Oxygen‐Assisted Chemical Reaction Enables Buried Interface Optimization for Efficient CsPbI₃ Perovskite Solar Cells

All‐inorganic CsPbI 3 perovskite solar cells (PSCs) have attracted dramatic attention in both single‐junction and tandem solar cells due to their exceptional thermal stability and superior optoelectronic properties. However, the unstable phase structure, especially under moisture, and the undesirable interfacial charge transfer severely hinder their further development in photovoltaic applications. To simultaneously address these issues, 2‐(1‐cyclohexenyl)ethylamine (CHEA) is employed to modify the buried interface of perovskites, which drastically improves the photovoltaic performance of the inverted CsPbI 3 PSCs fabricated in ambient air. Oxygen‐assisted chemical reaction together with the subsequent intermolecular condensation is found during annealing to form multiple functional groups, including C = O and N–H, which establish a robust chemical bridge between the hole transport layer PEDOT:PSS and perovskites. This not only optimizes the structural and electronic properties of PEDOT:PSS but also facilitates the accelerated and less‐defective growth of the upper CsPbI 3 perovskite films, significantly mitigating the unfavorable effects of ambient moisture. Furthermore, the optimized energy level alignment remarkably reduces the interfacial energy offset, favoring the reduction of the interfacial charge recombination. Consequently, the inverted CsPbI 3 PSCs achieve a remarkably enhanced efficiency of 21.19% along with an enhanced stability with over 98% initial efficiency retained after 600‐hour operation.