Evaluation of degradation in amorphous silicon solar cells

It is pointed out that lack of understanding of light-induced degradation in a-Si:H has prevented not only its reduction, but also its basic characterization. That is, there has been no quantitative index of material stability that can be used as a criterion of material quality, thus precluding evaluation of preparation conditions or comparative stability of different materials. From the recently developed stretched-exponential description of degradation, it is shown that the saturated behavior is the only reliable criterion, although it is little used. A recent atomic model for the metastable defects that is consistent with this description and explains most defect properties is summarized. This model is an adaptation of a model for the metastable DX centers in GaAs and differs from the usual Si-Si bond-breaking models in a-Si:H in that it is based on foreign atoms