From soft harmonic phonons to fast relaxational dynamics in CH$_{3}$NH$_{3}$PbBr$_{3}$

TL;DR

This study investigates the phonon and relaxational dynamics in CH₃NH₃PbBr₃ using neutron scattering, revealing strong coupling between framework and molecular motions that may influence its photovoltaic efficiency.

Contribution

It provides new insights into the coupling of phonons and relaxational dynamics in CH₃NH₃PbBr₃, linking microscopic dynamics to photovoltaic properties.

Findings

Identification of soft harmonic phonons associated with structural transitions.

Observation of a transition from molecular excitations to relaxational dynamics at 150 K.

Evidence of dynamics that could facilitate indirect band gap processes.

Abstract

The lead-halide perovskites, including CH$_{3}$NH$_{3}$PbBr$_{3}$, 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. CH$_{3}$NH$_{3}$PbBr$_{3}$ undergoes a series of structural transitions associated with orientational order of the CH$_{3}$NH$_{3}$ (MA) molecular cation and tilting of the PbBr$_{3}$ 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 PbBr$_{3}$ framework and the quasistatic CH$_{3}$NH$_{3}$ dynamics at low energy transfers. At higher energy transfers, we observe a PbBr$_{6}$ 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 $\sim$ 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.