Local polar fluctuations in lead halide perovskite crystals

TL;DR

This study reveals that local polar fluctuations are intrinsic to lead-halide perovskites, driven by anharmonic motions, and are not solely caused by organic cations, as shown by Raman scattering and molecular dynamics simulations.

Contribution

It demonstrates that local polar fluctuations are inherent to lead-halide perovskites and are driven by specific atomic motions, independent of organic cation presence.

Findings

Strong central peak observed in Raman spectra indicates intrinsic polar fluctuations.

Molecular dynamics simulations identify Cs motion coupled with Br face expansion as the fluctuation mechanism.

Polar fluctuations occur on a few hundred femtoseconds timescale.

Abstract

Hybrid lead-halide perovskites have emerged as an excellent class of photovoltaic materials. Recent reports suggest that the organic molecular cation is responsible for local polar fluctuations that inhibit carrier recombination. We combine low frequency Raman scattering with first-principles molecular dynamics (MD) to study the fundamental nature of these local polar fluctuations. Our observations of a strong central peak in both hybrid (CH$_3$NH$_3$PbBr$_3$) and all-inorganic (CsPbBr$_3$) lead-halide perovskites show that anharmonic, local polar fluctuations are intrinsic to the general lead-halide perovskite structure, and not unique to the dipolar organic cation. MD simulations show that head-to-head Cs motion coupled to Br face expansion, on a few hundred femtosecond time scale, drives the local polar fluctuations in CsPbBr$_3$.