Charge carrier photogeneration in oligo(phenylenevinylene) thin films: A quantitative study

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Abstract
We analyze the generation and recombination of charged states in oligo(phenylenevinylene)s at high photoexcitation densities, using femtosecond pump-probe spectroscopy as a direct probe for the time-dependent density of photogenerated singlet, triplet, and doublet states. Pairs of charged states are formed during the exciting laser pulse, partly by a two-photon process. Geminate recombination of these pairs is found in the first ps after formation, after which a nongeminate dispersive mechanism prevails. The application of high electric field strengths opens up an additional channel for charge carrier formation, involving field-induced electron transfer between two singlet excitons. This process is competing with bimolecular singlet exciton annihilation. Field-induced formation of triplet states is governed by nongeminate recombination of field-induced charged states. We give values for cross sections of the photogenerated species, the efficiency of field-induced charge carrier photogeneration and for the early charge carrier mobility.