Green Sonochemical Synthesis and Thin‐Film Processing of Cs ₃ Bi ₂ I ₉ Nanocrystals

Lead halide perovskite nanocrystals (NCs) have attracted considerable attention for solar‐energy applications. However, solar cells integrating these materials raise concerns because of the well‐known toxicity of lead. As such, lead‐free bismuth halide perovskites have been explored as an alternative, although their reported performance remains well below that of lead‐based counterparts. A challenge in this area is the limited availability of non‐conventional chemical strategies for synthesizing nanocrystalline bismuth halide perovskites, particularly when compared with the well‐established methods for preparing lead halide perovskites and when considering implementation under green‐chemistry principles. In this work, we report the first sonochemistry‐assisted synthesis of Cs 3 Bi 2 I 9 perovskite‐like nanocrystals (NCs) using a vegetable oil as a green solvent. The optical, morphological, and structural properties of the resulting nanocrystals were then evaluated before processing into thin films. Although surface‐ligand exchange is crucial for processing perovskite NCs into thin films for solar cells, it has not yet been explored for lead‐free systems. Here, we investigate the thin‐film processing of Cs 3 Bi 2 I 9 NCs prepared in the green solvent and using different antisolvents/purification cycles prior solar‐cell fabrication.