Transient localization of charge carriers in organic semiconductors

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In this collaborative research project we want to elucidate the charge-transport mechanisms in organic semiconductors and their intrinsic limitations. The focus will be on the recent suggestion that dynamical molecular disorder governs the optical properties and charge-carrier transport, with very specific predictions on the dc mobility and low-frequency optical absorption of the charge carriers injected or excited across the energy gap. The dynamical nature of molecular disorder prevents localization of the carriers at long times; i.e. fluctuations of the disorder potential destroy the quantum interferences responsible for Anderson localization.Our team of three high-profile groups completely covers the full range from material purification and crystal growth, sample characterization, device fabrication, optical spectroscopy, investigations of the charge-carrier dynamics, modelling and theoretical description. The projected experiments will be performed on high-quality molecular single crystals showing preferred hole (rubrene, pentacene, etc.) or electron conduction (dichloro-naphthalene-diimde, etc.) and which can be charge carrier-doped by either field-effect or photo-excitations. We plan to measure the optical conductivity, the FET mobility as well as the noise spectral density as a function of crystallographic direction, temperature, frequency, carrier density and doping-induced, static disorder. The experiments will provide quantitative evidence for the occurrence of transient localization in the electronic transport of molecular semiconductors and thus will allow us to improve and advance theoretical concepts and modelling of the dynamical charge carrier motion in this material class.