A Novel Solution Processable Electron Acceptor, C 60 (CN) 2 , for Bulk Heterojunction Photovoltaic Applications
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A Novel Solution Processable Electron Acceptor, C60(CN)2, for Bulk Heterojunction Photovoltaic Applications Vaishali R. Koppolu1, Mool C. Gupta1, Will Bagienski1, Yang Shen1, Chunying Shu2, Harry W. Gibson2, Harry C. Dorn2 1 Charles L. Brown Department of Electrical and Computer Engineering, 351 McCormick Road, University of Virginia, Charlottesville, VA 22904, U.S.A 2 Department of Chemistry, 107 Davidson Hall, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, U.S.A. ABSTRACT Photovoltaic devices based on soluble conjugated polymers have gained great interest in recent years because of the potential low cost of production and ease of fabrication. PCBM ([6,6]-phenyl-C61-butyric acid methyl ester), a fullerene derivative, has been extensively investigated as a solution processable electron acceptor for bulk-heterojunction (BHJ) photovoltaic devices blended with conjugate polymers like P3HT [poly(3-hexylthiophene)]. Here, we investigated a novel solution processable organic semiconductor, C60(CN)2, as an electron acceptor for bulk heterojunction photovoltaic applications. Optical and electrical properties of C60(CN)2 are studied and compared with PCBM. Blend devices with P3HT and C60(CN)2 have been fabricated and compared with P3HT-PCBM devices. The effect of thermal annealing on the device performance is evaluated. Open circuit voltage, short circuit current, fill factor and total efficiency data are compared with PCBM based devices. INTRODUCTION Bulk heterojunction (BHJ) solar cells fabricated by blending conjugated polymers with fullerenes have resulted in great improvements in the energy conversion efficiencies in recent years. The most commonly and extensively used materials for the fabrication of BJHs are P3HT [poly(3-hexylthiophene)] and PCBM ([6,6]-phenyl-C61-butyric acid methyl ester). Various studies have shown that the device performance is strongly related to nanomorphology of blends, processing conditions, solvents used and on thermal treatment [1,2]. With optimized device structure and fabrication conditions, P3HT:PCBM-based BHJ solar cells can generate efficiencies up to ~5% [1,2]. At present substituted poly(p-phenylene vinylene)s (PPVs) and polythiophenes (PThs) are typically used as donors and fullerene derivatives primarily are used as acceptors for these photovoltaic devices [3]. For further improvement of organic photovoltaic power conversion efficiencies, new donor-acceptor materials need to be examined. Here we investigated a solution processable organic semiconductor, C60(CN)2, as a novel electron acceptor for BHJ applications. Cyclic voltammetry has shown that C60(CN)2 is a stronger electron acceptor than the parent C60 [4]. Although this molecule was first synthesized by Keshavaraz-K et al. in 1995 [5], its electrical properties and photovoltaic applications were never investigated. In this study we investigated C60(CN)2 as a solution processable electron acceptor for the first time for BJH photovoltaic applications. Solar cells based on a blend of P3HT a
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