Enhancing efficiency of Cu(In, Ga)Se2 solar cells on flexible stainless steel foils using NaF co-evaporation

•Ferritic SS foils type 430 is more suitable than SS series 300 for CIGS solar cells.•Co-evaporation of NaF can reduce the total fabrication time.•Co-evaporation of NaF also induced Ga-double-grading composition.•CIGS solar cells on SS foils type 430 have comparable efficiency to those on SLG.•QE in the long wavelength regions depends upon the methods of Na addition.

The fabrication of Cu(In, Ga)Se2 (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 Å to 200 Å 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 Å 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.