Light induced changes in two distinct defect states at and below midgap in a-Si:H

The inconsistencies associated with the common simple method of evaluating the quality and stability of hydrogenated amorphous silicon (a-Si:H) materials for solar cell applications from just the magnitude of subgap absorption (/spl alpha/(E)) are addressed in this study. The approach taken in this study is to characterize the entire /spl alpha/(E) spectrum by its derivatives, d[/spl alpha/(E)]/dE. Results are presented and discussed for two a-Si:H thin film materials differing in deposition rate by an order of magnitude, which illustrate the utility of this approach. The presence of two distinctly different light induced defect states at and below midgap are clearly identified and their evolution characterized. The results are not only consistent with the corresponding electron mobility-lifetime products but also illustrate the inadequacy of the simple approach, particularly when applied to a-Si:H materials which are deposited at fast rates.