Vacancy Manipulation for Thermoelectric Enhancements in GeTe Alloys

Scattering Intensity Regulation via Intrinsic S Vacancies in [CuS2▫] Motif for Optimized Initial Thermoelectric Performance in Cu─Sb─S System

The Cu─Sb─S system has garnered significant attention as thermoelectric (TE) material due to the low lattice thermal conductivity (κlat), cost-effectiveness, and low toxicity. Herein, an intrinsic switch is demonstrated to optimize the initial properties before doping, which controls the intensity of phonon and carrier scattering simultaneously. The degree of filling of the S-site at the co-vertex junction of the [CuS3] triangles plays a crucial role in determining both electrical and thermal transport properties. Scattering intensity reaches a maximum with the formation of [CuS2▫] motifs and a minimum when the [CuS2▫] is fully filled. However, neither condition achieves optimal performance. The partially filled sample, Cu3SbS3.1, exhibits enhanced carrier transport while facilitating phonon scattering at a high level by preserving significant thermal vibration of Cu atoms. This optimal balance achieved by modulating the scattering intensity endows the pristine Cu3SbS3.1 with an intrinsic zT value of 0.7, which is far superior to that of other members in the Cu─Sb─S system. Additionally, zT and its average value are improved through Cd doping, reaching 1.07 and 0.70, respectively. This strategy regulates intrinsic scattering intensity to enhance initial TE properties and provide optimized materials for further development.

Advanced GeTe-Based Thermoelectrics: Charting the Path from Performance Optimization to Devices

Thermoelectric (TE) materials can interconvert electricity into heat, rendering them versatile for refrigeration and power generation. GeTe as a distinguished TE material has attracted considerable focus owing to its excellent TE performance. Herein, the milestones of research progress on GeTe are reviewed. The intrinsic potentials of GeTe are elaborated, mainly focusing on crystal structure, band structure and microstructures. The path of GeTe-based thermoelectrics from performance optimization to the devices is attempted to chart, referring to its shortcomings and characteristics. Primarily, optimization of the synthesis process is implemented to inhibit the generation of Ge precipitates and phonon migration. Furthermore, the thermoelectric performance of GeTe is enhanced through its features, including phase transition, multiple valence bands, and various microstructures via doping and alloying. Subsequently, the advancements of GeTe thermoelectric devices are presented from the aspect of device integration. Eventually, the prospect and challenges for the future direction of GeTe-based materials are proposed, offering a roadmap to inject vitality into further developments.

High Thermoelectric Figure of Merit (zT) in β-Ag2Se via Aliovalent Doping

High-performance thermoelectric materials are essential for efficient low-temperature (300-400 K) heat energy harvesting, with n-type Ag2Se being a promising candidate. To further enhance the thermoelectric figure of merit (zT) of Ag2Se, aliovalent doping has emerged as a key strategy. However, achieving wet-chemical aliovalent doping of Ag2Se at ambient temperature has proven challenging. In this work, a high zTmax of 1.57 at 398 K is reported for an optimally Cd(II)-doped Ag2Se sample, specifically in the structurally phase-pure Ag1.98Cd0.02Se, which is successfully synthesized via an aqueous-based method at room-temperature (300 K). The Ag1.98Cd0.02Se sample also exhibits an impressive average zTavg of 1.12 over the temperature range of 315-400 K. Density functional theory (DFT) calculations for both the pristine and doped samples reveal significant changes in the electronic band structures, including notable modulations in the density of states near the Fermi energy, particularly for the Ag-3d states. The remarkable thermoelectric performance of Ag1.98Cd0.02Se is attributed to an optimization of charge carrier induced by the Cd(II)-doping.

Achieving Extraordinary Power Factors in GeTe Epitaxial Films through Carrier Transport Engineering

GeTe-based films have attracted tremendous attention from the thermoelectric community owing to their excellent thermoelectric performance. It is vital to reduce the hole density and maintain a high carrier mobility for GeTe films; however, this remains a significant challenge. To overcome this drawback, we succeeded in fabricating high-crystalline quality GeTe-based films and remarkably improve their electrical properties using molecular beam epitaxy under a low substrate temperature and optimized Te/GeTe flux ratios. The Bi2Te3/GeTe double-layer buffer facilitated the reliable fabrication of high-quality GeTe films. The hole density and carrier mobility were synergistically optimized under a relatively low substrate temperature of 503 K and Te/GeTe flux ratio of 0.25/1 that suppress the formation of Ge vacancies, as well as a trace amount of Sb2Te3 incorporation that introduces SbTe substitutional defects. The best (GeTe)24/(Sb2Te3)0.25 film acquires a very low hole density of 2.57 × 1020 cm-3 and, simultaneously, a high carrier mobility of 96.53 cm2 V-1 s-1, which leads to an extraordinary power factor of 3.36 mW m-1 K-2 at room temperature as well as an average power factor of 4.15 mW m-1K-2 within 300-475 K, outperforming the values of GeTe from previous reports. This work provides valuable insights for fabricating high-performance GeTe-based films to promote their future applications near room temperature.

Significant Enhancement in the Thermoelectric Performance of the MnSb2Te4 Topological Insulator through Vacancy Regulation and Lattice-Softening Strategies

The topological insulator MnSb2Te4 shows promising potential in thermoelectric applications due to its intrinsically low lattice thermal conductivity. However, its thermoelectric performance is limited by the high carrier concentration, of which the origin is still unclear. In this work, the carrier concentration is successfully tuned from 2.24 × 1021 cm-3 to as low as 9.1 × 1019 cm-3. Transmission electron microscopy and positron annihilation measurements suggest that large amounts of Mn vacancies exist in the septuple layer of MnSb2Te4, which are responsible for the high carrier concentration. The Mn vacancies are suppressed by the excess Mn element and AgSbTe2 alloying, which not only reduces the carrier concentration but also weakens the carrier scattering and thus improves the mobility. The decrease in carrier concentration also leads to reduced electronic thermal conductivity. The excess Mn atoms introduce a strain field in the Mn layer, which enhances phonon scattering. Furthermore, the substitution of Ag for Mn causes lattice softening by weakening the chemical bonds in MnSb2Te4, which leads to reduced phonon velocity and, therefore, further reduction in lattice thermal conductivity. As a result, an extremely low lattice thermal conductivity of 0.44 W m-1 K-1 was obtained at 300 K and it further decreased to 0.17 W m-1 K-1 at 798 K. Finally, a record zT value of 1.53 at 798 K was achieved in Mn1.06Sb2Te4(AgSbTe2)0.04, and the optimal carrier concentration is about 2 × 1020 cm-3.

Chemical modulation and defect engineering in high-performance GeTe-based thermoelectrics

Thermoelectric technology plays an important role in developing sustainable clean energy and reducing carbon emissions, offering new opportunities to alleviate current energy and environmental crises. Nowadays, GeTe has emerged as a highly promising thermoelectric candidate for mid-temperature applications, due to its remarkable thermoelectric figure of merit (ZT) of 2.7. This review presents a thorough overview of the advancements in GeTe thermoelectric materials, meticulously detailing the crystal structure, chemical bonding characteristics, band structure, and phonon dynamics to elucidate the underlying mechanisms that contribute to their exceptional performance. Moreover, the phase transition in GeTe introduces unique degrees of freedom that enable multiple pathways for property optimization. In terms of electrical properties, noticeable enhancement can be realized through strategies such as band structure modulation, carrier concentration engineering, and vacancy engineering. For phonon transport properties, by incorporating defect structures with varying dimensions and constructing multi-scale hierarchical architectures, phonons can be effectively scattered across different wavelengths. Additionally, we provide a summary of current research on devices and modules of GeTe. This review encapsulates historical progress while projecting future development trends that will facilitate the practical application of GeTe in alignment with environmentally sustainable objectives.

Advances in Mg3Sb2 thermoelectric materials and devices

Thermoelectric technology offers a green-viable and carbon-neutral solution for energy problems by directly converting waste heat to electricity. For years, Bi2Te3-based compounds have been the main choice materials for commercial thermoelectric devices. However, Bi2Te3 comprises scarce and toxic tellurium (Te) elements, which might limit its large-scale application. Recently, Mg3Sb2 compounds have drawn increasing attention as an alternative to Bi2Te3 thermoelectrics due to their excellent thermoelectric performance. Enabled by effective strategies such as optimizing carrier concentration, introducing point defects, and manipulating carrier scattering mechanisms, Mg3Sb2 compounds have realized an improved thermoelectric performance. In this review, optimizing strategies for both Mg3Sb2-based thermoelectric materials and devices are discussed. Moreover, the flexibility and plasticity of Bi-alloyed Mg3Sb2 mainly stemming from the dense dislocations are outlined. The above strategies summarized here for enhancing Mg3Sb2 thermoelectrics are believed to be applicable to many other thermoelectrics.

Simultaneous Suppression of Phonon Transport and Carrier Concentration for Efficient Rhombohedral GeTe Thermoelectric

Superior electronic performance due to the highly degenerated Σ valence band (Nv∼12) makes rhombohedral GeTe a promising low-temperature (<600 K) thermoelectric candidate. Minimizing lattice thermal conductivity (κL) is an essential route for enhancing thermoelectric performance, but the temperature-dependent κL, corelated to T-1, makes its reduction difficult at low temperature. In this work, a room-temperature κL of ≈0.55 W m-1-K-1, the lowest ever reported in GeTe-based thermoelectric, is realized in (Ge1- ySbyTe)1- x(Cu8GeSe6)x, primarily due to strong phonon scattering induced by point defects and precipitates. Simultaneously, Cu8GeSe6-alloying effectively suppresses the precipitation of Ge, enabling the optimization of carrier concentration with the additional help of aliovalent Sb doping. As a result, an extraordinary peak zT of up to 2.3 and an average zTavg. of ≈1.2 within 300-625 K are achieved, leading to a conversion efficiency of ≈9% at a temperature difference of 282 K. This work robustly demonstrates its potential as a promising component in thermoelectric generator utilizing low-grade waste heat.

Reducing Domain Density Enhances Conversion Efficiency in GeTe

Incorporating dilute doping and controlled synthesis provides a means to modulate the microstructure, defect density, and transport properties. Transmission electron microscopy (TEM) and geometric phase analysis (GPA) have revealed that hot-pressing can increase defect density, which redistributes strain and helps prevent unwanted Ge precipitates formation. An alloy of GeTe with a minute amount of indium added has shown remarkable TE properties compared to its undoped counterpart. Specifically, it achieves a maximum figure-of-merit zT of 1.3 at 683 K and an exceptional TE conversion efficiency of 2.83% at a hot-side temperature of 723 K. Significant zT and conversion efficiency improvements are mainly due to domain density engineering facilitated by an effective hot-pressing technique applied to lightly doped GeTe. The In-GeTe alloy exhibits superior TE properties and demonstrates notable stability under significant thermal gradients, highlighting its promise for use in mid-temperature TE energy generation systems.

High Thermoelectric Performance in Rhombohedral GeSe-LiBiTe2

GeSe, an analogue of SnSe, shows promise in exhibiting exceptional thermoelectric performance in the Pnma phase. The constraints on its dopability, however, pose challenges in attaining optimal carrier concentrations and improving ZT values. This study demonstrates a crystal structure evolution strategy for achieving highly doped samples and promising ZTs in GeSe via LiBiTe2 alloying. A rhombohedral phase (R3m) can be stabilized in the GeSe-LiBiTe2 system, further evolving into a cubic (Fm3̅m) phase with a rising temperature. The band structures of GeSe-LiBiTe2 in the rhombohedral and cubic phases feature a similar multiple-valley energy-converged valence band of L and Σ bands. The observed high carrier concentration (∼1020 cm-3) reflects the effective convergence of these bands, enabling a high density-of-states effective mass and an enhanced power factor. Moreover, a very low lattice thermal conductivity of 0.6-0.5 W m-1 K-1 from 300 to 723 K is achieved in 0.9GeSe-0.1LiBiTe2, approaching the amorphous limit value. This remarkably low lattice thermal conductivity is related to phonon scattering from point defects, planar vacancies, and ferroelectric instability-induced low-energy Einstein oscillators. Finally, a maximum ZT value of 1.1 to 1.3 at 723 K is obtained, with a high average ZT value of over 0.8 (400-723 K) in 0.9GeSe-0.1LiBiTe2 samples. This study establishes a viable route for tailoring crystal structures to significantly improve the performance of GeSe-related compounds.

Screening Weldable Metal Electrodes for Ag2Se Thermoelectric Devices below 300 °C

Thermoelectricity has been considered as the most important solution of generating electricity, particularly from low-grade heat below 300 °C. Despite efforts in recent years on exploring alternative materials to only commercialized Bi2Te3, the practical implementation of these new materials has been hindered by inadequate investigation into device design. Given that the utilization of weldable electrodes offers advantages in technical compatibility for a large-scale assembly of thermoelectric elements into modules, a thorough investigation into the potential of weldable metal electrodes at T < 300 °C is imperative. In this work, the diffusion of 11 kinds of thermoelectric materials in common weldable metals (Ni, Fe, Cu, and Ag) was screened. Ag is sorted out as a promising weldable electrode that can directly bond to thermoelectric Ag2Se in this temperature range, leading to a minimization of an interfacial contact resistivity down to 11 μΩ cm2 in a design of the Ag/Ag2Se/Ag structure. The conversion efficiency of ∼3% at ΔT of 95 K with an excellent stability indicates Ag2Se as a top alternative to n-type Bi2Te3 for low-grade heat recovery.

Vacancies tailoring lattice anharmonicity of Zintl-type thermoelectrics

While phonon anharmonicity affects lattice thermal conductivity intrinsically and is difficult to be modified, controllable lattice defects routinely function only by scattering phonons extrinsically. Here, through a comprehensive study of crystal structure and lattice dynamics of Zintl-type Sr(Cu,Ag,Zn)Sb thermoelectric compounds using neutron scattering techniques and theoretical simulations, we show that the role of vacancies in suppressing lattice thermal conductivity could extend beyond defect scattering. The vacancies in Sr2ZnSb2 significantly enhance lattice anharmonicity, causing a giant softening and broadening of the entire phonon spectrum and, together with defect scattering, leading to a ~ 86% decrease in the maximum lattice thermal conductivity compared to SrCuSb. We show that this huge lattice change arises from charge density reconstruction, which undermines both interlayer and intralayer atomic bonding strength in the hierarchical structure. These microscopic insights demonstrate a promise of artificially tailoring phonon anharmonicity through lattice defect engineering to manipulate lattice thermal conductivity in the design of energy conversion materials.

Inverse-Perovskite Ba3 BO (B = Si and Ge) as a High Performance Environmentally Benign Thermoelectric Material with Low Lattice Thermal Conductivity

High energy-conversion efficiency (ZT) of thermoelectric materials has been achieved in heavy metal chalcogenides, but the use of toxic Pb or Te is an obstacle for wide applications of thermoelectricity. Here, high ZT is demonstrated in toxic-element free Ba3 BO (B = Si and Ge) with inverse-perovskite structure. The negatively charged B ion contributes to hole transport with long carrier life time, and their highly dispersive bands with multiple valley degeneracy realize both high p-type electronic conductivity and high Seebeck coefficient, resulting in high power factor (PF). In addition, extremely low lattice thermal conductivities (κlat ) 1.0-0.4 W m-1  K-1 at T = 300-600 K are observed in Ba3 BO. Highly distorted O-Ba6 octahedral framework with weak ionic bonds between Ba with large mass and O provides low phonon velocities and strong phonon scattering in Ba3 BO. As a consequence of high PF and low κlat , Ba3 SiO (Ba3 GeO) exhibits rather high ZT = 0.16-0.84 (0.35-0.65) at T = 300-623 K (300-523 K). Finally, based on first-principles carrier and phonon transport calculations, maximum ZT is predicted to be 2.14 for Ba3 SiO and 1.21 for Ba3 GeO at T = 600 K by optimizing hole concentration. Present results propose that inverse-perovskites would be a new platform of environmentally-benign high-ZT thermoelectric materials.

Charge transfer engineering to achieve extraordinary power generation in GeTe-based thermoelectric materials

By the fine manipulation of the exceptional long-range germanium-telluride (Ge─Te) bonding through charge transfer engineering, we have achieved exceptional thermoelectric (TE) and mechanical properties in lead-free GeTe. This chemical bonding mechanism along with a semiordered zigzag nanostructure generates a notable increase of the average zT to a record value of ~1.73 in the temperature range of 323 to 773 K with ultrahigh maximum zT ~ 2.7. In addition, we significantly enhanced the Vickers microhardness numbers (H_v) to an extraordinarily high value of 247 H_v and effectively eliminated the thermal expansion fluctuation at the phase transition, which was problematic for application, by the present charge transfer engineering process and concomitant formation of microstructures. We further fabricated a single-leg TE generator and obtained a conversion efficiency of ~13.4% at the temperature difference of 463 K on a commercial instrument, which is located at the pinnacle of TE conversion.

Ultralow Lattice Thermal Conductivity and High Thermoelectric Performance in Ge1-x-yBixCayTe with Ultrafine Ferroelectric Domain Structure

GeTe and its derivatives emerging as a promising lead-free thermoelectric candidate have received extensive attention. Here, a new route was proposed that the minimization of κ_L in GeTe through considerable enhancement of acoustic phonon scattering by introducing ultrafine ferroelectric domain structure. We found that Bi and Ca dopants induce strong atomic strain disturbance in the GeTe matrix because of large differences in atom radius with host elements, leading to the formation of ultrafine ferroelectric domain structure. Furthermore, large strain field and mass fluctuation induced by Bi and Ca codoping result in further reduced κ_L by effectively shortening the phonon relaxation time. The co-existence of ultrafine ferroelectric domain structure, large strain field, and mass fluctuation contribute to an ultralow lattice thermal conductivity of 0.48 W m^-1 K^-1 at 823 K. Bi and Ca codoping significantly enhances the Seebeck coefficient and power factor through reducing the energy offset between light and heavy valence bands of GeTe. The modified band structure boosts the power factor up to 47 μW cm^-1 K^-2 in Ge_0.85Bi_0.09Ca_0.06Te. Ultimately, a high ZT of ∼2.2 can be attained. This work demonstrates a new design paradigm for developing high-performance thermoelectric materials.

Colloidal Ternary Telluride Quantum Dots for Tunable Phase Change Optics in the Visible and Near-Infrared

A structural change between amorphous and crystalline phase provides a basis for reliable and modular photonic and electronic devices, such as nonvolatile memory, beam steerers, solid-state reflective displays, or mid-IR antennas. In this paper, we leverage the benefits of liquid-based synthesis to access phase-change memory tellurides in the form of colloidally stable quantum dots. We report a library of ternary M_xGe_1-xTe colloids (where M is Sn, Bi, Pb, In, Co, Ag) and then showcase the phase, composition, and size tunability for Sn-Ge-Te quantum dots. Full chemical control of Sn-Ge-Te quantum dots permits a systematic study of structural and optical properties of this phase-change nanomaterial. Specifically, we report composition-dependent crystallization temperature for Sn-Ge-Te quantum dots, which is notably higher compared to bulk thin films. This gives the synergistic benefit of tailoring dopant and material dimension to combine the superior aging properties and ultrafast crystallization kinetics of bulk Sn-Ge-Te, while improving memory data retention due to nanoscale size effects. Furthermore, we discover a large reflectivity contrast between amorphous and crystalline Sn-Ge-Te thin films, exceeding 0.7 in the near-IR spectrum region. We utilize these excellent phase-change optical properties of Sn-Ge-Te quantum dots along with liquid-based processability for nonvolatile multicolor images and electro-optical phase-change devices. Our colloidal approach for phase-change applications offers higher customizability of materials, simpler fabrication, and further miniaturization to the sub-10 nm phase-change devices.

All Cubic-Phase δ-TAGS Thermoelectrics Over the Entire Mid-Temperature Range

GeTe-based pseudo-binary (GeTe)_x (AgSbTe₂ )_{100- x} (TAGS-x) is recognized as a promising p-type mid-temperature thermoelectric material with outstanding thermoelectric performance; nevertheless, its intrinsic structural transition and metastable microstructure (due to Ag/Sb/Ge localization) restrict the long-time application of TAGS-x in practical thermoelectric devices. In this work, a series of non-stoichiometric (GeTe)_x (Ag_{1- δ} Sb_{1+ δ} Te_{2+ δ} )_{100- x} (x = 85∼50; δ = ≈0.20-0.23), referred to as δ-TAGS-x, with all cubic phase over the entire testing temperature range (300-773 K), is synthesized. Through optimization of crystal symmetry and microstructure, a state-of-the-art ZT_max of 1.86 at 673 K and average ZT_avg of 1.43 at ≈323-773 K are realized in δ-TAGS-75 (δ = 0.21), which is the highest value among all reported cubic-phase GeTe-based thermoelectric systems so far. As compared with stoichiometric TAGS-x, the remarkable thermoelectric achieved in cubic δ-TAGS-x can be attributed to the alleviation of highly (electrical and thermal) resistive grain boundary Ag₈ GeTe₆ phase. Moreover, δ-TAGS-x exhibits much better mechanical properties than stoichiometric TAGS-x, together with the outstanding thermoelectric performance, leading to a robust single-leg thermoelectric module with η_max of ≈10.2% and P_max of ≈0.191 W. The finding in this work indicates the great application potential of non-stoichiometric δ-TAGS-x in the field of mid-temperature waste heat harvesting.

Theoretical Study of Intrinsic and Extrinsic Point Defects and Their Effects on Thermoelectric Properties of Cu2SnSe3

Current understanding of the intrinsic point defects and potential extrinsic dopants in p-type Cu₂SnSe₃ is limited, which hinders further improvement of its thermoelectric performance. Here, we show that the dominant intrinsic defects in Cu₂SnSe₃ are Cu_Sn and V_Cu under different chemical conditions, respectively. The presence of V_Cu will damage the hole conduction network and reduce hole mobility. Besides, we find that the substitution of Al, Ga, In, Cd, Zn, Fe, and Mn for Sn can inhibit the formation of V_Cu; introducing Cu_Sn, Fe_Sn, Mn_Sn, and Ni_Cu defects can significantly enhance electronic density of states near the Fermi level due to the contribution of 3d orbitals. Therefore, increasing the Cu content and/or introducing the above beneficial dopants appropriately are expected to cause enhancement of carrier mobility and/or thermopower of Cu₂SnSe₃. Furthermore, introducing Ag_Cu, Al_Sn, Zn_Sn, Ge_Sn, and Mn_Sn defects can induce large mass and strain field fluctuations, lowering lattice thermal conductivity remarkably. Present results not only deepen one's insights into point defects in Cu₂SnSe₃ but also provide us with a guide to improve its thermoelectric properties.

High-Power Factor Enabled by Efficient Manipulation Interaxial Angle for Enhancing Thermoelectrics of GeTe-Cu2Te Alloys

The emerged strategy of manipulating the rhombohedral crystal structure provides another new degree of freedom for optimizing the thermoelectric properties of GeTe-based compounds. However, the concept is difficult to be effectively measured and often depends on heavy doping that scatters carriers severely. Herein, we synergistically manipulate lattice distortion and vacancy concentration to promote the excellent electrical transport of GeTe-Cu2Te alloys and quantify the interaxial angle-dependent density of state effective mass. Distinct from the conventional electronic coupling effect, about 2% substitution of Zr4+ significantly increases the interaxial angle, thereby enhancing the band convergence effect and improving the Seebeck coefficient. In addition, Ge-compensation attenuates the mobility deterioration, leading to improved power factor over the whole temperature range, especially exceeding ∼22 μW cm-1 K-2 at 300 K. Furthermore, the Debye-Callaway model elucidates low lattice thermal conductivity due to strong phonon scattering from Zr/Ge substitutional defects. As a result, the highest figure of merit zT of ∼1.6 (at 650 K) and average zTave of ∼0.9 (300-750 K) are obtained in (Ge1.01Zr0.02Te)0.985(Cu2Te)0.015. This work demonstrates the effective band modulation of Zr on GeTe-based materials, indicating that the modification of the interaxial angle is a deep pathway to improve thermoelectrics.

Screening Metal Electrodes for Thermoelectric PbTe

Historically, both p- and n-type PbTe show extraordinary thermoelectric figures of merit within 300-600 °C for power generation applications. A full realization of the potential of these high-performance thermoelectric materials on a device level largely depends on the electrical and thermal contacts with the metal electrodes. Chemical inertness with a slow diffusion could be an important criterion for the selection of metal electrodes. In this work, the diffusion of the total 12 potential metal electrodes in PbTe diffusion couples are focused on and sorted, suggesting the superiority of Co as an electrode for its low diffusion coefficient and interfacial contact resistivity, inertial to PbTe and compatibility in temperature for sintering. The strategy used in this work is believed to be applicable to the selection of electrodes for other thermoelectric materials.

Enhanced Density of States Facilitates High Thermoelectric Performance in Solution-Grown Ge- and In-Codoped SnSe Nanoplates

SnSe single crystals have gained great interest due to their excellent thermoelectric performance. However, polycrystalline SnSe is greatly desired due to facile processing, machinability, and scale-up application. Here, we report an outstanding high average ZT of 0.88 as well as a high peak ZT of 1.92 in solution-processed SnSe nanoplates. Nanosized boundaries formed by nanoplates and lattice strain created by lattice dislocations and stacking faults effectively scatter heat-carrying phonons, resulting in an ultralow lattice thermal conductivity of 0.19 W m^-1 K^-1 at 873 K. Ultraviolet photoelectron spectroscopy reveals that Ge and In incorporation produces an enhanced density of states in the electronic structure of SnSe, resulting in a large Seebeck coefficient. Ge and In codoping not only optimizes the Seebeck coefficient but also substantially increases the carrier concentration and electrical conductivity, helping to maintain a high power factor over a wide temperature range. Benefiting from an enhanced power factor and markedly reduced lattice thermal conductivity, high average ZT and peak ZT are achieved in Ge- and In-codoped SnSe nanoplates. This work achieves an ultrahigh average ZT of 0.88 in polycrystalline SnSe by adopting nontoxic element doping, potentially expanding its usefulness for various thermoelectric generator applications.

Advances in Versatile GeTe Thermoelectrics from Materials to Devices

Driven by the intensive efforts in the development of high-performance GeTe thermoelectrics for mass-market application in power generation and refrigeration, GeTe-based materials display a high figure of merit of >2.0 and an energy conversion efficiency beyond 10%. However, a comprehensive review on GeTe, from fundamentals to devices, is still needed. In this regard, the latest progress on the state-of-the-art GeTe is timely reviewed. The phase transition, intrinsic high carrier concentration, and multiple band edges of GeTe are fundamentally analyzed from the perspectives of the native atomic orbital, chemical bonding, and lattice defects. Then, the fabrication methods are summarized with a focus on large-scale production. Afterward, the strategies for enhancing electronic transports of GeTe by energy filtering effect, resonance doping, band convergence, and Rashba band splitting, and the methods for strengthening phonon scatterings via nanoprecipitates, planar vacancies, and superlattices, are comprehensively reviewed. Besides, the device assembly and performance are highlighted. In the end, future research directions are concluded and proposed, which enlighten the development of broader thermoelectric materials.

Evolution of defect structures leading to high ZT in GeTe-based thermoelectric materials

GeTe is a promising mid-temperature thermoelectric compound but inevitably contains excessive Ge vacancies hindering its performance maximization. This work reveals that significant enhancement in the dimensionless figure of merit (ZT) could be realized by defect structure engineering from point defects to line and plane defects of Ge vacancies. The evolved defects including dislocations and nanodomains enhance phonon scattering to reduce lattice thermal conductivity in GeTe. The accumulation of cationic vacancies toward the formation of dislocations and planar defects weakens the scattering against electronic carriers, securing the carrier mobility and power factor. This synergistic effect on electronic and thermal transport properties remarkably increases the quality factor. As a result, a maximum ZT > 2.3 at 648 K and a record-high average ZT (300-798 K) were obtained for Bi_0.07Ge_0.90Te in lead-free GeTe-based compounds. This work demonstrates an important strategy for maximizing the thermoelectric performance of GeTe-based materials by engineering the defect structures, which could also be applied to other thermoelectric materials.

The intrinsic high-concentration Ge vacancies in GeTe-based thermoelectric materials hinder their performance maximization. Here, the authors find that defect structure engineering strategy is effective for performance enhancement.

Chemistry in Advancing Thermoelectric GeTe Materials

ConspectusThe ever-growing energy crisis and the deteriorated environment caused by carbon energy consumption motivate the exploitation of alternative green and sustainable energy supplies. Because of the unique advantages of zero-emission, no moving parts, accurate temperature control, a long steady-state operation period, and the ability to operate in extreme situations, thermoelectrics, enabling the direct conversion between heat and electricity, is a promising and sustainable option for power generation and refrigeration. However, with increasing application potentials, thermoelectrics is now facing a major challenge: developing high-performance, Pb-free, and low-toxic thermoelectric materials and devices.As one group of promising candidates, GeTe derivatives have the potential to replace the widely used thermoelectric materials containing highly toxic elements. In this Account, we summarize our recent progress in developing high-performance GeTe-based thermoelectric materials via exploring innovative strategies to enhance electron transports and dampen phonon propagations. First, we fundamentally illustrate the underlying chemistry and physical reason for an intrinsically high carrier concentration in GeTe, which enormously restrains the thermoelectric performance of GeTe. From our theoretical calculations, the formation energy of Ge vacancy is the lowest among the defects in GeTe, energetically favoring Ge vacancies in the lattice and leading to intrinsically high carrier concentrations. Accordingly, aliovalent doping/alloying is proposed to increase the formation energy of Ge vacancies and decrease the carrier concentration to the optimal level. We then outline the newly developed method to refine the band structures of GeTe with tuned electronic transport. On the basis of the molecular orbital theory, the energy offset between two valence band edges at the L and Σ points in GeTe should be ascribed to the slightly different Ge_4s orbital characters at these two points, which guides the screening of dopants for band convergence. Besides, the Rashba spin splitting is explored to increase the band degeneracy of GeTe. Afterward, we analyze the dampened phonon propagation in GeTe to minimize its lattice thermal conductivity. Alloying with the heavy Sb atoms can shift the optical phonon modes toward low frequency and reinforce the interaction of optical and acoustic phonon modes so that the inherent phonon scattering is enhanced. In addition, planar vacancies and superlattice precipitates can significantly strengthen phonon scattering to result in ultralow lattice thermal conductivity. After that, we overview the finite elemental analysis simulations to optimize the device geometry for maximizing the device performance and introduce the as-developed prototype GeTe-based thermoelectric device. In the end, we point out future directions in the development of GeTe for device applications. The strategies summarized in this Account can serve as references for developing wide materials with enhanced thermoelectric performance.

Highly Thermoelectric ZnO@MXene (Ti3C2Tx) Composite Films Grown by Atomic Layer Deposition

Due to its unique high conductivity and flexibility, the two-dimensional MXene material (Ti₃C₂T_x) is expected to possess great potential in the thermoelectric field. However, the low thermoelectric performance from high thermal conductivity and a low Seebeck coefficient has limited its practical application. In this report, we demonstrate the uniform growth of ZnO layers on the laminar Ti₃C₂T_x membrane by atomic layer deposition (ALD). Benefiting from the low-temperature deposition characteristics of the ALD technique, the ZnO@Ti₃C₂T_x composite films maintain the basic apparent morphology of the original films after the deposition. We reveal that the Schottky barrier formed between ZnO and Ti₃C₂T_x exhibits an energy-filtering effect, significantly enhancing the Seebeck coefficient to result in more than a double increase in the power factor. Meanwhile, the strong phonon-interface scattering between ZnO and Ti₃C₂T_x is found to reduce the thermal conductivity of the composite films by a factor of four as compared to pure Ti₃C₂T_x ones, further improving the overall thermoelectric properties of the ZnO@Ti₃C₂T_x composite films. Our investigation provides an ALD-based strategy for growing wide band gap layers on the narrow band gap films to improve the thermoelectric performance of various MXene materials.

Synergistically Optimized Thermal Conductivity and Carrier Concentration in GeTe by Bi-Se Codoping

The GeTe compound has been revealed to be an outstanding thermoelectric compound, while its inherent high thermal conductivity restricts further improvement in its performance. Herein, we report a study on the synergistic optimization of the thermoelectric performance of GeTe by Bi-Se codoping. It is shown that the introduction of Bi decreases the carrier concentration and increases the structural parameter of the interaxial angle. With Se doping in the Te site, the lattice thermal conductivity is markedly reduced, while the carrier mobility is slightly influenced. Compared with the singly Se-doped GeTe, the Ge_1-xBi_xTe_1-ySe_y samples are more closed to a cubic phase, as indicated by the larger interaxial angle. On account of the reduction of carrier concentration and thermal conductivity, a ZT_max of 1.80 at 665 K and a high ZT_ave of 1.39 (400-800 K) are obtained in Ge_0.94Bi_0.06Te_0.85Se_0.15. This work reveals that the interaxial angle is vital to the performance optimization of rhombohedral GeTe.

High Thermoelectric Performance Achieved in Sb-Doped GeTe by Manipulating Carrier Concentration and Nanoscale Twin Grains

Lead-free and eco-friendly GeTe shows promising mid-temperature thermoelectric applications. However, a low Seebeck coefficient due to its intrinsically high hole concentration induced by Ge vacancies, and a relatively high thermal conductivity result in inferior thermoelectric performance in pristine GeTe. Extrinsic dopants such as Sb, Bi, and Y could play a crucial role in regulating the hole concentration of GeTe because of their different valence states as cations and high solubility in GeTe. Here we investigate the thermoelectric performance of GeTe upon Sb doping, and demonstrate a high maximum zT value up to 1.88 in Ge_0.90Sb_0.10Te as a result of the significant suppression in thermal conductivity while maintaining a high power factor. The maintained high power factor is due to the markable enhancement in the Seebeck coefficient, which could be attributed to the significant suppression of hole concentration and the valence band convergence upon Sb doping, while the low thermal conductivity stems from the suppression of electronic thermal conductivity due to the increase in electrical resistivity and the lowering of lattice thermal conductivity through strengthening the phonon scattering by lattice distortion, dislocations, and twin boundaries. The excellent thermoelectric performance of Ge_0.90Sb_0.10Te shows good reproducibility and thermal stability. This work confirms that Ge_0.90Sb_0.10Te is a superior thermoelectric material for practical application.

Achieving Ultralow Lattice Thermal Conductivity and High Thermoelectric Performance in GeTe Alloys via Introducing Cu2Te Nanocrystals and Resonant Level Doping

The binary compound of GeTe emerging as a potential medium-temperature thermoelectric material has drawn a great deal of attention. Here, we achieve ultralow lattice thermal conductivity and high thermoelectric performance in In and a heavy content of Cu codoped GeTe thermoelectrics. In dopants improve the density of state near the surface of Femi of GeTe by introducing resonant levels, producing a sharp increase of the Seebeck coefficient. In and Cu codoping not only optimizes carrier concentration but also substantially increases carrier mobility to a high value of 87 cm² V^-1 s^-1 due to the diminution of Ge vacancies. The enhanced Seebeck coefficient coupled with dramatically enhanced carrier mobility results in significant enhancement of PF in Ge_1.04-x-yIn_xCu_yTe series. Moreover, we introduce Cu₂Te nanocrystals' secondary phase into GeTe by alloying a heavy content of Cu. Cu₂Te nanocrystals and a high density of dislocations cause strong phonon scattering, significantly diminishing lattice thermal conductivity. The lattice thermal conductivity reduced as low as 0.31 W m^-1 K^-1 at 823 K, which is not only lower than the amorphous limit of GeTe but also competitive with those of thermoelectric materials with strong lattice anharmonicity or complex crystal structures. Consequently, a high ZT of 2.0 was achieved for Ge_0.9In_0.015Cu_0.125Te by decoupling electron and phonon transport of GeTe. This work highlights the importance of phonon engineering in advancing high-performance GeTe thermoelectrics.

An Overview of the Strategies for Tin Selenide Advancement in Thermoelectric Application

Chalcogenide, tin selenide-based thermoelectric (TE) materials are Earth-abundant, non-toxic, and are proven to be highly stable intrinsically with ultralow thermal conductivity. This work presented an updated review regarding the extraordinary performance of tin selenide in TE applications, focusing on the crystal structures and their commonly used fabrication methods. Besides, various optimization strategies were recorded to improve the performance of tin selenide as a mid-temperature TE material. The analyses and reviews over the methodologies showed a noticeable improvement in the electrical conductivity and Seebeck coefficient, with a noticeable decrement in the thermal conductivity, thereby enhancing the tin selenide figure of merit value. The applications of SnSe in the TE fields such as microgenerators, and flexible and wearable devices are also discussed. In the future, research in low-dimensional TE materials focusing on nanostructures and nanocomposites can be conducted with the advancements in material science technology as well as microtechnology and nanotechnology.

Optimizing Electronic Quality Factor toward High-Performance Ge1- x - y Tax Sby Te Thermoelectrics: The Role of Transition Metal Doping

Owing to high intrinsic figure-of-merit implemented by multi-band valleytronics, GeTe-based thermoelectric materials are promising for medium-temperature applications. Transition metals are widely used as dopants for developing high-performance GeTe thermoelectric materials. Herein, relevant work is critically reviewed to establish a correlation among transition metal doping, electronic quality factor, and figure-of-merit of GeTe. From first-principle calculations, it is found that Ta, as an undiscovered dopant in GeTe, can effectively converge energy offset between light and heavy conduction band extrema to enhance effective mass at high temperature. Such manipulation is verified by the increased Seebeck coefficient of synthesized Ge_{1- x - y} Ta_x Sb_y Te samples from 160 to 180 µV K^-1 at 775 K upon doping Ta, then to 220 µV K^-1 with further alloying Sb. Characterization using electron microscopy also reveals the unique herringbone structure associated with multi-scale lattice defects induced by Ta doping, which greatly hinder phonon propagation to decrease thermal conductivity. As a result, a figure-of-merit of ≈2.0 is attained in the Ge_0.88 Ta_0.02 Sb_0.10 Te sample, reflecting a maximum heat-to-electricity efficiency up to 17.7% under a temperature gradient of 400 K. The rationalized beneficial effects stemming from Ta doping is an important observation that will stimulate new exploration toward high-performance GeTe-based thermoelectric materials.

Regulating Te Vacancies through Dopant Balancing via Excess Ag Enables Rebounding Power Factor and High Thermoelectric Performance in p-Type PbTe

Thermoelectric properties are frequently manipulated by introducing point defects into a matrix. However, these properties often change in unfavorable directions owing to the spontaneous formation of vacancies at high temperatures. Although it is crucial to maintain high thermoelectric performance over a broad temperature range, the suppression of vacancies is challenging since their formation is thermodynamically preferred. In this study, using PbTe as a model system, it is demonstrated that a high thermoelectric dimensionless figure of merit, zT ≈ 2.1 at 723 K, can be achieved by suppressing the vacancy formation via dopant balancing. Hole-killer Te vacancies are suppressed by Ag doping because of the increased electron chemical potential. As a result, the re-dissolution of Na2 Te above 623 K can significantly increase the hole concentration and suppress the drop in the power factor. Furthermore, point defect scattering in material systems significantly reduces lattice thermal conductivity. The synergy between defect and carrier engineering offers a pathway for achieving a high thermoelectric performance by alleviating the power factor drop and can be utilized to enhance thermoelectric properties of thermoelectric materials.

Stabilizing the Optimal Carrier Concentration in Al/Sb-Codoped GeTe for High Thermoelectric Performance

GeTe is a promising thermoelectric material and has attracted growing research interest recently. In this study, the effect of Al doping and Al&Sb codoping on the thermoelectric properties of GeTe was investigated. Due to the presence of a high concentration of intrinsic Ge vacancies, pristine GeTe exhibited a very high hole concentration and unpromising thermoelectric performance. By Sb doping in GeTe, the hole concentration can be effectively reduced, thus improving the thermoelectric performance. Aluminum, as a p-type dopant in GeTe, will increase the hole concentration and lattice thermal conductivity; thus, it has long been considered as an unfavorable dopant for the optimization of GeTe-based materials. However, when Al and Sb were codoped into GeTe, the hole concentration was effectively suppressed, and the lattice thermal conductivity can be reduced. Eventually, a maximum zT of ∼2.0 at 773 K was achieved in Al&Sb-codoped Al_0.01Sb_0.1Ge_0.89Te.

Enhancing Near-Room-Temperature GeTe Thermoelectrics through In/Pb Co-doping

The traditional thermoelectric material GeTe has drawn much attention recently because of the reported high thermoelectric performance of the rhombohedral phase in low-temperature ranges, where the split L and Σ band can be reconverged to have a small energy offset and thus high density of state effective mass according to the rhombohedral angle. In addition, In doping in GeTe is also reported to enhance the density of effective mass and therefore increase the Seebeck coefficient because of the induced resonant levels. In this work, In and Pb are doped in GeTe, and In doping leads to an increase in the rhombohedral angle and thus enhanced density of state effective mass in addition to the resonant effect. However, the improved Seebeck coefficient result from In doping is compensated for by a sharp reduction of Hall mobility, leading to no significant enhancement of electronic performance in the rhombohedral phase. By further Pb/Ge doping in the matrix Ge_0.95In_0.05Te for the optimization of carrier concentration and reduction of lattice thermal conductivity (as low as 0.7 W/mK), a zT as high as ∼1.2 at 550 K and average zT of ∼0.75 between 300 and 550 K are realized in this work, demonstrating GeTe as a promising candidate for near-room-temperature application.

Enhanced Thermoelectric Performance in Ge0.955- x Sbx Te/FeGe2 Composites Enabled by Hierarchical Defects

Manipulations of carrier and phonon scatterings through hierarchical structures have been proved to be effective in improving thermoelectric performance. Previous efforts in GeTe-based materials mainly focus on simultaneously optimizing the carrier concentration and band structure. In this work, a synergistic strategy to tailor thermal and electrical transport properties of GeTe by combination with the scattering effects from both Ge vacancies and other defects is reported. The addition of Fe in GeTe-based compounds introduces the secondary phase of FeGe₂ , synchronously increasing the concentration of Ge vacancies and arousing more Ge planar defects. These hierarchical defects contribute to a large scattering factor, leading to a significant enhancement of Seebeck coefficient and further a splendid power factor. Meanwhile, benefiting from the reinforced phonon scatterings by multiscale hierarchical structures, an extremely low lattice thermal conductivity is successfully achieved. With simultaneously optimized electrical and thermal transport properties, a maximum figure of merit, zT, value of 2.1 at 750 K and an average zT value of 1.5 in 400-800 K are realized in Ge_0.875 Sb_0.08 Te/1.5%FeGe₂ . This work demonstrates that manipulation of hierarchical defects is an effective strategy to optimize the thermoelectric properties.

Anomalous Thermopower and High ZT in GeMnTe2 Driven by Spin's Thermodynamic Entropy

Na _x CoO₂ was known 20 years ago as a unique example in which spin entropy dominates the thermoelectric behavior. Hitherto, however, little has been learned about how to manipulate the spin degree of freedom in thermoelectrics. Here, we report the enhanced thermoelectric performance of GeMnTe₂ by controlling the spin's thermodynamic entropy. The anomalously large thermopower of GeMnTe₂ is demonstrated to originate from the disordering of spin orientation under finite temperature. Based on the careful analysis of Heisenberg model, it is indicated that the spin-system entropy can be tuned by modifying the hybridization between Te-p and Mn-d orbitals. As a consequent strategy, Se doping enlarges the thermopower effectively, while neither carrier concentration nor band gap is affected. The measurement of magnetic susceptibility provides a solid evidence for the inherent relationship between the spin's thermodynamic entropy and thermopower. By further introducing Bi doing, the maximum ZT in Ge_0.94Bi_0.06MnTe_1.94Se_0.06 reaches 1.4 at 840 K, which is 45% higher than the previous report of Bi-doped GeMnTe₂. This work reveals the high thermoelectric performance of GeMnTe₂ and also provides an insightful understanding of the spin degree of freedom in thermoelectrics.

Ultralow Lattice Thermal Conductivity and Superhigh Thermoelectric Figure-of-Merit in (Mg, Bi) Co-Doped GeTe

High-efficiency thermoelectric (TE) technology is determined by the performance of TE materials. Doping is a routine approach in TEs to achieve optimized electrical properties and lowered thermal conductivity. However, how to choose appropriate dopants with desirable solution content to realize high TE figure-of-merit (zT) is very tough work. In this study, via the use of large mass and strain field fluctuations as indicators for low lattice thermal conductivity, the combination of (Mg, Bi) in GeTe is screened as very effective dopants for potentially high zTs. In experiments, a series of (Mg, Bi) co-doped GeTe compounds are prepared and the electrical and thermal transport properties are systematically investigated. Ultralow lattice thermal conductivity, about 0.3 W m^-1 K^-1 at 600 K, is obtained in Ge_0.9 Mg_0.04 Bi_0.06 Te due to the introduced large mass and strain field fluctuations by (Mg, Bi) co-doping. In addition, (Mg, Bi) co-doping can introduce extra electrons for optimal carrier concentration and diminish the energy offset at the top of the valence band for high density-of-states effective mass. Via these synthetic effects, a superhigh zT of ≈2.5 at 700 K is achieved for Ge_0.9 Mg_0.04 Bi_0.06 Te. This study sheds light on the rational design of effective dopants in other TE materials.

High thermoelectric performance by chemical potential tuning and lattice anharmonicity in GeTe1−xIx compounds

Electronic ZT value with chemical potential for rhombohedral α- (black line) and cubic β-phase (red line) (a) and the temperature-dependent ZT value of GeTe_1−xI_x compounds with reference data (b).

Generation of multi-dimensional defect structures for synergetic engineering of hole and phonon transport: enhanced thermoelectric performance in Sb and Cu co-doped GeTe

The thermoelectric performance of GeTe can be enhanced by Sb/Cu codoping due to the generation of complex defect structures.

Compositional Fluctuations Locked by Athermal Transformation Yielding High Thermoelectric Performance in GeTe

Phase transition in thermoelectric (TE) material is a double-edged sword-it is undesired for device operation in applications, but the fluctuations near an electronic instability are favorable. Here, Sb doping is used to elicit a spontaneous composition fluctuation showing uphill diffusion in GeTe that is otherwise suspended by diffusionless athermal cubic-to-rhombohedral phase transition at around 700 K. The interplay between these two phase transitions yields exquisite composition fluctuations and a coexistence of cubic and rhombohedral phases in favor of exceptional figures-of-merit zT. Specifically, alloying GeTe by Sb₂ Te₃ significantly suppresses the thermal conductivity while retaining eligible carrier concentration over a wide composition range, resulting in high zT values of >2.6. These results not only attest to the efficacy of using phase transition in manipulating the microstructures of GeTe-based materials but also open up a new thermodynamic route to develop higher performance TE materials in general.

Thermoelectric properties of flexible PEDOT:PSS-based films tuned by SnSe via the vacuum filtration method

In this study, flexible thermoelectric tin selenide (SnSe)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composite films have been fabricated by the vacuum filtration method, and their thermoelectric properties were investigated. The electrical conductivities of the composite films have a tendency to decrease with an increase in the SnSe content, while their Seebeck coefficients have an inverse tendency. The electrical conductivities decrease gradually with an increase in temperature, while the Seebeck coefficients show a tendency to increase first and then decrease with the increase in temperature. The maximum power factor (PF = S 2 σ) of the composite film is obtained when the SnSe content is 10 wt%, which is 24.42 μW m-1 K-2 at 353 K. Besides, the 10 wt% SnSe/PEDOT:PSS film exhibited excellent stability with only a 9% increase in resistance after 1000 bends under a bending radius of 4 mm. When the temperature gradient is 50 K, a flexible thermoelectric generator fabricated by 3 legs of the 10 wt% SnSe/PEDOT:PSS film has an open-circuit voltage and maximum output electrical power of 3.2 mV and 13.73 nW, respectively, which demonstrates a great potential application to power wearable flexible electronic devices.

Ultralow Thermal Conductivity, Enhanced Mechanical Stability, and High Thermoelectric Performance in (GeTe)1-2x(SnSe)x(SnS)x

Thermoelectric (TE) energy conversion demands high performance crystalline inorganic solids that exhibit ultralow thermal conductivity, high mechanical stability, and good TE device properties. Pb-free germanium telluride (GeTe)-based material has recently attracted significant attention in TE power generation in mid temperatures, but pristine GeTe possesses significantly higher lattice thermal conductivity (κlatt) compared to that of its theoretical minimum (κmin) of ∼0.3 W/mK. Herein, we have demonstrated the reduction of κlatt of (GeTe)1-2x(SnSe)x(SnS)x very near to its κmin. The (GeTe)1-2x(SnSe)x(SnS)x system behaves as a coexistence of point-defect rich solid solution and phase separation. Initially, the addition of equimolar SnSe and SnS in the GeTe reduces the κlatt by effective phonon scattering because of the excess point defects and rich microstructures. In the second step, introduction of Sb-doping leads to additional phonon scattering centers and optimizes the p-type carrier concentration. Notably, 10 mol % Sb-doped (GeTe)0.95(SnSe)0.025(SnS)0.025 exhibits ultralow κlatt of ∼0.30 W/mK at 300 K. Subsequently, 10 mol % Sb-doped (GeTe)0.95(SnSe)0.025(SnS)0.025 exhibits a high TE figure of merit (zT) of ∼1.9 at 710 K. The high-performance sample exhibits a Vickers microhardness (mechanical stability) value of ∼194 HV that is significantly higher compared to the pristine GeTe and other state-of-the-art thermoelectric materials. Further, we have achieved a high output power, ∼150 mW for the temperature difference of 462 K, in single leg TE device based on 10 mol % Sb-doped (GeTe)0.95(SnSe)0.025(SnS)0.025.

Achieving Enhanced Thermoelectric Performance in (SnTe)1-x(Sb2Te3)x and (SnTe)1-y(Sb2Se3)y Synthesized via Solvothermal Reaction and Sintering

SnTe is proposed to be an intriguing low-toxicity alternative to PbTe. Herein, we report the diminished lattice thermal conductivity (κ_L) and enhanced zT of SnTe by way of vacancy engineering. (SnTe)_1-x(Sb₂Te₃)_x (x = 0.03, 0.06, and 0.10) and (SnTe)_1-y(Sb₂Se₃)_y (y = 0.03 and 0.06) were synthesized by blending and sintering their solution-synthesized nano/microstructures (i.e., SnTe octahedral particles, Sb₂Te₃ nanoplates, and Sb₂Se₃ nanorods). Benefiting from the chemical reactions during sintering, single-phase SnTe-based solid solutions were formed when x or y is not higher than 0.06, into which tunable concentrations of Sn vacancies were introduced. Such vacancies significantly enhance phonon scattering, leading to the sharply reduced room temperature κ_L of 1.40 and 1.26 W m^-1 K^-1 for x = 0.06 and y = 0.06 samples, respectively, as compared to 3.73 W m^-1 K^-1 for pristine SnTe. Enabled by point defects with the highest concentration and SnSb₂Te₄ secondary phase, (SnTe)_0.90(Sb₂Te₃)_0.10 sample obtains the lowest κ_L of 0.70 W m^-1 K^-1 at 813 K. Ultimately, maximum zT values of 0.6 and 0.7 at 813 K are achieved in (SnTe)_0.90(Sb₂Te₃)_0.10 and (SnTe)_0.94(Sb₂Se₃)_0.06, respectively. This study demonstrates the effectiveness of vacancy engineering in improving zT of SnTe-based materials.

Ternary Compounds Cu3RTe3 (R = Y, Sm, and Dy): A Family of New Thermoelectric Materials with Trigonal Structures

In this work, we report a series of Cu₃RTe₃ (R = Y, Sm, and Dy) ternary compounds with a trigonal structure (R3̅) as a family of new thermoelectric materials. First-principles calculations show that Cu₃RTe₃ (R = Y, Sm, and Dy) compounds are semiconductors with similar band structures and moderate band gaps (0.69-0.82 eV). The synthesized polycrystalline Cu₃RTe₃ (R = Y, Sm, and Dy) compounds possess moderate carrier concentrations (0.8-2.2 × 10²⁰ cm^-3) and density-of-state effective masses (around 1.1 m_e), yielding decent electrical transport performance. Furthermore, intrinsically low lattice thermal conductivities, below 1 W m^-1 K^-1 at 300-900 K, originating from the heavy average atomic masses and large number of atoms in the unit cell, are observed for Cu₃RTe₃ (R = Y, Sm, and Dy). Finally, Cu₃DyTe₃ demonstrates a peak dimensionless figure of merit of 0.9 at 900 K, which is among the highest reported for the Cu/Ag-based tellurides.

Advanced Thermoelectric Design: From Materials and Structures to Devices

The long-standing popularity of thermoelectric materials has contributed to the creation of various thermoelectric devices and stimulated the development of strategies to improve their thermoelectric performance. In this review, we aim to comprehensively summarize the state-of-the-art strategies for the realization of high-performance thermoelectric materials and devices by establishing the links between synthesis, structural characteristics, properties, underlying chemistry and physics, including structural design (point defects, dislocations, interfaces, inclusions, and pores), multidimensional design (quantum dots/wires, nanoparticles, nanowires, nano- or microbelts, few-layered nanosheets, nano- or microplates, thin films, single crystals, and polycrystalline bulks), and advanced device design (thermoelectric modules, miniature generators and coolers, and flexible thermoelectric generators). The outline of each strategy starts with a concise presentation of their fundamentals and carefully selected examples. In the end, we point out the controversies, challenges, and outlooks toward the future development of thermoelectric materials and devices. Overall, this review will serve to help materials scientists, chemists, and physicists, particularly students and young researchers, in selecting suitable strategies for the improvement of thermoelectrics and potentially other relevant energy conversion technologies.

Improved Figure of Merit of Cu2SnSe3 via Band Structure Modification and Energy-Dependent Carrier Scattering

As an ecofriendly thermoelectric material with intrinsic low thermal conductivity, ternary diamond-like Cu₂SnSe₃ (CSS) has attracted much attention. Nevertheless, its figure of merit, ZT, is limited by its small thermopower (S) and power factor (PF). Here, we show that an increase in thermopower by 63% and a carrier-mobility rise of 81% at 300 K can be simultaneously achieved through 5% substitution of Fe for Sn due to both enhancement of electronic density of states and degeneracy of multiple valence band maxima, which lead to high PF = 10.3 μW·cm^-1·K^-2 at 823 K for Fe-doped CSS (CSFS). Besides, an ultrahigh PF of 14.8 μW·cm^-1·K^-2 (at 773 K) and 45% reduction of lattice thermal conductivity (at 823 K) are realized for CSFS-based composites with 0.125 wt % of MgO nanoinclusions, owing to further enhancement of S via energy-dependent scattering and strong phonon scattering by the embedded nanoparticles. Consequently, a maximum ZT = 1 at 823 K is reached for the CSFS/f MgO composite samples with f = 0.125 wt %, which is around 2.5 times larger than that of the CSS compound.

Vacancy-Based Defect Regulation for High Thermoelectric Performance in Ge9Sb2Te12-x Compounds

Defect engineering is the core strategy for improving thermoelectric properties. Herein, cation doping along with modulation of cation vacancy has been developed in GeTe-based materials as an effective method to induce vacancy-based defects to boost their thermoelectric performance. A series of ternary compounds of Ge₉Sb₂Te_12-x (x = 0, 0.03, 0.06, 0.09, 0.12, 0.15) was prepared by vacuum-melting and annealing combined with the spark plasma sintering (SPS) process. The role of Sb doping and cation vacancy on thermoelectric properties was systematically investigated. It is found that alloying Sb₂Te₃ into GeTe increases the concentration of cation vacancies, which is corroborated by both positron annihilation measurements and theoretical calculations. The vacancies, stacking faults, and planar defect interactions determine the thermoelectric transport properties. Adjusting the deficiency of Te effectively tunes the concentration of cation vacancies and dopant defects in the structure. In turn, this tunes the carrier concentration close to its optimum. A high power factor of 32.6 μW cm^-1 K^-2 is realized for Ge₉Sb₂Te_11.91 at 725 K. Moreover, large strains induced by the defect structures, including Sb dopant, vacancy, staking faults, as well as planar defects intensify phonon scattering, leading to a significant decrease in the thermal conductivity from 7.6 W m^-1 K^-1 for pristine GeTe to 1.18 W m^-1 K^-1 for Ge₉Sb₂Te_11.85 at room temperature. All of the above contribute to a high ZT value of 2.1 achieved for the Ge₉Sb₂Te_11.91 sample at 775 K.

Al-Si Alloy as a Diffusion Barrier for GeTe-Based Thermoelectric Legs with High Interfacial Reliability and Mechanical Strength

To build high-performance thermoelectric (TE) devices for power generation, a suitable diffusion-barrier layer between the electrodes and the TE materials in a TE device is generally required for achieving good interfacial connection with high reliability, high mechanical strength but low electrical and thermal contact resistivities. GeTe-based materials have attracted great attention recently due to their high TE performance in the mid-temperature range, but studies on their TE devices are still limited. Here, we selected the Al₆₆Si₃₄ alloy as a diffusion barrier for GeTe-based TE legs based on the matching test of the coefficient of thermal expansion. The good connection between Al₆₆Si₃₄ and Ge_0.9Sb_0.1TeB_0.01 is realized by the interfacial reaction, where the randomly distributed Al₂Te₃ and Ge precipitates are formed at the interface of the joint. The as-prepared interfacial electrical contact resistivity can be as low as 20.7 μΩ·cm² and only slightly increases to 26.1 μΩ·cm² after 16 days of aging at 500 °C. Moreover, the shear strength of the joints can be as high as 26.6 MPa and unexpectedly increases to 41.7 MPa after 16 days of aging. The thickness of the reaction layer tends to be stabilized after 8 days of aging and nearly does not change after further aging to 16 days, which may be ascribed to the drag effect from Si and the secondary Ge phases. These results demonstrate the great potential of the Al-Si alloy as a diffusion barrier for GeTe-based TE devices with high performance.

High-Performance Thermoelectric SnSe: Aqueous Synthesis, Innovations, and Challenges

Tin selenide (SnSe) is one of the most promising candidates to realize environmentally friendly, cost-effective, and high-performance thermoelectrics, derived from its outstanding electrical transport properties by appropriate bandgaps and intrinsic low lattice thermal conductivity from its anharmonic layered structure. Advanced aqueous synthesis possesses various unique advantages including convenient morphology control, exceptional high doping solubility, and distinctive vacancy engineering. Considering that there is an urgent demand for a comprehensive survey on the aqueous synthesis technique applied to thermoelectric SnSe, herein, a thorough overview of aqueous synthesis, characterization, and thermoelectric performance in SnSe is provided. New insights into the aqueous synthesis-based strategies for improving the performance are provided, including vacancy synergy, crystallization design, solubility breakthrough, and local lattice imperfection engineering, and an attempt to build the inherent links between the aqueous synthesis-induced structural characteristics and the excellent thermoelectric performance is presented. Furthermore, the significant advantages and potentials of an aqueous synthesis route for fabricating SnSe-based 2D thermoelectric generators, including nanorods, nanobelts, and nanosheets, are also discussed. Finally, the controversy, strategy, and outlook toward future enhancement of SnSe-based thermoelectric materials are also provided. This Review guides the design of thermoelectric SnSe with high performance and provides new perspectives as a reference for other thermoelectric systems.