The Ultrafast Dynamics of Electronic Excitations in Pentacene Thin Films
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The Ultrafast Dynamics of Electronic Excitations in Pentacene Thin Films Henning Marciniak1, Bert Nickel2, and Stefan Lochbrunner1 1 Institut für Physik, Universität Rostock, Universitätsplatz 3, 18055 Rostock, Germany 2 Fakultät für Physik und CeNS, Ludwig-Maximilians-Universität, Geschwister-Scholl-Platz 1, 80539 München, Germany ABSTRACT Pentacene films which are model systems for organic electronics exhibit contrary to the pentacene monomer extremely little photoluminescence. This points to an ultrafast relaxation mechanism. We apply pump-probe absorption spectroscopy with a time resolution of 30 fs to microcrystalline pentacene films. It is found that the primarily excited Frenkel excitons decay within 70 fs to a non fluorescing singlet species. Contrary to expectations fission into triplets is only a minor channel. We propose that the ultrafast relaxation of the photoexcited excitons leads to a species very similar to excimers. The radiative transition of the excimer to the ground state is electric dipole forbidden due to symmetry reasons. The excimer formation provides therefore an efficient mechanism to turn off the emission. INTRODUCTION Pentacene is one of the most promising candidates for applications in organic electronics like transistors because of its extraordinary high hole mobility in the crystalline phase of more than 1 cm2/Vs [1]. Therefore pentacene serves as model compound to understand the electronic properties of organic crystals. In optoelectronic devices the behavior of electronic excitations like excitons is crucial for the performance. While the pentacene monomer fluoresces with a high quantum yield, pentacene films show only an extremely low photoluminescence pointing to the relevance of ultrafast relaxation processes. To investigate this dynamics, we apply pump-probe absorption spectroscopy with a time resolution of 30 fs to microcrystalline pentacene films. EXPERIMENT Microcrystalline thin film samples are prepared by vacuum deposition of pentacene on a transparent polymer substrate (TOPAS; thermoplastic olefin polymer of amorphous structure). The film consists of closely packed crystalline grains with a diameter of about 1 µm and a height of roughly 30 monolayers [2]. The samples are kept under nitrogen for storage and during the optical experiments. In the pump probe measurements the samples are excited by 30 fs long pulses which are generated with a noncollinearly phase matched optical parametric amplifier (NOPA) pumped by a 1 kHz regenerative Ti:sapphire amplifier system (CPA 2001; Clark MXR). The absorption changes are probed over the whole visible spectral range with a whitelight continuum generated in a 3 mm thick sapphire substrate. The photoexcitation is performed at the lowest absorption band with an excitation wavelength of 669 nm.
RESULTS AND DISCUSSION Transient spectra Figure 1 shows the steady state absorption spectrum of the investigated pentacene film and transient absorption spectra measured with different polarization geometries 8 ps after photo excita
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