Effects of Alkali and Oxidation Treatments on Efficiency and Stability of CdS/CIGS Solar Cells

Three types of CIGS devices with varying treatments of RbF post-deposition treatment (PDT) and sodium are subjected to accelerated stress test (AST) conditions at elevated temperature (65 °C), voltage bias (short- vs open-circuit), and illumination (AM1.5 vs dark). RbF treatment with reduced CdS thickness shows an improvement in Voc and efficiency in this sample series, and also results in high FF and doping ~ 1016cm−3. Cells with reduced sodium show a decrease in Voc and infrared QE results suggest a higher optical minimum bandgap. Heat and light soaking experiments at 50, 65, and 75 °C, open and short-circuit junction bias under AM1.5G illumination suggest stabilization of CIGS solar cells with addition of Na and RbF. SCAPS-1D simulations suggest reduction in thermally ionized defect density in ordered vacancy compound (OVC) and changes in conduction band offset with RbF post-deposition treatment. Further, simulations show an increase in shallow acceptor and donor density after open-circuit and short-circuit AST respectively. Preliminary light soaking results for oxidized CIGS with and without Na are also discussed.