When Band Convergence is Not Beneficial for Thermoelectrics.

Band convergence is considered a clear benefit to thermoelectric performance because it increases the charge carrier concentration for a given Fermi level, enhancing electrical conductivity while preserving the Seebeck coefficient. However, this advantage hinges on the assumption that there is no significant additional interband scattering of carriers to other bands resulting from the band convergence. With first-principles treatment of electron-phonon scattering in CaMg2Sb2-CaZn2Sb2 Zintl alloy and full Heusler Sr2SbAu, we demonstrate that the benefit of band convergence indeed depends on the manner in which bands converge and the extent of interband scattering. Critically, multi-band convergence at a single k-point (as in the Zintl alloy) is much less beneficial, if at all, compared to multi-pocket convergence at distant k-points (as in the full Heusler). We find convergence at a single k-point is designed to induce strong interband scattering with no improvement in the effective mass or weighted mobility, limiting improvements in thermoelectric performance. Convergence of band pockets at distant k-points better preserves the initial scattering behavior, thereby successfully leading to improved performance.