Dresden 2011 – scientific programme

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 5: Organic Semiconductors I: Solar Cells A (jointly with HL, DS)

CPP 5.3: Talk

Monday, March 14, 2011, 14:45–15:00, ZEU 222

Efficiency-Limiting Processes in Bulk Heterojunction Organic Solar Cells — Ian Howard, Ralf Mauer, Fabian Etzold, Valentin Kamm, Michael Meister, Hannah Mangold, and •Frédéric Laquai — Max Planck Research Group for Organic Optoelectronics, MPI for Polymer Research, Mainz, Germany

Despite significant study, the efficiency-limiting processes that govern the efficiency of bulk heterojunction photovoltaic devices still remain ambiguous. In particular the role of interfacial charge-transfer (CT) states as potential intermediates of free charge carriers is diversely debated. In this contribution we directly observe charge generation and recombination processes in state-of-the-art polymer:methanofullerene photovoltaic blends by transient absorption spectroscopy and compare polythiophene (P3HT) of varying regioregularity and low-bandgap polymers as electron donor materials. We observe a common feature of these blends is ultrafast (< 100 fs) exciton dissociation at the donor-acceptor interface. However, a certain fraction of excitons create CT states that predominantly recombine geminately within a few nanoseconds. On the other hand the fraction of free charge carriers recombines bimolecularly on a time scale competing with charge extraction and can thus be swept out of the device as photocurrent. The results demonstrate the importance of ultrafast free carrier generation and suppression of interfacial CT state formation to achieve high power conversion efficiencies in various material systems. [1] I.A. Howard, R. Mauer, M. Meister, F. Laquai, J. Am. Chem. Soc. 2010, 132, 14866. [2] I.A. Howard, F. Laquai, Macromol. Chem. Phys. 2010, 211, 2063.