Lattice Softening and Band Convergence in GeTe-Based Alloys for High Thermoelectric Performance

GeTe-based alloys have been studied as promising TE materials in the midtemperature range as a lead-free alternate to PbTe due to their nontoxicity. Our previous study on GeTe1-xIx revealed that I-doping increases lattice anharmonicity and decreases the structural phase transition temperature, consequently enhancing the thermoelectric performance. Our current work elucidates the synergistic interplay between band convergence and lattice softening, resulting in an enhanced thermoelectric performance for Ge1-ySbyTe0.9I0.1 (y = 0.10, 0.12, 0.14, and 0.16). Sb doping in GeTe0.9I0.1 serves a double role: first, it leads to lattice softening, thereby reducing lattice thermal conductivity; second, it promotes a band convergence, thus a higher valley degeneracy. The presence of lattice softening is corroborated by an increase in the internal strain ratio observed in X-ray diffraction patterns. Doping also introduces phonon scattering centers, further diminishing lattice thermal conductivity. Additionally, variations in the electronic band structure are indicated by an increase in density of state effective mass and a decrease in carrier mobility with Sb concentration. Besides, Sb doping optimizes the carrier concentration efficiently. Through a two-band modeling and electronic band structure calculations, the valence band convergence due to Sb doping can be confirmed. Specifically, the energy difference between valence bands progressively narrows upon Sb doping in Ge1-ySbyTe0.9I0.1 (y = 0, 0.02, 0.05, 0.10, 0.12, 0.14, and 0.16). As a culmination of these effects, we have achieved a significant enhancement in zT for Ge1-ySbyTe0.9I0.1 (y = 0.10, 0.12, 0.14, and 0.16) across the entire range of measured temperatures. Notably, the sample with y = 0.12 exhibits the highest zT value of 1.70 at 723 K.

Achieving High Isotropic Figure of Merit in Cd and in Codoped Polycrystalline SnSe

Here, we combined Cd and In codoping with a simple hydrothermal synthesis method to prepare SnSe powders composed of nanorod-like flowers. After spark plasma sintering, its internal grains inherited well the morphological features of the precursor, and the multiscale microstructure included nanorod-shaped grains, high-density dislocations, and stacking faults, as well as abundant nanoprecipitates, resulting in an ultralow thermal conductivity of 0.15 W m-1 K-1 for the synthesized material. At the same time, Cd and In synergistically regulated the electrical conductivity and Seebeck coefficient of SnSe, leading to an enhanced power factor. Among them, Sn0.94Cd0.03In0.03Se achieved a peak ZT of 1.50 parallel to the pressing direction, representing an 87.5% improvement compared with pure SnSe. Notably, the material possesses isotropic ZT values parallel and perpendicular to the pressing direction, overcoming the characteristic anisotropy in thermal performance observed in previous polycrystalline SnSe-based materials. Our results provide a new strategy for optimizing the performance of thermoelectric materials through structural engineering.

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

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

Effects of isotropic strain on the structure and transport properties of half-Heusler alloy BiBaK: a first-principles investigation

In this study, using density functional and Boltzmann transport theories, we systematically investigated the effects of tensile and compressive strains on the elastic properties, phonon dispersion relation, electronic structure, and transport properties of the half-Heusler compound BiBaK. We calculated the elastic constants and phonon dispersion curves for BiBaK, which demonstrated its mechanical and thermodynamic stability, respectively, under different isotropic strains. Further, calculations showed that the electronic structure and energy bandgap of BiBaK changed with the application of isotropic strain. A high power factor and low thermal conductivity are key to improving the performance of thermoelectric materials. The figure of merit of BiBaK is 0.6 when it is unstrained and reaches a maximum value of 0.93 at -9% compressive strain and a temperature of 1200 K, indicating that under isotropic compressive strain, BiBaK compounds are efficient thermoelectric materials for high-temperature applications.

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

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

Advances in Versatile GeTe Thermoelectrics from Materials to Devices

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

Synergistically Optimized Carrier and Phonon Transport Properties in Bi-Cu2S Coalloyed GeTe

GeTe is an emerging lead-free thermoelectric material, but its excessive carrier concentration and high thermal conductivity severely restrict the enhancement of thermoelectric properties. In this study, the synergistically optimized thermoelectric properties of p-type GeTe through Bi-Cu₂S coalloying are reported. It can be found that the donor behavior of Bi and the substitution-interstitial defect pairs of Cu⁺ ions effectively reduce the hole concentration to an optimal level with carrier mobility less affected. At the same time, Bi-Cu₂S coalloying induces many phonon scattering centers involving stacking faults, nanoprecipitations, grain boundaries and tetrahedral dislocations and suppresses the lattice thermal conductivity to 0.64 W m^-1 K^-1. Consequently, all effects synergistically yield a peak ZT of 1.9 at 770 K with a theoretical conversion efficiency of 14.5% (300-770 K) in the (Ge_0.94Bi_0.06Te)_0.988(Cu₂S)_0.012 sample, which is very promising for mid-low temperature range waste heat harvest.

Synergistic Effect of Work Function and Acoustic Impedance Mismatch for Improved Thermoelectric Performance in GeTe-WC Composite

The preparation of composite materials is a promising methodology for concurrent optimization of electrical and thermal transport properties for improved thermoelectric (TE) performance. This study demonstrates how the acoustic impedance mismatch (AIM) and the work function of components decouple the TE parameters to achieve enhanced TE performance of the (1-z)Ge0.87Mn0.05Sb0.08Te-(z)WC composite. The simultaneous increase in the electrical conductivity (σ) and Seebeck coefficient (α) with WC (tungsten carbide) volume fraction (z) results in an enhanced power factor (α2σ) in the composite. The rise in σ is attributed to the creation of favorable current paths through the WC phase located between grains of Ge0.87Mn0.05Sb0.08Te, which leads to increased carrier mobility in the composite. Detailed analysis of the obtained electrical properties was performed via Kelvin probe force microscopy (work function measurement) and atomic force microscopy techniques (spatial current distribution map and current-voltage (I-V) characteristics), which are further supported by density functional theory (DFT) calculations. Furthermore, the difference in elastic properties (i.e., sound velocity) between Ge0.87Mn0.05Sb0.08Te and WC results in a high AIM, and hence, a large interface thermal resistance (Rint) between the phases is achieved. The correlation between Rint and the Kapitza radius depicts a reduced phonon thermal conductivity (κph) of the composite, which is explained using the Bruggeman asymmetrical model. Moreover, the decrease in κph is further validated by phonon dispersion calculations that indicate the decrease in phonon group velocity in the composite. The simultaneous effect of enhanced α2σ and reduced κph results in a maximum figure of merit (zT) of 1.93 at 773 K for (1-z)Ge0.87Mn0.05Sb0.08Te-(z)WC composite for z = 0.010. It results in an average thermoelectric figure of merit (zTav) of 1.02 for a temperature difference (ΔT) of 473 K. This study shows promise to achieve higher zTav across a wide range of composite materials.

Achieving High-Performance Ge0.92 Bi0.08 Te Thermoelectrics via LaB6 -Alloying-Induced Band Engineering and Multi-Scale Structure Manipulation

In this work, a LaB₆ -alloying strategy is reported to effectively boost the figure-of-merit (ZT) of Ge_0.92 Bi_0.08 Te-based alloys up to ≈2.2 at 723 K, attributed to a synergy of La-dopant induced band structuring and structural manipulation. Density-function-theory calculations reveal that La dopant enlarges the bandgap and converges the energy offset between the sub-valence bands in cubic-structured GeTe, leading to a significantly increased effective mass, which gives rise to a high Seebeck coefficient of ≈263 µV K^-1 and in turn a superior power factor of ≈43 µW cm^-1 K^-2 at 723 K. Besides, comprehensive electron microscopy characterizations reveal that the multi-scale phonon scattering centers, including a high density of planar defects, Boron nanoparticles in tandem with enhanced boundaries, dispersive Ge nanoprecipitates in the matrix, and massive point defects, contribute to a low lattice thermal conductivity of ≈0.67 W m^-1 K^-1 at 723 K. Furthermore, a high microhardness of ≈194 H_v is witnessed in the as-designed Ge_0.92 Bi_0.08 Te(LaB₆ )_0.04 alloy, derived from the multi-defect-induced strengthening. This work provides a strategy for developing high-performance and mechanical robust middle-temperature thermoelectric materials for practical thermoelectric applications.

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

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

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

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

Low-Pressure CVD of GeE (E = Te, Se, S) Thin Films from Alkylgermanium Chalcogenolate Precursors and Effect of Deposition Temperature on the Thermoelectric Performance of GeTe

The homologous series [GeⁿBu₃(EⁿBu)] (E = Te, Se, S; (1), (3) and (4)) and [GeⁿBu₂(TeⁿBu)₂] (2) have been synthesized as mobile oils in excellent yield (72-93%) and evaluated as single-source precursors for the low-pressure chemical vapor deposition (LPCVD) of GeE thin films on silica. Compositional and structural characterizations of the deposits have been performed by grazing-incidence X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray analysis, and Raman spectroscopy, confirming the phase purity and stoichiometry. Electrical characterization via variable-temperature Hall effect measurements is also reported. Given the strong interest in GeTe and its alloys for thermoelectric applications, variable-temperature Seebeck data were also investigated for a series of p-type GeTe films. The data show that it is possible to tune the thermoelectric response through intrinsic Ge vacancy regulation by varying the deposition temperature, with the highest power factor (40 μW/K²cm@629 K) and effective ZT values observed for the films deposited at higher temperatures.

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

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

Realizing a 14% single-leg thermoelectric efficiency in GeTe alloys

GeTe alloys have recently attracted wide attention as efficient thermoelectrics. In this work, a single-leg thermoelectric device with a conversion efficiency as high as 14% under a temperature gradient of 440 K was fabricated on the basis of GeTe-Cu2Te-PbSe alloys, which show a peak thermoelectric figure of merit (zT) > 2.5 and an average zT of 1.8 within working temperatures. The high performance of the material is electronically attributed to the carrier concentration optimization and thermally due to the strengthened phonon scattering, the effects of which all originate from the defects in the alloys. A design of Ag/SnTe/GeTe contact successfully enables both a prevention of chemical diffusion and an interfacial contact resistivity of 8 microhm·cm2 for the realization of highly efficient devices with a good service stability/durability. Not only the material's high performance but also the device's high efficiency demonstrated the extraordinariness of GeTe alloys for efficient thermoelectric waste-heat recovery.

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

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

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

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

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

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

Semiconductor glass with superior flexibility and high room temperature thermoelectric performance

Most crystalline inorganic materials, except for metals and some layer materials, exhibit bad flexibility because of strong ionic or covalent bonds, while amorphous materials usually display poor electrical properties due to structural disorders. Here, we report the simultaneous realization of extraordinary room temperature flexibility and thermoelectric performance in Ag₂Te_1-x S _x -based materials through amorphization. The coexistence of amorphous main phase and crystallites results in exceptional flexibility and ultralow lattice thermal conductivity. Furthermore, the flexible Ag₂Te_0.6S_0.4 glass exhibits a degenerate semiconductor behavior with a room temperature Hall mobility of ~750 cm² V^-1 s^-1 at a carrier concentration of 8.6 × 10¹⁸ cm^-3, which is at least an order of magnitude higher than other amorphous materials, leading to a thermoelectric power factor also an order of magnitude higher than the best amorphous thermoelectric materials known. The in-plane prototype uni-leg thermoelectric generator made from this material demonstrates its potential for flexible thermoelectric device.

Is LiI a Potential Dopant Candidate to Enhance the Thermoelectric Performance in Sb-Free GeTe Systems? A Prelusive Study

As a workable substitute for toxic PbTe-based thermoelectrics, GeTe-based materials are emanating as reliable alternatives. To assess the suitability of LiI as a dopant in thermoelectric GeTe, a prelusive study of thermoelectric properties of GeTe1−xLiIx (x = 0–0.02) alloys processed by Spark Plasma Sintering (SPS) are presented in this short communication. A maximum thermoelectric figure of merit, zT ~ 1.2, was attained at 773 K for 2 mol% LiI-doped GeTe composition, thanks to the combined benefits of a noted reduction in the thermal conductivity and a marginally improved power factor. The scattering of heat carrying phonons due to the presumable formation of Li-induced “pseudo-vacancies” and nano-precipitates contributed to the conspicuous suppression of lattice thermal conductivity, and consequently boosted the zT of the Sb-free (GeTe)0.98(LiI)0.02 sample when compared to that of pristine GeTe and Sb-rich (GeTe)x(LiSbTe2)2 compounds that were reported earlier.

Promising and Eco-Friendly Cu2 X-Based Thermoelectric Materials: Progress and Applications

Due to the nature of their liquid-like behavior and high dimensionless figure of merit, Cu₂ X (X = Te, Se, and S)-based thermoelectric materials have attracted extensive attention. The superionicity and Cu disorder at the high temperature can dramatically affect the electronic structure of Cu₂ X and in turn result in temperature-dependent carrier-transport properties. Here, the effective strategies in enhancing the thermoelectric performance of Cu₂ X-based thermoelectric materials are summarized, in which the proper optimization of carrier concentration and minimization of the lattice thermal conductivity are the main focus. Then, the stabilities, mechanical properties, and module assembly of Cu₂ X-based thermoelectric materials are investigated. Finally, the future directions for further improving the energy conversion efficiency of Cu₂ X-based thermoelectric materials are highlighted.

Realization of Both n- and p-Type GeTe Thermoelectrics: Electronic Structure Modulation by AgBiSe2 Alloying

Successful applications of a thermoelectric material require simultaneous development of compatible n- and p-type counterparts. While the thermoelectric performance of p-type GeTe has been improved tremendously in recent years, it has been a challenge to find a compatible n-type GeTe counterpart due to the prevalence of intrinsic Ge vacancies. Herein, we have shown that alloying of AgBiSe₂ with GeTe results in an intriguing evolution in its crystal and electronic structures, resulting in n-type thermoelectric properties. We have demonstrated that the ambient rhombohedral structure of pristine GeTe transforms into cubic phase in (GeTe)_100-x(AgBiSe₂)_x for x ≥ 25, with concurrent change from its p-type electronic character to n-type character in electronic transport properties. Such change in structural and electronic properties is confirmed from the nonmonotonic variation of band gap, unit cell volume, electrical conductivity, and Seebeck coefficient, all of which show an inflection point around x ∼ 20, as well as from the temperature variations of synchrotron powder X-ray diffractions and differential scanning calorimetry. First-principles density functional theoretical (DFT) calculations explain that the shift toward n-type electronic character with increasing AgBiSe₂ concentration arises due to increasing contribution of Bi p orbitals in the conduction band edge of (GeTe)_100-x(AgBiSe₂)_x. This cubic n-type phase has promising thermoelectric properties with a band gap of ∼0.25 eV and ultralow lattice thermal conductivity that ranges between 0.3 and 0.6 W/mK. Further, we have shown that (GeTe)_100-x(AgBiSe₂)_x has promising thermoelectric performance in the mid-temperature range (400-500 K) with maximum thermoelectric figure of merit, zT, reaching ∼1.3 in p-type (GeTe)₈₀(AgBiSe₂)₂₀ at 467 K and ∼0.6 in n-type (GeTe)₅₀(AgBiSe₂)₅₀ at 500 K.

Enhancing thermoelectric performance by Fermi level tuning and thermal conductivity degradation in (Ge1-xBix)Te crystals

In this work, a high thermoelectric figure of merit, zT of 1.9 at 740 K is achieved in Ge_1−xBi_xTe crystals through the concurrent of Seebeck coefficient enhancement and thermal conductivity reduction with Bi dopants. The substitution of Bi for Ge not only compensates the superfluous hole carriers in pristine GeTe but also shifts the Fermi level (E_F) to an eligible region. Experimentally, with moderate 6–10% Bi dopants, the carrier concentration is drastically decreased from 8.7 × 10²⁰ cm⁻³ to 3–5 × 10²⁰ cm⁻³ and the Seebeck coefficient is boosted three times to 75 μVK⁻¹. In the meantime, based on the density functional theory (DFT) calculation, the Fermi level E_F starts to intersect with the pudding mold band at L point, where the band effective mass is enhanced. The enhanced Seebeck coefficient effectively compensates the decrease of electrical conductivity and thus successfully maintain the power factor as large as or even superior than that of the pristine GeTe. In addition, the Bi doping significantly reduces both thermal conductivities of carriers and lattices to an extremely low limit of 1.57 W m⁻¹K⁻¹ at 740 K with 10% Bi dopants, which is an about 63% reduction as compared with that of pristine GeTe. The elevated figure of merit observed in Ge_1−xBi_xTe specimens is therefore realized by synergistically optimizing the power factor and downgrading the thermal conductivity of alloying effect and lattice anharmonicity caused by Bi doping.

Stacking Fault-Induced Minimized Lattice Thermal Conductivity in the High-Performance GeTe-Based Thermoelectric Materials upon Bi2Te3 Alloying

Materials with low lattice thermal conductivity (κ_lat) are crucial for the applications of thermal insulation and thermoelectric (TE) energy conversion. Stacking fault (SF)-induced phonon scattering within interfaces has been put forward theoretically by Klemens in 1950s. However, unlike other traditional defects such as point defects, grain boundaries, and dislocations, the role of SF for reducing κ_lat remains poorly understood and is yet to be revealed experimentally. The layered Bi₂Te₃ with a van der Waals gap shows different stacking structures than the nonlayered GeTe, which is used to introduce SFs into the GeTe-based alloys in this work. On the basis of the experimental and theoretical modeling results, this paper reveals the significant contribution of SF phonon scattering for minimizing the κ_lat. Besides the achieved extremely low κ_lat (∼0.39 W m^-1 K^-1 at 573 K), optimized carrier density and band convergence are also realized in the GeTe-based alloys upon Bi₂Te₃ alloying, leading to a significant high TE figure of merit ZT > 2 at 773 K and an averaged ZT > 1.4 within 300-773 K. This SF engineering strategy provides a different avenue to reduce the κ_lat for enhancing the performance of thermal insulation and TE materials.

Flexible Organic Thermoelectric Materials and Devices for Wearable Green Energy Harvesting

In the past few decades, organic thermoelectric materials/devices, which can exhibit remarkable potential in green energy conversion, have drawn great attention and interest due to their easy processing, light weight, intrinsically low thermal conductivity, and mechanical flexibility. Compared to traditional batteries, thermoelectric materials have high prospects as alternative power generators for harvesting green energy. Although crystalline inorganic semiconductors have dominated the fields of thermoelectric materials up to now, their practical applications are limited by their intrinsic fragility and high toxicity. The integration of organic polymers with inorganic nanoparticles has been widely employed to tailor the thermoelectric performance of polymers, which not only can combine the advantages of both components but also display interesting transport phenomena between organic polymers and inorganic nanoparticles. In this review, parameters affecting the thermoelectric properties of materials were briefly introduced. Some recently developed n-type and p-type thermoelectric films and related devices were illustrated along with their thermoelectric performance, methods of preparation, and future applications. This review will help beginners to quickly understand and master basic knowledge of thermoelectric materials, thus inspiring them to design and develop more efficient thermoelectric devices.

Thermoelectric GeTe with Diverse Degrees of Freedom Having Secured Superhigh Performance

Driven by the ability to harvest waste heat into reusable electricity and the exclusive role of serving as the power generator for deep spacecraft, intensive endeavors are dedicated to enhancing the thermoelectric performance of ecofriendly materials. Herein, the most recent progress in superhigh-performance GeTe-based thermoelectric materials is reviewed with a focus on the crystal structures, phase transitions, resonant bondings, multiple valance bands, and phonon dispersions. These features diversify the degrees of freedom to tune the transport properties of electrons and phonons for GeTe. On the basis of the optimized carrier concentration, strategies of alignment of multiple valence bands and density-of-state resonant distortion are employed to further enhance the thermoelectric performance of GeTe-based materials. To decrease the thermal conductivity, methods of strengthening intrinsic phonon-phonon interactions and introducing various lattice imperfections as scattering centers are highlighted. An overview of thermoelectric devices assembled from GeTe-based thermoelectric materials is then presented. In conclusion, possible future directions for developing GeTe in thermoelectric applications are proposed. The achieved high thermoelectric performance in GeTe-based thermoelectric materials with rationally established strategies can act as a reference for broader materials to tailor their thermoelectric performance.

Recent Advances in Organic Thermoelectric Materials: Principle Mechanisms and Emerging Carbon-Based Green Energy Materials

Thermoelectric devices have recently attracted considerable interest owing to their unique ability of converting heat to electrical energy in an environmentally efficient manner. These devices are promising as alternative power generators for harvesting electrical energy compared to conventional batteries. Inorganic crystalline semiconductors have dominated the thermoelectric material fields; however, their application has been restricted by their intrinsic high toxicity, fragility, and high cost. In contrast, organic thermoelectric materials with low cost, low thermal conductivity, easy processing, and good flexibility are more suitable for fabricating thermoelectric devices. In this review, we briefly introduce the parameters affecting the thermoelectric performance and summarize the most recently developed carbon-material-based organic thermoelectric composites along with their preparation technologies, thermoelectric performance, and future applications. In addition, the p- and n-type carbon nanotube conversion and existing challenges are discussed. This review can help researchers in elucidating the recent studies on carbon-based organic thermoelectric materials, thus inspiring them to develop more efficient thermoelectric devices.

Realizing a stable high thermoelectric zT ∼ 2 over a broad temperature range in Ge1−x−yGaxSbyTe via band engineering and hybrid flash-SPS processing

We report a remarkably high and stable thermoelectric zT ∼ 2 by manipulating the electronic bands in hybrid flash-SPSed Ga–Sb codoped GeTe.

Boosting the Thermoelectric Performance of Pseudo-Layered Sb2Te3(GeTe) n via Vacancy Engineering

An ultrahigh figure of merit ZT value ≈2.4 at 773 K for p-type pseudo-layered Sb2Te3(GeTe)17 along the parallel direction is reported by synergistically optimizing its electrical and thermal properties via vacancy engineering. The microstructural origin of thermoelectric property enhancement is studied by spherical aberration corrected transmission electron microscopy and its in situ mode. The results reveal that upon annealing, Ge vacancy gaps in quenched samples tend to migrate and recombine into long-range gaps in order to minimize the elastic and electrostatic energies. The recombination of Ge gaps would lead to an overall reduction of carrier concentration and electrical thermal conductivity. The detailed study of Ge vacancies migration via heat treatment and its effects on thermoelectric performance in pseudo-layered Sb2Te3(GeTe)17 materials can provide enlightening clues for future research in a number of thermoelectric materials of similar structures.

Detrimental Effects of Doping Al and Ba on the Thermoelectric Performance of GeTe

GeTe-based materials are emerging as viable alternatives to toxic PbTe-based thermoelectric materials. In order to evaluate the suitability of Al as dopant in thermoelectric GeTe, a systematic study of thermoelectric properties of Ge_1−xAl_xTe (x = 0–0.08) alloys processed by Spark Plasma Sintering are presented here. Being isoelectronic to Ge_1−xIn_xTe and Ge_1−xGa_xTe, which were reported with improved thermoelectric performances in the past, the Ge_1−xAl_xTe system is particularly focused (studied both experimentally and theoretically). Our results indicate that doping of Al to GeTe causes multiple effects: (i) increase in p-type charge carrier concentration; (ii) decrease in carrier mobility; (iii) reduction in thermopower and power factor; and (iv) suppression of thermal conductivity only at room temperature and not much significant change at higher temperature. First principles calculations reveal that Al-doping increases the energy separation between the two valence bands (loss of band convergence) in GeTe. These factors contribute for Ge_1−xAl_xTe to exhibit a reduced thermoelectric figure of merit, unlike its In and Ga congeners. Additionally, divalent Ba-doping [Ge_1−xBa_xTe (x = 0–0.06)] is also studied.

Realization of non-equilibrium process for high thermoelectric performance Sb-doped GeTe

Pristine GeTe shows inferior thermoelectric performance around unit due to the large carrier concentration induced by the presence of intrinsic high concentration of Ge vacancy. In this study, we report a thermoelectric figure of merit ZT of 1.56 at 700 K, realized in Sb-doped GeTe based thermoelectric (TE) materials via combined effect of suppression of intrinsic Ge vacancy and Sb doping. The non-equilibrium nature during melt spinning process plays very important role. For one thing, it promotes the homogeneity in Ge_1-xSb_xTe samples and refines the grain size of the product. Moreover the persistent Ge precipitated as impurity phase in the traditional synthesis process is found to be dissolved back into the GeTe sublattice, accompanying with a drastic suppression of Ge vacancies concentration which in combination with Sb electron doping significantly reduced the inherent carrier concentration in GeTe. Low carrier concentration, approaching the optimum carrier concentration ∼3.74 × 10^-20 cm^-3 and a high power factor of 4.01 × 10^-3 W m^-1 K^-2 at 750 K are achieved for Ge_0.98Sb_0.02Te sample. In addition, the enhanced grain boundary phonon scattering by refining the grain size through melt spinning (MS) process, coupled with the intensified alloying phonon scattering via Sb doping leads to low thermal conductivity of 1.53 W m^-1 K^-1 at 700 K for Ge_0.94Sb_0.06Te sample. All those contribute to a high ZT value, representing over 50% improvement in the ZT value compared to the Sb free samples, which provides an alternative way for ultrafast synthesis of high performance GeTe based thermoelectric material.

Rhombohedral to Cubic Conversion of GeTe via MnTe Alloying Leads to Ultralow Thermal Conductivity, Electronic Band Convergence, and High Thermoelectric Performance

In this study, a series of Ge_1-xMn_xTe (x = 0-0.21) compounds were prepared by a melting-quenching-annealing process combined with spark plasma sintering (SPS). The effect of alloying MnTe into GeTe on the structure and thermoelectric properties of Ge_1-xMn_xTe is profound. With increasing content of MnTe, the structure of the Ge_1-xMn_xTe compounds gradually changes from rhombohedral to cubic, and the known R3m to Fm-3m phase transition temperature of GeTe moves from 700 K closer to room temperature. First-principles density functional theory calculations show that alloying MnTe into GeTe decreases the energy difference between the light and heavy valence bands in both the R3m and Fm-3m structures, enhancing a multiband character of the valence band edge that increases the hole carrier effective mass. The effect of this band convergence is a significant enhancement in the carrier effective mass from 1.44 m₀ (GeTe) to 6.15 m₀ (Ge_0.85Mn_0.15Te). In addition, alloying with MnTe decreases the phonon relaxation time by enhancing alloy scattering, reduces the phonon velocity, and increases Ge vacancies all of which result in an ultralow lattice thermal conductivity of 0.13 W m^-1 K^-1 at 823 K. Subsequent doping of the Ge_0.9Mn_0.1Te compositions with Sb lowers the typical very high hole carrier concentration and brings it closer to its optimal value enhancing the power factor, which combined with the ultralow thermal conductivity yields a maximum ZT value of 1.61 at 823 K (for Ge_0.86Mn_0.10Sb_0.04Te). The average ZT value of the compound over the temperature range 400-800 K is 1.09, making it the best GeTe-based thermoelectric material.

Low Thermal Conductivity and High Thermoelectric Performance in (GeTe)1-2x(GeSe)x(GeS)x: Competition between Solid Solution and Phase Separation

GeTe and its derivatives constituting Pb-free elements have been well known as potential thermoelectric materials for the last five decades, which offer paramount technological importance. The main constraint in the way of optimizing thermoelectric performance of GeTe is the high lattice thermal conductivity (κ_lat). Herein, we demonstrate low κ_lat (∼0.7 W/m·K) and a significantly high thermoelectric figure of merit (ZT = 2.1 at 630 K) in the Sb-doped pseudoternary (GeTe)_1-2x(GeSe)_x(GeS)_x system by two-step strategies. The (GeTe)_1-2x(GeSe)_x(GeS)_x system provides an excellent podium to investigate competition between an entropy-driven solid solution and enthalpy-driven phase separation. In the first step, small concentrations of Se and S were substituted simultaneously in the position of Te in GeTe to reduce the κ_lat by phonon scattering due to mass fluctuations and point defects. When the Se/S concentration increases significantly, the system deviates from a solid solution, and phase separation of the GeS_1-xSe_x (5-20 μm) precipitates in the GeTe_1-xSe_x matrix occurs, which does not participate in phonon scattering. In the second stage, κ_lat of the optimized sample is further reduced to 0.7 W/m·K by Sb alloying and spark plasma sintering (SPS), which introduce additional phonon scattering centers such as excess solid solution point defects and grain boundaries. The low κ_lat in Sb-doped (GeTe)_1-2x(GeSe)_x(GeS)_x is attributed to phonon scattering by entropically driven solid solution point defects rather than conventional endotaxial nanostructuring. As a consequence, the SPS-processed Ge_0.9Sb_0.1Te_0.9Se_0.05S_0.05 sample exhibits a remarkably high ZT of 2.1 at 630 K, which is reproducible and stable over temperature cycles. Moreover, Sb-doped (GeTe)_1-2x(GeSe)_x(GeS)_x exhibits significantly higher Vickers microhardness (mechanical stability) compared to that of pristine GeTe.

Ultrahigh Average Thermoelectric Figure of Merit, Low Lattice Thermal Conductivity and Enhanced Microhardness in Nanostructured (GeTe)x (AgSbSe2 )100-x

Waste heat sources are generally diffused and provide a range of temperatures rather than a particular temperature. Thus, thermoelectric waste heat to electricity conversion requires a high average thermoelectric figure of merit (ZT_avg ) of materials over the entire working temperature along with a high peak thermoelectric figure of merit (ZT_max ). Herein an ultrahigh ZT_avg of 1.4 for (GeTe)₈₀ (AgSbSe₂ )₂₀ [TAGSSe-80, T=tellurium, A=antimony, G=germanium, S=silver, Se=selenium] is reported in the temperature range of 300-700 K, which is one of the highest values measured amongst the state-of-the-art Pb-free polycrystalline thermoelectric materials. Moreover, TAGSSe-80 exhibits a high ZT_max of 1.9 at 660 K, which is reversible and reproducible with respect to several heating-cooling cycles. The high thermoelectric performance of TAGSSe-x is attributed to extremely low lattice thermal conductivity (κ_lat ), which mainly arises due to extensive phonon scattering by hierarchical nano/meso-structures in the TAGSSe-x matrix. Addition of AgSbSe₂ in GeTe results in κ_lat of ≈0.4 W mK^-1 in the 300-700 K range, approaching to the theoretical minimum limit of lattice thermal conductivity (κ_min ) of GeTe. Additionally, (GeTe)₈₀ (AgSbSe₂ )₂₀ exhibits a higher Vickers microhardness (mechanical stability) value of ≈209 kgf mm^-2 compared to the other state-of-the-art metal chalcogenides, making it an important material for thermoelectrics.

Thermoelectric Properties of Highly-Crystallized Ge-Te-Se Glasses Doped with Cu/Bi

Chalcogenide semiconducting systems are of growing interest for mid-temperature range (~500 K) thermoelectric applications. In this work, Ge₂₀Te₇₇Se₃ glasses were intentionally crystallized by doping with Cu and Bi. These effectively-crystallized materials of composition (Ge₂₀Te₇₇Se₃)_100−xM_x (M = Cu or Bi; x = 5, 10, 15), obtained by vacuum-melting and quenching techniques, were found to have multiple crystalline phases and exhibit increased electrical conductivity due to excess hole concentration. These materials also have ultra-low thermal conductivity, especially the heavily-doped (Ge₂₀Te₇₇Se₃)_100−xBi_x (x = 10, 15) samples, which possess lattice thermal conductivity of ~0.7 Wm⁻¹ K⁻¹ at 525 K due to the assumable formation of nano-precipitates rich in Bi, which are effective phonon scatterers. Owing to their high metallic behavior, Cu-doped samples did not manifest as low thermal conductivity as Bi-doped samples. The exceptionally low thermal conductivity of the Bi-doped materials did not, alone, significantly enhance the thermoelectric figure of merit, zT. The attempt to improve the thermoelectric properties by crystallizing the chalcogenide glass compositions by excess doping did not yield power factors comparable with the state of the art thermoelectric materials, as these highly electrically conductive crystallized materials could not retain the characteristic high Seebeck coefficient values of semiconducting telluride glasses.