Evolution of structural and optoelectronic properties in fluorine–aluminum co-doped zinc oxide (FAZO) thin films and their application in CZTSSe thin-film solar cells

Abstract

The window layer is an important layer that transmits sunlight and generates electric current as the carriers move. Aluminum (Al) -doped zinc oxide (AZO) has been widely applied as the window layer in thin-film solar cells (TFSCs) owing to its favorable optoelectronic properties, as its properties can be controlled and further improved with suitable dopants. In this study, we strategically co-dope Al and fluorine (F) in ZnO thin films (FAZO) to improve the transmittance with controlled carrier concentration. To study the influence of F dopant on the structural, optical, and electrical properties of FAZO thin films, the F doping concentration is varied (0–1.5%) while keeping the Al content fixed (2%). With increasing F doping concentration, the carrier concentration, transmittance, and bandgap values increase, whereas, the sheet resistance and resistivity of the single-layer FAZO thin films decrease, compared to the reference AZO film. The addition of a small amount of F alters the surface roughness with increasing concentration. The properties of the single-layer FAZO thin films show deteriorated properties at the highest doping concentration (1.5%). Under the optimum condition of 1% F, FAZO thin films exhibit the lowest sheet resistance of 5.84 Ω/sq, in addition to the highest transmittance of 89.3% in the visible light region. Furthermore, the FAZO thin films applied in Cu2ZnSn(S,Se)4 (CZTSSe) device shows significant enhancements in the short-circuit current density (Jsc) and fill factor (FF), resulting in improved device performance from ~8 to 9.57%.