Superior electron transport in the single-crystalline TiCoSb-based half-Heuslers

Half-Heuslers emerged as promising candidates for medium- and high-temperature thermoelectric power generation. However, polycrystalline half-Heuslers inevitably suffer from the defect-dominated scattering of electrons that greatly limits the optimization of their electronic properties. Herein, high-quality TiCoSb-based single-crystals with a dimension above 1 cm have been obtained. Benefitting from the improved electron mobility, an average power factor of ~37 μW cm^-1 K^-2 in the temperature range between 307 and 973 K has been realized in the n-type single-crystalline Ti_1-xNb_xCoSb. In addition, Hf alloying results in the expansion of the weighted scattering phase space and enhances the anharmonic scattering rate, thereby effectively suppressing the lattice thermal conductivity. Eventually, co-doping of Nb/Ta and alloying of Hf effectively elevate the thermoelectric performance of TiCoSb single crystal, and a peak zT above 1.0 has been realized, which outperforms the previously reported polycrystalline (Ti, Zr, Hf)CoSb-based and ZrCoBi-based materials. Importantly, a single leg of TiCoSb-based single crystals exhibits a heat-to-electricity energy conversive efficiency of ~10.2% at a temperature difference of 700 K. Here, our findings reveal the promise of TiCoSb-based single crystals for thermoelectric power generation, and can potentially guide the future explorations of other single-crystalline half-Heuslers.