Electronic Structure Modulation in GeTe by Hg and Sb Codoping Leads to High Thermoelectric Performance

Electronic band convergence and the introduction of doping-induced midgap states near the Fermi level offer a compelling mechanism for modulating the electronic structure to achieve high thermoelectric performance. Germanium telluride (GeTe), with its unique crystal and electronic structure, holds great promise for thermoelectric (TE) power generation. However, a high p-type carrier concentration coupled with high lattice thermal conductivity limits its TE performance. Herein, we report an impressive thermoelectric figure of merit (zT) of ∼2.4 (∼2.6 with Dulong-Petit C_p) at 727 K in Hg and Sb codoped GeTe, achieved through the synergistic effects of electronic structure optimization and lattice thermal conductivity reduction. Hg doping in GeTe boosts the Seebeck coefficient by facilitating the valence band convergence. Importantly, a hybridized midgap band emerges upon Hg doping, through the antibonding interaction of Hg 6s and p orbitals of Te and Ge. Further codoping of Sb makes the midgap state more localized and shifts the E_F up to pin the midgap electronic band, resulting in an enhanced electronic density of states near E_F, as validated by first-principles density functional theory (DFT) calculations of the electronic band structure and experimental Pisarenko analysis. This leads to a significant enhancement of the Seebeck coefficient in Hg and Sb codoped GeTe. Further, when Hg doping exceeds the solid solution limit, it forms HgTe nanoprecipitates in the GeTe matrix, suppressing the lattice thermal conductivity. We have constructed a double-leg TE device using the developed material as a p-type leg, which exhibits a promising output power density of 0.77 W/cm² for the ΔT = 440 K, underscoring the material's potential for high-performance TE applications.