Safe hole trapping, light soaking and secondary photocurrent transients in amorphous silicon (solar cells)

An analysis is developed for long secondary photocurrent transients in a-Si:H which gives the distribution of trapped holes in valence band tail states. Thermally assisted tunneling to dangling bonds is implicated as the rate-limiting step in hole recombination. Light soaking causes the energetically deeper hole traps with the longer residence times to be lost first and in the same number as would be expected for the increase in dangling bonds. This result supports a model which has hole trapping in valence tail states as a precursor to light-induced dangling bonds. Two types of safe hole traps (SHTs) are considered, and it is shown that hole trapping in either kind of SHT is an intermediate step in bond breaking or defect conversion.