The fabrication of high efficiency Cu(In,Ga)Se₂ (CIGS) thin film solar cells on flexible metallic foils generally requires the deposition of an insulating or diffusion barrier layer to reduce the diffusion of contaminations from the metallic substrate into the CIGS absorber layer. In this work, the Al₂O₃ thin film is chosen as a diffusion barrier, due to its high density, strong ionic bond, coefficient of thermal expansion (CTE) closely matched with the stainless steel (SS type 430) substrate and its ability to block the impurities (Fe and other elements) from the SS into the CIGS absorber. The Al₂O₃ thin film is deposited on 3x3 cm² and 0.1 mm thick SS foils by the RF magnetron sputtering from a pure ceramic target. The thicknesses of the Al₂O₃ layer are varied in order to study its contribution to the efficiency of the CIGS solar cells. The Na enhancement is one of the most important factors affecting the structure and the quality of the CIGS absorber. The effect of Na is studied by using the NaF co-evaporation in the three-stage deposition process. The morphology and thickness of the films are investigated by atomic force microscopy and optical reflectance spectroscopy, respectively. The solar cells with Al/ZnO(Al)/CdS/CIGS/Mo structure are then fabricated on top of the Al₂O₃ layer. The results show that the thicknesses of the Al₂O₃ layer and Na content in the CIGS film have a direct influence to the energy conversion efficiency of the solar cells, determined from the current density – voltage (J-V) characteristics of the CIGS solar cells. The methods of Na addition in the CIGS are compared based on the results of the performance of the solar cells.