High Thermoelectric Power Factor Research Articles

C3N based heterobilayers: a potential platform to explore optoelectronic and thermoelectric properties

We theoretically investigate the full thermal transport and optoelectronic features of two established van der Waals heterostructures based on the recently synthesized monolayer of C3N using the machinery of the Boltzmann transport equation and GW+BSE calculations. Among the structures, C3N/hBN tends to exhibit a small indirect gap semiconducting nature with an admixture of comparatively higher ‘flat-and-dispersiveness’ and band degeneracy in the conduction band minima. A nearly comparable high thermoelectric power factor is observed for both carrier types at 300 K and 900 K at specific concentrations. The other material, C3N/Graphene however maintains a low Seebeck coefficient with large electrical conductivity which correctly manifests its metallic character. A combination of low atomic mass, higher anharmonicity and longer lifetime of acoustic phonons in C3N/hBN results in an intermediate lattice thermal conductivity (196 W m−1 K−1) at room temperature as compared to its constituent monolayers. Under heavy n-type doping, C3N/hBN hetero-bilayer displays a figure of merit value of 0.13 (and 0.36) at room temperature (and at 900 K). As per the optical signatures are concerned, C3N/hBN reveals two distinct absorption peaks with a high electron–hole quasiparticle interaction energy correction. Besides both the heterostructures display a much better absorption throughout the spectrum as compared to graphene. We expect these findings will motivate future research in designing thermoelectric and optoelectronic materials made of light mass, earth-abundant and non-toxic elements.

Tailoring stress relaxation for dopant-free ZnO thin films with high thermoelectric power factor

In this study, the effects of stress relaxation on the thermoelectric properties (carrier concentration n, Hall mobility μH, weighted mobility μW, density-of-state mass md*, Seebeck coefficient S, and thermopower factor PF) of undoped ZnO films were rationalized in terms of native defects (VO-related defects and Zni-related donors) induced through the deposition temperature (TD) during the sputtering process. All investigated ZnO films exhibited compressive stress and tended to become less compressive with increasing TD. The stress relaxation at high TD resulted in improved film crystallization and decreased native defect concentration, thus significantly enhancing md* through the reduction of intrinsic lattice defects, while less carriers were trapped and scattered by defects. Therefore, n and μ increased simultaneously (by 28 times and one order of magnitude, respectively), markedly enhancing the PF of dopant-free ZnO films.

Computational identification of 2D TlPt2X3 (X = S, Se, Te) for thermoelectric and photocatalytic applications

Thermoelectric (TE) device can convert heat into electricity, and suitable photocatalyst can split water to produce hydrogen, which makes them have great potential in the field of clean energy and environmental protection. In this paper, we investigated three 2D materials, TlPt2X3 (X = S, Se, Te), which are indirect semiconductors with band-gaps of 2.11 eV, 1.85 eV, and 1.14 eV, respectively. The three single-layers show moderate mobilities (102 – 103 cm2/Vs), large Seebeck coefficients (0.66 – 1.86 mV/K), and high TE power factor (13.73 – 25.20 mW/K2m) at 300 K. Besides, due to the weak bonding and “Rattling” vibrations caused by Tl atoms, the single-layers have strong phonon–phonon scattering, and thus possess low lattice thermal conductivities of 2.34 – 4.46 W/mK. As a result, they can deliver high TE figure-of-merit of 0.53/0.63/0.52 at 300 K, and rise even further to 2.72/3.05/2.38 at 700 K. They also have appropriate REDOX potential, which satisfy the band structure requirement for overall water splitting. Also, the three single-layers also have strong absorption coefficient of ∼ 105 cm−1 in vision-ultraviolet region, which can efficiently carry out photocatalytic water splitting to produce H2. Ultimately, 2D TlPt2X3 are suitable candidates for both TE and photocatalytic applications.

Janus 2D B2P6: A promising anisotropic thermoelectric material with high power factor

A two-dimensional (2D) material with high thermoelectric (TE) performance is of great interest for the energy conversion device. In this study, by using first-principles calculations, we show that the Janus 2D B2P6 has an extremely high TE power factor of 0.10 and 0.07 W/mK2 for p-type and n-type at room temperature, respectively, which are higher than that of conventional 2D TE materials. We also investigate lattice thermal conductivity, κph, of B2P6 by using the Boltzmann transport theory with phonon–phonon interaction. With κph of 41.45 and 133.3 W/mK for the armchair and zigzag directions, respectively, the optimized values of the TE figure of merit are up to 0.7 at 900 K. Moreover, the Janus 2D B2P6 exhibits anisotropic mechanical properties, in which the ideal strength of B2P6 is 10.7 and 18.63 N/m for the armchair and zigzag directions, respectively. Thus, our results suggest that the Janus 2D B2P6 is the potentially promising flexible anisotropic TE material.

Thermal analysis of thermoelectric active cooling including external thermal resistances

Thermoelectric active cooling uses nontraditional thermoelectric materials with high thermal conductivity, high thermoelectric power factor, and relatively low figure of merit (ZT) to transfer large heat flows from a hot object to a cold heat sink. However, prior studies have not considered the influence of external thermal resistances associated with the heat sinks or contacts, making it difficult to design active cooling thermal systems or compare the use of low-ZT and high-ZT materials. Here, we perform a non-dimensionalized analysis of thermoelectric active cooling under forced heat flow boundary conditions, including arbitrary external thermal resistances. We identify the optimal electrical currents to minimize the heat source temperature and find the crossover heat flows at which low-ZT active cooling leads to lower source temperatures than high-ZT and even ZT→+∞ thermoelectric refrigeration. These optimal parameters are insensitive to the thermal resistance between the heat source and thermoelectric materials, but depend strongly on the heat sink thermal resistance. Finally, we map the boundaries where active cooling yields lower source temperatures than thermoelectric refrigeration. For currently considered active cooling materials, active cooling with ZT < 0.1 is advantageous compared to ZT→+∞ refrigeration for dimensionless heat sink thermal conductances larger than 15 and dimensionless source powers between 1 and 100. Thus, our results motivate further investigation of system-level thermoelectric active cooling for applications in electronics thermal management.

High Thermoelectric Power Factors in Plastic/Ductile Bulk SnSe2 -Based Crystals.

The recently discovered plastic/ductile inorganic thermoelectric (TE) materials open a new avenue for the fabrication of high-efficiently flexible TE devices, which can utilize the small temperature difference between human body and environment to generate electricity. However, the maximum power factor (PF) of current plastic/ductile TE materials is usually around or less than 10 µW cm-1 K-2 , much lower than the classic brittle TE materials. In this work, a record-high PF of 18.0 µW cm-1 K-2 at 375 K in plastic/ductile bulk SnSe2 -based crystals is reported, superior to all the plastic inorganic TE materials and flexible organic TE materials reported before. The origin of such high PF is from the modulation of material's stacking forms and polymorph crystal structures via simultaneously doping Cl/Br at Se-site and intercalating Cu inside the van der Waals gap, leading to the significantly enhanced carrier concentrations and mobilities. An in-plane fully flexible TE device made of the plastic/ductile SnSe2 -based crystals is successfully developed to show a record-high normalized maximum power density to 0.18W m-1 under a temperature difference of 30 K. This work indicates that the plastic/ductile material can realize high TE power factor to achieve large output electric power density in flexible TE technology.

Enhanced thermoelectric performance with improved mechanical strength in Bi2S3/graphite composites

In spite of possessing high Seebeck coefficient and low thermal conductivity, Bi2S3 suffers from poor electrical conductivity, which makes it challenging to use for application in thermoelectric power generator (TEG). In the present work, we have used graphite (G) to improve the electron transport in Bi2S3 by making a composite. Here, we have obtained ZT∼0.15 at room temperature in n-type Bi2S3, which is much higher than that reported for sulphides at room temperature. The inclusion of graphite in Bi2S3, provides a conductive pathway for the easy transport of electrons, and leads to a surge in electrical conductivity. As a result, a high thermoelectric power factor ∼643 μW/m-K2 is attained, which is 30 times higher than that of pristine Bi2S3 at room temperature. The maximum ZT ∼0.3 is obtained for 1.5 wt% graphite-Bi2S3 composite at 705 K temperature. Furthermore, the mechanical strength of Bi2S3 is found to be significantly improved by graphite inclusion, which offers better load bearing capacity for practical application as TEG.

Dopant induced band convergence leading to high thermoelectric power factor in Mg2Sn

Band convergence is a common technique for enhancing the power factor of thermoelectric (TE) materials. This is usually achieved by solid solution formation in compounds having specific electronic band features. In this work, we show that a similar band convergence effect can be achieved in Mg2Sn by doping with small amounts of Bi. To identify this effect and the possible reason behind it, a detailed investigation of the electronic band structure (EBS) has been carried out using a recently developed multi-band refinement technique (MBRT). Nominal compositions of Mg2.2Sn1-xBix (x = 0,0.005, 0.01, 0.0125, 0.015, 0.02) have been synthesized by induction melting followed by compaction using uniaxial hot-pressing. Measurement of Seebeck coefficient (S), electrical conductivity (σ) and Hall Coefficient (RH) have been performed between 300K–700K and the data used for multi-band refinement. Results indicate changes in the inter-band separation of conduction bands with doping content and temperature. Significantly lower inter-band separation compared to reported undoped data is found even for the lowest doped composition, resulting in both conduction bands contributing to the power factor (S2σ). Further, for the heavily doped compositions (x = 0.0125, 0.015, 0.02) band-flipping between the conduction bands has been observed. Thus, the light conduction band is found to constitute the conduction band edge in the heavily doped compositions. This is contrary to the reported EBS of undoped Mg2Sn in which the heavy conduction band has a lower energy. Also, increasing temperature results in band convergence in the heavily doped compositions. A plausible cause for the band-flipping behaviour is provided based on merging of the impurity band with the light conduction band in heavily doped Mg2Sn. The band-flipping and the subsequent band convergence results in an even higher S2σ in the heavily doped compositions. The work highlights the beneficial influence of Bi doping on the TE properties of Mg2Sn and opens the possibility of utilizing dopants for achieving band convergence in TE materials.

Highly Conductive n‐Type Polymer Fibers from the Wet‐Spinning of n‐Doped PBDF and Their Application in Thermoelectric Textiles

AbstractThe field of electronic textiles currently lacks n‐type polymer fibers that can complement the more established p‐type polymer fibers. Here, a highly conductive n‐type polymer fiber is obtained via wet‐spinning of n‐doped poly(3,7‐dihydrobenzo[1,2‐b:4,5‐b’]difuran‐2,6‐dione) (n‐PBDF). The electrical conductivity of the fibers increases from 1000 to 1600 S cm−1 with increased draw during processing and correlates well with Young's modulus. Wide‐angle X‐ray scattering reveals the existence of a bimodal orientation of the polymer chains, favoring parallel alignment to the fiber axis with increased draw. After 14 d in 80% humid air, fiber conductivity stabilizes maintaining 81% of the initial conductivity. Although the electrical conductivity drops slightly over time, the Seebeck coefficient increases, resulting in the highest thermoelectric power factor being measured at 91 µW m−1 K−2 for the most drawn fiber 14 d after its fabrication. A proof‐of‐concept two‐couple thermoelectric textile is crafted by embroidering bundles of n‐type PBDF fibers and p‐type PEDOT:PSS fibers. The device generates 2.40 nW at a 22 °C temperature gradient. This work represents the initial steps and a crucial advancement toward fabricating high‐performance n‐type polymer fibers that can complement their p‐type counterparts to close the existing performance gap.

Solution‐Doped Donor–Acceptor Copolymers Based on Diketopyrrolopyrrole and 3, 3′‐Bis (2‐(2‐(2‐Methoxyethoxy) Ethoxy) ethoxy)‐2, 2′‐Bithiophene Exhibiting Outstanding Thermoelectric Power Factors with p‐Dopants

AbstractThe design of polymeric semiconductors exhibiting high electrical conductivity (σ) and thermoelectric power factor (PF) will be vital for flexible large‐area electronics. In this work, four polymers based on diketopyrrolopyrrole (DPP), 2,3‐dihydrothieno[3,4‐b][1,4]dioxine (EDOT), thieno[3,2‐b]thiophene (TT), and 3, 3′‐bis (2‐(2‐(2‐methoxyethoxy) ethoxy) ethoxy)‐2, 2′‐bithiophene (MEET) are investigated as side‐chains, with the MEET polymers newly synthesized for this study. These polymers are systematically doped with tetrafluorotetracyanoquinodimethane ( F4TCNQ), CF3SO3H, and the synthesized dopant Cp(CN)3‐(COOMe)3, differing in geometry and electron affinity. The DPP‐EDOT‐based polymer containing MEET as side‐chains exhibits the highest conductivity (σ) ≈700 S cm−1 in this series with the acidic dopant (CF3SO3H). This polymer also shows the lowest oxidation potential by cyclic voltammetry (CV), the strongest intermolecular interactions evidenced by differential scanning calorimetry (DSC), and has the most oxygen‐based functionality for possible hydrogen bonding and ionic screening. Other polymers exhibit high σ ≈300–500 S cm−1 and power factor up to 300 µW m−1 K−2. The mechanism of conductivity is predominantly electronic, as validated by time‐dependent conductance studies and transient thermo voltage monitoring over time, including for those doped with the acid. These materials maintain significant thermal stability and air stability over ≈6 weeks. Density functional theory calculations reveal molecular geometries and inform about frontier energy levels. Raman spectroscopy, in conjunction with scanning electron microscopy (SEM‐EDS) and x‐ray diffraction, provides insight into the solid‐state microstructure and degree of phase separation of the doped polymer films. Infrared spectroscopy enables this study to further quantify the degree of charge transfer from polymer to dopant.

Two-band Model with High Thermoelectric Power Factor and Its Application to FeSe Thin Film

Two-band Model with High Thermoelectric Power Factor and Its Application to FeSe Thin Film

Thermoelectric properties of B-doped nanostructured bulk diamond with lowered thermal conductivity

Diamond exhibits high wear resistance and high chemical resistance. If diamond exhibits high thermoelectric performance, the thermoelectric application will be expanded for various situations such as the use of friction heat on turbines. However, bulk diamond exhibits low electrical conductivity and high thermal conductivity, leading to the low thermoelectric performance. In this study, to solve the above problem, we propose a strategy of heavy impurity doping in nanostructured bulk diamond which brings two things: (1) higher thermoelectric power factor, and (2) phonon scattering enhancement at impurities and nanostructure interfaces reducing the thermal conductivity drastically. We investigate the thermoelectric properties of heavily B-doped nanostructured bulk diamonds prepared by direct conversion sintering under ultra-high pressure and temperature using B-doped graphite as a starting material. This nanostructured bulk diamond exhibited high electrical conductivity due to heavy B doping, resulting in high power factor of 4 μWcm−1 K−2 at 1050 K. Furthermore, we found that this nanostructured bulk diamond exhibited drastic thermal conductivity reduction, resulting in quite low thermal conductivity among bulk diamond ever reported. This comes from phonon scattering both at nanostructure interfaces and by substitutional B atoms. From these results, the present study would be informative for expanding thermoelectric application to various situations such as the use of friction heat.

Enhanced in-plane thermoelectric properties achieved through the vertical van der Waals stacking of black phosphorus and Ti2C

Vertical van der Waals heterostructures (vdWHs) exhibit considerable freedom in integrating two-dimensional layered materials for achieving excellent performance. In this study, we construct a double-layer Ti2C/BP vdWH by stacking pristine monolayer black phosphorus (BP) and Ti2C vertically. The in-plane thermoelectric properties of BP, Ti2C and Ti2C/BP vdWHs at different temperatures are investigated using density functional theory combined with the non-equilibrium Green's function method. Results showed that the thermoelectric figure of merit (ZT) of Ti2C/BP vdWHs at room temperature (300 K) was approximately 103 and 104 times that of Ti2C and BP, respectively. Because of the mutual compensation of the electronic energy bands and interfacial phonon scattering, Ti2C/BP vdWHs have higher electron conductance and lower phonon conductance than both BP and Ti2C, which significantly enhances the ZT value of these structures. More importantly, the prominent thermoelectric properties of Ti2C/BP vdWHs maintain greater stability as the temperature rises (300–900 K), which leads to a high thermoelectric power factor and ZT value without relying on high temperatures. These results provide a new idea for designing two-dimensional functionalised nanoscale devices with excellent thermoelectric performance.

Room temperature high thermoelectric performance of Bi-based full-Heusler compounds CsxRb[formula omitted]Bi with strong anharmonicity

Bismuth-based compounds have been identified as a class of promising candidates for the realization of high-performance thermoelectrics, derived from their exceptional electrical conductivity and intrinsic low lattice thermal conductivity κL. Here, we employed first-principles calculations, self-consistent phonon theory, and compressive sensing technique in conjunction with the Boltzmann transport equation to investigate the anharmonic thermoelectric properties of full-heusler bismuth compounds CsxRb3−xBi. As the Cs content increases, the κL of CsxRb3−xBi decreases due to impurity scattering effects, and then increases. Among them, at 300 K, Rb3Bi has the largest κL of 0.69 Wm−1K−1, while Cs2RbBi has the smallest κL of 0.45 Wm−1K−1, both of which are lower than the κL of traditional bismuth-based thermoelectric materials. The ultralow κL originates from the strong lattice anharmonicity. In addition, the coexistence of high dispersion and flat band edges leads to high thermoelectric power factor at optimal doping concentration. As a result, the largest thermoelectric figure of merit ZT values of 2.59 (4.19) at 300 (500) K were obtained for CsRb2Bi at optimal hole doping level. These findings suggest that CsxRb3−xBi compounds are superior for thermal management and thermoelectric applications.

Study on the thermoelectric characteristics of the ternary alkali metal compound XMgSb (X = K, Rb, Cs)

We conducted a comprehensive analysis of the mechanical, transport, and thermoelectric (TE) characteristics of XMgSb (X = K, Rb, Cs) alkali metal compounds using first-principles calculations, self-consistent phonon theory, pressive sensing lattice dynamics, and the Boltzmann transport equation. Our results show that the very low lattice thermal conductivity of these materials is mainly attributed to the strong anharmonicity of the alkali metal atoms, and the lattice thermal conductivity is negatively correlated with the mass of X atoms. Furthermore, we consider five scattering mechanisms to accurately calculate transport properties and electron relaxation times. Notably, we observed that the large electronic dispersion band near the valence band maximum leads to the high conductivity of these three compounds, with a flat electronic energy band at the Γ-Z point, which generally implies a larger Seebeck coefficient. The coexistence of good electronic band dispersion and flat electronic bands leads to high conductivity and large Seebeck coefficient, resulting in high TE power. Low lattice thermal conductivity and high TE power factor jointly determine the development potential of alkali metal compound XMgSb (X = K, Rb, Cs) in the field of TE.

Enhanced thermoelectric performance of P-type SnTe thin film through Sr doping and Post-Annealing treatment

Lead-free tin telluride (SnTe) has gained significant attention for its potential use in mid-temperature thermoelectric power generation. In this work, we systematically analyzed the thermoelectric power factor of Sn0.7Sr0.3Te thin films at various post-annealing temperatures through structural, morphological, and thermoelectric property measurements. Our findings show that Sr-doping in the SnTe thin film matrix and post-annealing treatment can significantly enhance the thermoelectric properties by modifying the band structure and introducing an energy-filtering effect at grain boundaries. We achieved a maximum power factor of 29.4 μWm-1K−2 at 523 K for Sn0.7Sr0.3Te thin films post-annealed at 1073 K, which is three times greater than that of pure SnTe. This work demonstrates that Sr-doping and post-annealing effects can tune the band gap, induce band degeneration, and reduce grain size, resulting in a high thermoelectric power factor of the SnTe thin film.

High thermoelectric power factor of n-type Bi2(Se,S)3 via synergetic influence on the charge carrier concentration and energy filtering effect

Excellent thermoelectric performance is sought to address essential energy concerns, including the impact of energy consumption on the environment. In this work, we have synthesized the Bi2Se3 thin film using a simple thermal evaporation route and post‑sulfurized it for different durations at (773 K). The crystal structure change from rhombohedral to orthorhombic with different post‑sulfurization times has been confirmed via x-ray diffraction. The atomic vibrational modes of Bi2(Se,S)3 thin films have been examined by Raman spectroscopy. The surface morphology of Bi2(Se,S)3 thin films have been observed using a scanning electron microscope. The Seebeck coefficient values enhanced from 155.1 to 203.5 μV/K with post‑sulfurization time due to increased charge carrier concentration and energy filtering effect. The electrical conductivity improved from 90.2 to 118.7 S/m of the as-deposited sample to the highest post‑sulfurized sample due to improving charge carrier concentration and changing the crystal structure from rhombohedral to orthorhombic structure. We have achieved the maximum power factor of Bi2(Se,S)3 thin film of the 4.86 μWcm−1 K−2 at room temperature. Obtaining the highest power factor has a straightforward approach to fabricating commercial thermoelectric material.

High thermoelectric power factor of Ag and Nb co-substituted SrTiO3 perovskite nanostructures

Ag substituted and Ag & Nb co-substituted SrTiO3 were synthesized by hydrothermal method and their structural, morphological and thermoelectric properties were studied. The crystal structure and crystallinity of the samples were examined by X-ray Diffraction analysis. The morphology of the samples was analysed using FESEM and TEM analysis. X-ray Photoelectron Spectroscopic analysis revealed the presence of Ag and the binding state of each element present in the prepared samples. The variations of electrical resistivity with Ag content was studied in Ag substituted SrTiO3. The electrical resistivity of Ag & Nb co-substituted SrTiO3 was relatively higher than that of pure SrTiO3 and Ag alone substituted samples. The Seebeck coefficient increased as the Ag content increases in the sample. Higher Seebeck coefficient was observed for Ag & Nb co-substituted SrTiO3 when compared with other samples. The power factor of the sample significantly improved with Ag content due to high Seebeck Coefficient. The Ag & Nb co-substituted samples exhibited high power factor compared to other samples. Moreover, the Ag-substituted SrTiO3 shows relatively low thermal conductivity and thereby high figure of merit of 0.15 compared to the reported values of similar materials. The experimental results demonstrated that Ag and Nb substitution plays an important role in enhancing the thermoelectric properties of SrTiO3.

Thermoelectric Properties Investigation of Ni/Co Doped ZrCoBi Half-Heusler Alloy

Half-Heusler (HH) thermoelectric (TE) composites have been extensively inspected due to their excellent TE properties in the medium- to high-temperature range. First-principle calculations make it easier to discover or improve more HH compounds. This article presents an ab initio theoretical evaluation of TE properties of Half-Heusler alloy, when doped with Nickel (Ni), using FP-LAPW and the semi classic Boltzmann theory. Thermoelectric parameters were calculated using BoltzTraP code, like Seebeck coefficient ( ), electrical conductivity to relaxation time ratio ( ), electronic thermal conductivity to relaxation time ratio ( ), thermoelectric power factor to relaxation time ratio ( ), and the dimensionless figure-of-merit ( ) in a temperature range of . Calculated Seebeck coefficient reveals that the studied alloys show a tendency to conduct as p-type with balanced TE performance between both charge carriers (holes and electrons). A high electronic thermal conductivity value is found, which predicts a potential use in heat sink applications for the investigated alloys. Obtained results, such as a high thermoelectric power factor and , postulate that alloys could have potential thermoelectric applications.

Boosting Thermoelectric Performance in Epitaxial GeTe Film/Si by Domain Engineering and Point Defect Control

This study demonstrates a simultaneous realization of ultralow thermal conductivity and high thermoelectric power factor in epitaxial GeTe thin films/Si substrates by a combination of the interface introduction by domain engineering and the suppression of Ge vacancy generation by point defect control. We formed epitaxial Te-poor GeTe thin films having low-angle grain boundaries with a misorientation angle close to 0° or twin interfaces with a misorientation angle close to 180°. The control of interfaces and point defects gave rise to ultralow lattice thermal conductivity of ∼0.7 ± 0.2 W m-1 K-1. This value was the same in the order of magnitude as the theoretical minimum lattice thermal conductivity of ∼0.5 W m-1 K-1 calculated by the Cahill-Pohl model. At the same time, the GeTe thin films exhibited a high thermoelectric power factor because of the suppression of Ge vacancy generation and a small contribution of grain boundary carrier scattering. The outstanding combined technique of domain engineering and point defect control can be a great approach for developing high-performance thermoelectric films.