Stable lead-free bismuth perovskite solar cells via thermally activated bilayer conversion

Lead-free bismuth-based metal halide perovskites are promising photovoltaic candidates due to their low toxicity, favorable bandgaps and intrinsic air stability. Scalable, reproducible fabrication remains challenging. We study post-deposition annealing of methylammonium bismuth iodide (MBI) bilayer films prepared by low-temperature thermal vapor deposition (LTTV), where BiI 3 and MAI are sequentially evaporated at substrate temperatures as low as 40 ∘ C under moderate vacuum in ambient air. Unlike solution or high-vacuum co-evaporation routes, LTTV provides a practical dry pathway to crystalline, converted MBI absorbers. Annealing above 200 ∘ C drives thermally activated bilayer conversion to a monoclinic C 2 / c MBI phase with strong (111) texture and micrometer-scale grain growth, yielding power conversion efficiencies up to 0.53%, fill factor 0.63 and short-circuit current density 1.68 mA cm −2 . Forward and reverse J–V scans are statistically indistinguishable across the device population, indicating hysteresis-free operation attributed to limited ionic disorder from the dry conversion pathway. Non-encapsulated devices retain stable photovoltaic performance during extended ambient dark storage, confirming the ambient storage stability of LTTV-deposited MBI absorbers. These results demonstrate LTTV as a scalable route to stable bismuth perovskite absorbers for solar energy applications.