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

CsPbI₃ 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 DMAPbI₃ 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⁺-[PbI₃] ^- 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 CsPbI₃ 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 cm² 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.