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We present a scaling theory for charge transport in molecular semiconductors with Gaussian energy disorder, which extends standard percolation theory by including bonds with conductances close to the percolating one in the random-resistor network of bonds representing charge hopping. A general and compact expression is given for the charge mobility as a function of temperature and charge concentration, with parameters that are determined for Miller-Abrahams and Marcus hopping on different lattices from numerically exact results. A universal dependence on charge concentration is found and a temperature dependence that differs significantly from other reports.

In non-steady-state experiments, the electrical response of devicesbased on disordered organic semiconductors often shows a large transient contribution due to relaxation of the out-of-equilibrium charge-carrier distribution. We have developed a model describing this process, based only on the parameters describing the d.c. mobility andon Monte Carlo calculations of the effective conduction energy level. The model successfully predicts the relaxation-enhancement of thedifferential capacitance of sandwich-type devices based on a polyfluorene-copolymer. Generalization of the approach is expected to enable efficient modeling of relaxation effects in other types of experiments.