Effects of Sodium on Cu(In, Ga)Se 2 -Based Thin Films and Solar Cells

Cu(In, Ga)Se 2 (CIGS) films were deposited with intentionally incorporated Na 2 Se by a multi-step process onto SiO x /soda-lime glass substrates at elevated temperatures. The effects of sodium on film properties were investigated using X-ray diffraction, X-ray photoelectron spectroscopy and Hall effect measurements. By the addition of Na 2 Se, the hole concentration of the CIGS-based films (Ga/(In+Ga) ratio=0.37–0.39) increased dramatically to the 10 16 –10 17 c m -3 range for a wide range of Cu/(In+Ga) ratios from 0.4 to 0.8. The increased hole concentration resulted in improved CIGS-based solar cells with efficiencies of 10–13.5% over an extremely wide Cu/(In+Ga) ratio range of 0.51–0.96. P-type Cu(In, Ga) 3 Se 5 phase films with hole concentrations high enough to be used as absorber layers of photovoltaic devices were obtained for the first time by the Na control technique. The possibility of a new type of solar cell with a ZnO:Al/buffer/ p-Cu(In, Ga) 3 Se 5 heterojunction structure is suggested.