The effect of rapid thermal annealing (RTA) on the compositional ratio of atoms and interfacial zone of Cu(In,Ga)Se-2 (CIGS) solar cells was systematically studied using X-ray photoelectron spectroscopy. The RTA treatment induced the diffusion of Al, Zn, and Se atoms, which increased the compositional ratio of these atoms in the corresponding layer. The ratio and characteristics of Cu/(In + Ga) and Ga/(In + Ga) were further modulated to attain higher efficiency in the devices. Furthermore, the width of the interfacial zone decreased at the Mo/CIGS interface because the diffusion depth of the Se atoms contracted from 1300 nm to 1200 nm. After RTA treatment, the absolute efficiency of the CIGS solar cells was enhanced by 1.2%. Our results suggest that RTA treatment reduces series resistance, optimizes the position of the minimum bandgap in the CIGS thin film, and improves the crystallinity of the CIGS thin film, which is responsible for the improvement in conversion efficiency. (C) 2012 Elsevier B. V. All rights reserved.