| Summary: | Summary: Dissociation of organic molecules is one critical factor leading to degradation of perovskite solar cells. The much-improved thermal stability of formamidinium (FA) can significantly reduce molecular dissociation; however, FA-based perovskite suffers from high defect density, which affects efficiency and long-term stability. In this work, based on a precise ion-exchange reaction at the grain surface, we fabricate a Cs0.15FA0.85PbI3/CsxFA1-xPbI3 heterostructure with a Cs-rich CsxFA1-xPbI3 quasi-shell structure. The Cs-rich inorganic structure on the perovskite surface increases defect formation energy and reduces defect density, leading to a power conversion efficiency of 20.7%. The encapsulated device maintains 95% of its initial efficiency after 1,000 h of continuous operation, corresponding to a calculated lifetime approaching 2 years. The device can operate at 60°C for 250 h and at 85°C for over 3,000 min, which is one of the best operational stabilities for CsxFA1-xPbI3-based devices.
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