optical properties of the test structures. These results demonstrate that significant, spectrally tunable, excitation enhancement can be achieved in organic polymer films using plasmon-resonant silver nanoprisms and should serve as useful guidelines for the design of next-generation plasmon-enhanced organic photovoltaics.
12:45 PM - S12.11
Energy Transfer Enhancement by Perturbation of Spin-Forbidden Electronic Transitions in Multicomponent System for Sensitized Upconversion.
Angelo Monguzzi 1 , Riccardo Tubino 1 , Francesco Meinardi 1 , Matteo Salamone 1
1 Scienza dei Materiali, Univ. Milano Bicocca, Milano, Milano, Italy
Show AbstractUltra low power photon upconversion based on sensitized triplet-triplet annihilation which has been recently proposed emerges now as a promising wavelength-shifting strategy with potential applications in the field of low cost plastic photovoltaic technology. In these multicomponent systems, the upconverted fluorescence is the result of several intermediate processes: i) absorption of the light by a donor molecule producing singlet excited states, ii) intersystem crossing (ISC) switching the excitation from singlet to triplet states, iii) energy transfer (ET) processes toward metastable triplet states of the acceptor molecule, and iv) TTA giving rise to high-energy singlet excited states of the acceptor moiety from which the upconverted emission takes place. This strategy has proven exceedingly effective when late transition metal based sensitizers such as metallated porphyrins are combined with aromatic hydrocarbon based anthracene derivatives triplet acceptors/annihilators; therefore, the analysis of sensitized upconversion phenomenology is an ideal approach for the study of fundamental photophysical properties an interactions of π-conjugated molecular systems, in particular of lowest metastable triplet states which are usually difficult to deal. The multistep process described has been successfully modellized to investigate its assets and limits in order to estimate possible applications. In particular, the ET transfer step has been found to be well described by Dexter theory approximations for exchange forces driven transfer processes, as expected by considering spin-forbidden electronic transitions involved Anyway, the high ET rate observed in commonly proposed systems seems to be overestimated if only pure Dexter type processes are considered, since in this case the donor/acceptor molecular orbitals overlap is a necessary condition which usually determines short interaction radii. It will be presented how this phenomenon can be strictly related to the presence of molecular oxygen, which exchange coupling with the acceptor moiety perturbs its electronic structure generating in a such a way a finite transition probability for its lowest spin forbidden; consequently, the ET rate increases due the arised dipole-dipole coupling between donor and acceptor singlet-triplet transitions, which evolves the short range Dexter transfer to a combined Dexter/Förster type process.