Methylammonium Governs Structural and Optical Properties of Hybrid Lead Halide Perovskites through Dynamic Hydrogen Bonding

Methylammonium lead halide perovskites have emerged as ideal materials for photovoltaics and other optoelectronic applications. Their optimal properties stem from complex mechanisms at an atomic level. In these structures, methylammonium molecules undergo continuous temperature-induced reorientations. The underlying atomistic mechanism of this phenomenon and its effect on the material properties are unexplored in many aspects. Here, we address this issue through extensive first-principles and force-field calculations. We show that the interplay between thermal energy and the energetics of molecular orientations fully explains the orthorhombic-tetragonal-cubic phase transitions of the material. The close links among methylammonium orientations, electronic structures, and optical properties are identified. Hydrogen bonding between organic and inorganic sublattices is demonstrated to be the key player in these relationships. We validate our findings against temperature-dependent one-photon and two-photon photoluminescence spectra of CH3NH3PbBr3 and results from the literature. The gained insights can explain the diversity of spectral features observed in the experimental data.