Synergistically Improved Electronic and Thermal Transport Properties in Nb-Doped Nb_yMo_1–ySe_2–2xTe_2x Solid Solutions Due to Alloy Phonon Scattering and Increased Valley Degeneracy

The layered 2H-MoSe 2 -based compounds have recently attracted considerable attention as novel thermoelectric (TE) materials for medium-to-high temperature power generation. In this research, for the first time, dissolving Te in binary MoSe 2 and thus forming MoSe 2–2 x Te 2 x solid solutions is shown to be very effective for reducing lattice thermal conductivity (κ L ) due to strong alloy scattering of phonons. Along the ⊥P direction, MoSe 1.2 Te 0.8 achieves the lowest κ L of 2.87 W m –1 K –1 at room temperature among all solid solutions, an 85% decrease from that of pristine MoSe 2 (18.69 W m –1 K –1 ). Band structure calculations and experiments have verified that Te alloying effectively increases the density-of-states effective mass of MoSe 2–2 x Te 2 x solid solutions via increased valley degeneracy, leading to an enhanced Seebeck coefficient. Moreover, Nb doping modulates the density of holes of Nb y Mo 1– y Se 1.2 Te 0.8 samples to its optimum level and gives rise to the maximum power factor of 0.85 mW m –1 K –2 at 823 K. On account of synergistic optimization of the electronic and thermal transport, Nb 0.05 Mo 0.95 Se 1.2 Te 0.8 has achieved the highest ZT value of 0.34 at 823 K, representing a 70% enhancement as compared to the best result previously reported for MoSe 2 . This research documents that alloying combined with doping is an effective avenue to greatly enhance the TE performance of Nb y Mo 1– y Se 2–2 x Te 2 x solid solutions.