Condensed point defects enhance thermoelectric performance of rare-earth Lu-doped GeTe

Heavy rare-earth element doping can effectively strengthen phonon scattering, suppress the lattice thermal conductivity, and enhance the overall thermoelectric performance of GeTe. However, the large electronegativity difference between rare-earth elements (such as La, Eu, and Gd) and Ge refrains the doping limit of rare-earth elements below 1 mol.% in GeTe. Here, compared with other rare earth elements, Lu was found to have a relatively small radius and electronegativity difference with Ge, which can induce a high doping level in GeTe. The result shows that Lu doping effectively reduces the lattice thermal conductivity from 0.77 W m−1 K−1 of GeTe to 0.35 W m−1 K−1 of Ge0.98Lu0.02Te at 673 K, and further induces a high zT value of 1.5 in Ge0.98Lu0.02Te at 673 K. Extra Sb alloying optimizes the carrier concentration from 1.02 × 1021 cm−3 of Ge0.98Lu0.02Te to 1.77 × 1020 cm−3 of Ge0.90Lu0.02Sb0.08Te, which results in a reasonable power factor of 33.82 µW cm−1 K−2 and a low electrical thermal conductivity of 0.75 W m−1 K−1 at 673 K in Ge0.90Lu0.02Sb0.08Te. Correspondingly, a peak zT of 1.75 at 673 K and an average zT of 0.92 within the temperature range of 303–723 K are obtained in Ge0.9Lu0.02Sb0.08Te. This study indicates that Lu and Sb co-doping can effectively boost the thermoelectric performance of GeTe-based thermoelectric materials.