Charged defect states in hydrogenated amorphous silicon materials for solar cells

A detailed study was carried out to obtain new insights into the nature and densities of gap states in a-Si:H materials. A novel methodology was used to characterize the gap states using photoconductivity and subband gap absorption measurements over a wide range of generation rates on a variety of a-Si:H films. It is found that in these high quality materials there are significant differences in the functional dependence of mobility-lifetime products on the generation rate. It was also found that not only different values of subgap absorption are present in these materials but they also exhibit distinctly different dependences on photon energy as well as their sensitivity to bias illumination. The results obtained are clearly inconsistent with the conventional gap state distributions which include only the neutral dangling bond defect states. An improved gap state distribution, which includes not only the neutral defect states but also the charged defect states, is proposed and successfully used to selfconsistently analyze the results on different films. Using an improved subgap absorption model, "operational parameters" are obtained for distributions of gap states which are a better representation of that proposed recently by the defect pool and potential fluctuation models. The light induced changes in a-Si:H materials were also investigated and also found to be inconsistent if charged defects are not included. Several examples are presented for a-Si:H and a-Si:D materials on how the presence of charged defect states manifests itself in both the annealed and degraded states. Examples are also presented of how "operational parameters" can be obtained for these states only from the self-consistent analysis of such a wide range of detailed measurements. This characterization of the charged defects in a-Si:H, carried out for the first time in this work, has important consequences on improving the understanding of the Staebler-Wronski effect, its effect on solar cell performance as well as their realistic modeling .