High-pressure synthesis and multi-element doping engineering promote thermoelectric performance of anisotropic WSe2 bulk material

Abstract

Transition metal dichalcogenides possess thermoelectric conversion potential based on their nontoxic, low-cost, earth-abundant compositions and high Seebeck coefficient. The challenges are the intrinsically poor electrical conductivity and high lattice thermal conductivity. Here, we take WSe2 for example to demonstrate the systematic strategy of high-pressure synthesis and multi-element doping to tailor carrier transport and phonon scattering. The high-pressure technique enables effective multi-element doping and rapid crystal growth with layered grains reaching the 10 μm scale in 30 min through its kinetic conditions. A series of chemical compositions W1–x–yNbxMoySe2−zSz (0 ≤ x ≤ 0.06, 0 ≤ y ≤ 0.7, 0 ≤ z ≤ 0.3) are rationally designed with a doping sequence of Nb, Mo, and S elements. Both temperature-dependent carrier and phonon transport behaviors change from acoustic phonon scattering for the pristine sample to alloy scattering after multi-element doping. The carrier concentration is improved by four orders of magnitude to 1020 cm−3 as Nb content is above x ≥ 0.02. This approach achieves a power factor of 1006 μW m−1 K−2 at 823 K for W0.67Nb0.03Mo0.3Se1.8S0.2 in the perpendicular direction, which is among the highest reported for WSe2-based bulk materials. Guided by phonon scattering analysis, the multiple substitutions achieve 75% and 68% reduction in lattice thermal conductivity at room temperature along the parallel and perpendicular directions, where mass fluctuation plays a dominant role in disorder scattering. Based on the anisotropic analysis, maximum zT values of 0.27 and 0.23 at 823 K are achieved for the W0.67Nb0.03Mo0.3Se1.8S0.2 sample. This work provides a feasible strategy of high-pressure synthesis combined with multi-element doping to promote electrical transport and phonon scattering for layered thermoelectric materials.