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Herein, wooden-stick-based thermoelectric legs are introduced as a new type of thermoelectric legs. As a proof of concept, graphite- and bismuth-coated wooden sticks are fabricated with a green method, and their use as thermoelectric elements is successfully demonstrated by constructing a test thermoelectric generator. The generator based on 20 pairs of p–n legs shows a maximum power output of ∼18 nW on heating one side of the device to 70°C. The strategy can suitably be extended to other good thermoelectric materials. The approach can be promising due to the ultralow thermal conductivity of the wooden substrate pillars in contrast to other typical substrates such as glass, and hence, better thermal gradients across the device can be achieved. These environmentally friendly, non-toxic and low-cost wooden-stick-based thermoelectric devices with good thermoelectric material coatings can be conveniently used for low-grade industrial waste heat conversion applications.

Calculations of AgSbTe2 thermoelectric material and 2d CdS/TiS2 and their heterostructures were carried out using Density Functional Theory in Cambridge Serial Total Energy Package code as implemented in Material Studio 2018 software. The work function, thermal transport, electronic and optical properties were calculated. The results revealed that the heterostructures are possible to be achieved with improved properties. The electronic and thermal transport properties were likened with the description of equations derived from Boltzmann transport theory and Mott expressed in the maximum achievable Figure merit. Orbital contributions from the electron movement show valence and conduction band atomic shells.

Bi2Te3 based alloys have been the most widely used thermoelectric material at low temperature for many decades. Here we report Se doped n-type Mg3Bi2 based materials with a thermoelectric figure-of-merit ZT of 0.82 at 300 K and a peak ZT of 1.24 at 498 K, which is comparable to the n-type Bi2Te3 and Te doped Mg3Bi1.4Sb0.6. The improved thermoelectric performance is benefited from the high carrier concentration and mobility as well as the thermal conductivity reduction. The reduced resistivity increased the power factor at all measured temperatures, leading to a higher engineering ZT (ZTeng) and engineering power factor (PFeng) for n-type Mg3Bi2. The n-type Mg3Bi1.4Sb0.6 materials are promising for thermoelectric power generation and cooling applications near room temperature.

Recently, oxide-based materials have proven to be potential thermoelectric materials at high temperatures. In this work, the thermoelectric properties of one-step solid-state sintered Sn1-xSbxO2 (x = 0, 0.005, 0.01, 0.02, 0.03, 0.04) ceramic pellets were investigated in detail. It was confirmed that the addition of Sb significantly alters the thermoelectric properties of SnO2 due to the increase in the carrier concentration, which increases the electrical conductivity. The Seebeck coefficient values of all the solid solutions were negative, which indicates that these samples have n-type conduction. The thermoelectric performance of the material was evaluated by determining the zT value and the best composition was Sn0·98Sb0·02O2 with zT ∼0.06 at 1073 K.

CuPbBi5S9 compounds have been investigated as gladite for years. However, there have been no significant studies on their physical and chemical properties. This work demonstrates that upon alloying with moderate Cu, Pb, Bi, and S using an appropriate preparation method, quaternary CuPbBi5S9 compounds can exhibit excellent figure of merit ZT within the temperature range 298–723 K. A low average velocity, low Young’s modulus and Debye temperature, and large Gruneisen parameter, determined experimentally, indicate strong lattice anharmonicity in CuPbBi5S9 crystals. Furthermore, density functional theory calculations (local vibration of low-frequency acoustic phonons) justify the low lattice thermal conductivity of CuPbBi5S9 compounds. Because of the low thermal conductivity (0.514 W m−1K−1) and a relatively high power factor (293 μW m−1K−2), a maximum ZT of 0.42 was achieved at 723 K for CuPbBi5S9 prepared by mechanical alloying combined with solid-state melting. Thus, CuPbBi5S9 materials are promising candidates for use as high-performance thermoelectric materials in the intermediate-temperature range.

Thermoelectric (TE) materials are receiving increasing attention due to their ability to directly converting heat to electricity. They are used to harvest electrical energy from the wasted heat in order to increase the efficiency of global energy. Polymer-based TE materials are particularly fascinating to wearable and mobile devices due to their low density, good flexibility, and low toxicity. This review summarizes the recent breakthroughs and optimization strategies of polymer-based TE materials. Among a large number of different organic TE materials, those with remarkable TE performance are selected and divided into three categories, which are poly(3,4-ethylenedioxythiophene) derivatives, carbon nanotube/conductive polymer composites, and inorganic semiconductive nanomaterial/polymer composites. The effect of components and structures on the power factor are presented and discussed. Finally, some challenges are described and suggestions are provided for preparing the next-generation of polymer-based materials with high TE performance.

Thermoelectric devices enable direct conversion between thermal and electrical energy. Recent studies have indicated that the thin film/substrate heterostructure is effective in achieving high thermoelectric performance via decoupling the Seebeck coefficient and electrical conductivity otherwise adversely inter-dependent in homogenous bulk materials. However, the mechanism underlying the thin film/substrate heterostructure thermoelectricity remains unclear. In addition, the power output of the thin film/substrate heterostructure is limited to the nanowatt scale to date, falling short of the practical application requirement. Here, we fabricated the CrN/SrTiO3−x heterostructures with high thermoelectric output power and outstanding thermal stability. By varying the CrN film thickness and the reduction degree of SrTiO3−x substrate, the optimized power output and the power density have respectively reached 276 μW and 108 mW/cm2 for the 30 nm CrN film on a highly reduced surface of SrTiO3−x under a temperature difference of 300 K. The performance enhancement is attributed to the CrN/SrTiO3−x heterointerface, corroborated by the band bending as revealed by the scanning Kelvin probe microscopy. These results will stimulate further research efforts towards interface thermoelectrics.

Composites were prepared, through hot pressing, using carbon materials with different pore size distributions as additives for commercial Bi0.5Sb1.5Te3 thermoelectric material (BST, p-type). Thermoelectric properties of the composites were measured in a temperature range of 298‒473 K. Thermal conductivity of the composites, especially lattice thermal conductivity, was effectively decreased due to the mesoporous properties of the incorporated carbon additives. The electrical conductivity of the composites slightly decreased due to the electron scattering at the interface between the carbon material and the commercial BST matrix. The composite with 0.2 vol.% mesoporous carbon powder (36% mesoporosity) exhibited a figure of merit value approximately 10.7% higher than that of commercial BST without additives. This behavior resulted in 116% improved output power in the composite block-based single element compared with a bare BST thermoelectric block. The enhanced figure of merit was attributed to the effective reduction of lattice thermal conductivity by acoustic phonons scattering at the interface between the BST matrix and the mesoporous carbon as well as at the pore surfaces within the mesoporous carbon. By utilizing mesoporous carbon materials used in this study, the shortcomings and economic difficulties of the composite process with low dimensional carbon additives (carbon nanotubes, graphene, and nanodiamond) can be overcome for extensive practical applications. Mesoporous carbon powder with a tailored porosity distribution revealed the validity of bulk-type carbon additives to enhance the figure of merit of commercial thermoelectric materials.

Various strategies for thermoelectric material optimization have been widely studied and used for promoting electrical transport and suppressing thermal transport. As a nontraditional method, pressure has shown great potential, as it has been applied to obtain a high thermoelectric figure of merit, but the microscopic mechanisms involved have yet to be fully explored. In this study, we focus on r-GeTe, a low-temperature phase of GeTe, and investigate the pressure effects on the electronic structure, electrical transport properties and anharmonic lattice dynamics based on density functional theory (DFT), the Boltzmann transport equations (BTEs) and perturbation theory. Electronic relaxation times are obtained based on the electron-phonon interaction and the constant relaxation time approximation. The corresponding electrical transport properties are compared with those obtained from previous experiments. Hydrostatic pressure is shown to increase valley degeneracy, decrease the band effective mass and enhance the electrical transport property. At the same time, the increase in the low-frequency phonon lifetime and phonon group velocity leads to an increase in lattice thermal conductivity under pressure. This study provides insight into r-GeTe under hydrostatic pressure and paves the way for a high-pressure strategy to optimize transport properties.

Electronic structures, which play a key role in determining electrical and optical properties of π-conjugated organic materials, have attracted tremendous interest. Efficient thermoelectric (TE) conversion of organic materials has rigorous requirements on electronic structures. Recently, the rational design and precise modulation of electronic structures have exhibited great potential in exploring state-of-the-art organic TE materials. This review focuses on the regulation of electronic structures of organic materials toward efficient TE conversion. First, we present the basic knowledge regarding electronic structures and the requirements for efficient TE conversion of organic materials, followed by a brief introduction of commonly used methods for electronic structure characterization. Next, we highlight the key strategies of electronic structure engineering for high-performance organic TE materials. Finally, an overview of the electronic structure engineering of organic TE materials, along with current challenges and future research directions, are provided.

Cu- and Ag-based superionic conductors are promising thermoelectric materials due to their good electrical properties and intrinsically low thermal conductivity. However, the poor electrical and thermal stability restrict their application. In this work, n-type pure phase Ag2Te compound is synthesized by simply grinding elemental powders at room temperature and compacted by spark plasma sintering. It is found that, because of the migration of Ag+ after the phase transition around 425 K, submicron pores are formed inside the samples during the electrical performance measurement, resulting in poor electrical stability and repeatability of Ag2Te samples. However, Pb-doped Ag2-xPbxTe (x = 0–0.05) specimens exhibit improved electrical stability by the precipitation of the secondary phase PbTe in the Ag2Te matrix, which is confirmed via cyclic electrical property measurement and microstructure characterization. A maximum zT = 0.72 is obtained at 570 K for x = 0.03 mainly due to the increased power factor.

Metal oxide as a thermoelectric material has shown great attention in recent years due to its outstanding thermal and chemical stability, good oxidation resistance, low cost, etc. This article reports the successful synthesis of tungsten oxide (WO3) nanoparticles and tungsten oxide-reduced graphene oxide (WO3-rGO) nanocomposites and studies of their thermoelectric properties. The structural characteristics of the WO3 nanoparticles and WO3-rGO nanocomposites were examined using XRD and Raman techniques. At 313 K, WO3-rGO nanocomposites exhibit an ~ 8.8 times enhancement in the thermoelectric material figure of merit (zT). These excellent performances are further supported by up to ~ 13.7 times enhanced electrical conductivity and a ~ 56% reduction in thermal conductivity compared to WO3 nanoparticles. It has been concluded that reduce graphene oxide helps in forming a conducting pathway in the WO3-rGO nanocomposites, allowing the easy passage of charge carriers in the matrix.

Co0.91Ni0.09Sb3 embedded with Cu nanoparticles (NPs) was synthesized by applying a hydrothermal process followed by spark plasma sintering. The effects of Cu NPs on the electrical and thermal transport properties of the Co0.91Ni0.09Sb3 matrix were investigated in the temperature range of 303–773 K. Cu NPs introduced into the Co0.91Ni0.09Sb3 matrix slightly decreased the electrical conductivity and significantly enhanced the Seebeck coefficient by the energy filtering effect, leading to an improved power factor at 773 K. Furthermore, a significant decrease in the thermal conductivity was realized by increasing the number of grain boundaries. These favorable effects contributed to the improvement in ZT to a value ~57% higher than that of pristine Co0.91Ni0.09Sb3. The highest ZT value of 1.02 was achieved by incorporating 4.5 wt% of Cu NPs in Co0.91Ni0.09Sb3 at 773 K. Thus, decoupling the electrical and thermal transport properties of thermoelectric materials is a viable strategy for optimizing the ZT value.

The discovery of band convergence has opened an effective avenue for significantly enhancing thermoelectric performance of SnTe, while alloying CdTe in SnTe is evidenced efficient for improving the valley degeneracy. However, the thermoelectric transport properties are limited due to the low solubility of CdTe in SnTe (∼3%). Inspired by the improvement of dimensionless figure of merit zT in Cu or Se-doped SnTe, investigating the effect of Cu2Se on the electronic and phonon transport properties of SnTe-CdTe alloys is highly desired. Traditionally, improving the quality factor can trigger an increase of the potential of a compound for higher zT, which is of importance for design of thermoelectric materials. Here, alloyed 3% Cu2Se in SnTe-3%CdTe system enables an increased peak zT, which is attributed by the optimization of electronic performance (∼21 μW cm−1 K-2 at 800 K), as well as the decreased lattice thermal conductivity owing to the enhanced mass and strain fluctuations. More importantly, alloying Cu2Se not only improves the quality factor from ∼0.25 to ∼0.45, resulting in a higher maximum potential zT, but also effectively preserves the Fermi energy in a relative optimized level. The current findings demonstrate the role of Cu2Se for manipulating thermoelectrics in SnTe.

As an eco-friendly thermoelectric material, SnTe has attracted extensive attention. In this study, we use a stepwise strategy to enhance the thermoelectric performance of SnTe. Firstly, AgCl is doped into SnTe to realize band convergence and enlarge the band gap of AgCl-doped SnTe. AgCl-doping also induces dense point defects, strengthens the phonon scattering, and reduces the lattice thermal conductivity. Secondly, Sb is alloyed into AgCl-doped SnTe to further optimize the carrier concentration and simultaneously reduce the lattice thermal conductivity, leading to improved thermoelectric dimensionless figure of merit, ZT. Finally, (Sn0.81Sb0.19Te)0.93(AgCl)0.07 has approached the ZT value as high as ∼0.87 at 773 K, which is 272 % higher than that of pristine SnTe. This study indicates that stepwise AgCl-doping and Sb-alloying can significantly improve thermoelectric performance of SnTe due to synergistic band engineering, carrier concentration optimization and defect engineering.

Flexible thermoelectric materials are presented with potential applications in electronic devices and energy conversion due to their convenient preparation, good flexibility, and various forms. However, as ductility is rarely observed in inorganic semiconductors and ceramic insulators, reports on applications of inorganic oxide materials in flexible thermoelectric materials are sparse. Here, we report a new method for the synthesis of a flexible Na1.4Co2O4 thermoelectric material based on Na1.4Co2O4 bulk materials, which are prepared by a self-flux method and painted on print paper. Seebeck coefficient and power factor of the obtained thermoelectric material are 78–102 μVK-1 and 159–223 μWm-1K-2, respectively, in a temperature range of 303–522 K, which are superior to those values of other conductive polymers and their compounds. More interestingly, the n-type Na1.4Co2O4 flexible material is obtained in the painting process at higher pressure with Seebeck coefficients of –109 to –183 μVK-1 in a temperature range of 303–522 K. The convenient preparation method of these novel flexible thermoelectric materials may be expanded to the synthesis of other flexible thermoelectric materials, which will be the focus of future work.

Skutterudite CoSb3 thermoelectric materials were synthesized by a combination of melt spinning (MS) and consolidation by spark plasma sintering (SPS). Microstructural analysis was carried out on the MS-SPS samples using various analytical techniques such as XRD, SEM, EBSD and DSC/TGA. In the melt-spun ribbons, the initial microstructure consisted of a multiphase mixture of Sb and CoSbx (x = 1, 2, or 3) where CoSb3 is the only thermoelectric phase. Subsequent SPS processing caused conversion of other phases present (Sb, CoSb and CoSb2) into single-phase CoSb3 and recrystallization concurrently. The phase conversion mechanism has been explained in the present work using a thermodynamic interpretation. Thermoelectric transport measurement results show that the thermal conductivity of MS-SPS sample was significantly decreased as compared with that of the sample prepared by traditional methods combined with SPS (TM-SPS) due to the reduction of its grain size. Compared with TM-SPS sample, MS-SPS sample has a high ZT value at room temprerature because of its high electrical conductivity and low lattice thermal conductivity.

Ag/Cu-based argyrodites have recently received extensive attention as promising thermoelectric materials. This can be largely understood by the hierarchical chemical bonding structure that allows the existence of high-concentration and highly mobile Ag+/Cu+ ions distributed in the rigid polyanionic framework, since the diffusive cations enable an effective phonon scattering for phononically insulating as that in woods while the rigid framework ensures an electronic charge transport as that in trivial thermoelectric semiconductors. This review focused firstly on the synthesis of both single-crystalline and poly-crystalline argyrodites as well as the chemical compositions, phases and crystallographic features; secondly on the origins of the low lattice thermal conductivity including crystal structure complexity, weak bonded ions, strong lattice anharmonicity and low sound velocity; and thirdly on the electronic band structure characteristics including effective mass and band degeneracy for an evaluation on the electronic transport properties, as well as performance stability. These together lead a perspective on future development of thermoelectric argyrodites to emphasize the enhancement of electronic performance such as by carrier concentration and by manipulating the electronic band structure.

Vanadium dioxide (VO2) is known as a typical 3d-orbital transition metal oxide exhibiting the metal-to-insulator-transition (MIT) property near room temperature. However, their electronic applications have been challenged by the quality and uniformity of VO2 thin films. In this work, we demonstrate the high sensitivity in the valence charge of vanadium and the MIT properties of the VO2 thin films to the deposition temperature. This observation indicates the necessity to eliminate the inhomogeneity in the temperature distribution of substrate during the vacuum-deposition process of VO2. In addition, a high thermoelectric power factor (PF, e.g., exceeding 1 μWcm−1K−2) was achieved in the metallic phase of the VO2 thin films and this value is comparable to typical organic or oxide thermoelectric materials. We believe this high PF enriches the potential functionality in thermoelectric energy conversions beyond the existing electronic applications of the current vacuum-grown VO2 thin films.

Tin telluride (SnTe) overwhelmingly continues to be studied owing to its promising thermoelectric properties, tunable electronic structure, and its potential as an alternate to toxic lead telluride (PbTe) based materials. In this research, we engineer the electronic properties of SnTe by co-doping Mn and Bi below their individual solubility limit. The First principles density functional theory studies reveal that both Bi and Mn introduce resonance states, thereby increasing the density of states near the Fermi level leading to enhanced Seebeck coefficient. This unique combination of using two resonant dopants to introduce flatter bands is effective in achieving higher performance at lower temperatures manifesting into a large Seebeck value of ∼91 μV/K at room temperature in the present case. Both elements optimally co-doped results in a very high power factor value of ∼24.3 μW/cmK2 at 773 K when compared to other high performance SnTe based materials. A zT of ∼0.93 at 773 K is achieved by tuning the proportion of the co-dopants Mn and Bi in SnTe. The hardness value of pristine SnTe was also seen to increase after doping. As a result, synergistic optimized doping proves to be a suitable means for obtaining thermoelectric materials of superior characteristics without the need for heavy doping.

Nature materials (2021). doi:10.1038/s41563-021-01064-6, Published by Nature Publishing Group, Basingstoke

The management of thermal conductivity is of significant scientific interest, particularly for thermal barrier coatings (TBCs). Multifarious strategies have been used to regulate heat transportation, but it is hard to achieve limit thermal conductivity at elevated temperatures. A systematical investigation of weberite (La1-xGdx)3NbO7 was thus performed, and multiphonon scattering mechanisms were introduced to achieve limit thermal conductivity (0.92 W m−1 K−1). Phonon point defect scattering process accounted for thermal conductivity reduction at low temperatures. Additionally, lattice softening strongly contributed to the reduction of high-temperature thermal conductivity, and solid and stiff chemical bonds were beneficial for inhibiting thermal radiative conductivity. A novel strategy was presented to modify thermal transportation property of weberite RE3NbO7 ceramics. Also, the hardness, toughness, and modulus were improved to promote engineering applications of weberite RE3NbO7. This study also illuminates novel paths for thermal management and mechanical properties manipulation of TBCs, thermoelectric materials, and microelectronics.

Thermoelectric materials transfer heat and electrical energy, being useful for power generation or cooling applications. Many of these materials have narrow bandgaps, especially for cooling applications where this property has been seen as particularly important for enhancing the thermoelectric properties. We developed SnSe crystals with a wide bandgap Eg ~ 33 kBT with attractive thermoelectric properties through Pb alloying. The momentum and energy multiband alignment promoted by Pb alloying resulted in an ultra-high power factor ~75 μWcm–1K–2 at 300 K, and a ZTave ~ 1.90. We show that a 31-pair thermoelectric device can produce a power generation efficiency ~4.4% and a cooling ΔTmax ~ 45.7 K. These results demonstrate that wide bandgap compounds can be used for thermoelectric cooling applications.

The performance of thermoelectric materials is determined by their electrical and thermal transport properties that are very sensitive to small modifications of composition and microstructure. Discovery and design of next-generation materials are starting to be accelerated by computational guidance. We review progress and challenges in the development of accurate and efficient first-principles methods for computing transport coefficients and illustrate approaches for both rapid materials screening and focused optimization. Particularly important and challenging are computations of electron and phonon scattering rates that enter the Boltzmann transport equations, and this is where there are many opportunities for improving computational methods. We highlight the first successful examples of computation-driven discoveries of high-performance materials and discuss avenues for tightening the interaction between theoretical and experimental materials discovery and optimization.

Intrinsic partial occupation of Cu induces strong phonon scattering for an extremely low lattice thermal conductivity in TmCuTe2, which leads this material to be a promising candidate for thermoelectric applications. This work focus on the electronic and phononic transport properties of TmCuTe2 with Ag-alloying on the Cu site. Such a Ag/Cu substitution is found to enable a decrease in Hall carrier concentration from 14 × 1019 to 8 × 1019 cm−3 for an evaluation of the charge transport based on a single parabolic band (SPB) model with acoustic scattering. In addition, Ag-substitution simultaneously introduces Cu/Ag point defects for extra phonon scattering, leading the lattice thermal conductivity to be as low as ∼0.25 W/m-K in a broad temperature. The optimization in carrier concentration and the reduction in lattice thermal conductivity contribute to a ∼40% improvement in average thermoelectric figure of merit (zT). The determination of band parameters enables a guidance for further advancements in this promising thermoelectric material.

In a search for new thermoelectric materials, indium oxide (In2O3) was selected as a candidate for high-temperature thermoelectric oxide materials due to its intrinsically low thermal conductivity (

Nanocrystallization has been an important approach for reducing thermal conductivity in thermoelectric materials due to limits on phonon mean-free path imposed by the characteristic structural size. We report on thermal conductivity as low as 0.3 Wm−1K−1 of nanocrystalline silicon thin films prepared by plasma-enhanced chemical-vapor deposition as grain size is reduced to 2.8 nm by controlling hydrogen dilution of silane gas during growth. A multilayered film composed by alternating growth conditions, with layer thicknesses of 3.6 nm, is measured to have a thermal conductivity 30% and 15% lower than its two constituents. Our quantitative analysis attributes the strong reduction of thermal conductivity with decreasing grain size to the magnifying effect of porosity which occurs concomitantly due to increased mass density fluctuations. Our results demonstrate that ultrasmall grain sizes, multilayering, and porosity, all at a similar nanometer-size scale, may be a promising way to engineer thermoelectric materials. Thermoelectric materials convert heat into electricity and their performance is determined by their figure of merit ZT, which is generally too small in many materials for practical applications. Here, the authors demonstrate that a reduction in grain size for nanocrystalline Si can reduce thermal conductivity and potentially be used as a method to engineer greater ZT in Si for thermoelectric applications.

Nb1-xTixFe1.02Sb half-Heusler thermoelectric materials were synthesized trough arc melting and subsequent spark plasma sintering (SPS). Doping effect of Ti at Nb-site results in high power factor due to the optimization of hole carrier concentration. Further, the lattice thermal conductivity reduced as the Ti content increased, which mainly resulted from point defects and mass fluctuation by Ti doping as well as coherent nano-scale second phase, Ti-rich and Fe-deficient Ti1.18Fe0.57Sb. For the sample of Nb0.6Ti0.4Fe1.02Sb, a minimum lattice thermal conductivity of ∼2.08 W/mK was obtained. This is attributed to the enhanced phonon scattering at the alternating white and dark lamellar interphase boundaries of (Nb0.6,Ti0.4)FeSb half-Heusler and Nb-doped Ti1.18Fe0.57Sb. A maximum thermoelectric figure of merit, ZT, of ∼0.81 was obtained at 973 K, which is comparable with values previously reported for this system.

Optoelectronic and thermoelectric properties of A3AsN (A ¼ Mg, Ca, Sr and Ba) compounds in cubic and orthorhombic crystallographic phases are investigated using FP-LAPW method along with GGA and GGA-mBJ potentials based on DFT. Electronic properties shows that all the compounds both in cubic and orthorhombic crystallographic phases are direct band gap semiconductor at central symmetry. The calculated band gap for the cubic and orthorhombic phases ranging from2.43 to 0.084eV through GGA and GGA-mBJ. The calculated band gap not only decreases with cation replacement but also diereses going from cubic to orthorhombic phase. The result revel that all the compounds are optically active in visible and infrared region of electromagnetic spectrum. Due to the narrow band gap semiconducting nature of these compounds their thermoelectric parameters are also calculated which shows that these compounds are efficient to use as the active thermoelectric materials.

SnSe emerges as one of the most promising Te-free thermoelectric materials due to its strong anharmonicity and multiple valence bands structure. Recently, compositing has been proven effective in optimizing thermoelectric performance of various metal chalcogenides. Herein, a series of SnSe-xCu2S (x = 0, 0.5%, 1%, 3%, 5%) materials have been fabricated via solution synthesis, particle blending, and spark plasma sintering in sequence. After incorporating Cu2S, the materials become SnSe based composites with Cu doping, S substitution and Cu2SnSe3 secondary phase. We elucidate that the power factor of polycrystalline SnSe can be tuned and enhanced at varied temperature ranges through adjusting the addition amount of Cu2S. Additionally, the composites achieve suppressed lattice thermal conductivity when compared to SnSe itself, as the introduced point defects and SnSe/Cu2SnSe3 interfaces intensify phonon scattering. Consequently, SnSe-0.5%Cu2S and SnSe-3%Cu2S achieve a peak zT of 0.70 at 830 K (intermediate temperature range) and a highly increased zT of 0.28 at 473 K (low temperature range), respectively, which are ∼130% and 200% of values reached by SnSe at the corresponding temperatures. The study demonstrates that our approach, which combines compositing with elemental doping and substitution, is effective in optimizing the thermoelectric performance of SnSe at varied temperature ranges.

GeTe that exhibits a strong anharmonicity and a ferroelectric phase transition between the rhombohedral and cubic structures has emerged as one of the leading thermoelectric materials. Herein, combining molecular dynamics simulations and inelastic neutron scattering measurements, the lattice dynamics in GeTe have been investigated to reveal the soft-mode mechanisms across the phase transition. We have constructed a first-principles-based machine-learning interatomic potential, which successfully captures the dynamical ferroelectric phase transition of GeTe by adopting the neural network technique. Although the low-energy acoustic phonons remain relatively unaffected at elevated temperatures, the high-energy optical, and longitudinal acoustic phonons demonstrate strong renormalizations as evidenced from the vibrational phonon spectra, which are attributed to the large anharmonicity accompanying the phase transition. Furthermore, our results reveal a nonmonotonic temperature dependence of the soft-modes beyond the perturbative regime. The insight provided by this work into the soft-modes may pave the way for further phonon engineering of GeTe and the related thermoelectrics.

Dopants play an important role in synthesizing materials to improve target materials properties or stabilize the materials. In particular, the dopants are essential to improve thermoelectic performances of the materials. However, existing machine learning methods cannot accurately predict the materials properties of doped materials due to severely nonlinear relations with their materials properties. Here, we propose a unified architecture of neural networks, called DopNet, to accurately predict the materials properties of the doped materials. DopNet identifies the effects of the dopants by explicitly and independently embedding the host materials and the dopants. In our evaluations, DopNet outperformed existing machine learning methods in predicting experimentally measured thermoelectric properties, and the error of DopNet in predicting a figure of merit (ZT) was 0.06 in mean absolute error. In particular, DopNet was significantly effective in an extrapolation problem that predicts ZTs of unknown materials, which is a key task to discover novel thermoelectric materials.

As a unique class of molecular electronic materials, organic donor–acceptor complexes now exhibit tantalizing prospect for heat–electricity interconversion. Over the past decades, in design of these materials for thermoelectric applications, consistent efforts have been made to synthesize a wide variety of structures and to characterize their properties. However, hitherto, one of the paramount conundrums, namely lack of systematic molecular design principles, has not been addressed yet. Here, based on ab initio calculations, and by comprehensively examining the underlying correlation among thermoelectric power factors, non-intuitive transport processes, and fundamental chemical structures for 13 prototypical organic donor–acceptor complexes, we establish a unified roadmap for rational development of these materials with increased thermoelectric response. We corroborate that the energy levels of frontier molecular orbitals in the isolated donor and acceptor molecules control the charge transfer, electronic property, charge transport, and thermoelectric performance in the solid-state complexes. Our results demonstrate that tailoring a suitable energy-level difference between donor’s highest occupied molecular orbital and acceptor’s lowest unoccupied molecular orbital holds the key to achieving an outstanding power factor. Moreover, we reveal that the charge-transfer-caused Coulomb scattering governs the charge and thermoelectric transport in organic donor–acceptor complexes.

Solid solution alloying is a promising strategy to establish high performance thermoelectrics. By alloying different elements, phase structures and phase compositions may vary accompanied by appearance of variety of interesting microstructures including mass fluctuation, lattice strain, nano-scale defects and spinodal decomposition, all of which may greatly influence the electrical and specifically the thermal transport of the material. In the present study, atomic structures of Cu2S0.5Se0.5 solid solution have been examined by using atom-resolved electron microscopy in order to investigate the structure-correlated physical insights for the abnormal thermal transport in this solid solution. Then the exceptional intergrowth nanostructures were observed. The solid solution consists of two high symmetrical phases, i.e. the hexagonal and cubic phase, which alternately intergrow to form highly oriented ultra-thin lamellae of nano or even, unit cell scales. The compositional oscillation in Se/S atomic ratio during alloying is responsible for the phase stability and intergrowth nanostructures. The unique binary phase intergrowth nanostructures make great contribution to the ultra-low lattice thermal conductivity comparable to glass and extremely short phonon mean free path of only 1.04 A, peculiar continuous hexagonal-to-cubic structural transformation without a critical transition temperature and its corresponding abnormal changes of thermal characters with temperatures. The present study further evokes the unlimited possibilities and potentials for tailoring nanostructures by alloying for improved thermoelectric performance.

Facing the increasingly serious problem of environmental pollution and energy waste, the thermoelectric generator has been attracting more and more attention owing to its advantages including low cost, no pollution, and good stability. The family of thermoelectric material is constantly extended with enhanced performance. Note that nanostructuring can enhance thermoelectric performance. However, the most recent excellent material with effective thermoelectric transformation reported from bulk materials has definite benefits to the practical application compared to nanomaterials. In this work, a nanostructure integrated macroscale thermoelectric chip, that is an alloyed band gap gradient macroscale chip (1.0 cm × 2.0 cm) composed of CdSSe nanowires, has been proven as an excellent thermoelectric generator for the first time. A high Seebeck coefficient of -152.4 μV/K and the average output voltage of 10.8 mV are obtained after optimizing the electrode patterns and distance between electrodes. More interestingly, upon illumination by white light from a xenon lamp, a photo-thermoelectric output voltage is greatly elevated to 45 mV due to the high concentration of photogenerated carriers. The CdSSe thermoelectric chip also shows good repeatability and high stability with a relative error of

Filling guest atoms into the nanovoids of skutterudite compounds provides effective scattering for low-frequency phonons to reduce the lattice thermal conductivity. However, it is still difficult to simultaneously realize the full-spectrum phonon scattering and band engineering in the n-type skutterudites with higher thermoelectric performance. Here, we reveal that the combination of five types of element atoms in the lattice nanovoids brings about dense dislocations, abundant precipitated nanoparticles, and electronic band convergence in the n-type skutterudites. The Seebeck coefficient shows an increase with little deterioration on the carrier mobility due to the enhanced density of states near the Fermi level, leading to a 11% enhancement in the power factor at 823 K. The lattice thermal conductivity is significantly reduced to approach the glass limit due to the full-spectrum phonon scattering. As a result, a peak ZT value of about 1.7 at 823 K and an average ZT value of about 1.2 from 323 to 823 K are obtained. High thermoelectric performance combined with the structural optimization enables the simulated maximum energy conversion efficiency of the skutterudite module to reach up to 15.0%. Our finding opens a new dimension for doping atoms to achieve simultaneous optimization of electrical and thermal properties in polynary thermoelectric materials.

As an eco-friendly thermoelectric material, Cu2SnSe3 has recently drawn much attention. However, its high electrical resistivity ρ and low thermopower S prohibit its thermoelectric performance. Herein, we show that a widened band gap and the increased density of states are achieved via S alloying, resulting in 1.6 times enhancement of S (from 170 to 277 μV/K). Moreover, doping In at the Sn site can cause a 19-fold decrease of ρ and a 2.2 times enhancement of S (at room temperature) due to both multivalence bands' participation in electrical transport and the further enhancement of the density of states effective mass, which allows a sharp increase in the power factor. As a result, PF = 9.3 μW cm-1 K-2 was achieved at ∼800 K for the Cu2Sn0.82In0.18Se2.7S0.3 sample. Besides, as large as 44% reduction of lattice thermal conductivity is obtained via intensified phonon scattering by In-doping-induced formation of multidimensional defects, such as Sn vacancies, dislocations, twin boundaries, and CuInSe2 nanoprecipitates. Consequently, a record high figure of merit of ZT = 1.51 at 858 K is acquired for Cu2Sn0.82In0.18Se2.7S0.3, which is 4.7-fold larger than that of pristine Cu2SnSe3.

Band convergence is considered a clear benefit to thermoelectric performance because it increases the charge carrier concentration for a given Fermi level, which typically enhances charge conductivity while preserving the Seebeck coefficient. However, this advantage hinges on the assumption that interband scattering of carriers is weak or insignificant. With first-principles treatment of electron-phonon scattering in the CaMg2Sb2-CaZn2Sb2 Zintl system and full Heusler Sr2SbAu, we demonstrate that the benefit of band convergence can be intrinsically negated by interband scattering depending on the manner in which bands converge. In the Zintl alloy, band convergence does not improve weighted mobility or the density-of-states effective mass. We trace the underlying reason to the fact that the bands converge at a one k-point, which induces strong interband scattering of both the deformation-potential and the polar-optical kinds. The case contrasts with band convergence at distant k-points (as in the full Heusler), which better preserves the single-band scattering behavior thereby successfully leading to improved performance. Therefore, we suggest that band convergence as thermoelectric design principle is best suited to cases in which it occurs at distant k-points., Band convergence is a strategy to enhance a material’s thermoelectric performance, as it increases the charge carrier concentration for a given Fermi level. Here, the authors find that the benefit of band convergence can be negated by interband scattering depending on the manner in which bands converge.

Doped Tin (IV) Oxide has excellent potential in high-temperature thermoelectrics because of its large bandgap. It has been thoroughly experimentally studied for high-temperature thermoelectric application. Low electron mobility limits the thermoelectric performance of oxides. Understanding the temperature dependence of mobility help increase thermoelectric performance. This is rarely performed. In this work, we study antimony doped tin dioxide. Combining the calculated transport properties and the existing experimental results, we obtain other transport properties. Unlike in bulk materials, the grain boundary scattering competes against the acoustic phonon scattering. The grain size, temperature, and carrier concentration determine the mobility. The small grain, low carrier concentration, and high temperature are beneficial to the mobility. Antimony doping also causes the Fermi level into the conduction band as deep as 0.59 eV, making the large bandgap SnO2 metallic. Furthermore, the conductivity effective mass demonstrates the doping effect. These findings might help design new oxide thermoelectric materials by decrease the grain size.

Toward high-performance thermoelectric energy conversion, the electrons and holes must work jointly like two wheels of a cart: if not longitudinally, then transversely. The bipolar effect — the main performance restriction in the traditional longitudinal thermoelectricity, can be manipulated to be a performance enhancer in the transverse thermoelectricity. Here, we demonstrate this idea in semimetal Mg2Pb. At 30 K, a giant transverse thermoelectric power factor as high as 400 μWcm−1K−2 is achieved, a 3 orders-of-magnitude enhancement than the longitudinal configuration. The resultant specific heat pumping power is ~ 1 Wg−1, higher than those of existing techniques at 10~100 K. A large number of semimetals and narrow-gap semiconductors making poor longitudinal thermoelectrics due to severe bipolar effect are thus revived to fill the conspicuous gap of thermoelectric materials for solid-state applications. Heat pumping is in high demand at cryogenic temperature, but whether thermoelectricity can take on cryogenic heat pumping is an open question. Here, the authors answer this question by leveraging bipolar effect to enhance transverse thermoelectricity in semimetal Mg2Pb for cryogenic heat pumping.

SrTiO3(STO) is a thermoelectric (TE) material with a wide band gap. It is usually either sintered or annealed in a reducing atmosphere to obtain semiconducting behavior. In this work, non-stoichiometric SrTiO2.8 was synthesized by the high temperature and high pressure (HPHT) method in a reducing environment to obtain improved electrical and thermal properties. The reducing environment was obtained by the application of high pressure and due to the strong ability of Ti to capture oxygen at high temperatures. The thermoelectric properties of the samples synthesized at high temperature and pressure were studied by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and UV spectroscopy. The results show that with increasing of synthesis pressure, the band gap of the samples decreases, improving their electrical properties; at the same time, the high pressure also changes the micro-morphology of the samples and improves their thermal properties. The zT value of the sample synthesized at 5 GPa reached the maximum value of 0.20 at 973 K. It was found that the mechanical strength of the synthesized samples is superior to that of the traditional alloy-based thermoelectric materials. Our work shows that HPHT provides a new variable of pressure for the optimization of the synthesis process of TE materials and provides a new approach for the synthesis and performance optimization of thermoelectric materials.

With the ever-growing development of multifunctional and miniature electronics, the exploring of high-power microwatt-milliwatt self-charging technology is highly essential. Flexible thermoelectric materials and devices, utilizing small temperature difference to generate electricity, exhibit great potentials to provide the continuous power supply for wearable and implantable electronics. In this review, we summarize the recent progress of flexible thermoelectric materials, including conducting polymers, organic/inorganic hybrid composites, and fully inorganic materials. The strategies and approaches for enhancing the thermoelectric properties of different flexible materials are detailed overviewed. Besides, we highlight the advanced strategies for the design of mechanical robust flexible thermoelectric devices. In the end, we point out the challenges and outlook for the future development of flexible thermoelectric materials and devices.

Distinct ‘hidden’ phases of a technologically relevant thermoelectric material, which are identical in terms of composition and periodic crystal structure, but differ on a local scale, are observed and can be controlled through synthesis conditions. The local structure is explained in terms of a vacancy repulsion model, and relaxations around vacancies are characterized., Although crystalline solids are characterized by their periodic structures, some are only periodic on average and deviate on a local scale. Such disordered crystals with distinct local structures have unique properties arising from both collective and localized behaviour. Different local orderings can exist with identical average structures, making their differences hidden to Bragg diffraction methods. Using high-quality single-crystal X-ray diffuse scattering the local order in thermoelectric half-Heusler Nb1−x CoSb is investigated, for which different local orderings are observed. It is shown that the vacancy distribution follows a vacancy repulsion model and the crystal composition is found always to be close to x = 1/6 irrespective of nominal sample composition. However, the specific synthesis method controls the local order and thereby the thermoelectric properties thus providing a new frontier for tuning material properties.

As a promising thermoelectric material, aluminum-doped zinc oxide (AZO) exhibits high thermal conductivity, which limits its use in high-temperature thermoelectric applications. Here, we report an effective route for forming a porous architecture and nanoinclusions by introducing nano-SiC to reduce the thermal conductivity. The simultaneous formation of a porous architecture and nanoinclusions promotes enhanced phonon scattering, resulting in fairly low thermal conductivity of approximately 3.3 W⋅ m−1⋅ K−1. Meanwhile, the Seebeck coefficient shows the significant improvement due to energy filtration effect caused by porous architecture and nanoinclusions. The resultant dimensionless figure of merit of approximately 0.2 at 1050 K was 1.5 times higher than that of AZO ceramic without nano-SiC, which is attributed to the combined factors of increased Seebeck coefficient and reduced thermal conductivity.

Funding: The Oxford Atom Probe facility is funded by EPSRC (EP/M022803/1) and the Glasgow plasma focused ion beam system was funded by EPSRC grant EP/P001483/1. Thermoelectric materials were developed under joint EPSRC grants EP/N017218/1 and EP/N01717X/1. Cu-doping and crystallographic site occupations within the half-Heusler (HH) TiNiSn, a promising thermoelectric material, have been examined by atom probe tomography. In particular, this investigation aims to better understand the influence of atom probe analysis conditions on the measured chemical composition. Under a voltage-pulsing mode, atomic planes are clearly resolved and suggest an arrangement of elements in-line with the expected HH (F-43m space group) crystal structure. The Cu dopant is also distributed uniformly throughout the bulk material. For operation under laser-pulsed modes, the returned composition is highly dependent on the selected laser energy, with high energies resulting in the measurement of excessively high absolute Ti counts at the expense of Sn and in particular Ni. High laser energies also appear to be correlated with the detection of a high fraction of partial hits, indicating nonideal evaporation behavior. The possible mechanisms for these trends are discussed, along with suggestions for optimal analysis conditions for these and similar thermoelectric materials. Postprint

Oxide thermoelectric materials are considered promising for high-temperature thermoelectric applications in terms of low cost, temperature stability, reversible reaction, and so on. Oxide materials have been intensively studied to suppress the defects and electronic charge carriers for many electronic device applications, but the studies with a high concentration of defects are limited. It desires to improve thermoelectric performance by enhancing its charge transport and lowering its lattice thermal conductivity. For this purpose, here, we modified the stoichiometry of cation and anion vacancies in two different systems to regulate the carrier concentration and explored their thermoelectric properties. Both cation and anion vacancies act as a donor of charge carriers and act as phonon scattering centers, decoupling the electrical conductivity and thermal conductivity.

Fabrication of nanoparticle-dispersed composites is an effective strategy for enhancing the performance of thermoelectric materials, and in particular SiC nanoparticles have been often used to create composites with Bi2Te3-based applied thermoelectric materials. However, the effect of particle size on the thermoelectric performance is unclear. This work systematically investigated the electrical and thermal properties of a series of (Bi,Sb)2Te3-based nanocomposites containing dispersed SiC nanoparticles of different sizes. It was found that particle size has a significant impact on the electrical properties with smaller SiC nanoparticles giving rise to higher electrical conductivity. Even though the dispersed SiC nanoparticles enhanced the Seebeck coefficient, no apparent dependence of the enhancement on the particle size was observed. It was also found that smaller SiC nanoparticles scatter phonons to some extent while the larger nanoparticles contribute to increased thermal conductivity. Eventually, the highest ZT value of 1.12 was obtained in 30 nm-SiC dispersed sample, corresponding to an increase by 18% from 0.95 for the matrix made from commercial scraps, and then the ZT was further boosted to 1.33 by optimizing the matrix composition and expelling excess Te during the optimized spark plasma sintering process. This work proves that the dispersion of smaller SiC nanoparticles in p-type (Bi,Sb)2Te3 materials is more effective than the dispersion of larger nanoparticles. In addition, it is revealed that additional compositional and/or processing optimization is vital and effective for obtaining further performance enhancement for nanocomposites of SiC nanoparticles dispersed in (Bi,Sb)2Te3.

As more than 60% of worldwide consumed energy is unused and becomes waste heat every year, high-efficiency waste heat to power technologies are highly demanded for the conversion of wasted heat to electricity. Thermoelectrics which can convert the wasted heat directly into electricity represent a promising approach for energy recovery. Thermoelectric technology has existed for several decades, but its usage has been limited due to low efficiencies. Recent advances in nanotechnology have enabled the improving of thermoelectric properties which open up the thermoelectrics’ feasibility in industry. In this paper, we present an overview of recent progress in increasing the porosity of thermoelectric materials from atomic scale to microscale, leading to the enhancement of figure of merit.

Innovative material configurations obtained by incorporating two-dimensional (2D) layers of MoS2 (a two-dimensional material) with Fe doped ZnO (Fe:ZnO) thin films are found to exhibit high thermoelectric properties at lower temperature. The low dimensionality of material (MoS2) is expected to enhance the thermoelectric power factor because of the strong confinement of charge carriers. MoS2 layers have been incorporated in the Fe:ZnO thin film by depositing MoS2 layers over Fe:ZnO thin film and by fabricating the multilayered structure of MoS2 layers with Fe:ZnO. The fabricated structures were characterized for their structural, optical and morphological properties using the available characterization tools. The multilayer configuration of the MoS2 and Fe:ZnO (FZnMFZn) sample was found to exhibit higher values of power factor of 1.06 × 10−3 W/mK2 and figure of merit (ZT) of 3.11 × 10−2 at a very low operating temperature of 300 K. The obtained results highlight the importance of charge confinement in improving the thermoelectric properties of the multilayered structure (FZnMFZn thin film sample) indicating that it is a promising candidate for practical applications.

Abstract The superionic α ↔ β phase transition in Cu1.96Se thermoelectric material is investigated by means of thermal analysis (DSC) and measurements of Seebeck coefficient and electrical conductivity. Results of the DSC measurements with 1–10 K/min heating and cooling rates show that the material is close to the equilibrium phase composition during the transformation. However, the kinetic limitation of the process exists, which is indicated by supercooling. At the beginning of the β → α transition, the most significant kinetic delay was attributed to the nucleation of the α phase. During the phase transformation, the Seebeck coefficient was lower than in a stabilised material (measured with 0.1 K/min heating/cooling rate). During cooling, a decrease from 130 μV/K (in a stabilised measurement) to 7 μV/K (5 K/min cooling rate) was observed. The deviation from the expected values of the Seebeck coefficient was correlated with the difference between the actual and equilibrium phase compositions. Graphical abstract