Subgap modulated photocurrent spectroscopy and its application to the study of the solar cell absorber defect distributions

Abstract

In this paper, a theoretical background of subgap modulated photocurrent experiment is presented. It allows the investigation of the density of states ͑DOS͒ distribution, directly from the active region of a semiconductor heterojunction device. The junction is illuminated with a modulated subgap light excitation ͑i.e., light with photon energy lower than the band gap of the active layer͒. Under specific considerations for the applied reverse bias voltage and the bias-light level, a simple theoretical relation of the imaginary part of the photocurrent versus the modulation angular frequency allows the determination of the energy profile of the gap states. This technique has been successfully applied to a Ga free Cu͑In, Ga͒Se 2 based solar cell to investigate the DOS distribution in the band gap of the absorber. Two distinct defect distributions have been exhibited in the absorber layer of the studied solar cell.