Transient conductivity and photoconductivity ina-Si:H

Transient conductivity and photoconductivity are explored in amorphous silicon (a-Si:H). The breakdown of the time-of-flight method is demonstrated in doped samples of sufficient bulk conductivity. The transient response is then shown to contain no information about the drift mobility or carrier lifetimes, but instead is governed by the contact depletion-layer capacitance and the bulk series resistance. Analysis of the results gives a new method of determining the density of shallow occupied states in a-Si:H and quantitative results are given for some n- and p-type samples. Measurements of gap-cell photoconductivity are shown to have similar contact effects which causes a decay artifact that can extend up to {~}1 sec. It is argued that the artifact may have been mistakenly interpreted as bulk dispersive transport and recombination in some published data.