Charge carrier mobility and diffusion in conjugated polymers

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A significant progress has been made in recent years for the understanding of charge transport in organic disordered semiconductors such as conjugated polymers, particularly in view of the charge carrier mobility. The charge carrier diffusion coefficient, however, has been mostly neglected—the reason being that usually the Einstein relation D/µ = kT/q was implicitly assumed to be valid. Up to now, charge carrier mobility µ and diffusion coefficient D and their temperature dependence have not been reported together for organic disordered semiconductors, with one exception, where the Einstein relation was found not to be valid. A systematic approach concerning the direct determination of D and µ, also accounting for the temperature dependence, is lacking. This is despite the fact that the Einstein relation has a clear impact on the device performance, as diode characteristics and nongeminate recombination dynamics—which are proportional to the diffusion coefficient, not the charge carrier mobility—are influenced by deviations from its usual form. Within this proposal, we will attempt to investigate the charge carrier diffusion coefficient and the mobility in organic semiconductors, in order to experimentally determine the generalized form of the Einstein relation, D/µ = Χ • kT/q. For a more fundamental understanding, we will complement these experiments with calculations of the hopping master equation as well as by macroscopic simulations. With the latter, also the influence of a generalised Einstein relation on the device performance of organic electronics will be evaluated. The goal will therefore be to directly determine the Einstein relation and its impact on the performance of organic electronic devices, which is a requirement for a guided optimisation, and an important step to a fundamental understanding.