LAUSR

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 bandgap limit their applications in perovskite solar cells (PSCs). We herein show that large grained all inorganic bismuth-based perovskites with a narrow bandgap can be obtained from the Lewis acid-base adduct reaction under ambient conditions. Thiourea (CH4N2S) is utilized as a Lewis base to interact with BiI3, confirmed with infrared spectra (IR). The strong coordination between thiourea and the Bi3+ center could slow down the perovskite crystallization and promote the prefered 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 extra thick grains, higher surface coverage, extended light absorption, higher crystallinity and similar air stablility as 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.