Darmstadt 2008 – scientific programme
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MO: Fachverband Molekülphysik
MO 3: Femtosekundenspektroskopie I
MO 3.1: Invited Talk
Monday, March 10, 2008, 14:00–14:30, 3G
Dynamics and Mobility of Electronic Excitations in Organic Materials — •Stefan Lochbrunner — Institut für Physik, Universität Rostock, Universitätsplatz 3, 18055 Rostock
In organic materials and thin layer systems designed for optoelectronics applications complex energy transport and electronic relaxation processes govern the performance. Since many of them contribute it is difficult to derive a microscopic understanding purely from measuring device parameters. Femtosecond spectroscopy provides complementary information since it probes the primary processes initiated by the absorption of a photon and is predominantly sensitive to local effects. Our pump-probe experiments on different systems with 30 fs time resolution show that it is possible to discriminate between different relaxation scenarios and to provide absolute numbers for the local mobility. For thin microcrystalline pentacene films we find that the originally excited Frenkel excitons decay within 70 fs to a non fluorescing species which is still mobile. Its exact nature is currently under investigation. The subsequent decay of the exciton population can be modeled assuming that diffusion driven exciton-exciton annihilation takes place and a significant concentration of traps exists which can immobilize the excitons. A diffusion constant of 5·104 cm2/s, a trap concentration of 2·1019 cm3, and a lifetime of 850 ps for the trapped excitons are obtained.[1]
In unordered polymers the large spreading of site energies restricts the exciton diffusion length. To circumvent this problem perylene bisimide dyes are incorporated in a polymer matrix with a high concentration to allow for exciton migration via Förster energy transfer. A fast transfer of 1.5 ps between the dye molecules is derived from the anisotropy decay indicating efficient exciton migration.[2] However, the lifetime of the excitons is limited due to trapping by dimers. In the next step we want to avoid the dimer formation and to achieve diffusion lengths in the order of the layer thickness.
To look into the dynamics of one dimensional systems perylene bisimide aggregates are investigated. Here an ultrafast relaxation associated with weak signal oscillations is found pointing to the formation of excimers. On the picosecond time scale no further signal changes are observed indicating that the excimers are immobile and annihilation processes do not occur.
H. Marciniak, M. Fiebig, M. Huth, S. Schiefer, B. Nickel, F. Selmaier, and S. Lochbrunner, Phys. Rev. Lett. 99 (2007) 176402.
M. Schlosser and S. Lochbrunner, J. Phys. Chem. B 110 (2006) 6001.