Chemi-Mechanically Peeling the Unstable Surface States of α-FAPbI 3 .

Surface states are one of the crucial factors determining the phase stability of formamidinium-based perovskites. Compared with other compositions, exclusive lattice strain in FAPbI 3 perovskite generates defects at the surface more readily, making them more vulnerable at the surface and easier to trigger the phase transition from α-phase to the non-perovskite δ-phase. In order to regulate the surface quality, here, a chemi-mechanical cleavage approach is reported, i.e., tape peel-zone (PZ), implemented by attaching and peeling off the ordinary Kapton Tapes. The PZ approach can simultaneously eliminate the surface defects of perovskite and siliconize the film surface with hydrophobic silicone compounds. These two functionalities endow α-FAPbI 3 perovskite with a robust hydrophobic surface, which can sustain for 30 days under a relative humidity of 60% and withstand the high temperature up to 240 °C. The unencapsulated PZ-treated cells show 80.3% of initial performance after 90 h of continuous operation in ambient air, which is 31.4 times more stable than the pristine cell.