Investigation of deep-level defects in CuGaSe₂ thin-film solar cells using photocapacitance methods

Properties of deep-level defects in CuGaSe₂ thin-film solar cells were investigated using photocapacitance methods. By measuring the transient photocapacitance spectra, a deep-level defect centered at around 0.8 eV above the valence band and a defect band located around 1.54 eV above the valence band were determined. A configuration coordinate model was used to explain the thermal quenching effect of the two defects. By measuring the steady-state photocapacitance, a fast increase, followed by a slow increase, was observed in the photocapacitance transient when the sample was illuminated by light with a photon energy of 0.8 eV at low temperature. Upon re-exposure by sub-bandgap light, an extra slow decrease in photocapacitance transient was observed. These observations were interpreted using a configuration coordinate model assuming two states for the 0.8 eV defect: a stable state D and a metastable state D∗ with a large lattice relaxation. The variation of the photocapacitance transients was attributed to the different optical transition processes of carriers between the two states of the 0.8 eV defect and the valence and conduction bands.