Band convergence and thermoelectric transport properties of <i>p</i>-type PbTe

The alignment of the valence band maxima at L and Σ (or “band convergence”) in p-type PbTe and related materials gives rise to a large thermoelectric figure of merit ZT [1]. It is still unclear why the band convergence improves ZT. Here we present a first principles model of electron-phonon scattering and thermoelectric transport in the L and Σ valleys of p-type PbTe. Our model accounts for the thermally induced renormalization of those valleys [2], which enables us to quantify the effects of the L and Σ valley convergence at ~620 K [3] on ZT. We find that even though the increased intervalley scattering due to the band convergence reduces the electrical conductivity, it significantly increases the Seebeck coefficient. The Σ valleys also provide additional transport channels which further improve the Seebeck coefficient. This results in the enhanced values of the thermoelectric power factor near the band convergence temperature and large ZT. Our findings thus support the idea that materials with high valley degeneracy may be good thermoelectrics.

[1] Y. Pei et al, Nature 473, 66 (2011)
[2] R. D’Souza et al, Phys. Rev. B 102, 115204 (2020)
[3] J. D. Querales-Flores et al, Phys. Rev. Mater. 3, 055405 (2019)