Thermoelectric Power Factor Under Strain-Induced Band-Alignment in the Half-Heuslers NbCoSn and TiCoSb

Abstract

Band convergenceband convergence is an effective strategy to improve the thermoelectric performance of complex bandstructure thermoelectric materials. Half-HeuslersHalf-Heuslers are good candidates for band convergenceband 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 convergenceband convergence employ various approximations for the carrier scattering relaxation timesrelaxation time (the most common being the constant relaxation timerelaxation time approximation) due to the high computational complexity involved in extracting them accurately. Here, we compare the outcome of strain-induced band convergenceband convergence under two such scattering scenarios: (i) the most commonly used constant relaxation timerelaxation time approximation and (ii) energy dependent inter- and intra-valley scattering considerations for the half-HeuslersHalf-Heuslers NbCoSn and TiCoSb. We show that the outcome of band convergenceband convergence on the power factor depends on the carrier scattering assumptions, as well as the temperature. For both materials examined, band convergenceband convergence improves the power factor. For NbCoSn, however, band convergenceband convergence becomes more beneficial as temperature increases, under both scattering relaxation timerelaxation time assumptions. In the case of TiCoSb, on the other hand, constant relaxation timerelaxation 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 convergenceband convergence studies to predict more accurate trends.