Investigation of deep-level defects in Cu(In,Ga)Se₂ thin films by a steady-state photocapacitance method

The properties of defect levels located 0.8 eV above the valence band in Cu(In_1-x,Ga_x)Se₂ thin films were investigated by a steady-state photocapacitance method. When illuminated by light with a photon energy of 0.8 eV at 60 K, a fast increase, followed by a slow increase, was observed in the photocapacitance transients of all samples. Upon being re-exposed, samples with a low bandgap energy showed a slow decrease in photocapacitance transients. These observations were interpreted using a configuration coordinate model assuming two states for the 0.8 eV defect: a stable state D and its metastable state D∗ with a large lattice relaxation. The difference in the evolution mechanisms of the photocapacitance transients was attributed to the difference in the optical transition of carriers between the two states of the 0.8 eV defect and the valence and conduction bands.