Thermoelectric power factor under strain-induced band-alignment in the half-Heuslers NbCoSn and TiCoSb

TL;DR

This study compares how strain-induced band convergence affects thermoelectric power factors in NbCoSn and TiCoSb, highlighting the importance of scattering assumptions for accurate predictions across temperatures.

Contribution

It demonstrates that the impact of band convergence on power factor varies with scattering models and temperature, emphasizing the need for detailed scattering considerations in thermoelectric predictions.

Findings

Band convergence improves power factor in both materials.

Temperature influences the benefit of band convergence differently under scattering models.

Scattering assumptions significantly affect the predicted trends of thermoelectric performance.

Abstract

Band convergence is an effective strategy to improve the thermoelectric performance of complex bandstructure thermoelectric materials. Half-Heuslers are good candidates for band convergence studies because they have multiple bands near the valence bad edge that can be converged through various band engineering approaches providing power factor improvement opportunities. Theoretical calculations to identify the outcome of band convergence employ various approximations for the carrier scattering relaxation times (the most common being the constant relaxation time approximation) due to the high computational complexity involved in extracting them accurately. Here, we compare the outcome of strain-induced band convergence under two such scattering scenarios: i) the most commonly used constant relaxation time approximation and ii) energy dependent inter- and intra-valley scattering considerations for the half-Heuslers NbCoSn and TiCoSb. We show that the outcome of band convergence on the power factor depends on the carrier scattering assumptions, as well as the temperature. For both materials examined, band convergence improves the power factor. For NbCoSn, however, band convergence becomes more beneficial as temperature increases, under both scattering relaxation time assumptions. In the case of TiCoSb, on the other hand, constant relaxation time considerations also indicate that the relative power factor improvement increases with temperature, but under the energy dependent scattering time considerations, the relative improvement weakens with temperature. This indicates that the scattering details need to be accurately considered in band convergence studies to predict more accurate trends.