High Thermoelectric Performance of p-Type PbTe Enabled by the Synergy of Resonance Scattering and Lattice Softening

Band Flattening and Strain Field Assists an Excellent Thermoelectric Performance of n-type Bi2Se3 for Room to Mid-Temperature Application

Bismuth selenide, (Bi2Se3) is a typical n-type V-VI chalcogenide-based thermoelectric (TE) material with high lattice thermal conductivity, which limits its TE performance. Single element doping is generally adopted to enhance the TE performance, since it is not affect significantly. Herein, it is revealed that the impact of the co-doping strategy boost the TE performance of Bi2Se3 through tuning the band structure and phonon transport properties. The synergy of isovalent Ga (gallium) and aliovalent Ge (germanium) co-doping produces a noteworthy improvement in carrier tuning from -1.1 × 1018 to -3.56 × 1018 cm-3. Bi1.95Ga0.05Ge0.033Se3 sample reached the enhanced power factor of 1403 µWm-1K-2 obtained at 303 K. The reduced low lattice thermal conductivity of 0.24 Wm-1K-1 is occurred by the multi-scale phonon scattering mechanisms through the dissemination of strain field raised from various defects. The combined strategy of flat band structure and low lattice thermal conductivity significantly optimized the thermoelectric zT of 0.7 is achieved at 513 K and the highest zTaverage of 0.68 is obtained. The maximum zT values leads to attain a theoretical efficiency of 5.9% in the temperature range between 333-573 K. This work shows a practical approach to improve the zT of n-type Bi2Se3 via co-doping.

Polycrystalline Films of Indium-Doped PbTe on Amorphous Substrates: Investigation of the Material Based on Study of Its Structural, Transport, and Optical Properties

Nowadays, polycrystalline lead telluride is one of the premier substances for thermoelectric devices while remaining a hopeful competitor to current semiconductor materials used in mid-infrared photonic applications. Notwithstanding that, the development of reliable and reproducible routes for the synthesis of PbTe thin films has not yet been accomplished. As an effort toward this aim, the present article reports progress in the growth of polycrystalline indium-doped PbTe films and their study. The introduction foregoing the main text presents an overview of studies in these and closely related research fields for seven decades. The main text reports on the electron-beam-assisted physical vapor deposition of n-type indium-doped PbTe films on two different amorphous substrates. This doping of PbTe is unique since it sets electron density uniform over grains due to pinning the Fermi level. In-house optimized parameters of the deposition process are presented. The films are structurally characterized by a set of techniques. The transport properties of the films are measured with the original setups described in detail. The infrared transmission spectra are measured and simulated with the original optical-multilayer modeling tool described in the appendix. Conclusions of films' quality in terms of these properties altogether are drawn.

Atomistic insight into the promising thermoelectric performance of the copper-based ternary phosphide CaCuP

A first-principles study on the thermoelectric properties of the copper-based ternary phosphide CaCuP is presented. Self-energy relaxation time approximation and unified theory for lattice transport provide an accurate description of electron-phonon and phonon-phonon scattering. Our work provides an atomistic insight into its high thermoelectric performance, highlighting that nano-structuring can increase the thermoelectric figure of merit ZT by reducing the lattice thermal conductivity.

Controlled Phonon Transport via Chemical Bond Stretching and Defect Engineering: The Case Study of Filled β-Mn-Type Phases

Controlling the elastic properties of the material could become a powerful tool for tuning the thermal transport in solids. Nevertheless, the impact of the crystal structure, chemical bonding, and elastic properties on the lattice thermal conductivity remains to be elucidated. This is a pivotal issue for the advancement of thermoelectric (TE) materials. In this context, the influence of cation substitution in tetrahedral voids on the structural, thermal, and TE properties of α- and β-PbyGa6-xInxTe10─filled β-Mn-type phases─is reported here. The investigated materials show semiconducting behavior and a change from p- to n-type conductivity, depending on the chemical composition and temperature. Our findings indicate that the electronic transport in β-Mn-type phases is largely influenced by the substantial distortion of the Te framework, which causes the low weighted mobility and strong scattering of charge carriers. The presence of a significant anharmonicity of lattice vibrations results in the ultralow lattice thermal conductivity of PbyGa6-xInxTe10 materials. With increasing x, κL decreases from 0.59 to an extremely low value of 0.36 W m-1 K-1 at 298 K due to the decrease of bonding energy, intensification of anharmonic thermal vibrations of atoms, and formation of point defects. This work demonstrates the efficacy of utilizing the crystal structure and elastic properties to regulate phonon transport in functional materials.

In-Plane Overdamping and Out-Plane Localized Vibration Contribute to Ultralow Lattice Thermal Conductivity of Zintl Phase KCdSb

Zintl phases typically exhibit low lattice thermal conductivity, which are extensively investigated as promising thermoelectric candidates. While the significance of Zintl anionic frameworks in electronic transport properties is widely recognized, their roles in thermal transport properties have often been overlooked. This study delves into KCdSb as a representative case, where the [CdSb4/4]- tetrahedrons not only impact charge transfer but also phonon transport. The phonon velocity and mean free path, are heavily influenced by the bonding distance and strength of the Zintl anions Cd and Sb, considering the three acoustic branches arising from their vibrations. Furthermore, the weakly bound Zintl cation K exhibits localized vibration behaviors, resulting in strong coupling between the high-lying acoustic branch and the low-lying optical branch, further impeding phonon diffusion. The calculations reveal that grain boundaries also contribute to the low lattice thermal conductivity of KCdSb through medium-frequency phonon scattering. These combined factors create a glass-like thermal transport behavior, which is advantageous for improving the thermoelectric merit of zT. Notably, a maximum zT of 0.6 is achieved for K0.84Na0.16CdSb at 712 K. The study offers both intrinsic and extrinsic strategies for developing high-efficiency thermoelectric Zintl materials with extremely low lattice thermal conductivity.

2D Self-Assembly of Large-Sized Inorganic Nanosheets Leading to Enhanced Power Factors in Earth-Abundant Cu3SbSe4-Based Flexible Hybrid Films

Fabrication of large-sized inorganic nanosheets is an efficient strategy to promote carrier transportation in flexible thermoelectric (TE) films. Herein, we report the self-assembly of large-sized Cu3SbSe4 nanosheets by using a Se nanowire template via wet chemical synthesis and then vacuum-assisted filter these plate-like microcrystals on nylon to prepare Cu3SbSe4 flexible thermoelectric (TE) hybrid films. SEM reveals that the as-synthesized Cu3SbSe4 powders by using Se nanowires as selenium sources presented 2D plate-like micron structures uniformly and tightly self-assembled by acute triangle-like nanoparticles. Furthermore, XPS evidences that extra Sb vacancies are generated in the unit cell of Cu3SbSe4 crystals synthesized by using the Se NW template, resulting in the shrinkage of the unit cell and the narrowing interplanar spacing, which are characterized by XRD and TEM. As a result, both carrier concentration and carrier mobility have been significantly improved. The high carrier concentration is proved to originate from the extra carriers induced by Sb vacancies, and the high carrier mobility of the film is mainly ascribed to its continuous grain boundaries in the plate-like microcrystal morphology. The large-sized nanosheet Cu3SbSe4/nylon hybrid film (CSS MPs) exhibits a high power factor (PF) of 235.45 μW m-1 K-2 at 400 K, which is 4.23 times higher than that of the Cu3SbSe4/nylon hybrid film (CSS NPs) where Cu3SbSe4 crystals are synthesized by using raw Se particles. This work reveals a novel approach to prepare plate-like Se-based semiconductors, which requires both high carrier concentration and high carrier mobility.

Improved Thermoelectric Performance of p-Type PbTe by Entropy Engineering and Temperature-Dependent Precipitates

Entropy engineering is aneffective scheme to reduce the thermal conductivity of thermoelectric materials, but it inevitably deteriorates the carrier mobility. Here, we report the optimization of thermoelectric performance of PbTe by combining entropy engineering and nanoprecipitates. In the continuously tuned compounds of Pb0.98Na0.02Te(1-2x)SxSex, we show that the x = 0.05 sample exhibits an exceptionally low thermal conductivity relative to its configuration entropy. By introducing Mn doping, the produced temperature-dependent nanoprecipitates of MnSe cause the high-temperature thermal conductivity to be further reduced. A very low lattice thermal conductivity of 0.38 W m-1 K-1 is achieved at 825 K. Meanwhile, the carrier mobility of the samples is only slightly influenced, owing to the well-controlled configuration entropy and the size of nanoprecipitates. Finally, a high peak zT of ∼2.1 at 825 K is obtained in the Pb0.9Na0.04Mn0.06Te0.9S0.05Se0.05 alloy.

Achieving high thermoelectric conversion efficiency in Bi2Te3-based stepwise legs through bandgap tuning and chemical potential engineering

In this study, we show that the energy conversion efficiency in thermoelectric (TE) devices can be effectively improved through simultaneous optimization of carrier concentration, bandgap tuning, and fabrication of stepwise legs. n- and p-type Bi2Te3-based materials were selected as examples for testing the proposed approach. At first, the Boltzmann transport theory was employed to predict the optimal temperature-dependent carrier concentration for high thermoelectric performance over a broad temperature range. Then, the synthesized n-Bi2Te3-xSex and p-Bi2-xSbxTe3 solid solutions were tested to evaluate their suitability for fabricating the stepwise thermoelectric legs. The output energy characteristics of the designed TE devices were estimated using numerical modeling employing the finite element method. The theoretical simulation revealed an improvement in the conversion efficiency between the best homogeneous and stepwise TE legs from 8.8% to 10.1% and from 9.9% to 10.8% in p-type and n-type legs, respectively, which is much higher than the efficiency of the industrial thermoelectric modules (3-6%). The measured conversion efficiency of the fabricated n- and p-type stepwise legs reached very high values of 9.3% and 9.0%, respectively, at the relatively small temperature gradient of 375 K. This work suggests carrier concentration and bandgap engineering accompanied by the stepwise leg approach as powerful methods for achieving high energy conversion efficiency in thermoelectric converters.

Structure Evolution and Bonding Inhomogeneity toward High Thermoelectric Performance in Cu2CoSnS4-xSex Materials

Lightweight diamond-like structure (DLS) materials are excellent candidates for thermoelectric (TE) applications due to their low costs, eco-friendly nature, and property stability. The main obstacles restricting the energy-conversion performance by the lightweight DLS materials are high lattice thermal conductivity and relatively low carrier mobility. By investigating the anion substitution effect on the structural, microstructural, electronic, and thermal properties of Cu₂CoSnS_4-xSe_x, we show that the simultaneous enhancement of the crystal symmetry and bonding inhomogeneity engineering are effective approaches to enhance the TE performance in lightweight DLS materials. Particularly, the increase of x in Cu₂CoSnS_4-xSe_x makes the DLS structure with the ideal tetrahedral bond angles of 109.5° favorable, leading to better crystal symmetry and higher carrier mobility in samples with higher selenium content. In turn, the phonon transport in the investigated DLS materials is strongly disturbed due to the bonding inhomogeneity between anions and three sorts of cations inducing large lattice anharmonicity. The increase of Se content in Cu₂CoSnS_4-xSe_x only intensified this effect resulting in a lower lattice component of the thermal conductivity (κ_L) for Se-rich samples. As a result of the enhanced power factor S²ρ^-1 and the low κ_L, the dimensionless thermoelectric figure of merit ZT achieves a high value of 0.75 for Cu₂CoSnSe₄ DLS material. This work demonstrates that crystal symmetry and bonding inhomogeneity play an important role in the transport properties of DLS materials and provide a path for the development of new perspective materials for TE energy conversion.

Lead Vacancy Promotes Sodium Solubility to Achieve Ultra-High zT in Only Ternary Pb1- x Nax Te

Chemical doping of sodium is an indispensable means to optimize thermoelectric properties of PbTe materials, while a bottleneck is that an aliovalent atom doping leads to spontaneous intrinsic defects in the PbTe matrix, resulting in low dopant solubility. Therefore, it is urgent to improve the doping efficiency of Na for maximizing optimization. Here, an amazing new insight that the intentionally introduced Pb vacancies can promote Na solubility in ternary Pb_{1- x} Na_x Te is reported. Experimental analysis and theoretical calculations provide new insights into the inherent mechanism of the enhancement of Na solubility. The Pb vacancies and the resultant more dissolved Na not only synergistically optimize the carrier concentration and further facilitate the band convergence, but also induce a large number of dense dislocations in the grains. Consequently, benefiting from the self-enhancement of Seebeck coefficient and the minimization of lattice thermal conductivity, an 18% growth is obtained for the figure of merit zT in vacancy-containing Pb_0.95 Na_0.04 Te sample, reaching maximum zT_max  ≈ 2.0 at 823 K, which achieves an ultra-high performance in only Na-doped ternary Pb_{1- x} Na_x Te materials. The strategy utilized here provides a novel route to optimize PbTe materials and represents an important step forward in manipulating thermoelectrics to improve dopant solubility.

NaCdSb: An Orthorhombic Zintl Phase with Exceptional Intrinsic Thermoelectric Performance

Many Zintl phases are promising thermoelectric materials owning to their features like narrow band gaps, multiband behaviors, ideal charge transport tunnels, and loosely bound cations. Herein we show a new Zintl phase NaCdSb with exceptional intrinsic thermoelectric performance. Pristine NaCdSb exhibits semiconductor behaviors with an experimental hole concentration of 2.9×10¹⁸  cm^-3 and a calculated band gap of 0.5 eV. As the temperature increases, the hole concentration rises gradually and approaches its optimal one, leading to a high power factor of 11.56 μW cm^-1  K^-2 at 673 K. The ultralow thermal conductivity is derived from the small phonon group velocity and short phonon lifetime, ascribed to the structural anharmonicity of Cd-Sb bonds. As a consequence, a maximum zT of 1.3 at 673 K has been achieved without any doping optimization or structural modification, demonstrating that NaCdSb is a remarkable thermoelectric compound with great potential for performance improvement.

Dramatic Enhancement of Thermoelectric Performance in PbTe by Unconventional Grain Shrinking in the Sintering Process

Nanostructure engineering is a key strategy for tailoring properties in the fields of batteries, solar cells, thermoelectrics, and so on. Limited by grain coarsening, however, the nanostructure effect gradually degrades during the materials' manufacturing and in-service period. Herein, a strategy of cleavage-fracture for grain shrinking is developed in a Pb_0.98 Sb_0.02 Te sample during sintering, and the grain size remains stable after repeated tests. Moreover, the initial grain boundary is filled by fractured slender grains and enriched by dislocations, evolving into a hierarchical grain-boundary structure. The lattice thermal conductivity (k_lat ) is greatly reduced to approach the amorphous limit. As a result, a record-high ZT value of ≈1.9 is obtained at 815 K in the n-type Pb_0.98 Sb_0.02 Te sample and a decent efficiency of 6.7% in thermoelectric device. This strategy for grain shrinking will shed light on the application of nanostructure engineering under high temperature and extreme conditions in other material systems.

Processing High-Performance Thermoelectric Materials in a Green Way: A Proof of Concept in Cold Sintered PbTe0.94Se0.06

For years, most of the advanced polycrystalline thermoelectric (TE) materials are fabricated by spark plasma sintering (SPS) in the research field, mainly because of its high processing efficiency. However, issues like high energy consumption and an expensive apparatus have prevented the application of this strategy in industry. Herein, taking PbTe_0.94Se_0.06 (PTS) as a typical n-type mid-temperature material, we demonstrate that the cold sintering process (CSP) can serve as a green and cost-effective technology for preparing advanced TE materials. By selecting the solvothermal precursors as liquid sintering aids, the CSP-densified PTS shows a maximum figure of merit of 0.96 at 700 K, which is on par with, if not better than, the reported similar materials prepared by SPS. This remarkable performance is ascribed to the distinct densification procedure in the CSP: (1) the ultralow temperature alleviates the precipitation of Pb, which preserves the high carrier concentration of PTS; (2) the transient liquid phase forms intimate grain boundaries comparable to the high-temperature sintered one, leading to a high carrier mobility; (3) the dissolution-precipitation process greatly restrains the coarsening of precipitates, which effectively suppresses the bipolar effect and lattice thermal conductivity due to enhanced scattering. We believe that these results can greatly encourage the application of CSP in the future development of TE materials.

Lone-Pair-Like Interaction and Bonding Inhomogeneity Induce Ultralow Lattice Thermal Conductivity in Filled β-Manganese-Type Phases

Finding a way to interlink heat transport with the crystal structure and order/disorder phenomena is crucial for designing materials with ultralow lattice thermal conductivity. Here, we revisit the crystal structure and explore the thermoelectric properties of several compounds from the family of the filled β-Mn-type phases M 2/n n+Ga6Te10 (M = Pb, Sn, Ca, Na, Na + Ag). The strongly disturbed thermal transport observed in the investigated materials originates from a three-dimensional Te-Ga network with lone-pair-like interactions, which results in large variations of the Ga-Te and M-Te interatomic distances and substantial anharmonic effects. In the particular case of NaAgGa6Te10, the additional presence of different cations leads to bonding inhomogeneity and strong structural disorder, resulting in a dramatically low lattice thermal conductivity (∼0.25 Wm-1 K-1 at 298 K), being the lowest among the reported β-Mn-type phases. This study offers a way to develop materials with ultralow lattice thermal conductivity by considering bonding inhomogeneity and lone-pair-like interactions.

Ultralow Lattice Thermal Conductivity and Improved Thermoelectric Performance in Cl-Doped Bi2Te3-xSex Alloys

Bi2Te3-based alloys are the main materials for the construction of low- and medium-temperature thermoelectric modules. In this work, the microstructure and thermoelectric properties of Cl-doped Bi2Te3-xSex alloys were systematically investigated considering the high anisotropy inherent in these materials. The prepared samples have a highly oriented microstructure morphology, which results in very different thermal transport properties in two pressing directions. To accurately separate the lattice, electronic, and bipolar components of the thermal conductivity over the entire temperature range, we employed a two-band Kane model to the Cl-doped Bi2Te3-xSex alloys. It was established that Cl atoms act as electron donors, which tune the carrier concentration and effectively suppress the minority carrier transport in Bi2Te3-xSex alloys. The estimated value of the lattice thermal conductivity was found to be as low as 0.15 Wm-1 K-1 for Bi2Te3-x-ySexCly with x = 0.6 and y = 0.015 at 673 K in parallel to the pressing direction, which is among the lowest values reported for crystalline materials. The large reduction of the lattice thermal conductivity in both pressing directions for the investigated Bi2Te3-xSex alloys is connected with the different polarities of the Bi-(Te/Se)1 and Bi-(Te/Se)2 bonds, while the lone-pair (Te/Se) interactions are mainly responsible for the extremely low lattice thermal conductivity in the parallel direction. As a result of the enhanced power factor, suppressed bipolar conduction, and ultralow lattice thermal conductivity, a maximum ZT of 1.0 at 473 K has been received in the Bi2Te2.385Se0.6Cl0.015 sample.

Development of a High Perfomance Gas Thermoelectric Generator (TEG) with Possibible Use of Waste Heat

A huge concern regarding global warming, as well as the depletion of natural fuel resources, has led to a wide search for alternative energy sources. Due to their high reliability and long operation time, thermoelectric generators are of significant interest for waste heat recovery and power generation. The main disadvantage of TEGs is the low efficiency of thermoelectric commercial modules. In this work, a unique design for a multilayer TE unicouple is suggested for an operating temperature range of 50–600 °C. Two types of thermoelectric materials were selected: «low temperature» n-and p-type TE materials (for the operating temperature range of 50–300 °C) based on Bi2Te3 compounds and «middle temperature» (for the operating temperature range of 300–600 °C) n- and p-type TE materials based on the PbTe compound. The hot extrusion technology was applied to fabricate n- and p-type low-temperature TE materials. A unique design of multilayer TEG was experienced to achieve an efficiency of up to 15%. This allows for the possibility of extracting this amount of electrical power from the heat generated for domestic and water heating.

Crystal Structure and Thermoelectric Properties of Novel Quaternary Cu2MHf3S8 (M-Mn, Fe, Co, and Ni) Thiospinels with Low Thermal Conductivity

Uncovering of the origin of intrinsically low thermal conductivity in novel crystalline solids is among the main streams in modern thermoelectricity. Because of their earth-abundant nature and environmentally friendly content, Cu-based thiospinels are attractive functional semiconductors, including thermoelectric (TE) materials. Herein, we report the crystal structure, as well as electronic and TE properties of four new Cu2MHf3S8 (M-Mn, Fe, Co, and Ni) thiospinels. The performed density functional theory calculations predicted the decrease of the band gap and transition from p- to n-type conductivity in the Mn-Fe-Co-Ni series, which was confirmed experimentally. The best TE performance in this work was observed for the Cu2NiHf3S8 thiospinel due to its highest power factor and low thermal conductivity. Moreover, all the discovered compounds possess very low lattice thermal conductivity κlat over the investigated temperature range. The κlat for Cu2CoHf3S8 has been found to be as low as 0.8 W m-1 K-1 at 298 K and 0.5 W m-1 K-1 at 673 K, which are significantly lower values compared to the other Cu-based thiospinels reported up to date. The strongly disturbed phonon transport of the investigated alloys mainly comes from the peculiar crystal structure where the large cubic unit cells contain many vacant octahedral voids. As it was evaluated from the Callaway approach and confirmed by the speed of sound measurements, such a crystal structure promotes the increase in lattice anharmonicity, which is the main reason for the low κlat. This work provides a guideline for the engineering of thermal transport in thiospinels and offers the discovered Cu2MHf3S8 (M-Mn, Fe, Co, and Ni) compounds, as new promising functional materials with low lattice thermal conductivity.

Phase Analysis and Thermoelectric Properties of Cu-Rich Tetrahedrite Prepared by Solvothermal Synthesis

Because of the large Seebeck coefficient, low thermal conductivity, and earth-abundant nature of components, tetrahedrites are promising thermoelectric materials. DFT calculations reveal that the additional copper atoms in Cu-rich Cu14Sb4S13 tetrahedrite can effectively engineer the chemical potential towards high thermoelectric performance. Here, the Cu-rich tetrahedrite phase was prepared using a novel approach, which is based on the solvothermal method and piperazine serving both as solvent and reagent. As only pure elements were used for the synthesis, the offered method allows us to avoid the typically observed inorganic salt contaminations in products. Prepared in such a way, Cu14Sb4S13 tetrahedrite materials possess a very high Seebeck coefficient (above 400 μVK-1) and low thermal conductivity (below 0.3 Wm-1K-1), yielding to an excellent dimensionless thermoelectric figure of merit ZT ≈ 0.65 at 723 K. The further enhancement of the thermoelectric performance is expected after attuning the carrier concentration to the optimal value for achieving the highest possible power factor in this system.