Abstract The two‐step sequential deposition method exhibits favorable operability for processing perovskite films. Due to the growth of the perovskite films largely depends on the pre‐deposited PbI 2 films, the porous and rough PbI 2 films are expected to facilitate the penetration of the organic amine salts. However, in air conditions, the porous and rough PbI 2 films also facilitate the penetration of the water molecules, thus leading to diminished crystallinity. Despite substantial efforts aimed at inhibiting the decomposition of perovskite films, the performance of air‐processed perovskite solar cells (PSCs) remains unsatisfactory. Herein, the study presents that the PbI 2 films with high exposure of layered planes and vanished of non‐layered planes have more effective to resist water erosion and promote the crystallization process of perovskite films. An amphoteric Lewis acid‐base molecule (cephalothin, a type of antibiotic) is added in PbI 2 precursor solution to induce this highly layer‐oriented PbI 2 film. Consequently, the perovskite films can be processed under humidity condition and yield the champion PSC with an outstanding power conversion efficiency of 24.45%. In addition, the unencapsulated devices maintain 80% of their initial power conversion efficiency after 1000 h storage in air and exhibit high thermal stability after 100 cycles at 25–70 °C.
