Highest ZT Value Research Articles

Unraveling the Origin of Donor‐Like Effect in Bismuth–Telluride‐Based Thermoelectric Materials

The donor‐like effect, depicting the uncontrollable increase of electron density that can significantly alter the thermoelectric performance of both p‐type and n‐type polycrystalline Bi2Te3‐based materials, has long been an intriguing phenomenon, while its origin is still elusive. Herein, it is found that different from the common argument, the donor‐like effect in Bi2Te3‐based polycrystals is a result of the oxygen‐adsorption‐induced evolution of the point defects. The dominant point defect in stoichiometric zone‐melted Bi2Te3 ingot is the acceptor‐like . During the fabrication of high‐strength polycrystals, the exposure of the powders to the air leads to their absorption of oxygen and the formation of secondary phase Bi2TeO5 in the following sintering process. This brings about a change of local chemical equilibrium and promotes the evolution of the intrinsic point defect from acceptor‐like to donor‐like . Notably, if the fabrication process is strictly controlled to minimize oxygen adsorption, the evolution of the point defects will be avoided, whereby the donor‐like effect disappears. Consequently, a reproducible high zT value of 1.0 at 325 K can be achieved in Bi2Te2.7Se0.3‐based polycrystals. These results highlight the importance of understanding the evolution of point defects, which is crucial for developing high‐performance Bi2Te3‐based polycrystals and corresponding fabrication processes.

High Thermoelectric Performance in Cu2SnS3 by Control Over Phase‐Dependent Mobility Edge

AbstractComplex thermoelectric materials composed of multiple phases with order/disorder mixing feature have drawn intense attention due to their high performance. However, some physical mechanisms relevant to thermoelectric properties including the relationship between carrier mobility and disorder extent in these materials are still elusive. Here, a phase‐dependent mobility edge in (Cu2Sn1‐yS3)0.85(CuGaS2)0.15 solid solutions is demonstrated. By control over the mobility edge in these solid solutions by increasing the ratio of tetragonal phase with less disorder, a weakened localization of electronic states and promoted carrier mobility are realized. Combined with reduced lattice thermal conductivity, a record high zT value of 1.22 is attained at 823 K for (Cu2Sn0.92S3)0.85(CuGaS2)0.15 with the lowered mobility edge. The findings shed light on a new route to tuning the electrical and thermal transport properties for complex thermoelectric materials with multiple phases.

Construct Schottky interface containing energy-filtering effect: An efficient strategy to decouple thermopower and conductivity

Organic/inorganic thermoelectric hybrids demonstrate great potential for wearable applications. However, their scalability is hindered by an inferior power factor (S2σ). Nowadays, achieving deep optimization of S2σ necessitates a strategy to decouple the Seebeck coefficient (S) and electrical conductivity (σ). In this work, we propose a strategy to break the coupling between S and σ by constructing a Schottky interface that exhibits an energy-filtering effect. We validate the feasibility of this approach using a PANI/TiN–TiO2/carbon paper. The results demonstrate a 1.16-fold increase in σ and a 1.08-fold increase in S in PANI/TiN–TiO2/carbon paper achieved through the construction of a Schottky-type TiN/TiO2 interface. The separation of hole/electron at the TiN/TiO2 interface serves as the scattering center for ionized impurity scattering and facilitates the transport pathway for charge carriers. These factors are crucial in determining the simultaneous optimization of S and σ, respectively. Additionally, the energy-filtering effect of the TiN/TiO2 interface plays a positive role in the ionized impurity scattering mechanism by selectively filtering out low-energy carriers. This further strengthens decoupling of the thermoelectric properties. The 14.9% PANI/11.2% TiN–14.5% TiO2/59.44% carbon paper displays the highest S2σ and achieves a high ZT value of 223.6 μVm−1 K−2 and 0.31 at 300 K, highlighting the advantages of PANI-based thermoelectric hybrids. This work provides valuable guidance for the design of thermoelectric hybrids incorporating multi-interface morphology.

Raising n-Type Thermoelectric Performance in (Te, Zn)-Codoped CuAgSe

We have successfully fabricated undoped and (Te, Zn)-codoped n-type CuAgSe polycrystals. For doping studies, we kept 5% Te over Se and changed from 0 to 10% Zn over Cu. (Te, Zn) codoping on CuAgSe reduces the lattice thermal conductivity to about 0.3 W m–1 K–1 at 673 K and increases the electrical conductivity. In particular, a ZT value of 0.91 was achieved at 623 K, which is the highest ZT value in n-type CuAgSe. Our results thus indicate that Te and Zn codoping is a potential way to improve the thermoelectric performance of n-type CuAgSe.

Enhancement of thermoelectric performance in n-type Si90Ge10-based alloy by metallic Zn doping

Silicon–germanium (SiGe) alloy has become one of the representative high-temperature thermoelectric (TE) materials due to its advantages of stability, non-toxicity, oxidation resistance, and high mechanical strength. However, the high thermal conductivity and expensive Ge greatly limit the enhancement of zT value and its application. In this paper, n-type Si90Ge10P2Znx nanocomposites were prepared by ball milling and spark plasma sintering. By adjusting the Zn content and sintering time, multiple phonon-scattering centers, such as Zn precipitates, nano-pores, and layered structures, have been introduced into the SiGe matrix. The thermal conductivity was significantly reduced to 2.59 W m−1 K−1 without deteriorate power factor (PF), thus leading to a high zT value of 1.23 at 873 K. At 323–873 K, the average zT value (zTavg) also reached 0.6, increased by approximately 25% in comparison to the reported value using the same ratio of Si90Ge10. Compared with the conventional radioisotope TE generator with Si80Ge20 composition, the zTavg value increased by nearly 30% with only half of Ge, giving strong impetus to the application of SiGe-based TE materials.

High thermoelectric properties of Cu2Te[sbnd]Ag2Te composite with Fe addition and non-stoichiometric Te

Cu2Te-based materials are a type of superionic conductor belonging to the class of phonon-liquid electron-crystal materials and have achieved high ZT values by doping and nanostructuring. However, it is easy to form copper vacancies in Cu2Te which leads to an excessive carrier concentration and then results in a low Seebeck coefficient. Hence, controlling copper ion migration and optimizing carrier concentration is essential to improve the thermoelectric performance of Cu2Te. This paper reports high-performance Cu2TeAg2Te composite with high application value in the low-middle temperature region, which is achieved by fine tuning the carrier concentration using Fe addition and non-stoichiometric Te, as well as controlling the thermal conductivity of composite. A high ZT of ∼1.2 is obtained in AgCu0.97Fe0.03Te0.96 at a low temperature of 573 K. Meanwhile, the phase transition mechanism of Cu2TeAg2Te and its effect on the thermoelectric transport performance are revealed that go beyond nanostructuring and single-doping, which provides a strong theoretical basis for research and performance improvement of thermoelectric materials in this system.

Engineering of Thermoelectric Composites Based on Silver Selenide in Aqueous Solution and Ambient Temperature.

The direct, solid state, and reversible conversion between heat and electricity using thermoelectric devices finds numerous potential uses, especially around room temperature. However, the relatively high material processing cost limits their real applications. Silver selenide (Ag2Se) is one of the very few n-type thermoelectric (TE) materials for room-temperature applications. Herein, we report a room temperature, fast, and aqueous-phase synthesis approach to produce Ag2Se, which can be extended to other metal chalcogenides. These materials reach TE figures of merit (zT) of up to 0.76 at 380 K. To improve these values, bismuth sulfide (Bi2S3) particles also prepared in an aqueous solution are incorporated into the Ag2Se matrix. In this way, a series of Ag2Se/Bi2S3 composites with Bi2S3 wt % of 0.5, 1.0, and 1.5 are prepared by solution blending and hot-press sintering. The presence of Bi2S3 significantly improves the Seebeck coefficient and power factor while at the same time decreasing the thermal conductivity with no apparent drop in electrical conductivity. Thus, a maximum zT value of 0.96 is achieved in the composites with 1.0 wt % Bi2S3 at 370 K. Furthermore, a high average zT value (zTave) of 0.93 in the 300-390 K range is demonstrated.

Rapid Printing of Pseudo-3D Printed SnSe Thermoelectric Generators Utilizing an Inorganic Binder.

There has been much interest in tin selenide (SnSe) in the thermoelectric community since the discovery of the record zT in the material in 2014. Manufacturing techniques used to produce SnSe are largely energy-intensive (e.g., spark plasma sintering); however, recently, in previous work, SnSe has been shown to be produced via a low embodied energy printing technique, resulting in 3D samples with high zT values (up to 1.7). Due to the additive manufacturing technique, the manufacturing time required was substantial. In this work, 3D samples were printed using the inorganic binder sodium metasilicate and reusable molds. This facilitated a single-step printing process that substantially reduced the manufacturing time. The printed samples were thermally stable through multiple thermal cycles, and a peak zT of 0.751 at 823 K was observed with the optimum binder concentration. A proof-of-concept thermoelectric generator produced the highest power output of any reported printed Se-based TEG to date.

Enhancing Thermoelectric and Mechanical Properties of p-Type (Bi, Sb)2Te3 through Rickardite Mineral (Cu2.9Te2) Incorporation

Bi2Te3-based alloys are widely utilized in Peltier coolers owing to their highest thermoelectric performance at near-room-temperatures. However, their peak dimensionless thermoelectric figure of merit, zT, is limited to a narrow temperature window due to minority carrier excitation emerging upon heating at around 400 K. Here, we show how this issue can be overcome by incorporating a synthetic rickardite mineral, Cu3–xTe2, in p-type (Bi, Sb)2Te3. The significant enhancement of the electronic and thermal properties could be achieved due to small Cu incorporation into the crystal structure of (Bi, Sb)2Te3 and homogeneous precipitation of Cu3–xTe2 at the grain boundaries. This leads to a high average zT value (zTave) of 1.22 between 350 and 500 K for two compositions, Bi0.5Sb1.5Te3 (BST-5) and Bi0.3Sb1.7Te3 (BST-3), with peak zT values of 1.32 at 467 K and 1.30 at 400 K, respectively. These high zT values result in a considerably high maximum device ZT of ca. 1.15 and a theoretical efficiency of up to 7% between 325 and 525 K. Additionally, room-temperature micro-hardness is substantially improved, which is desirable for constructing reliable and durable thermoelectric modules.

Contrasting roles of trivalent dopants M (M = In, Sb, Bi) in enhancing the thermoelectric performance of Ge0.94M0.06Te

Thermoelectric materials can achieve the interconversion of electricity and heat, providing potential for power generation. It is a challenge to elevate conversion efficiency due to the coupled thermoelectric parameters. Herein, the high thermoelectric performance is successfully achieved in GeTe-based thermoelectric materials by single doping trivalent dopant M (M=In, Sb, Bi). The high conversion efficiency of ∼10.3% and power density of ∼ 0.6 W cm−2 under a temperature difference of 419 K are realized in a segmented single-leg device. And the remarkable hardness of ∼ 2.92 GPa in M-doped GeTe provides a prospect for device application. The advanced performance in M-doped GeTe is mainly ascribed to its high ZT value through manipulating deformation potential. The three trivalent dopants facilitate band convergence due to the enhanced symmetry, which increases effective mass without scarifying carrier mobility. Compared with In and Sb, Bi element can be the most effective dopant to decrease deformation potential from 25.3 eV to 17.2 eV. The low deformation potential by doping with Bi leads to high weighted mobility, further optimizing the power factor. The In-doped GeTe exhibits low lattice thermal conductivity at ambient temperature because of the softened phonons, while the lattice thermal conductivity of Bi-doped GeTe is significantly decreased at high temperature due to the larger atomic mass and size of Bi than that of the other two series. Consequently, the highest ZT of ∼1.9 at 723 K is attained in Ge0.94Bi0.06Te which outperforms other single-doped GeTe-based thermoelectric materials.

First-principles analysis of the physical properties of XAcTe2 (X = Li, Na) Heusler alloys for optoelectronic and thermoelectric devices

Materials with diverse properties are in high demand due to their utilization in multipurpose devices. The current work presents the systematical analysis of electronic structure, mechanical, optoelectronic and transport properties of new Heusler XAcTe2 (X = Li, Na) alloys. Density functional theory (DFT) based calculations are carried out first time to compute the physical properties of the alloys. The exchange–correlation potential is solved with modified Becke-Johnson (mBJ) approximation. Structural stability is obtained with ground state optimized lattice parameters, negative formation energy values, and phonon dispersion curves. A brittle nature with an anisotropic character is observed for both alloys. The band structures show a direct bandgap for both alloys, which is also reflected in the density of states. The evaluated optoelectronic response shows the high absorption in the ultraviolet region, confirming their ability to use them in optoelectronic devices. The analysis of thermoelectric parameters shows the high electrical and minimum thermal conductivities for both alloys. The electronic and lattice vibrations dependent high ZT values are achieved for LiAcTe2 and NaAcTe2 alloys. The optoelectronic and thermoelectric responses of the alloys show that they could be promising candidates for use in optoelectronic and renewable energy applications.

Combinatorial screening via high-throughput preparation: Thermoelectric performance optimization for n-type Bi–Te–Se film with high average zT > 1

Thermoelectric materials have drawn extensive interest due to the direct conversion between electricity and heat, however, it is usually a time-consuming process for applying traditional “sequential” methods to grow materials and investigate their properties, especially for thermoelectric films that typically require fine microstructure control. High-throughput experimental approaches can effectively accelerate materials development, but the methods for high-throughput screening of the microstructures require further study. In this work, a combinatorial high-throughput optimization solution of material properties is proposed for the parallel screening and optimizing of composition and microstructure, which involves two distinctive types of high-throughput fabrication approaches for thin films, along with a new portable multiple discrete masks based high-throughput preparation platform. Thus, Bi2Te3–xSex thin film library with 196 throughputs for locating the optimized composition is obtained in one growth cycle. In addition, another thin film library composed of 31 materials with traceable process parameters is built to further investigate the relationship between microstructure, process, and thermoelectric performance. Through high-throughput screening, the Bi2Te2.9Se0.1 film with (00l) orientation is prepared with a peak zT value of 1.303 at 353 K along with a high average zT value of 1.047 in the interval from 313 to 523 K. This method can be also extended to the discovery of other functional thin films with a rapid combinatorial screening of the composition and structure to accelerate material optimization.

Structural, Thermoelectric, Electronic, and Magnetic Properties of Pristine Intermetallic Rare-Earth-Based XMn2Si2 (X=Dy, Er) Compounds

Detailed Structural, thermoelectric, electronic and magnetic properties of the ternary rare-Earth based XMn2Si2 (X=Dy, Er) Compounds, are investigated using the full-potential linearized augmented-plane wave (FP-LAPW) method with generalized gradient approximation (GGA+U) in ferromagnetic phase. The basic calculations of optimization are found with the support of (PBE-GGA) to realize theoretical consistency with existing experimental consequences, although for the enhancement of magneto-electronic part the (GGA+U) technique is employed. We have identified theoretically that the ferromagnetic is the most suitable phase among three studied phases for these compounds agree well with previous experimental works. The electronic band structure indicates that these compounds are metallic through both spin channels in the FM phase. A secure hybridization occurs between the elements Dy/Er-f and Mn-d states in the valence band and the Si-p state in the conduction band. The total magnetic moments verify that the rare-Earth based DyMn2Si2 ternary inter-metallic compound showcases stronger ferromagnetic behavior patterns than the ErMn2Si2 compound. We estimated the Seebeck coefficient S, electrical and thermal conductivities, and the ZT in this study over the temperature range of 0 to 800 K. The ErMn2Si2 is a viable contender for high-temperature applications in waste heat management because of its high ZT values in the high-temperature region in thermoelectric devices.

Enhancement of Thermoelectric Performance for CuCl Doped P-Type Cu2Sn0.7Co0.3S3

Cu2SnS3 (CSS) has gained great attraction due to its constitutive earth-abundant elements and intrinsic low lattice thermal conductivity, κl, potentially providing high quality factor, B, and high zT value. However, the lack of band convergence is the bottleneck to enhancing the thermoelectric performance of Cu2SnS3 when performing the band engineering. To study the doping effect on the band structure and the thermoelectric performance, the composite Cu2Sn0.7Co0.3S3-xCuCl (x = 0, 0.1, 0.2, 0.3) (CSCS-xCuCl) has been investigated for the first time. The samples showed excellent data repeatability at high temperatures of up to 700 K. It was found that CuCl could compensate the Cu loss, enhance the phonon scattering and minimize the adverse effect on the power factor, PF. The ultralow lattice thermal conductivity could reach 0.38 W m-1 K-1 for the nominal composition of CSCS-0.3CuCl at 700 K. A peak zT of 0.56 (evaluated with no cold finger effect) was realized at 700 K when x = 0.3, which is almost double the performance of pristine samples.

Pressure induced bands convergence and strength enhancement in thermoelectric semiconductor β-InSe

The semiconductor InSe possesses an exceptional ductility at room temperature, but its weak electronic transport properties leads to the low thermoelectric properties, limiting its applications in flexible thermoelectric devices. In this work, we applied first principles to find that the hydrostatic pressure can remarkably improve the thermoelectric and mechanical properties of β-InSe. The multiple conduction bands convergence is observed at the hydrostatic pressure of 5 GPa, resulting in a maximum zT value of 0.235 at 300 K, which is 170 % higher than the intrinsic zT value of 0.087. The point defect calculations suggest that In0.96Pb0.04Se or InSe0.96Cl0.04 could obtain the optimal carrier concentration of 7.65 × 1019 cm−3 with the highest zT value. In addition, we find that the (001)/<110>slip system of InSe exhibits the lowest ideal shear strength of 0.68 GPa, due to the weak interlayered van der Waals SeSe bonds. While the hydrostatic pressure can reduce the bond lengths and strengthen the van der Waals SeSe bonds. At 5 GPa, a maximum stretching force constant of van der Waals SeSe bond is obtained, giving rise to a maximum shear ideal strength of 1.45 GPa, which is 113 % higher than that without pressure. This hydrostatic pressure induced strain engineering strategy provides a theoretical guidance to synthesize high-performance thermoelectric β-InSe with robust mechanical properties.

Two-Dimensional AMgB (A = Na, K; B = P, As, Sb, Bi) with Promising Optoelectronic and Thermoelectric Performances

For the purpose of solar energy harvesting, we have predicted eight kinds of stable two-dimensional materials: AMgB (A = Na, K; B = P, As, Sb, Bi) based on first-principles calculations. The band gaps of AMgB monolayers (MLs for short) cover a wide range from 0.39 to 1.68 eV and all exhibit direct or quasi-direct band features. More encouragingly, AMgB MLs possess high acoustic phonon-limited carrier mobilities ranging from 103 to 107 cm2 V–1 s–1. In addition, noticeable optical absorption from the visible light to ultraviolet regimes has been predicted for AMgB MLs and the power conversion efficiencies (PCEs) are up to 22.06 and 16.26% for the proposed NaMgAs/NaMgSb and KMgP/KMgAs type-II heterojunctions. Moreover, combined with low thermal conductivity and fairly large power factor, considerable thermoelectric properties have been theoretically confirmed in AMgB MLs with the figure of merit (ZT) up to 1.15 (NaMgBi ML). Desired band gap, ultrahigh carrier mobility, excellent light absorption capacity, and high ZT values could render AMgB MLs promising candidates for low-dimensional optoelectronic and thermoelectric applications.

Thermoelectric response of 1T-ZrS2 monolayer: Ab-initio study

The development of highly efficient thermoelectric materials has greatly assisted by two-dimensional transition metal dichalcogenides. The present work deals with the study of structural, electronic, and thermoelectric properties of 1T-ZrS2 monolayer by employing ab-initio calculations in combination with Boltzmann transport equation. The performance of thermoelectric materials is represented by the value of the figure of merit (ZT). The electronic band structure of 1T-ZrS2 monolayer is confirmed its semiconducting nature with indirect band gap value of 1.20 eV. Phonon dispersion curve indicates the monolayer stability. Low lattice thermal conductivity is primarily caused by the strong interaction of low frequency acoustic modes with optical modes, which leads to a high ZT value for the suggested monolayer. The Seebeck coefficient, which is higher for n-type doping than p-type doping, indicates that n-type doping ZT value is more effectively optimised than p-type doping. At room temperature, a high value of ZT (=0.9) is attained, and its value varies with temperature. Based on the above study, 1T-ZrS2 monolayer may act as the potential candidates for its use as thermoelectric materials.

Enhanced Thermoelectric Properties of Zr0.85–xHfxNb0.15–yTayCoSb Medium-Entropy Alloys: Tradeoff between “What to Alloy” and “How Much to Alloy”

Although configurational entropy is regarded to be a gene-like performance indicator for thermoelectric (TE) materials, increasing the configurational entropy alone does not guarantee a high TE figure of merit (ZT). Therefore, determining protocols for designing medium- and high-entropy TE materials with high ZT values is imperative. Herein, we provide a strategy for designing high ZT n-type ZrCoSb-based medium-entropy (ME) half-Heusler (HH) alloys and highlight the tradeoff between “what to alloy” and “how much to alloy”. As expected, an intrinsically low lattice thermal conductivity of ∼2.24 W m–1 K–1 was obtained at 923 K for the Zr0.45Hf0.4Nb0.06Ta0.09CoSb ME HH alloy owing to the synergy between Zr-site disorder, anharmonicity, refined grain size, and entropy-driven multiscale defects. Entropy-driven quantum traps lead to energy-filter effects that concurrently provide optimized Seebeck coefficients and power factors. These favorable modifications enable the Zr0.45Hf0.4Nb0.06Ta0.09CoSb ME HH alloy to exhibit a high peak ZT of ∼0.57. The developed design concept provides an effective strategy for enhancing the ZT values of medium- and high-entropy TE materials.

Pressure-induced enhancement of thermoelectric performance of CoP3 by the structural phase transition

Using particle swarm optimization and first-principles calculations, we discovered a novel high-pressure orthorhombic structure of CoP3 with the space group of Pnma, which is stable above 24.6 GPa. According to our calculations based on Boltzmann transport theory, the Seebeck coefficient of the Pnma phase is 6.5 times as great as that of the ambient Im-3 phase at a hole concentration of 1 × 1020 cm−3. The calculation further indicates the Pnma-CoP3 exhibits a maximal figure of merit ZT of 0.56 and 0.74 for p- and n-type at 800 K, respectively. Especially for p-type, the Pnma-CoP3 is quintuple better than the ambient phase which has a ZT of 0.1. The higher ZT value can be attributed to the deeper projected density of states generated by hybridization between P p states and Co d states near the Fermi level. Therefore, the high pressure mainly improves the ZT value by modulating electron properties. Current results demonstrate that the Pnma-CoP3 material is a competitive candidate for thermoelectric applications, especially under high temperature condition (800 K). This work provides a convenient alternative route to increase the thermoelectric performance of skutterudites.

Enhanced Thermoelectric Performance of Mg-Doped AgSbTe2 by Inhibiting the Formation of Ag2Te.

The existence of Ag2Te has always been an obstacle for p-type thermoelectric material AgSbTe2 to improve its thermoelectric performance. In this work, AgSb1-xMgxTe2 samples are synthesized by melting-slow-cooling and then spark plasma sintering (SPS). Through increasing the solubility of Ag2Te in the AgSbTe2 matrix by Mg doping, the formation of Ag2Te is inhibited. Density functional theory calculations confirm more valence bands are involved in electrical transport due to Mg doping. Therefore, the electrical conductivity of AgSb1-xMgxTe2 samples has been greatly improved due to the reduction of Ag2Te with n-type electrical conductivity. Moreover, the downward trend of ZT, which is caused by the structural transition of Ag2Te at about 418 K, disappears. Meanwhile, lattice defects form in the AgSb0.98Mg0.02Te2 sample, and Mg doping improves the configurational entropy change, resulting in a decrease in lattice thermal conductivity over the entire temperature range of measurement. Finally, a high ZT value of 1.31 at 523 K is achieved for the AgSb0.98Mg0.02Te2 sample. This study demonstrates that Mg doping can effectively improve AgSbTe2 thermoelectric performance by inhibiting the formation of the Ag2Te impurity phase.