Versatile vanadium doping induces high thermoelectric performance in GeTe via band alignment and structural modulation

Owing to the moderate energy offset between light and heavy band edges of the rock-salt structured GeTe, its figure-of-merit (ZT) can be enhanced by the rational manipulation of electronic band structures. In this study, density functional theory calculations are implemented to predict that V is an effective dopant for GeTe to enlarge the bandgap and converge the energy offset, which suppresses the bipolar conduction and increases the effective mass. Experimentally, V-doped Ge${}_{1-x}$V${}_x$Te samples are demonstrated to have an enhanced Seebeck coefficient from ≈163 to ≈191 µV K${}^{-1}$. Extra alloying with Bi in Ge${}_{1-x-y}$V${}_x$Bi${}_y$Te can optimize the carrier concentration to further enhance the Seebeck coefficient up to ≈252 µV K${}^{-1}$, plus an outstanding power factor of ≈43 µW cm${}^{-1}$ K${}^{-2}$. Comprehensive structural characterization results also verify the refinement of grain size by V-doping, associated with highly dense grain boundaries, stacking faults, nanoprecipitates, and point defects, reinforcing the wide-frequency phonon scattering and in turn, securing an ultralow thermal conductivity of ≈0.59 W m${}^{-1}$ K${}^{-1}$. As a result, the Ge${}_{0.9}$V${}_{0.02}$Bi${}_{0.08}$Te sample shows a peak ZT of >2.1 at 773 K, with an average plateaued average ZT of >2.0 from 623 and 773 K, which extends better thermoelectric behavior for GeTe over a wider temperature range. This study clarifies the multiple benefits of V-doping in GeTe-based derivatives and provides a framework for a new-type of high-performance middle-temperature thermoelectric material. (© 2021 Wiley-VCH GmbH)