Azetidinium Chloride‐Assisted Early Crystallization and Defect Passivation via Wet‐Film Treatment in CsPbI 3 Perovskite Solar Cells

CsPbI 3 perovskite solar cells (PSCs) offer high thermal stability and promising efficiency, making them strong candidates for next‐generation photovoltaics. However, the black‐phase CsPbI 3— responsible for these excellent optoelectronic properties—is metastable at room temperature and prone to phase degradation, which limits practical deployment. A widely adopted route involves the use of DMAPbI 3 as a processing intermediate, but its sluggish and incomplete DMA + removal hampers crystallization and induces defect‐prone microstructures. Here, we develop a wet‐film post‐treatment strategy using azetidinium chloride (AzCl) to accelerate black‐phase formation by promoting DMA + extraction in top‐down crystallization process. Unlike conventional surface treatment strategies, this method enables Az + to directly participate in the crystallization process—weakening DMA + –[PbI 3 ] − interactions and templating oriented growth from the surface. This dual role facilitates early black‐phase crystallization, improves crystallographic alignment, and suppresses defect formation, enhancing device performance and operational stability. As a result, the optimized CsPbI 3 PSCs deliver a power conversion efficiency of 21.82% and retain 94.7% of their initial efficiency after 500 h in ambient air without encapsulation. For larger‐area 1 cm 2 devices, the PCE remains high at 17.64%, while under low‐light conditions (2956 K, 1062 lux), the PCE reaches to 39.94%, demonstrating excellent weak‐light response.