Experimental Study of the Factors Governing the Staebler-Wronski Photodegradation Effect in a-Si:H Solar Cells: Final Subcontract Report

This report describes continuing studies on electroluminescence (EL), field profile, and hydrogen microstructure by the University of North Carolina, Chapel Hill, during the third year and the extension period. Based on systematic studies of the EL, we developed a complete model to explain the unique features of the EL as dispersive- transport-controlled, nongeminate recombination processes. Thismodel can explain the main features of the EL, not only in hydrogenated amorphous silicon (a-Si:H), but also in other types of trap-rich materials. By employing the forward current and EL temperature-dependence studies, information of both the localized tail states and the deep defect states in real solar-cell structures were obtained concomitantly, which is crucial for the device performance.We measured the internal electric field profile in p- i-n and n-i-p cells by a null-current method; we studied the structure of the hydrogen clusters in hot-wire a-Si:H films, both theoretically and experimentally, and show a clear evidence of improved structural order in hot-wire a-Si:H, which is an important factor leading to more stable materials. To link the film microstructure to themetastability, we also started the film stress measurements.