Achieving High Thermoelectric Performance by NaSbTe₂ Alloying in GeTe for Simultaneous Suppression of Ge Vacancies and Band Tailoring

GeTe alloys have attracted wide attention due to their high conversion efficiency. However, pristine GeTe possesses intrinsically massive Ge vacancies, leading to a very high hole concentration (10²¹ cm⁻³). Herein, a decreased carrier concentration is realized by alloying NaSbTe₂ in GeTe due to the increased formation energy of Ge vacancies. This alloying also lowers energy separation between the valence bands in the rhombohedral GeTe and induces two extra valence band pockets around the Fermi surface along Γ-L and L-W in the cubic GeTe, all of which contributes to the higher power factors over a wide temperature range. Combined with the low lattice thermal conductivities due to plenty of dislocations and strains as a result of the crystallographic disorder of Na, Ge, and Sb, a maximum zT ≈ 2.35 at 773 K and a zT_ave of 1.33 from 300 to 773 K are achieved in (GeTe)₉₀(NaSbTe₂)₁₀.