Wide Band Gap Solar Cells with High Stabilized Performance: Final Subcontract Report, 15 July - 15 March 1998

This report presents highlights of research performed by the Pennsylvania State University Center for Thin Film Devices under this subcontract. Significant progress was made in obtaining new insights into the nature of light-induced defects in different a-Si:H materials. This enabled, for the first time, direct, quantitative correlation to be made between the properties of intrinsic materials andtheir corresponding p-i-n solar cells and, in particular, their degraded states in the diluted materials and cells which are reached in less than 100 hours of AM1.5 illumination. Researchers also demonstrated that although the annealed state a-Si1-xCx:H i-layers prepared with H2-dilution of source gases (i.e., SiH4+CH4) exhibit similar photoconductive properties and densities of gap states tothose of H2-diluted materials, the gains made through H2-dilution are largely lost in the degraded state. In the optimization of p-layers, they found that a-Si1-xCx:H p-layers deposited using a trimethylboron gas source under their standard conditions for p-i-n solar cells are structurally unstable and release H when held at the substrate temperature of 200 deg. C. Detailed studies were alsocarried out on the effects that the p/i interfaces in different customized cells have on various solar cell characteristics. In the area of solar cell optimization for p- and i-layer performance, researchers developed a multilayer analysis that uses a 3-layer model for each of the components of the cell, including an interface roughness layer. They also undertook a new approach to devicemodeling in which the modeling of simple solar cell structures is carried out first to establish material parameters that are self-consistent with those derived in detailed studies on intrinsic material thin films.