Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session B22: Focus Session: Organic Electronics and Photonics - Polymer Photovoltaics |
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Sponsoring Units: GERA DPOLY Chair: Enrique Gomez, Pennsylvania State University Room: 407 |
Monday, March 3, 2014 11:15AM - 11:27AM |
B22.00001: Impact of Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation (RIR-MAPLE) on Morphology and Charge Conduction in Conjugated Polymer and Bulk Heterojunction Thin Films Adrienne Stiff-Roberts, Ryan McCormick, Ayomide Atewologun An approach to improve organic photovoltaic efficiency is to increase vertical charge conduction by promoting out-of-plane $\pi $-$\pi $ stacking in conjugated polymers. Resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) features multiple growth parameters that can be varied to achieve a desired organic thin film property. In addition, RIR-MAPLE enables nanoscale domains in blended polymeric films and multi-layer polymeric films regardless of constituent solubility. Thus, RIR-MAPLE deposition is compared to solution-cast films as a possible approach to increase out-of-plane charge transport in polymers and bulk heterojunctions. Two common, solar cell polymers are investigated: P3HT and PCPDTBT. Materials characterization includes grazing-incidence, wide angle x-ray scattering (GIWAXS) for structural information and two techniques to determine hole mobility: organic field effect transistors to measure in-plane mobility and charge extraction by linearly increasing voltage to measure out-of-plane mobility. Initial indications are that the RIR-MAPLE films have a fundamentally different morphology compared to solution-cast films. In the case of P3HT, an enhancement in out-of-plane $\pi $-$\pi $ stacking was observed by GIWAXS in RIR-MAPLE films compared to solution-cast films. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B22.00002: The Role of Aromatic Structural Units of Conjugated Copolymers in Reaching High Solid-State Order and Optoelectronic Performances Chien-Lung Wang, Chain-Shu Hsu, Kuan-Yi Wu, Tien-Hsin Lee Solid-state order of conjugated polymers is determinative in converting molecular properties into useful optoelectronic performances. The rapid development in donor-acceptor conjugated copolymers not only prompted device performances of polymeric optoelectronics, but also created wide varieties of complicate aromatic structural units, whose role in the solid-state order remains under studied. The roles of two widely used axisymmetrical aromatic units- 5,6-difluorobenzo-2,1,3-thiadiazole, and dithienocyclopentacarbazole will be discussed in this presentation. 2-dimensional X-ray diffraction, electron diffraction and theoretical molecular simulation showed that ordered solid-state structures were reached in copolymers with strong interchain interaction and good backbone linearity. The enhanced interchain interaction was supported by higher melting temperature and dis-aggregation temperature in the solution. High mobility of 0.29 cm2/Vs and power conversion efficiency of 6.82\% were reached in copolymers possess ordered solid-state structure with long correlation lengths. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B22.00003: Photovoltaic Cells involving Nonconjugated Conductive Polymer, Iodine-doped cis-Polyisoprene (Natural Rubber) S. Jaju, M. Thakur Photovoltaic cells have been fabricated using titanium dioxide/doped cis-polyisoprene/carbon on ITO glass-substrates. Photocurrents and photo-voltages for different intensities of light (from a white light bulb, emission at 300-700 nm) have been measured. Use of the iodine-doped nonconjugated conductive polymer film (absorption $\sim$ 250 to 700 nm) has led to significant enhancement of photocurrent compared to previous reports which included undoped polymer in a different cell-structure. A maximum photocurrent of about 0.20 mA was observed for a light intensity of $\sim$ 5 mW/cm$^{2}$. The maximum photo-voltage as observed was about 0.70 V for the same light intensity. Natural rubber being inexpensive these cells may provide cheaper alternatives to other reported cell structures. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B22.00004: Engineering Molecular Order for Enhanced Stability of Organic Solar Cells Anne Guilbert, Jenny Nelson, Joao Cabral The microstructure of organic photovoltaics (OPV) is often unstable and very sensitive to processing parameters. It is known that fullerene crystallisation has a huge impact on device behaviour. However, the observation of different size and shape of fullerene crystals in different polymer matrices is not yet understood. We select as a model system amorphous regiorandom poly(3-hexylthiophene-2,5-diyl) (RRa-P3HT) and crystalline phenyl-C61-butyric acid methyl ester (PCBM). We study the nucleation and growth of PCBM crystals into RRa-P3HT matrix as a function of fullerene loading, solution concentration, film thickness, supercooling and surface energy (tuned by UV ozonolysis) using optical and atomic force microscopy, and DSC. We show that the shape of PCBM crystallites can be tuned from needles to spheroidal crystals by increasing undercooling. We argue that the different behaviour of polymer:PCBM blends can be rationalised in term of undercooling, viscosity and fragility of the composite. We finally evaluate the role of additives on the crystallisation of PCBM. By mapping the impact of the processing parameters in this model system, we establish a simple strategy towards controlling PCBM crystallisation at relevant lengthscales for OPV performance. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B22.00005: Influence of Amorphous PCPDTBT on the Morphology of Ternary Blend Solar Cell Based on P3HT/PCPDTBT/PCBM Yu Gu, Cheng Wang, Feng Liu, Jihua Chen, Ondrej Dyck, Gerd Duscher, Thomas Russell To push the efficiency of organic photovoltaic devices to a higher level, ternary blend solar cells were fabricated. Only a few successful ternary systems have been reported, which have higher efficiency than the binary references. One of these is based on P3HT/PCPDTBT/PCBM. We used x-ray scattering methods in combination with transmission electron microscopies to determine the morphology of thin films of this blend. Different morphologies were generated by varying the molecular weight of P3HT, blending ratio and thermal annealing time. It was found that P3HT crystallized under the confinement of PCPDTBT. The bundles of P3HT fibrils, composed of the oriented P3HT crystal blocks, formed a network; and a PCBM-rich phase with amorphous PCPDTBT and P3HT filled in the space between the P3HT fibrils. Such a multi-length-scale morphology produced a parallel device structure. The extended absorption and the photosensitization of PCPDTBT at the interface were attributed to the improved device performance relative to the binary references. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B22.00006: Characterization of solution structure and its importance in thin film ordering of conjugated block copolymers for organic semiconductor devices Michael Brady, Sung-Yu Ku, Justin Cochran, Cheng Wang, Craig Hawker, Edward Kramer, Michael Chabinyc Fully conjugated diblock copolymers (CBCPs) form intriguing materials alternatives to polymer-small molecule blends for their control of mesoscopic order in low-cost organic semiconductor devices. In both bulk heterojunction (BHJ) photovoltaics, consisting of an interpenetrating network with high donor-acceptor interfacial area, and ambipolar transistors, the transport of charge carriers through continuous p- and n-type paths in thin films is a controlling factor in device performance. AFM, GIWAXS, NEXAFS spectroscopy, and RSoXS are used to probe the structure of films of CBCPs with a p-type P3HT block and an n-type DPP block. Thermal annealing in the P3HT melt after casting creates ordered domains with $\sim$ 50 nm in-plane lamellar spacings, as confirmed with GISAXS and RSoXS. GIWAXS diffraction from the (h00) alkyl-stacking and (010) pi-stacking planes shows primarily edge-on orientation for crystals of both P3HT and DPP blocks. In addition, temperature-dependent solution SAXS and UV-Vis spectroscopy are used to probe the size and conformation of casting solution aggregates. Fibrillar DPP aggregates direct the crystallization of P3HT-$b$-DPP following film casting and enable the formation of wormlike domains after annealing and thus ideal morphologies for transport in organic devices. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B22.00007: Influence of copolymer additives on the morphology and performance of bulk heterojunction organic photovoltaics Anton Li, Jojo Amonoo, Bingyuan Huang, Peter Goldberg, Anne McNeil, Peter Green Device performance of polymer:fullerene photovoltaics is intimately connected to their complex bulk heterojunction morphologies. We incorporated varying amounts a fully-conjugated random copolymer, poly((3-hexylthiophene)-$r$-(3-(hexyl(oxylmethyl)thiophene))), into a blend of poly(3-hexylthiophene) and indene-C$_{\mathrm{60}}$ bisadduct, achieving up to a 20{\%} increase in power conversion efficiency. We attribute part of the improved device performance to a decrease in bimolecular recombination, as measured by photo-induced charge extraction by linearly increasing voltage. Superior carrier transport and collection are in turn correlated to the altered structure of the active layer, both internally and near the electrode interface, as revealed by energy-filtered transmission electron microscopy and atomic force microscopy. These findings illustrate the potential for conjugated copolymers to be versatile tools for tailoring the morphology and energetics of organic semiconductor blends for photovoltaic applications. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B22.00008: Correlated molecular orientation in all-polymer solar cells and its role in free charge generation Brian A. Collins, Marcel Schubert, Steffen Roland, Riccardo Di Pietro, Robbert Steyrleuthner, Koen Vandewal, Alberto Salleo, Wolfram Schindler, Konstantinos Fostiropoulos, Zhihua Chen, Antonio Facchetti, Harald Ade, Dean Delongchamp, Dieter Neher New polymers with high electron mobilities have spurred research in organic solar cells using polymer rather than fullerene acceptors due to their potential of increased diversity, stability and scalability. However, all-polymer solar cells have thus far struggled to keep up with their polymer-fullerene counterparts. We examine this issue by investigating the effect of additives on morphology correlated to optoelectronic properties within poly(3-hexylthiphene), P(NDI2OD-T2) blend devices. Resonant X-ray microscopy and scattering along with energy-filtered electron microscopy monitor the evolution of domain size and purity, while X-ray diffraction and a novel polarized X-ray scattering technique reveal a reorientation of the molecules with respect to the donor-acceptor interface, correlating well with device photocurrent. In addition, low efficiency in separating directly excited charge transfer states ties the low device efficiency to geminate recombination. Thus, anisotropic polymer electronic orbitals may necessitate correlated donor-acceptor molecular orientation for efficient charge generation and represent an extra hurtle compared with the isotropic fullerene in realizing efficient devices. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B22.00009: Designing high efficiency organic photovoltaics by controlling the ordering at the donor-acceptor interface Aditya Mohite, Wanyi Nie, Gautam Gupta, Brian Crone, Chenyu Kuo, Hsinhan Tsai, Darryl Smith, Paul Ruden, Feilong Liu, Hsing-lin Wang, Sergei Tretiak The overall power conversion efficiency in an organic solar cell depends on the balance between the rates of exciton dissociation, recombination and separation at the donor acceptor interface. Inability to design, control and engineer these interfaces remains a key bottleneck in their widespread use for the next generation organic electronic devices. Here, we show that we can control the ordering at the P3HT/C60 interface in bilayer device geometry by inserting a monolayer of oligothiophenes, which leads to a complete suppression in the exciplex (or charge transfer state) recombination. We observe that the photocurrent increases by 500{\%}, which in turn results in an increase in the overall power conversion efficiency by an order of magnitude. Moreover, we find that the oligothiophene with an odd number of rings (ter and penta oligothiophene) exhibit a much higher increase in the photocurrent in comparison to the oligothiophene with an even number of rings (tetra oligothiphene). STM measurements reveal that the oligothiophene with odd and even number of rings differ in their ordering respectively, that has a big effect on the overall device performance. We also find that this ordering is highly dependent on the side functional groups in the oligothiophenes. The mechanism of photocurrent generation will be discussed and a simple transport model will be used to explain the change in the charge transfer and recombination rates and predict current-voltage curves. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B22.00010: Origins of Reduced Nongeminate Recombination in P3HT:PCBM Organic Solar Cells Michael Heiber, Julien Gorenflot, Vladimir Dyakonov, Carsten Deibel Understanding the nongeminate recombination processes that are the dominant loss mechanisms in organic solar cells is critical to improving device performance. In P3HT:PCBM blends, nongeminate recombination has been found to be significantly slower than expected from Langevin theory and also exhibits super-second order kinetics. Several theories for this behavior have been proposed, but a complete model has not yet been reached. To shed light on this problem, we have used a combination of transient absorption spectroscopy experiments and kinetic Monte Carlo simulations. By modeling the temperature dependence of the polaron transients measured in both neat P3HT films and annealed P3HT:PCBM blend films, we demonstrate the effects of phase separation, carrier trapping, and charge transfer states on the magnitude of the recombination rate. Furthermore, we show that while neat P3HT films exhibit second order recombination and mobility behavior indicating a Gaussian density of states (DOS), P3HT:PCBM blends are complicated by super-second order recombination that is indicative of an exponential DOS and mobility measurements that are consistent with a Gaussian DOS. To unify these observations, we show that a separate distribution of charge transfer states must be included. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B22.00011: Interpreting impedance spectra of organic photovoltaic cells - Extracting charge transit and recombination rate constants Tyler Mullenbach, Yunlong Zou, Russell Holmes Impedance spectroscopy has been widely used to extract the electron-hole recombination rate constant in organic photovoltaic cells (OPVs). This technique is typically performed on OPVs held at open-circuit. Under these conditions, the analysis is simplified with recombination as the only pathway for the decay of excess charge carriers; transit provides no net change in charge density. In this work we generalize the application and interpretation of impedance spectroscopy for bulk heterojunction OPVs at any operating voltage. This, in conjunction with reverse bias external quantum efficiency measurements, permits the extraction of both recombination and transit rate constants, offering a more complete picture of charge carrier dynamics in the device. Using this approach, both rate constants are determined for OPVs with varying electron donor-acceptor pairings and compositions. It is found that neither rate constant individually is sufficient to characterize the efficiency of charge collection in an OPV. A large recombination rate constant is not, on its own, detrimental if it coincides with a large transit rate constant. The technique presented here permits a detailed understanding of how OPV architecture and processing conditions impact the transient behavior of charge carriers, elucidating the origin of optimum device configurations. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B22.00012: Impedance Spectroscopy and Electroabsorption Studies of PCPDTBT-PCBM Bulk-Heterojunction Solar Cells Christopher Green, Zane Cohick, Marian Tzolov Bulk heterojunction polymer solar cells, consisting of PCPDTBT and PCBM, offer the potential for high light absorption and effective charge collection in sub-micron thick films. An understanding of the behavior of photo-generated charge transfer complexes in the polymer/small molecule blend is needed for further increases in the efficiency of such devices. In this study, we present experimental results on impedance spectroscopy indicating the formation of trapped electrical charges upon illumination. The population of the related energy states depends on the applied voltage. These results are supported by electroabsorption spectroscopy of the solar cells and of the films of the constituting materials. They indicate an additional electroabsorption feature not present in the individual films of the PCPDTBT and PCBM. The voltage dependence of the electroabsorption signal resulted in estimation of the built-in voltage in our solar cells of 1.1 eV. Our comparison of the photocurrent spectrum with the optical absorption spectrum of the solar cells indicate more efficient energy conversion for the light absorbed in the PCPDTBT compared to the light absorbed in PCBM. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B22.00013: Large Perturbation Transient Photovoltage for the Study of Lifetime and Order of Recombination in Organic Photovoltaics Lindsay Elliott, James Basham, Kurt Pernstich, Pragya Shrestha, Lee Richter, Dean DeLongchamp, David Gundlach Organic photovoltaics remain a topic of tremendous research interest and continue to show promise for use in applications requiring flexible and portable energy harvesting solutions. Measurements methods to accurately quantify charge generation and recombination over a large dynamic range are crucial to understanding and improving device operation and performance. We report here on the combination of two measurements: large perturbation transient photovoltage (LTPV) and impedance spectroscopy. These techniques are employed to compare the populations of mobile and trapped charge carriers over a range of open circuit voltages. Charge carrier lifetime and order of recombination are extracted from the combined techniques for the relatively large change in charge carrier density over which the measurements are valid. To demonstrate the utility of our approach we apply these combined measurements to the well-studied polymer-fullerene pair, P3HT:PC61BM. Together, these optoelectronic techniques provide better understanding of the involvement of free-free versus free-trapped recombination and the difference between total population of generated charge and mobile charge carriers. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B22.00014: Device models for bilayer organic solar cells using interface rate equations Non Thongprong, Phillip Duxbury Although the generalized Shockley diode equation is often used to fit the electrical response of organic photovoltaic devices, however developing device models to relate these parameters to atomistic processes is more difficult yet essential to fundamental understanding. A useful device model for organic heterojunctions was developed by Giebink et al. [1], where the heterointerface is treated using a rate equation approach and the electric field in the donor and acceptor regions is assumed to be constant. We have developed models and computational tools combining the bilayer interface model of Giebink et al. with methods to include non-uniform electric fields in the donor and acceptor regions of the material [2]. Injection barriers and trap effects in the donor and acceptor regions are also incorporated in our computational tools. Here the model and computational methods will be briefly outlined and results for the effects of low mobility in the donor or acceptor regions will be summarized. In these models, the series resistance in the generalized Shockley equation is interpreted as a sum of total bulk resistivity of materials and barriers at each layer's contact, while the parallel resistance mainly stems from dissociation efficiency of charge transfer states at the interface of doner and acceptor. \\[4pt] [1] N. C. Giebink, G. P. Wiederrecht, M. R. Wasielewski, S. R. Forrest, Phys. Rev. B (2010, APS).\\[0pt] [2] L. J. A. Koster, E. C. P. Smits, V. D. Mihailetchi, P. W. M. Blom, Phys. Rev. B (2005, APS). [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B22.00015: Characterization of phosphorescent organic light-emitting diodes using current noise cross-correlated spectroscopy Thaddee Kamdem Djidjou, Sergey Li, Andrey Rogachev Carrier injection and transport mechanism in small-molecule phosphorescent organic light-emitting diodes (PhOLED) have been investigated using current noise spectroscopy. The PhOLED devices studied consist of multilayers having the structure ITO / NPB / NPB:Irphq / Balq / Bpen:CsCO$_{3}$/ Al. We found that in high bias regime, the noise spectral density can be described by two terms, 1/ f$^{1.3}$ and 1/f$^{2.8}$. The first term disappears below 2.5 V, as does the luminance; this suggests that this term is related to bimolecular recombination in the devices. The second term is more pronounced al low frequencies and its magnitude is linearly proportional to the current in the device. This term, which exists in all bias range, is likely related to the presence of traps with a distributed time constant. For applied voltages greater than 2.4 V, the frequency-independent noise is dominated by the shot noise. The Fano factor is one in the range 2.4 - 2.5 V, and decreases to a constant value of 0.4 at higher biases. This indicates the presence of a barrier for carrier injection into the device. Our overall results confirm the utility of noise measurements for OLED characterization. [Preview Abstract] |
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