Achieving High Carrier Mobility And Thermal Stability in Plainified Rhombohedral GeTe Thermoelectric Materials with zT > 2

GeTe is a very promising thermoelectric material, but the presence of massive intrinsic Ge vacancies leads to an overhigh hole concentration and poor thermal stability. Counter doping is commonly employed to reduce the hole concentration, which, however, unavoidably deteriorates the carrier mobility. Here, it is found that the intrinsic hole concentration in the rhombohedral phase is much lower than that in the cubic phase, owing to the higher formation energy of Ge vacancy in the former. With this recognition, the hole concentration of GeTe can be tuned to its optimum value simply by annealing below the phase transition temperature. As a result, "compositional plainification" is realized in the high-performance GeTe-based thermoelectrics with significantly reduced amounts of counter dopants and hetero-alloys. A high carrier mobility of 150 cm(2) V-1 s(-1) is realized in GeTe at 300 K, which is much higher than that in conventional counter-doped ones (<= 60 cm(2) V-1 s(-1)). More importantly, GeTe-based compounds, with suppressed intrinsic vacancies, exhibit good thermal stability and reproducibility of thermoelectric performance. A high peak figure of merit, zT, of 2.14 at 670 K is obtained in Ge0.93Bi0.03Pb0.04Te. This work highlights the importance of understanding and regulating the intrinsic vacancy for high-performance GeTe thermoelectrics.