Transient photoconductivity and demarcation energies in amorphous semiconductors

In the theoretical analysis of transient photoconductivity data on amorphous semiconductors in terms of multiple trapping, a number of recent publications use the concept of a time-dependent demarcation energy E_d(t) such that, at time t, the excess charge carriers occupying localized states between the mobility edge and this energy are in quasi-thermal equilibrium with the free carriers in extended states, while those occupying states with energies beyond E_d(t) have had no time to leave such states. Results are reported of calculations of the occupancy of the localized states for a current transient of the form Ct(^1-α) that is often observed. The results show that such a simplistic interpretation of E_d(t) involves appreciable numerical errors, so that the concept of a demarcation energy is only useful for the qualitative, rather than the quantitative, analysis of experimental results.