Anomalous time-independent diffusion of charge carriers in a random potential under a bias field

We present Monte Carlo simulation results for time-independent drift and spatial spreading of a packet of charge carriers executing a random walk within a manifold of hopping sites under a bias field. The shape of the density-of-states profile is assumed to be Gaussian (width a). The ratio eD/μkT, equal to unity for ordinary diffusion, is found to increase dramatically with increasing disorder and bias field. For σ/kT= 3.5 and fields of order 10⁶Vcm^-1, eD/μkT reaches values of order 10³. The effect accounts for the anomalous spreading of time-of-flight profiles often seen in photocarrier transit time studies on various polymeric systems. The role of disorder in charge-carrier transport is emphasized.