The effects of sodium on off-stoichiometric Cu(In,Ga)Se-2 (CIGS)-based thin films and solar cells were investigated. The CIGS-based films were deposited with intentionally incorporated Na2Se on Mo-coated SiOx/soda-lime glass substrates by a multi-step process. By sodium control technique high-efficiency ZnO : Al/CdS/CIGS solar cells with efficiencies of 10-13.5% range were obtained over an extremely wide Cu/(In + Ga) ratio range of 0.51-0.96, which has great merit for the large-area manufacturing process. The improved efficiency in the off-stoichiometric regions is mainly attributed to the increased acceptor concentration and the formation of the Cu(In,Ga)(3)Se-5 phase films with p-type conductivity.A new type of solar cell with p-type Cu(In,Ga)(3)Se-5 phase absorber materials is also suggested.