This paper discusses the device physics of as-deposited and light-soaked hydrogenated amorphous silicon solar cells. The staebler-Wronski effect is the key issue in this paper. The metastability of a-Si: H solar cell is explained by a modified hydrogen collision model to explain the degradation of a-Si: H after some time light soaking. The key parameters are studied by experiments as well as computer simulation, both reach the agreement for our theory. Valence band tail model is used for our theory and simulation. For light soaked state, a defect is used for modeling.