| Hydrogenated amorphous silicon (a-Si:H), one of the least expensive solar cell materials, possess an excellent ecological balance sheet. However, the ecological and economic promise of a-Si:H solar cells is limited by the light induced degradation of its electronic properties known as the Staebler-Wronski Effect (SWE). Numerous models have been proposed to explain (and thus control) SWE, with most based on the breaking of weak silicon bonds to create neutral dangling bonds (D0). Evidence, however, has been found for the presence of other defects but such defects have not been characterized in detail. Thus, the predominant application of single defect models has limited the understanding of SWE and the ability to improve a-Si:H materials. This and the lack of correlation between the properties of a-Si:H thin films and corresponding solar cells have had a serious negative effect on the systematic improvement of a-Si:H solar cell performance. This thesis addresses several important issues regarding the properties of protocrystalline a-Si:H materials, the most stable a-Si:H, and their incorporation in cell structures. To improve the understanding of SWE several issues regarding the presence of multiple light induced defects are addressed with detailed studies on thin films. Utilizing AFM, TEM, and real time spectroscopic ellipsometry to track the phase of Si:H, the evolutionary nature (protocrystallinity) and substrate dependence of its growth were further characterized. This allowed highly controlled cell structures to be prepared in which the contributions of the carrier recombination from the p/i interface regions and the bulk could be identified and their effects on cell characteristics separated. For the first time direct correlations between the 1 sun light induced changes in films and solar cells were established in their kinetics at different temperatures. The presence of both 'fast' and 'slow' light induced defects were identified in these degradation kinetics from two clear regimes of degradation for both Fill Factors (FF) in cells and electron mobility-lifetime (mutau) products in thin films. Evidence was found in these studies that not only does the ratio of 'fast' to 'slow' defect states depend on the degradation temperature but also that the 'fast' defects are more negatively charged. A novel approach was developed in analyzing the subgap absorption spectra, alpha(E), extensively used to characterize the light induced defects in a-Si:H. (Abstract shortened by UMI.). |
