When Band Convergence is Not Beneficial for Thermoelectricity

Band convergence is known to generally benefit thermoelectric performance for its capability to increase carrier concentration for given Fermi level, i.e., increase conductivity for given Seebeck coefficient. With explicit treatment of electron-phonon scattering, we show that this is not necessarily the case and the degree of attainability of the said benefit depends on the dominant scattering mechanism and the manner in which bands converge. Multi-band convergence at a single k-point under deformation scattering is less beneficial (if at all) than multi-pocket convergence under polar-optical scattering at distant k-points. In the former case, one band gains while the other band loses phase space, and the increasingly disparate pocket lifetimes and mobilities can lower the Seebeck coefficient and render higher power factor inaccessible. In the latter case, the convergence preserves the scattering behavior, thereby successfully leading to higher power factor. We establish these by performing state-of-the-art first-principles studies on CaMg2Sb2-CaZn2Sb2 Zintl alloy and full-Heusler Sr2SbAu.