Weak electron-phonon coupling contributing to enhanced thermoelectric performance in n-type TiCoSb half-Heusler alloys

Abstract

n-type TiCoSb half-Heusler (HH) alloys show lower thermoelectric performance may be notably owing to the low valley degeneracy of the conduction band. Here, we show that this drawback can be counterbalanced by a decrease in the deformation potential coefficient and hence, weak electron-phonon coupling strength driven by the substitution of Nb for Ti in the n-type alloys Ti1-xNbxCoSb0.96Bi0.04. The combined substitutions of Nb and Bi yield a reduction of ∼86% in the deformation potential of the conduction band minimum, with the lowest value of ∼5 eV at 300 K achieved in Ti0.85Nb0.15CoSb0.96Bi0.04. This effect leads to enhanced power factor from ∼0.018 mW m−1 K−2 for x = 0 to ∼1.69 mW m−1 K−2 for x = 0.15 at RT due to the resulting strong increase in electron mobility by one order of magnitude. Both the isovalent Bi and aliovalent Nb substitutions further contribute to decrease the lattice thermal conductivity owing to enhanced mass and strain field fluctuations. The beneficial combined effects of weaker electron-phonon coupling and enhanced point-defect phonon scattering results in a higher dimensionless thermoelectric figure of merit ZT, with a peak value ∼ 0.37 at 870 K in Ti0.85Nb0.15CoSb0.96Bi0.04, representing a ∼ 375% improvement with respect to pristine TiCoSb.