High Thermoelectric Figure of Merit of FeSb2−x Thin Films via Defect Engineering for Low-Temperature Cooling Applications

In general, both the thermal and electrical conductivity of metal/metalloid/semimetal materials decrease with increasing temperature; that is, these two parameters are in lockstep. Defect engineering has recently been proven to have tremendous potential for improving the thermoelectric performance of topological materials with two-dimensional layered structures. We have explored the effect of Sb point-defect engineering on the thermoelectric performance of FeSb2−x (x = 0, 0.1, 0.2, and 0.3) thin films at low temperatures. Thin-film surface and grain-boundary phonon scattering contribute to the overall decrease in the thermal conductivity (κ) compared with single crystals. The current results indicate that FeSb1.8 shows the lowest κ value, the main reason being that its nanostructure scatters more phonons over a wider wavelength range. In addition, the presence of Sb point defects enables an enhancement of the Seebeck coefficient (S) to 357 μV/K at 45 K for FeSb1.8 thin films, coupled with an imperceptible decrease in the corresponding electrical conductivity (σ). Despite these results, the power factor (PF) is much lower than that of single crystals. The optimal peak thermoelectric figure of merit (ZT) of ~ 0.071 at 55 K is one order of magnitude higher than the ZTmax of low-temperature FeSb2 single crystals. These characteristics indicate that FeSb2 thin films are promising materials for use in low-temperature thermoelectric devices.