Charge Carrier Transport in Disordered Solids: Dispersive Versus Gaussian

Abstract

We present a critical review of numerous time-of-flight and radiation-induced conductivity measurements in disordered solids as well as of the theoretical models used to explain them. Recent experiments employing both techniques simultaneously resulted in conflicting conclusions about charge carrier transport in molecularly doped polymers. Whereas the latter method gives a consistent representation of it as a highly non-equilibrium phenomenon of yet unspecified duration (dispersive process), the former is notorious for a number of ambiguous results, some of which are still present, and it is also known to generate the Gaussian disorder model predicting the prevalent steady-state mobility (Gaussian transport). A way to reconcile these seemingly contradictory data has been suggested. This consists of taking due account of surface effects arising from the presence of surface regions (layers) in the dielectric sample with somewhat inferior electronic properties compared with the bulk. Preliminary calculations lend credit to this idea. It is argued that the time-of-flight method should be supplemented by some bulk excitation technique, which allows us to minimize or even remove surface effects. The methodological basis of the former method should be revisited. On the weight of the available information, preference should be given to the dispersive rather than the Gaussian transport but a major problem still exists.