Intermolecular motions as an explanation for exciton trapping in organic photovoltaics |
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| Volker Settels | |
| Universität Würzburg | |
| Exciton trapping was figured out as a considerably limitation for internal quantum efficiency of organic solar cells using perylene based dyes. In experiments it was found that excitons relax into stable and immobile intermolecular states after excitation. These states are populated rapidly on a picoseconds time scale. A general understanding of the trapping mechanism is not achieved yet. A deeper insight would help to overcome the limitations of perylene derivatives as materials for organic solar cells. We introduce an exciton trapping mechanism based on highly anharmonic intermolecular motions of perylene based molecules. For this second order correlated ab inito calculations on dimers were performed by considering the surrounding via mechanical embedding. Exciton trapping results due to a relaxation into an optically dark excited state. The lost of energy and a low transition dipole moment to the ground state yield a trap for Förster resonant energy transfer, which should be dominant for exciton transport at room temperature. Wave packet dynamics yield a low picoseconds timescale for exciton trapping, which agrees with experimental findings. This trapping mechanism can be generalized for a wide range of perylene based dyes. Exciton diffusion lengths were calculated by a hopping approach. The results of this method compare favourably with experiments for herring bone like crystals structures, but predict way too large exciton mobilities for π-stacked structures. In the latter type of structures the exciton trapping comes into play whereas in herring bone type structures it is hardly possible. This provides evidence that the small exciton diffusion length in many organic materials based on perylene derivatives is caused by exciton trapping, e. g. the small exciton diffusion length in ?-PTCDA compared to DIP can be explained by fast exciton trapping. |