Enhanced Photovoltaic Response in Ionically Self-Assembled Monolayer Thin-Film Devices
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Enhanced Photovoltaic Response in Ionically Self-Assembled Monolayer Thin-Film Devices Daniela Marciu1, Michael B. Miller1, Carrie Kozikowski1, J. R. Heflin2, Sung Cho2, Beth A. Reid2, Kaori Kuroda2, Willi Graupner2, Hong Wang3, Harry W. Gibson3, and Richey M. Davis4 1 Luna Innovations, Inc., P. O. Box 11704 Blacksburg, VA 24062-1704, U.S.A. 2 Department of Physics, Virginia Tech Blacksburg, VA 24061-0435, U.S.A. 3 Department of Chemistry, Virginia Tech Blacksburg, VA 24061-0212, U.S.A. 4 Department of Chemical Engineering, Virginia Tech Blacksburg, VA 24061-0211, U.S.A. ABSTRACT We describe detailed studies of ionically self-assembled monolayer (ISAM) photovoltaic (PV) devices incorporating various electron acceptor materials, such as fullerenes and phthalocyanines. Excitons are generated when the conducting polymer is irradiated, and the electron acceptors aid in dissociating the electron/hole pairs before they can radiatively recombine, thus improving the efficiency of the PV process. The ISAM technique allows the deposition of conducting polymer and electron acceptor materials in alternating layers of nanometer-scale thickness. This ensures that every photoexcited electron-hole pair is in proximity to an electron acceptor, thus minimizing electron-hole recombination and increasing the photocurrent. The individual thickness of each monolayer and the interpenetration of adjacent layers can be precisely controlled through the parameters of the electrolyte solutions. Using the ISAM technique, we have demonstrated that it is possible to create ultrathin films (100 nm) of PV material that have enhanced efficiencies. INTRODUCTION Photovoltaic (PV) devices convert optical energy into electrical energy. Compared to the traditional technologies of generating electricity, photovoltaic technology offers the benefits of low maintenance, modularity, and low construction costs, as well as being a clean energy alternative. Furthermore, the fabrication of ionically self-assembled monolayers (ISAMs) allows for precise placement of the individual components (electron donor and electron acceptor) with respect to one another, and simplifies the production of lightweight, large area, flexible devices. A major advance in polymer PV devices was demonstrated in 1992 by Heeger et al. [1]. Since the discovery of photoinduced electron transfer in blends of conducting polymers and fullerenes and their derivatives, much attention has been given to the development of organic photovoltaic devices [2-6]. This approach provides a new alternative for fabricating photovoltaic devices and offers many advantages over the traditional inorganic semiconducting materials: tunability of electronic and optical properties, easy processing, light weight, large area, flexibility, a large selection of materials, and low manufacturing costs. In this paper we present enhanced photoresponse from polymer based photovoltaic devices fabricated by the ISAM method. Polymeric photovoltaic devices rely on efficient photoexcited electron transfer from the conducting pol
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