Realization of High Thermoelectric Figure of Merit in GeTe by Complementary Co-doping of Bi and In

Abstract

GeTe and its derivatives have recently attracted wide attention as promising thermoelectric materials. The principle challenge in optimizing the thermoelectric figure of merit, zT, is the low Seebeck coefficient (S) and high thermal conductivity of GeTe. Here, we report a high zT of $2.1 at 723 K in In and Bi codoped GeTe along with an extremely high TE conversion efficiency of $12.3% in a single-leg thermoelectric generator for the temperature difference of 445 K. In and Bi play a distinct but complementary role. In doping significantly enhances the S through the formation of resonance level, which is confirmed with first-principles density functional theory calculations and Pisarenko plot considering two valance band model. However, Bi doping markedly reduces the lattice thermal conductivity due to the formation of extensive solid solution point defects and domain variants. Moreover, a high value of Vickers microhardness ($200 H v , H v = kgf/mm 2 ) reveals excellent mechanical stability.

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°.…”

Features of the High-Temperature Structural Evolution of GeTe Thermoelectric Probed by Neutron and Synchrotron Powder Diffraction

“…It is an open question whether this behavior is correlated to the significant Ge deficiency observed in our sample, since this anomaly is not reported in other works [5,42,43]. However, the aforementioned shoulder is present in other reports of the S vs. T curve [12,44,45] but it had never been correlated to structural features. We describe, therefore, a phenomenological relationship between crystal structure and properties that can be extrapolated to other GeTe-like materials.…”

“…Owing to their excellent electrical transport properties, the thermal transport of GeTe materials have been optimized to show a high-thermoelectric figure of merit, evaluated as zT = Sσ κ T, where T is the absolute temperature, S is the Seebeck coefficient, σ is the electrical conductivity, and κ is the total thermal conductivity [6][7][8][9]. The parent GeTe compound presents high-carrier mobilities and concentration [10,11], which are tuned by dopants such as In, Bi, and Sb, to reduce the carrier concentration to an optimum value in order to obtain high power factors and to reduce the lattice thermal conductivity by enhancing phonon scattering mechanisms [12][13][14]. This phonon scattering in GeTe is also promoted by domain boundaries due to the herringbone microstructure commonly found in the rhombohedral phase [15,16].…”

Is LiI a Potential Dopant Candidate to Enhance the Thermoelectric Performance in Sb-Free GeTe Systems? A Prelusive Study

“…Employing Li + as a "pseudo-vacancy" might have created a substantial mass difference between the atoms involved, which can potentially disturb the lattice vibrational modes and retard the lattice phononic contribution to the thermal conductivity. Nanostructuring can also be a reason for suppression of lattice contribution, as SPS processing is known to create nano-scale inclusions, defects, dislocations, misfit strains, displacement layers, precipitates and micro/nano-porosity [33][34][35][36][37][38][39], all of which, depending on their sizes, are known to scatter phonons of different mean free paths. It can be noted that at around 673 K, there is a change of trend in both electrical and thermal transport properties (for both pristine and LiI-doped GeTe), which is an indication noting the structural transition from the rhombohedral to cubic phase (at ~673 K).…”