Regulated Grain Growth for Lead-Free Cs ₃ Bi ₂ I ₉ and Self-Powered Photodetectors

High crystallinity and low trap density are fundamental prerequisites for perovskite materials to achieve high performance in optoelectronic applications. Compared with their well-developed lead (Pb)-based counterparts, bismuth (Bi)-based perovskites exhibit inferior performance, primarily due to poor crystallization and high defect concentrations in polycrystalline films. The fabrication of smooth, compact Cs 3 Bi 2 I 9 films with distinctly visible large-sized grains remains a significant challenge. Herein, we prepared high-quality Cs 3 Bi 2 I 9 films with large grains by promoting ripening growth of small grains via a modified antisolvent strategy. The resulting Cs 3 Bi 2 I 9 films exhibited enhanced crystallinity and absorption, along with reduced defect density and suppressed nonradiative recombination, which are conducive to the fabrication of high-performance photodetectors. Self-powered photodetectors assembled from these Cs 3 Bi 2 I 9 films demonstrated an on/off ratio of up to 1.2 × 10 5, a responsivity (R) of 0.44 mA/W, and a specific detectivity (D*) of 4.12 × 10 10 Jones under 1 sun illumination. This work paves the way for the preparation of high-quality Bibased perovskite films and highlights their potential in optoelectronic applications.