Bismuth-Iodide-Assisted Crystallization and Stability in Cs-Containing and Cs-Free Hybrid Perovskites

This study investigates the structural, morphological, and chemical effects of bismuth(III) iodide (BiI3) incorporation on perovskite films containing Cs+, FA+, and MA+ cations. Cs+-containing (CsFAMA) and Cs+-free (FAMA) structures were comparatively evaluated at 1, 3, and 5 wt % BiI3. X-ray diffraction (XRD) revealed that low doping levels enhanced phase purity, while higher concentrations induced morphological degradation, as confirmed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The incorporation of BiI3 was confirmed by energy-dispersive X-ray spectroscopy (EDX), while X-ray photoelectron spectroscopy (XPS) analyses further verified the chemical states of Bi and Pb. In the FAMA structure, the Bi0 signals indicated reduction tendencies, whereas the presence of Bi3+ suggested partial incorporation through doping. BiI3 suppressed the Pb0 states, improving oxidative stability and structural integrity. XRD analysis of the ∼2θ 12.2° peak showed that without BiI3, intermediate phases formed at low temperatures, whereas BiI3 altered crystallization kinetics, stabilizing this phase at higher temperatures. Cs+ addition further suppressed PbI2 and nonphotoactive δ-FAPbI3 phases, enhancing thermal stability. Two-week aging tests revealed that CsFAMA films with BiI3 maintained their crystal phase longer and exhibited reduced PbI2 formation. These results demonstrate that BiI3 provides both chemical and structural modification, with Cs+ significantly influencing its impact. This work highlights the synergistic role of Cs+ and Bi in stabilizing perovskites, offering new insights for durable perovskite solar cells.