Multiple-trapping model of anomalous transit-time dispersion in $a-\mathrm{S}\mathrm{e}$

A generalized model of multiple trapping from a band of extended or localized states is used to study timedependent charge transport in amorphous solids. The model differs from a conventional multiple-trapping model by including a distribution of trap release rates for a constant trap energy. An extensive analysis of transient photocurrent experiments on $a-\mathrm{S}\mathrm{e}$ is carried out to determine the transport parameters for this case. It is found that a small number of parameters can be used to analyze the experimental results over a wide range of temperature and sample thickness. The results of the analysis are interpreted in terms of trapcontrolled hopping, which is a special case of the generalized multiple-trapping model. The asymptotic value of the theoretical photocurrent transient is obtained for the multiple-trapping model, and the results of Scher and Montroll are recovered for the case of extreme or anomalous dispersion, which occurs for $a-\mathrm{S}\mathrm{e}$ at low temperature ($T\simeq 140$ K). The density of trapping sites is estimated, and the difficulties associated with considering a continuous distribution of trap release rates are discussed. It is concluded that the generalized multiple-trapping model, defined by simple first-order rate equations, is capable of describing detailed shapes of photocurrent transients, including dispersive and nondispersive charge transport.