The intensity dependence of the steady-state photoconductivity in amorphous semiconductors

The dependence of the steady-state photoconductivity sigma _ph of an amorphous semiconductor on the intensity F of the incident light often differs from that expected from simple monomolecular or bimolecular recombination mechanisms. This difference is associated with the existence of localised states at a variety of energies between the mobility edges of the valence and conduction bands, since the number of these states that are effective as recombination centres depends on the light intensity. The author presents an analysis of the calculation of the exponent gamma such that sigma _ph is proportional to F^Ϝ, for systems containing sets of states with up to two different values of the ratio of the capture cross sections for electrons and for holes. It is found that only under very restrictive conditions can the density of the traps for the majority carrier be deduced unambiguously from the value of gamma and its temperature dependence. However, the value of the exponent gamma can readily be used to test the validity and to establish the values of the parameters of any assumed model density of localised states deduced from other experimental results.