Low Thermal Conductivity and High Thermoelectric Performance in Sb and Bi Codoped GeTe: Complementary Effect of Band Convergence and Nanostructuring

Complementary and beneficial effects of Sb and Bi codoping in GeTe are shown to generate high thermoelectric figure of merit, zT, of 1.8 at 725 K in Ge1‑x‑y Bi x Sb y Te samples. Sb and Bi codoping in GeTe facilitates the valence band convergence enhancing the Seebeck coefficient as supported by density functional theoretical (DFT) calculations. Further, Sb and Bi codoping in GeTe releases the rhombohedral strain and increases its tendency to be cubic in structure, which ultimately enhances the valence band degeneracy. At the same time, Bi forms nanoprecipitates of size ∼5–20 nm in GeTe matrix and Sb doping increases solid solution point defects greatly, which altogether scatter low-to mid wavelength phonons and result in reduced lattice thermal conductivity down to 0.5 W/mK in the 300–750 K range.