Defect states in the intrinsic layer of amorphous silicon solar cells studied by the constant-photocurrent method

The constant-photocurrent method (CPM), which has been widely used for the study of the defect density in the gap of hydrogenated amorphous silicon (a-Si: H) films, is applied to study the defect density in the intrinsic layer of a-Si: H-based solar cells. The analysis of the CPM-derived absorption spectrum in p-i-n devices indicates that reliable measurements can be performed under short-circuit or reverse-bias conditions. In these conditions the CPM measurement is limited by the transport of holes and therefore is a good indicator of the quality of the intrinsic layer. Moreover, we show that CPM is sensitive to that part of the i layer in which the Fermi level is around midgap. Comparison of CPM spectra in films and cells is used to determine the conditions under which the measurements in the solar cell are relative to the properties of the intrinsic layer or sensitive to interface effects. Deconvolution of the CPM spectrum according to a standard model of the defect density distribution in a-Si: H yields similar values of the defect density and disorder parameter for films and devices deposited under the same conditions. A comparison of the kinetics of creation of metastable defects by light soaking in films and devices shows that metastability studies performed in films are relevant to devices.