Hamburg 2009 – scientific programme

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MO: Fachverband Molekülphysik

MO 7: Femtosekundenspektroskopie 1

MO 7.6: Talk

Tuesday, March 3, 2009, 11:45–12:00, VMP 6 HS-F

Ultrafast Exciton Dynamics in Organic Thin Films — •Henning Marciniak¹, Bert Nickel², and Stefan Lochbrunner¹ — ¹Institut für Physik, Universität Rostock — ²Fakultät für Physik und CeNS, LMU München

Thin films of microcrystalline pentacene frequently serve as active layers in prototypical organic electronic devices. The nature and dynamics of their electronic excitations determine to a large extent the device behavior. We investigate pentacene films with a thickness of about 30 monolayers by femtosecond absorption spectroscopy. The measurements show a decay of the stimulated emission with a sub-100-fs time constant [1]. A possible path is fission into triplet excitons. By tilting the pentacene layers relative to the incident laser beams one can distinguish between singlet and triplet absorption since the corresponding transition dipoles are orientated perpendicular to each other. We find that the triplet absorption rises significantly slower than the emission decays and adopts relatively weak signal strength, indicating that only a small fraction of triplet excitons is formed. We suggest that the dominant relaxation path leads to excimer-like excitons with a strongly reduced transition dipole moment to the ground state. These singlet excitons determine the subsequent dynamics on the picoseconds timescale, which is found to be excitation intensity dependent. Modeling the kinetic traces shows that the dynamics is governed by diffusion controlled processes, in particular by exciton-exciton-annihilation.

[1] H. Marciniak, M. Fiebig, M. Huth, S. Schiefer, B. Nickel, F. Selmaier, and S. Lochbrunner, Phys. Rev. Lett. 99 (2007), 176402.