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

Abstract

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 0.8 Pb 0.1 Bi 0.1 Te 1+x . The Ge vacancies can cause lattice shrinkage and promote rhombohedral-to-cubic phase transition. As revealed by firstprinciple 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 0.8 Pb 0.1 Bi 0.1 Te 1.06 can be obtained, demonstrating the great significance of utilizing vacancy-type defects for enhancing ZT.

Slowing down the heat in thermoelectrics

“…Vacancies originate from the loss of atoms in a crystal lattice, which are generally intrinsic and thus determine some inner properties of thermoelectric systems including SnSe, 173,175–177 PbTe, 162,178,179 SnTe, 138,180 and GeTe 171,181 . Regulation of vacancies is performed by compensating the increase in μ caused by vacancies or introducing more vacancies to optimize n .…”

High Power Factor and Enhanced Thermoelectric Performance in Sc and Bi Codoped GeTe: Insights into the Hidden Role of Rhombohedral Distortion Degree

“…[ 38–40 ] Due to the similarly beneficial band structure and strong phonon anharmonicity with PbTe arising from the same chemical bonding mechanism, [ 41,42 ] comparable high peak ZTs ≈2 have continuously been reported in the system of GeTe. [ 29,43–56 ] One important difference between these two compounds is that GeTe undergoes the structural phase transition from the low‐temperature rhombohedral to the high‐temperature cubic phase at a critical temperature around 700 K. [ 57,58 ] The rhombohedral structure refers to a unit cell with parameters ( a = b = c and α = β = γ < 90°), belonging to the hexagonal crystal family. It can be regarded as a slightly distorted rock‐salt lattice along the [111] direction with a interaxial angle less than 90°.…”

Realizing zT Values of 2.0 in Cubic GeTe

“…Albeit having a favourable band structure and high quality factor, GeTe suffers from high concentration of holes in pristine form [64–68] . Therefore, counter‐doping strategies is commonly used to optimize the carrier concentration with a great degree of success [69–71] .…”