Carrier thermalization and zero-point bandgap renormalization in halide perovskites from the Urbach tails of the emission spectrum

TL;DR

This study analyzes the temperature-dependent emission spectra of halide perovskites to understand exciton behavior, phonon interactions, and zero-point bandgap renormalization, revealing weak electron-phonon coupling and ruling out small polaron formation.

Contribution

It introduces new techniques to analyze Urbach tails in emission spectra, revealing insights into exciton localization, thermalization, and electron-phonon interactions in halide perovskites.

Findings

Zero-point bandgap renormalization is about 33 meV.

Weak electron-phonon coupling is indicated.

Small polaron formation is ruled out.

Abstract

We develop techniques to study the temperature dependent localization, thermalization, and the effects of phonon scattering on the excitons in halide perovskites from the analysis of the emission spectra. The excitonic Urbach edge, when inferred from the low energy tails of the temperature dependent luminescence spectra, is shown to be sensitive to the electron distribution and thermalization. A method to observe the Urbach focus is devised for halide perovskites where the temperature dependence of the excitonic gap is anomalous. The value of the zero-point bandgap renormalization is inferred to be about 33 meV. This small value of the bandgap renormalization rules out the formation of small polarons and points to weak electron-phonon coupling. The experiments are performed on the nanosheets of the archetypal halide perovskite, CsPbBr$_3$.