Invited Talk at E-MRS Spring Meeting in Strasbourg, France. Symposium L "New materials for organic electronics: from synthesis to processing, characterization and device physics"
Organic bulk heterojunction solar cells contain a complex morphology of two organic semiconductors. The energetic distribution of localised states depends strongly on both, the properties of the neat materials and the blend morphology. Therefore, energetics and morphology play a major tole for the physical processes involved in photocurrent generation, for instance the recombination of charge carriers. This nongeminate recombination occurs across the donor-acceptor interface through charge transfer states. In disordered materials with low charge carrier mobilities it is usually described by the so called reduced Langevin rate. The latter is often orders of magnitude smaller than predicted by the Langevin rate, which is proportional to the sum of electron and hole mobility. Based on kinetic Monte Carlo simulations, we find that for typical phase dimensions, the nongeminate recombination is governed rather by the geometric mean of mobilities [1,2]. Another property of nongeminate recombination in organic blend systems is that the recombination order is often, particularly at low temperatures, increased above the expected order of two as two particles are involved. We present transient absorption data of organic semiconductor blends and show how the order or recombination relates to both, the diode ideality factor  and the energetic disorder. We will discuss what is required to bring these different perspectives into one unified picture for recombination in organic solar cells.
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