Medium-temperature thermoelectric GeTe: vacancy suppression and band structure engineering leading to high performance

GeTe is a promising thermoelectric material at medium temperature, but its carrier concentration tends to go beyond the optimal range for thermoelectrics. This work realized a significant ZT enhancement from 1.0 to 2.0 by suppressing the formation of Ge vacancies and band convergence. By simply optimizing the amount of excessive Ge, the hole carrier concentration is greatly reduced. It is demonstrated that the suppression of Ge vacancies can not only optimize the carrier concentration but also recover the mobility to a high value of 90 cm(2) V-1 s(-1), which well exceeds the previously reported data and guarantees superior electrical transport properties, leading to a ZT of 1.6. Further Bi doping facilitates band convergence as featured by the increased band effective mass and high mobility, which in turn yields large power factors and low electronic thermal conductivity. Bi doping induced mass and strain fluctuation also favors the reduction of the lattice thermal conductivity. Consequently, a maximum ZT of approximate to 2.0 at 650 K with an average ZT of over 1.2 is achieved in the nominal composition Bi0.05Ge0.99Te, which is one of the best thermoelectric materials for medium temperature applications.