Enhanced thermoelectric performance of rhombohedral GeTe by Yb-Bi Co-doping

GeTe-based materials are promising for medium-temperature energy conversion, but its practical use is limited by the phase transition, which deteriorates both thermoelectric performance and mechanical stability. This work introduces a novel doping strategy to stabilize the GeTe lattice structure and enhance thermoelectric efficiency. Yb doping enhances the band degeneracy at the valence band maximum, which in turn increases the effective mass of holes, ultimately leading to an elevated Seebeck coefficient. The combination of Yb-Bi co-doping and rapid solidification process is concluded to be of two main functions: (i) increasing the homogeneity of Yb in GeTe matrix, and (ii) facilitating the formation of nano-sized Yb-rich secondary phases. The resulting defect structure, characterized by refined grains, nanoprecipitates, domain boundary and dense dislocation networks, intensifies phonon scattering. As a result, a decoupling of thermoelectric parameters was achieved in Yb_0.010Bi_0.07Ge_0.9Te—with increased power factor and decreased thermal conductivity, below 623K. Finally, compared with Bi monodoping sample, a 16 % enhancement of ZT_ave (323–623K) was obtained in Yb_0.005Bi_0.07Ge_0.9Te and Yb_0.010Bi_0.07Ge_0.9Te. The results demonstrate the benefits of dilute Yb doping in Bi_0.07Ge_0.9Te, which is ideal for advanced sustainable energy applications.