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 similar 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.
|Number of pages||5|
|Journal||Physical review B: Condensed matter and materials physics|
|Publication status||Published - 8 Sep 2015|
- ORGANIC-INORGANIC PEROVSKITES
- METHYLAMMONIUM LEAD IODIDE