Hydrogen-Bonding Evolution during the Polymorphic Transformations in CH₃NH₃PbBr₃: Experiment and Theory

Hydrogen bonding exists in all hybrid organic-inorganic lead halide perovskites MAPbX₃ (X = Cl, Br, or I). It has a strong influence on the structure, stability, and electronic and optical properties of this perovskite family. The hydrogen-bonding state between the H atoms of the methylammonium (MA) cation and the halide ions is resolved by combining ab initio calculations with temperature-dependent Raman scattering and powder X-ray diffraction measurements on MAPbBr₃ hybrid perovskites. When the compounds are cooled, the H-bonding in the NH₃ end of the MA group shows sequential changes while the H atoms in the CH₃ end form H bonds with only the Br ions in the orthorhombic phase, leading to a decrease in the degree of rotational freedom of MA and a narrowing for MA Raman modes. Hydrogen bonding drives the evolution of temperature-dependent rotations of the MA cation and the concomitant tilting of PbX₆ octahedra with the consequent dynamical change in the electronic band structures, from indirect bandgap to direct bandgap along with ∼60-fold PL emission enhancement upon cooling. We experimentally and theoretically reveal the evolution of hydrogen bonding during polymorphic transformations and how the different types of hydrogen bonding lead to specific optoelectronic properties and device applications of hybrid perovskites.