Achieving Superior Thermoelectric Performance in Ge4Se3Te via Symmetry Manipulation with I–V–VI2 Alloying

Abstract Although orthorhombic GeSe is predicted to have an ultrahigh figure of merit, ZT ≈ 2.5, up to now, the highest experimental value is ≈0.2 due to the low carrier concentration ( n H ≈ 10 18 cm −3 ). Improving symmetry is an effective approach for enhancing the ZT of GeSe‐based materials. With Te‐alloying, Ge 4 Se 3 Te displays the two‐dimensional hexagonal structure and high n H ≈ 1.23 × 10 21 cm −3 . Interestingly, Ge 4 Se 3 Te transformed from the hexagonal into the rhombohedral phase with only ≈2% I–V–VI 2 ‐alloying (I = Li, Na, K, Cu, Ag; V = Sb, Bi; VI = Se, Te). According to the calculated results of Ge 0.82 Ag 0.09 Bi 0.09 Se 0.614 Te 0.386 single‐crystal grown via AgBiTe 2 ‐alloying, it exhibits a higher valley degeneracy than the hexagonal Ge 4 Se 3 Te. For instance, AgBiTe 2 ‐alloying induces a strong band convergence and band inversion effect, resulting in a significantly enhanced Seebeck coefficient and power factor with a similar n H from 17 µV K −1 and 0.63 µW cm −1 K −2 for pristine Ge 4 Se 3 Te to 124 µV K −1 and 5.97 µW cm −1 K −2 for 12%AgBiTe 2 ‐alloyed sample, respectively. Moreover, the sharply reduced phonon velocity, nano‐domain wall structure, and strong anharmonicity lead to low lattice thermal conductivity. As a result, a record‐high average ZT ≈0.95 over 323–773 K with an excellent ZT ≈ 1.30 is achieved at 723 K.