Düsseldorf 2007 – scientific programme
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MO: Fachverband Molekülphysik
MO 52: Femtosecond Spectroscopy IV
MO 52.7: Talk
Thursday, March 22, 2007, 15:30–15:45, 6B
Ultrafast Exciton Dissociation in Micro Crystalline Pentacene Films — •Henning Marciniak1, Stefan Lochbrunner1, Martin Huth2, Stefan Schiefer2, and Bert Nickel2 — 1Lehrstuhl für BioMolekulare Optik, LMU München — 2Department für Physik und CeNS, LMU München
Micro crystalline thin films of organic molecules are frequently used in organic electronics applications. Their electronic structure and dynamics determine to a large extent the device behavior. We investigate with femtosecond absorption spectroscopy pentacene films prepared by vapor deposition which are used in many prototype applications. They consist of closely packed grains that are microcrystals formed by several monolayers of pentacene molecules [1]. 30 fs long pump pulses are applied to generate singlet excitons in the film. Transient spectra are measured with a white light as probe beam and time traces with compressed pulses resulting in a time resolution of 30 fs. The polarization dependent bleach spectra point to a fairly strong charge transfer character of the primary excitations. We find that the original exciton emission decays within 100 fs indicating that an ultrafast dissociation of the excitons into polarons or triplet excitons takes place. The recovery kinetics exhibits two components. One depends on the excitation energy and can be modeled with annihilation processes. A second component is interpreted as occupied traps which have a limited lifetime.
[1] B. Nickel, R. Barabash, R. Ruiz, N. Koch, A. Kahn, L. C. Feldman, R. F. Haglund, and G. Scoles, Phys. Rev. B 70 (2004), 125401.