The effect of Se/(Ga+In) ratio, expressed as Se/III, on the electrical, optical and structural properties of Cu(In,Ga)Se-2, abbreviated as CIGS, thin film and on the solar cell performance was studied. CIGS films with various Se/III ratios were grown by the molecular beam epitaxy (MBE) process. With decreasing Se/III ratio, the resistivity of CIGS film was found to be increased and majority carrier hole concentration was decreased. This is probably due to the formation of Se-vacancy and/or vacancy complex. Again, the band-gap energy of CIGS film increased and the Cu content in the bulk of the film decreased with decreasing Se/III ratio despite the Ga/III ratio in the film remained almost unchanged. These results may suggest the formation of anti-site defects such as In-Cu in CIGS. Solar cell performance degraded with decreasing Se/III ratio in CIGS absorber layer and the cause of the degradation was discussed. (c) 2008 Elsevier B.V. All rights reserved.