Direct Observation of Dynamic Symmetry Breaking above Room Temperature in Methylammonium Lead Iodide Perovskite

TL;DR

This study reveals persistent anharmonic rotational instabilities of PbI_6 octahedra in methylammonium lead iodide perovskite at room temperature, providing insights into its exceptional photovoltaic properties.

Contribution

It provides direct experimental evidence of dynamic symmetry breaking in MAPI at room temperature, linking local structural disorder to its outstanding optoelectronic performance.

Findings

Persistent anharmonic rotational instabilities of PbI_6 octahedra at room temperature

Coupling between methylammonium orientations and lattice modes

Local non-centrosymmetric structure explains material properties

Abstract

Lead halide perovskites such as methylammonium lead triiodide (MAPI) have outstanding optical and electronic properties for photovoltaic applications, yet a full understanding of how this solution processable material works so well is currently missing. Previous research has revealed that MAPI possesses multiple forms of static disorder regardless of preparation method, which is surprising in light of its excellent performance. Using high energy resolution inelastic X-ray (HERIX) scattering, we measure phonon dispersions in MAPI and find direct evidence for another form of disorder in single crystals: large amplitude anharmonic zone-edge rotational instabilities of the PbI_6 octahedra that persist to room temperature and above, left over from structural phase transitions that take place tens to hundreds of degrees below. Phonon calculations show that the orientations of the methylammonium couple strongly and cooperatively to these modes. The result is a non-centrosymmetric, instantaneous local structure, which we observe in atomic pair distribution function (PDF) measurements. This local symmetry breaking is unobservable by Bragg diffraction, but can explain key material properties such as the structural phase sequence, ultra low thermal transport, and large minority charge carrier lifetimes despite moderate carrier mobility.