High Thermoelectric Efficiency of n‐type PbS

PbS shares several features with the other lead chalcogenides PbX (X: Te, Se), which are good thermoelectric materials. PbS has a potential advantage in that it is quite earth abundant and inexpensive. In this work we tune the transport properties in n‐type, single‐phase polycrystalline PbS_1‐xCl_x (x ≤ 0.008) with different carrier densities. Lead chloride provides a nearly 100% efficient doping control up to 1.2 × 10²⁰ cm^?3. The maximum zT achieved at 850 K is 0.7 with a predicted zT ～ 1 at 1000 K. This is about twice as high as what was previously reported (～0.4) for binary PbS. Compared with the other lead chalcogenides the higher effective mass and higher lattice thermal conductivity makes binary PbS an inferior thermoelectric material. However this study also predicts greater potential of zT improvement in PbS by material engineering such as alloying or nanostructuring compared to PbSe or PbTe. Considering their abundance and low cost, PbS based materials are quite competitive among the lead chalcogenides for thermoelectric applications.     

Grain Boundary Scattering of Charge Transport in n‐Type (Hf,Zr)CoSb Half‐Heusler Thermoelectric Materials

High thermoelectric figure of merit zT of ≈1.0 has been reported in both n‐ and p‐type (Hf,Zr)CoSb‐based half‐Heusler compounds, and further improvement of thermoelectric performance relies on the insightful understanding of electron and phonon transport mechanisms. In this work, the thermoelectric transport features are analyzed for (Hf_0.3Zr_0.7)_1?xNb_xCoSb (x = 0.02–0.3) with a wide range of carrier concentration. It is found that, although both temperature and energy dependencies of charge transport resemble ionized impurity scattering, the grain boundary scattering is the dominant scattering mechanism near room temperature. With increasing carrier concentration and grain size, the influence of the grain boundary scattering on electron transport weakens. The dominant scattering mechanism changes from grain boundary scattering to acoustic phonon scattering as temperature rises. The lattice thermal conductivity decreases with increasing Nb doping content due to the increased strain field fluctuations. These results provide an in‐depth understanding of the transport mechanisms and guidance for further optimizing thermoelectric properties of half‐Heusler alloys and other thermoelectric systems.     

Self‐Tuning the Carrier Concentration of PbTe/Ag2Te Composites with Excess Ag for High Thermoelectric Performance

Thermoelectric materials can be optimized by tuning the carrier concentration with chemical doping. However, because the optimum dopant concentration typically increases with temperature, the optimum efficiency can not normally be achieved for a uniform material in a temperature gradient. Here, we show Ag‐doped PbTe/Ag₂Te composites exhibit high thermoelectric performance (～50% greater than La‐doped composites) because of a temperature induced gradient in the doping concentration caused by the temperature‐dependent solubility of Ag in the PbTe matrix. This demonstrates a new mechanism to achieve a higher thermoelectric efficiency afforded by a given material system, and should be applicable to other thermoelectric materials.     

Subtle Roles of Sb and S in Regulating the Thermoelectric Properties of N‐Type PbTe to High Performance

A high ZT (thermoelectric figure of merit) of ≈1.4 at 900 K for n‐type PbTe is reported, through modifying its electrical and thermal properties by incorporating Sb and S, respectively. Sb is confirmed to be an amphoteric dopant in PbTe, filling Te vacancies at low doping levels (<1%), exceeding which it enters into Pb sites. It is found that Sb‐doped PbTe exhibits much higher carrier mobility than similar Bi‐doped materials, and accordingly, delivers higher power factors and superior ZT . The enhanced electronic transport is attributed to the elimination of Te vacancies, which appear to strongly scatter n‐type charge carriers. Building on this result, the ZT of Pb_0.9875Sb_0.0125Te is further enhanced by alloying S into the Te sublattice. The introduction of S opens the bandgap of PbTe, which suppresses bipolar conduction while simultaneously increasing the electron concentration and electrical conductivity. Furthermore, it introduces point defects and induces second phase nanostructuring, which lowers the lattice thermal conductivity to ≈0.5 W m^?1 K^?1 at 900 K, making this material a robust candidate for high‐temperature (500–900 K) thermoelectric applications. It is anticipated that the insights provided here will be an important addition to the growing arsenal of strategies for optimizing the performance of thermoelectric materials.     

Sodium‐Doped Tin Sulfide Single Crystal: A Nontoxic Earth‐Abundant Material with High Thermoelectric Performance

Lead‐free tin sulfide (SnS), with an analogous structure to SnSe, has attracted increasing attention because of its theoretically predicted high thermoelectric performance. In practice, however, polycrystalline SnS performs rather poorly as a result of its low power factor. In this work, bulk sodium (Na)‐doped SnS single crystals are synthesized using a modified Bridgman method and a detailed transport evaluation is conducted. The highest zT value of ≈1.1 is reached at 870 K in a 2 at% Na‐doped SnS single crystal along the b‐axis direction, in which high power factors (2.0 mW m^?1 K^?2 at room temperature) are realized. These high power factors are attributed to the high mobility associated with the single crystalline nature of the samples as well as to the enhanced carrier concentration achieved through Na doping. An effective single parabolic band model coupled with first‐principles calculations is used to provide theoretical insight into the electronic transport properties. This work demonstrates that SnS‐based single crystals composed of earth‐abundant, low‐cost, and nontoxic chemical elements can exhibit high thermoelectric performance and thus hold potential for application in the area of waste heat recovery.     

n‐Type SnSe2 Oriented‐Nanoplate‐Based Pellets for High Thermoelectric Performance

It is reported that electron doped n‐type SnSe₂ nanoplates show promising thermoelectric performance at medium temperatures. After simultaneous introduction of Se deficiency and Cl doping, the Fermi level of SnSe₂ shifts toward the conduction band, resulting in two orders of magnitude increase in carrier concentration and a transition to degenerate transport behavior. In addition, all‐scale hierarchical phonon scattering centers, such as point defects, nanograin boundaries, stacking faults, and the layered nanostructures, cooperate to produce very low lattice thermal conductivity. As a result, an enhanced in‐plane thermoelectric figure of merit ZT_max of 0.63 is achieved for a 1.5 at% Cl doped SnSe_1.95 pellet at 673 K, which is much higher than the corresponding in‐plane ZT of pure SnSe₂ (0.08).     

Weak Electron Phonon Coupling and Deep Level Impurity for High Thermoelectric Performance Pb1−xGaxTe

High ZT of 1.34 at 766 K and a record high average ZT above 1 in the temperature range of 300‐864 K are attained in n‐type PbTe by engineering the temperature‐dependent carrier concentration and weakening electron–phonon coupling upon Ga doping. The experimental studies and first principles band structure calculations show that doping with Ga introduces a shallow level impurity contributing extrinsic carriers and imparts a deeper impurity level that ionizes at higher temperatures. This adjusts the carrier concentration closer to the temperature‐dependent optimum and thus maximizes the power factor in a wide temperature range. The maximum power factor of 35 µW cm⁻¹ K⁻² is achieved for the Pb_0.98Ga_0.02Te compound, and is maintained over 20 µWcm⁻¹ K⁻² from 300 to 767 K. Band structure calculations and X‐ray photoelectron spectroscopy corroborate the amphoteric role of Ga in PbTe as the origin of shallow and deep levels. Additionally, Ga doping weakens the electron–phonon coupling, leading to high carrier mobilities in excess of 1200 cm² V⁻¹ s⁻¹. Enhanced point defect phonon scattering yields a reduced lattice thermal conductivity. This work provides a new avenue, beyond the conventional shallow level doping, for further improving the average ZT in thermoelectric materials.     

Eco‐Friendly Higher Manganese Silicide Thermoelectric Materials: Progress and Future Challenges

As a promising thermoelectric material, higher manganese silicides are composed of earth‐abundant and eco‐friendly elements, and have attracted extensive attention for future commercialization. In this review, the authors first summarize the crystal structure, band structure, synthesis method, and pristine thermoelectric performance of different higher manganese silicides. After that, the strategies for enhancing electrical performance and reducing lattice thermal conductivity of higher manganese silicides as well as their synergism are highlighted. The application potentials including the chemical and mechanical stability of higher manganese silicides and their energy conversion efficiency of the assembled thermoelectric modules are also summarized. By analyzing the current advances in higher manganese silicides, this review proposes that potential methods of further enhancing zT of higher manganese silicides, lie in enhancing electrical performance while simultaneously reducing lattice thermal conductivity via reducing effective mass, optimizing carrier concentration, and nanostructure engineering.     

High Thermoelectric Figure of Merit in PbTe Alloys Demonstrated in PbTe–CdTe

PbTe alloys have been the most efficient materials for thermoelectric power generation since the 1950s. In addition to the observed lattice thermal conductivity reduction by alloy scattering, recent efforts targeting electronic‐transport enhancement through band structure engineering have demonstrated a design route to achieve extraordinary thermoelectric performance. Guided by these recent results, the current work discusses the influence of alloying CdTe in p‐type PbTe where the solubility is strongly temperature‐dependent. Beneficial changes to the band structure and effective scattering of phonons are concurrently inferred when the PbTe–CdTe precipitate composite is expected to become a solid solution at high temperature, enhancing the peak thermoelectric figure of merit to ~1.7. This work emphasizes the importance of solid solution alloys of PbTe for thermoelectric power‐generation applications and also provides an upper limit of about 1 micrometer to the average distance between interfaces above which phonon scattering by composite interfaces is ineffectual.     

Effect of Dislocation Arrays at Grain Boundaries on Electronic Transport Properties of Bismuth Antimony Telluride: Unified Strategy for High Thermoelectric Performance

Taming electronic and thermal transport properties is the ultimate goal in the quest to achieve unprecedentedly high performance in thermoelectric (TE) materials. Most state‐of‐the‐art TE materials are inherently narrow bandgap semiconductors, which have an inevitable contribution from minority carriers, concurrently decreasing Seebeck coefficient and increasing thermal conductivity. Nevertheless, the restraint control of minority carrier transport is seldom considered as a key element to enhance the TE figure of merit (zT). Herein, it is verified that the localized dislocation arrays at grain boundaries enable the suppression of minority carrier contribution to electronic transport properties, resulting in an increase of the Seebeck coefficient and the carrier mobility in bismuth antimony tellurides. It is also suggested that the suppression of minority carriers via the generation of dislocation arrays at grain boundaries is an effective and noninvasive strategy to optimize overall electronic transport properties without sacrificing predominant characteristics of majority carriers in TE materials.     

Heterogeneous Distribution of Sodium for High Thermoelectric Performance of p‐type Multiphase Lead‐Chalcogenides

Despite the effectiveness of sodium as a p‐type dopant for lead chalcogenides, its solubility is shown to be very limited in these hosts. Here, a high thermoelectric efficiency of ≈2 over a wide temperature range is reported in multiphase quaternary (PbTe)_0.65(PbS)_0.25(PbSe)_0.1 compounds that are doped with sodium at concentrations greater than the solubility limits of the matrix. Although these compounds present room temperature thermoelectric efficiencies similar to sodium doped PbTe, a dramatically enhanced Hall carrier mobility at temperatures above 600 K for heavily doped compounds results in significantly enhanced thermoelectric efficiencies at elevated temperatures. This is achieved through the composition modulation doping mechanism resulting from heterogeneous distribution of the sodium dopant between precipitates and the matrix at elevated temperatures. These results can lead to further advances in designing high performance multiphase thermoelectric materials with intrinsically heterogeneous dopant distributions.     

High Band Degeneracy Contributes to High Thermoelectric Performance in p‐Type Half‐Heusler Compounds

Half‐Heusler (HH) compounds are important high temperature thermoelectric (TE) materials and have attracted considerable attention in the recent years. High figure of merit zT values of 0.8 to 1.0 have been obtained in n‐type ZrNiSn‐based HH compounds. However, developing high performance p‐type HH compounds are still a big challenge. Here, it is shown that a new p‐type HH alloy with a high band degeneracy of 8, Ti‐doped FeV_0.6Nb_0.4Sb, can achieve a high zT of 0.8, which is one of the highest reported values in the p‐type HH compounds. Although the band effective mass of this system is found to be high, which may lead to a low mobility, its low deformation potential and low alloy scattering potential both contribute to a reasonably high mobility. The enhanced phonon scattering by alloying leads to a reduced lattice thermal conductivity. The achieved high zT demonstrates that the p‐type Ti doped FeV_0.6Nb_0.4Sb HH alloys are promising as TE materials and offer an excellent TE performance match with n‐type ones for high temperature power generation.     

Unique Role of Refractory Ta Alloying in Enhancing the Figure of Merit of NbFeSb Thermoelectric Materials

NbFeSb‐based half‐Heusler alloys have been recently identified as promising high‐temperature thermoelectric materials with a figure of merit zT > 1, but their thermal conductivity is still relatively high. Alloying Ta at the Nb site would be highly desirable because the large mass fluctuation between them could effectively scatter phonons and reduce the lattice thermal conductivity. However, practically it is a great challenge due to the high melting point of refractory Ta. Here, the successful synthesis of Ta‐alloyed (Nb_1?xTa_x)_0.8Ti_0.2FeSb (x = 0 – 0.4) solid solutions with significantly reduced thermal conductivity by levitation melting is reported. Because of the similar atomic sizes and chemistry of Nb and Ta, the solid solutions exhibit almost unaltered electrical properties. As a result, an overall zT enhancement from 300 to 1200 K is realized in the single‐phase Ta‐alloyed solid solutions, and the compounds with x = 0.36 and 0.4 reach a maximum zT of 1.6 at 1200 K. This work also highlights that the isoelectronic substitution by atoms with similar size and chemical nature but large mass difference should reduce the lattice thermal conductivity but maintain good electrical properties in thermoelectric materials, which can be a guide for optimizing the figure of merit by alloying.     

Enhancement of Thermoelectric Figure of Merit by the Insertion of MgTe Nanostructures in p‐type PbTe Doped with Na2Te

The thermoelectric properties of crystalline melt‐grown ingots of p‐type PbTe–xMgTe (x = 1–3 mol%) doped with Na₂Te (1–2 mol%) were investigated over the temperature range of 300 K to 810 K. While the powder X‐ray diffraction patterns show that all samples crystallize in the NaCl‐type structure with no MgTe or other phases present, transmission electron microscopy reveals ubiquitous MgTe nanoprecipitates in the PbTe. The very small amounts of MgTe in PbTe have only a small effect on the electrical transport properties of the system, while they have a large effect on thermal transport significantly reducing the lattice thermal conductivity. A ZT of 1.6 at 780 K is achieved for the PbTe containing 2% MgTe doped with 2% Na₂Te.     

Low Electron Scattering Potentials in High Performance Mg2Si0.45Sn0.55 Based Thermoelectric Solid Solutions with Band Convergence

Understanding the electron and phonon transport characteristics is crucial for designing and developing high performance thermoelectric materials. Weak scattering effects on charge carriers, characterized by deformation potential and alloy scattering potential, are favorable for thermoelectric solid solutions to enable high carrier mobility and thereby promising thermoelectric performance. Mg₂(Si,Sn) solid solutions have attracted much attention due to their low cost and environmental compatibility. Usually, their high thermoelectric performance with ZT ～ 1 is ascribed to the band convergence and reduced lattice thermal conductivity caused by alloying. In this work, both a low deformation potential Ξ = 13 eV and a low alloy scattering potential U = 0.7 eV are found for the thermoelectric alloys by characterizing and modeling of thermoelectric transport properties. The band convergence is also verified by the increased density‐of‐states effective mass. It is proposed that, in addition to band convergence and reduced lattice thermal conductivity, the low deformation potential and alloy scattering potential are additional intrinsic features that contribute to the high thermoelectric performance of the solid solutions.     

High Efficiency Half‐Heusler Thermoelectric Materials for Energy Harvesting

Half‐Heusler (HH) compounds have gained ever‐increasing popularity as promising high temperature thermoelectric materials. High figure of merit zT of ≈1.0 above 1000 K has recently been realized for both n‐type and p‐type HH compounds, demonstrating the realistic prospect of these high temperature compounds for high efficiency power generation. Here, recent progress in advanced fabrication techniques and the intrinsic atomic disorders in HH compounds, which are linked to the understanding of the electrical transport, is discussed. Thermoelectric transport features of n‐type ZrNiSn‐based HH alloys are particularly emphasized, which is beneficial to further improving thermoelectric performance and comprehensively understanding the underlying mechanisms in HH thermoelectric materials. The rational design and realization of new high performance p‐type Fe(V,Nb)Sb‐based HH compounds are also demonstrated. The outlook for future research directions of HH thermoelectric materials is also discussed.     

High‐Performance GeTe‐Based Thermoelectrics: from Materials to Devices

High‐performance GeTe‐based thermoelectrics have been recently attracting growing research interest. Here, an overview is presented of the structural and electronic band characteristics of GeTe. Intrinsically, compared to low‐temperature rhombohedral GeTe, the high‐symmetry and high‐temperature cubic GeTe has a low energy offset between L and Σ points of the valence band, the reduced direct bandgap and phonon group velocity, and as a result, high thermoelectric performance. Moreover, their thermoelectric performance can be effectively enhanced through either carrier concentration optimization, band structure engineering (bandgap reduction, band degeneracy, and resonant state engineering), or restrained lattice thermal conductivity (phonon velocity reduction or phonon scattering). Consequently, the dimensionless figure of merit, ZT values, of GeTe‐based thermoelectric materials can be higher than 2. The mechanical and thermal stabilities of GeTe‐based thermoelectrics are highlighted, and it is found that they are suitable for practical thermoelectric applications except for their high cost. Finally, it is recognized that the performance of GeTe‐based materials can be further enhanced through synergistic effects. Additionally, proper material selection and module design can further boost the energy conversion efficiency of GeTe‐based thermoelectrics.     

Engineering Temperature‐Dependent Carrier Concentration in Bulk Composite Materials via Temperature‐Dependent Fermi Level Offset

Precise control of carrier concentration in both bulk and thin‐film materials is crucial for many solid‐state devices, including photovoltaic cells, superconductors, and high mobility transistors. For applications that span a wide temperature range (thermoelectric power generation being a prime example) the optimal carrier concentration varies as a function of temperature. This work presents a modified modulation doping method to engineer the temperature dependence of the carrier concentration by incorporating a nanosize secondary phase that controls the temperature‐dependent doping in the bulk matrix. This study demonstrates this technique by de‐doping the heavily defect‐doped degenerate semiconductor GeTe, thereby enhancing its average power factor by 100% at low temperatures, with no deterioration at high temperatures. This can be a general method to improve the average thermoelectric performance of many other materials.     

Crystal Structure Induced Ultralow Lattice Thermal Conductivity in Thermoelectric Ag9AlSe6

Recent discoveries of novel thermoelectric materials largely rely on an intrinsic low lattice thermal conductivity. This results from various mechanisms including low sound velocity, complex crystal structure, liquid‐like ions, and lattice anharmonicity. Here semiconducting Ag₉AlSe₆ with many weakly bonded and highly disordered cations is shown to be a promising novel thermoelectric material, due to its ultralow lattice thermal conductivity (κ_L) of ≈0.3 W m^?1 K^?1 in the entire temperature range. Such a low κ_L is believed to be a result of its (1) complex crystal structure for a small population of acoustic phonons, (2) soft bonding for an overall low sound velocity (1300 m s^?1), and (3) massive disordering of Ag ions. Its electronic transport properties can be well understood by a single parabolic band model with acoustic scattering. The achieved thermoelectric figure of merit (zT) can be as high as unity, which is unlike conventional thermoelectric materials, which rely heavily on a high power factor. This work not only demonstrates Ag₉AlSe₆ as a promising thermoelectric material, but also paves the way for the exploration of novel thermoelectrics with a complex crystal structure with weakly bonded and highly disordered constituent elements in the structure.     

Improving Thermoelectric Performance of α‐MgAgSb by Theoretical Band Engineering Design

α‐MgAgSb is recently discovered to be a new class of thermoelectric material near room temperature. A competitive ZT of 1.4 at 525 K is achieved in Ni‐doped α‐MgAgSb, and the measured efficiency of energy conversion reaches a record value of 8.5%, which is even higher than that of the commercially applied material bismuth telluride. On the other hand, the band structure of α‐MgAgSb is believed to be unprofitable to the power factor, owing to the less degenerate valence valleys. Here, this paper reports a systematic theoretical study on the thermoelectric properties by using the electron/phonon structure and transport calculations. Based on the careful analysis of Fermi surface, a principled scheme is presented to design band engineering in α‐MgAgSb. Following the given rules, several effective dopants are predicted. As two examples, Zn‐ and Pd‐doped α‐MgAgSb are numerically confirmed to exhibit an extraordinary ZT value of 2.0 at 575 K and a high conversion efficiency of 12.6%, owing to the effects of band convergence. This work develops an applicable scheme for the purposive design of band engineering, and the idea can be simply applied to more thermoelectric materials.