Enhanced thermoelectric performance through crystal field engineering in transition metal–doped GeTe

Abstract

The change of multivalence band structure configuration between rhombodedral and cubic phase in GeTe offers additional dimension to modify its thermoelectric properties. Here, we report the synergetic optimization of electronic and thermal transport properties in rhombohedral GeTe doped with transition metal Ti. The Seebeck coefficient of Ge 1-x Ti x Te is significantly increased and the corresponding thermal conductivity is decreased. The structure refinement shows that Ti doping could reduce the lattice constant c/a ratio. Density functional theory calculations demonstrate that the affected crystal field rather than Ti orbitals is contributing to the valence band convergence and a Seebeck coefficient enhancement. Further optimization incorporates the effects of Bi substitution for reducing the carrier concentrations and introducing more point defects. This work not only confirms the transition metal elements as promising dopants for GeTe-based materials, but also indicates that the strategy of manipulating the crystal field effect can be exploited as a direct but effective route for improving thermoelectric performance.

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

“…Temperature‐dependent thermoelectric transport properties for Cr doping samples Ge 1– x Cr x Te ( x = 0, 0.02, 0.03, 0.04, and 0.05): a) electrical resistivity, b) Seebeck coefficient, c) power factor, d) total thermal conductivity, e) lattice thermal conductivity of Ge 1– x Cr x Te (this work) in comparison with reported Ti x Ge 1– x Te, [ 52 ] In x Ge 1– x Te, [ 59 ] Mn x Ge 1– x Te, [ 43 ] Ag x Ge 1– x Te, [ 60 ] and f) figure of merit (ZT).…”

“…Doping with Bi is found to be more effective than Sb doping for decreasing the lattice c ‐axis parameter and increasing the lattice a ‐axis parameter (Table S2, Supporting Information), which indicates modified band structure by crystal field engineering as reported before. [ 52 ] The successful Bi/Sb substitution along with Cr at Ge site also effectively reduces the carrier concentration as low as ≈2 × 10 20 cm −3 .…”

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

“…Very recently, Li et al proposed a new perspective of tuning the crystal symmetry by Pb alloying and Bi doping to enable the convergence of the split low‐symmetry valence bands, which contributed to a record ZT ≈2.4 at 600 K for the rhombohedral phase. [ 29 ] Similarly, some transition metals, e.g., Mn, [ 65,66 ] Ti, [ 67,68 ] and Cd, [ 69–71 ] are also thought to be effective in reducing the valence band offset of rhombohedral phase, leading to the increased Seebeck coefficient. However, most of the previous work neglected the role of Ge vacancies in the properties optimization.…”

“…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.…”

“…[ 39 ] The most important observation here is that the increased interaxial angle α leads to the reduced power factor, which is contradicted to the conventional understanding of the symmetry engineering in GeTe. [ 29,67,69 ] It has to be noted, however, that the previous studies do not take into account the simultaneously increased Ge‐vacancies concentration during the structural change. As a result of the compromise between the optimized carrier concentration and the reduced charge carrier mobility from the increased Ge‐vacancies concentration, the observed high power factor is confined in a relatively small range, indicated by the red color in Figure 2b.…”