Ultralow Thermal Conductivity, Multiband Electronic Structure and High Thermoelectric Figure of Merit in TlCuSe

Weak Chemical Bonds and High Electrical Conductivities Lead to High Thermoelectric Performance in Zintl Compounds Sr2CdX2 (X = As, Sb, and Bi)

Zintl semiconductors are known to exhibit exceptional thermoelectric properties, due to their high electron transport and low lattice thermal conductivity (κL). In this work, thermoelectric properties of Zintl compounds Sr2CdX2 (X = As, Sb, Bi) were studied by using first-principles calculations in conjunction with Boltzmann transport theory. Both Sr2CdSb2 and Sr2CdBi2 adopt the same structure as Sr2CdAs2 (Cmc21) and are synthesizable. All of these compounds demonstrate low lattice thermal conductivities, ranging from 2.3 to 0.5 Wm1- K-1 at 300 K, attributed to their low sound velocity and significant three-phonon scattering, which originate from the weak chemical bonds and strong interaction between acoustic phonons and low-lying optical phonons. The low polar-optical phonon scattering is a result of the relatively small contributions of ions to dielectric constants. Notably, the electrical conductivities (σ) and κL of these compounds exhibit significant anisotropy, with the highest and lowest values being along the x- and z-axis, respectively. Consequently, the highest ZT values of Sr2CdAs2 (1.39), Sr2CdSb2 (1.97), and Sr2CdBi2 (1.74) are achieved along the x-direction at 800 K under n-type doping, respectively, which is comparable or even superior to the well-studied thermoelectric material Mg3Sb2, positioning them as promising candidates for thermoelectric applications.

s-d Coupling-Induced Dynamic Off-Centering of Cu Drives High Thermoelectric Performance in TlCuS

Seeking new and efficient thermoelectric materials requires a detailed comprehension of chemical bonding and structure in solids at microscopic levels, which dictates their intriguing physical and chemical properties. Herein, we investigate the influence of local structural distortion on the thermoelectric properties of TlCuS, a layered metal sulfide featuring edge-shared Cu-S tetrahedra within Cu2S2 layers. While powder X-ray diffraction suggests average crystallographic symmetry with no distortion in CuS4 tetrahedra, the synchrotron X-ray pair distribution function experiment exposes concealed local symmetry breaking, with dynamic off-centering distortions of the CuS4 tetrahedra. The Cu off-centering is driven by the on-site coupling of filled 3d and unoccupied 4s orbitals (s-d) of Cu through a second-order Jahn-Teller mechanism. Bond softening by the p-d* (S-3p and Cu-3d) antibonding interaction coupled with dynamic off-centering distortions causes significant anharmonicity in the lattice, which acts as a phonon-blocking mechanism conducive to ultralow lattice thermal conductivity (κlat) (∼0.35-0.23 W m-1 K-1 in the ∼296-573 K temperature range) in TlCuS. The synergy between low κlat and multiband electronic structure leads to thermoelectric figure-of-merits (zT) of ∼1 and ∼1.3 at 573 K for pristine TlCuS and TlCu0.98S, respectively, underscoring the potential of metal sulfides as promising high-performance thermoelectrics.

Organic Porous Materials and Their Nanohybrids for Next-Generation Thermoelectric Application

Thermoelectricity offers a promising solution for reducing carbon emissions by efficiently converting waste heat into electrical energy. However, high-performance thermoelectric materials predominantly consist of rare, toxic, and costly inorganic compounds. Therefore, the development of alternating material systems for high-performance thermoelectric materials is crucial for broader applications. A significant challenge in this field is the strong interdependence of the various thermoelectric parameters, which complicates their simultaneous optimization. Consequently, the methods for decoupling these parameters are required. In this respect, composite technology has emerged as an effective strategy that leverages the advantages of diverse components to enhance the overall performance. After elaborating on the fundamental concepts of thermoelectricity and the challenges in enhancing the thermoelectric performance, the present review provides a comparative analysis of inorganic and organic materials and explores various methods for decoupling the thermoelectric parameters. In addition, the benefits of composite systems are emphasized and a range of low thermal conductivity materials with microporous to macroporous structures are introduced, highlighting their potential thermoelectric applications. Furthermore, the current development obstacles are discussed, and several cutting-edge studies are highlighted, with a focus on the role of high electrical conductivity fillers in enhancing the performance and mechanical properties. Finally, by combining low thermal conductivity materials with high electrical conductivity fillers can achieve superior thermoelectric performance. These insights are intended to guide future research and development in the field of organic porous materials and their nanohybrids in order to promote more sustainable and efficient energy solutions.

Defect Engineering Advances Thermoelectric Materials

Defect engineering is an effective method for tuning the performance of thermoelectric materials and shows significant promise in advancing thermoelectric performance. Given the rapid progress in this research field, this Review summarizes recent advances in the application of defect engineering in thermoelectric materials, offering insights into how defect engineering can enhance thermoelectric performance. By manipulating the micro/nanostructure and chemical composition to introduce defects at various scales, the physical impacts of diverse types of defects on band structure, carrier and phonon transport behaviors, and the improvement of mechanical stability are comprehensively discussed. These findings provide more reliable and efficient solutions for practical applications of thermoelectric materials. Additionally, the development of relevant defect characterization techniques and theoretical models are explored to help identify the optimal types and densities of defects for a given thermoelectric material. Finally, the challenges faced in the conversion efficiency and stability of thermoelectric materials are highlighted and a look ahead to the prospects of defect engineering strategies in this field is presented.

CsPbBrxCl3-x Solid Solutions: Understanding the Relationship between Lattice Expansion and Optical Properties

All-inorganic halide perovskite semiconductors have received extensive attention due to their excellent photoelectronic conversion efficiency. Prior studies have reported on compounds CsPbBr3 and CsPbCl3. However, the transition phases between them have not been systematically studied. Here, a series of large-size single crystals of CsPbBrxCl3-x (x = 0-3) were successfully grown by the Bridgman method, which proves that the Br and Cl atoms can be miscible in any proportion in the solid solution system, and the change of lattice parameters conforms to Vegard's law. Also, the bandgap and light emission were studied. It is found that the band gap (2.90-2.29 eV) and photoluminescence characteristics (from blue light to green light) can be effectively tuned by adjusting the content of the Br atom. These results provide valuable guidance for the development and optimization of photoelectronic semiconductors that can meet different practical demands.

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

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

Excellent thermoelectric properties of the Tl2S3 monolayer for medium-temperature applications

Exploring materials with high thermoelectric (TE) performance can alleviate energy pressure and protect the environment, and thus, TE materials have attracted extensive attention in the new energy field. In this paper, we systematically study the TE properties of Tl₂S₃ using first-principles combined with Boltzmann transport theory (BTE). The calculation results show an excellent power factor (1.12 × 10^-2 W m^-1 K^-2) and ultra-low lattice thermal conductivity (k_l = 0.88 W m^-1 K^-1) at room temperature. Through analysis, we attribute the ultra-low k_l of Tl₂S₃ to the lower phonon group velocity (v_g) and larger phonon anharmonicity. Meanwhile, discussion of chemical bonding showed that the filling of the anti-bonding state leads to the weakening of the Tl-S chemical bond, resulting in low v_g. Furthermore, this research also investigates the scattering processes (the out-of-plane acoustic mode (ZA) + optical mode (O) → O (ZA + O → O), the in-plane transverse acoustic mode (TA) + O → O (TA + O → O), and the in-plane longitudinal acoustic mode (LA) + O → O (LA + O → O)), from which we find that 2D Tl₂S₃ possesses strong acoustic-optical scattering. Based on the analysis of electron transport properties under electron-phonon coupling, 2D Tl₂S₃, as a novel TE material, exhibits a ZT value as high as 2.8 at 400 K. Our calculations suggest that Tl₂S₃ is a potential TE material at medium temperature.

Enhanced Thermoelectric Performance and Low Thermal Conductivity in Cu2GeTe3 with Identified Localized Symmetry Breakdown

Highly efficient and eco-friendly thermoelectric generators rely on low-cost and nontoxic semiconductors with high symmetry and ultralow lattice thermal conductivity κ_L. We report the rational synthesis of the novel cubic (Ag, Se)-doped Cu₂GeTe₃ semiconductors. A localized symmetry breakdown (LSB) was found in the composition of Cu_1.9Ag_0.1GeTe_1.5Se_1.5 (i.e., CAGTS15) with an ultralow κ_L of 0.37 W/mK at 723 K, the lowest value outperforming all Cu₂GeCh₃ (Ch = S, Se, and Te). A joint investigation of synchrotron X-ray techniques identifies the LSB embedded into the cubic CAGTS15 host matrix. This LSB is an Ångström-scale orthorhombic symmetry unit, characteristic of multiple bond lengths, large anisotropic atomic displacements, and distinct local chemical coordination of anions. Computational results highlight that such an unusual orthorhombic symmetry demonstrates low-frequency phonon modes, which become softer and more predominant with increasing temperatures. This unconventional LSB promotes bond complexity and phonon scattering, highly beneficial for extraordinarily low lattice thermal conductivity.

A quasi-one-dimensional bulk thermoelectrics with high performance near room temperature

Pursuing efficient thermoelectricity from low-dimensional materials has been highly motivated since the seminal work of Hicks and Dresselhaus. In fact, many superior thermoelectric materials like Bi₂Te₃, Mg₃Sb₂/Mg₃Bi₂ and SnSe are quasi-two-dimensional (q2D), though the advantages of two-dimensionality appear to be diverse and sometimes controversial. Here, we report on a remarkably high thermoelectric performance in TlCu₃Te₂, which is quasi-one-dimensional (q1D) with a further reduced dimension. The thermoelectric figure of merit zT along its q1D axis amounts to 1.3 (1.5) at 300 (400) K, rivaling the best ever reported at these temperatures. The high thermoelectric performances benefit from, on one hand, large power factors derived from a center-hollowed, pancake-like Fermi pocket with q1D dispersion at the edge of a narrow band gap, and on the other hand, small lattice thermal conductivities caused by the large and anharmonic q1D lattice consisting of heavy, lone-pair-electron bearing (Tl⁺) and weakly-bonded (Cu⁺) ions. This compound represents the first bulk material with quasi-uniaxial thermoelectric transport of application level, offering a renewed opportunity to exploit reduced dimensionality for high-performance thermoelectricity.

Relativistic effects in complex compounds containing heavy elements: ternary Cu-Tl-X (X = S/Se/Te)

Cu-chalcogenides are a large group of multifunctional compounds traditionally used in photovoltaics and optoelectronics. Their bandgap sizes usually decrease with the element masses, e.g., 2.68, 1.68, and 1.04 eV for CuAlSe₂, CuGaSe₂, and CuInSe₂, respectively. Cu-Tl-X (X = S/Se/Te) with even heavier thallium (Tl) has received recent attention in topological insulators and high-performance thermoelectric converters. While the novel applications may be related to Tl relativistic effects, first-principles investigations are scarce for these complex compounds. Here, we reveal the relativistic effects in Cu-Tl-X using a tailored density-functional-theory approach. Three relativistic terms of mass-velocity, Darwin, and spin-orbit-coupling play distinct roles. In diamond-like CuTlX₂, the mass-velocity correction reduces the conduction band position and contributes to minimizing the bandgaps. The relativistic bandgap of CuTlS₂ of 0.11 eV is substantially smaller than 1.7 eV without considering the relativistic effects. In CuTlTe₂, the spin-orbit-coupling splits the valence bands, resulting in an exotic band inversion. CuTlSe₂ lies on the boundary of normal and inverted band topologies. Interestingly, the relativistic core contraction is so strong that it may favor non-centrosymmetric defective structures with stereoactive lone-pair electrons. The bandgap of the defective structure is much larger, leaving the system little chance to develop an inverted band topology. Our work provides deep insights into understanding the relativistic band topologies of the complex Cu-Tl-X compounds.

Symmetry-Guaranteed High Carrier Mobility in Quasi-2D Thermoelectric Semiconductors

Quasi-2D semiconductors have garnered immense research interest for next-generation electronics and thermoelectrics due to their unique structural, mechanical, and transport properties. However, most quasi-2D semiconductors experimentally synthesized so far have relatively low carrier mobility, preventing the achievement of exceptional power output. To break through this obstacle, a route is proposed based on the crystal symmetry arguments to facilitate the charge transport of quasi-2D semiconductors, in which the horizontal mirror symmetry is found to vanish the electron-phonon coupling strength mediated by phonons with purely out-of-plane vibrational vectors. This is demonstrated in ZrBeSi-type quasi-2D systems, where the representative sample Ba_1.01 AgSb shows a high room-temperature hole mobility of 344 cm² V^-1 S^-1 , a record value among quasi-2D polycrystalline thermoelectrics. Accompanied by intrinsically low thermal conductivity, an excellent p-type zT of ≈1.3 is reached at 1012 K, which is the highest value in ZrBeSi-type compounds. This work uncovers the relation between electron-phonon coupling and crystal symmetry in quasi-2D systems, which broadens the horizon to develop high mobility semiconductors for electronic and energy conversion applications.

Phase-engineered high-entropy metastable FCC Cu2-y Ag y (In x Sn1-x )Se2S nanomaterials with high thermoelectric performance

Crystal-phase engineering to create metastable polymorphs is an effective and powerful way to modulate the physicochemical properties and functions of semiconductor materials, but it has been rarely explored in thermoelectrics due to concerns over thermal stability. Herein, we develop a combined colloidal synthesis and sintering route to prepare nanostructured solids through ligand retention. Nano-scale control over the unconventional cubic-phase is realized in a high-entropy Cu2-y Ag y (In x Sn1-x )Se2S (x = 0-0.25, y = 0, 0.07, 0.13) system by surface-ligand protection and size-driven phase stabilization. Different from the common monoclinic phase, the unconventional cubic-phase samples can optimize electrical and thermal properties through phase and entropy design. A high power factor (0.44 mW m-1 K-2), an ultralow thermal conductivity (0.25 W m-1 K-1) and a ZT value of 1.52 are achieved at 873 K for the cubic Cu1.87Ag0.13(In0.06Sn0.94)Se2S nanostructured sample. This study highlights a new method for the synthesis of metastable phase high-entropy materials and gives insights into stabilizing the metastable phase through ligand retention in other research communities.

Ultralow lattice thermal conductivity at room temperature in 2D KCuSe from first-principles calculations

Ultralow lattice thermal conductivity is crucial for achieving a high thermoelectric figure of merit for thermoelectric applications. In this work, using first-principles calculations and the phonon Boltzmann transport theory, we investigate the phonon thermal transport properties of 2D KCuSe. Our calculations indicate that the strong acoustic-optical coupling, the low-lying acoustic phonon modes and the strong lattice anharmonic effect with a large Grüneisen parameter and phase space volume result in an ultralow lattice thermal conductivity of 0.021 W m^-1 K^-1 at 300 K for monolayer KCuSe, which is lower than those of recently reported KAgSe (0.26 W m^-1 K^-1 at 300 K) and TlCuSe (0.44 W m^-1 K^-1 at 300 K). Importantly, although the Coulomb interactions and the tensile biaxial strain lead to the increase of lattice thermal conductivity due to the increasing relaxation time (0.056 and 0.28 W m^-1 K^-1 at 300 K without and with 6% tensile strain, respectively), it is still lower than those of most 2D thermoelectric materials. The advantages of being cheap, environmentally friendly and having low lattice thermal conductivity compared to the KAgSe and TlCuSe derivatives make KCuSe a promising candidate for thermoelectric applications, which will stimulate more efforts toward theoretical and experimental studies on this class of 2D ternary semiconductors.

A Tunable Structural Family with Ultralow Thermal Conductivity: Copper-Deficient Cu1-x□xPb1-xBi1+xS3

Understanding the mechanism that connects heat transport with crystal structures and order/disorder phenomena is crucial to develop materials with ultralow thermal conductivity (κ), for thermoelectric and thermal barrier applications, and requires the study of highly pure materials. We synthesized the n-type sulfide CuPbBi₅S₉ with an ultralow κ value of 0.6-0.4 W m^-1 K^-1 in the temperature range 300-700 K. In contrast to prior studies, we show that this synthetic sulfide does not exhibit the ordered gladite mineral structure but instead forms a copper-deficient disordered aikinite structure with partial Pb replacement by Bi, according to the chemical formula Cu_1/3□_2/3Pb_1/3Bi_5/3S₃. By combining experiments and lattice dynamics calculations, we elucidated that the ultralow κ value of this compound is due to very low energy optical modes associated with Pb and Bi ions and, to a smaller extent, Cu. This vibrational complexity at low energy hints at substantial anharmonic effects that contribute to enhance phonon scattering. Importantly, we show that this aikinite-type sulfide, despite being a poor semiconductor, is a potential matrix for designing novel, efficient n-type thermoelectric compounds with ultralow κ values. A drastic improvement in the carrier concentration and thermoelectric figure of merit have been obtained upon Cl for S and Bi for Pb substitution. The Cu_1-x□_xPb_1-xBi_1+xS₃ series provides a new, interesting structural prototype for engineering n-type thermoelectric sulfides by controlling disorder and optimizing doping.