Large-Grained All-Inorganic Bismuth-Based Perovskites with Narrow Band Gap via Lewis Acid-Base Adduct Approach

Bismuth halide perovskites have recently been considered a potential alternative to lead halide analogues due to their low toxicity and high stability. However, the layered flake structure and wide band gap limit their applications in perovskite solar cells (PSCs). We herein show that large-grained all-inorganic bismuth-based perovskites with a narrow band gap can be obtained from a Lewis acid-base adduct reaction under ambient conditions. Thiourea (CH₄N₂S) is utilized as a Lewis base to interact with BiI₃, confirmed with infrared (IR) spectra. The strong coordination between thiourea and the Bi³⁺ center could slow down the perovskite crystallization and promote the preferred orientation of the perovskite crystals with a hexagonal phase. The morphology of the perovskite films varies dramatically with an increase of molar ratio of BiI₃ and thiourea in the precursor. The perovskites derived from a BiI₃/thiourea ratio of 1:2 display extrathick grains, higher surface coverage, extended light absorption, higher crystallinity, and similar air stability compared to the pristine sample. The power conversion efficiency (PCE) of the thiourea-induced bismuth perovskite solar cells is significantly enhanced due to the higher surface coverage and the broader absorption of the perovskite film.