High Thermoelectric Power Research Articles

Data-driven prediction of higher-order structure in carbon nanotube films for high thermoelectric power factor

We optimized the higher-order structures and semiconducting purity of single-walled carbon nanotubes (SWCNTs) to enhance the thermoelectric power factor PF by combining the thermoelectric random stick network (TE-RSN) method and a genetic algorithm. The PF of the optimized films was increased approximately fivefold for initial random structures. In addition, while the random structures showed the maximum PF when the ratio of semiconducting to metallic SWCNTs R s exceeded 0.98, the optimized structures converged to an R s of approximately 0.9. The optimized structures exhibited an increased local density and the peak of alignment angle distribution, leading to an increase in both the electrical conductivity and the Seebeck coefficient. We interpreted the increase in the Seebeck coefficient using a serial model. The results indicated that the reduction in the number of contacts within the paths and the subsequent increase in temperature difference on semiconducting SWCNTs led to the increase in the Seebeck coefficient.

Effects of W alloying on the electronic structure, phase stability, and thermoelectric power factor in epitaxial CrN thin films

CrN-based alloy thin films are of interest as thermoelectric materials for energy harvesting. Ab initio calculations show that dilute alloying of CrN with 3 at. % W substituting Cr induce flat electronic bands and push the Fermi level EF into the conduction band while retaining dispersive Cr 3d bands. These features are conducive for both high electrical conductivity σ and high Seebeck coefficient α and, hence, a high thermoelectric power factor α2σ. To investigate this possibility, epitaxial CrWxNz films were grown on c-sapphire by dc-magnetron sputtering. However, even films with the lowest W content (x = 0.03) in our study contained metallic h-Cr2N, which is not conducive for a high α. Nevertheless, the films exhibit a sizeable power factor of α2σ ∼ 4.7 × 10−4 W m−1 K−2 due to high σ ∼ 700 S cm−1, and a moderate α ∼ − 25 μV/K. Increasing h-Cr2N fractions in the 0.03 < x ≤ 0.19 range monotonically increases σ, but severely diminishes α leading to two orders of magnitude decrease in α2σ. This trend continues with x > 0.19 due to W precipitation. These findings indicate that dilute W additions below its solubility limit in CrN are important for realizing a high thermoelectric power factor in CrWxNz films.

Unusual Electrical Conductivity Enhancement in Stable n-Type Carbon Nanotube Networks.

Organic molecule-doped n-type single-walled carbon nanotube (SWCNT) networks are promising candidates for advanced energy applications, such as flexible thermoelectrics and photovoltaics. Yet charge transport in n-type SWCNTs is limited by two factors: i) charge localization impeding inter-tube transport caused by disordered mesostructure of randomly oriented SWCNTs and ii) reduction of charge carrier concentration driven by oxidation. Herein, studied the relationship between the mesostructure and thermoelectric properties of n-type SWCNTs obtained by surfactant-functionalization and polymer-dopant grafting. Surprisingly, the electrical conductivity of the polymer-doped SWCNTs keeps increasing with increasing polymer content, even after the saturation of carrier concentration, resulting in 12x higher conductivity on polymer-doping compared to surfactant-functionalization. While hopping transport typically dominates in disordered systems, it is shown that a bridging effect from the polymer causes unusual band-like conduction in polymer-doped SWCNTs. Additionally, since surfactants are essential to prevent oxidation and retain n-type over a long duration, shows that SWCNTs obtained through a dual-functionalization strategy using both polymer-dopant and surfactant, demonstrates a long-term stable high n-type thermoelectric power factor, when the surfactant amount is carefully controlled. Besides thermoelectrics, the findings are of general interest to developing stable and conductive n-type SWCNTs for various energy and electronic applications.

Wedge-shape Al and Mg co-doped zinc oxide thin films: A Facile Route to achieve high thermoelectric power factor

Thin film thermoelectric technology has immense potential to become a next-generation power source for small-scale electronics. However, the rapid development of thin film thermoelectric still lacks a controllable structural design to improve the performance of thermoelectric devices for required temperature applications. This work presented a controlled thermoelectric design of ZnO-based thin films by Al and Mg co-doping using ALD. Herein, the Mg-doped ZnO sample exhibits the highest Seebeck coefficient of −311.83 μVK−1 at 500oC, while the maximum thermoelectric power factor of 3.68 μWcm−1K−2 is obtained at 300oC, which drops for higher temperatures. This decrease in power factor resulted from the reduction in electrical conductivity above 300oC. Similarly, the Al and Mg co-doped ZnO sample (Mg/Al ≈2 at%) exhibits a high thermoelectric power factor of 3.85 μWcm−1K−2 for ZnO based material due to its moderate electrical conductivity and Seebeck coefficient value at 325oC and behave monotonically for elevated temperatures. Scanning electron microscope (SEM) images show the formation of wedge-shaped structures for Al-incorporated samples, which helps to boost the overall thermoelectric performance. Additionally, we performed XRD, UV–Vis, Hall, and AFM analysis to get a deep insight into structural, optical, electrical, and surface features of the grown thin films and their linkage with the thermoelectric properties.

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.

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.

Solvent effect induced charge polarity switching from p- to n-type in polyaniline and carbon nanotube hybrid films with a high thermoelectric power factor

Understanding and controlling the polarity of organic materials is essential for a broad range of organic electronic 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.

High Performance Thermoelectric Power of Bi0.5Sb1.5Te3 Through Synergistic Cu2GeSe3 and Se Incorporations.

Bi2Te3-based alloys are the benchmark for commercial thermoelectric (TE) materials, the widespread demand for low-grade waste heat recovery and solid-state refrigeration makes it imperative to enhance the figure-of-merits. In this study, high-performance Bi0.5Sb1.5Te3 (BST) is realized by incorporating Cu2GeSe3 and Se. Concretely, the diffusion of Cu and Ge atoms optimizes the hole concentration and raises the density-of-states effective mass (md *), compensating for the loss of "donor-like effect" exacerbated by ball milling. The subsequent Se addition further increases md *, enabling a total 28% improvement of room-temperature power factor (S2σ), reaching 43.6µWcm-1K-2 compared to the matrix. Simultaneously, the lattice thermal conductivity is also significantly suppressed by multiscale scattering sources represented by Cu-rich nanoparticles and dislocation arrays. The synergistic effects yield a peak ZT of 1.41 at 350K and an average ZT of 1.23 (300-500K) in the Bi0.5Sb1.5Te2.94Se0.06 + 0.11 wt.% Cu2GeSe3 sample. More importantly, the integrated 17-pair TE module achieves a conversion efficiency of 6.4%, 80% higher than the commercial one at ΔT=200K. These results validate that the facile composition optimization of the BST/Cu2GeSe3/Se is a promising strategy to improve the application of BST-based TE modules.

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

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.