Promising Thermoelectric Properties Research Articles

Theoretical investigation into potential thermoelectric material Monolayer InI with direct bandgap and ultralow lattice thermal conductivity

Monolayer metal iodides are garnering widespread attention due to their ultralow lattice thermal conductivity and their potential applications in the field of thermoelectrics. Here, we utilized first-principles calculations and Boltzmann transport theory to thoroughly investigate the thermoelectric behavior of monolayer InI. Results indicate that this material functions as a direct bandgap semiconductor and features an exceptionally low lattice thermal conductivity of 0.27 W m−1K−1 at 300 K, a result of its low group velocities and short phonon lifetimes. Furthermore, transport coefficients were calculated with enhanced precision through the Wannier interpolation method, incorporating the HSE06 hybrid functional and spin-orbit coupling (SOC) effects. Remarkably, the maximum thermoelectric figure of merit (ZT) values for monolayer InI demonstrate substantial growth with increased temperatures, surging from 0.31 at 300 K to 1.20 at 450 K for p-type, and escalating from 0.26 at 300 K to 0.72 at 450 K for n-type. These promising low-temperature thermoelectric properties exhibited by monolayer InI suggest its suitability for integration into the production of thermoelectric devices.

Grain boundary density on realizing anisotropic thermoelectric properties of Ca3Co4O9-based ceramics with excellent texturation

Texturation and anisotropic performance of Ca3Co4O9-based ceramics fabricated by spark plasma sintering were reported, showing promising thermoelectric properties. With template seeds simultaneous additives, the high degree of texturing was obtained. Electrical resistivity decreased from 16.9 to 6.1 mΩ cm and Seebeck coefficient increased from 75.8 to 187.6 μV/K for the sample with 10 wt% templates, which were dominated by a synergistic combination of the grain orientation and grain boundary density. The obvious anisotropic power factors presented, mainly coming from the texturing characteristic. A low total thermal conductivity of 1.16 W/(m·K) were greatly suppressed by tuning broad wavelength phonon scattering, which was ascribed to the layered nanostructures at grain boundaries. Furthermore, the ZT value of 0.31 was obtained at 1073 K for the sample perpendicular to pressure direction, yielding 138 % enhancement compared with that of the other direction. The work confirmed clear evidence that the textured ceramics with template seeds addition have a great influence on thermoelectricity in oxides materials.

Large thermoelectric transport in magnetically coupled CrI3/1T-MoS2 vdW heterostructure via spin–charge interconversion

Low-dimensional materials with prominent thermoelectric (TE) effect play a pivotal role in realizing state-of-the-art nanoscale TE devices. The fusion of TE effect with the magnetism through seamless integration of TE and magnetic materials in the 2D limit offers access to control longitudinal as well as transverse TE properties via magnetic proximity effect. Herein, we design a van der Waals (vdW) heterostructure of metallic 1T-MoS2 with promising TE properties and a layer-dependent magnetic CrI3 material. The result highlights exotic electronic and magnetic configurations of the designed monolayer-CrI3/1T-MoS2 vdW heterostructure, which show magnetically-coupled TE characteristics. The observed remarkable magnetic proximity stems from large magnetic anisotropy energy and spin polarization, which are found to be 2.21 meV Cr−1 and 12.30%, respectively. To this end, the semiconducting CrI3 layer with intrinsic magnetism leads to efficient control and tunability of the observed spin-correlated anomalous Nernst effect. Moreover, a large dimensionless figure of merit of ∼6 and a power factor of ∼3.8×1011/τ∘ Wm−1K−2s−1 near the Fermi level at 300 K endorse the rejuvenated TE effect. The strong relativistic spin–orbit coupling validates the significant correlation of TE properties with intrinsic magnetic configuration. The present study underscores the significance of the magnetic proximity-governed TE effect in vdW heterostructures to engineer low-dimensional TE devices.

Low-temperature electrical conductivity of ion-beam irradiated Bi–Sb films

Bismuth-antimony alloys are among the most studied topological insulators and also have very promising thermoelectric properties. In addition, in the amorphous state they exhibit superconductivity with critical temperatures in the range 6.0–6.4 K. In this work, we have prepared and studied different polycrystalline films of Bi100–xSbx (x = 0, 5, 10, 15), and we have induced, through ion beam irradiation, significant damage in their internal structure with the aim of amorphizing the material. Specifically, we have irradiated Bi ions in the 10–30 MeV range, exploiting the capabilities of a 5 MV ion beam accelerator of tandem type. We have characterized the Bi–Sb films before and after irradiation from a morphological and structural point of view and measured their electrical resistivity from room temperature to near 2 K, to evaluate the influence of the preparation method and degree of disorder. We have found that the studied Bi–Sb system always behaves as a small energy gap semiconductor that follows the empirical Meyer–Neldel rule, which correlates the conductivity prefactor with the exponential value of the energy gap.

Integration of layered group IV selenides: From SnSe–SnSe2-xSx core-shell crystals to complex (SnSe–SnSe2-xSx)-GeSe van der waals heterostructures

The layered semiconductor tin selenide (SnSe) has received extensive interest due to its promising thermoelectric and ferroelectric properties. Integrating SnSe with other layered crystals in heterostructures can enable the modification of charge- and thermal transport, electrical polarization, and other properties such as chemical stability, optoelectronics, and photonics. Here, we demonstrate the vapor transport synthesis of single-crystalline SnSe monochalcogenide flakes that are spontaneously encapsulated in a thin layered SnSe2-xSx dichalcogenide shell. In a second growth step, such SnSe–SnSe2-xSx heterostructures are integrated with the monochalcogenide GeSe, which is laterally stitched to the SnSe side facets while preserving the dichalcogenide shell across the basal facets. This architecture is confirmed by optical microscopy, electron microscopy and diffraction, energy dispersive X-ray and Raman spectroscopies, as well as cathodoluminescence spectroscopy. The results extend our capabilities for materials integration by forming complex heterostructures with both vertical van der Waals interfaces and covalent lateral interfaces between layered semiconductors.

Mechanochemically Enabled Metastable Niobium Tungsten Oxides.

Metastable compounds have greatly expanded the synthesizable compositions of solid-state materials and have attracted enormous amounts of attention in recent years. Especially, mechanochemically enabled metastable materials synthesis has been very successful in realizing cation-disordered materials with highly simple crystal structures, such as rock salts. Application of the same strategy for other structural types, especially for non-close-packed structures, is peculiarly underexplored. Niobium tungsten oxides (NbWOs), a class of materials that have been under the spotlight because of their diverse structural varieties and promising electrochemical and thermoelectric properties, are ideally suited to fill such a knowledge gap. In this work, we develop a new series of metastable NbWOs and realize one with a fully cation-disordered structure. Furthermore, we find that metastable NbWOs transform to a cation-disordered cubic structure when applied as a Li-ion battery anode, highlighting an intriguing non-close-packed-close-packed conversion process, as evidenced in various physicochemical characterizations, in terms of diffraction, electronic, and vibrational structures. Finally, by comparing the cation-disordered NbWO with other trending cation-disordered oxides, we raise a few key structural features for cation disorder and suggest a few possible research opportunities for this field.

Unveiling heavy heterovalent doping-modulated microstructure and thermoelectric performance in bulk hybrid perovskite single crystals

Organic-inorganic hybrid metal halide perovskites have emerged as a promising candidate for low-temperature thermoelectric conversion. However, challenges remain in heavily doping bulk perovskite single crystals without phase segregation or self-doping compensation issues to increase the carrier concentration. Previous efforts to enhance their thermoelectric performance rely on photoexcitation or surface/interface doping, which is not suitable for bulk devices to work in dark. This study achieves high monovalent and trivalent doping concentrations of up to 5 % in bulk single crystals of MAPbI3 and MAPbBr3 without phase segregation or defect formation. For monovalent doping, K+ and Li+ are incorporated into the perovskite lattice as interstitials. The resulting lattice distortion/microstrain enhances the electrical conductivity by 10 ∼ 100 times of undoped samples at 420 K. Trivalent doping using SbCl3, BiBr3, and BiI3 significantly enhances the carrier concentration and electrical conductivity, up to 1012 cm−3 and 0.038 S m−1 for MAPbBr3 with 10 % BiI3 at 465 K, respectively, which is 105 times of the undoped samples and 10 times of the best reports on doped MAPbBr3. Interestingly, the Bi3+ dopant oxidizes the deep defect state Pb0, resulting in Bi+ and Pb2+ formation and contributing to the enhanced electrical conductivity. Enhanced thermoelectric figure of merit (ZT) is achieved in MAPbBr3 with 5 % BiI3 at 315 K, almost 100 times of the best reports on doped MAPbBr3. This study provides fundamental understanding of doping-modulated microstructural, charge carrier, and thermoelectric properties of hybrid halide perovskites, offering potential doping strategies to realize the theoretically predicted promising thermoelectric properties in their bulk single crystals for practical thermoelectric devices. The doping-modulated carrier transport will also guide the optimization of optoelectronic applications of halide perovskites for solar cells, light emitters, and photodetectors.

Investigation of optoelectronic and thermoelectric properties of novel BaCd2X2 (X = P, As, Sb) Zintl-phase for energy harvesting applications

The quest for new Zintl phases with promising thermoelectric properties has gained significant momentum recognitions to the precision of computational predictions. Our study presents an in-depth investigation of fundamental characteristics like structural, optoelectronic, optical, and transport aspects of BaCd2X2 (X = P, As, Sb) Zintl compounds. The stability of all compounds is firmly established through their optimized negative (-ve) formation energies and phonon dispersion spectra. Our findings affirm their robust structural integrity. Notably, we observe a semiconductor nature in these compounds, with calculated bandgap values of 1.35 eV for BaCd2P2, 0.85 eV for BaCd2As2, and 0.23 eV for BaCd2Sb2. Probing into optical properties, we uncover their potential utility in optoelectronic devices, as indicated by the optical response of these phases. We employ semi-classical Boltzmann theory, executed via BoltzTraP code, to explore their thermal behavior. Impressively higher values of Seebeck are attained, both at RT and elevated temperatures. Furthermore, the power factor exhibits an increasing trend with increasing temperature. Crucially, the analysis of the figure of merit (ZT) reveals promising values, 0.90, 0.81, and 0.72 for BaCd2X2 (X = P, As, Sb) at 300 K, respectively. These favorable optical and thermoelectric characteristics position these phases as attractive candidates for applications in optoelectronics and thermoelectric systems.

A first-principles study on the promising thermoelectric properties of SnX (X = S, Se, Te) compounds.

SnSe has emerged as an outstanding thermoelectric material due to its exceptional performance. In this study, first-principles calculations are employed to investigate the thermoelectric properties of materials within the SnX family, where X can be either S, Se, or Te. Initially, we assessed the stability of SnX (X = S, Se, Te). We found that SnS exhibits better mechanical and thermal stability than SnSe and SnTe. We then conduct phonon and electronic transport analysis. Following the general rule that heavier atoms have lower thermal conductivity, SnTe demonstrates lower thermal conductivity due to its low group velocity compared with SnS and SnSe. Regarding electrical transport properties, the band gaps for SnS, SnSe, and SnTe are 0.56, 0.54, and 0.35 eV, respectively. Notably, the small band gap and higher degeneracy in its band valleys for SnTe make it more effective for achieving a high power factor. The maximum ZT values are determined to be 1.41, 1.41, and 1.87 for SnS, SnSe, and SnTe, respectively. Remarkably, ZTmax of SnTe exceeds that of SnSe by 32.6%. Overall, the results clearly demonstrate that SnTe exhibits superior thermoelectric properties compared to SnSe and SnS. This study provides valuable insights into the electronic structure, thermal conductivity, and mechanical and thermal stability of materials within the SnSe family, such as SnS or SnTe, without the need for extensive and costly experimental work.

Ternary system based on polyaniline, graphene, and acrylic matrix applied to thermoelectric systems

AbstractThermoelectric (TE) materials have attracted attention for offering a green option for power generation, due to their ability to convert thermal energy into electricity. In recent years, a promising way to achieve efficiency in TE properties has been proposed based on composites of conjugated polymers, such as polyaniline (PANI), and carbon nanomaterials such as graphene (GR). Since polyaniline and GR composites are promising fillers for organic thermoelectric materials (OTE), we expanded their investigations for a ternary system (TS), providing materials with multiple functionalities, and high performance. In this research work, a TS based on an acrylic matrix (ACR), GR, and PANI was successfully prepared through the combination of in situ polymerization of aniline in contact with GR and mechanical mixture of the resulting hybrid with an ACR. Structural and morphological characterization confirmed that GR affected PANI morphology and crystallinity. The band gap determination by Tauc's relation indicated the occurrence of π‐π interaction between the chains and an increase of the electrical conductivity of the composites allowed to infer a synergistic effect. The measured Seebeck coefficient reached a maximum value of −17.02 μVK−1 and the highest power factor obtained was 4.94 μWm−1 K−2 for the ACR/PANI sample, indicating a material with promising thermoelectric properties.

Structural, Optical, Electrical, and Thermoelectric Properties of Bi2Se3 Films Deposited at a High Se/Bi Flow Rate.

Low-temperature synthesis of Bi2Se3 thin film semiconductor thermoelectric materials is prepared by the plasma-enhanced chemical vapor deposition method. The Bi2Se3 film demonstrated excellent crystallinity due to the Se-rich environment. Experimental results show that the prepared Bi2Se3 film exhibited 90% higher transparency in the mid-IR region, demonstrating its potential as a functional material in the atmospheric window. Excellent mobility of 2094 cm2/V·s at room temperature is attributed to the n-type conductive properties of the film. Thermoelectrical properties indicate that with the increase in Se vapor, a slight decrease in conductivity of the film is observed at room temperature with an obvious increase in the Seebeck coefficient. In addition, Bi2Se3 thin film showed an enhanced power factor of as high as 3.41 μW/cmK2. Therefore, plasma-enhanced chemical vapor deposition (PECVD)-grown Bi2Se3 films on Al2O3 (001) substrates demonstrated promising thermoelectric properties.

Zintl Phases: From Curiosities to Impactful Materials.

The synthesis of new compounds and crystal structures remains an important research endeavor in pursuing technologically relevant materials. The Zintl concept is a guidepost for the design of new functional solid-state compounds. Zintl phases are named in recognition of Eduard Zintl, a German chemist who first studied a subgroup of intermetallics prepared with electropositive metals combined with main-group metalloids from groups 13-15 in the 1930s. Unlike intermetallic compounds, where metallic bonding is the norm, Zintl phases exhibit a combination of ionic and covalent bonding and are typically semiconductors. Zintl phases provide a palette for iso- and aliovalent substitutions that can each contribute uniquely to the properties. Zintl electron-counting rules can be employed to interrogate a structure type and develop a foundation of structure-property relationships. Employing substitutional chemistry allows for the rational design of new Zintl compounds with technological properties, such as magnetoelectronics, thermoelectricity, and other energy storage and conversion capabilities. Discovering new structure types and compositions through this approach is also possible. The background on the strength and innovation of the Zintl concept and a few highlights of Zintl phases with promising thermoelectric properties in the context of structural and electronic design will be provided.

Overdamped Phonon Diffusion and Nontrivial Electronic Structure Leading to a High Thermoelectric Figure of Merit in KCu5Se3.

Thermoelectric copper selenides are highly attractive owing to not only their constituent nontoxic, abundant elements but also their ultralow liquid-like lattice thermal conductivity (κlat). For the first time, the promising thermoelectric properties of the new KCu5Se3 are reported herein, showing a high power factor (PF = 9.0 μWcm-1 K-2) and an intrinsically ultralow κlat = 0.48 Wm-1 K-1. The doped K1-xBaxCu5Se3 (x = 0.03) realizes a figure-of-merit ZT = 1.3 at 950 K. The crystallographic structure of KCu5Se3 allows complex lattice dynamics that obey a rare dual-phonon transport model well describing a high scattering rate and an extremely short phonon lifetime that are attributed to interband phonon tunneling, confinement of the transverse acoustic branches, and temperature-dependent anharmonic renormalization, all of which generate an unprecedently high contribution of the diffusive phonons (70% at 300 K). The overall weak chemical bonding feature of KCu5Se3 gives K+ cations a quiescence behavior that further blocks the heat flux transfer. In addition, the valence band edge energy dispersion of KCu5Se3 is quasilinear that allows a large Seebeck coefficient even at high hole concentrations. These in-depth understandings of the ultralow lattice thermal conductivity provide new insights into the property-oriented design and synthesis of advanced complex chalcogenide materials.

Dynamical approach to the atomic and electronic structures of the ductile semiconductor Ag2S.

Silver sulfide in monoclinic phase (α-Ag2S) has attracted significant attention owing to its metal-like ductility and promising thermoelectric properties near room temperature. However, first-principles studies on this material by density functional theory calculations have been challenging as both the symmetry and atomic structure of α-Ag2S predicted from such calculations are inconsistent with experimental findings. Here, we propose that a dynamical approach is imperative for correctly describing the structure of α-Ag2S. The approach is based on a combination of abinitio molecular dynamics simulation and deliberately chosen density functional considering both proper treatment of the van der Waals interaction and on-site Coulomb interaction. The obtained lattice parameters and atomic site occupations of α-Ag2S are in good agreement with experimental data. A stable phonon spectrum at room temperature can be obtained from this structure, which also yields a bandgap in accord with experimental measurements. The dynamical approach thus paves the way for studying this important ductile semiconductor in not only thermoelectric but also optoelectronic applications.

Cation Substitution and Size Effects in Ca2ZnSb2 and Yb2MnSb2: Crystal and Electronic Structures and Thermoelectric Properties

Zintl compounds often feature complex structural fragments and small band gaps, favoring promising thermoelectric properties. In this work, a new phase Ca2ZnSb2 is synthesized and characterized to be a LiGaGe-type structure. It is isotypic to Yb2MnSb2 with half vacancies at transition metal sites and undergoes a phase transition to Ca9Zn4+xSb9 after annealing. Interestingly, Ca2ZnSb2 and Yb2MnSb2 are amenable to diverse doping mechanisms at different sites. Here, by substituting smaller Li on cation sites, two novel layered compounds Ca1.84(1)Li0.16(1)Zn0.84(1)Sb2 and Yb1.82(1)Li0.18(1)Mn0.96(1)Sb2 with the P63/mmc space group are discovered, which can be viewed as derivatives of LiGaGe type. Despite having lower occupancy, the structural stability is improved compared with the prototype compounds owing to the reduced interlayered distances. Besides, the band structure analyses demonstrate that the bands near the Fermi level are mainly governed by the interlayered interaction. Due to the highly disordered structure, Yb1.82Li0.18Mn0.96Sb2 features ultralow thermal conductivity from 0.79 to 0.47 W·m-1·K-1 among the testing range; in addition, a remarkable Seebeck coefficient of 270.77 μV·K-1 at 723 K is observed. The discovery of the Ca2ZnSb2 phase enriches the 2-1-2 map, and the size effect induced by cations provides new ideas for material designing.

Insights into structural, elastic, mechanical, opto-electronic, and thermoelectric properties of rubidium-based fluoroperovskites RbXF3 (X = Zn, Cd, Hg)

In this study, we investigate the structural, electronic, optical, thermoelectric, elastic, and mechanical characteristics of Rb-based perovskites RbXF3 (X = Zn, Cd, Hg) by using ab-initio calculations based on density functional theory and semi-classical Boltzmann theory through the FP-LAPW method in WIEN2k. The ground-state computations were performed by using three approximations of the GGA, i.e., PBE-GGA, WC-GGA, PBE-sol GGA, and LSDA, as exchange correlation potentials. The optimized structural data obtained were consistent with the results for similar compounds. All samples were found to be indirect band gap semiconductors along the direction of high Γ-M symmetry, and the conclusions were consistent with those obtained for similar compounds in past studies. Furthermore, we explored the elastic constants of the fluoroperovskites to inspect their mechanical behavior by using the IRelast approach. The dependencies of ε(ω), optical conductivity σ(ω), loss function L(ω), and refractive index n(ω) of the materials on the spectral frequency provide a fundamental basis for attaining optical characteristics that are suitable for use in applications of optoelectronics. The Seebeck coefficient, electrical and thermal conductivities, power factor, and figure of merit of the samples were also calculated by using semi-classical Boltzmann theory in BoltzTraP code. The results of thermoelectric computations showed that the Rb-based fluoroperovskite compounds investigated here have promising thermoelectric properties.

Recent Advances in Multicomponent Organic Composite Thermoelectric Materials

AbstractOrganic thermoelectric materials have the potential to be used as power supplies for wearable electronics, implantable medical devices, and sensors in Internet of Things. The past 10 years has witnessed the rapid development of the research on organic thermoelectric materials, different material systems and optimizing strategies are developed, and the ZT values of some organic materials have approached that of many inorganic thermoelectric material systems in the low‐temperature region. Among different material systems, organic composite materials, especially multicomponent organic composite materials show very promising thermoelectric properties and offer more tunability compared to single component thermoelectric materials. Multicomponent organic composite thermoelectric materials can not only further improve the ZT values, but also have unique advantages of combining the merits of different materials, thus rendering the composites with better processing, mechanical, or other properties on demand. This review summarizes the concepts, the design criteria, the research progress of organic–inorganic and all‐organic multicomponent thermoelectric materials which contain three or more different ingredients, as well as their applications. Furthermore, the challenges and prospects are also analyzed to provide guidelines for the development of multicomponent organic composite thermoelectric materials.

Molybdenum disulfide under extreme conditions: An ab initio study on its melting

Crystalline molybdenum disulfide has become a central actor in the 2D-materials community due to its promising optoelectronic and thermoelectric properties. Despite the extensive work made in investigating these properties, a vast area of knowledge remains unknown on the structure and dynamics of its disordered phases such as liquid and amorphous. Thus, the goal of this work is to investigate the melting of bulk molybdenum disulfide using ab initio molecular dynamics based on density functional theory. We employ the two-phase and Z-methods to evaluate the melting in a number of conditions. Our results at 1 bar reveal that the two-phase procedure is preferred since it predicts a melting point of 2266.92 K that is directly computed using simulations at constant pressure and energy. In contrast, this temperature is indirectly estimated at 2154.01 K with the Z-method using an interpolation of simulations at constant volume and energy. Nevertheless, we find that both methods are complementary as they allow computing different thermodynamic and structural properties. For instance, we estimate a melting heat of 0.67 eV/atom with the two-phase coexistence route, which shows very good agreement with the value of 0.75 eV/atom obtained from the difference of the internal energies of separate crystalline and liquid ensembles at the same conditions of 1 bar and 2266.92 K. In contrast, the Z-method allows us to determine the influence of pressure on the melting temperature, density, and coordination number with a lower computational cost.

Low-temperature structure and thermoelectric properties of ductile Ag2S0.4Te0.6

Ductile Ag2S0.4Te0.6 shows promising thermoelectric properties and interesting phase transitions and structural features above room temperature. In this work, our experiments reveal that Ag2S0.4Te0.6 is composed of a Ag2Te-based glass and a Ag2S-based supercooled liquid at room temperature. The Ag2Te-based glass preserves its amorphous state in the measured temperature range (20–303 K), while the Ag2S-based supercooled liquid crystallizes into a triclinic phase below 183 K, leading to respectively the glass and crystalline-like thermal transport of Ag2S0.4Te0.6 near room temperature and at low temperature. At the crystallization point of the Ag2S-based phase, the electrical conductivity of Ag2S0.4Te0.6 shows an anomalous change. The extremely wide supercooling region (183–420 K) of the Ag2S-based phase leads to the unprecedented ductility of Ag2S0.4Te0.6. The thermoelectric figure of merit zT of Ag2S0.4Te0.6 increases monotonically with increasing temperature and reaches ∼0.17 at 300 K, showing the great potential as a ductile thermoelectric inorganic.

Dual Post‐Treatments Boost Thermoelectric Performance of PEDOT:PSS Films and Their Devices

AbstractOwing to intrinsically high electrical conductivity and low thermoelectric conductivity, poly(3,4‐ethylenedioxithiophene):poly(styrenesulfonate) (PEDOT:PSS) shows promising thermoelectric properties. However, its relatively low power factor limits the practical applications of PEDOT:PSS. Here, unique dual post‐treatments by sodium sulfite (Na2SO3) and formamide (CH3NO) to boost the thermoelectric performance of flexible PEDOT:PSS films with an optimized power factor of 74.09 µW m–1 K–2 are used. Comprehensive characterizations confirm that CH3NO reduces the excessive insulating PSS and thereby increases the electrical conductivity, while Na2SO3 lowers the reduction of the doping level of PEDOT, leading to an increased Seebeck coefficient. Furthermore, the rationally post‐treated PEDOT:PSS films are assembled into a flexible thermoelectric device that exhibits an open‐circuit voltage of 2.8 mV using the heat from the human arm and an output power density of 2.56 µW cm–2 by a temperature difference of 25 K, indicating great potential for practical applications on sustainably charging low‐grade wearable electronics.