Four-Phonon Enhanced the Thermoelectric Properties of ScSX (X = Cl, Br, and I) Monolayers

Recently, the FeOCl-type two-dimensional materials have attracted significant attention owing to their versatile applications in fields such as thermoelectricity and photocatalysis. This study aims to systematically investigate the thermoelectric properties of ScSX (X = Cl, Br, and I) monolayers by a combination of the first-principles calculations and the machine-learning interatomic potential approach. These monolayers are indirect semiconductors with band gaps of 3.22 (ScSCl), 3.27 (ScSBr), and 2.87 eV (ScSI), respectively. The lattice thermal conductivity is decreased by 25.72% (20.90%), 44.05% (40.00%), and 30.96% (34.76%) for ScSCl, ScSBr, and ScSI along the x-axis (y-axis) when the four-phonon scattering is introduced, indicating its important role in phonon transport. Anharmonic phonon scattering yields high Grüneisen parameter and scattering rate values, hence causing these low lattice thermal conductivities. Additionally, the large Seebeck coefficients and electrical conductivities of n-type doped ScSX monolayers contribute to their excellent power factors (24.69, 25.66, and 24.99 mW/K2·m for ScSCl, ScSBr and ScSI at 300 K, respectively). Based on the excellent power factor and low thermal conductivity, the maximum values of the figure of merit are calculated to be 2.68, 3.39, and 3.21 for ScSCl, ScSBr, and ScSI monolayers at 700 K, respectively. Our research provides valuable insights into the phonon thermal transport of ScSX monolayers and suggests a promising approach to address high-order anharmonicity.

High Thermoelectric Performance in Bismuth Telluride via Constructing MoSe2-2D Heterojunction

Currently, the only thermoelectric (TE) materials commercially available at room temperature are those based on bismuth telluride. However, their widespread application is limited due to their inferior thermoelectric and mechanical properties. In this study, a strategy of growing a rigid second phase of MoSe2 is employed, in situ within the matrix phase to achieve n-type bismuth telluride-based materials with exceptional mechanical and thermoelectric properties. The in situ grown second phase contributes to both the electronic and lattice thermal conductivities. This is primarily attributed to the strong energy filtering effect, as the second phase forms a semi-common lattice interfacial structure with the matrix phase during growth. Furthermore, for composites containing 2 wt% MoSe2, a maximum zT value of 1.24 at 373 K can be achieved. On this basis, 8-pair TE module is fabricated and 1-pair TE module is optimized using a homemade p-type material. The optimized 1-pair TE module generates a maximum output power of 13.6 µW, which is twice that of the 8-pair TE module and four times more than the 8-pair TE module fabricated by commercial material. This work facilitates the development of the TE module by presenting a novel approach to obtaining bismuth telluride-based thermoelectric materials with superior thermoelectric and mechanical properties.

Improvement of Mechanical and Thermoelectric Properties of AgCuTe by Pinning Effect and Enhanced Liquid-Like Behavior via SiC Alloying

With the development and application of thermoelectric (TE) devices, it requires not only high-performance of TE materials but also high mechanical properties. Here, we report a medium-temperature liquid material, AgCuTe, with high mechanical properties. The results demonstrate that AgCuTe possesses a multiphase structure characterized by abundant grain boundaries, resulting in reduced lattice thermal conductivity and inherently high mechanical strength. Furthermore, nano-SiC was alloyed into the AgCuTe material to further improve its mechanical and TE properties. Nano-SiC exhibited a button-like distribution within the grain boundaries, introducing a pinning effect that significantly elevated the Vickers hardness of the samples. Additionally, nano-SiC induced strong lattice distortion energy in the vicinity, which promotes Ag/Cu ions to escape from the lattice and enhances the liquid-like behavior of Ag/Cu ions. Finally, these enhancements led to a 21% improvement in the mechanical properties and a 40% improvement in the TE properties for AgCuTe. Notably, AgCuTe achieved its peak TE performance, with a latest peak ZT value of 1.32 at 723 K. This research expands the potential applications of AgCuTe.

High Thermoelectric Performance of n-type BiTeSe-Based Composites Incorporated with Both Inorganic and Organic Nanoinclusions

N-type Bi2Te2.7Se0.3 (BTS) alloy has relatively low thermoelectric performance as compared to its p-type counterpart, which restricts its widespread applications. Herein, we designed and prepared a novel composite system, which consists of an n-type BTS matrix incorporated with both inorganic and organic nanoinclusions. The results indicate that the thermopower of the composite samples can be enhanced by more than 19% upon incorporating inorganic nanophase AgBi3S5 (ABS) due to the energy-dependent carrier scattering, which ensures a high power factor. On the other hand, further incorporation of organic nanophase polypyrrole (PPy) can drastically reduce its lattice thermal conductivity owing to the strong scattering of mid- and low-frequency phonons at these nanoinclusions. As a result, high figures of merit ZTmax = 1.3 at 348 K and ZTave = 1.17 (300-500 K) are achieved with improved mechanical properties in BTS-based composites incorporated with 1.5 wt % ABS and 0.5 wt % PPy, demonstrating that the incorporation of both inorganic and organic nanoinclusions is an effective way to improve its thermoelectric performance.

Enhanced Thermoelectric Performance of P-Type (Bi,Sb)2 Te3 by Incorporating Non-Stoichiometric Ag5 Te3 and Refining Te-Se Ratio

Power generation modules utilizing thermoelectric (TE) materials are suitable for recycling widespread low-grade waste heat (<600 K), highlighting the immediate necessity for advanced Bi2 Te3 -based alloys. Herein, the substantial enhancement in TE performance of the p-type Bi0.4 Sb1.6 Te3 (BST) sintered sample is realized by subtly incorporating the non-stoichiometric Ag5 Te3 and counteractive Se. Specifically, Ag atoms diffused into the BST lattice improve the density-of-states effective mass (md * ) and boost the hole concentration for the suppressed bipolar effect. The addition of Se further improves md * prompting the room-temperature power factor upgrade to 46 W cm-1  K-2 . Concurrently, the lattice thermal conductivity is considerably lowered by multiple scattering sources exemplified by Sb-rich nanoprecipitates and dense dislocations. These synergistic results yield a high peak ZT of 1.44 at 375 K and an average ZT of 1.28 between 300 and 500 K in the Bi0.4 Sb1.6 Te2.95 Se0.05 + 0.05 wt.% Ag5 Te3 sample. More significantly, when coupled with n-type zone-melted Bi2 Te2.7 Se0.3 , the integrated 17-pair TE module achieves a competitive conversion efficiency of 6.1% and an output power density of 0.40 W cm-2 at a temperature difference of 200 K, demonstrating great potential for practical applications.

Strong Intervalley Scattering-Induced Renormalization of Electronic and Thermal Transport Properties and Selection Rule Analysis in 2D Tellurium

The electron-phonon interaction (EPI) and phonon-phonon interactions are ubiquitous in promising two-dimensional (2D) semiconductors, determining both electronic and thermal transport properties. In this work, based on ab initio calculations, the effects of intervalley scattering on EPI and higher-order four-phonon interactions of α-Te and β-Te are investigated. Through the proposed selection rules for scattering channels and calculations of full electron-phonon scattering rates, we demonstrate that multiple nearly degenerate local valleys/peaks produce more scattering channels, resulting in stronger intervalley scattering over intravalley scattering. The lattice thermal conductivities of α-Te and β-Te are decreased by as much as 10.9% and 30.8% by considering EPI under the carrier concentration of 2 × 1013 cm-2 (n-type) at 300 K compared to those limited by three-phonon scattering, respectively. However, when further considering four-phonon scattering, EPI reduces the lattice thermal conductivities by 2.6% and 19.4% for α-Te and β-Te, respectively. Furthermore, it is revealed that the four-phonon interaction is more dominant in phonon transport for α-Te than that for β-Te due to the presence of an acoustic-optical phonon gap in α-Te. Finally, we demonstrate strong intervalley scattering induces significant renormalization effects from EPI on all the constituent parameters of thermoelectric performance. Our results show the contributions of intervalley scattering to the electronic properties as well as thermal transport properties in band-convergent thermoelectric materials are essential and highlight the potential of monolayer tellurium as a promising candidate for advanced thermoelectric applications.

Study on the Effect of Sn, In, and Se Co-Doping on the Thermoelectric Properties of GeTe

GeTe and Ge0.99-xIn0.01SnxTe0.94Se0.06 (x = 0, 0.01, 0.03, and 0.06) samples were prepared by vacuum synthesis combined with spark plasma sintering (SPS). The thermoelectric properties of GeTe were coordinated by multiple doping of Sn, In, and Se. In this work, a maximum zT(zT = S2σT/κ) of 0.9 and a power factor (PF = S2σ) of 3.87 μWmm-1 K-2 were obtained in a sample of Ge0.99In0.01Te0.94Se0.06 at 723K. The XRD results at room temperature show that all samples are rhombohedral phase structures. There is a peak (~27°) of the Ge element in GeTe and the sample (x = 0), but it disappears after Sn doping, indicating that Sn doping can promote the dissolution of Ge. The scattering mechanism of the doped samples was calculated by the conductivity ratio method. The results show that phonon scattering Is dominant in all samples, and alloy scattering is enhanced with the increase in the Sn doping amount. In doping can introduce resonance energy levels and increase the Seebeck coefficient, and Se doping can introduce point defects to suppress phonon transmission and reduce lattice thermal conductivity. Therefore, the thermoelectric properties of samples with x = 0 improved. Although Sn doping will promote the dissolution of Ge precipitation, the phase transition of the samples near 580 K deteriorates the thermoelectric properties. The thermoelectric properties of Sn-doped samples improved only at room temperature to ~580 K compared with pure GeTe. The synergistic effect of multi-element doping is a comprehensive reflection of the interaction between elements rather than the sum of all the effects of single-element doping.

Comprehensive investigation of electronic structure, phonon spectrum and thermoelectric performance of LuMSb (M = Ni, Pd, Pt) half Heusler compounds from first principles

We studied the structural, electronic, phonon spectrum and thermoelectric properties of ternary LuMSb (M = Ni, Pd, Pt) half Heusler compounds by using first principles method. The electronic properties are calculated via energy band structure and density of states by using GGA + U approximation. The calculations reveal that the replacement of Ni with Pd and Pt, energy gap decreases and LuNiSb, LuPdSb are found to have narrow indirect band gaps and exhibit semiconducting nature, while LuPtSb is found to be a gapless semiconductor. Phonon band structure calculations give only positive values of phonon frequency indicating the dynamically stability of these compounds. The thermoelectric properties have been computed using semi-classical Boltzmann transport theory. We found high Seebeck coefficient (S) and high power factor (PF) for LuNiSb and LuPdSb compounds in the whole temperature range. The ZT values of LuNiSb and LuPdSb are high in general and reach a maximum of 0.67 and 0.69 at 450 K, respectively, whereas 0.39 is the maximum ZT value for LuPtSb at the same temperature. These findings propose LuNiSb and LuPdSb compounds as promising materials for thermoelectric applications at room temperature.

Synergetic Enhancement of Strength-Ductility and Thermoelectric Properties of Ag2 Te by Domain Boundaries

Simultaneously improving the mechanical and thermoelectric (TE) properties is significant for the engineering applications of inorganic TE materials. In this work, a novel nanodomain strategy is developed for Ag2 Te compounds to yield 40% and 200% improved compressive strength (160 MPa) and fracture strain (16%) when compared to domain-free samples (115 MPa and 5.5%, respectively). The domained samples also achieve a 45% improvement in average ZT value. The domain boundaries (DBs) provide extra sites for dislocation nucleation while pinning the dislocation movement, resulting in superior strength and ductility. In addition, phonon scattering induced by DBs suppresses the lattice thermal conductivity of Ag2 Te and also reduces the weighted mobility. These findings provide new insights into grain and DB engineering for high-performance inorganic semiconductors with robust mechanical properties.

Extremely Low Lattice Thermal Conductivity Leading to Superior Thermoelectric Performance in Cu4TiSe4

Low thermal conductivity is crucial for obtaining a promising thermoelectric (TE) performance in semiconductors. In this work, the TE properties of Cu₄TiS₄ and Cu₄TiSe₄ were theoretically investigated by carrying out first-principles calculations and solving Boltzmann transport equations. The calculated results reveal a lower sound velocity in Cu₄TiSe₄ compared to that in Cu₄TiS₄, which is due to the weaker chemical bonds in the crystal orbital Hamilton population (COHP) and also the larger atomic mass in Cu₄TiSe₄. In addition, the strong lattice anharmonicity in Cu₄TiSe₄ enhances phonon-phonon scattering, which shortens the phonon relaxation time. All of these factors lead to an extremely low lattice thermal conductivity (κ_L) of 0.11 W m^-1 K^-1 at room temperature in Cu₄TiSe₄ compared with that of 0.58 W m^-1 K^-1 in Cu₄TiS₄. Owing to the suitable band gaps of Cu₄TiS₄ and Cu₄TiSe₄, they also exhibit great electrical transport properties. As a result, the optimal ZT values for p (n)-type Cu₄TiSe₄ are up to 2.55 (2.88) and 5.04 (5.68) at 300 and 800 K, respectively. For p (n)-type Cu₄TiS₄, due to its low κ_L, the ZT can also reach high values over 2 at 800 K. The superior thermoelectric performance in Cu₄TiSe₄ demonstrates its great potential for applications in thermoelectric conversion.

Thermoelectric Properties of n-Type Bi4O4SeX2 (X = Cl, Br)

The multiple anion superlattice Bi₄O₄SeCl₂ has been reported to exhibit extremely low thermal conductivity along the stacking c-axis, making it a promising material for thermoelectric applications. In this study, we investigate the thermoelectric properties of Bi₄O₄SeX₂ (X = Cl, Br) polycrystalline ceramics with different electron concentrations by adjusting the stoichiometry. Despite optimizing the electric transport, the thermal conductivity remained ultra-low and approached the Ioffe-Regel limit at high temperatures. Notably, our findings demonstrate that non-stoichiometric tuning is a promising approach for enhancing the thermoelectric performance of Bi₄O₄SeX₂ by refining its electric transport, resulting in a figure of merit of up to 0.16 at 770 K.

Spin-Orbit-Coupling-Induced Topological Transition and Anomalously Strong Intervalley Scattering in Two-Dimensional Bismuth Allotropes with Enhanced Thermoelectric Performances

The convergence of multivalley bands is originally believed to be beneficial for thermoelectric performance by enhancing the charge conductivity while preserving the Seebeck coefficients, based on the assumption that electron interband or intervalley scattering effects are totally negligible. In this work, we demonstrate that β-Bi with a buckled honeycomb structure experiences a topological transition from a normal insulator to a Z₂ topological insulator induced by spin-orbit coupling, which subsequently increases the band degeneracy and is probably beneficial for enhancement of the thermoelectric power factor for holes. Therefore, strong intervalley scattering can be observed in both band-convergent β- and aw-Bi monolayers. Compared to β-Bi, aw-Bi with a puckered black-phosphorus-like structure possesses high carrier mobilities with 318 cm²/(V s) for electrons and 568 cm²/(V s) for holes at room temperature. We also unveil extraordinarily strong fourth phonon-phonon interactions in these bismuth monolayers, significantly reducing their lattice thermal conductivities at room temperature, which is generally anomalous in conventional semiconductors. Finally, a high thermoelectric figure of merit (zT) can be achieved in both bismuth monolayers, especially for aw-Bi with an n-type zT value of 2.2 at room temperature. Our results suggest that strong fourth phonon-phonon interactions are crucial to a high thermoelectric performance in these materials, and two-dimensional bismuth is probably a promising thermoelectric material due to its enhanced band convergence induced by the topological transition.

High-Performance Stretchable Thermoelectric Generator for Self-Powered Wearable Electronics

Wearable thermoelectric generators (TEGs), which can convert human body heat to electricity, provide a promising solution for self-powered wearable electronics. However, their power densities still need to be improved aiming at broad practical applications. Here, a stretchable TEG that achieves comfortable wearability and outstanding output performance simultaneously is reported. When worn on the forehead at an ambient temperature of 15 °C, the stretchable TEG exhibits excellent power densities with a maximum value of 13.8 µW cm^-2 under the breezeless condition, and even as high as 71.8 µW cm^-2 at an air speed of 2 m s^-1 , being one of the highest values for wearable TEGs. Furthermore, this study demonstrates that this stretchable TEG can effectively power a commercial light-emitting diode and stably drive an electrocardiogram module in real-time without the assistance of any additional power supply. These results highlight the great potential of these stretchable TEGs for power generation applications.

Theoretical Prediction of Thermoelectric Performance for Layered LaAgOX (X = S, Se) Materials in Consideration of the Four-Phonon and Multiple Carrier Scattering Processes

Inspired by the experimental achievement of layered LaCuOX (X = S, Se) with superior thermoelectric (TE) performance, the TE properties of Ag-based isomorphic LaAgOX are systemically investigated by the first-principles calculation. The LaAgOS and LaAgOSe are direct semiconductors with wide bandgaps of ≈2.50 and ≈2.35 eV. Essential four-phonon and multiple carrier scattering mechanisms are considered in phonon and electronic transport calculations to improve the accuracy of the figure-of-merit (ZT). The p-type LaAgOX (X = S, Se) shows excellent TE performance on account of the large Seebeck coefficient originated from the band convergency and low thermal conductivity caused by the strong phonon-phonon scattering. Consequently, the optimal ZTs along the out-of-plane direction decrease in the order of n-type LaAgOSe (≈2.88) > p-type LaAgOSe (≈2.50) > p-type LaAgOS (≈2.42) > n-type LaAgOS (≈2.27) at 700 K, and the optimal ZTs of ≈1.16 and ≈1.29 are achieved for p-type LaAgOS and LaAgOSe at the same temperature. The present work would provide a deep insight into the phonon and electronic transport properties of LaAgOX (X = S, Se), but also could shed light on the way for the rational design of state-of-the-art heteroanionic materials for TE application.

Band Structure and Phonon Transport Engineering Realizing Remarkable Improvement in Thermoelectric Performance of Cu2SnSe4 Incorporated with In2Te3

Cu₂SnSe₄ (CTS) ternary chalcogenides have potential applications in thermoelectrics for they crystallize in a high-symmetry cubic structure and consist of earth-abundant and eco-friendly elements. However, the pristine CTS does not have optimal thermoelectric (TE) performance (ZT = 0.35 at ∼700 K), so further investigation is required in this regard. In this work, we propose an incorporation of In₂Te₃ with a defect zinc-blende cubic structure into CTS, aiming to regulate the electronic and phonon transport mechanism simultaneously. The first-principles calculation reveals that the element In favors the residing at a vacancy site as an interstitial atom while Te at the Se site, which leads to band convergence and degeneracy, respectively. As a result, the electrical property improves with a 22% increase in the power factor (PF), and at the same time, the lattice thermal conductivity (κ_L) reduces to 0.31 W K^-1 m^-1 at 718 K. Synergistic engineering realizes a remarkable improvement in TE performance with the highest figure of merit (ZT) of 0.92 at 718 K. This value is ∼3 times that of the pristine CTS and stands among the highest in the Cu₂SnSe₄ family so far, which proves that the incorporation of In₂Te₃ into CTS is a good proposal.

Phonon-drag thermopower and thermoelectric performance of MoS2monolayer in quantizing magnetic field

We present a theory of phonon-drag thermopower,Sxxg, in MoS₂monolayer at a low-temperature regime in the presence of a quantizing magnetic fieldB. Our calculations forSxxgconsider the electron-acoustic phonon interaction via deformation potential (DP) and piezoelectric (PE) couplings for longitudinal (LA) and transverse (TA) phonon modes. The unscreened TA-DP is found to dominateSxxgover other mechanisms. TheSxxgis found to oscillate with the magnetic field where the lifting effect of the valley and spin degeneracies in MoS₂monolayer has been clearly observed. An enhancedSxxgwith a peak value of∼1mV K^-1at aboutT = 10 K is predicted, which is closer to the zero field experimental observation. In the Bloch-Grüneisen regime the temperature dependence ofSxxggives the power-lawSxxg∝Tδe, whereδ_evaries marginally around 3 and 5 for unscreened and screened couplings, respectively. In addition,Sxxgis smaller for larger electron densityn_e. The power factor PF is found to increase with temperatureT, decrease withn_e, and oscillate withB. The prediction of an increase of thermal conductivity with temperature and the magnetic field is responsible for the limit of the figure of merit (ZT). At a particular magnetic field and temperature,ZTcan be maximized by optimizing electron density. By fixingne=1012cm^-2, the highestZTis found to be 0.57 atT = 5.8 K andB = 12.1 T. Our findings are compared with those in graphene and MoS₂for the zero-magnetic field.

Mediating Point Defects Endows n-Type Bi2 Te3 with High Thermoelectric Performance and Superior Mechanical Robustness for Power Generation Application

Bi₂ Te₃ -related alloys dominate the commercial thermoelectric market, but the layered crystal structure leads to the dissociation and intrinsic brittle fracture, especially for single crystals that may worsen the practical efficiency. In this work, point defect configuration by S/Te/I defects engineering is engaged to boost thermoelectric and mechanical properties of n-type Bi₂ Te₃ alloy, which, coupled with p-type BiSbTe, shows a competitive conversion efficiency for the fabricated module. First, as S alloying suppresses the intrinsic B i T e , antisite defects and forms a donor-like effect, electronic transport properties are optimized, associated with the decreased thermal conductivity due to the point defect scattering. The periodide compound TeI₄ is afterward adopted to further tune carrier concentration for the realization of an optimal ZT. Finally, an advanced average ZT of 1.05 with ultra-high compressive strength of 230 MPa is achieved for Bi₂ Te_2.9 S_0.1 (TeI₄ )_0.0012 . Based on this optimum composition, a fabricated 17-pair module demonstrates a maximum conversion efficiency of 5.37% under the temperature difference of 250 K, rivaling the current state-of-the-art Bi₂ Te₃ modules. This work reveals the novel mechanism of point defect reconfiguration in synergistic enhancement of thermoelectric and mechanical properties for durably commercial application, which may be applicable to other thermoelectric systems.

Nanotwins Strengthening High Thermoelectric Performance Bismuth Antimony Telluride Alloys

Bi₂ Te₃ based thermoelectric alloys have been commercialized in solid-state refrigeration, but the poor mechanical properties restrict their further application. Nanotwins have been theoretically proven to effectively strengthen these alloys and could be sometimes constructed by strong deformation during synthesis. However, the obscure underlying formation mechanism restricts the feasibility of twin boundary engineering on Bi₂ Te₃ based materials. Herein, thorough microstructure characterizations are employed on a series of Bi_0.4 Sb_1.6 Te_{3+ δ} alloys to systematically investigate the twins' formation mechanism. The results show that the twins belong to the annealing type formed in the sintering process, which is sensitive to Te deficiency, rather than the deformation one. The Te deficiency combined with mechanical deformation is prerequisite for constructing dense nanotwins. By reducing the δ below -0.01 and undergoing strong deformation, samples with a high density of nanotwins are obtained and exhibit an ultrahigh compressive strength over 250 MPa, nearly twice as strong as the previous record reported in hierarchical nanostructured (Bi, Sb)₂ Te₃ alloy. Moreover, benefitting from the suppressed intrinsic excitation, the average zT value of this robust material could reach near 1.1 within 30-250 °C. This work opens a new pathway to design high-performance and mechanically stable Bi₂ Te₃ based alloys for miniature device development.

Improved Thermoelectric Performance of p-Type Argyrodite Cu8GeSe6 via the Simultaneous Engineering of the Electronic and Phonon Transports

Guided by the concept of "phonon-liquid electron-crystal", many n-type argyrodite compounds have been developed as candidates for thermoelectric (TE) materials. In recent years, the p-type Cu₈GeSe₆ (CGS) compound has attracted some attention in TEs due to the presence of very strong atomic vibrational arharmonicity inside the sublattice, which is caused by the weak bonding between Cu ions and [GeSe₆]^8-. However, its TE performance is still poor, with a ZT value of only 0.2 at 623 K. Therefore, in this work, we propose to engineer both the electronic and phonon transports in CGS by incorporating the species In₂Te₃. This strategy tunes the carrier concentration and at the same time increases the phonon scattering on the point defects (In_Ge, In_interstitial, and Te_Se) and randomly distributed tetrahedra ([InSe₄]^5- and [GeTeSe₃]^4-). As a result, the phase transformation at 329 K in CGS is eliminated, and the peak ZT value is enhanced from 0.27 for CGS to ∼0.92 for (Cu₈SnSe₆)_0.9(In₂Te₃)_0.1 at 774 K; this thus proves that the incorporation of In₂Te₃ in CGS is an effective way of regulating its TE performance.

High Thermoelectric Performance SnTe with a Segregated and Percolated Structure

A nanostructure has a significant role in enhancing the power factor and preventing the heat propagation for thermoelectric materials. Herein, we propose a unique segregated and percolated (SP) microphase-separated structure to enhance the thermoelectric performance of SnTe. The SP structure is composed of insoluble SnTe and AgCuTe, in which AgCuTe with ultralow lattice thermal conductivity undergoes a solid-phase welding during a spark plasma sintering process and forms continuous percolated layers at the interface of isolated SnTe. The SP structure achieved a simultaneous scattering for low energy holes due to the energy offset of the valence band maximum between SnTe and AgCuTe and for phonons due to the noncoherent interfaces between SnTe and AgCuTe, resulting in a high Seebeck coefficient of ∼219.4 μV/K and a low lattice thermal conductivity of ∼1.1 W m^-1 K^-1 at 800 K for (SnTe)_0.55(AgCuTe)_0.45. The thermoelectric performance was further enhanced by means of the cosubstitution of In and Mn for Sn in the SnTe lattice, inducing resonance levels and extra phonon scattering. As a result, the SP structure combined with In/Mn codoping enable us to achieve a low lattice thermal conductivity of 0.47 W m^-1 K^-1, a peak ZT of ∼1.45 at 800 K, and a high average ZT of ∼0.73 (400-800 K) for (Sn_0.98In_0.01Mn_0.01Te)_0.75(AgCuTe)_0.25.

Improved Thermoelectric Performance of P-type SnTe through Synergistic Engineering of Electronic and Phonon Transports

SnTe has been regarded as a potential alternative to PbTe in thermoelectrics because of its environmentally friendly features. However, it is a challenge to optimize its thermoelectric (TE) performance as it has an inherent high hole concentration (n_H∼2 × 10²⁰ cm^-3) and low mobility (μ_H∼18 cm² V^-1 s^-1) at room temperature (RT), arising from a high intrinsic Sn vacancy concentration and large energy separation between its light and heavy valence bands. Therefore, its TE figure of merit is only 0.38 at ∼900 K. Herein, both the electronic and phonon transports of SnTe were engineered by alloying species Ag_0.5Bi_0.5Se and ZnO in succession, thus increasing the Seebeck coefficient and, at the same time, reducing the thermal conductivity. As a result, the TE performance improves significantly with the peak ZT value of ∼1.2 at ∼870 K for the sample (SnGe_0.03Te)_0.9(Ag_0.5Bi_0.5Se)_0.1 + 1.0 wt % ZnO. This result proves that synergistic engineering of the electronic and phonon transports in SnTe is a good approach to improve its TE performance.

Enhancing Thermoelectric Performance of AgSbTe2-Based Compounds via Microstructure Modulation Combining with Entropy Engineering

Modulation of the microstructure and configurational entropy tuning are the core stratagem for improving thermoelectric performance. However, the correlation of evolution among the preparation methods, chemical composition, structural defects, configurational entropy, and thermoelectric properties is still unclear. Herein, two series of AgSbTe₂-based compounds were synthesized by an equilibrium melting-slow-cooling method and a nonequilibrium melting-quenching-spark plasma sintering (SPS) method, respectively. The equilibrium method results in coarse grains with a size of >300 μm in the samples and a lower defect concentration, leading to higher carrier mobility of 10.66 cm² V^-1 s^-1 for (Ag₂Te)_0.41(Sb₂Te₃)_0.59 compared to the sample synthesized by nonequilibrium preparation of 1.83 cm² V^-1 s^-1. Moreover, tuning the chemical composition of nonstoichiometric AgSbTe₂ effectively improves the configurational entropy and creates a large number of cation vacancies, which evolve into dense dislocations in the samples. Owing to all of these in conjunction with the strong inharmonic vibration of lattice, an ultralow thermal conductivity of 0.51 W m^-1 K^-1 at room temperature is achieved for the (Ag₂Te)_0.42(Sb₂Te₃)_0.58 sample synthesized by the equilibrium preparation method. Due to the enhanced carrier mobility, optimized carrier concentration, and low thermal conductivity, the (Ag₂Te)_0.42(Sb₂Te₃)_0.58 sample synthesized by the equilibrium preparation method possesses the highest ZT of 1.04 at 500 K, more than 60% higher than 0.64 at 500 K of the same composition synthesized by nonequilibrium preparation.

Enhancing the Thermoelectric and Mechanical Properties of Bi0.5Sb1.5Te3 Modulated by the Texture and Dense Dislocation Networks

Bi₂Te₃-based materials are dominating thermoelectrics for almost all of the room-temperature applications. To meet the future demands, both their thermoelectric (TE) and mechanical properties need to be further improved, which are the requisite for efficient TE modules applied in areas such as reliable micro-cooling. The conventional zone melting (ZM) and powder metallurgy (PM) methods fall short in preparing Bi₂Te₃-based alloys, which have both a highly textured structure for high TE properties and a fine-grained microstructure for high mechanical properties. Herein, a mechanical exfoliation combined with spark plasma sintering (ME-SPS) method is developed to prepare Bi_0.5Sb_1.5Te₃ with highly improved mechanical properties (correlated mainly to the dislocation networks), as well as significantly improved thermoelectric properties (correlated mainly to the texture structure). In the method, both the dislocation density and the orientation factor (F) can be tuned by the sintering pressure. At a sintering pressure of 20 MPa, an exceptional F of up to 0.8 is retained, leading to an excellent power factor of 4.8 mW m^-1 K^-2 that is much higher than that of the PM polycrystalline. Meanwhile, the method can readily induce high-density dislocations (up to ∼10¹⁰ cm^-2), improving the mechanical properties and reducing the lattice thermal conductivity as compared to the ZM ingot. In the exfoliated and then sintered (20 MPa) sample, the figure-of-merit ZT = 1.2 (at 350 K), which has increased by about ∼20%, and the compressive strength has also increased by ∼20%, compared to those of the ZM ingot, respectively. These results demonstrate that the ME-SPS method is highly effective in preparing high-performance Bi₂Te₃-based alloys, which are critical for TE modules in commercial applications at near-room temperature.

Synergistic Optimization of the Electronic and Phonon Transports of N-Type Argyrodite Ag8Sn1-xGaxSe6 (x = 0-0.6) through Entropy Engineering

The argyrodite compound, Ag₈SnSe₆ (ATS), which is one of the promising thermoelectric (TE) candidates, is receiving growing attention in thermoelectrics recently. However, its TE performance is still low and phases are unstable as the temperature varies. In this work, inspired by entropy engineering, we eliminate the β/γ phase transformation at ∼355 K via alloying Ga, thus extending its high-temperature cubic phase from 320 to 730 K. In the meantime, the power factor (PF) enhances by 10% and lattice thermal conductivity (κ_L) reduces by 40% at 723 K. As a result, the ZT value is boosted to ∼1.15 for Ag₈Sn_0.5Ga_0.5Se₆, which stands high among the ATS systems. This proves that the entropy engineering is an effective approach to extend the high-temperature range for the cubic γ-phase and improve its TE performance simultaneously.

Mechanical Properties and Thermal Stability of the High-Thermoelectric-Performance Cu2Se Compound

The Cu₂Se compound possesses extraordinary thermoelectric performance at high temperatures and shows great potential for the application of waste heat recycling. However, a thermoelectric device usually undergoes mechanical vibration, mechanical and/or thermal cycling, and thermal shock in service. Therefore, mechanical properties are of equal importance as thermoelectric performance. However, the mechanical performance and stability of the Cu₂Se compound during long-term service at high temperatures have rarely been reported. In this study, we systematically investigated the mechanical properties of Cu₂Se compounds synthesized by three varied methods (melting (M), self-propagating high-temperature synthesis (SHS), and a combination of SHS and ultrasonic treatment (UT)) and investigated the thermal stability of the SHS-UT compound under different annealing temperatures. The SHS-UT process effectively refines the grain size from 19 μm for the melting sample to 5 μm for the SHS-UT sample. The high density of grain boundaries in the SHS-UT sample effectively dissipates the energy of crack propagation; thus, the mechanical properties are greatly improved. The compressive strength, bending strength, and Vickers hardness of the SHS-UT sample are 147 MPa, 52.6 MPa, and 0.46 GPa, respectively, which are 21.5, 16.6, and 35.3% higher than those of the melting sample, respectively. Moreover, excellent thermal stability is achieved in the compound prepared by SHS and ultrasonication at a temperature below 873 K. After annealing at temperatures up to 873 K for 7 days, the excellent thermoelectric performance of the Cu₂Se compound is well maintained with a ZT value exceeding 1.80 at 873 K. However, with further increasing the annealing temperature to 973 K, the volatilization of Se and the precipitation of Cu result in the instability and significantly deteriorated thermoelectric performance of the material. This work provides an avenue for boosting the mechanical properties and commercial application of Cu₂Se.

High ZT Value of Pure SnSe Polycrystalline Materials Prepared by High-Energy Ball Milling plus Hot Pressing Sintering

The research of thermoelectric materials is of great significance to solve the energy crisis and environmental problems. In this work, a series of pure SnSe bulk crystals were prepared by melting, high-energy ball milling, and hot pressing processes. The results show that the ZT value of the prepared pure SnSe polycrystalline material is up to 2.1 at 873 K. On the one hand, due to the reduction of grain size and lattice distortion caused by long-time high-energy ball milling, the lattice thermal conductivity is significantly reduced, which is only 0.18 W K^-1 m^-1 at 873 K. On the other hand, high-energy ball milling leads to the increase of Sn vacancies, which improves the conductivity of SnSe polycrystalline materials. Since the whole process of ball milling was carried out in a closed ball milling tank filled with high-purity argon, no oxidation of the SnSe powder is also a guarantee to obtain pure SnSe polycrystalline materials with high ZT value.

Liquid-Phase Manipulation Securing Enhanced Thermoelectric Performance of Ag2Se

Developing n-type materials with high peak and/or average ZT (ZT is the figure of merit) is an urgent need for the lower ZT of the existing n-type BiTeSe materials compared with the p-type BiSbTe materials. Here, we demonstrate that liquid-phase sintering can lead to lowered thermal conductivity and an improved power factor in n-type Ag₂Se, which originates from the greatly lowered electronic thermal conductivity attributed to the decreased mobility and improved Seebeck coefficients because of increased effective mass. Benefiting from this, the maximum ZT (ZT_max) of ∼1.21 and the average ZT (ZT_ave) of 1.06 are successfully achieved in polycrystalline Ag₂Se. In this work, ZT_ave is the highest reported value, being 26% larger than that of Ag₂Se reported. Our work shows that liquid-phase sintering to achieve improved thermoelectric (TE) performance opens a great opportunity for designing prospective thermoelectrics.

Improved Thermoelectric Performance of Cu12Sb4S13 through Gd-Substitution Induced Enhancement of Electronic Density of States and Phonon Scattering

Cu₁₂Sb₄S₁₃ has aroused great interest because of its earth-abundant constituents and intrinsic low thermal conductivity. However, the applications of Cu₁₂Sb₄S₁₃ are hindered by its poor thermoelectric performance. Herein, it is shown that Gd substitution not only causes a significant increase in both electrical conductivity σ and thermopower S but also leads to dramatic drop in lattice thermal conductivity κ_L. Consequently, large ZT reaches 0.94 at 749 K for Cu_11.7Gd_0.3Sb₄S₁₃, which is ∼41% higher than the ZT value of undoped sample. Rietveld refinements of XRD results show that accompanying inhibition of impurity phase Cu₃SbS₄, the number of Cu vacancies increases substantially with substituted content x (x ≤ 0.3), which leads to reduced κ_L owing to intensive phonon scattering by the point defects and increased σ arising from the charged defects (V_Cu^'). Crucially, synchrotron radiation photoelectron spectroscopy reveals substantial increment of electronic density of states at Fermi level upon Gd substitution, which is proven, by our first-principle calculations, to originate from contribution of Gd 4f orbit, resulting in enhancement of S. Our study provides us with a new path to enhance thermoelectric performance of Cu₁₂Sb₄S₁₃.

Boosting Thermoelectric Properties of AgBi3(SeyS1-y)5 Solid Solution via Entropy Engineering

AgBi₃S₅ is an environmentally friendly n-type thermoelectric material composed of earth-abundant and nontoxic elements. It has a complex monoclinic structure with distorted NaCl-type fragments, which provide its intrinsically low thermal conductivity. However, poor electrical properties limit its overall performance. Configurational entropy engineering is an effective method to enhance thermoelectric properties. With the increase of configurational entropy, phonon point defect scattering is amplified, yielding lower lattice thermal conductivity, while the structure symmetry can also be improved, which leads to the enhanced electrical transport property. In this study, we combine carrier modulation and entropy engineering, utilizing melting-annealing and spark plasma sintering, to synthesize a series of AgBi₃(Se_yS_1-y)_5.08 bulks. Se substitution effectively increases the configurational entropy and thus dramatically decreases the thermal conductivity. Moreover, anion deficiency modulation effectively optimizes the carrier concentration and the electrical transport properties. Due to a power factor of 2.7 μW/(cm·K²) and a low thermal conductivity of 0.45 W/(m·K) at 723 K, the AgBi₃(Se_0.9S_0.1)_5.08 sample possesses the highest ZT of 0.42 at 723 K, nearly double the value of AgBi₃S_5.08 or pristine AgBi₃S₅. Our work demonstrates that apart from carrier optimization, entropy engineering opens a new avenue for enhancing the thermoelectric properties of a given material.

Doping Effect on Cu2Se Thermoelectric Performance: A Review

Cu2Se, owing to its intrinsic excellent thermoelectric (TE) performance emerging from the peculiar nature of "liquid-like" Cu+ ions, has been regarded as one of the most promising thermoelectric materials recently. However, the commercial use is still something far from reach unless effective approaches can be applied to further increase the figure of merit (ZT) of Cu2Se, and doping has shown wide development prospect. Until now, the highest ZT value of 2.62 has been achieved in Al doped samples, which is twice as much as the original pure Cu2Se. Herein, various doping elements from all main groups and some transitional groups that have been used as dopants in enhancing the TE performance of Cu2Se are summarized, and the mechanisms of TE performance enhancement are analyzed. In addition, points of great concern for further enhancing the TE performance of doped Cu2Se are proposed.

Symmetry Breaking Induced Anisotropic Carrier Transport and Remarkable Thermoelectric Performance in Mixed Halide Perovskites CsPb(I1-xBrx)3

We present a combination of first-principles calculations and the Boltzmann transport theory to understand the carrier transport and thermoelectric performance of mixed halide perovskite alloys CsPb(I_1-xBr_x)₃ with different Br compositions. Our computational results correlate the conduction band splitting in CsPb(I_1-xBr_x)₃ to the significant anisotropy in their carrier transport properties, such as effective masses and deformation potential constants. Such band splitting originates from the symmetry-broken crystal structures of CsPb(I_1-xBr_x)₃ polymorphs: with residue stresses/strains in asymmetric CsPb(I_1-xBr_x)₃, nondegenerate orbitals reconstruct the conduction band and reduce the Pb-halide antibonding character along certain directions. While the Seebeck coefficient (S) and the relaxation time-normalized electrical conductivity (σ/τ) show weak directional anisotropy, the carrier relaxation time (τ) is highly direction-dependent. The reconstruction of the conduction band finally leads to significantly anisotropic and enhanced thermoelectric power factors (PF = S²σ) in CsPb(I_1-xBr_x)₃ compared to those in pure CsPbI₃ and CsPbBr₃, showing anomalous nonlinear alloy behavior. A delicate balance between S²σ and combined measurement of the carrier effective mass and deformation potential constant, m*E_DP, is confirmed. The lattice thermal conductivities of CsPb(I_1-xBr_x)₃ are significantly suppressed compared to those of their pure counterparts due to strong mass disordering and strain fields upon halogen substitution. As a result, symmetry breaking in CsPb(I_1-xBr_x)₃ leads to anisotropy in carrier transport, high PF, and scattered phonon transport (ultralow thermal conductivity), concurrently contributing to their promising thermoelectric figures of merit (ZT) up to 1.7 at room temperature. The principles behind the asymmetry-induced factors would serve as new design concepts to tailor the thermoelectric properties of alloys, mixtures, superlattices, and low-dimensional materials.

Enhanced Thermoelectric Performance of Bi0.46Sb1.54Te3 Nanostructured with CdTe

Cd-containing polycrystalline Bi_0.46Sb_1.54Te₃ samples with precisely controlled phase composition were synthesized by conventional melting-quenching-annealing technique and a melt-spinning method. The pseudo ternary phase diagram for Cd-Bi/Sb-Te in the region near Bi_0.46Sb_1.54Te₃ was systematically studied. Cd serves as an acceptor dopant contributing holes, whereas for samples doped with CdTe, the combined effects of the substitution of Sb/Bi with Cd and the formation of Sb/Bi_Te antisite defects leads to the increase in hole concentration. Moreover, upon doping with Cd, the lattice thermal conductivity decreases significantly owing to the intensified point defect phonon scattering. The sample with Cd content of 0.01 attains the maximum ZT of 1.15 at 425 K. The utilization of melt-spinning method brings about the in situ nanostructured CdTe and grain size refinement, which further reduce the lattice thermal conductivity while preserving excellent electrical performance. As a result, a higher ZT of 1.30 at 425 K is realized with CdTe content x = 0.005.

Non-Rigid Band Structure in Mg2Ge for Improved Thermoelectric Performance

Magnesium silicide and its solid solutions are among the most attractive materials for thermoelectric generators in the temperature range of 500-800 K. However, while n-type Mg2(Si,Ge,Sn) materials show excellent thermoelectric performance, the corresponding p-type solid solutions are still inferior, mainly due to less favorable properties of the valence bands compared to the conduction bands. Here, Li doped Mg2Ge with a thermoelectric figure of merit zT of 0.5 at 700 K is reported, which is four times higher than that of p-type Mg2Si and double than that of p-type Mg2Sn. The reason for the excellent properties is an unusual temperature dependence of Seebeck coefficient and electrical conductivity compared to a standard highly doped semiconductor. The properties cannot be captured assuming a rigid band structure but well reproduced assuming two parabolic valence bands with a strong temperature dependent interband separation. According to the analysis, the difference in energy between the two bands decrease with temperature, leading to a band convergence at around 650 K and finally to an inversion of the band positions. The finding of a combination of a light and a heavy band that are non-rigid with temperature can pave the way for further optimization of p-type Mg2(Si,Ge,Sn).

Precise Regulation of Carrier Concentration in Thermoelectric BiSbTe Alloys via Magnetic Doping

The Bi₂Te₃-based alloy is the best commercial thermoelectric material around room temperature, although it is extremely difficult to further improve its thermoelectric performance. In this work, we demonstrate that magnetic doping is an effective strategy to regulate the thermoelectric performance of p-type Bi_0.5Sb_1.5Te₃. According to our experiments, it is much more difficult for ferromagnetic Fe/Co to enter the Bi_0.5Sb_1.5Te₃ lattice in comparison with diamagnetic Pb, which can be understood by the "like dissolves like" rule. At the same doping content, Fe and Co provide much lower hole carriers than Pb due to their larger carrier thermal activation energies, indicating that Fe and Co as dopants are very applicable for the fine regulation of the carrier concentration. The Fe/Co-doped samples have higher Seebeck coefficients but less carrier mobilities than the Pb-doped sample since the doped magnetic atoms induce additional carrier scattering. Beyond the solid solubility limit, excess Fe/Co represents as the impurity, which can maintain a high carrier concentration due to the metal-semiconductor contact. Finally, the zT values of ∼1.05 and 1.15 near room temperature have been achieved for the samples with 1.71 at. % Co and 1.80 at. % Fe, respectively.

Efficient interlayer charge release for high-performance layered thermoelectrics

Many layered superlattice materials intrinsically possess large Seebeck coefficient and low lattice thermal conductivity, but poor electrical conductivity because of the interlayer transport barrier for charges, which has become a stumbling block for achieving high thermoelectric performance. Herein, taking BiCuSeO superlattice as an example, it is demonstrated that efficient interlayer charge release can increase carrier concentration, thereby activating multiple Fermi pockets through Bi/Cu dual vacancies and Pb codoping. Experimental results reveal that the extrinsic charges, which are introduced by Pb and initially trapped in the charge-reservoir [Bi₂O₂]²⁺ sublayers, are effectively released into [Cu₂Se₂]^2- sublayers via the channels bridged by Bi/Cu dual vacancies. This efficient interlayer charge release endows dual-vacancy- and Pb-codoped BiCuSeO with increased carrier concentration and electrical conductivity. Moreover, with increasing carrier concentration, the Fermi level is pushed down, activating multiple converged valence bands, which helps to maintain a relatively high Seebeck coefficient and yield an enhanced power factor. As a result, a high ZT value of ∼1.4 is achieved at 823 K in codoped Bi_0.90Pb_0.06Cu_0.96SeO, which is superior to that of pristine BiCuSeO and solely doped samples. The present findings provide prospective insights into the exploration of high-performance thermoelectric materials and the underlying transport physics.

Effects of Sb Deviation from Its Stoichiometric Ratio on the Micro- and Electronic Structures and Thermoelectric Properties of Cu12Sb4S13

Thermoelectric material tetrahedrite Cu₁₂Sb₄S₁₃ has attracted much attention because of its intrinsic low lattice thermal conductivity, excellent electrical transport property, and environment-friendly constituents. However, its thermoelectric figure merit, ZT, is limited because of the low Seebeck coefficient (S) and power factor (PF). Hence, it is indispensable to enhance its S and PF to increase its ZT. Here, we show that when Sb deviation from its stoichiometric ratio in the Cu₁₂Sb₄S₁₃ band structure is modulated, it gives rise to increased density of states and enhancement of the Seebeck coefficient. Moreover, carrier concentration is tuned by changing sulfur and copper vacancies through controlling the Cu₃SbS₄ phase with an atomic ratio of Sb, leading to increased electrical conductivity. In addition, as large as ∼60% reduction of lattice thermal conductivity is obtained by intensified phonon scattering using an impurity phase/element and vacancy-like defects induced by different Sb contents. As a result, a high ZT = 0.86 is achieved at 723 K for the Cu₁₂Sb_4+δS₁₃ sample with δ = 0.2, which is ∼50% larger than that of stoichiometric Cu₁₂Sb₄S₁₃ studied here, indicating that ZT of Cu₁₂Sb₄S₁₃ can be improved through simple modulation of the Sb stoichiometric ratio.

Microstructure Evolution and Mechanical Properties of Melt Spun Skutterudite-based Thermoelectric Materials

The rapid solidification of melt spinning has been widely used in the fabrication of high-performance skutterudite thermoelectric materials. However, the microstructure formation mechanism of the spun ribbon and its effects on the mechanical properties are still unclear. Here, we report the microstructure evolution and mechanical properties of La–Fe–Co–Sb skutterudite alloys fabricated by both long-term annealing and melt-spinning, followed by sintering approaches. It was found that the skutterudite phase nucleated directly from the under-cooled melt and grew into submicron dendrites during the melt-spinning process. Upon heating, the spun ribbons started to form nanoscale La-rich and La-poor skutterudite phases through spinodal decomposition at temperatures as low as 473 K. The coexistence of the micron-scale grain size, the submicron-scale dendrite segregation and the nanoscale spinodal decomposition leads to high thermoelectric performance and mechanical strength. The maximum three-point bending strength of the melt spinning sample was about 195 MPa, which was 70% higher than that of the annealed sample.

High Thermoelectric Performance of Co-Doped P-Type Polycrystalline SnSe via Optimizing Electrical Transport Properties

This work systematically investigated the thermoelectric properties of p-type Na and M (M = K, Li, Ag) codoped polycrystalline SnSe. It is found that the electrical properties of polycrystalline SnSe can be improved significantly for (Na, Ag) codoped samples, contributed by the enhanced carrier concentration. Specifically, a carrier concentration of 6.23 × 10¹⁹ cm^-3 was obtained in Sn_0.98Na_0.016Ag_0.004Se sample at 335 K, an increase of 18% compared with that of the Na single-doped sample (5.22 × 10¹⁹ cm^-3). The power factor reached ∼0.73 mW m^-1 K^-2 for the Sn_0.98Na_0.016Ag_0.004Se sample at 785 K, enhanced by ∼26% compared with Na single-doped one. In addition, Sn-rich and Ag-rich particles/areas observed in the matrix of Sn_0.98Na_0.016Ag_0.004Se contribute to the reduction of lattice thermal conductivity from 0.61 W m^-1 K^-1 for Sn_0.98Ag_0.02Se to 0.47 W m^-1 K^-1 at 785 K. The combination of simultaneously enhanced power factor and depressed thermal conductivity leads to a maximum ZT ≈ 1.2 at 785 K and a high average ZT ≈ 0.74 at 335-785 K for Sn_0.98Na_0.016Ag_0.004Se, and generating a high theoretical conversion efficiency of ∼11%. These illuminating discoveries could provide routes to enhance the thermoelectric performance in p-type polycrystalline SnSe.

Ultralow Thermal Conductivity and Extraordinary Thermoelectric Performance Realized in Codoped Cu3SbSe4 by Plasma Spark Sintering

Cu₃SbSe₄-based materials have attracted much attention for thermoelectric power generation in the mid-temperature range due to their low cost, ecofriendliness, and abundant elements on the earth. However, the peak figure of merit (ZT) for the Cu₃SbSe₄-based system prepared by the fusion method is usually smaller than unity because of its high thermal conductivity. Here, we show that through a coprecipitation method combined with spark plasma sintering ultrafine-grained Cu₃Sb_0.94Sn_0.06Se_4-yS_y (y = 0, 0.5) embedded with Cu₃SbSe₃ nanoprecipitates can be prepared. Due to the ultralow thermal conductivity and enhanced Seebeck coefficient, a record-high ZT value of 1.32 is achieved for the sample Cu₃Sb_0.94Sn_0.06Se_3.5Se_0.5. The ultralow thermal conductivity is attributed to the enhanced phonon scattering caused by the nanoprecipitates and fine grains of the samples, and the improved Seebeck coefficient originates from the enhancement of electronic density-of-state effective mass. Present results demonstrate that excellent thermoelectric performance can be realized in dual-substituted and fine-grained Cu₃Sb_0.94Sn_0.06Se_4-yS_y with nanoprecipitates.

Achieving an Ultrahigh Power Factor in Sb2Te2Se Monolayers via Valence Band Convergence

An efficient approach to improve the thermoelectric performance of materials is to converge their electronic bands, which is known as band engineering. In this regard, lots of effort has been made to further improve the thermoelectric efficiency of bulk and exfoliated monolayers of Bi₂Te₃ and Sb₂Te₃. However, ultrahigh band degeneracy and thus significant improvement of the power factor have not yet been realized in these materials. Using first-principles methods, we demonstrate that the valley degeneracy of Bi₂Te₃ and Sb₂Te₃ can be largely improved upon substitution of the middle-layer Te atoms with the more electronegative S or Se atoms. Our detailed analysis reveals that in this family of materials, two out of four possible valence band valleys merely depend on the electronegativity of the middle-layer chalcogen atoms, which makes the independent modulation of the valleys' position feasible. As such, band alignment of Bi₂Te₃ and Sb₂Te₃ largely improves upon substitution of the middle-layer Te atoms with more electronegative, yet chemically similar, S and Se ones. A superior valence band alignment is attained in Sb₂Te₂Se monolayers where three out of four possible valleys are well aligned, resulting in a giant band degeneracy of 18 that holds the record among all thermoelectric materials. As a result, an outstanding power factor for the hole-doped monolayers is achieved, indicating a highly efficient p-type thermoelectric material.

Boosting Thermoelectric Performance of SnSe via Tailoring Band Structure, Suppressing Bipolar Thermal Conductivity, and Introducing Large Mass Fluctuation

Here, we report a peak ZT of 1.85 at 873 K for sulfur and Pb codoped polycrystalline SnSe by boosting electrical transport properties while suppressing the lattice thermal conductivity. Compared with single sulfur doped samples, the carrier concentration is improved 1 order of magnitude by Pb incorporation, thereby contributing to improved electrical conductivity and power factor. Moreover, the introduction of sulfur and Pb suppresses the bipolar thermal conductivity by enlarging the band gap. The lattice thermal conductivity significantly decreased as low as 0.13 W m^-1 K^-1 at 873 K due to the synergic approach involving suppressing bipolar thermal conductivity, large mass fluctuation induced by sulfur incorporation, and nanoprecipitates. We demonstrate that the combination of tailoring band structure, suppressing bipolar thermal conductivity, and introducing large mass fluctuation contributes to the high thermoelectric performance in SnSe. The high performance was achieved through boosting electrical transport properties while maintaining ultralow thermal conductivity. Our findings offer a new strategy for achieving high performance thermoelectric materials.

Enhancement of Thermoelectric Properties in Pd-In Co-Doped SnTe and Its Phase Transition Behavior

SnTe has attracted more and more attention due to the similar band and crystal structure with high performance thermoelectric materials PbTe. Here, we introduced Pd into SnTe and the valence band convergence was confirmed by first-principles calculation. In the experimental process, we found that Pd-doped SnTe exhibit a reduced thermal conductivity because of softening chemical bonds and grain refining effects. To further improve the thermoelectric performance, Pd-In codoped SnTe samples were prepared, and the abnormal change of thermal conductivity was observed. The results of synchrotron powder diffraction suggest that the local phase transition (local structural distortions) near 400 K results in the first turn on thermal conductivity. Similarly, the second local phase transition in near 600 K observed by neutron powder diffraction lead to a decrease thermal conductivity of the sample. Finally, a peak thermoelectric figure of merit (ZT) ≈ 1.51 has been obtained in Sn_0.98Pd_0.025In_0.025Te at 800 K.

Manipulating Localized Vibrations of Interstitial Te for Ultra-High Thermoelectric Efficiency in p-Type Cu-In-Te Systems

Thermoelectric materials are of imperative need on account of the worldwide energy crisis. However, their efficiency is limited by the interplay of high electrical and lower thermal conductivities, that is, the figure of merit (ZT). Owing to their unique crystal structures, Cu-In-Te-based chalcogenides are suitable for both and thus have attracted much attention recently as potential thermoelectrics. Here we explore a newly developed Cu-In-Te derivative compound Cu_3.52In_4.16Te₈. With a proper adjustment of Cu₂Te doping, this material shows an ultralow lattice thermal conductivity (κ_L) (0.3 WK^-1m^-1) and, consequently, a figure of merit (ZT) as high as 1.65(±0.15) at 815 K: the highest value reported for p-type Cu-In-Te to date. The reduction in κ_L is directly related to the alteration of local symmetry around the interstitial Te, resulting in an effectively optimized phonon transport through localized "rattling" of the same. Although the Hall carrier concentration reduces upon Cu₂Te addition due to the unpinning of the Fermi level (E_Fermi) toward the conduction band minimum, the power factor remains stable. The knowledge depicted here not only demonstrates the potential of Cu_3.52In_4.16Te₈-based alloys as a promising TE, but also provides guidelines for developing further high-performance thermoelectric materials by enhancing the electronic conductivity.

Efficient DMSO-Vapor Annealing for Enhancing Thermoelectric Performance of PEDOT:PSS-Based Aerogel

Conducting polymer-based composite aerogel film is desired to be used as thermoelectric (TE) materials due to its good flexibility and ultralow thermal conductivity. Here, we proposed the simple freeze drying method to fabricate free-standing poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS)-based aerogel films without any crosslinker addition. The evolutions of morphology and TE performance were systemically investigated with various organic solvent addition. Furthermore, a series of the PEDOT:PSS/tellurium nanowires (Te-NWs) composite aerogel films was prepared, and the relationship between the structure and the charge-transport mechanism of the binary complex system was explored based on series and parallel models. Finally, an efficient dimethyl sulfoxide-vapor annealing was employed to further optimize the TE performance of PEDOT:PSS/Te-NWs composite aerogel films. The ZT value was estimated to be 2.0 × 10^-2 at room temperature. On the basis of the flexibility and highly enhanced TE performance, a prototype TE generator consisting of p-type PEDOT:PSS/Te-NWs aerogel films and n-type carbon nanotube fibers as legs has been fabricated with an acceptable output power of 1.28 μW at a temperature gradient of 60 K, which could be potentially applied in wearable electronics.

Thermoelectric Performance of Sb2Te3-Based Alloys is Improved by Introducing PN Junctions

Interface engineering has been demonstrated to be an effective strategy for enhancing the thermoelectric (TE) performance of materials. However, a very typical interface in semiconductors, that is, the PN junction (PNJ), is scarcely adopted by the thermoelectrical community because of the coexistence of holes and electrons. Interestingly, our explorative results provide a definitively positive case that appropriate PNJs are able to enhance the TE performance of p-type Sb₂Te₃-based alloys. Specifically, owing to the formation of the charge-depletion layer and built-in electric field, the carrier concentration and transport can be optimized and thus the power factor is improved and the electronic thermal conductivity is decreased. Meanwhile, PNJs provide scattering centers for phonons, leading to a reduced lattice thermal conductivity. Consequently, the p-type (Bi₂Te₃)_0.15-(Sb₂Te₃)_0.85 composites comprising PNJs achieve a ∼131% improvement of the ZT value compared with the pure Sb₂Te₃. The increased ZT demonstrates the feasibility of improving the TE properties by introducing PNJs, which will open a new and effective avenue for designing TE alloys with high performance.

The Effects of Excess Co on the Phase Composition and Thermoelectric Properties of Half-Heusler NbCoSb

NbCoSb with nominal 19 valence electrons, and is supposed to be metallic, has recently been reported to also exhibit the thermoelectric properties of a heavily doped n-type semiconductor. In this study, we prepared Co-rich NbCo_1+xSb samples (x = 0, 0.2, 0.3, 0.4, 0.5), and their phase compositions, microstructures and thermoelectric properties were investigated. The Seebeck coefficient increased a great deal with increasing x, due to decreasing carrier concentration, and the total thermal conductivity reduced mainly because of declining κ_e. Finally, a peak thermoelectric figure of merit, ZT, was about 0.46 for NbCo_1.3Sb at 973 K. This enhancement was mainly attributed to the reduction of electric thermal conductivity and the increase of Seebeck coefficient. The excess Co had effects on the carrier concentration, deformation potential E_def and DOS effective mass m^*. Adding an excessive amount of Co leads to a very high E_def, which was detrimental for transport characteristics.

Titanium Trisulfide Monolayer as a Potential Thermoelectric Material: A First-Principles-Based Boltzmann Transport Study

Good electronic transport capacity and low lattice thermal conductivity are beneficial for thermoelectric applications. In this study, the potential use as a thermoelectric material for the recently synthesized two-dimensional TiS₃ monolayer is explored by applying first-principles method combined with Boltzmann transport theory. Our work demonstrates that carrier transport in the TiS₃ sheet is orientation-dependent, caused by the difference in charge density distribution at band edges. Due to a variety of Ti-S bonds with longer lengths, we find that the TiS₃ monolayer shows thermal conductivity much lower compared with that of transition-metal dichalcogenides such as MoS₂. Combined with a high power factor along the y-direction, a considerable n-type ZT value (3.1) can be achieved at moderate carrier concentration, suggesting that the TiS₃ monolayer is a good candidate for thermoelectric applications.

Engineering Band Structure via the Site Preference of Pb(2+) in the In(+) Site for Enhanced Thermoelectric Performance of In6Se7

Although binary In-Se based alloys have in recent years gained interest as thermoelectric (TE) candidates, little attention has been paid to In6Se7-based compounds. Substituting Pb in In6Se7, preference for Pb(2+) in the In(+) site has been observed, allowing Fermi level (Fr) shift toward the conduction band, where the localized state conduction becomes dominant. Consequently, the Hall carrier concentration (nH) has been significantly enhanced with the highest nH value being about 2-3 orders of magnitude higher than that of the Pb-free sample. Meanwhile, the lattice thermal conductivity (κL) tends to be reduced as the nH value increases, owing to an increased phonon scattering on carriers. As a result, a significantly enhanced TE performance has been achieved with the highest TE figure of merit (ZT) of 0.4 at ∼850 K. This ZT value is 27 times that of intrinsic In6Se7 (ZT = 0.015 at 640 K), which proves a successful band structure engineering through site preference of Pb in In6Se7.

Upgrade Recycling of Cast Iron Scrap Chips towards β-FeSi₂ Thermoelectric Materials

The upgrade recycling of cast-iron scrap chips towards β-FeSi₂ thermoelectric materials is proposed as an eco-friendly and cost-effective production process. By using scrap waste from the machining process of cast-iron components, the material cost to fabricate β-FeSi₂ is reduced and the industrial waste is recycled. In this study, β-FeSi₂ specimens obtained from cast iron scrap chips were prepared both in the undoped form and doped with Al and Co elements. The maximum figure of merit (ZT) indicated a thermoelectric performance of approximately 70% in p-type samples and nearly 90% in n-type samples compared to β-FeSi₂ prepared from pure Fe and other published studies. The use of cast iron scrap chips to produce β-FeSi₂ shows promise as an eco-friendly and cost-effective production process for thermoelectric materials.