Positive Effect of Ge Vacancies on Facilitating Band Convergence and Suppressing Bipolar Transport in GeTe‐Based Alloys for High Thermoelectric Performance

Because the intrinsic Ge vacancies in GeTe usually lead to high hole concentration beyond the optimal range, many previous studies tend to consider Ge vacancies as negative effects on increasing the figure of merit ZT of GeTe‐based alloys, and consequently have proposed various approaches to suppress Ge vacancies. However, in this work, it is demonstrated that the Ge vacancies can have great positive effects on enhancing the ZT of GeTe‐based alloys when the hole concentration falls into the optimal range. First, hole concentration of GeTe is reduced close to the optimal range by co‐alloying of Pb and Bi, and then the Ge vacancies are increased by adding excess Te into the Ge_0.8Pb_0.1Bi_0.1Te_1+x. The Ge vacancies can cause lattice shrinkage and promote rhombohedral‐to‐cubic phase transition. As revealed by first‐principle calculations, theoretical simulations, and experimental tests, Ge vacancies can facilitate the band convergence, suppress the bipolar transport at higher temperature range, and reduce the lattice thermal conductivity. Combining these effects, a peak ZT of 1.92 at 637 K and an average ZT of 1.34 within 300–773 K in Ge_0.8Pb_0.1Bi_0.1Te_1.06 can be obtained, demonstrating the great significance of utilizing vacancy‐type defects for enhancing ZT.