The past two decades have witnessed the rapid growth of thermoelectric (TE) research. Novel concepts and paradigms are described here that have emerged, targeting superior TE materials and higher TE performance. These superior aspects include band convergence, "phonon-glass electron-crystal", multiscale phonon scattering, resonant states, anharmonicity, etc. Based on these concepts, some new TE materials with distinct features have been identified, including solids with high band degeneracy, with cages in which atoms rattle, with nanostructures at various length scales, etc. In addition, the performance of classical materials has been improved remarkably. However, the figure of merit zT of most TE materials is still lower than 2.0, generally around 1.0, due to interrelated TE properties. In order to realize an "overall zT > 2.0," it is imperative that the interrelated properties are decoupled more thoroughly, or new degrees of freedom are added to the overall optimization problem. The electrical and thermal transport must be synergistically optimized. Here, a detailed discussion about the commonly adopted strategies to optimize individual TE properties is presented. Then, four main compromises between the TE properties are elaborated from the point of view of the underlying mechanisms and decoupling strategies. Finally, some representative systems of synergistic optimization are also presented, which can serve as references for other TE materials. In conclusion, some of the newest ideas for the future are discussed.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/adma.201702816

Compromise and Synergy in High-Efficiency Thermoelectric Materials.

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.

Point Defect Engineering of High-Performance Bismuth-Telluride-Based Thermoelectric Materials

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.

https://onlinelibrary.wiley.com/doi/epdf/10.1002/advs.201600004

New Insights into Intrinsic Point Defects in V2VI3 Thermoelectric Materials

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 Bi0.3Sb1.7Te3 alloy was reached at 380 K, whereas the average ZTav of 1.18 was found in the range of 300–480 K, indicating potential for application in low-temperature TE power generation. Thermoelectric materials, which convert temperature differences and electric voltage into each other, serve in refrigeration or power generation applications. Currently, bismuth telluride (Bi2Te3) and its alloys are the most widely used thermoelectric materials. Tie-Jun Zhu, Xin-Bing Zhao and co-workers from Zhejiang University, China, have now investigated the effect of antimony (Sb) alloying on bismuth tellurides through a series of polycrystalline solid solutions of Bi2-xSbxTe3—where x varies between 1.4 and 1.8—prepared by hot deformation. Systematic tuning of the alloy composition showed that higher antimony content raised the material's optimal conversion temperature by repressing undesirable conduction. This effect arises from an increase in both the hole concentration and the band gap in the material. For a composition where x is 1.7, the alloy showed optimal performances at 380 kelvin—a suitable temperature for low-temperature power generation from the waste heat generated by industry or vehicles. The p-type bismuth telluride-based polycrystalline materials suiting for low-temperature power generations (near 380 K) have been obtained through Sb-alloying and HD, which suppresses the detrimental effect of intrinsic conduction at elevated temperature via increasing the hole concentration and band gap. The hot-deformed Bi0.3Sb1.7Te3 alloy, not usual composition Bi0.5Sb1.5Te3, shows a maximum ZT of 1.3 at 380 K, indicating a bright application potential in low-temperature power generations.

https://cyberleninka.org/article/n/1030749.pdf

Shifting up the optimum figure of merit of p -type bismuth telluride-based thermoelectric materials for power generation by suppressing intrinsic conduction

Topological surface states, a new kind of electronic state of matter, have recently been observed on the cleaved surfaces of crystals of a handful of small band gap semiconductors. The underlying chemical factors that enable these states are crystal symmetry, the presence of strong spin–orbit coupling, and an inversion of the energies of the bulk electronic states that normally contribute to the valence and conduction bands. The goals of this review are to briefly introduce the physics of topological insulators to a chemical audience and to describe the chemistry, defect chemistry, and crystal structures of the compounds in this emergent field.

M. Stölzer

Papers

Antisite defects in Sb2-xBixTe3 mixed crystals