Suppressing Intrinsic Ge Vacancies Enables High Thermoelectric Performance in Rhombohedral GeTe

GeTe is a promising thermoelectric material, yet the abrupt change in the coefficient of thermal expansion of GeTe during phase transition leads to high internal stress and device failure. Utilizing the low-temperature rhombohedral phase can solve this issue, but its performance is limited by high carrier concentration and low mobility caused by massive intrinsic Ge vacancies. Herein, we introduce an effective fabrication process that integrates high-energy ball milling with low-temperature annealing for the synergistic optimization of carrier concentration and mobility. High-energy ball milling effectively fragments Ge second phases in the ingot and uniformly envelops it around the matrix. Annealing at the low-temperature range with high Ge vacancy formation energy promotes the redissolution of the Ge secondary phase back into the matrix, and thus reduces the content of Ge vacancies. Furthermore, the co-doping of Bi/Sb is employed to reduce the carrier concentration and lattice thermal conductivity. Consequently, a maximum ZT of 1.95 at 650 K with an average ZT of 1.0 within 300-673 K is achieved in the rhombohedral Ge_0.96Sb_0.02Bi_0.02Te. This work emphasizes the importance of controlling intrinsic defects in enhancing thermoelectric performance through the fabrication process.