Boosting the thermoelectric performance of GeTe via vacancy control and engineering sintering parameters

Pristine GeTe is a p-type degenerate semiconductor with very low thermoelectric performance due to its large carrier concentration induced by large Ge vacancy. In this work, Ge vacancy was effectively decreased through the melt quenching process and also non-equilibrium reaction caused by the spark plasma sintering (SPS) consolidation. The effects of excess Ge doping were studied on the polycrystalline and single-crystal specimens to understand the excess doping effect on the carrier concentration. The p-type carrier concentration for the Ge1.03Te polycrystalline specimen was significantly reduced to 2.43 x 10(20) cm(-3) from 8.9 x 10(20) cm(-3) of pristine GeTe. This work realized that the zT of the single-crystal specimen was lower than that of pellets prepared by the spark plasma sintering due to its high carrier concentration. Meanwhile, polycrystalline specimen sintered by spark plasma sintering exhibits lower thermal conductivity due to the reduction of carrier concentration and phonon scattering centers such as Ge precipitates and nanostructured defects. We also systematically studied the different SPS sintering conditions on thermoelectric properties of the Ge1.03Te polycrystalline specimen. Eventually, superior thermoelectric performance zT= 1.8 at 700 K was obtained in 3% Ge excess Ge1.03Te polycrystalline specimen sintered at 773 K with 30 MPa pressure. The zT enhancement results from the synergistic contribution of the enhanced Seebeck coefficient by excess Ge doping and reduced thermal conductivity through SPS sintering pressure. This work demonstrates that the zT value of GeTe can be effectively enhanced by adding excess Ge with optimal SPS conditions. Our results provide an enlightening strategy to improve thermoelectric material performance.