Characterization of Bi-doped FAPbI perovskite films investigated by X-ray absorption spectroscopy.

A thorough investigation of perovskite structures formed through doping is essential for advancing the efficiency and stability of perovskite solar cells. In this study, Bi-doped FAPbI perovskite films with varying Bi concentrations (0.5-2%) were fabricated using a spin-coating technique on ITO glass substrates. Then the films' phase structure, local structure, and optical characteristics were analyzed. X-ray diffraction (XRD) analysis revealed that the pristine FAPbI film exhibited both hexagonal and cubic phases, indicating structural instability. In contrast, Bi-doped FAPbI films predominantly displayed a cubic perovskite structure, with a notable reduction in the XRD peak intensity corresponding to the hexagonal phase. UV-Vis spectroscopy showed that the undoped FAPbI film had an absorption edge in the visible-near infrared range, while Bi-doping caused a redshift, indicating a reduction in the optical band gap. The calculated results show that optical band gaps decrease with increasing Bi, from a value of 1.49 (pure) to 1.43 (2% Bi) eV. X-ray absorption near edge structure (XANES) analysis confirmed the oxidation states of Pb and Bi ions across all samples, with Bi ions replacing Pb in the local structure. Photoluminescence (PL) measurements revealed an increased PL intensity with 1% Bi doping (7 × 10) compared with pristine FAPbI (4.7 × 10), suggesting a reduction in carrier recombination. These findings demonstrate the potential of Bi-doping to stabilize perovskite structures with improved optoelectronic properties.