A conversion efficiency of 13.6% has been achieved in Cu(In,Ga)Se-2 (CIGS) thin film solar cell with absorber layer fabricated by sequentially RF sputtering (In,Ga)(2)Se-3 and CuSe targets and further annealing in Se vapor. The significant improvement, comparing with the efficiency of 10.8% for CIGS solar cell sputtering from a quaternary CIGS target, was attributed to smoother surface, better crystallinity, and more compact structure of the CIGS film. The reaction pathway of (In,Ga)(2)Se-3/CuSe bilayer was discussed, and such a bilayer design was demonstrated to be energetically favorable to form a better-crystallized CIGS film. (C) 2013 Elsevier B.V. All rights reserved.