Cu(In,Ga)S-2 (CIGS) thin film was synthesized on ITO glass substrate via electrodeposition of Cu-Ga-S precursor layer followed by thermal annealing treatment. Our results show that annealing temperature played an important role on the formation of CIGS crystallites. The pure quaternary chalcopyrite CIGS crystal phase in good crystallization with an uniform and compact surface morphology was reproducibly achieved after sintering at 400 degrees C. The metallic In atom diffused from the ITO substrate was found to incorporate to the Cu-Ga-S precursor film and allow the conversion of the quaternary chalcopyrite structure. Several characterization methods including X-ray diffraction (XRD), scanning electron microscope (SEM), energy diffraction spectrum (EDS) and high-resolution transmission electron microscopy (HRTEM) certified the incorporation of In. A possible growth mechanism for explaining the formation of CIGS thin films is proposed and briefly discussed. Completed CIGS solar cell device achieved a 5.75% total area power conversion efficiency under a simulated AM 1.5 illumination. (C) 2014 The Electrochemical Society. All rights reserved.