Role of the A-site cation in determining the properties of the hybrid perovskite CH3NH3PbBr3

The presence of a molecule at the A site of an organic perovskite leads to unusual behavior compared to its inorganic counterpart. Considering the case of CH3NH3, we find that it is both the size of the molecule as well as its orientation in the cage formed by the Pb and Br atoms which determine the favored structure. At the microscopic level, the basic energetics which come into play are steric effects as well as hydrogen bonding. While the molecule is asymmetrically placed in the cuboctahedral cavity, a mapping of the ab initio band structure to a tight-binding model reveals that the movement of the amine end of the molecule towards the Br atoms is driven primarily by electrostatic considerations. While the hydrogen bonding is responsible for driving the octahedral tilts, the energy lowering considerations do not follow a simple prescription of minimizing H-Br bond lengths. The presence of several competing energetics results in a complex low-energy landscape with deep valleys and high barriers between them which could explain the glassy dynamics seen even at low temperatures in the orthorhombic structure where the dipoles are believed to be frozen.