Solar Cell Applications of Solution-Processed AgInGaSe₂ Thin Films and Improved Properties by Sodium Doping

Binary nanoparticle inks comprising Ag₂Se, In₂Se₃, and Ga₂Se₃ were fabricated via a wet ball-milling method and were further used to fabricate AgInGaSe₂ (AIGS) precursors by sequentially spraying the inks onto a Mo-coated substrate. AIGS precursors were annealed under a Se atmosphere for 1 h at 570 °C. Na₂Se thin layers of varying thicknesses (0, 5, 10, and 20 nm) were vacuum-evaporated onto the Mo layer prior to the AIGS precursors being fabricated to investigate the influence on AIGS solar cells. Sodium plays a critical role in improving the material properties and performance of AIGS thin-film solar cells. The grain size of the AIGS films was significantly improved by sodium doping. Secondary ion mass spectroscopy illustrated slight surficial sodium segregation and heavy sodium segregation at the AIGS/Mo interface. Double-graded band profiles were observed in the AIGS films. With the increase in Na₂Se thickness, the basic photovoltaic characteristics of the AIGS solar cells were significantly improved. The highest solar cell conversion efficiency of 6.6% (open-circuit voltage: 775.6 mV, short-circuit current: 15.5 mA/cm², fill factor: 54.9%, area: 0.2 cm²) was obtained when the Na₂Se thickness was 20 nm.