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 Na2Se by a multi-step process onto SiO2/soda-lime glass substrates at elevated temperatures. The effects of sodium on Blm properties were investigated using X-ray diffraction, X-ray photoelectron spectroscopy and Hall effect measurements. By the addition of Na2Se, the hole concentration of the CIGS-based films (Ga/(In+Ga) ratio=0.37-0.39) increased dramatically to the 10(16)-10(17) cm(-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.