Thermoelectric Figure Of Merit Research Articles

A Universal, Highly Stable Dopant System for Organic Semiconductors Based on Lewis-Paired Dopant Complexes.

Chemical doping of organic semiconductors is an essential enabler for applications in electronic and energy-conversion devices such as thermoelectrics. Here, Lewis-paired complexes are advanced as high-performance dopants that address all the principal drawbacks of conventional dopants in terms of limited electrical conductivity, thermal stability, and generality. The study focuses on the Lewis acid B(C6F5)3 (BCF) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) bearing Lewis-basic -CN groups. Due to its high electron affinity, BCF:F4TCNQ dopes an exceptionally wide range of organic semiconductors, over 20 of which are investigated. Complex activation and microstructure control lead to conductivities for poly(3-hexylthiophene) (P3HT) exceeding 300 and 900 S cm-1 for isotropic and chain-oriented films, respectively, resulting in a 10 to 50 times larger thermoelectric power factor compared to those obtained with neat dopants. Moreover, BCF:F4TCNQ-doped P3HT exhibits a 3-fold higher thermal dedoping activation energy compared to that obtained with neat dopants and at least a factor of 10 better operational stability.

Significant Improvement in Thermoelectric Properties of Carbon-Reinforced Cementitious Composites Achieved by Regulating the Graphitization Degree of Expanded Graphite.

Building structures are exposed to direct sunlight for a long time, accumulating a large amount of low-grade thermal energy, which aggravates environmental pollution and energy consumption. Thermoelectric cement-based composites can realize the interconversion of thermal and electrical energy, showing great potential benefits in large-scale heat collection and energy conversion. Although a lot of exploration and research has been carried out on the thermoelectric properties of cement-based composites reinforced with carbon materials, the contribution of the characteristics of carbon materials, such as the graphitization degree, to the thermoelectric properties of cement-based composites is still unclear. In this article, the graphitization degree of expanded graphite (EG) was modulated by etching EG with an acid solution. The low graphitization degree improves the effective mass of carriers and aggravates the electron and phonon scattering at the interface of EG/cement-based composites. Low thermal conductivity was obtained while increasing the Seebeck coefficient of EG/cement-based composites. The power factor (17.1 μW m-1 K-2) and thermoelectric figure of merit (2.95 × 10-3) of the sample are increased by 18.6 times and 44.2 times, respectively, achieving the highest thermoelectric performance in cement-based composites reinforced with carbon materials. This study provides a direction for improving the thermoelectric properties of cement-based composites by structural regulation of carbon materials.

Studying supercapacitance and thermoelectric performance of semiconductor BaTiO[formula omitted] along with its halfmetal alloy of BaTiFO[formula omitted]: A DFT+U investigation

In the present work, the electrical, supercapacitance, and thermoelectric properties of BaTiO3 semiconductor perovskite and its alloy with F atoms (in replacement of one of the O atoms) have been investigated using density functional theory and implementing general gradient approximation by considering Hubbard’s potential (U). The electronic density of state results for the BaTiO3 exhibit energy band gaps of 1.833 and 2.143 eV without and with considering U, respectively. Moreover, by implementing the F atom and because of the existence of Ti atoms in the compound the ferromagnetic property appeared in the alloy compound with the total magnetization of 0.97 Bohr mag/cell. The halfmetallic behavior emerged in the alloy compound by using GGA+U and the spin down showed an energy band gap of 3.32 eV while the spin up exhibited metallic properties. Furthermore, the most significant peak of the areal quantum capacitance was 871.71 μF/cm2 for BaTiO3 by the GGA method, and by considering U it was 765.78 μF/cm2. In addition, the most prominent peak of thermoelectric power factor per relaxation time at room temperature belonged to BaTiO3 with a value of 42.05 × 1016μW m−1 s−1 K−2 with considering U, and without considering U, it was 34.7 × 1016μW m−1 s−1 K−2.

Semiconducting Carbon Nanotube Extraction Enabled by Alkylated Cellulose Wrapping.

With the growing demand for postsilicon electronics, the purification of single-walled carbon nanotubes (SWCNTs) in terms of their chirality, which defines their atomic and electronic structure, is becoming increasingly important. Herein, we demonstrate the selective extraction of high-quality semiconducting SWCNTs using alkyl cellulose as a dispersant in organic solvents. We investigated the separation factors of dispersant structures, such as the degree of substitution (DS) and molecular weight, and clarified the appropriate dispersant structures, such as moderately substituted hexyl cellulose, for selective semiconducting SWCNT extraction. Due to the improved purity and quality of the semiconducting SWCNTs obtained by this method, their films exhibit excellent thermoelectric power factors, outperforming not only unsorted SWCNTs but also conducting polymer-sorted SWCNTs. This sorting technology paves the way for supplying high-quality semiconducting SWCNTs in a viable manner.

High thermoelectric power factor of SbPbTe thin alloy film grown by thermal evaporation and post-annealing treatment

Simple chemical compositions are frequently preferred in practical thermoelectric material applications due to their high growth success rates and increased stability. In this study, an n-type SbPbTe thin alloys film with equal amounts of all elements was grown, and the effect of equal contributions on thermoelectric properties and post-annealing treatment for 1–4 h was investigated. The as-grown SbPbTe thin alloys film and post annealed sample exhibited uniform composition distributions, single-phase microstructures, and non-uniform grain sizes at both micro- and nanoscales. The 4-h post annealed SbPbTe thin alloys film demonstrated maximum thermoelectric power factor of 21.2 μWcm−1K−2 at 500 K. This improvement was attributed to a decrease in electrical conductivity and the maintenance of a relatively maximum Seebeck coefficient achieved through the adjustment of carrier concentrations. The small grain size distribution over the surface contributed to strengthened grain boundary scattering, enhanced the charge carrier density and energy filtering effect. The thermoelectric power factor for the 4-h annealed sample peaks at 500 K, indicating optimal thermoelectric performance, as calculated from the Seebeck coefficient and electrical conductivity values. These findings provide insights into the structural and thermoelectric properties of SbPbTe thin alloy films, pointing to potential ways to improve their thermoelectric efficiency.

Fast preparation and thermoelectric performance of AgSTe based materials via plasma activated sintering

AgSTe based materials have recently been identified as promising thermoelectric materials that have favorable properties at room temperature. They are usually obtained by conventional processes involving melting, annealing, and sintering, which incur significant energy and time expenditures. The present investigation involved the synthesis of Ag20S7Te3 compounds by a fast non-equilibrium methodology, i.e., plasma activated sintering from raw elemental powders. The monoclinic Ag20S7Te3 phase crystal was successfully formed at a temperature of 673 K, accompanied by the presence of a secondary monoclinic phase, Ag2S. The Ag20S7Te3 phase predominated in the monoclinic form, constituting a significant percentage at temperature of 873 K. Furthermore, the thermoelectric figure of merit (zT) of the AgSTe phase at 873 K was determined to be 0.42. The rapid sample synthesis described herein has the potential to significantly reduce the duration required for material synthesis, hence presenting a conspicuous advantage in the context of manufacturing on a wide scale.

Counterion docking: a general approach to reducing energetic disorder in doped polymeric semiconductors

Molecular doping plays an important role in controlling the carrier concentration of organic semiconductors. However, the introduction of dopant counterions often results in increased energetic disorder and traps due to the molecular packing disruption and Coulomb potential wells. To date, no general strategy has been proposed to reduce the counterion-induced structural and energetic disorder. Here, we demonstrate the critical role of non-covalent interactions (NCIs) between counterions and polymers. Employing a computer-aided approach, we identified the optimal counterions and discovered that NCIs determine their docking positions, which significantly affect the counterion-induced energetic disorder. With the optimal counterions, we successfully reduced the energetic disorder to levels even lower than that of the undoped polymer. As a result, we achieved a high n-doped electrical conductivity of over 200 S cm−1 and an eight-fold increase in the thermoelectric power factor. We found that the NCIs have substantial effects on doping efficiency, polymer backbone planarity, and Coulomb potential landscape. Our work not only provides a general strategy for identifying the most suitable counterions but also deepens our understanding of the counterion effects on doped polymeric semiconductors.

Tuning of thermoelectric performance by modulating vibrational properties in Ni-doped Sb2Te3

Sb2Te3, a binary chalcogenide-based 3D topological insulator, attracts significant attention for its exceptional thermoelectric performance. We report the vibrational properties of magnetically doped Sb2Te3 thermoelectric material. Ni doping induces defect/disorder in the system and plays a positive role in engineering the thermoelectric properties through tuning the vibrational phonon modes. Synchrotron powder x-ray diffraction study confirms good crystalline quality and single-phase nature of the synthesized samples. The change in structural parameters, including B iso and strain, further corroborate with structural disorder. Detailed modification of phonon modes with doping and temperature variation is analysed from temperature-dependent Raman spectroscopic measurement. Compressive lattice strain is observed from the blue shift of Raman peaks owing to Ni incorporation in Sb site. An attempt is made to extract the lattice thermal conductivity from total thermal conductivity estimated through optothermal Raman studies. Hall concentration data support the change in temperature-dependent resistivity and thermopower. Remarkable increase in thermopower is observed after Ni doping. Simulation of the Pisarenko model, indicating the convergence of the valence band, explains the observed enhancement of thermopower in Sb2−x Ni x Te3. The energy gap between the light and heavy valence band at Γ point is found to be 30 meV (for Sb2Te3), which is reduced to 3 meV (in Sb1.98Ni0.02Te3). A significant increase in thermoelectric power factor is obtained from 715 μWm−1K−2 for pristine Sb2Te3 to 2415 μWm−1K−2 for Ni-doped Sb2Te3 sample. Finally, the thermoelectric figure of merit, ZT is found to increase by four times in Sb1.98Ni0.02Te3 than that of its pristine counterpart.

Enhancing thermoelectric efficiency of benzodithiophene-thienothiophene copolymers through doping-induced charge transfer

Organic thermoelectric materials have been gaining increasing interest due to their capacity to convert waste heat into electricity. This study explores the impact of doping-induced charge transfer states on the thermoelectric properties of benzodithiophene-thieno[3,2-b]thiophene-based polymers (PBDTTT) doped with 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). Detailed structural, optical, and electrical analyses were conducted on two specific polymers, PBDTTT:C (P1) and PBDTTT:EFT (P2). The investigation revealed distinct doping behaviors: P1 formed a partial charge transfer (PCT) complex, whereas P2 predominantly formed an integer charge transfer (ICT) complex. This distinction is attributed to the inherent structural differences and varying aggregation capabilities of the polymers. As a result, P2 exhibited enhanced electrical conductivity and a superior thermoelectric power factor compared to P1. This study emphasizes the critical role of polymer aggregation and charge-transfer mechanisms in optimizing the thermoelectric performance of conjugated polymers. It offers valuable insights for the development of efficient thermoelectric materials and the advancement of energy conversion technologies.

Ultralow thermal conductivity and high thermoelectric performance induced by dimensionality reduction in chain-like compounds X2PtSe2 (X=K, rb)

Balancing low lattice thermal conductivity (κL) with a high power factor is a major challenge in the research of thermoelectric (TE) materials. In this paper, we report on a class of chain-like compounds, X2PtSe2(X = K, Rb), characterized by strong lattice anharmonicity, weak bonds, and low phonon group velocities. Using first-principles calculations in conjunction with self-consistent phonon theory and the Boltzmann transport equation, we systematically investigate the thermal and electrical transport properties of X2PtSe2(X = K, Rb). At room temperature, the κL of K2PtSe2 and Rb2PtSe2 in the direction perpendicular to the Pt–Se chains is only 0.15 and 0.10 Wm−1K−1, respectively. Additionally, due to the low-dimensional nature of the Pt–Se chains, these compounds exhibit a ‘pudding-mold type’ band structure, contributing to a TE power factor. At 300K, under n-type doping, K2PtSe2 and Rb2PtSe2 achieve TE figures of merit (ZT) values of 1.66 and 1.67, respectively, in the direction perpendicular to the Pt–Se chains. At 500K, the ZT value of Rb2PtSe2 approaches 4, far surpassing traditional TE materials. These results indicate that the chain-like compounds X2PtSe2(X = K, Rb) exhibit excellent TE performance, achieving energy conversion efficiencies comparable to commercial transducer devices.

Epitaxial growth of high-quality Mg3Sb2 thin films on annealed c-plane Al2O3 substrates and their thermoelectric properties

High-quality epitaxial Mg3Sb2 thin films are promising thermoelectric materials to enable practical applications of compact and environmentally friendly thermoelectric conversion at RT. In this study, high-quality single-crystal Mg3Sb2 with high c-plane orientation was epitaxially grown directly on annealed c-Al2O3 substrates without passive layers. These thin films exhibited about three times higher thermoelectric power factor than any previously reported values due to high carrier mobility. The ultra-smooth surface of the annealed c-Al2O3 substrate facilitated the formation of high-quality Mg3Sb2 thin films without passive layers or polycrystalline interfaces that could be carrier scatters.

Investigation on the electrical and thermal properties of Cr-doped FeSe2 alloys

FeSe2 is a p-type semiconductor with a narrow band gap, which can be considered a potential thermoelectric material. Herin, the electrical and thermal properties of Cr-doped FeSe2, Fe1-xCrxSe2 (x = 0, 0.03, 0.06, and 0.09), polycrystalline alloys are investigated. All compositions show single-phase marcasite structures of FeSe2 with gradual decreases in lattice parameters with Cr doping x. With increasing x, the electrical conductivity gradually increases from 13 (x = 0) to 46 S/cm (x = 0.09), owing to a large increase in carrier concentration originated from the substitution of Cr2+ and Cr3+ at the Fe site. However, the magnitude of Seebeck coefficient decreases considerably at 300 K, resulting in decrease in power factor. In addition, the density-of-state effective mass decreases significantly from 6.0 m0–1.04 m0. At 700 K, the decrease in Seebeck coefficient is less severe. Although the total and lattice thermal conductivity decrease at all temperatures owing to additional phonon scattering of the extrinsic Cr ions, a thermoelectric figure of merit, zT, generally decreases compared to that of the pristine sample due to power factor decrease at 300 K. Meanwhile, a slight increase of zT to 0.11 is observed for x = 0.06 at 700 K.

Self-powered PEDOT: PSS/CSWCNT/Mxenes thermoelectric films and wearable electronics devices

In this work, Ti3C2–Cl is introduced into poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonic acid) / single-walled carbon nanotubes (PEDOT: PSS/SWCNT, noted as PPSC) films to prepare ternary thermoelectric films of PEDOT: PSS/SWCNT/Ti3C2–Cl (PPSCT). The microscopic morphology, structures, and thermoelectric properties of PPSC and PPSCT films show that the introduction of Ti3C2–Cl results in a tendency of decreasing electrical as well as thermal conductivity, and the Seebeck coefficient is increased. When the content of Ti3C2–Cl reaches 8 wt%, the conductivity, Seebeck coefficient, and thermal conductivity of the PPSCT films reach the optimum values of 1683 S/cm, 21.69 μV/K, and 14.97 W/mK, respectively, and the power factor (PF) and thermoelectric figure of merit (ZT) are 79.23 μW/mK2 and 1.58 × 10–3. The UPS and Hall effect results reveal that the improvement of the Seebeck effect of PPSCT mainly results from the ternary interfacial energy barrier as well as the change of carrier concentration. In addition, the wearable electronics devices demonstrate an output voltage of 4.5 mV and an output power of 54.36 nW under a 50 K temperature difference. The device displays a good monitoring sensitivity by reaching an output voltage of 0.6 mV at a 7 K temperature difference between the skin and the environment.

Order-N calculations for thermoelectric power factor based on linear response theory

We present an order-N quantum transport calculation methodology to evaluate thermoelectric transport coefficients, such as electric conductivity and Seebeck coefficient. Different from a conventional method using the electric conductivity spectrum, it obtains the coefficients directly from the correlation function between heat and electric current based on linear response theory. As an example, we apply the methodology to a two-dimensional square-lattice model with static disorder and confirm that the calculated results are consistent with those obtained by the conventional method. The proposed methodology provides an effective approach to evaluate the thermoelectric performance of micron-scale materials based on quantum mechanics from an atomistic viewpoint.

Combination of Counterion Size and Doping Concentration Determines the Electronic and Thermoelectric Properties of Semiconducting Polymers.

In both chemical and electrochemical doping of organic semiconductors (OSCs), a counterion, either from the electrolyte or ionized dopant, balances the charge introduced to the OSC. Despite the large influence of this counterion on OSC optical and electronic response, there remains substantial debate on how a fundamental parameter, ion size, impacts these properties. This work resolves much of this debateby accounting for two doping regimes. In the low-doping regime, the Coulomb binding energies between charge carrierson the OSC and the counterions are significant, and larger counterions lead to decreased Coulomb interactions, more delocalized charge carriers, and higher electrical conductivities. In the high-doping regime, the Coulomb binding energies become negligible due to the increased dielectric constant of the films and a smoothing of the energy landscape; thereby, the electrical conductivities depend primarily on the extent of morphological disorder in the OSC. Moreover, in regioregular poly(3-hexylthiophene), rr-P3HT, smaller counterions lead to greater bipolaron concentrations in the low-doping regime due to the increased Coulomb interactions. Emphasizing the impact of the counterion size, it is shown that larger counterions can lead to increased thermoelectric power factors for rr-P3HT.

Nanocrystal-based thermoelectric SnTe-NaSbSe2 alloys with strengthened band convergence and reduced thermal conductivity

Ternary I-V-VI2 colloidal NaSbSe2 nanocrystals are herein used to improve the performance of lead-free SnTe thermoelectric materials. We showcase a versatile bottom-up engineering approach to produce nanocrystal-based SnTe-NaSbSe2 alloys from the rapid hot press of colloidal nanocrystal building blocks. The incorporation of NaSbSe2 nanocrystals significantly enhances the Seebeck coefficient of SnTe. The band convergence and simultaneous increasing band gap of SnTe-NaSbSe2 alloys are certified by the first-principles density functional theory calculations. Besides, defect engineering generated by the incorporation of NaSbSe2 nanocrystals such as Sn vacancies, substitution point defects, dense dislocations, and strains generated by the NaSbSe2 nanoparticles incorporation result in a dramatic reduction of the lattice thermal conductivity below the amorphous limit of pure SnTe, down to 0.38 W m−1 K−1. As a consequence, power factors enhance up to 1.77 mW m−1 K−2, which is ∼193 % higher than that of the pristine SnTe, and thermoelectric figures of merit up to 1.15 at 823 K for (SnTe)0.85(NaSbSe2)0.15 are achieved.

Enhanced thermoelectric properties of carbon nanotubes/polyaniline fibers through engineering doping level and orientation

The rapid progress of miniaturized wearable electronics has put forward great requirements for organic fiber-based thermoelectric (TE) generators. Despite polyaniline (PANI) exhibits many outstanding attributes such as facile synthesis and low cost, as well as good environmental and thermal stability, only a few PANI-based fibers were fabricated and their TE efficiency needs to be further improved. In this work, the TE performance of wet-spun carbon nanotubes (CNTs)/PANI fibers was improved by synergistic engineering doping level of PANI and orientation of the fibers. The doping degree was optimized by varying coagulation baths, bath durations, and dopant loadings in the spinning solution, followed by fixing process during air drying to decrease shrinkage and enhance orientation of the fiber. Hexane coagulated CNTs/PANI fibers exhibited a higher doping degree of PANI compared to that of acetone and ethyl acetate, resulting in a maximum TE power factor of 77.4 μW m−1K−2 for 71 wt% CNTs/PANI fibers at PANI/dopant molar ratio of 2:1.25. Further fixing process induced a more oriented structure along the fibers, facilitating carrier transport and contributing to a significantly increased conductivity of 2155 S cm−1. Consequently, the CNTs/PANI fibers reached an optimal power factor of 91.8 μW m−1K−2. With outstanding TE performance and mechanical properties, the resultant fibers were assembled to fabricate a flexible TE generator, which generated a high output power of 2.5 nW with a temperature gradient of 10 K. These results demonstrate the potential of high-performance CNTs/PANI fibers to harvest body heat for the power supply of the wearable electronics.

Achieving thermoelectric properties of ultra-high-performance concrete using carbon nanotubes and fibers

The study investigated the impact of carbon nanotubes (CNTs), carbon fibers (CFs), steel fibers, and varying water-to-binder (W/B) ratios on the thermoelectric and mechanical properties of ultra-high-performance concrete (UHPC). Flowability tests revealed reduced flow with decreased water content and the addition of CNTs or CFs, particularly pronounced at a lower W/B ratio. Thermal gravimetric analysis and Fourier-transform infrared spectroscopy demonstrated differences in peak intensities and shifts in peaks related to the hydration products of the UHPC by the incorporation of the CNTs and CFs. The compressive strength and tensile performance increased with reduced W/B ratios and the inclusion of steel fibers, whereas the CNTs and CFs affected the strength differently based on their dispersion and interaction with other components that influenced porosity. The presence of steel fibers reduces the percolation threshold for CNTs and CFs, indicating a synergistic effect that enhances electron transport connectivity. The thermal conductivity increased with the addition of CNTs, CFs, and steel fibers, enhancing heat transfer within the UHPC. The thermoelectric figure of merit (ZT) values highlighted the combined impact of CNTs, CFs, steel fibers, and W/B ratios on the thermoelectric efficiency of the UHPC, showing significant improvements with the inclusion of steel fibers and the interplay between the CNTs and W/B ratios. Ultimately, upon introducing 1.5 % steel fibers and 0.3 % CNTs, a substantial enhancement in the thermoelectric ZT was observed, surpassing the standard UHPC values by 12 orders of magnitude.

Extraordinary Thermoelectric Properties of Topological Surface States in Quantum-Confined Cd3As2 Thin Films.

Topological insulators and semimetals have been shown to possess intriguing thermoelectric properties promising for energy harvesting and cooling applications. However, thermoelectric transport associated with the Fermi arc topological surface states on topological Dirac semimetals remains less explored. This work systematically examines thermoelectric transport in a series of topological Dirac semimetal Cd3As2 thin films grown by molecular beam epitaxy. Surprisingly, significantly enhanced Seebeck effect and anomalous Nernst effect are found at cryogenic temperatures when the Cd3As2 layer is thin. In particular, a peak Seebeck coefficient of nearly 500µVK-1 and a corresponding thermoelectric power factor over 30mWK-2m-1 are observed at 5K in a 25-nm-thick sample. Combining angle-dependent quantum oscillation analysis, magnetothermoelectric measurement, transport modeling, and first-principles simulation, the contributions from bulk and surface conducting channels are isolated and the unusual thermoelectric properties are attributed to the topological surface states. The analysis showcases the rich thermoelectric transport physics in quantum-confined topological Dirac semimetal thin films and suggests new routes to achieving high thermoelectric performance at cryogenictemperatures.

Wenzhou TE: A First-Principle-Calculated Thermoelectric Materials Database.

Since the implementation of the Materials Genome Project by the Obama administration in the United States, the development of various computational materials' databases has fundamentally expanded the choice of industries such as materials and energy. In the field of thermoelectric materials, the thermoelectric figure of merit (ZT) quantifies the performance of the material. From the viewpoint of calculations for vast materials, the ZT values are not easily obtained due to their computational complexity. Here, we show how to build a database of thermoelectric materials based on first-principle calculations for the electronic and heat transport of materials. Firstly, the initial structures are classified according to the values of bandgap and other basic properties using the clustering algorithm K-means in machine learning, and high-throughput first principle calculations are carried out for narrow-bandgap semiconductors which exhibit a potential thermoelectric application. The present framework of calculations mainly includes a deformation potential module, an electrical transport performance module, a mechanical and a thermodynamic properties module. We have also set up a search webpage for the calculated database of thermoelectric materials, providing search facilities and the ability to view the related physical properties of materials. Our work may inspire the construction of more computational databases of first-principle thermoelectric materials and accelerate research progress in the field of thermoelectrics.