High-performance Thermoelectric Devices Research Articles

InP3 facilitating high thermoelectric performance in Te-based composites via high pressure

Thermoelectric devices as eco-friendly candidates for energy conversion have attracted increasing interest as a sustainable and emission free solution to solve the environmental problem. Tellurium, is expected to be a suitable candidate, nevertheless, due to the poor electrical property, it has severely hampered the further development in practical application. This work proposes a new fabrication route for bulk Te via high pressure and high temperature method to enhance its power factor by compositing a certain quality of InP3. As InP3 decomposed into indium and black phosphorus under high pressure conditions, both the carrier concentration and Seebeck coefficient was enhanced significantly, leading to a maximum power factor of 11.4 μWcm-1K-2 at 460K, which increase by 44.2% compared to the peak value of pristine Te. Moreover, the induced defects, such as point defects, abundant grain boundaries and heterojunctions, strongly influence the thermoelectric performance, with a substantial decrease in lattice thermal conductivity. Consequently, the ZT values are sharply enhanced by InP3 compositing with the highest value reaching ∼0.76 of Te + 0.25wt.% InP3 at 490K. Importantly, the average ZT value within the 300K - 600K range reaches to 0.48, which is 3.5 times that of the pristine Te. This work mark a step toward developing high performance thermoelectric devices for energy conversion, making elemental Te more competitive with high-performance in medium temperature range.

Creation of flexible spin-caloritronic material with giant transverse thermoelectric conversion by nanostructure engineering

Functional materials such as magnetic, thermoelectric, and battery materials have been revolutionized through nanostructure engineering. However, spin caloritronics, an advancing field based on spintronics and thermoelectrics with fundamental physics studies, has focused only on uniform materials without complex microstructures. Here, we show how nanostructure engineering enables transforming simple magnetic alloys into spin-caloritronic materials displaying significantly large transverse thermoelectric conversion properties. The anomalous Nernst effect, a promising transverse thermoelectric phenomenon for energy harvesting and heat sensing, has been challenging to utilize due to the scarcity of materials with large anomalous Nernst coefficients. We demonstrate a remarkable ~ 70% improvement in the anomalous Nernst coefficients (reaching ~ 3.7 µVK−1) and a significant ~ 200% enhancement in the power factor (reaching ~ 7.7 µWm−1K−2) in flexible Fe-based amorphous materials by nanostructure engineering without changing their composition. This surpasses all reported amorphous alloys and is comparable to single crystals showing large anomalous Nernst effect. The enhancement is attributed to Cu nano-clustering, facilitating efficient transverse thermoelectric conversion. This discovery advances the materials science of spin caloritronics, opening new avenues for designing high-performance transverse thermoelectric devices for practical applications.

Synthesis, Characterization and Power Factor Estimation of SnSe Thin Film for Energy Harvesting Applications

In this work, a simple, cost-effective successive ionic layer adsorption and reaction (SILAR) deposition technique has been used to deposit a high-quality tin selenide (SnSe) thin film onto a glass substrate. Structural, morphologic, and thermoelectric properties have been characterized for the prepared thin film. X-ray diffraction (XRD) results of the SnSe thin film reveal an orthorhombic structure phase. The morphological properties of the prepared thin films have been studied using field emission scanning electron microscopy (FESEM). The stoichiometric composition of the deposited thin film and the elemental binding energies of the Sn and Se elements have been investigated with energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The Fourier transformation infrared (FTIR) spectrum of the SnSe thin film displays vibrational modes of chalcogenides bonds. These results suggest that the developed thin film is crystalline, uniform, and without impurities and is appropriate for energy harvesting applications. The prepared thin film’s Seebeck coefficient and electrical resistivity were estimated through ZEM-3 from room temperature to 600 K. The power factor was evaluated. A substantially high electrical conductivity is observed, which decreases somewhat with temperature, suggesting a semimetal conducting transport—the absolute values of the Seebeck coefficient increase with temperature. The resulting power factor showed the highest values near room temperature and a somewhat decreasing trend as the temperature increased. Despite lower values of the Seebeck coefficient, the substantially enhanced power factor is due to the higher electrical conductivity of the thin film, superior to that reported previously. This precursor study demonstrates promising results for developing high-performance flexible thermoelectric devices via a simple and facile SILAR strategy.

Hexylation Stabilises Twisted Backbone Configurations in the Prototypical Low-Bandgap Copolymer PCDTBT.

Conjugated donor-acceptor copolymers hold great potential as materials for high-performance organic photovoltaics, organic transistors and organic thermoelectric devices. Their low optical bandgap is achieved by alternation of donor and acceptor moieties along the polymer chain, leading to a pronounced charge-transfer character of electronic excitations. However, the influence of appended side chains and of chemical defects of the backbone on their photophysical and conformational properties remains largely unexplored on the level of individual chains. Here, we employ room temperature single-molecule photoluminescence spectroscopy on four compounds based on the prototypical copolymer PCDTBT with systematically changed chemical structure. Our results show that an increasing density of statistically added hexyl chains to the TBT comonomer distorts the molecular conformation, likely through the increase of average dihedral angles along the backbone. We find that, although the conformation becomes more twisted with high hexyl density, the side chains appear to stabilize the backbone in this twisted conformation. In addition, we demonstrate that homocoupling defects along the backbone barely influence the PL spectra of single chains, and thus intra-chain electronic properties.

Thermal transport and thermoelectric properties of transition metal dichalcogenides MoX 2 from first-principles calculation

The properties of two-dimensional (2D) materials have been extensively studied and applied in various applications. Our interest is to theoretically investigate the thermal transport and thermoelectric properties of the 2D transition metal dichalcogenides MoX 2 (X=S, Se, Te). We employ density functional theory and Boltzmann transport theory with relaxation-time approximation to calculate the electronic and transport properties. We also implemented the kinetic-collective model to improve the calculation of lattice thermal conductivity. Our calculations indicate that MoTe2 has the highest ZT of 2.77 among the other MoX 2 at 550 K due to its low thermal conductivity and high electrical conductivity. Consequently, we suggest that MoX 2 monolayers hold promise as materials for energy conversion devices due to their relatively high ZT. Moreover, these results could be beneficial to design 2D material-based high-performance thermoelectric devices.

High-performance flexible thermoelectric devices with a copper foam heatsink for personal thermal management

A Bi2Te3-based thermoelectric device with high thermoelectric performance and flexibility for human body waste heat recovery and temperature management.

Thermoelectric Power of a Single van der Waals Interface between Carbon Nanotubes.

Control of van der Waals interfaces is crucial for fabrication of nanomaterial-based high-performance thermoelectric devices because such interfaces significantly affect the overall thermoelectric performances of the device due to their relatively high thermal resistance. Such interfaces could induce different thermoelectric power from the bulk, i.e., interfacial thermoelectric power. However, from a macroscopic point of view, a correct evaluation of the interfacial thermoelectric power is difficult owing to various interface configurations. Therefore, the study of the thermoelectric properties at a single interface is crucial to address this problem. Herein, we used in situ transmission electron microscopy and nanomanipulation to investigate the thermoelectric properties of carbon nanotubes and their interfaces. The thermoelectric power of the bridged carbon nanotubes was individually measured. The existence of the interfacial thermoelectric power was determined by systematically changing the contact size between the two parallel nanotubes. The effect of interfacial thermoelectric power was qualitatively supported by Green's function calculations. When the contact length between two parallel nanotubes was less than approximately 100 nm, the experimental results and theoretical calculations indicated that the interface significantly contributed to the total thermoelectric power. However, when the contact length was longer than approximately 200 nm, the total thermoelectric power converged to the value of a single nanotube. The findings herein provide a basis for investigating thermoelectric devices with controlled van der Waals interfaces and contribute to thermal management in nanoscale devices and electronics.

Enhancing conductivity on p-type (Bi,Sb)2Te3 surface by introducing slip trace

Structural modification can effectively tune and optimize the thermoelectric properties of Bi2Te3-based alloys. Herein, we present an innovative and controllable approach to enhance the conductivity of the Bi2Te3-based alloy by introducing slip traces. The effect of slip traces on the conductivity of the Bi2Te3-based alloy was comprehensively investigated using advanced techniques such as Electron Backscatter Diffraction (EBSD) and conductive Atomic Force Microscopy (c-AFM). The results revealed that slip traces were readily formed in (Bi,Sb)2Te3 grains when a load was applied perpendicular to the prismatic plane. The Point Discharge Effect (PDE) was found to promote carriers moving towards the slip traces, leading to the accumulation of positively charged holes. As a result of this process, a higher current intensity was detected at the slip traces. This innovative strategy offers a convenient and effective way to fabricate high-conductivity thermoelectric materials and thus will inspire new technological possibilities for high-performance thermoelectric devices.

Towards energy filtering in Mg2X-based composites: Investigating local carrier concentration and band alignment via SEM/EDX and transient Seebeck microprobe analysis

A comprehensive understanding of multi-phase materials requires the characterization of transport properties at the micro/nano-scale. Optimized alignments of the conduction bands and valence bands at the interfaces of multi-phase materials can enhance the properties of thermoelectric materials by filtering out undesired charge carriers and is a promising path towards high performance thermoelectric devices. Here, a micro-scale characterization technique using transient Seebeck microprobe analysis, scanning electron microscopy with energy-dispersive X-ray spectroscopy, and electronic transport modelling based on the Boltzmann transport equation modeling is introduced to assess the band diagram and estimate the band offset in multi-phase materials. This characterization technique is applied to a composite of magnesium silicide-based materials. This material class is prone to form self-assembling nano-structured composites driven by a miscibility gap in the solid solutions series. Our analysis reveals changes in carrier concentration upon composite formation and considerable band offsets for both conduction and valence band when synthesizing nominally undoped composites. Employing Bi as dopant we show that Bi exhibits a preference for the Sn-rich phase, changing the carrier concentration differently in the Sn-rich matrix phase compared to the Si-rich secondary phase, effectively altering the band alignment of the composite. This demonstrates that our approach can be utilized to measure and manipulate the band offset within the composite to achieve optimal thermoelectric performance.

High-frequency complex impedance analysis of the two-dimensional semiconducting MXene Ti2CO2

The two-dimensional compound group of MXenes, which exhibit unique optical, electrical, chemical, and mechanical properties, are an exceptional class of transition metal carbides and nitrides. In addition to traditional applications in Li-S, Li-ion batteries, conductive electrodes, hydrogen storage, and fuel cells, the low lattice thermal conductivity coupled with high electron mobility in the semiconducting oxygen-functionalized MXene Ti2CO2 has led to the recent interests in high-performance thermoelectric and nanoelectronic devices. Apart from the above dc- transport applications, it is crucial to also understand ac- transport across them, given the growing interest in applications surrounding wireless communications and transparent conductors. In this work, we investigate using our recently developed ab initio transport model, the real and imaginary components of electron mobility and conductivity to conclusively depict carrier transport beyond the room temperature for frequency ranges upto the terahertz range. We also contrast the carrier mobility and conductivity with respect to the Drude’s model to depict its inaccuracies for a meaningful comparison with experiments. Our calculations show the effect of acoustic deformation potential scattering, piezoelectric scattering, and polar optical phonon scattering mechanisms. Without relying on experimental data, our model requires inputs calculated from first principles using density functional theory. Our results set the stage for providing ab initio based ac- transport calculations given the current research on MXenes for high-frequency applications.

Enhanced electrical and mechanical properties of Bi2Te3-based thermoelectric thick films enabled by a practical dynamic regulation strategy

The application of high-density and high-performance micro thermoelectric devices is still in its infancy, mainly restricted by the low performance of Bi2Te3-based thick film as well as the limited device integration. In this study, we proposed a dynamic regulation strategy to simultaneously strengthen the thermoelectric and mechanical properties for n-type Bi2Te3-based thick films. The effects of growth temperature and time on thermoelectric properties have been firstly explored. As the thermoelectric properties exhibit consistent degradation with increasing thickness at static growth temperature, an effective rising temperature method is introduced to dynamically regulate the nucleation rate and growing diffusion ability. Thus, the grain refinement with compact texture structure leads to a relatively large carrier mobility (77.1 cm2·V−1·s−1) and appropriate concentration (5.25 × 1019 cm−3) as well as further 12% improvement of power factor with an average value up to 12.0 μW·cm−1·K−2 over a wide temperature ranging from 313 K to 453 K. Furthermore, significant enhancement of mechanical property is also achieved with high elastic modules (56.03 GPa), hardness (0.63 GPa) and large energy dissipation capacity to prevent micro-cracks. This study provides a practical solution with dynamic temperature control to fabricate high-performance Bi2Te3 thick films with enhanced mechanical property and processing feasibility for micro thermoelectric devices.

Rapid growth of high-performance Bi2Te3 thin films by laser annealing at room temperature

Bismuth telluride, (Bi2Te3), is of great scientific and technical importance due to its excellent thermoelectric performance near room temperature. However, deposition of Bi2Te3 thin films suffers from composition deviation due to preferential loss of Te at a high-temperature deposition environment. We demonstrated a fast and efficient laser annealing method to synthesize high-quality stoichiometric Bi2Te3 thin films at room temperature from Te/Bi stacking films. The production rate of Bi2Te3 was around 180 times higher than conventional thermal annealing. The formation of Bi2Te3 thin films was studied with respect to annealing laser energy density. Bi2Te3 thin films with an Bi:Te atomic ratio of 2:3 were achieved at an annealing laser energy density of 6.6 mJ/cm2. The structural, chemical, and thermoelectric performance of laser-annealed films were compared with conventional thermally annealed films. The Seebeck coefficient of laser-annealed films reached −167 µV/K, which was1.3 higher than that of thermally annealed films, evidencing the superiority of the laser annealing method in producing high quality Bi2Te3 stochiometric films. Rapid growth of stoichiometric Bi2Te3 thin films by laser annealing holds great promise for the development of high-performance thermoelectric devices.

High-performance Sb2Si2Te6 thermoelectric device

High-performance thermoelectric (TE) devices are promising for waste-heat recovery and power generation. Recently, Sb2Si2Te6 has been demonstrated as a promising p-type TE material with high performance and low cost. However, Sb2Si2Te6-based TE devices have not yet been fabricated. The performance of TE devices depends on not only TE materials, but also electrode materials and inner interfaces. Herein, we utilize the high throughput strategy to screen the appropriate electrode material and find that cobalt exhibits a low contact resistance (2.47 μΩ cm−2) and a high shear strength (18.03 MPa). Furthermore, we successfully fabricate one eight-couple Sb2Si1.99Ge0.01Te6 (p-type)/Yb0.3Co4Sb12 (n-type) TE module with optimized geometric parameters by the finite element method. The module exhibits a high output power density (0.92 W cm−2) and a decent energy conversion efficiency (4.2%) under a temperature difference of 500 K. Our studies will promote further development of Sb2Si2Te6 materials and Sb2Si2Te6-based devices.

Boosting thermoelectric performance of carbon nanotube-based materials and devices by radical-containing molecules

Organic thermoelectric materials are attracting growing research interests as they are seen as environmentally friendly functional materials with tremendous potential for use in waste heat recovery. The absence of low electrical conductivity has somewhat slowed down the development of most organic thermoelectric materials. Blending the organic semiconductor materials with single walled carbon nanotube (SWCNT) has become an extremely important strategy to boost the conductivity and thermoelectric performance of organic materials. Therefore, in this paper, novel arylimide compounds NDI-2 T and PDI-2 T with radical substituents of TEMPO are designed from molecular engineering, complexing with SWCNT as organic thermoelectric composites. Compared with the non-radical compounds of NDI-2TP and PDI-2TP, the introduction of radicals in NDI-2 T and PDI-2 T can significantly improve the performance of the thermoelectric composites. Radical-containing composite films exhibit outstanding thermoelectric performance compared to the film based on non-radical molecules. The best power factor reaching 278.2 ± 8.2 μW m−1 K−2 for p-type films, while n-type films can attain 64.6 ± 13.5 μW m−1 K−2. The NDI-2 T/SWCNT devices can obtain a maximum output power of 0.94 μW when the temperature gradient is around 60 K, which is 5 times higher of NDI-2TP/SWCNT device. The significantly enhanced thermoelectric performance of the composite film based on organic radical compounds is resulted from the increased interactions between the radical compounds and SWCNTs and modulated carrier concentrations in the composite films. This work provides a new direction for the design of high-performance thermoelectric materials and devices.

Does a Blatter Radical Retain Its Open-Shell Character When Incorporated into Gold–Molecule–Gold Junctions?

One of the key issues for the use of organic radicals in high-performance molecular electronic, spintronic and thermoelectric devices is the retention, at room temperature, of their open-shell character when they are in contact with the metal electrodes of a junction. By means of first-principles quantum transport calculations, we have investigated the stability and the electronic transport properties of single-molecule junctions incorporating a Blatter radical (BR) with thiomethyl linker groups. Our calculations suggest that the BR can unequivocally retain its open-shell nature even when bound to gold electrodes through undercoordinated gold adatoms. Such a state cannot be destroyed by the surrounding solvent molecules and the applied bias. The calculated low-bias conductance values of junctions with BR, an oxidized BR derivative, and a closed-shell stilbene molecule are in quantitative agreement with the available experimental measurements. The small conductance of the BR can be associated with the localization of its π-type singly occupied and singly unoccupied molecular orbitals (SOMO and SUMO), leading to an asymmetrically weak electronic coupling to the two gold electrodes, even though these two orbitals lie very close to the Fermi energy. Furthermore, the nonmonotonic modulation of the junction conductance controlled by a charge gate can be used in experiments to further verify the presence of the unpaired electron. This proof may be realized in practice with an electrostatic solid-state or an electrochemical gate. Our findings deepen the current understanding of the radical–metal interfacial properties and facilitate the design of future radical-based multifunctional molecular devices.

Minimization of the electrical contact resistance in thin-film thermoelectric device

High electrical contact resistance refrains the performance of thin-film thermoelectric devices at the demonstrative level. Here, an additional Ti contact layer is developed to minimize the electrical contact resistance to ∼4.8 Ω in an as-assembled thin-film device with 50 pairs of p–n junctions. A detailed interface characterization demonstrates that the low electrical contact resistance should be mainly attributed to the partial epitaxial growth of Bi2Te3-based thin-film materials. Correspondingly, the superlow electrical contact resistance facilitates the applicability of the out-of-plane thin-film device and results in an ultrahigh surface output power density of ∼81 μW cm−2 at a low temperature difference of 5 K. This study illustrates the Ti contact layer that strengthens the contact between Cu electrodes and Bi2Te3-based thermoelectric thin films mainly through partial epitaxial growth and contributes to high-performance thin-film thermoelectric devices.

Metal Complex Molecular Junctions as Thermoelectric Devices.

Single-molecule junctions and self-assembled monolayer junctions are attractive architectures for thermoelectric devices. However, given the poor thermoelectric performance of organic molecules investigated so far, molecules characterized by high conductance and Seebeck coefficient values are desired to be explored. Metal complexes are promising possible active components of high-performance thermoelectric devices given that metal-ligand combinations and functions can be made to vary to modulate the transmission functions, thus strongly influencing conductance and Seebeck coefficient. In this concept article are described recent studies wherein thermoelectric measurements were conducted on metal complex junctions. Furthermore, the potential for the use of the junctions in thermoelectric devices is discussed.

Synergistically enhanced thermoelectric and mechanical performance of Bi2Te3 via industrial scalable hot extrusion method for cooling and power generation applications

To meet the fast-expanding needs of thermoelectric cooling for high heat flux systems, high-performance thermoelectric devices must be constructed with materials possessing both high thermoelectric properties and mechanical strength. However, the synergistic improvement of thermoelectric and mechanical properties is challenging because of the anisotropic thermal and electrical transports in the layered Van der Waals structure of Bi2Te3, particularly for n-type Bi2(TexSe1-x)3 material. The hot extrusion (HE) technique was believed as the most efficient approach to decoupling the thermoelectric and mechanical properties of Bi2Te3. However, the mechanism and performance of Bi2Te3 materials prepared by the HE method have been elusive. In this work, we developed an industrial-scalable HE technique to manipulate the texture ordering in Bi2Te3 material, achieving simultaneous improvement of thermoelectric and mechanical properties. Compared to SPS samples, our HE samples not only show 19% enhancement of the peak ZT, but also exhibit record high compressive strength of 205 MPa and flexural strength of 79 MPa, respectively. Furthermore, 7-pairs thermoelectric modules were fabricated to verify the cooling and power generation performance. A maximum temperature difference of 72 K (Th = 300 K) compared to 68 K for commercial devices and a maximum conversion efficiency of 5% at Th ≤ 500 K were realized.

Anisotropy engineering in solution-derived nanostructured Bi2Te3 thin films for high-performance flexible thermoelectric devices

Flexible thermoelectric (TE) technology can convert human-body heat into electricity, showing great promise for powering wearable devices. Achieving simultaneous excellent flexibility and high TE performance is still a big challenge for both inorganic and organic-based flexible TE materials. Here, to overcome this challenge in the well-known Bi2Te3-based materials, we propose an anisotropy engineering strategy triggered by nanostructuring design and excess Te addition. A two-step solution-synthesis method is developed to prepare patchwork-like Bi2Te2.7Se0.3 nanorods, which can be assembled into flexible thin films by the screen-printing and spark-plasma-sintering process. Owing to the weakened anisotropy by nanostructuring, our nanorods-derived Bi2Te2.7Se0.3 thin films exhibit excellent bending flexibility. Further introducing excess Te can optimize the anisotropy as well as the interfacial connecting and oxidation resistance of these thin films. Consequently, a high power factor of ∼ 745 μWm−1K−2 at room temperature can be achieved in the n-type Bi2Te2.7Se0.3 thin film with 10 % excess Te. These n-type thin films are further assembled with p-type legs (Bi0.5Sb1.5Te3 with 4 % excess Te) into a 5-pair TE device with good flexibility and stability, showing a maximum power density of ∼ 6.06 Wm−2 at a temperature difference of ∼ 28.3 K. This work indicates that anisotropy engineering in solution-derived nanostructured thin films can be a facile way to balance the flexibility and TE properties, further advancing flexible TE devices.

Thermoelectric Figure of Merit of a Superatomic Crystal Re6Se8I2 Monolayer

Superatomic materials are newly emerging candidates for high-performance thermoelectric (TE) devices due to their intrinsic ultralow thermal conductivities. However, the low TE power factor becomes a huge obstacle to reaching the required dimensionless figure of merit (ZT) values for practical applications. Here, motivated by the recently synthesized superatomic ${\mathrm{Re}}_{6}{\mathrm{Se}}_{8}{\mathrm{I}}_{2}$ monolayer [He et al. J. Am. Chem. Soc. 144, 74 (2022)], we study its superior TE properties by using density functional theory combined with phonon Boltzmann transport theory and deep potential molecular dynamics. We show that the large mass and anharmonic $\mathrm{Re}---\mathrm{I}$ bonds introduce strong phonon scattering and result in a low lattice thermal conductivity of 1.20 W m${}^{\ensuremath{-}1}$ K${}^{\ensuremath{-}1}$ at 300 K, while the strong and harmonic $\mathrm{Re}---\mathrm{Se}$ network ensures a high TE power factor of 4344 \textmu{}W m${}^{\ensuremath{-}1}$ K${}^{\ensuremath{-}2}$ in the b direction for n-type doping. This is an order of magnitude higher than those of other cluster-based materials. Owing to the low thermal conductivity and high TE power factor, ${\mathrm{Re}}_{6}{\mathrm{Se}}_{8}{\mathrm{I}}_{2}$ exhibits higher ZT values of 1.20 (500 K) and 1.43 (900 K) with n-type doping along the b direction compared with other cluster-based TE materials reported so far.