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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.

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.

From a numerically exact solution of the Master equation for hoppingtransport in a disordered energy landscape with a Gaussian densityof states, we determine the dependence on temperature, carrier density, and electric field of the charge carrier mobility. Experimentalspace-charge limited currents in semiconducting polymer-based devices are excellently reproduced with this unified description of the mobility. At room temperature it is mainly the dependence on carrier density that plays an important role, whereas at low temperatures andhigh fields the electric field dependence becomes important. Omission of the carrier-density dependence has led to an underestimation of the hopping distance and the width of the density of states in these polymers.