In Situ Forming of a 2D Inorganic Perovskite Capping\nLayer by Surface Reconstruction for Efficient and Stable CsPbI<sub>2</sub>Br Perovskite Solar Cells

Although inorganic perovskite solar\ncells (PSCs) have made remarkable\nprogress, ambient instability and serious nonradiative recombination\nloss greatly impede their further development. Herein, we develop\na novel surface reconstruction process to in situ grow a 2D inorganic\nperovskite capping layer on a 3D CsPbI<sub>2</sub>Br perovskite surface\nvia the dynamic methanol treatment and subsequent thermal annealing\nfor simultaneously enhancing the stability and suppressing nonradiative\nrecombination of inorganic CsPbI<sub>2</sub>Br PSCs. The dynamic methanol\ntreatment removes the surface defective regions of CsPbI<sub>2</sub>Br perovskite and results in forming excessive PbI<sub>2</sub> on\nthe CsPbI<sub>2</sub>Br perovskite surface, and the subsequent thermal\nannealing triggers the surface reconstruction reaction of excessive\nPbI<sub>2</sub> with CsPbI<sub>2</sub>Br that leads to in situ forming\nof a 2D CsPb<sub>2</sub>I<sub>4</sub>Br layer on the CsPbI<sub>2</sub>Br perovskite surface, which effectively decreases the defect density\nand enhances the stability of CsPbI<sub>2</sub>Br perovskite. As a\nresult, the fabricated carbon-based CsPbI<sub>2</sub>Br PSC displays\na power conversion efficiency of 14.29%. Moreover, the CsPbI<sub>2</sub>Br device with a 2D CsPb<sub>2</sub>I<sub>4</sub>Br layer displays\nsuperior stability, and the efficiency of the cell without encapsulation\nremains at over 90% of the original value after storing in ambient\nconditions for 900 h.