Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session J16: Focus Session: Polymers and Energy: Photovoltaics I |
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Sponsoring Units: DPOLY GERA DMP Chair: Karim Alamgir, The University of Akron Room: B115 |
Tuesday, March 16, 2010 11:15AM - 11:51AM |
J16.00001: Directing the Assembly of Semiconductors for PV Applications Invited Speaker: Segregated structures for photovoltaic devices are currently created by the use of lamination techniques or by the controlled multi-step layer deposition of the semiconductors by vacuum processing techniques. Both these techniques have their limitations. Most organic conjugated materials have low vapor pressure and are not easy to vacuum process. As a result, the deposition rate is not uniform, the thickness of the layers is also not uniform and the packing is not controlled. This results in very low charge mobility and in poor overall device efficiency. Moreover, while several studies have focused on that packing of pristine charge carrier conductors, no strategy exists for the controlling their packing when they are both present in a device. Therefore, there is an imminent need for strategies to assemble hole-conducting and electron-conducting $\pi $-conjugated organic moieties into segregated structures, with appropriate packing for charge mobility. Two questions that arise are (1) how do we obtain segregated structures of electron rich and electron poor semiconductors through self-assembly? and (2) within the segregate stacks, what packing is needed for high mobility of the charges? This talk will focus on our approaches using self-assembly to direct the packing of organic semiconductors for photovoltaic applications. [Preview Abstract] |
Tuesday, March 16, 2010 11:51AM - 12:03PM |
J16.00002: Synthesis and Microstructure of a Fullerene-Terminated Poly(3-hexylthiophene) Bryan W. Boudouris, Francesc Molins, David A. Blank, Marc A. Hillmyer, C. Daniel Frisbie End-functionalized poly(3-hexylthiophene) (P3HT) was synthesized such that both polymer chain ends were terminated with fullerene units to create an internal electron accepting-donating-accepting molecule, methylfulleropyrrolidine-poly(3-hexylthiophene)-methylfulleropyrrolidine (C$_{60}$-P3HT-C$_{60})$. The molecular properties of the polythiophene were characterized to confirm covalent linkage of the fullerene units to the polymer ends. Differential scanning calorimetry (DSC) and x-ray diffraction (XRD) experiments were used to study the microstructure of the polymers, and revealed that microphase separation occurs between the main polymer chain and the fullerene end groups. This suggests the creation of two distinct semicrystalline regimes in C$_{60}$-P3HT-C$_{60}$ that are akin to those seen in a compositionally similar blend of P3HT and C$_{60}$. This comparable domain formation, coupled with the possibility of enhanced charge transfer generally associated with internal electron donating-accepting species, makes C$_{60}$-P3HT-C$_{60}$ a promising candidate for organic electronic applications. [Preview Abstract] |
Tuesday, March 16, 2010 12:03PM - 12:15PM |
J16.00003: Synthesis and Characterization of C60-Porphyrin Derivatives for Enhanced Photovoltaic Performance through Efficient Charge Generation and Transport. Chien-Lung Wang, Ryan M. Van Horn, Wenbin Zhang, David A. Modarelli, Stephen Z. D. Cheng Although organic photovoltaics has been developed for over 30 years, low device efficiency still hinders it from wide use. To efficiently convert solar radiation into electricity, a material must convert solar radiation to charges and charge transport channels are required. C60-porphyrin dyads are highly efficient charge generation media. However, making transport channels by packing such dyads into an ordered structure remains challenging. In this study, a C60-porphyrin dyad (MonoC60-Por) capable of forming hexagonal columnar phase has been synthesized. DSC thermal diagrams show one main phase transition process in MonoC60-Por. Wide-angle X-ray diffraction and electron diffraction results reveal that the phase transition is between isotropic melt and a hexagonal columnar phase. Significant quenching of fluorescence of the porphyrin core in the dyad is observed by steady-state fluorescence measurement, which is indicative of potential high charge generation. The ordered structure formed by MonoC60-Por therefore holds the potential to enhance organic photovoltaic efficiency through efficient charge generation and transport. [Preview Abstract] |
Tuesday, March 16, 2010 12:15PM - 12:27PM |
J16.00004: Electronic structure study of Diketo-Pyrrolo-Pyrrole Polymers for Photovoltaic Applications Simon Levesque, Paolo E. Trevisanutto, Jean Frederic Laprade, Michel Cote Using density-functional theory, we investigate the electronic properties of polymers that contain diketo-pyrrolo-pyrrole (DPP) in the repeating units. We will present our calculated results for the band gap and LUMO energy levels that can be related to the measured optical gap and electron affinity. Some of the polymers studied have LUMO energy similar to the C70-PCBM, or even lower, making them promising candidates for electron transport in organic photovoltaic devices. The homopolymer of DPP is predicted to have a band gap around 1.2 eV and shows a good dispersion of the conduction band, suggesting it should be a good charge carrier.~Finally, we use approximations of the quantum field theory of many-body systems to get more accurate results and obtain the absorption spectrum of some polymers. [Preview Abstract] |
Tuesday, March 16, 2010 12:27PM - 12:39PM |
J16.00005: Design of low band gap polymers for photovoltaic applications: a theoretical study Nicolas Berube, Michel Cote Organic photovoltaic cells received a great interest in the last few years as they offer an environmentally clean and low-cost solution to the world's rising energy needs. Studies show that this technology holds the potential to reach a power conversion efficiency of 10 {\%}, which would hold on as a competitor to actual inorganic devices based on silicon. Experimental results of solar cells using PCPDTBT (pentadithiophene-benzothiadiazole) or DPP (diketo-pyrrolo-pyrrole) as the electron donor and PCBM as the acceptor already show power conversion efficiencies of over 5 {\%}. Those results are limited mostly by a misalignment of the HOMO (Highest Occupied Molecular Orbital) and the LUMO (Lowest Unoccupied Molecular Orbital) energy level of the different components of the solar cell. In this presentation, we will discuss how different modifications on those polymers can affect and improve their electronic properties. Our calculations based on density-functional theory using the B3LYP functional on a DPP-based copolymer with a ITN (isothianaphtene) unit, indicate a bandgap as low as 1.2 eV and a HOMO level comparable to P3HT, making a promising candidate for photovoltaic applications. [Preview Abstract] |
Tuesday, March 16, 2010 12:39PM - 12:51PM |
J16.00006: Photovoltaic Response of an Adaptive Mix-conducting \textit{p-n} Junction Fuding Lin, Ethan Walker, Mark Lonergan Most photovoltaic junctions are based on materials that contain only electronic charge carriers, and they rely on a built-in electronic asymmetry to separate photo-generated carriers. In this work we study the photovoltaic response and the photo-induced NIR absortion of a mix-conducting \textit{p-n} junction constructed from two polyacetylene ionomers with symmetric Au electrodes. The two ionomers share the same semiconducting backbone but are dissimilarly functionalized so that all anions are fixed in the anionic ionomer, and all cations are fixed in the cationic ionomer. Based on our experimental observations, we propose that the response of the junction can be explained by an adaptive photochemical doping process enabled by the built-in ionic asymmetry. According to this model, the simultaneous equilibration of ionic and electronic charge carriers across the junction upon contact leads to spontaneous \textit{p}-doping of the anionic ionomer and \textit{n}-doping of the cationic ionomer in the dark. When the junction is illuminated, the doping levels of both ionomers change adaptively with light intensity as a result of interaction between ionic and electronic carriers. This adaptive photochemical doping process is a unique property of mix-conducting junctions with built-in ionic asymmetry. [Preview Abstract] |
Tuesday, March 16, 2010 12:51PM - 1:03PM |
J16.00007: Nanoparticle distribution in polymer solar cells B.J. Kirby, J.W. Kiel, B.B. Maranville, C.F. Majkrzak, M.E. Mackay Polymer based solar cells (PSC) hold the promise of cheap, versatile devices for harnessing solar energy. A widely studied PSC is poly-3-hexylthiophene (P3HT) blended with [6,6] - phenyl-C61 - butyric acid methyl ester (PCBM) nanoparticles. The acceptor PCBM is needed to inhibit exciton recombination, thus, proper PCBM distribution is critical for photovoltaic performance. However, determining this distribution is challenging, as PCBM is extremely difficult to distinguish from P3HT via standard techniques like microscopy or x-ray diffraction. Neutron scattering presents a solution, as the scattering potential for PCBM is $\sim $5 x that of P3HT. Thus, we have studied PCBM:P3HT thin film samples using neutron reflectometry, which is sensitive to the compositional depth profile.[1] Measurements were conducted both with a weak scatterer (air) and then with a strong scatterer (D2O) backing the sample, such that the depth profile could be calculated from the reflectometry data with no fitting parameters, and/or model fitted with virtually no ambiguity. We find that PCBM aggregates near the substrate and surface interfaces of the P3HT film, implying that the PCBM is not optimally distributed for best photovoltaic performance. In general, this work demonstrates the extreme utility of neutron reflectometry for studying this class of materials. [1] Kiel, et al. Soft Matter, DOI:10.1039/B920979D (2009). [Preview Abstract] |
Tuesday, March 16, 2010 1:03PM - 1:15PM |
J16.00008: The effect of chain architecture and additive processing on the performance of BHJ solar cells James T. Rogers, Kristin Schmidt, Jeff Peet, Robert Coffin, Guillermo C. Bazan, Edward J. Kramer The bulk heterojunction morphology (BHJ) of certain polymer-fullerene blends has enabled the development of solution processable plastic solar cells which promise to make solar energy a more economically viable renewable energy source. To study the morphological changes induced by additive processing of these devices, a novel class of donor-acceptor type polymers has been developed whose performance is greatly improved upon the incorporation of an additive. Synthetic control over three highly analogous polymers has enabled a systematic and quantifiable comparison of the influence of several important polymer structural perturbations on the morphology of additive processed films. The distribution of components and interfacial structure of these additive processed devices was investigated using resonant soft x-ray reflectivity (RSoXR) and near edge x-ray absorption fine structure (NEXAFS) techniques. These techniques reveal how the changes in chain architecture affect chain orientation relative to each interface and how additives influence the depth distribution of components. [Preview Abstract] |
Tuesday, March 16, 2010 1:15PM - 1:27PM |
J16.00009: High efficiency morphologies for bulk heterojunction solar cells Kitran Colwell, Lucas Wagner, Jeffrey Grossman Inexpensive high efficiency solar cells represent a potentially transformative technology in energy production. One possible route to these devices is to construct them of two inexpensive materials in a type II heterojunction. The mixing morphology becomes important due to a bound exciton that can only separate at an interface and the need for good charge carrier transport. The optimum morphology is determined by the competition between these two loss mechanisms, and is currently unknown for the general problem. We use a grid-based kinetic Monte Carlo model to explore the morphology space in these devices. We find that it is possible to attain a high efficiency without a large degree of ordering, which may be a route towards manufacturing higher efficiency bulk heterojunctions inexpensively. [Preview Abstract] |
Tuesday, March 16, 2010 1:27PM - 1:39PM |
J16.00010: Hierarchical Mixing of PCBM:P3HT Bulk Heterojunctions as Determined by Neutron Scattering: Implications for Organic Photovoltaics Mark Dadmun, Nathan Henry, Wen Yin, Kai Xiao The current model for the ideal morphology of a conjugated polymer bulk heterojunction organic photovoltaic (OPV) is a phase-separated structure that consists of two pure phases, one an electron donor, the other an acceptor, that form an interpenetrating, bicontinuous, percolating network on the length scale of 10-20 nm. In this work, we use neutron scattering to demonstrate that the two phases in one of the most common conjugated polymer bulk heterojunctions, poly[3-hexylthiophene] (P3HT) and surface-functionalized fullerene 1-(3-methyloxycarbonyl)propy(1-phenyl [6,6]) C$_{61}$ (PCBM), are far from pure. The implications of this finding on the ideal morphology of conjugated polymer bulk heterojunctions will be discussed. This hierarchical mixing picture provides a favorable amount of surface area for exciton dissociation, and if properly designed, sufficient pathways for charge transport, and presents a new paradigm in the definition of an ideal bulk heterojunction material. [Preview Abstract] |
Tuesday, March 16, 2010 1:39PM - 1:51PM |
J16.00011: Oligothiophene/fullerene interfaces: Molecular Simulations of Organic Photovoltaic Materials Sridhar Yerusu, Vikram Kuppa Atomistically detailed molecular dynamics simulations are used to study the interface between quaterthiophene and fullerene, prototypical molecules in organic bulk heterojunction solar cells. The simulation mimics the morphology that provides optimized generation of free charges from photogenerated excitons in such assemblies, as studied by experiment. The effect of temperature and the presence of a rigid fullerene phase on the behavior of the conjugated polymers are explored. Order parameters of chain orientation demonstrate that significant fractions of the oligothiophene molecules align along directions of the form $<$110$>$, parallel and adjacent to the C60 substrate. It is seen that the surface influences the stacking of thiophene rings, which is crucial to facilitating effective charge transport in organic semiconductors, due to the resulting overlap of $\pi $-electron clouds. The distribution of ring orientations reveals the relative stacking of rings as a function of the distance from the fullerene phase. The simulations also investigate the dynamics of backbone dihedral libration in the oligothiophenes. Our goal in performing these simulations is to provide fundamental insights into the mechanisms that govern the overall efficiency of polymeric optoelectronic devices. [Preview Abstract] |
Tuesday, March 16, 2010 1:51PM - 2:03PM |
J16.00012: XPS investigations of P3HT/Al interface for the organic bulk-heterojunction solar cells Bakhtyar Ali, Brian Reindl, Conan Weiland, Robert Opila, Syed Shah Solution-processed organic solar cells (OSCs) have gained significant attention due to the increasing demand for the low cost power production. However, the efficiencies of these devices are still low for commercialization. One of the efficiency limiting parameters is believed to be the poor polymer/metal electrode contact interface. In our paper we discuss the X-ray Photoelectron Spectroscopy (XPS) results obtained for the poly(3-hexylthiophene) (P3HT) and Aluminum (Al) interface. Al was deposited in-situ by thermal evaporation at 10 {\AA}/min. The study of high resolution C1s XPS peaks (binding energy$\sim $184.6 eV), S 2p (B.E$\sim $ 164 eV) and Al 2p (BE$\sim $ 79 eV) reveals the presence of Al-S bonds as well as the existence of Al-C bonds with Al evaporation. This shows the altered chemistry at the polymer/metal interface which limits the power conversion efficiency of the solar cells. The use of LiF prior to evaporation of Al results in improved performance of the devices. Our preliminary results demonstrate that Al preferentially deposit on LiF and thus providing a better contact for the carriers to be collected at the cathode. [Preview Abstract] |
Tuesday, March 16, 2010 2:03PM - 2:15PM |
J16.00013: Ultrafast Spectroscopy of Charge Generation in Polythiophene/Fullerene Blends for Organic Photovoltaic Applications Sanjeev Singh, Josh Holt, Bill Pandit, Zeev Vardeny We present a detailed spectroscopy study of ultrafast photogeneration of charges in polymer/fullerene blends, such as regio-regular polythiophene (RR-P3HT), regio-random polythiophene (RRa-P3HT) and fullerene derivatives such as C$_{61}$ (PCBM), using the transient pump-probe photomodulation (PM) spectroscopy with $\sim $ 100 fs resolution. These blends serve as active layers in organic photovoltaic devices with high power conversion quantum yield (up to 6 {\%}), due to a photoinduced charge transfer (PCT) reaction between the polymer and the fullerene molecules. Our transient PM spectrum spans a broad energy range from 0.1-2.4 eV, and this allows us to monitor the transient behavior of the various photoinduced absorption (PA) bands of polarons and excitons; as well as photobleaching (PB) of the ground state. In order to understand the nano-morphology of these active layers, which also affects their photophysics, TEM images are also presented. Various processes such as PCT, geminate recombination, and energy transfer from the polymer to the fullerene phase are unraveled. [Preview Abstract] |
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