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27 September 2007 Tailored heterojunctions for efficient thin-film organic solar cells: a photoinduced absorption study
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Recently, we have demonstrated an open circuit voltage of 1.0V and a power conversion efficiency of 3.4% in thin film solar cells, utilizing a new acceptor-substituted oligothiophene with an optical gap of 1.77 eV as donor and C60 as acceptor. Stimulated by this result, we systematically study the energy and electron transfer processes taking place at the oligothiophene:fullerene heterojunction along a homologous series of these oligothiophenes. The heterojunction is modified by tuning the HOMO level using different oligothiophene chain lengths, while the LUMO level is essentially fixed by the choice of the acceptor-type end-groups (dicyanovinyl) attached to the oligothiophene. We study electron transfer at the heterojunction to C60 using photoinduced absorption. The observed transitions are unambiguously identified by TD-DFT calculations. With increasing the effective energy gap of the donor-acceptor pair, charge carrier dissociation following the photoinduced electron transfer is eventually replaced by recombination into the triplet state, which alters the photovoltaic operation conditions. Therefore, the optimum open-circuit voltage of a solar cell is a trade-off between an efficient charge separation at the interface and a maximized effective gap. We conclude that values between 1.0 and 1.1 V for the open-circuit voltage in our solar cell devices present an optimum, as higher voltages were only achieved with concomitant losses in charge separation efficiency.
© (2007) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
R. Schueppel, K. Schmidt, C. Uhrich, K. Schulze, D. Wynands, J. L. Brédas, B. Maennig, M. Pfeiffer, K. Leo, E. Brier, E. Reinold, H.-B. Bu, and P. Baeuerle "Tailored heterojunctions for efficient thin-film organic solar cells: a photoinduced absorption study", Proc. SPIE 6656, Organic Photovoltaics VIII, 66560G (27 September 2007);

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