Nanoscale Control of the Polymer-Fullerene Interface in Photovoltaic Devices by Thermally-Controlled Interdiffusion
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Nanoscale Control of the Polymer-Fullerene Interface in Photovoltaic Devices by Thermally-Controlled Interdiffusion M. Drees a, R.M. Davis b,D. Marciu c, K. Premaratne a, W. Graupnera+, M. Miller c, J.R. Heflin a a Department of Physics, Virginia Tech, Blacksburg, VA 24061-0435 b Department of Chemical Engineering, Virginia Tech, Blacksburg, VA 24061-0211 c Luna Innovations, Inc., P.O. Box 11704, Blacksburg, VA 24062-1704 + Present address: austriamicrosystems AG, concerto!; Schloss Premstaetten, A 8141 Austria ABSTRACT Ultrafast photoinduced charge transfer from conjugated polymers to fullerenes has led to extensive studies of these systems as photovoltaic devices. The charge transfer process prevents radiative electron-hole recombination, resulting in free, mobile charges. A limiting factor is the exciton diffusion distance, which is of the order of 10 nm. If the fullerene is not within this distance of the optical excitation site, no charge separation will take place. The simplest system for such devices is a bilayer system in which a film of C60 is sublimed onto a spin-cast film of MEH-PPV, for example. We describe studies in which the polymer is heated above the glass transition temperature in an inert environment, inducing an interdiffusion of the polymer and the fullerene layers. With this process, a controlled, bulk, gradient heterojunction is created. The photoluminescence and the short circuit currents of the devices show a dramatic decrease in photoluminescence and concomitant increase in short circuit currents, demonstrating the improved interface. INTRODUCTION As part of the worldwide effort towards renewable energy sources, organic photovoltaic materials are intensely studied because of the potential for lightweight, flexible, inexpensive, efficient solar cells. A major advance in polymeric photovoltaic devices was achieved with the observation of photogenerated charge separation in poly(para-phenylenevinylene) (PPV)-C60 composites [1]. Upon photoexcitation, rapid electron transfer occurs from the polymer to the high electron affinity C60 [2]. However, photoexcited electron-hole pairs at distances larger than ~10 nm from the fullerene acceptor recombine before charge separation occurs, yielding photoluminescence [3,4]. Significant improvements of the photovoltaic efficiency have been achieved by providing improved donor/acceptor proximity throughout the device using interpenetrating polymer networks [5,6] and polymer/fullerene blends [7], resulting in "bulk heterojunctions". Nanoscale compositional control of the electron donor and acceptor species is clearly important to optimizing the perfomance of polymeric photovoltaics. Here, we demonstrate a new approach for controlling the nanoscale composition of a polymer/fullerene film. Starting from a standard bilayer system of 100 nm of spin-cast MEHPPV followed by ~200 nm of evaporated C60, the film is then heated above the MEH-PPV glass transition temperature to enhance the diffusion of the fullerene into the polymer, resulting in a concentration gradient
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