Anharmonic Fluctuations Govern the Band Gap of Halide Perovskites

TL;DR

This study shows that anharmonic thermal vibrations significantly influence the band gap of halide perovskites, explaining temperature-dependent electronic properties beyond harmonic phonon models.

Contribution

It provides the first detailed analysis linking anharmonic vibrations to band gap variations in halide perovskites using first-principles simulations and experimental evidence.

Findings

Anharmonic fluctuations cause a band gap change of up to 450 meV at 425 K.

Harmonic phonon models cannot explain the temperature dependence of the band gap.

Experimental data supports the importance of anharmonic effects in electronic structure.

Abstract

We determine the impact of anharmonic thermal vibrations on the fundamental band gap of CsPbBr$_3$, a prototypical model system for the broader class of halide perovskite semiconductors. Through first-principles molecular dynamics and stochastic calculations, we find that anharmonic fluctuations are a key effect in the electronic structure of these materials. We present experimental and theoretical evidence that important characteristics, such as a mildly changing band-gap value across a temperature range that includes phase-transitions, cannot be explained by harmonic phonons thermally perturbing an average crystal structure and symmetry. Instead, the thermal characteristics of the electronic structure are microscopically connected to anharmonic vibrational contributions to the band gap that reach a fairly large magnitude of 450 meV at 425 K.