Effect of Na doping using Na2S on the structure and photovoltaic properties of CIGS solar cells

Na doping in CIGS film is one of the most crucial factors that affects the performance of CIGS solar cells. However, the role of Na doping is not yet well understood. In this study, in order to find an effective Na doping method, Na was doped into CIGS film using Na2S precursor at three different points in the three-stage co-evaporation and its effect on CIGS film was investigated. The microstructures, Na doping profiles and bonding states were examined using scanning electron microscope, x-ray diffraction, secondary ion mass spectroscopy and x-ray photoelectron spectroscopy. It was found that the external sodium doping with Na2S changed the microstructure and the Na doping profile. The performance of a CIGS solar cell with the external Na doping on Mo/Corning glass was worse than that of a conventional CIGS solar cell fabrication. The best performance achieved by means of Na external doping was 14.2% efficiency, which was achieved when Na was doped by depositing a Na2S precursor layer on a Mo metal layer.