Buckminsterfullerene (C60) and Copper(II) Phthalocyanine (CuPc) have been widely used as a Donor-accepetor pair in Organic Photovoltaic (OPV) devices with good efficiencies of up to 5% [1]. To date, crystallinity and molecular orientation have not been optimized for OPV performance. We will describe an approach to this optimization that takes advantage of our recent work creating crystalline flat-lying CuPc film structures on graphite[2] to promote highly crystalline layer-by-layer growth of C60 films. Using grazing incidence wide-angle x-ray scattering and atomic force microscopy, we observe a nearly ideal morphology for bilayer films of C60 / CuPc / HOPG,. We hypothesize that these bilayers will improve the Voc, Jsc, and Fill Factor of solar cells. Flat lying CuPc allows a strong co-facial interface with C60, which induces layer-by-layer C60 growth in highly ordered FCC crystals, covering the flat lying CuPc single crystal domains. Very similar morphologies are observed when varying the film thickness of either material from 5 nm up to more than 100 nm, suggesting that this advantageous morphology is not only robust, but may actually be preferred by these materials. The large, ordered domains observed in both materials and the co-facial interface should greatly increase charge carrier mobility leading to increased Fill factor. We envision that bilayer films such as described here can use very high quality graphene as a transparent conducting electrode to simultaneously optimize crystallinity and molecular orientation for solar cell performance. This work was equally supported by the U.S. Department of Energy, OS, BES, MSE (DE-FG02−98ER45737) and NSF CAREER award DMR-1056861. TM and EHG were partially supported by GAANN Fellowships.
References [1] Xue, J., Rand, B. P., Uchida, S., & Forrest, S. R. (2005). A Hybrid Planar-Mixed Molecular Heterojunction Photovoltaic Cell. Advanced Materials, 17(1), 66–71. doi:10.1002/adma.200400617 [2] McAfee, T., Gann, E., Guan, T., Stuart, S. C., Rowe, J., Dougherty, D. B., & Ade, H. (2014). Toward Single-Crystal Hybrid-Carbon Electronics : Impact of Graphene Substrate Defect Density on Copper Phthalocyanine Film Growth.