Anharmonic stabilization and band gap renormalization in the perovskite CsSnI$_3$

TL;DR

This study uses self-consistent phonon theory to show how anharmonic interactions stabilize the cubic phase of CsSnI$_3$ and reveals temperature-dependent band gap renormalization due to vibrational effects.

Contribution

It introduces a method to account for anharmonic stabilization and electronic band gap changes in CsSnI$_3$ using self-consistent phonon calculations.

Findings

Anharmonic phonon interactions stabilize the cubic phase at experimental conditions.

Vibrations cause an opening of the band gap at elevated temperatures.

Temperature plays a crucial role in the electronic properties of CsSnI$_3$.

Abstract

Amongst the X(Sn,Pb)Y$_3$ perovskites currently under scrutiny for their photovoltaic applications, the cubic B-$\alpha$ phase of CsSnI$_3$ is arguably the best characterized experimentally. Yet, according to the standard harmonic theory of phonons, this deceptively simple phase should not exist at all due to rotational instabilities of the SnI$_6$ octahedra. Here, employing self-consistent phonon theory we show that these soft modes are stabilized at experimental conditions through anharmonic phonon-phonon interactions between the Cs ions and their iodine cages. We further calculate the renormalization of the electronic energies due to vibrations and find an unusual opening of the band gap, estimated as 0.24 and 0.11 eV at 500 and 300 K, which we attribute to the stretching of Sn-I bonds. Our work demonstrates the important role of temperature in accurately describing these materials.