Simultaneous Enhancement of the Power Factor and Phonon Blocking in Nb-Doped WSe₂

Transition-metal dichalcogenide WSe₂ is a potentially good thermoelectric (TE) material due to its high thermopower (S). However, the low electrical conductivity (σ), power factor (PF), and relatively large lattice thermal conductivity (κ_L) of pristine WSe₂ degenerate its TE performance. Here, we show that through proper substitution of Nb for W in WSe₂, its PF can be increased by ∼10 times, reaching 5.44 μW cm^-1 K^-2 (at 850 K); simultaneously, κ_L lowers from 1.70 to 0.80 W m^-1 K^-1. Experiments reveal that the increase of PF originates from both increased hole concentration due to the replacement of W⁴⁺ by Nb³⁺ and elevated thermopower (S) caused by the enhanced density of states effective mass, while the reduced κ_L comes mainly from phonon scattering at point defects Nb_W. As a result, a record high figure of merit ZT_max ∼0.42 is achieved at 850 K for the doped sample W_0.95Nb_0.05Se₂, which is ∼13 times larger than that of pristine WSe₂, demonstrating that Nb doping at the W site is an effective approach to improve the TE performance of WSe₂.