Effects of bismuth-doping on the properties of Cu(In, Al)Se2 thin films prepared by selenization of sputtered stacked precursors

The influence of bismuth (Bi) doping on the properties of Cu(In, Al)Se2 thin films by pre-depositing a thin layer of bismuth has been investigated systematically via the sputtering approach followed by a selenization treatment. It is demonstrated that the crystallinity of CIAS thin films has been affected by various content of Bi doping with no impurity phases presented by XRD patterns and Raman spectra. Moreover, the grain size in the obtained films has been significantly increased by the incorporation of Bi with less grain boundaries, which can be attributed to the low melting point of Bi, a fluxing agent during the grain growth process. In addition, all the thin films with various content of Bi doping have estimated band gaps around 0.95 eV, revealing that Bi doping has a faint effect on the optical properties in CIAS structure.