All-inorganic CsPbI₃ perovskite has a near-ideal band gap, high thermal stability, and simple material composition, thus presenting a promising option for developing perovskite/Si tandem solar cells. However, CsPbI₃ undergoes a rapid phase transition under exposure to moisture and exhibits a significant performance gap relative to other perovskite compounds, particularly in the p-i-n structure favored for perovskite/Si tandems. Here, we demonstrate highly efficient and stable p-i-n-structured CsPbI₃ perovskite solar cells by surface engineering the CsPbI₃ layer with oxidized Ti₃C₂T_x MXene (OMXene) nanoplates via spray coatings. OMXene provides a physical barrier against moisture and improves charge separation at the perovskite-electron transporting layer interface via an enhanced electric field. Consequently, we demonstrated CsPbI₃/OMXene-based p-i-n devices with efficiencies of 19.69% for 0.096-cm² cells and 14.64% for 25-cm² minimodules. The encapsulated minimodule showed good stability, retaining ∼85% of the initial efficiency under simultaneous damp heat (85°C/85% relative humidity) and 1-sun light soaking for over 1,000 h.