Rare three-valence-band convergence leading to ultrahigh thermoelectric performance in all-scale hierarchical cubic SnTe

Band convergence is an important approach for improving the thermoelectric performance, as it can significantly increase the Seebeck coefficient without obviously sacrificing the electrical conductivity. Herein, we report a rare three-valence-band convergence achieved in doped p-type cubic SnTe to enhance the power factor and obtain a record-high ZT. Ternary MnCdTe2 alloying activates the lower-energy-lying Λ valence band (with a large degeneracy of Nv = 8), and additional Ge doping promotes energy alignment between the Λ and L bands, leading to a rare three-valence-band (i.e., L, Σ and Λ) convergence that pronouncedly enhances the power factor (PF = 29.3 μW cm−1 K−2 at 900 K). Ge doping also greatly enhances the solid solubility of MnCdTe2 in the SnTe matrix and effectively tunes the precipitate size, producing all-scale hierarchical structures, which generate a full spectrum of phonon scattering, especially low-frequency acoustic–optical scattering, leading to an ultralow lattice thermal conductivity (κL = 0.26 W m−1 K−1 at 670 K). Collectively, this gives a record-high ZT of 1.97 at 900 K and an average ZTave of 0.8 in the temperature range of 300 to 900 K for 8% MnCdTe2-doped SnTe with 3.2% Ge. Furthermore, a single-leg TE module based on (SnTe)0.92(MnCd0.6Ge0.4Te2)0.08 outputs a power density of 800 mW cm−2 for a ΔT of 446 K, which is competitive with those of devices based on state-of-the-art mid-temperature materials (600–900 K) within the same ΔT, demonstrating great potential for future applications.