Exploring the cation dynamics in lead-bromide hybrid perovskites

Density functional theory including a many-body treatment of dispersive forces is used to describe the interplay between structure and electronic properties of two prototypical Br-based hybrid perovskites, namely, CH3NH3PbBr3 and HC(NH2)2PbBr3. We find that, like for some of their iodine-based counterparts, the molecules' orientation plays a crucial role in determining the shape of both the conduction and valence bands around the band edges. This is mostly evident in the case of CH3NH3PbBr3, which is a direct band-gap semiconductor when the CH3NH3 group is oriented along the (111) direction but turns indirect when the orientation is (100). We have constructed a simple dipole model, with parameters all evaluated from ab initio calculations, to describe the molecules' depolarization dynamics. We find that, once the molecules are initially orientated along a given high-symmetry direction, their room-temperature depolarization depends on the specific material investigated. In particular we find that the ratio between the polarization decay constant of CH3NH3PbBr3 and that of HC(NH2)2PbBr3 is about 2 at room temperature. With these results at hand we suggest a simple luminescence decay experiment to prove our findings and establish a correlation between optical activity and the molecules' dynamics in these materials.