Phase Morphology in Poly(thiophene)-Fullerene Thin Film Devices
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Phase Morphology in Poly(thiophene)-Fullerene Thin Film Devices D. G. Bucknalla, N Deba, M Skodab, B Sumpterc and A. Karimd a
Georgia Institute of Technology, b ISIS, STFC, c Oak Ridge National Laboratory, d University of Akron
Abstract Nanoscale heterojunction systems consisting of fullerenes blended with conjugated polymers are promising materials candidates for achieving high performance organic photovoltaic (OPV) devices. In order to understand the phase behaviour in these thin film devices, we have used neutron reflectivity to determine the behavior of model conjugated polymer-fullerene mixtures. Neutron reflectivity is particularly useful for these types of thin film studies since the fullerenes generally have a higher scattering contrast with respect to most polymers. We are studying model bulk heterojunction (BHJ) films based on mixtures of poly(3-hexyl thiophene)s (P3HT), a widely used photoconductive polymer, and different fullerenes (C60, PCBM and bis-PCBM). We have used neutron reflection measurements to determine the film morphology normal to the film surfaces in real device configurations. The novelty of the approach over previous studies is that the BHJ layer is measured with the confining films of PEDOT/PSS and Al in place. Using this model system, we have measured the effect of typical thermal annealing processes on the film development as a function of the polythiophene-fullerene mixtures. Introduction Solar energy devices can potentially serve as an alternative source that could provide a large fraction of the future global energy requirements. Solar energy devices can be made of materials which are organic, inorganic or composite based. Organic based solar devices (OPVs) offer the potential for cheaply produced, flexible large area panels. The current most efficient organic based solar devices consist of donor and acceptor materials mixed together to form what is known as a bulk heterojunction (BHJ). However, even the best OPVs have device efficiencies that are below those of inorganic based devices and despite significant research and technological development is still not a plausible option to satisfy domestic energy requirements. The efficiency of the OPVs depend upon a number of factors including material choice, bulk heterojunction morphology and thin film processing. Arguably the least understood of the various contributing factors is the hierarchical structure of the devices and how this relates to device performance. The lack of a complete understanding of structure-processing-property relationships is a contributing impediment to significant advancement in the performance of OPVs. The goal of our research is to conduct fundamental studies to determine how the phase behaviour of bulk heterojunction thin film geometry affects device performance. We also wish to understand the effect of the chemical structure (in this case through modification of the fullerene) on the morphology of the heterojunction, before and after annealing. To determine this thin film
morphology we utilized neutron
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