AbstractThe in situ formation of reduced dimensional perovskite layer via post‐synthesis ion exchange has been an effective way of passivating organic‐inorganic hybrid perovskites. In contrast, cesium ions in Cs‐based inorganic perovskite with strong ionic binding energy cannot exchange with those well‐known organic cations to form reduced dimensional perovskite. Herein, we demonstrate that tetrabutylammonium (TBA⁺) cation can intercalate into CsPbI₃ to effectively substitute the Cs cation and to form one‐dimensional (1D) TBAPbI₃ layer in the post‐synthesis TBAI treatment. Such TBA cation intercalation leads to in situ formation of TBAPbI₃ protective layer to heal defects at the surface of inorganic CsPbI₃ perovskite. The TBAPbI₃‐CsPbI₃ perovskite exhibited enhanced stability and lower defect density, and the corresponding perovskite solar cell devices achieved an improved efficiency up to 18.32 % compared to 15.85 % of the control one.