Grain Boundary Complexions Enable a Simultaneous Optimization of Electron and Phonon Transport Leading to High‐Performance GeTe Thermoelectric Devices

Abstract

The heat-to-electricity conversion efficiency depends on the dimensionless figure-of-merit of materials, defined as ZT = S 2 σT/κ, where S, σ, and T are the Seebeck coefficient, the electrical conductivity, and the absolute temperature, respectively. The total thermal conductivity (κ) consists of the electronic thermal conductivity (κ e ) and the lattice thermal conductivity (κ l ). The ZT value is directly proportional to several physical para meters summarized in the so-called materials quality factor, B∝µ W /κ l , at a given carrier concentration (n), where the weighted mobility (µ W ) and κ l are determined by the transport properties of electrons and phonons, respectively. [5] Hence, it is desirable to maximize the weighted mobility and minimize the lattice thermal conductivity. Yet, similarities in performance change of thermal and electrical transport for thermoelectrics impede the improvement of B factor for high ZT, which is the key challenge in optimizing TE materials. [6] Sustained efforts have been undertaken to improve the TE properties of materials, such as band convergence, [7] resonant doping, [8] energy filtering, [9] high entropy, [10] and hierarchical microstructure engineering. [11] For polycrystalline bulk samples, the most ubiquitous defects are grain boundaries (GBs).Grain boundaries (GBs) form ubiquitous microstructures in polycrystalline materials which play a significant role in tuning the thermoelectric figure of merit (ZT). However, it is still unknown which types of GB features are beneficial for thermoelectrics due to the challenge of correlating complex GB microstructures with transport properties. Here, it is demonstrated that GB complexions formed by Ga segregation in GeTe-based alloys can optimize electron and phonon transport simultaneously. The Ga-rich complexions increase the power factor by reducing the GB resistivity with slightly improved Seebeck coefficients. Simultaneously, they lower the lattice thermal conductivity by strengthening the phonon scattering. In contrast, Ga 2 Te 3 precipitates at GBs act as barriers to scatter both phonons and electrons and are thus unable to improve ZT. Tailoring GBs combined with the beneficial alloying effects of Sb and Pb enables a peak ZT of ≈2.1 at 773 K and an average ZT of 1.3 within 300-723 K for Ge 0.78 Ga 0.01 Pb 0.1 Sb 0.07 Te. The corresponding thermoelectric device fabricated using 18-pair p-n legs shows a power density of 1.29 W cm −2 at a temperature difference of 476 K. This work indicates that GB complexions can be a facile way to optimize electron and phonon transport, further advancing thermoelectric materials.

Doping by Design: Enhanced Thermoelectric Performance of GeSe Alloys Through Metavalent Bonding

“…[19,20] Taking PbTe as an example, the band convergence with aligned band extrema is realized by doping with Se, [11] Eu, [21] and Mg; [22] and the band anisotropy with different longitudinal and transverse band effective masses can be modified by doping with Mn. [23] Similarly, the TE performance of GeTe can be largely improved by doping with Sb, [24] In, [24] Bi, [25] Pb, [25,26] Mn, [27] Cd, [28] etc. It seems that chemical doping is a universal approach to increase zT.…”

Grain Boundary Phases in NbFeSb Half‐Heusler Alloys: A New Avenue to Tune Transport Properties of Thermoelectric Materials

“…As the GB scattering term does not follow the same temperature dependence, this is consistent with the major discrepancy from the electron–phonon T −3/2 trend (metallic behavior) at lower temperatures. The GB‐dominated electron scattering has been reported in many fine‐grained thermoelectric materials, including Mg 3 Sb 2 , [ 52 ] ZrCoSb, [ 42 ] NbCoSn, [ 81 ] GeTe, [ 68 ] and TiCoSb. [ 82 ] At larger grain sizes (>1 µm), the GBs are sufficiently far apart so that their contribution to the scattering term becomes negligible.…”

Doping Copper Selenide for Tuning the Crystal Structure and Thermoelectric Performance of Germanium Telluride-Based Materials

“…Anion I as a dopant shows a brilliant TE performance due to its multimechanisms accounting for carrier and phonon transport characteristics . Grain boundary complexions formed by Ga-doped GeTe compounds optimize the electrical and thermal transport simultaneously leading to a lower κ lat by strengthening the phonon scattering . However, the phase transition of GeTe still occurs, which has a serious impact on the practical application of the material.…”

“…24 Grain boundary complexions formed by Gadoped GeTe compounds optimize the electrical and thermal transport simultaneously leading to a lower κ lat by strengthening the phonon scattering. 25 However, the phase transition of GeTe still occurs, which has a serious impact on the practical application of the material. In addition, for a thermoelectric material, a structure of high symmetry often has an intrinsically high energy band degeneracy, which enables high thermoelectric performance.…”