Ab initio optimization of phonon drag effect for lower-temperature thermoelectric energy conversion

TL;DR

This paper demonstrates first-principles computation of phonon drag effects in heavily doped materials, proposing nanostructuring and phonon filtering to significantly improve thermoelectric efficiency at lower temperatures.

Contribution

It introduces a novel ab initio method to optimize phonon drag for thermoelectric enhancement, revealing spectral phonon contributions and proposing an ideal phonon filter for silicon.

Findings

Phonon drag can be quantitatively identified and optimized.

Nanostructuring reduces phonon thermal conductivity.

zT can be increased by a factor of 20 at room temperature and 70 at 100K.

Abstract

While the thermoelectric figure of merit zT above 300K has seen significant improvement recently, the progress at lower temperatures has been slow, mainly limited by the relatively low Seebeck coefficient and high thermal conductivity. Here we report, for the first time, success in first-principles computation of the phonon drag effect - a coupling phenomenon between electrons and non-equilibrium phonons - in heavily doped region and its optimization to enhance the Seebeck coefficient while reducing the phonon thermal conductivity by nanostructuring. Our simulation quantitatively identifies the major phonons contributing to the phonon drag, which are spectrally distinct from those carrying heat, and further reveals that while the phonon drag is reduced in heavily-doped samples, a significant contribution to Seebeck coefficient still exists. An ideal phonon filter is proposed to enhance zT of silicon at room temperature by a factor of 20 to around 0.25, and the enhancement can reach 70 times at 100K. This work opens up a new venue towards better thermoelectrics by harnessing non-equilibrium phonons.