Role of Polar Phonons in the Photo Excited State of Metal Halide Perovskites

TL;DR

This study uses first-principles calculations to investigate exciton binding energies in methylammonium lead iodide perovskites, showing that polar phonons do not significantly influence ionic screening at room temperature.

Contribution

It provides a detailed first-principles analysis demonstrating that polar phonons are not responsible for exciton binding energy reduction at room temperature in these perovskites.

Findings

Exciton binding energies at low temperature agree with large experimental values.

Polar phonons have negligible impact on ionic screening at room temperature.

Ionic screening is not the cause of reduced exciton binding energy at room temperature.

Abstract

The development of high efficiency perovskite solar cells has sparked a multitude of measurements on the optical properties of these materials. For the most studied methylammonium(MA)PbI$_3$ perovskite, a large range (6-55 meV) of exciton binding energies has been reported by various experiments. The existence of excitons at room temperature is unclear. For the MAPb$X_3$ perovskites we report on relativistic $GW$-BSE calculations. This method is capable to directly calculate excitonic properties from first-principles. At low temperatures it predicts exciton binding energies in agreement with the reported 'large' values. For MAPbI$_3$, phonon modes present in this frequency range have a negligible contribution to the ionic screening. By calculating the polarisation in time from finite temperature molecular dynamics, we show that at room temperature this does not change. We therefore exclude ionic screening as an explanation for the experimentally observed reduction of the exciton binding energy at room temperature.