Light-induced changes, adsorbate effects and surface photovoltage in a-Si : H

Surface photovoltage (SPV) and dark conductance, G, in amorphous silicon (a-Si : H) have been measured using pulsed light, after exposing the heat-dried samples to cycles of light exposure (Staebler-Wronski effect) and moist nitrogen. The changes in SPV and G upon exposure to moisture can be understood only if surface states are assumed to dominate SPV. Thus, the sign of SPV does not give the direction of band bending at the surface. Furthermore, the final value of the SPV after a cycle is found to depend strongly on the sequence of light exposure and moisture. In contrast, the final G is almost the same for the two sequences. This suggests that light-soaking changes surface states as well as the bulk. Results on lithium-doped a-Si : H are also presented and are found to be qualitatively similar.