Transient current in thin layers of disordered organic materials under conditions of nonequilibrium charge carrier transport

The theoretical model of nonequilibrium hopping charge-carrier transport in disordered organic materials based on the concept of the transport level is applied to the case of Gaussian energy distribution of localized states. As limiting cases, the model includes quasi-equilibrium and extremely nonequilibrium (dispersive) transport modes at long and short times, respectively. An analytic description of the intermediate regime of moderately nonequilibrium transport is proposed. In this regime, the quasi-equilibrium value of the mobility is already established, whereas the coefficient of field-assisted diffusion continues to increase with time. The results of the calculations are in good agreement both with the data of time-of-flight experiments for photocon-ducting molecularly doped polymers and with the results of numerical simulation for the Gaussian disorder model. Transient currents under the conditions of near-electrode and spatially uniform charge-carrier generation are compared.