Optical gap and Urbach edge slope in a-Se

Good correspondence between the temperature shift of the optical gap and the Urbach edge slope has been found for a-Se. Such coherence, however, does not exist for photoinduced changes of the gap and Urbach edge slope measured at 77 K. The kinetics of photoinduced changes of the gap takes the form of first-order process, while the kinetics of photoinduced changes of Urbach edge obeys stretched exponential form. It is supposed that overheating due to illumination is most probably not responsible for photoinduced changes. Rather an increase in density of localized states at band edges, together with formation of self-trapped exciton-like states could explain the photoinduced changes observed in the gap and Urbach edge slope.