National Research Council of Canada. Security and Disruptive Technologies
The lead-halide perovskites, including CH₃NH₃PbBr₃, are components in cost effective, highly efficient photovoltaics, where the interactions of the molecular cations with the inorganic framework are suggested to influence the electronic and ferroelectric properties. CH₃NH₃PbBr₃ undergoes a series of structural transitions associated with orientational order of the CH₃NH₃ (methylammonium) molecular cation and tilting of the PbBr₃ host framework. We apply high-resolution neutron scattering to study the soft harmonic phonons associated with these transitions, and find a strong coupling between the PbBr₃ framework and the quasistatic CH₃NH₃ dynamics at low energy transfers. At higher energy transfers, we observe a PbBr₆ octahedra soft mode driving a transition at 150 K from bound molecular excitations at low temperatures to relatively fast relaxational excitations that extend up to ∼50–100 meV. We suggest that these temporally overdamped dynamics enables possible indirect band gap processes in these materials that are related to the enhanced photovoltaic properties.