Robust Protection via In Situ Formation of Lead Sulfate and Defect Suppression for Stable Air‐Fabricated CsPbl ₃ Solar Cells

Inorganic CsPbI 3 perovskites show compelling advantages for application in photovoltaics due to their exceptional optoelectronic properties and solution processability. However, the poor phase stability limits their further applications, especially under moisture atmosphere. Here, a surface engineering strategy is developed by using sodium ethylxanthate (NaEX) to effectively improve the phase stability of air‐fabricated CsPbI 3 . Apart from crystallization regulation and defect passivation via the coordination of S atoms with Pb 2+ ions, a solid water‐insoluble PbSO 4 layer is in situ formed to block the moisture ingress. These synergistically promote the phase stability of CsPbI 3 films and boost the device power conversion efficiency (PCE) to 21.13%, representing one of the highest values in inverted inorganic PSCs, even though fabricated in ambient air. Additionally, the device stability is also improved with >95% PCE retention after 3,800 h in N 2 . This work provides an effective route for enhancing the phase stability of all‐inorganic perovskites for photovoltaic applications.