Limitations of the constant photocurrent method: a comprehensive experimental and modeling study

The common pratice of using the constant photocurrent method (CPM) to measure the subgap absorption coefficient of amorphous semiconductors is erroneous. Users of CPM assume a constant generation rate under the constant photocurrent. However, computer modeling of the subgap absorption coefficient of undoped hydrogenated amorphous silicon demonstrates that CPM underestimates the absorption coefficient and the density of midgap states. Electrons have a nonconstant lifetime in the 0.8-1.2 eV energy range. The analysis indicated that CPM is applicable to the study of doped amorphous silicon.