Distinguishing thermal fluctuations from polaron formation in halide perovskites

TL;DR

This paper demonstrates that thermal fluctuations, rather than polaron formation, explain the effective mass enhancement observed in halide perovskites, emphasizing the importance of including thermal effects in first-principles calculations for such materials.

Contribution

The study shows that thermal fluctuations account for effective mass enhancements in halide perovskites, challenging previous polaron-based interpretations and highlighting the need to consider temperature effects in theoretical models.

Findings

Thermal fluctuations significantly influence effective mass in halide perovskites.

First-principles calculations at finite temperature match experimental data.

Polaron formation is not necessary to explain effective mass enhancements.

Abstract

Recent angle-resolved photoelectron spectroscopy (ARPES) measurements of the hole effective mass in CsPbBr$_3$ revealed an enhancement of $\sim$50 % compared to the bare mass computed from first principles for CsPbBr$_3$ at $T = 0 K$. This large enhancement was interpreted as evidence of polaron formation. Employing accurate finite-temperature first-principles calculations, we show that the calculated hole effective mass of CsPbBr$_3$ at $T = 300 K$ can explain experimental results without invoking polarons. Thermal fluctuations are particularly strong in halide perovskites compared to conventional semiconductors such as Si and GaAs, and cannot be ignored when comparing with experiment. We not only resolve the debate on polaron formation in halide perovskites, but also demonstrate the general importance of including thermal fluctuations in first-principles calculations for strongly anharmonic materials.