Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session B20: Focus Session: Polymers and Energy: Photovoltaics, Fuel Cells, Batteries I |
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Sponsoring Units: DPOLY Chair: Bumjoon Kim, Korea Advanced Institute of Science and Technology Room: 321 |
Monday, March 16, 2009 11:15AM - 11:51AM |
B20.00001: Polymers for new battery technologies. Invited Speaker: The chemical and electrochemical reactivity of the components comprising today's lithium batteries has severely limited their lifetime and stability, and attempts to push the limits on energy density have exacerbated these stability issues. The weakest link in terms of safety and stability of Li ion systems is the organic liquid electrolyte that facilitates the Li$^{+}$ ion transport between the electrodes. The electrolyte is flammable and electrochemically unstable against the graphitic anode. It is the continuous electrochemical degradation of the electrolyte at the electrodes that leads to poor cycle life of the batteries, and in some cases runaway reactions that lead to explosions. Dry polymer electrolytes alleviate the electrochemical stability problem by offering a stable electrode-electrolyte interface. The absence of flammable liquids prevents runaway reactions. The main hurdle that has prevented dry polymer electrolytes from being commercialized is low ionic conductivity, and challenges in interfacing with the electrode materials. We demonstrate a novel approach towards addressing these challenges that renders batteries with excellent cycle lives, and thermal stability. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B20.00002: Control of Domain Orientation in Block Copolymer Electrolyte Membranes at the Interface with Humid Air Moon Jeong Park, Suhan Kim, Andrew M. Minor, Nitash P. Balsara Access to ion transporting channels in polymer electrolyte membranes depends crucially on the orientation of hydrophobic and hydrophilic domains at the surface. We demonstrate that domain orientation of polymer electrolyte membranes made from poly(styrenesulfonate-b-methylbutylene) (PSS-PMB) copolymers can be tuned by controlling sulfonation level and moisture content of the air. At low sulfonation levels, highly ordered hydrophobic PMB cylinders oriented perpendicular to the film surface are obtained, when the film is contacted with humid air. Increasing the sulfonation level results in a transition from perpendicular to parallel orientation. Our conclusion is based on three-dimensional characterization of membranes using electron microscopy of samples prepared by the shadow focused ion beam technique, grazing incident small angle x-ray scattering, and electron tomography. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B20.00003: Vertical phase-separation due to differences in surface energies in bulk heterojunction polymer solar cells Sarah Cowan, Anshuman Roy, Ji Sun Moon, Sung Heum Park, Alan Heeger The synthesis and testing of new photoactive polymers is steadily improving the light conversion efficiencies of organic bulk heterojunction solar cells. Understanding the physical interactions between the polymer donor material and the electron acceptor is critical in controlling and optimizing the morphology of the blend. While interactions between the donor and acceptor in the blend determine the scale and stability of lateral phase separation, interactions between the constituents of the blend and the neighboring device layers are equally important. In this work, we demonstrate that bulk heterojunction constituents in a polymer solar cell tend to vertically phase-separate due to differences in surface energies leading to surface-directed spinodal decomposition and/or a wetting layer. Using a combination of cross-sectional transmission electron microscopy (TEM), variable angle spectroscopic ellipsometry (VASE), and a contact angle study, we probe the vertical phase separation in poly(3-hexylthiophene) : [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) and poly[N-9$\prime $-heptadecanyl-2,7-carbazole-alt-5,5-(4$\prime $,7$\prime $-di-2-thienyl-2$\prime $,1$\prime $,3$\prime $-benzothiadiazole)] : [6,6]-phenyl-C71-butyric acid methyl ester (PCDTBT:PC70BM). [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B20.00004: Investigation of the bulk heterojunction structure of organic photovoltaics using neutron reflectivity Jonathan Kiel, Brian Kirby, Michael Mackay Organic photovoltaics have received much attention recently due to their promise of affordable and flexible solar power. A major component of these devices is the bulk heterojunction: an interconnected mixture of an electron donator, a highly conjugated polymer, and an electron acceptor, generally a fullerene derivative. We have performed neutron reflectivity experiments on 200 nm thick films of poly(3-hexylthiophene) and [6,6]-phenyl-C61- butyric acid methyl ester (PCBM) to investigate the structure of this bulk heterojunction. We observe a gradient of PCBM throughout the film that depends on processing conditions, ratio of polymer to PCBM and choice of solvent. These results are compared to working devices to show which bulk heterojunction structures are more suitable to highly efficient solar cells. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B20.00005: Size- and Shape-dependent efficiency of PbSe nanocrystal and nanowire doped organic semiconductor photovoltaics Wenting Li, Christopher Murray, Cherie Kagan Hybrid solar cells based on nanocomposite organic semiconductors and IR sensitive PbSe nanocrystals (NCs) and nanowires (NWs) are fabricated and serve as a model system to test in PV devices. Wet chemical routes are used to synthesize PbSe NCs tunable in size, from 6 to 12nm in diameter, and in shape by tailoring the reaction temperature and selection of surfactants. PbSe NWs are also synthesized through oriented attachment in solution of NC building blocks to form straight, zigzag, helical, and branched NWs. We integrate PbSe NCs and NWs with the organic semiconductors P3HT and pentacene. We are able to fabricate organic-inorganic bulk heterojunctions with pentacene using a solution-processable precursor that is thermally converted to pentacene. We investigate the role of the organic semiconductor pentacene in the solar cell, both as a conductivity booster and as a more stable alternative to P3HT. We find that ligand exchange significantly increases photocurrent by replacing oleic acid ligands used in NC synthesis with shorter pyridine or octylamine ligands. We also report that tailoring the size and shape of the NCs and controlling the deposition and annealing conditions of the nanocomposites enhances the solar cell performance. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B20.00006: Transient photovoltaic behavior of air-stable inverted organic solar cells with solution-processed electron transport layer and high work function top electrode Chang Su Kim, Yueh-Lin (Lynn) Loo In this study, we made air-stable inverted organic solar cells comprising sol-gel derived TiOx as the electron transport layer and Au as the high work function top electrode. The highly transparent TiOx layer, placed between the ITO cathode and the active layer, smooths out ITO and provides better alignment of energy levels for electron transport. The conductivity of TiOx is known to increase with increasing exposure time to light as the excited electrons fill up shallow traps during illumination. The short circuit current of our inverted solar cells thus increases from 1.41mA/cm2 to 8.13mA/cm2 under continuous illumination for 10 minutes. In addition, when our inverted solar cells are stored in air for extended periods of time, the open circuit voltage increases due to oxygen doping of poly(3-hexylthiophene). Exposure to air for 2 days, for example, increases the open circuit voltage from -0.38V to -0.53V. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B20.00007: Controlling Photovoltaic Loss: Recombination of Dissociated Electrons and Holes in Organic Solar Cells Zhihua Xu, Huidong Zang, Bin Hu This presentation reports the studies of charge-transfer complex states formed from the recombination of dissociated electrons and holes at the donor-acceptor interfaces in bulk-heterojunction organic solar cells based on magnetic field effects of photocurrent. Our studies indicate that the formation of charge-transfer complex states is determined by the competition between Coulombic attraction and electrical drifting. Externally, applying electric field can clearly decrease the density of charge-transfer complex states through electrical drifting. Internally, morphology can change the competition between Coulombic attraction and electric drifting through dielectric fields and charge mobilities, and consequently affects the formation of charge-transfer complex states. As a result, changing internal dielectric fields and charge mobilities through internal Coulomb interaction and electrical drifting presents as two mechanisms to control the formation of charge-transfer complex states towards the improvement of photovoltaic efficiencies in organic bulk-heterojunction solar cells. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B20.00008: Photo-induced improvement of Bulk Heterojunction Polymeric Solar Cells Kamil Mielczarek, Alexander Cook, Anvar Zakhidov The effectiveness of BHJ polymeric solar cells depends highly on the formation of continuous three dimensional interconnecting networks of electron donor (typically RR P3HT) and acceptor (typically PCBM) materials. This process is controlled by post-processing heat treatment to induce phase separation of the materials. We demonstrate in this presentation, that in-situ photo-excitation of the BHJ structure during the annealing process controls both the maximal photocurrent and filling factor of the BHJ solar cell. We have found that variations in intensity and spectral composition of the photo excitation affect the resulting morphology of BHJ. The increased diffusivity of constituents and photo-modulation of the carrier recombination upon annealing is discussed as one of the causes of the observed morphology improvement. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B20.00009: High-Vacuum Annealing of Polythiophene:Methanofullerene Bulk Heterojunction Solar Cells Jennifer Segui, Ioana Gearba, Miriam Rafailovich, Charles Black Solar cell device architectures incorporating photoactive layers of immiscible blends of organic semiconductors achieve improved photovoltaic power conversion efficiency compared to planar device geometries. We have fabricated bulk heterojunction solar cells with active layer blends of poly-3 hexylthiophene (P3HT) and the fullerene derivative, [6,6] phenyl C61-butyric acid methyl ester (PCBM). Spin casting the blend from a chlorobenzene solution forms nanometer-scale domains of electron donor and acceptor phases in the device active layer. We solution process the active layers in ambient atmospheric conditions prior to aluminum contact evaporation resulting in inevitable oxygen adsorption in the P3HT bulk and interfaces. We have investigated several device post-fabrication thermal treatments for driving oxygen from the device active layer, including different temperatures, times, and vacuum pressures. We evaluate the efficacy of this technique in improving Al contact quality, film morphology, solar cell efficiency, and reproducibility via analysis of device current-voltage characteristics and tapping mode atomic force microscopy. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B20.00010: Optimizing Ionic Electrolytes for Dye-Sensitized Solar Cells Xiaojuan Fan, Sarah Hall Dye-sensitized solar cells DSSCs provide next generation, low cost, and easy fabrication photovoltaic devices based on organic sensitizing molecules, polymer gel electrolyte, and metal oxide semiconductors. One of the key components is the solvent-free ionic liquid electrolyte that has low volatility and high stability. We report a rapid and low cost method to fabricate ionic polymer electrolyte used in DSSCs. Poly(ethylene oxide) (PEO) is blended with imidazolinium salt without any chemical solvent to form a gel electrolyte. Uniform and crack-free porous TiO$_{2}$ thin films are sensitized by porphrine dye covered by the synthesized gel electrolyte. The fabricated DSSCs are more stable and potentially increase the photo-electricity conversion efficiency. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B20.00011: Improved polythiophene conductivity by thermal crosslinking for solar cell applications I.R. Gearba, C.-Y. Nam, R. Pindak, C.T. Black Organic photovoltaic device power conversion efficiencies are limited in part by low charge mobility within the constituent active layer. For example, the p-type polythiophene polymers used in the highest efficiency organic photovoltaic devices have transverse hole mobilities of only 10$^{-4}$-10$^{-5}$ cm$^{2}$/V-s, despite showing significantly higher values ($\sim $0.1 cm$^{2}$/V-s) in a lateral FET geometry. This mobility anisotropy is caused by poor overlap of $\pi -\pi $ orbitals in the transverse direction, which impedes charge hopping between polymer chains. We have improved the transverse hole conductivity by as much as three times by incorporating the radical initiator di-tert-butyl peroxide into polythiophene thin films. The initiator promotes thermal crosslinking upon annealing at 170C. Crosslinked polythiophene films maintain a similar absorption spectrum to the uncrosslinked material. Grazing incidence X-ray measurements correlate film structural changes to the measured electronic properties, and reveal two possible mechanisms for increased $\pi -\pi $ overlap in crosslinked films. We have increased the power conversion efficiency of planar photovoltaic devices composed of p-type polythiophene and n-type C60 by approx three times (from 0.09{\%} to 0.27{\%}) by crosslinking the polythiophene material. Moreover, crosslinked polythiophene films are rendered insoluble and thus amenable to the further material processing. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B20.00012: Modeling photocurrent transients in organic solar cells Inchan Hwang, Neil Greenham We investigate the transient photocurrents of organic photovoltaic devices by numerical modeling of the drift-diffusion equations. Understanding charge transport in organic solar cells is one of the major interesting issues relevant to improving performance of organic devices. We demonstrate the simulation of the transient photocurrents in a response to a sharp turn-on illumination. Our results show the transient time in photocurrents is determined not only by free charge transport, but also by geminate charge pair dynamics. The dissociation probability of geminate charge pairs is a key parameter in determining the performance of organic devices, controlling the efficiency at low intensity, and also governing the fate of charge pairs formed by bimolecular recombination at high intensity. Bimolecular recombination appears to shorten the typical distance traveled by free charges from where they are generated to the electrode, leading to a reduced turn-on time at high intensity. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B20.00013: Conjugated Polymer Organic Solar Cells made using Low Bandgap Vinylene-linked Benzothiadiazole-thiophene N. C. Heston, J. Mei, S. Vasilyeva, J. R. Reynolds With over 70{\%} of the solar photon flux occurring at wavelengths beyond 700 nm, the broad absorption spectra of low bandgap conjugated polymers offers an additional path towards improving organic photovoltaic efficiencies. Here, we report on polymer solar cells fabricated using a vinylene-linked benzothiadiazole-thiophene polymer and [6,6]-phenyl-C$_{61}$-butyric acid methyl ester (PCBM) blends. We have fabricated cells with various blend film polymer to PCBM ratios as well as film thicknesses and architectures. The performance of these cells was investigated using both AM 1.5 and incident photon to current efficiency measurements. Surface morphologies were characterized using atomic force microscopy. A strong correlation was observed between the percentage of polymer in the blend and the resulting film morphology. We observed photon-generated currents at wavelengths greater than 800nm, though we have not yet obtained high overall power conversion efficiencies. [Preview Abstract] |
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