Manipulating the Ge Vacancies and Ge Precipitates through Cr Doping for Realizing the High‐Performance GeTe Thermoelectric Material

Abstract

GeTe alloy is a promising medium‐temperature thermoelectric material but with highly intrinsic hole carrier concentration by thermodynamics, making this system to be intrinsically off‐stoichiometric with Ge vacancies and Ge precipitations. Generally, an intentional increase of formation energy of Ge vacancy by element substitution will lead to an effective dissolution of Ge precipitates for reduction in hole concentration. Here, an opposite direction of decreasing the formation energy of Ge vacancies is demonstrated by substituting Cr at Ge site. This strategy produces more but nearly homogenously distributed Ge precipitations and Ge vacancies, which provides enhanced phonon scattering and effectively reduces the lattice thermal conductivity. Furthermore, Cr atom carries one more electron than Ge and serves as an electron donor for decreasing the hole carrier concentrations. Further optimization incorporates the effect of Bi substitution for facilitating band convergence. A maximum figure of merit (ZT) of 2.0 at 600 K with average ZT of over 1.2 is achieved in the sample of Ge0.92Cr0.03Bi0.05Te, making it one of the best thermoelectric materials for medium‐temperature application.

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°.…”

“…For instance, it is found that Cr doping significantly reduced the formation energy from 1.02 to −0.08 eV at the valence band maximum (VBM). [ 53 ] As is known, suppressing the Ge vacancies is essential to ensure high charge carrier mobility that will be beneficial to achieve a high power factor. [ 47,50,66,72 ]…”

“…As a consequence of native Ge vacancies, most of previous studies utilized the aliovalent dopant on the Ge site to reduce the hole concentration, such as Bi, [ 61,63,64,66 ] Sb, [ 62,73 ] In, [ 74,75 ] and transition metals (Ti, [ 67,68 ] Cr, [ 53 ] Cd, [ 69–71 ] and Cu [ 51,54 ] ). Among them, some dopants also obviously contributed to modifying the structure symmetry from rhombohedral to cubic, represented by means of the increased interaxial angle α.…”

“…Analysis and understanding of high power factor of Sc‐doped GeTe by comparison with previous literature results of rhombohedral GeTe‐based materials, including Ti doping, [ 67 ] In doping, [ 74 ] Bi doping, [ 64 ] Ag doping, [ 76 ] Cr doping, [ 53 ] Mn alloying, [ 65 ] Cd alloying, [ 70 ] and Pb alloying. [ 29 ] a) Temperature dependent power factor.…”

“…As above‐mentioned, this is due to the significantly decreased carrier concentration after Sc doping. Nevertheless, the charge carrier mobility still shows the increased tendency (Figure 4b), which may be related to the microstructural modulation, e.g., Ge precipitates, [ 50,53 ] Ge‐vacancies thin layer, [ 54 ] and nanoscale domains, [ 50,52,72 ] to reduce the grain boundary scattering for charge carriers. Compared to other dopants, the unusual high values of the charge carrier mobility of Sc‐doped samples are caused by the weak scattering from fewer Ge vacancies.…”