Carrier-relaxation dynamics in model crystalline organic semiconductors

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This project is devoted to studying the relaxation dynamics in single-crystalline organic semiconductors. In particular, the relation between the molecular packing motifs and the optoelectronic response will be explored. The relaxation mechanism will be studied in three model systems perylene, pentacene, and perfluoropentacene. These prototypical small molecules all form various polymorphs of highly-ordered van.-der-Waals-bonded single crystals. On the one hand, this is of fundamental scientific interest as phenome such as delocalization of excitations and intermolecular coupling are explored. On the other hand the findings are expected to yield insights in the performance limitations of current organic optoelectronic devices as well as provide predictive capabilities towards optimized structure and materials design. Experimentally, these challenges will be addressed by polarization-resolved optical spectroscopy with high spatial resolution. The fundamental siglet and triplet-type transitions will be identified by modulated reflection and transmission spectroscopy and compared to results model calculations. In particular, the triplets ground states will become optically addressable in static external magnetic fields. Furthermore, emission spectra will be used to corroborate these findings. Time-resolve measurements help to distinguish direct-gap from indirect-gap transitions. A particular focus will be put on the microscopic mechanism of singlet-exciton fission, the conversion of one singlet-type exciton into two triplet states. And explore the efficiency of selective intersystem crossing. Furthermore, I will explore the role of enhanced next-neighbour coupling resulting in, e.g., excimer formation. And the related distortion of the crystalline lattice through the formation of this correlated molecule-pair excited state. Therefore, the individual molecules will be selectively changes by perfluorination or deuteration.