The CPM (constant photocurrent method) is used for the study of the density of states in the forbidden gap of hydrogenated amorphous silicon (a-Si:H). The a-Si:H film obtained at substrate temperature 250℃ has the smallest defect density of $7.7×10^{16} cm^{-3}$. In the light-soaked state, a-Si:H based Solar cell shows an increased defect density and the degraded conversion efficiency. This relationship reveals that CPM is the useful method for studying degradation phenomena of a-Si:H based solar cell.
