Elastic properties of hybrid organic-inorganic perovskites by Brillouin microspectroscopy and density functional theory

Hybrid organic-inorganic perovskites (HOIPs) have shown remarkable photovoltaic properties, including long carrier lifetimes. Furthermore, HOIPs can be fabricated from solution at room temperature, which makes them very attractive for efficient renewable energy applications. Despite the great amount of attention HOIPs have received for solar cell applications, their intrinsic properties are not fully known. Here, we present a joint experimental-theoretical study on the elastic properties of HOIPs, which are relevant for understanding the material stability and charge transport. For the experimental part, we chose single crystals of methylammonium lead chloride CH₃NH₃PbCl₃, methylammonium lead bromide CH₃NH₃PbBr₃ and their mixture CH₃NH₃PbBrxCl_1-x and used Brillouin microspectroscopy for their mechanical characterization. We compare our experimental results to elastic constants calculated using density functional theory (DFT), in which we take dispersion-corrections into account. We find good quantitative agreement between the experimental and theoretical data: chloride crystals are the stiffest, bromide crystals the softest, and the mixed crystals are mid-range. Furthermore, we present data on the directional acoustic speeds and discuss elastic anisotropy.