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 MAPbI3 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 [PbI6]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.