Band Engineering of Thermoelectric Materials

Lead chalcogenides have long been used for space‐based and thermoelectric remote power generation applications, but recent discoveries have revealed a much greater potential for these materials. This renaissance of interest combined with the need for increased energy efficiency has led to active consideration of thermoelectrics for practical waste heat recovery systems—such as the conversion of car exhaust heat into electricity. The simple high symmetry NaCl‐type cubic structure, leads to several properties desirable for thermoelectricity, such as high valley degeneracy for high electrical conductivity and phonon anharmonicity for low thermal conductivity. The rich capabilities for both band structure and microstructure engineering enable a variety of approaches for achieving high thermoelectric performance in lead chalcogenides. This Review focuses on manipulation of the electronic and atomic structural features which makes up the thermoelectric quality factor. While these strategies are well demonstrated in lead chalcogenides, the principles used are equally applicable to most good thermoelectric materials that could enable improvement of thermoelectric devices from niche applications into the mainstream of energy technologies.     

Lanthanide Contraction as a Design Factor for High‐Performance Half‐Heusler Thermoelectric Materials

Forming solid solutions, as an effective strategy to improve thermoelectric performance, has a dilemma that alloy scattering will reduce both the thermal conductivity and carrier mobility. Here, an intuitive way is proposed to decouple the opposite effects, that is, using lanthanide contraction as a design factor to select alloying atoms with large mass fluctuation but small radius difference from the host atoms. Typical half‐Heusler alloys, n‐type (Zr,Hf)NiSn and p‐type (Nb,Ta)FeSb solid solutions, are taken as paradigms to attest the validity of this design strategy, which exhibit greatly suppressed lattice thermal conductivity and maintained carrier mobility. Furthermore, by considering lanthanide contraction, n‐type (Zr,Hf)CoSb‐based alloys with high zT of ≈1.0 are developed. These results highlight the significance of lanthanide contraction as a design factor in enhancing the thermoelectric performance and reveal the practical potential of (Zr,Hf)CoSb‐based half‐Heusler compounds due to the matched n‐type and p‐type thermoelectric performance.     

Entropy as a Gene‐Like Performance Indicator Promoting Thermoelectric Materials

High‐throughput explorations of novel thermoelectric materials based on the Materials Genome Initiative paradigm only focus on digging into the structure‐property space using nonglobal indicators to design materials with tunable electrical and thermal transport properties. As the genomic units, following the biogene tradition, such indicators include localized crystal structural blocks in real space or band degeneracy at certain points in reciprocal space. However, this nonglobal approach does not consider how real materials differentiate from others. Here, this study successfully develops a strategy of using entropy as the global gene‐like performance indicator that shows how multicomponent thermoelectric materials with high entropy can be designed via a high‐throughput screening method. Optimizing entropy works as an effective guide to greatly improve the thermoelectric performance through either a significantly depressed lattice thermal conductivity down to its theoretical minimum value and/or via enhancing the crystal structure symmetry to yield large Seebeck coefficients. The entropy engineering using multicomponent crystal structures or other possible techniques provides a new avenue for an improvement of the thermoelectric performance beyond the current methods and approaches.     

Synergistic Impacts of Electrolyte Adsorption on the Thermoelectric Properties of Single‐Walled Carbon Nanotubes

Single‐walled carbon nanotubes are promising candidates for light‐weight and flexible energy materials. Recently, the thermoelectric properties of single‐walled carbon nanotubes have been dramatically improved by ionic liquid addition; however, controlling factors remain unsolved. Here the thermoelectric properties of single‐walled carbon nanotubes enhanced by electrolytes are investigated. Complementary characterization with absorption, Raman, and X‐ray photoelectron spectroscopy reveals that shallow hole doping plays a partial role in the enhanced electrical conductivity. The molecular factors controlling the thermoelectric properties of carbon nanotubes are systematically investigated in terms of the ionic functionalities of ionic liquids. It is revealed that appropriate ionic liquids show a synergistic enhancement in conductivity and the Seebeck coefficient. The discovery of significantly precise doping enables the generation of thermoelectric power factor exceeding 460 µW m^–1 K^–2.     

Solid‐State Explosive Reaction for Nanoporous Bulk Thermoelectric Materials

High‐performance thermoelectric materials require ultralow lattice thermal conductivity typically through either shortening the phonon mean free path or reducing the specific heat. Beyond these two approaches, a new unique, simple, yet ultrafast solid‐state explosive reaction is proposed to fabricate nanoporous bulk thermoelectric materials with well‐controlled pore sizes and distributions to suppress thermal conductivity. By investigating a wide variety of functional materials, general criteria for solid‐state explosive reactions are built upon both thermodynamics and kinetics, and then successfully used to tailor material's microstructures and porosity. A drastic decrease in lattice thermal conductivity down below the minimum value of the fully densified materials and enhancement in thermoelectric figure of merit are achieved in porous bulk materials. This work demonstrates that controlling materials' porosity is a very effective strategy and is easy to be combined with other approaches for optimizing thermoelectric performance.     

Carbon‐Nanotube‐Based Thermoelectric Materials and Devices

Conversion of waste heat to voltage has the potential to significantly reduce the carbon footprint of a number of critical energy sectors, such as the transportation and electricity‐generation sectors, and manufacturing processes. Thermal energy is also an abundant low‐flux source that can be harnessed to power portable/wearable electronic devices and critical components in remote off‐grid locations. As such, a number of different inorganic and organic materials are being explored for their potential in thermoelectric‐energy‐harvesting devices. Carbon‐based thermoelectric materials are particularly attractive due to their use of nontoxic, abundant source‐materials, their amenability to high‐throughput solution‐phase fabrication routes, and the high specific energy (i.e., W g^?1) enabled by their low mass. Single‐walled carbon nanotubes (SWCNTs) represent a unique 1D carbon allotrope with structural, electrical, and thermal properties that enable efficient thermoelectric‐energy conversion. Here, the progress made toward understanding the fundamental thermoelectric properties of SWCNTs, nanotube‐based composites, and thermoelectric devices prepared from these materials is reviewed in detail. This progress illuminates the tremendous potential that carbon‐nanotube‐based materials and composites have for producing high‐performance next‐generation devices for thermoelectric‐energy harvesting.     

Control of thermoelectric properties in ZrO2/CoSb3 nano‐dispersed composites

Nano‐dispersed ZrO₂/CoSb₃ composites of various composition were prepared by uniaxial hot pressing of nano‐sized powders of thermoelectric CoSb₃ intermixed with ceramic nano‐powders. The phase purity, the microstructure, and the temperature dependent transport parameters of the composites were investigated. Non‐dispersed samples from nano‐sized CoSb₃ powders show higher electrical conductivity compared to melt‐grown material which is attributed to the presence of a small excess of metallic Sb, but they exhibit lower thermal conductivity due to the fine‐grained structure. Addition of 5 at.‐% ZrO₂ enhances the ratio of electrical to thermal conductivity, whereas hardly affects the Seebeck coefficient. In this manner the nano‐dispersion method provides an effective approach to improving the material´s thermoelectric performance.     

Ag2Q-Based (Q = S,Se, Te) Silver Chalcogenide Thermoelectric Materials

Thermoelectric technology provides a promising solution to sustainable energy utilization and scalable power supply. Recently, Ag₂Q-based (Q = S, Se, Te) silver chalcogenides have come forth as potential thermoelectric materials that are endowed with complex crystal structures, high carrier mobility coupled with low lattice thermal conductivity, and even exceptional plasticity. This review presents the latest advances in this material family, from binary compounds to ternary and quaternary alloys, covering the understanding of multi-scale structures and peculiar properties, the optimization of thermoelectric performance, and the rational design of new materials. The “composition-phase structure-thermoelectric/mechanical properties” correlation is emphasized. Flexible and hetero-shaped thermoelectric prototypes based on Ag₂Q materials are also demonstrated. Several key problems and challenges are put forward concerning further understanding and optimization of Ag₂Q-based thermoelectric chalcogenides.     

Controllable Colloidal Synthesis of Tin(II) Chalcogenide Nanocrystals and Their Solution‐Processed Flexible Thermoelectric Thin Films

A systematic colloidal synthesis approach to prepare tin(II, IV) chalcogenide nanocrystals with controllable valence and morphology is reported, and the preparation of solution‐processed nanostructured thermoelectric thin films from them is then demonstrated. Triangular SnS nanoplates with a recently‐reported π‐cubic structure, SnSe with various shapes (nanostars and both rectangular and hexagonal nanoplates), SnTe nanorods, and previously reported Sn(IV) chalcogenides, are obtained using different combinations of solvents and ligands with an Sn⁴⁺ precursor. These unique nanostructures and the lattice defects associated with their Sn‐rich composition allow the production of flexible thin films with competitive thermoelectric performance, exhibiting room temperature Seebeck coefficients of 115, 81, and 153 μV K^?1 for SnS, SnSe, and SnTe films, respectively. Interestingly, a p‐type to n‐type transition is observed in SnS and SnSe due to partial anion loss during post‐synthesis annealing at 500 °C. A maximum figure of merit (ZT) value of 0.183 is achieved for an SnTe thin film at 500 K, exceeding ZT values from previous reports on SnTe at this temperature. Thus, a general strategy to prepare tin(II) chalcogenide nanocrystals is provided, and their potential for use in high‐performance flexible thin film thermoelectric generators is demonstrated.     

High Conductivity and Electron‐Transfer Validation in an n‐Type Fluoride‐Anion‐Doped Polymer for Thermoelectrics in Air

Air‐stable and soluble tetrabutylammonium fluoride (TBAF) is demonstrated as an efficient n‐type dopant for the conjugated polymer ClBDPPV. Electron transfer from F^? anions to the π‐electron‐deficient ClBDPPV through anion–π electronic interactions is strongly corroborated by the combined results of electron spin resonance, UV–vis–NIR, and ultraviolet photoelectron spectroscopy. Doping of ClBDPPV with 25 mol% TBAF boosts electrical conductivity to up to 0.62 S cm^?1, among the highest conductivities that have been reported for solution‐processed n‐type conjugated polymers, with a thermoelectric power factor of 0.63 µW m^?1 K^?2 in air. Importantly, the Seebeck coefficient agrees with recently published correlations to conductivity. Moreover, the F^?‐doped ClBDPPV shows significant air stability, maintaining the conductivity of over 0.1 S cm^?1 in a thick film after exposure to air for one week, to the best of our knowledge the first report of an air‐stable solution‐processable n‐doped conductive polymer with this level of conductivity. The result shows that using solution‐processable small‐anion salts such as TBAF as an n‐dopant of organic conjugated polymers possessing lower LUMO (lowest unoccupied molecular orbital), less than ?4.2 eV) can open new opportunities toward high‐performance air‐stable solution‐processable n‐type thermoelectric (TE) conjugated polymers.     

热电材料的第一性原理高通量研究

热电材料是一种新型能量转换材料, 在温差发电或通电制冷等领域具有广泛应用。热电优值ZT值是衡量热电材料能量转换效率的关键参数, ZT值要求热电材料具有优异的电输运性能及较低的热导率。传统第一性原理热电材料研究往往关注少量样本下的电热输运性质理解与优化, 很难得到系统性的规律, 也不利于新体系的设计优化。材料基因组计划力求通过大数据、高通量手段去加速材料设计与发现, 具有广阔的发展前景。在热电材料研究领域, 第一性原理高通量计算也将在新材料预测与性能优化等方面起到越来越重要的作用。另一方面, 高通量研究也带来了新的挑战, 譬如电热输运性质的高通量算法发展、大数据分析手段等等, 这些方面的问题决定了高通量方法在材料应用中的效率与准确性。本文综述了热电材料中现有的电热输运性质高通量计算方法, 介绍了这些方法具体的应用案例, 并对高通量与热电材料结合的未来发展趋势进行了展望。     

Carbon‐Based Photocathode Materials for Solar Hydrogen Production

Hydrogen is considered a promising environmentally friendly energy carrier for replacing traditional fossil fuels. In this context, photoelectrochemical cells effectively convert solar energy directly to H₂ fuel by water photoelectrolysis, thereby monolitically combining the functions of both light harvesting and electrolysis. In such devices, photocathodes and photoanodes carry out the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively. Here, the focus is on photocathodes for HER, traditionally based on metal oxides, III–V group and II–VI group semiconductors, silicon, and copper‐based chalcogenides as photoactive material. Recently, carbon‐based materials have emerged as reliable alternatives to the aforementioned materials. A perspective on carbon‐based photocathodes is provided here, critically analyzing recent research progress and outlining the major guidelines for the development of efficient and stable photocathode architectures. In particular, the functional role of charge‐selective and protective layers, which enhance both the efficiency and the durability of the photocathodes, is discussed. An in‐depth evaluation of the state‐of‐the‐art fabrication of photocathodes through scalable, high‐troughput, cost‐effective methods is presented. The major aspects on the development of light‐trapping nanostructured architectures are also addressed. Finally, the key challenges on future research directions in terms of potential performance and manufacturability of photocathodes are analyzed.     

聚3, 4-乙烯二氧噻吩: 聚苯乙烯磺酸盐基柔性复合热电材料研究进展

热电材料可以实现热能与电能的直接转化,是一种安全环保的新型能源材料。近年来,随着可穿戴电子设备的发展,柔性热电材料成为研究人员关注的焦点。传统无机热电材料具有优异的热电性能,但由于自身固有的脆性,限制了在柔性领域的发展。聚3, 4-乙烯二氧噻吩: 聚苯乙烯磺酸盐(PEDOT: PSS)具有高电导率、低热导率和良好的柔性,在柔性热电领域具有巨大的潜力。当选择合适的无机填料与PEDOT: PSS进行复合,可以得到优异的热电性能和良好的力学性能。本文综述了PEDOT: PSS基纳米复合薄膜的最新进展,并详细介绍了提高PEDOT: PSS基纳米复合薄膜热电性能的有效方法。最后,本文总结了实现高性能PEDOT: PSS基柔性热电材料的途径及面对的挑战。      

A Chemically Doped Naphthalenediimide‐Bithiazole Polymer for n‐Type Organic Thermoelectrics

The synthesis of a novel naphthalenediimide (NDI)‐bithiazole (Tz2)‐based polymer [P(NDI2OD‐Tz2)] is reported, and structural, thin‐film morphological, as well as charge transport and thermoelectric properties are compared to the parent and widely investigated NDI‐bithiophene (T2) polymer [P(NDI2OD‐T2)]. Since the steric repulsions in Tz2 are far lower than in T2, P(NDI2OD‐Tz2) exhibits a more planar and rigid backbone, enhancing π–π chain stacking and intermolecular interactions. In addition, the electron‐deficient nature of Tz2 enhances the polymer electron affinity, thus reducing the polymer donor–acceptor character. When n‐doped with amines, P(NDI2OD‐Tz2) achieves electrical conductivity (≈0.1 S cm^?1) and a power factor (1.5 µW m^?1 K^?2) far greater than those of P(NDI2OD‐T2) (0.003 S cm^?1 and 0.012 µW m^?1 K^?2, respectively). These results demonstrate that planarized NDI‐based polymers with reduced donor–acceptor character can achieve substantial electrical conductivity and thermoelectric response.     

In‐Plane Anisotropic Thermal Conductivity of Few‐Layered Transition Metal Dichalcogenide Td‐WTe2

2D Td‐WTe₂ has attracted increasing attention due to its promising applications in spintronic, field‐effect chiral, and high‐efficiency thermoelectric devices. It is known that thermal conductivity plays a crucial role in condensed matter devices, especially in 2D systems where phonons, electrons, and magnons are highly confined and coupled. This work reports the first experimental evidence of in‐plane anisotropic thermal conductivities in suspended Td‐WTe₂ samples of different thicknesses, and is also the first demonstration of such anisotropy in 2D transition metal dichalcogenides. The results reveal an obvious anisotropy in the thermal conductivities between the zigzag and armchair axes. The theoretical calculation implies that the in‐plane anisotropy is attributed to the different mean free paths along the two orientations. As thickness decreases, the phonon‐boundary scattering increases faster along the armchair direction, resulting in stronger anisotropy. The findings here are crucial for developing efficient thermal management schemes when engineering thermal‐related applications of a 2D system.     

Realizing zT of 2.3 in Ge1−x−ySbxInyTe via Reducing the Phase‐Transition Temperature and Introducing Resonant Energy Doping

GeTe with rhombohedral‐to‐cubic phase transition is a promising lead‐free thermoelectric candidate. Herein, theoretical studies reveal that cubic GeTe has superior thermoelectric behavior, which is linked to (1) the two valence bands to enhance the electronic transport coefficients and (2) stronger enharmonic phonon–phonon interactions to ensure a lower intrinsic thermal conductivity. Experimentally, based on Ge_1?xSb_xTe with optimized carrier concentration, a record‐high figure‐of‐merit of 2.3 is achieved via further doping with In, which induces the distortion of the density of states near the Fermi level. Moreover, Sb and In codoping reduces the phase‐transition temperature to extend the better thermoelectric behavior of cubic GeTe to low temperature. Additionally, electronic microscopy characterization demonstrates grain boundaries, a high‐density of stacking faults, and nanoscale precipitates, which together with the inevitable point defects result in a dramatically decreased thermal conductivity. The fundamental investigation and experimental demonstration provide an important direction for the development of high‐performance Pb‐free thermoelectric materials.     

Improvement of Low‐Temperature zT in a Mg3Sb2–Mg3Bi2 Solid Solution via Mg‐Vapor Annealing

Materials with high zT over a wide temperature range are essential for thermoelectric applications. n‐Type Mg₃Sb₂‐based compounds have been shown to achieve high zT at 700 K, but their performance at low temperatures (<500 K) is compromised due to their highly resistive grain boundaries. Syntheses and optimization processes to mitigate this grain‐boundary effect has been limited due to loss of Mg, which hinders a sample's n‐type dopability. A Mg‐vapor anneal processing step that grows a sample's grain size and preserves its n‐type carrier concentration during annealing is demonstrated. The electrical conductivity and mobility of the samples with large grain size follows a phonon‐scattering‐dominated T^?3/2 trend over a large temperature range, further supporting the conclusion that the temperature‐activated mobility in Mg₃Sb₂‐based materials is caused by resistive grain boundaries. The measured Hall mobility of electrons reaches 170 cm² V^?1 s^?1 in annealed 800 °C sintered Mg_{3 + δ}Sb_1.49Bi_0.5Te_0.01, the highest ever reported for Mg₃Sb₂‐based thermoelectric materials. In particular, a sample with grain size >30 mm has a zT 0.8 at 300 K, which is comparable to commercial thermoelectric materials used at room temperature (n‐type Bi₂Te₃) while reaching zT 1.4 at 700 K, allowing applications over a wider temperature scale.     

Structurally Controlled Cellular Architectures for High‐Performance Ultra‐Lightweight Materials

The design and synthesis of cellular structured materials are of both scientific and technological importance since they can impart remarkably improved material properties such as low density, high mechanical strength, and adjustable surface functionality compared to their bulk counterparts. Although reducing the density of porous structures would generally result in reductions in mechanical properties, this challenge can be addressed by introducing a structural hierarchy and using mechanically reinforced constituent materials. Thus, precise control over several design factors in structuring, including the type of constituent, symmetry of architectures, and dimension of the unit cells, is extremely important for maximizing the targeted performance. The feasibility of lightweight materials for advanced applications is broadly explored due to recent advances in synthetic approaches for different types of cellular architectures. Here, an overview of the development of lightweight cellular materials according to the structural interconnectivity and randomness of the internal pores is provided. Starting from a fundamental study on how material density is associated with mechanical performance, the resulting structural and mechanical properties of cellular materials are investigated for potential applications such as energy/mass absorption and electrical and thermal management. Finally, current challenges and perspectives on high‐performance ultra‐lightweight materials potentially implementable by well‐controlled cellular architectures are discussed.     

New Directions for Low‐Dimensional Thermoelectric Materials

Many of the recent advances in enhancing the thermoelectric figure of merit are linked to nanoscale phenomena found both in bulk samples containing nanoscale constituents and in nanoscale samples themselves. Prior theoretical and experimental proof‐of‐principle studies on quantum‐well superlattice and quantum‐wire samples have now evolved into studies on bulk samples containing nanostructured constituents prepared by chemical or physical approaches. In this Review, nanostructural composites are shown to exhibit nanostructures and properties that show promise for thermoelectric applications, thus bringing together low‐dimensional and bulk materials for thermoelectric applications. Particular emphasis is given in this Review to the ability to achieve 1) a simultaneous increase in the power factor and a decrease in the thermal conductivity in the same nanocomposite sample and for transport in the same direction and 2) lower values of the thermal conductivity in these nanocomposites as compared to alloy samples of the same chemical composition. The outlook for future research directions for nanocomposite thermoelectric materials is also discussed.     

Thermoelectric GeTe with Diverse Degrees of Freedom Having Secured Superhigh Performance

Driven by the ability to harvest waste heat into reusable electricity and the exclusive role of serving as the power generator for deep spacecraft, intensive endeavors are dedicated to enhancing the thermoelectric performance of ecofriendly materials. Herein, the most recent progress in superhigh‐performance GeTe‐based thermoelectric materials is reviewed with a focus on the crystal structures, phase transitions, resonant bondings, multiple valance bands, and phonon dispersions. These features diversify the degrees of freedom to tune the transport properties of electrons and phonons for GeTe. On the basis of the optimized carrier concentration, strategies of alignment of multiple valence bands and density‐of‐state resonant distortion are employed to further enhance the thermoelectric performance of GeTe‐based materials. To decrease the thermal conductivity, methods of strengthening intrinsic phonon–phonon interactions and introducing various lattice imperfections as scattering centers are highlighted. An overview of thermoelectric devices assembled from GeTe‐based thermoelectric materials is then presented. In conclusion, possible future directions for developing GeTe in thermoelectric applications are proposed. The achieved high thermoelectric performance in GeTe‐based thermoelectric materials with rationally established strategies can act as a reference for broader materials to tailor their thermoelectric performance.