The fabrication of Cu(In, Ga)Se-2 (CIGS) thin film solar cells on flexible stainless steel (SS) foils or Na free substrates needs the impurity blocking barrier to prevent the diffusion of undesired elements from the substrate into the CIGS as well as the addition of alkali doping especially Na in the CIGS absorber layer. The amount Na in terms of the thicknesses of NaF was varied from 30 angstrom to 200 angstrom in order to study its contributions to the efficiency of the CIGS solar cells. The results show that the Na content in the CIGS films has a direct influence to the open-circuit voltage leading to the energy conversion efficiency and affects the distribution of Ga in the CIGS film. The influence of Na was studied and compared, based on the results of the performance of the solar cells, by using the NaF co-evaporation in various steps during the CIGS deposition process. The optimum thickness of NaF is approximately 50 angstrom to achieve the maximum efficiency of 15.8% without antireflection coating. In addition, the quantum efficiency (QE) indicated different absorption in the long wavelength regions depending upon the methods of Na addition. (C) 2013 Elsevier Ltd. All rights reserved.