Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session K33: Polymers for Solar Energy ConversionFocus Industry
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Sponsoring Units: DPOLY FIAP Chair: Miriam Rafailovich, State University of New York, Stony Brook Room: 336 |
Wednesday, March 16, 2016 8:00AM - 8:36AM |
K33.00001: Dynamic Covalent Functionalization as a route to Controlling Self Assembly of Organic Molecules Invited Speaker: Emily Pentzer Efforts to optimize the optoelectronic properties of conjugated organic materials are ongoing across many fields of science and engineering. For example, in bulk heterojunction polymer solar cells, researchers seek to optimize absorption of the solar spectrum by the active materials, form interpenetrating domains of p-type and n-type materials to facilitate exciton dissociation, and improve interactions between electrode, charge blocking layers, and active layers to ensure rapid charge transport. One advantage of organic polymers compared to inorganic materials (e.g., silicon), is the low cost and ability process the materials in solution. Moreover, assembly of conjugated organic materials in solution or in the solid state (i.e., films) can be used to optimize both a material's optoelectronic properties and its interface with surfaces and other materials, addressing many of the concerns listed above. Unfortunately, such solution processability requires appendage of insulating alkyl chains to the conjugated frameworks, which don solubility, but are also insulating and thus can hurt device performance. This presentation will report recent results from the Pentzer Lab from Case Western Reserve University on using functional alkyl chains that serve to control self-assembly, control interfaces with other materials, or can be removed by an external stimulus as a route to optimizing the materials for solar cell applications. [Preview Abstract] |
Wednesday, March 16, 2016 8:36AM - 8:48AM |
K33.00002: Control of Crystallization to Promote Microphase Separation in Fully Conjugated Block Copolymers Youngmin Lee, Thinh P. Le, Zach Seibers, S. Michael Kilbey, II, Qing Wang, Enrique D. Gomez Donor$-$acceptor fully conjugated block copolymers, where donor and acceptor conjugated polymers are covalently bonded together, are interesting as single-component active-layer materials for photovoltaics because it can adopt mesoscale microphase separated structures with length scales comparable to the exciton diffusion length. Nevertheless, due to the strong crystallization of poly(3-hexylthiophene-2,5-diyl) (P3HT), morphologies of fully conjugated block copolymers containing P3HT are predominantly driven by crystallization as opposed to microphase separation. We control the crystallization in block copolymers to promote microphase separation in fully conjugated block copolymers through the addition of small amounts of 3-octylthiophene to the polymerization of P3HT. Poly(3-hexylthiophene-2,5-diyl-$r$-3-octylthiophene-2,5-diyl)-\textit{block}-poly((9,9-dioctylfluorene-2,7-diyl)-\textit{alt}-(4,7-di(thiophene-2-yl)-2,1,3-benzothiadiazole)-5',5''-diyl) (P3HT-$b$-PFTBT) copolymers were prepared by Grignard metathesis for the alkylthiophene block followed by chain extension through a Suzuki-Miyaura polycondensation. We compare the crystallization, self-assembly and performance in devices of P3HT-b-PFTBT with a few mole percent of 3-octylthiophene in the P3HT block. [Preview Abstract] |
Wednesday, March 16, 2016 8:48AM - 9:00AM |
K33.00003: Spectral analysis of resonant scattering to quantify phase behavior in organic blends Thomas Ferron, Jon Downing, Dean DeLongchamp, Brian Collins It is important in organic solar cells to understand how phase behavior and possible non-equilibrium states of device morphology relate to functionality. However, characterization of carbon based electronics can be a challenging endeavor due to low levels of crystallinity, anisotropic molecular structure leading to complex three dimensional morphologies, and low contrast that hamper traditional techniques such as electron microscopy or hard X-ray scattering. Recently, studies utilizing resonant x-rays have shown sensitivity to phase behavior but a quantitative characterization is vital to correlate device performance. Here we demonstrate a multi-domain analysis of spectrally resolved scattering using molecular optical constants to accurately calculate absolute domain volume fraction and composition of a model block co-polymer. Our methods address complex absorption and fluorescence that occur at the carbon edge in order to move current models beyond the Born approximation. Techniques developed have been applied to P3HT -- PCBM solar cells to draw conclusions of device morphology. [Preview Abstract] |
Wednesday, March 16, 2016 9:00AM - 9:12AM |
K33.00004: Ternary blend polymer solar cells with self-assembled structure for enhancing power conversion efficiency Zhenhua Yang, Hongfei Li, Chang-Yong Nam, Kim Kisslinger, Sushil Satija, Miriam Rafailovich Bulk heterojunction (BHJ) polymer solar cells are an area of intense interest due to their advantages such as mechanical flexibility. The active layer is typically spin coated from the solution of polythiophene derivatives (donor) and fullerenes (acceptor) and interconnected domains are formed because of phase separation. However, the power conversion efficiency (PCE) of BHJ solar cell is restricted by the disordered inner structures in the active layer, donor or acceptor domains isolated from electrodes. Here we report a self-assembled columnar structure formed by phase separation between (PCDTBT) and polystyrene (PS) for the active layer morphology optimization. The BHJ solar cell device based on this structure is promising for exhibiting higher performance due to the shorter carrier transportation pathway and larger interfacial area between donor and acceptor. The surface morphology is investigated with atomic force microscopy (AFM) and the columnar structure is studied by investigation of cross-section of the blend thin film of PCDTBT and PS under the transmission electron microscopy (TEM). The different morphological structures formed via phase segregation are correlated with the performance of the BHJ solar cells. [Preview Abstract] |
Wednesday, March 16, 2016 9:12AM - 9:48AM |
K33.00005: Enhancing efficiency in polymer-blend solar cells: Structural insights through scattering Invited Speaker: Vikram Kuppa All-polymer solar cells that employ blends of semiconducting polymers are capable of harnessing a greater portion of the incident solar spectrum than singly sensitized devices. However, they invariably show poor performance when compared with small-molecule bulk heterojunction cells. Following our successful approach in adding very small quantities of pristine graphene to the active layer to boost performance in P3HT/PCBM cells, we have recently reported a three-fold enhancement in efficiency of all-polymer (a blend of P3HT and F8BT) photovoltaic devices. These new cells exhibit more balanced transport of electrons and holes, strong dependence of recombination behavior on graphene content, and up to two orders of magnitude increase in mobility, resulting in a peak improvement of over 200% in the short-circuit current. However, active layer morphology must also be considered in designing high performance organic photovoltaic devices: structures deviating from the optimized bulk heterojunction (BHJ) structure are expected to show decreased efficiency. We therefore investigate the blend morphology and its dependence on graphene content via small angle neutron scattering (SANS) and neutron reflectivity (NR). SANS reveals that the structure in the cell devolves to a dispersion of disk-like crystals in an amorphous blend matrix, with crystal volume fraction being influenced by filler content. Graphene addition resulted in increased P3HT crystallinity, larger crystallites and a higher degree of ordering. We extend our methodology to other all-polymer solar cells (for example, those involving P3HT and PCPDTBT), and demonstrate how scattering data can be used to correlate morphological features with efficiency. Results on cell characteristics and recombination mechanisms are also reported, and indicate means of addressing fundamental problems in organic photovoltaic systems. [Preview Abstract] |
Wednesday, March 16, 2016 9:48AM - 10:00AM |
K33.00006: Magnetic field effects in a polymer/fullerene blend photovoltaic cell Hyuk-Jae Jang, James I. Basham, David J. Gundlach, Curt A. Richter Organic photovoltaic (OPV) systems based on blends of conjugated polymers and fullerene derivatives have shown great promise for low-cost and efficient photovoltaic applications. Recent findings suggest that a weak external magnetic field can disturb the spin configuration of excited states and subsequently change properties of OPV cells such as photocurrent. These changes are referred to as magnetic field effects (MFEs). In order to have a better understanding of the underlying mechanisms responsible for the MFEs in polymer/fullerene blend photovoltaic systems, we fabricated poly-3-hexylthiophene (P3HT):phenyl-C$_{61}$-butyric acid methyl ester (PC$_{61}$BM) cells and carried out photovoltaic device performance and impedance spectroscopy measurements with and without an externally applied magnetic field. A significant reduction in short circuit current (J$_{SC})$ as well as open circuit voltage (V$_{OC})$ was observed with an applied magnetic field of a 0.1 tesla compared to those measured without a magnetic field under the same intensity of illumination. Impedance spectroscopy data gives insights into the influence of an external magnetic field on charge generation and recombination near normal photovoltaic operating conditions. [Preview Abstract] |
Wednesday, March 16, 2016 10:00AM - 10:12AM |
K33.00007: Conductivity Scaling Relationships of Nanostructured Membranes based on Hydrated Protic Polymerized Ionic Liquids: Effect of Domain Spacing Gabriel Sanoja, Bhooshan Popere, Bryan Beckingham, Christopher Evans, Nathaniel Lynd, Rachel Segalman Elucidating the relationship between chemical structure, morphology, and ionic conductivity is essential for designing novel materials for electrochemical applications. In this work, the effect of lamellar domain spacing (d) on ionic conductivity ($\sigma )$ is investigated for a model system of hydrated block copolymer based on a protic polymerized ionic liquid. We present a strategy that allows for the synthesis of a well-defined series of narrowly dispersed PS-$b-$PIL with constant volume fraction of ionic liquid moieties (f$_{IL}\approx $0.39). These materials self-assemble into ordered lamellar morphologies with variable domain spacing (23-59 nm) as demonstrated by SAXS. PS-$b-$PIL membranes exhibit ionic conductivities above 10$^{-4}$ S/cm at room temperature, which are independent of domain spacing. The conductivity scaling relationship demonstrated in this work suggests that a mechanically robust membrane can be designed without compromising its ability to transport ions. In addition, PIL-based membranes exhibit lower water uptake ($\lambda =$10) in comparison with many proton-conducting systems reported elsewhere. The low water content of these materials makes them promising candidates for solar-fuels electrochemical devices. [Preview Abstract] |
Wednesday, March 16, 2016 10:12AM - 10:24AM |
K33.00008: Electronic structure properties as signatures of morphological motifs in organic photovoltaics Matthew Goldey, Giulia Galli The effect of polymer morphologies upon the efficiency of organic solar cells is difficult to determine experimentally; however, theoretical models may directly probe how structural changes affect electronic properties, such as band locations and band gaps and hence provide key insight into solar energy conversion processes. Using density functional and many body perturbation theory (G0W0) calculations, we investigated the dependence of the electronic states on order parameters such as backbone dihedrals of organic donor polymers. We focused on the donor polymers PTB7 and PID2 and the acceptor PC71BM for which recent experiments[1] reported promising photoconversion efficiencies of 8.22\% for ternary blend cells. Our results suggest that accurate predictions of the device performance must include a description of local disorder in the active layer. In particular, we found multiple possible configurations of the donor polymer with relative energies 0.06-0.3 eV/monomer above the lowest energy conformer whose electronic structure differ significantly. Using the lowest energy conformations found at zero T, calculated electronic energy levels are in good agreement with experimental values, with errors within 0.2 eV. [1] Lu, L., et. al. Nature Photon, 8(9), 716–722 (2014). [Preview Abstract] |
Wednesday, March 16, 2016 10:24AM - 10:36AM |
K33.00009: Enhancing the performance of BHJ solar cell via self-assembly templates in active layer Ying Liu, Hongfei Li, Zhenhua Yang, Chang-Yong Nam, Sushil Satija, Miriam Rafailovich The bulk heterojunction (BHJ) solar cell is an important example of a polymer solar cell technology that has been proposed in recent years. However, due to the disordered inner structures in the active layer, control of the inner structure within the active layer is required to enhance the efficiency. In our approach, a self-assembly of tertiary polymer blend film is confined between the air and solid interfaces. The principal has been proved using a blend of PMMA: P3HT: PCBM where we showed that the PMMA phase formed a column structure in the P3HT, which template the PCBM phase between the electrodes. Neutron reflectometry was used to demonstrate the confinement of PCBM at the interface between P3HT and PMMA in the active layer. The columnar structured template is investigated under atomic force microscopy (AFM) and transmission electron microscopy (TEM). SCLC mobility measurement indicated an obvious improvement on both hole and electron mobility. The different morphological structures formed via phase segregation are correlated with the performance of the PEV cells fabricated at the BNL-CFN and significant enhancement for the efficiency is observed. [Preview Abstract] |
Wednesday, March 16, 2016 10:36AM - 10:48AM |
K33.00010: Electroabsorption spectroscopy of bulk heterojunction solar cells Marian Tzolov, Zane Cohick, Christopher Green Bulk heterojunction solar cells were fabricated using PCPDTBT polymer and PCBM in inert atmosphere. Electroabsorption (EA) spectroscopy was performed in the spectral range 350-1000 nm. The first derivative of the optical absorbance spectra explains satisfactory most of the bands in the EA spectra. The only deviation is in the spectral range around 760 nm. Control experiments with devices containing only a PCPDTBT film confirm that the band at 760 nm is associated with the presence of PCBM, despite that pristine PCBM is not expected to have such band. Electrical DC bias strongly affects this band. Negative DC bias almost completely eliminates this band, while positive bias enhances it. We interpret the band at 760 nm as due to photogenerated charge carriers trapped at defect states or at interfaces within the composite PCPDTBT/PCBM film. The effective transfer of photogenerated charge carriers between PCPDTBT and PCBM is confirmed by the quenching of the photoluminescence in the composite film, while the emission of electroluminescence from the devices confirms that PCPDTBT is an effective medium for recombination of charges injected in it. [Preview Abstract] |
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