High band degeneracy and weak chemical bonds leading to enhanced thermoelectric transport properties in 2H–MoTe₂

2H-MoX₂ (X ​= ​S, Se and Te) based compounds are promising medium-to-high thermoelectric materials, with distinct advantages of excellent thermal stability and superior mechanical flexibility. As illustrated by our band structure calculations and electronic-thermal transport measurements, the p-type 2H–MoTe₂ achieves the best thermoelectric properties among the binary 2H-MoX₂ analogues, arising from the high band degeneracy and the weak Mo–Te chemical bonds. Multiple valence bands at the Γ, K Z, and H points converge in energy, and the impurity band induced by the doping of Nb contributes to the valence band convergence, leading to a large carrier effective mass of ~4.1 m₀ and thus the excellent power factors in 2H–Mo_1–xNb_xTe₂ with x ​> ​0.05. A hopping electrical conduction is observed in the 2H–Mo_1–xNb_xTe₂ with a slight amount of Nb, which could be ascribed to the formed discrete impurity levels in the vicinity of valence band edge. In addition, the doping of Nb enhances the defect scattering of phonons and thus markedly reduces the lattice thermal conductivity of 2H–Mo_1–xNb_xTe₂. Finally, 2H–Mo_1–xNb_xTe₂ with x ​= ​0.07 obtains the largest ZT value of 0.20 and 0.14 ​at 823 ​K, when measured perpendicular and parallel to the pressing directions, respectively.