Developing high‐performance thermoelectric materials is one of the crucial aspects for direct thermal‐to‐electric energy conversion. Herein, atomic scale point defect engineering is introduced as a new strategy to simultaneously optimize the electrical properties and lattice thermal conductivity of thermoelectric materials, and (Bi,Sb)2(Te,Se)3 thermoelectric solid solutions are selected as a paradigm to demonstrate the applicability of this new approach. Intrinsic point defects play an important role in enhancing the thermoelectric properties. Antisite defects and donor‐like effects are engineered in this system by tuning the formation energy of point defects and hot deformation. As a result, a record value of the figure of merit ZT of ≈1.2 at 445 K is obtained for n‐type polycrystalline Bi2Te2.3Se0.7 alloys, and a high ZT value of ≈1.3 at 380 K is achieved for p‐type polycrystalline Bi0.3Sb1.7Te3 alloys, both values being higher than those of commercial zone‐melted ingots. These results demonstrate the promise of point defect engineering as a new strategy to optimize thermoelectric properties.

Microstructure manipulation plays an important role in enhancing physical and mechanical properties of materials. Here a high figure of merit zT of 1.2 at 357 K for n‐type bismuth‐telluride‐based thermoelectric (TE) materials through directly hot deforming the commercial zone melted (ZM) ingots is reported. The high TE performance is attributed to a synergistic combination of reduced lattice thermal conductivity and maintained high power factor. The lattice thermal conductivity is substantially decreased by broad wavelength phonon scattering via tuning multiscale microstructures, which includes microscale grain size reduction and texture loss, nanoscale distorted regions, and atomic scale lattice distotions and point defects. The high power factor of ZM ingots is maintained by the offset between weak donor‐like effect and texture loss during the hot deformation. The resulted high zT highlights the role of multiscale microstructures in improving Bi2Te3‐based materials and demonstrates the effective strategy in enhancing TE properties.

The abundance of low-temperature waste heat produced by industry and automobile exhaust necessitates the development of power generation with thermoelectric (TE) materials. Commercially available bismuth telluride-based alloys are generally used near room temperature. Materials that are composed of p-type bismuth telluride, which are suitable for low-temperature power generation (near 380 K), were successfully obtained through Sb-alloying, which suppresses detrimental intrinsic conduction at elevated temperatures by increasing hole concentrations and material band gaps. Furthermore, hot deformation (HD)-induced multi-scale microstructures were successfully realized in the high-performance p-type TE materials. Enhanced textures and donor-like effects all contributed to improved electrical transport properties. Multiple phonon scattering centers, including local nanostructures induced by dynamic recrystallization and high-density lattice defects, significantly reduced the lattice thermal conductivity. These combined effects resulted in observable improvement of ZT over the entire temperature range, with all TE parameters measured along the in-plane direction. The maximum ZT of 1.3 for the hot-deformed Bi 0.3 Sb 1.7 Te 3 alloy was reached at 380 K, whereas the average ZT av of 1.18 was found in the range of 300-480 K, indicating potential for application in low-temperature TE power generation. Keywords: bismuth telluride; donor-like effect; hot deformation; low-temperature power generation; texture INTRODUCTION Thermoelectric (TE) devices have attracted extensive interest over the past few decades because of their potential use in direct thermal-toelectrical energy conversion and solid-state refrigeration. The TE conversion efficiency of a material can be gauged by the dimensionless figure of merit ZT ¼ a 2 sT/k, where a, s, k and T are the Seebeck coefficient, the electrical conductivity, the thermal conductivity and the operating temperature, respectively. 1 Continuous effort has been invested toward improving the ZT values of TE materials, resulting in significant advances through phonon engineering 2-9 and band engineering. [10][11][12][13][14] For example, remarkable increases in ZT have been achieved in bulk nanomaterials via the enhancement of phonon scattering at boundaries to reduce lattice thermal conductivities. 2,4,6,7 Currently, the best commercial TE materials near room temperature are still rhombohedral bismuth tellurides and related solid solutions fabricated by unidirectional crystal growth. [15][16][17] Nanostructuring strategies have been devised to prepare highperformance bismuth telluride-based alloys, including bottom-up

TE material research has attracted intense interest over the past few decades. [1] The TE performance of a material is measured by zT = σS 2 T/κ, where T, σ, S, and κ are the absolute temperature, electrical conductivity, Seebeck coefficient, and total thermal conductivity, respectively. Typically, κ = κ el + κ ph , where the κ el and κ ph are the carrier and lattice thermal conductivity, respectively. Since σ, S, and κ el are adversely interrelated whereas the κ ph is relatively independent of σ, S, and κ el , the stride toward high zT is in line with a two-pronged strategy, coined by Slack as "electron-crystal phonon-glass" (ECPG): [2] i) decoupling σ, S, and κ el through band structure engineering toward a high power factor (PF) = σS 2 ; [3,4] and ii) suppressing the κ ph via all-scale hierarchical microstructures. [5][6][7] Rooted in the core effects of high entropy alloys (HEAs), entropy engineering enables a synergy of band structure engineering and multiscale hierarchical microstructures through high entropy alloying. HEAs typically refer to the solid solutions in which more than five principal elements each in 5-35% molar ratio compete for the same crystallographic site, yielding high entropy of mixing and a wider variety of exciting properties. [8] HEA is a subset of multielement-doped materials. Neither the doping process nor the resulting composition would differentiate aThe core effects of high entropy alloys distinguish high entropy alloying from ordinary multielement doping, allowing for a synergy of band structure and microstructure engineering. Here, a systematic synthesis, structural, theoretical, and thermoelectric study of multi-principal-element-alloyed SnTe is reported. Toward high thermoelectric performance, the entropy of mixing needs to be high enough to make good use of the core effects, yet low enough to minimize the degradation of carrier mobility. It is demonstrated that high entropy of mixing extends the solubility limit of Mn while retaining the lattice symmetry, the enhanced Mn content elicits multiscale microstructures. The resulting ultralow lattice thermal conductivity of ≈0.32 W m −1 K −1 at 900 K in (Sn 0.7 Ge 0.2 Pb 0.1 ) 0.75 Mn 0.275 Te is not only lower than the amorphous limit of SnTe but also comparable to those thermoelectric materials with complex crystal structures and strong anharmonicity. Co-alloying of (Sn,Ge,Pb,Mn) also enhances band convergence and band effective mass, yielding good power factors. Further tuning of the Sn excess yields a state-of-the-art zT ≈1.42 at 900 K in (Sn 0.74 Ge 0.2 Pb 0.1 ) 0.75 Mn 0.275 Te. In view of the simple face-centeredcubic structure of SnTe-based alloys, these results attest to the efficacy of entropy engineering toward a new paradigm of high entropy thermoelecrics.

Defects and defect engineering are at the core of many regimes of material research, including the field of thermoelectric study. The 60‐year history of V2VI3 thermoelectric materials is a prime example of how a class of semiconductor material, considered mature several times, can be rejuvenated by better understanding and manipulation of defects. This review aims to provide a systematic account of the underexplored intrinsic point defects in V2VI3 compounds, with regard to (i) their formation and control, and (ii) their interplay with other types of defects towards higher thermoelectric performance. We herein present a convincing case that intrinsic point defects can be actively controlled by extrinsic doping and also via compositional, mechanical, and thermal control at various stages of material synthesis. An up‐to‐date understanding of intrinsic point defects in V2VI3 compounds is summarized in a (χ, r)‐model and applied to elucidating the donor‐like effect. These new insights not only enable more innovative defect engineering in other thermoelectric materials but also, in a broad context, contribute to rational defect design in advanced functional materials at large.

Understanding the electron and phonon transport characteristics is crucial for designing and developing high performance thermoelectric materials. Weak scattering effects on charge carriers, characterized by deformation potential and alloy scattering potential, are favorable for thermoelectric solid solutions to enable high carrier mobility and thereby promising thermoelectric performance. Mg2(Si,Sn) solid solutions have attracted much attention due to their low cost and environmental compatibility. Usually, their high thermoelectric performance with ZT ∼ 1 is ascribed to the band convergence and reduced lattice thermal conductivity caused by alloying. In this work, both a low deformation potential Ξ = 13 eV and a low alloy scattering potential U = 0.7 eV are found for the thermoelectric alloys by characterizing and modeling of thermoelectric transport properties. The band convergence is also verified by the increased density‐of‐states effective mass. It is proposed that, in addition to band convergence and reduced lattice thermal conductivity, the low deformation potential and alloy scattering potential are additional intrinsic features that contribute to the high thermoelectric performance of the solid solutions.

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Large size bismuth telluride based polycrystalline alloys have been prepared to investigate the texture-related anisotropy of thermoelectric properties. X-ray diffraction analysis indicated that the n-type alloy was more easily (001) oriented than the p-type one. Both the electrical and thermal conductivities showed strong anisotropy in the textured samples, while the Seebeck coefficient and the ZT were almost isotropic. However, as the electrical and thermal conductivities were measured along different directions, the ZT could be overestimated up to ∼60%. A relationship between the texture degree and the anisotropy of the thermal conductivity has been proposed.