Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session B41: Focus Session: Polymers for Solar Energy Conversion - Charge Transport in Organic Photovoltaics |
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Sponsoring Units: DPOLY Chair: Arthi Jayaraman, University of Delaware Room: 214A |
Monday, March 2, 2015 11:15AM - 11:27AM |
B41.00001: Mechanism of charge recombination in organic-inorganic hybrid perovskite solar cells Wenchao Yang, Yao Yao, Chang-Qin Wu In the recent popular organic-inorganic hybrid perovskite solar cells, the slowness of the charge recombination processes is found to be a key factor for contributing to their high efficiencies and open circuit voltages, but the underlying mechanism remains unclear. In this work we study the recombination mechanism in perovskite solar cells and its roles on determining the device performance. Based on macroscopic device model simulations, the recombination resistances ($R_{\mathrm{rec}})$ under different applied voltages are calculated to characterize the recombination mechanism, and the current density-voltage ($J-V)$ curves are simulated to describe the device performance under at the same time. Through comparison with the impedance spectroscopy (IS) extracted $R_{\mathrm{rec}}$ data, it is found that bimolecular recombination (BR) is the dominant recombination process in the whole applied voltage regime and can determine the open circuit voltage, while the trap-assisted SRH monomolecular recombination (MR) is only important if the trap density is high or the BR rate is significantly reduced. The different electron injection barriers at the contact can induce different patterns for the $R_{\mathrm{rec}}$-$V$ characteristics. Under the cases of increased band gap or decreased BR rate, the $R_{\mathrm{rec}}$'s are enhanced which leads to high open circuit voltages. [Preview Abstract] |
Monday, March 2, 2015 11:27AM - 11:39AM |
B41.00002: Au Nanocluster assisted PCE improvement in PEDOT: PSS - Si Hybrid Devices Manisha Sharma, Pushpa Raj Pudasaini, Arturo A. Ayon Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), a P-type organic polymer is frequently employed in the fabrication of heterojunction p-n solar cell devices due to its proper HOMO-LUMO band gap as well as its tunable conductivity. In this report we describe the incorporation of gold (Au) nanoclusters in the PEDOT:PSS blend and its influence on the power-conversion-efficiency (PCE) on planar silicon (Si) hybrid heterojunction solar cell devices. Specifically, the reference samples without the aforementioned nanoclusters, were measured to exhibit a 6.10{\%} PCE, value that increased to 7.55{\%} upon the addition of the Au nanoclusters. The observed increase in the PCE is attributed to the enhanced electrical conductivity of the PEDOT:PSS films due to the incorporation of the nanoclusters, which is directly reflected in their improved fill factor. It is further theorized that the presence of Au nanoclusters in the insulating PSS layer in the PEDOT:PSS blend have a positive influence in the charge collection effectiveness of the devices produced. Considering that the Au nanoparticles involved in this research exercise had an average size of only 4 nm, it is considered that plasmonic effects did not play a relevant role in the observed PCE improvement. [Preview Abstract] |
Monday, March 2, 2015 11:39AM - 11:51AM |
B41.00003: Co-Assembling P3HT/ZnO as Parallel-Lane Hybrid Nanowires for Photovoltaic Application Chi-An Dai, Yi-Huan Lee, Yang-Hui Chen, Leeyih Wang The fabrication of organic/inorganic hybrid materials based on conducting polymers and inorganic semiconducting nanoparticles has gathered great attentions recently due to its potential applications in optoelectronics such as solar cells. However, inorganic nanocrystals often suffer from macrophase separation from pi-conjugated polymers following increasing loading concentrations with decreasing interfacial area, leading to reduced efficiency. To counter this problem, we develop a facile in-situ co-assembly method to fabricate highly elongated P3HT nanowires lining along their long fibril-axis with continuous and highly crystalline ZnO nanocrystal pathways. The donor/acceptor (D/A) parallel-lane nanowire hybrid thus formed may act as efficient pathways for charge separation and transport. The optoelectronic property and the solar cell performance of the resulting hybrid will be discussed. [Preview Abstract] |
Monday, March 2, 2015 11:51AM - 12:03PM |
B41.00004: Fulleropyrrolidine interlayers lower cathode work function to raise organic solar cell efficiency Yao Liu, Zachariah Page, Volodimyr Duzhko, Todd Emrick, Thomas Russell A major challenge in organic solar cell design is the trade-off between oxidative stability and work function of the metal used as a cathode. Here we report that solution-based incorporation of fulleropyrrolidines with amine (\textbf{C}$_{\mathrm{\mathbf{60}}}$\textbf{-N}) or zwitterionic (\textbf{C}$_{\mathrm{\mathbf{60}}}$\textbf{-SB}) substituents as cathode-independent buffer layers conveniently surmounts this barrier in single junction polymer solar cells. Specifically, a thin layer of \textbf{C}$_{\mathrm{\mathbf{60}}}$\textbf{-N} reduced the effective work function of Ag, Cu, and Au electrodes to 3.65 eV. Power conversion efficiency (PCE) values exceeding 8.5{\%} were obtained for organic photovoltaics independent of the cathode selection (Al, Ag, Cu or Au). Such high efficiencies did not require precise control over interlayer thickness, as devices prepared with \textbf{C}$_{\mathrm{\mathbf{60}}}$\textbf{-N} and \textbf{C}$_{\mathrm{\mathbf{60}}}$\textbf{-SB} layers ranging from 5 to 55 nm functioned with high efficiency. [Preview Abstract] |
Monday, March 2, 2015 12:03PM - 12:15PM |
B41.00005: Design of Radical Polymers as Transparent Conductors in Organic Photovoltaic Devices Lizbeth Rostro, Si Hui Wong, Lucio Galicia, Bryan W. Boudouris Much of the interest in electronically-active macromolecules has focused on conjugated systems where electron delocalization facilitates charge transport. However, our recent work has demonstrated that radical polymers, an amorphous class of polymers containing stable radical sites pendant on the repeat unit, can efficiently transport charge in the solid state. Furthermore, we have established that a specific radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), can be tuned to have relatively high solid-state electrical conductivity values while remaining highly transparent (due to the lack of backbone conjugation) in the solid-state. As such, the optimized PTMA was incorporated into organic photovoltaic devices as the anodic modifier in inverted geometry devices. Due to PTMA's high transparency and charge transport ability, the fabricated devices demonstrated higher performance than devices fabricated in the absence of an anodic modifier. Specifically, devices with 15 nm of PTMA demonstrated the highest performance. Importantly, these devices retained their high performance stable after prolonged exposure to ambient conditions, and this performance also was demonstrated to be independent of reflective metal ($e.g.,$ gold or silver) deposited on top of the radical polymer interlayer. [Preview Abstract] |
Monday, March 2, 2015 12:15PM - 12:27PM |
B41.00006: The role of exciton ionization processes in bulk heterojunction organic photovoltaic cells Yunlong Zou, Russell Holmes Dissociating photogenerated excitons into their constituent charges is essential for efficient photoconversion in organic semiconductors. Organic photovoltaics cells (OPV) widely adopt a heterojunction architecture where dissociation is facilitated by charge transfer at a donor-acceptor (D-A) interface. Interestingly, recent work on MoO$_{\mathrm{x}}$/C$_{\mathrm{60}}$ Schottky OPVs has demonstrated that excitons in C$_{\mathrm{60}}$ may also undergo bulk-ionization to generate photocurrent, driven by the built-in field at the MoO$_{\mathrm{x}}$/C$_{\mathrm{60}}$ interface. Here, we show that bulk-ionization processes also contribute to the photocurrent in bulk heterojunction (BHJ) OPVs with fullerene-rich compositions. The short-circuit current density (J$_{\mathrm{SC}})$ in a MoO$_{\mathrm{x}}$/C$_{\mathrm{60}}$ Schottky OPVs shows almost no dependence on temperature down to 80 K. This characteristic of bulk-ionization allows the use of temperature-dependent measurements of J$_{\mathrm{SC}}$ to distinguish dissociation by bulk-ionization from charge transfer at a D-A interface. For BHJ OPVs constructed using the D-A pairing of boron subphthalocyanine chloride (SubPc)-C$_{\mathrm{60}}$, bulk-ionization is found to contribute \textgreater 10{\%} of the total photocurrent and \textgreater 30{\%} of the photocurrent from C$_{\mathrm{60}}$. We further find that fullerene-rich SubPc-C$_{\mathrm{60}}$ BHJ OPVs show a larger open-circuit voltage (V$_{\mathrm{OC}})$ than evenly mixed BHJs due to the presence of bulk-ionization. This talk will examine the dependence of J$_{\mathrm{SC}}$ and V$_{\mathrm{OC}}$ on the relative fraction of dissociation by charge transfer and bulk-ionization processes. [Preview Abstract] |
Monday, March 2, 2015 12:27PM - 1:03PM |
B41.00007: Novel solar energy harvesting options based on solution-processable inorganic/organic hybrid materials Invited Speaker: Natalie Stingelin The growing demand for energy and increasing concerns for the effect of the excessive abuse of fossil fuels on the environment force the scientific world to search for alternative, clean and safe energy sources. Finding ways to harvest solar energy is thereby one of the most appealing options. Here, we present a novel approach that exploits the versatile properties of recently developed, photoactive organic/inorganic hybrid fluids based on titanium oxide hydrates and polyalcohols for the production of versatile solar fuels. We will show that such systems can absorb light in the UV-near visible wave-length range. The sunlight's energy is then converted into chemical energy in the form of reduced titanium species, which can be re-oxidised by oxygen when required. Therefore, the absorbed energy is stored as long as oxygen is excluded by the hybrid system. We, furthermore, demonstrate that once discharged, the fluid can be activated again by exposing it to sunlight and recycled -- a property that is important technologically. The same hybrids can also be exploited to produce structures that permit efficient management of light. We will illustrate the potential of this class of materials based on some of our recent approaches to fabricate light-scattering and light in-coupling structures, and discuss future opportunities they open up. [Preview Abstract] |
Monday, March 2, 2015 1:03PM - 1:15PM |
B41.00008: Dark current of organic heterostructure devices with insulating spacer layers Sun Yin, Wanyi Nie, Aditya D. Mohite, Avadh Saxena, Darryl L. Smith, P. Paul Ruden The dark current density at fixed voltage bias in donor/acceptor organic planar heterostructure devices can either increase or decrease when an insulating spacer layer is added between the donor and acceptor layers. The dominant current flow process in these systems involves the formation and subsequent recombination of an interfacial exciplex state. If the exciplex formation rate limits current flow, the insulating interface layer can increase dark current whereas, if the exciplex recombination rate limits current flow, the insulating interface layer decreases dark current. We present a device model to describe this behavior and illustrate it experimentally for various donor/acceptor systems, e.g. P3HT/LiF/C$_{60}$. [Preview Abstract] |
Monday, March 2, 2015 1:15PM - 1:27PM |
B41.00009: Unpinning the Open-Circuit Voltage in Organic Solar Cells through Tuning Ternary Blend Active Layer Morphology Petr Khlyabich, Barry Thompson, Yueh-Lin Loo The use of ternary, as opposed to binary, blends having complementary absorption in active layers of organic bulk heterojunction solar cells is a simple approach to increase overall light absorption. While the open-circuit voltage ($V_{oc})$ of such solar cells have generally been shown to be pinned by the smallest energy level difference between the donor and acceptor constituents, there have been materials systems, that when incorporated into active layers of solar cells, exhibit composition dependent and tunable $V_{oc}$. Herein, we demonstrate that this $V_{oc}$ tunability in ternary blend solar cells is correlated with the morphology of the active layer. Chemical compatibility between the constituents in the blend, as probed by grazing-incidence X-ray diffraction (GIXD) measurements, affords $V_{oc}$ tuning. The constituents need not ``co-crystallize''; limited miscibility between the constituents in the active layers of solar cells affords $V_{oc}$ tunability. Poor physical interactions between the constituent domains within the active layers, on the other hand, result in devices that exhibit an invariant $V_{oc}$ that is pinned by the smallest energy level difference between the donor(s) and the acceptor(s). Our morphological studies thus support the proposed alloying model that was put forth originally. [Preview Abstract] |
Monday, March 2, 2015 1:27PM - 1:39PM |
B41.00010: Synthesis and Structure of Fully Conjugated Block Copolymers Utilized in Organic Photovoltaics Youngmin Lee, Melissa Aplan, Qing Wang, Enrique D. Gomez Fully conjugated block copolymers have the potential to overcome many of the limitations of mixtures and blends as photoactive layers in solar cells; furthermore, they may serve as model systems to study fundamental questions regarding optoelectric properties and charge transfer. However, the synthesis of fully conjugated block copolymers remains a challenging issue in the fieldchallenge. We have optimized the two-step synthesis of P3HT-b-PFTBT, which is composed comprised of Grignard metathesis for polymerization of P3HT followed by chain extension through a Suzuki-Miyaura polycondenstation. We find that the concentration of the Grignard reagent is critical for end-group control such that P3HT is terminated by H at one end and Br at the other. Furthermore, we can utilize an asymmetric feed ratio of monomers for the Suzuki-Miyaura reaction to minimize the amount of uncoupled homopolymers and to control the molecular weight of the second block. We investigated the chemical composition, structure and electrical characteristics of the polymers prepared by the different synthetic methods, and demonstrate that we can utilize these strategies for the synthesis of block copolymers beyond P3HT-b-PFTBT. [Preview Abstract] |
Monday, March 2, 2015 1:39PM - 1:51PM |
B41.00011: Comparison of Reverse Leakage Current Density in Bilayer and Bulk Heterojunction Organic Photodetectors Xin Xu, Ananth Dodabalapur Soft materials such as organic semiconducting polymers and small molecules will allow the development of next generation photodetectors. Their ease of manufacturing and ability to be placed on flexible substrates allow new innovations such flexible camera elements. While organic photodetectors are structurally similar to their solar cell counterparts, their operation under reverse bias is an important difference which leads to differences in optimization. Reverse leakage current within photodetectors are a key metric in their performance. Minimizing these leakage currents is an important research goal for the advancement of organic photodetectors. We have examined a variety of photodetector structures and material systems to study this topic. We will compare two different active layer structures, bilayer and heterojunctions, and their respective optimizations. We have examined how the HOMO-LUMO level alignments in our device structure will impact device performance. We have also examined different material systems such as ZnO/CuPc, ZnO/P3HT, and P3HT/PCBM. Using all of this data, we will present a broad picture on how to improve organic photodetector performance. [Preview Abstract] |
Monday, March 2, 2015 1:51PM - 2:03PM |
B41.00012: Analysis of Charge Carrier Transport in Organic Photovoltaic Active Layers Xu Han, Dimitrios Maroudas We present a systematic analysis of charge carrier transport in organic photovoltaic (OPV) devices based on phenomenological, deterministic charge carrier transport models. The models describe free electron and hole transport, trapping, and detrapping, as well as geminate charge-pair dissociation and geminate and bimolecular recombination, self-consistently with Poisson's equation for the electric field in the active layer. We predict photocurrent evolution in devices with active layers of P3HT, P3HT/PMMA, and P3HT/PS, as well as P3HT/PCBM blends, and photocurrent-voltage (I-V) relations in these devices at steady state. Charge generation propensity, zero-field charge mobilities, and trapping, detrapping, and recombination rate coefficients are determined by fitting the modeling predictions to experimental measurements. We have analyzed effects of the active layer morphology for layers consisting of both pristine drop-cast films and of nanoparticle (NP) assemblies, as well as effects on device performance of insulating NP doping in conducting polymers and of specially designed interlayers placed between an electrode and the active layer. The model predictions provide valuable input toward synthesis of active layers with prescribed morphology that optimize OPV device performance. [Preview Abstract] |
Monday, March 2, 2015 2:03PM - 2:15PM |
B41.00013: Ultrafast Measurement Confirms Charge Generation through Cold Charge Transfer States Bhoj Gautam, Robert Younts, Liang Yan, Evgeny Danilov, Harald Ade, Wei You, Kenan Gundogdu The role of excess energy in generation and extraction of charges through charge transfer (CT) states in polymer solar cells is a subject of debate. There are reports suggesting increase of charge generation yield with excess energy based on ultrafast experiments. On the other hand time delayed collection field measurements shows that excess photon energy has no effect in photovoltaic efficiency. Here we resolved this discrepancy by studying the dynamics of CT excitons and polarons in blends of medium gap copolymers. We found that low-lying charge transfer (CT) excitons can generate charges over a long time period (nanosecond) and contribute photocurrent on the bulk heterojunction devices. By performing resonant CT excitation as well as above gap excitation transient absorption measurements we investigated that the charges are generated more efficiently through low-lying CT states in efficient devices independent of excitation energy. [Preview Abstract] |
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