Promising Thermoelectric Material Research Articles

High-Performance AgSbTe2 Thermoelectrics: Advances, Challenges, and Perspectives.

Environmental-friendless and high-performance thermoelectrics play a significant role in exploring sustainable clean energy. Among them, AgSbTe2 thermoelectrics, benefiting from the disorder in the cation sublattice and interface scattering from secondary phases of Ag2Te and Sb2Te3, exhibit low thermal conductivity and a maximum figure-of-merit ZT of 2.6 at 573 K via optimizing electrical properties and addressing phase transition issues. Therefore, AgSbTe2 shows considerable potential as a promising medium-temperature thermoelectric material. Additionally, with the increasing demands for device integration and portability in the information age, the research on flexible and wearable AgSbTe2 thermoelectrics aligns with contemporary development needs, leading to a growing number of research findings. This work provides a detailed and timely review of AgSbTe2-based thermoelectrics from materials to devices. Principles and performance optimization strategies are highlighted for the thermoelectric performance enhancement in AgSbTe2. The current challenges and future research directions of AgSbTe2-based thermoelectrics are pointed out. This review will guide the development of high-performance AgSbTe2-based thermoelectrics for practical applications.

Enhanced thermoelectric properties of Se-doped quasi-one-dimensional van der Waals crystal Ta2PdS6

Recently, quasi-one-dimensional van der Waals crystal Ta2PdS6 has been reported as a promising thermoelectric material with an extraordinarily high power factor. However, element doping to tune the thermoelectric properties has not been studied yet. Here, we systematically investigated the effect of Se doping on the phase composition, charge transport properties and thermoelectric performance of Se-doped Ta2Pd(S1−xSex)6 (x = 0, 0.02, 0.05, and 0.07) polycrystalline bulk materials. Upon doping Se at the S sites to increase the carrier concentration and mobility, the electrical conductivity of Ta2Pd(S0.93Se0.07)6 is dramatically enhanced, while the S is slightly reduced, yielding a significantly improved power factor compared to that of pristine Ta2PdS6. Consequently, Ta2Pd(S0.93Se0.07)6 exhibits a peak ZT of 0.29 at 700 K when the doping content x = 0.07, which is more than twice that of pristine Ta2PdS6.

Optimization of thermoelectric properties for microwave sintered Fe-doped ZnO

ZnO is a promising thermoelectric ceramic material, but the low electrical conductivity and high thermal conductivity restrict its satisfactory thermoelectric performance. Chemical doping is an effective way to improve the thermoelectric properties of ZnO. Due to the valence states and similar ionic radii of Fe3+ and Zn2+, Fe was selected as the effective dopant for ZnO. In this work, Fe-doped ZnO samples were prepared using hydrothermal synthesis and microwave sintering technology under oxygen-depleted conditions. The electrical conductivity of sintered Fe-doped ZnO was greatly improved owing to the energy band adjustment. The maximum power factor of 3108 μW/mK2 at 800 K was obtained from Zn0.997Fe0.003O, which is about 4.4 times greater than that of pure ZnO. Meanwhile, the small grain size (300–500 nm) and residual pores resulted from microwave rapid sintering in an oxygen-depleted environment, as well as the point defects caused by Fe3+ substituting for Zn2+, significantly reduced the thermal conductivity. Finally, the highest ZT value of 0.91 has been achieved in Zn0.997Fe0.003O at 800 K.

Ion-Exchange Doping of Semiconducting Single-Walled Carbon Nanotubes.

Semiconducting single-walled carbon nanotubes (SWCNTs) are a promising thermoelectric material with high power factors after chemical p- or n-doping. Understanding the impact of dopant counterions on charge transport and thermoelectric properties of nanotube networks is essential to further optimize doping methods and to develop better dopants. This work utilizes ion-exchange doping to systematically vary the size of counterions in thin films of small and large diameter, polymer-sorted semiconducting SWCNTs with AuCl3 as the initial p-dopant and investigates the impact of ion size on conductivity, Seebeck coefficients, and power factors. Larger anions are found to correlate with higher electrical conductivities and improved doping stability, while no significant effect on the power factors is found. Importantly, the effect of counterion size on the thermoelectric properties of dense SWCNT networks is not obscured by morphological changes upon doping. The observed trends of carrier mobilities and Seebeck coefficients can be explained by a random resistor model for the nanotube network that accounts for overlapping Coulomb potentials leading to the formation of an impurity band whose depth depends on the carrier density and counterion size. These insights can be applied more broadly to understand the thermoelectric properties of doped percolating disordered systems, including semiconducting polymers.

Enhanced thermoelectric properties for eco-friendly CaTiO3 by band sharpening and atomic-scale defect phonon scattering

CaTiO3-based compounds have emerged as a promising thermoelectric material, renowned for their environmentally benign, thermally stable, and cost-efficient merits. Non-etheless, the pristine CaTiO3 manifests inherently low electronic transport properties. Herein, the thermoelectric properties of Ca1-xDyxTiO3 (x = 0, 0.05, 0.10, 0.15, 0.20) compounds are systematically investigated. The electrical transport properties are markedly enhanced by synergistic optimization of the carrier concentration, mobility, and density-of-states effective mass. Density functional theory results demonstrate that the conduction band tends to be sharper and that the lighter band participates in carrier transport after Dy doping. The large discrepancy in atomic mass results in considerable mass fluctuations, which give rise to intense phonon scattering. Benefitting from the modulated band structure and reduced thermal conductivity, the highest thermoelectric figure of merit (ZT) of 0.31 is achieved at 1073 K, enhanced by 287.5% in contrast with pristine CaTiO3 (ZTmax = 0.08). The defect and energy band modulation strategies proposed to optimize thermoelectric performance are applicable to other thermoelectric materials. This investigation inspires the exploration of high-performance and eco-friendly high-temperature thermoelectric material.

The A-Ni chemical bond in AIIINiSb (AIII=Sc, Y, Er) half-Heusler materials triggers the formation of anomalous vacancy defects

Defects exert a profound influence on thermoelectric materials by altering electronic band structures and significantly impacting their performance. Despite being a potentially promising thermoelectric material, the mechanism behind defect formation in half-Heusler (HH) compounds ABX remains unclear, impeding the enhancement of their thermoelectric properties. In this study, we investigated the intrinsic defect formation energies for AⅢNiSb (AⅢ = Sc, Y, Er) and other 9 HH compounds, namely AⅣNiSn (AⅣ = Ti, Zr, Hf), AⅤFeSb (AⅤ = V, Nb, Ta), and AⅣCoSb (AⅣ = Ti, Zr, Hf), using first-principles calculations and thermodynamics. The results reveal that AⅢNiSb (AⅢ = Sc, Y, Er) exhibits anomalous B (Ni) vacancy defects, with their formation energies being significantly lower than those of the corresponding B vacancy defects in the other 9 HH compounds. This anomaly can be attributed to the strength of the A-B bonds, where a decrease in bond strength leads to a decrease in the formation energy of B vacancies. This approach of exploring the influence of interatomic bond strength on defect formation is not only insightful for HH compounds but also holds potential applications in defect studies across various materials, offering a broader perspective on the fundamental mechanisms governing defect formation and stability.

Insight into phase stability and thermoelectric properties of semiconducting iron silicides with manganese substitution: β-Fe1−xMnxSi2 (0≤x≤0.05)

Metal-doped iron silicides (β-FeSi2) are promising thermoelectric materials for high-temperature applications. However, the addition of metal dopants usually causes the formation of secondary metallic phases, resulting in the degradation of thermopower. Here we report the stability of semiconducting β-phase (higher than 95 %) of Mn-doped β-FeSi2 obtained at Mn concentration from 1 % to 5 %, where the samples were prepared through direct arc melting and heat treatment process. The carrier density remarkably increases with Mn content, but the mobility decreases. The electrical resistivity significantly decreases with Mn content. The Seebeck coefficient of Mn-doped samples is improved and stable at high-temperature regions because of the reduction of the bipolar effect and β-phase stability. The thermal conductivity slightly increases with Mn. As a result, the maximum thermoelectric power factor PF = 970 μWm−1K−2 and dimensionless figure of merit ZT = 0.12 at 800 K are obtained in a 3 % Mn-doped sample. The stability of β-phase and the improved carrier density are the origins of enhanced thermoelectric properties, where the Seebeck coefficient is improved, and the electrical resistivity is reduced. Our study provides insight into the importance of phase stability for enhancing thermoelectric transport in Mn-doped β-FeSi2.

Biomass lignin as dispersion to substantially enhance carbon nanotubes thermoelectric converter for energy harvesting

Carbon nanotubes (CNTs) have attracted much attention as promising thermoelectric materials to harvest waste energy, however the agglomeration between nanotubes greatly limits their application. Despite various dispersants that have been devised to enhance the dispersion of CNTs, biomass lignin has been rarely employed to facilitate the dispersion of CNT for enhancing its TE properties. Herein, green lignin was incorporated with CNTs to synergistically improve the dispersion and TE properties of CNTs, in which π-π interactions between the aromatic ring structure of lignin and CNTs, as well as electrostatic charge brought by ionized groups of lignin promote the exfoliation of nanotubes and reduces CNTs entanglement. The well-dispersed structure of CNTs facilitates efficient charge transfer, and thereby enhances the thermoelectric properties. CNTs/lignin composite could reach a maximum power factor of 198.0 μW m−1K−2 at a lignin content of 11.8 wt%, which was then raised to 223.1 μW m−1K−2 with subsequent partial removal of lignin by alkali immersion treatment, higher than 98.6 μW m−1K−2 of pristine CNTs film. Furthermore, the flexible TE device with five pairs of CNTs/lignin and nickel wires generates a high power of 288 nW at ΔT = 30 K, demonstrating potential applications of lignin for sustainable energy harvesting.

First-principles calculations of structural and bonding properties of Li-doped tetrahedrite thermoelectrics

Tetrahedrite (copper antimony sulfosalt) is a promising p-type thermoelectric material due to its very low intrinsic thermal conductivity and moderately-high power factor, with one of the limitations being the lack of n-type variant to create a thermoelectric generator. In this paper, DFT calculations have been carried out to study tetrahedrite doped with Li into structural voids, LixCu12Sb4S13 (0≤x≤3). Enthalpies of formation show that the introduction of Li into both 6b and 24 g sites is energetically favorable. Dopants in those positions differently affect the rattling Cu(12e) behavior, as well as vary in the magnitude of induced local disorder. Topological analysis of charge density classifies tetrahedrite as a closed-shell, ionic system of interactions with some degree of covalency. The addition of Li increases bond strain and decreases structural stability. Electronic band structure shows that for x>2.0, material becomes n-type; however, results are not precisely conclusive on whether structure will be synthesizable, which should be determined experimentally.

Janus ScYCBr2 MXene as a Promising Thermoelectric Material.

Finding green energy resources that contribute to the battle against global warming and the pollution of our planet is an urgent challenge. Thermoelectric electricity production is a clean and efficient method of producing energy; consequently, scientists are currently researching and creating thermoelectric materials to increase the efficiency of thermoelectric electricity production and expand the potential of the thermoelectric effect for clean energy production. This work focuses on a comprehensive study of the thermoelectric properties of two-dimensional ScYCBr2. We report here a computational analysis of this Janus-like MXene, which is predicted to exhibit outstanding thermoelectric properties. The study uses density-functional theory to provide evidence of the important role played by symmetry breaking to promote low-thermal transport by favoring certain phonon scattering channels. Compared to its symmetric parent compounds, the asymmetric Janus-type ScYCBr2 displays additional phonon scattering channels reducing the thermal conductivity. An exhaustive investigation of the dynamical stability for both zero-temperature and high-temperature conditions was also performed to support the stability of ScYCBr2. Our analysis shows that thanks to its asymmetric structure, the ScYCBr2 MXene has thermoelectric properties that largely surpass those of its parent symmetric counterpart Sc2CBr2, being a material with a remarkable thermoelectric high figure of merit. Another advantage of ScYCBr2 is its high carrier mobility. This work not only demonstrates that this material is a promising thermoelectric material but also shows that ScYCBr2 can operate efficiently at high temperatures up to 1200 K.

Thermoelectric Performance Enhancement in SnS Polycrystals Owing to Hole Doping Combined with Textured Microstructures.

Recently, the earth-abundant tin sulfide (SnS) has emerged as a promising thermoelectric material due to its phonon and electron structure similar to that of tin selenide (SnSe). However, compared with SnSe, limited progress has been achieved in the thermoelectric property enhancement of SnS. Textured SnS polycrystals with an enhanced thermoelectric performance have been developed in this work. The high carrier mobility benefited from the enhanced texture through the repressing strategy of spark plasma sintering, improving the electrical conductivity. In addition, Sn atom deficiencies in the texture sample led to an increased hole concentration, further boosting the electrical conductivity and power factor. The power factor exceeded 4.10 μW/cm·K2 at 423 K and 5.50 μW/cm·K2 at 850 K. The phonon scattering was strengthened by adjusting the multiscale microstructures including dislocations, defect clusters, etc., leading to an ultralow lattice thermal conductivity of 0.23 W/m·K at 850 K. A figure of merit zT > 1.3 at 850 K and an average zTave of 0.58 in the temperature range 373-850 K were achieved in the SnS polycrystal.

Enhanced thermoelectric performance of Cr-doped ZnSb through lattice thermal conductivity reduction below the phonon glass limit

Zinc antimonide (ZnSb) is a promising thermoelectric material due to its economic viability, elemental abundance, and superior phase stability compared to other Zn-Sb compounds. However, its widespread application is hindered by a low figure-of-merit (zT). Extensive research has explored doping, alloying, and nanostructuring to improve zT. This study investigates the impact of Cr doping in CryZn1-ySb (y = 0.0 – 0.03). Cr doping with y = 0.01 significantly reduces lattice thermal conductivity below the phonon glass limit. Debye-Callaway model calculations suggest a combined effect of point defects and decreased grain size caused by Cr incorporation. This reduction translates to enhanced thermoelectric performance, with the y = 0.01 sample exhibiting a 70 % improvement in zT at 673 K, reaching ∼0.67. Exceeding the Cr substitution limit leads to the formation of a detrimental secondary CrSb2 phase, increasing thermal conductivity. The Single Parabolic Band model calculations predict further zT enhancement in y = 0.01 sample to ∼0.2 at 300 K through optimized carrier concentration (307 % improvement in zT). These findings demonstrate the potential for significant zT improvement in ZnSb via defect engineering and carrier concentration tuning.

Intrinsically Atomic-Disordered LuCuTe2 as a Promising Thermoelectric Material

Intrinsically Atomic-Disordered LuCuTe₂ as a Promising Thermoelectric Material

Thermodynamic modeling of Bi2Te3 in the defect energy formalism

Bi2Te3 is a promising thermoelectric material that is often touted as one of the best-performing low-temperature thermoelectric materials. As a result, it has been widely used commercially, both for clean energy generation and in cooling devices. Like many other thermoelectric materials, defects play a key role in the performance of Bi2Te3. As a result, numerous studies have attempted to experimentally and computationally map out the dominant defects in the phase, these include efforts to determine the dominant defect, estimate defect energies, and predict their concentration. The computer coupling of phase diagrams and thermochemistry (CALPHAD) is one of many tools under the auspices of the materials genome initiative (MGI) that enables the rapid design of new functional materials with improved properties. The Defect energy formalism (DEF) with a charged sublattice, an offshoot of the Compound energy formalism (CEF), provides a way to directly include first-principle defect energy calculations into CALPHAD descriptions of solid phases. The introduction of the charge sublattice enables the estimation of the free carrier concentrations in the phase. Here we apply the DEF to the Bi2Te3 system, emphasizing the robustness of the DEF in describing meaningful endmembers and the elimination of fitting parameters. Unlike previous assessments using the Wagner–Schottky defect model, we include the description of the charged defects in our assessment. The DEF with a charged sublattice provides a good prediction of the non-stoichiometry of the phase when compared with experimental data and also predicts a thermodynamic defect concentration at low temperature that is physically reasonable.

Chalcogenide perovskite BaZrS3 bulks for thermoelectric conversion with ultra-high carrier mobility and low thermal conductivity

Chalcogenide perovskites are expected to be promising thermoelectric materials, since they not only possess efficient carrier transport and defect tolerance, but also demonstrate unique advantages of high thermodynamic stability, eco-friendly and earth-abundant constituents. Especially, theoretical reports have predicted their “glass-like” thermal conductivities. However, experimental investigation on thermoelectric performances of chalcogenide perovskite BaZrS3 is extremely scarce due to the difficulty in preparing high-quality bulk samples, which originates from the brittle nature, high melting point, and the large difference in melting points between Ba/S and Zr. In this work, pure phase BaZrS3 bulks with high relative density reaching 100 % are realized by optimized sulfurization from low-cost BaZrO3 powders combined with fast spark plasma sintering. A maximum zT value of 0.37 at 623 K in BaZrS3 bulks is achieved, which is the record-high value among the reported sulfide, halide, and hybrid perovskite materials. A room-temperature electron mobility up to 385 cm2V−1s−1 is among the highest values for perovskites due to the high phase purity, dense morphology and corner-sharing ZrS6 octahedral three-dimensional network as effective carrier channels. Meanwhile, a measured low lattice thermal conductivity of 1.11 Wm−1K−1 at 623 K is attributed to the intense phonon scattering from the intrinsic distorted-perovskite structure and the lattice defects by sulfur deficiency. Moreover, the BaZrS3 bulks in this work are stable against moisture/air and high temperature test. This work provides new insights into the fundamental electrical and thermal properties of chalcogenide perovskites, and highlights their great potential in the practical thermoelectric applications.

Defect and dopant complex mediated high power factor in transparent selenium-doped copper iodide thin films

Copper iodide (CuI) is a promising p-type transparent thermoelectric material for near-room temperature energy harvesting. We report a high-power factor for selenium (Se)-doped CuI films. Ion beam-sputtered CuI films were doped using 30 keV 80Se+ implantation with Se concentration varying between 0.50% and 6.50%. Hall effect measurements showed a ∼34% increase in electrical conductivity (σ ≈ 36.1 Ω−1cm−1) due to a ∼54% increase in carrier density (pH ≈ 5.4 × 1019 cm−3) in the p-type γ-CuI film implanted with 5.0 × 1014 Se.cm−2. A high Seebeck coefficient, α ≈ 388.9 μVK−1−1, and moderate electrical conductivity, σ ≈ 29.1 Ω−1cm−1, yield a nearly 85% increase in the power factor, α2σ ≈ 439.7 μWm−1K−2, for a 1.0 × 1015 Se.cm−2 implanted film compared to the unimplanted film (α2σ ≈ 236.4 μWm−1K−2). Monte Carlo simulation and ab initio density functional theory calculations revealed that the increased displacement per atom values and the {SeI−VCu} defect complex-induced shallow acceptor could be attributed to the observed increase in hole density. Our results highlight that native defects and defect complexes are beneficial for enhancing the power factor in transparent CuI for thermoelectric applications.

Promising photovoltaic, optoelectronic and p-type thermoelectric Sr4Pn2O (Pn = Sb, Bi) compounds: A first principles study

Tetra-strontium di-pnictide oxides Sr4Pn2O (Pn = Sb, Bi) belong to the oxypnictide-type materials attracting a lot of attention due their unique properties which are very advantageous for photovoltaic and thermoelectric applications. In spite of the fact that structural features of Sr4Pn2O (Pn = Sb, Bi) are known, however, their ability for applying in renewable energy has not been discovered yet. Therefore, in this study, the electronic structure of the two compounds was studied in detail that provides a key knowledge on understanding their optical and thermoelectric properties. The present data indicate that Sr4Pn2O (Pn = Sb, Bi) oxides have suitable band structure for photovoltaic applications with high absorption rates of 105–106 cm−1. The two compounds are also promising p-type thermoelectric materials with the figure of merit of about 0.45–0.55. Sr4Pn2O materials have strong nonlinear characteristics, making them highly promising for optical switching devices. Our research provides further insights into the features of Sr4Sb2O and Sr4Bi2O compounds besides current experimental data, demonstrating that both compounds have promising characteristics as optoelectronic materials.

High wide-temperature-range thermoelectric performance in GeTe through hetero-nanostructuring

GeTe is emerging as promising medium-temperature thermoelectric material due to its highly competitive performance and good mechanical properties. Strong Rashba spin splitting was harnessed to markedly improve the Seebeck coefficient and power factor of Bi and Sn codoped GeTe at low-medium temperature. Moreover, it is found that Bi-Sn-Cu doping reduces the phase-transition temperature to extend better electrical transport behavior of cubic phase to low temperature. As a result, the electrical transport properties in low-medium temperature were overall enhanced. In the meanwhile, endotaxial hetero-nanostructures efficiently scatter phonons and play a dominant role on affecting phonon propagation. The lattice thermal conductivity was reduced to 0.2 W m−1K−1 at 673 K. Drive by strengthening Rashba effect and endotaxial hetero-nanostructures, a record-high average ZT (300–823 K) of 1.6 and a high ZT of 2.1 were obtained in lead-free GeTe-based compounds. The vast increase of ZT promotes GeTe as a promising candidate for a wide range of applications in waste heat recovery and power generation.

Ideal two-dimensional electronic structure in bulk lead titanate for superior thermoelectrics

An ideal two-dimensional (2D) electronic structure is revealed in n-type tetragonal perovskite PbTiO3 through first-principle calculations. This opens exciting prospects for its applications in thermal-to-electrical energy conversion. The electronic structure is regarded as ideally 2D when it is highly dispersive only along two axes, while along the remaining axis, it is nearly non-dispersive with a bandwidth smaller than 1 meV – insignificant when compared to thermal energy above room temperature (i.e., kBT ≥ 25.9 meV). As a basis for comparison, the electronic structure of the cubic perovskite SrTiO3 is calculated. It is nonideal 2D with the bandwidth along the less dispersive axis larger than the thermal energy. The ideal 2D electronic structure in PbTiO3 leads to a 2D-like density of states, resembling a step function around the conduction minimum while for SrTiO3, the density of states remains 3D-like with a square-root energy dependence. Consequently, n-type PbTiO3 exhibits superior effective band degeneracy within the Fermi window compared to SrTiO3, even though PbTiO3 is nondegenerate at the conduction band minimum, while cubic SrTiO3 is three-fold degenerate. This demonstrates the important role of the ideality in the 2D electronic structure. The ab-initio results demonstrate the potential of PbTiO3 as a promising thermoelectric material.

Effect of HPHT technology and Se doping on thermoelectric properties of In filled Fe doped CoSb3 materials

Skutterudite (CoSb3) is a kind of promising environment-friendly medium-temperature thermoelectric material. Indium-filled CoSb3 has been widely studied, but its bipolar effect, which is commonly generated between 600 and 700 K, seriously hinders the increase of Seebeck coefficient and zT value. The zT value not only directly reflects the overall thermoelectric performance of the material, but also reflects the thermoelectric conversion efficiency of the device. In this work, In filled and Fe, Se double doped CoSb3 materials were successfully prepared by high-temperature and high-pressure synthesis method (HPHT), with a significant shorten synthesis time of 30 mins for each sample. This research has shown that after Se doping via HPHT synthesis method, the bipolar effect of In-filled CoSb3 was significantly suppressed, thus the temperature limitations of electrical transport performance has been overcome. Through the further microstructure analysis, it was found that the crystal structure of sample In0.5Fe0.01Co3.99Sb11.91Se0.09 has the characteristics of multiple grain sizes and complex defect types. And these two microscopic morphological characteristics resulted in the sample obtaining a minimum lattice thermal conductivity of 0.94 Wm−1K−1 at a test temperature of 773.15 K. Through synergistic optimization of synthesis methods and element substitution, the maximum zT value of 1.21 of the sample In0.5Fe0.01Co3.99Sb11.91Se0.09 has been found at 773.15 K.