Deep levels in Cu(In(1-x)Ga(x))Se(2) (CIGS) are studied by transient photocapacitance (TPC) spectroscopy by varying the Ga concentration, x, from 0.38 to 0.7. The TPC spectra of CIGS thin-film solar cells at 140 K exhibited a defect level with an optical transition energy of about 0.8 eV. The spectrum shape in the subbandgap region is independent of the Ga concentration. Therefore, the optical transition energy to the defect level is almost constant with about 0.8 eV from the valence band. The TPC signals for defect level are quenched by increasing temperature. The activation energy of thermal quenching is estimated to be about 0.3 eV. The thermal and optical activation processes are explained using configuration coordinate diagram. (C) 2008 Elsevier B.V. All rights reserved.