Ab initio calculations of the thermoelectric figure of merit, within the relaxation time approximation

TL;DR

This paper introduces a computational framework using VASP and phono3py to calculate thermoelectric efficiency from electron-phonon interactions, applied to various materials, highlighting the influence of phonon frequencies and computational choices.

Contribution

The paper develops a novel computational approach combining electron-phonon and phonon-phonon interactions to predict thermoelectric figures of merit.

Findings

Computed figures of merit align with experiments but are not perfect.

Electron lifetimes vary across materials due to phonon frequency differences.

Defects and exchange-correlation functionals significantly affect results.

Abstract

In this paper, we propose a computational framework, based on the VASP and phono3py computer codes, to obtain the thermoelectric figure of merit from the electron-phonon and phonon-phonon interactions using finite displacements in supercells. Several numerical techniques are developed for efficiency. The method is applied to several thermoelectric materials. We found different behaviors for the lifetimes of the electrons in PbTe, PbSe, and in compounds of the half-Heusler and magnesium silicide family. This is traced back to the different frequencies of the phonons involved in the scattering around the Fermi level. They have a lower frequency in PbTe and PbSe. The magnitude of the thermoelectric figures of merit we computed compare well with experiments, but the agreement is far from perfect. The role of the defects, not explicitly considered in our calculations, but abundant in thermoelectric materials, is discussed as a possible explanation. It is also shown that the choice of the exchange-correlation functional can strongly impact the results.