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

ABSTRACT 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.