Space-charge transport in disordered media

The general equations ruling the transport of free excess space charge in disordered media within the continuous-time random walk approximation are inferred from previous works. The equations are shown to have the same structure as those in nondisordered media (for which a mobility may be defined) when written in the Laplace image space. Therefore, the solution in disordered media may be derived from the corresponding solution in nondisordered media if the same initial and boundary conditions are kept. Assuming the extreme dispersive hyperbolic decay of Scher and Montroll, expressions are given for the initial and final stages of the following processes: (1) surface potential decay of corona charged samples, (2) potential buildup for samples charged with a constant current, and (3) the current resulting from charge injection from an ohmic contact. For case (1) it is found that the time dependence of the time derivative of the potential is the same, in absolute value, as in a dispersive photocurrent signal, as far as those extreme stages are concerned. [S0163-1829(97)07841-7].