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Perturbation of Exciton Aggregate Coupling by Optical Excitation in Crystalline Perfluoropentacene Films

Carrier multiplication by singlet exciton fission enhances photovoltaic conversion efficiencies in organic solids. This decay of one singlet exciton into two triplet states promises to overcome the Shockley-Queisser limit as up to two electrons may be harvested per absorbed photon. Intermolecular coupling is deemed mandatory for both, singlet exciton fission and a band-like transport. Such a coupling is manifested, $e.g.$, by the Davydov-splitting of the lowest-energy exciton transition in crystalline organic solids. For the model system perfluoropentacene, the corresponding transitions in the experimental, polarisation-resolved absorption spectra are identified by theoretical calculations based on the concept of H- and J- aggregation. Optical injection into the first vibronic progression of the fundamental exciton transitions significantly perturbs the higher-energy transitions that are associated to H-type aggregates of the $S_0 \rightarrow S_3$ transition during and following efficient singlet exciton fission. These findings underline the necessity for efficient carrier extraction as triplet accumulation may be detrimental to both, singlet exciton fission and any potentially band like transport. More generally, our observations indicate that electronic excitations can perturb the electronic band structure in organic crystals and highlight their correlated nature by potentially distorting the lattice.