Interaction of Organic Cation with Water Molecule in Perovskite MAPbI₃: From Dynamic Orientational Disorder to Hydrogen Bonding

Microscopic understanding of interaction between H₂O and MAPbI₃ (CH₃NH₃PbI₃) is essential to further improve efficiency and stability of perovskite solar cells. A complete picture of perovskite from initial physical uptake of water molecules to final chemical transition to its monohydrate MAPbI₃·H₂O is obtained with in situ infrared spectroscopy, mass monitoring, and X-ray diffraction. Despite strong affinity of MA to water, MAPbI₃ absorbs almost no water from ambient air. Water molecules penetrate the perovskite lattice and share the space with MA up to one H₂O per MA at high-humidity levels. However, the interaction between MA and H₂O through hydrogen bonding is not established until the phase transition to monohydrate where H₂O and MA are locked to each other. This lack of interaction in water-infiltrated perovskite is a result of dynamic orientational disorder imposed by tetragonal lattice symmetry. The apparent inertness of H₂O along with high stability of perovskite in an ambient environment provides a solid foundation for its long-term application in solar cells and optoelectronic devices.