Hierarchical Defense and Self‐Healing Make Perovskite Single Crystals Impervious to Water

Despite their exceptional optoelectronic properties, organic‐inorganic halide perovskites have a fatal flaw of weak moisture instability. Here, we demonstrate that MAPbI 3 single crystals exhibit a unique combination of hierarchical defense to water and self‐healing capability, with their skeleton structure nearly unaffected by water. The crystals maintain structural integrity for over one month while floating on water and show a reversible black‐to‐yellow‐to‐black phase transition. Surface hydration reverses by natural drying, while bulk hydration can be completely recovered by solvent immersion. The surface microstructure enhanced hydrophobicity and the inherently water‐resistant (110) facet play key roles in blocking water ingress. Even upon water invasion, the crystal forms a hydrate that preserves the critical [PbI 6 ] 4− octahedral framework, providing the structural basis for subsequent dehydration and self‐healing. The devices with the self‐healed crystal have 98% efficiency of the devices with the original crystal, and they maintain 90% efficiency after 1300 and 2000 h in air, respectively. This work demonstrates the exceptional water tolerance of perovskite single crystals, promising long‐term stable optoelectronic devices.