When band convergence is not beneficial for thermoelectrics

Abstract

Band convergence is considered a clear benefit to thermoelectric performance because it increases the charge carrier concentration for a given Fermi level, which typically enhances charge conductivity while preserving the Seebeck coefficient. However, this advantage hinges on the assumption that interband scattering of carriers is weak or insignificant. With first-principles treatment of electron-phonon scattering in the CaMg2Sb2-CaZn2Sb2 Zintl system and full Heusler Sr2SbAu, we demonstrate that the benefit of band convergence can be intrinsically negated by interband scattering depending on the manner in which bands converge. In the Zintl alloy, band convergence does not improve weighted mobility or the density-of-states effective mass. We trace the underlying reason to the fact that the bands converge at a one k-point, which induces strong interband scattering of both the deformation-potential and the polar-optical kinds. The case contrasts with band convergence at distant k-points (as in the full Heusler), which better preserves the single-band scattering behavior thereby successfully leading to improved performance. Therefore, we suggest that band convergence as thermoelectric design principle is best suited to cases in which it occurs at distant k-points.

Anomalous thermoelectric transport phenomena from interband electron-phonon scattering

“…13 and 14 of Supplementary Material we dis-cuss other details of this case, such as estimates of S and the impact of band degeneracy. We note that earlier studies have observed that adding a heavier band is detrimental to electronic conductivity 80,81 , in agreement with our conclusions.…”

“…One of the strategies of using band engineering to design high-performance thermoelectric materials is based on the concept of band convergence, where it is proposed that high degeneracy near conduction or valence band edges is beneficial for increasing σ for a given S due to the presence of multiple conducting channels 3,23,[27][28][29][30][31][32][33][34][35][36][37] . However, several recent studies predicted that the interband scattering can complicate the conventional understanding of the effects of band convergence on the electronic transport properties and that it is not always beneficial for the PF optimization [79][80][81] . Here we revisit the concept of band convergence using the parabolic two-band model within the EPA approximation.…”

Conduction band engineering of half-Heusler thermoelectrics using orbital chemistry

“…from intervalley scattering. 41 Engineering the band edges to achieve high band degeneracy is therefore a clear strategy to improve thermoelectric performance. A simple model is needed to engineer the band convergence by doping or alloying.…”

Thermopower, figure of merit and Fermi integrals

“…As early raised by Chasmar and Stratton 6 , the transport coefficients which characterize the thermoelectric efficiency can be written as a function of Fermi integrals in the frame of the semi-classical treatment of the Boltzmann equation within the relaxation time approximation 7 . This description can be extended by including different scattering mechanisms 8 – 12 , bipolar effects 9 , multiple bands and valley degeneracy 13 – 17 . If one considers for instance that the relaxation time, the velocity and the density of states vary as a function of the energy according power laws 18 with the exponents , and respectively, then the electrical conductivity , the thermopower and the Lorentz number , with the electronic thermal conductivity , can be defined as it follows 6 , 8 – 10 .…”