Thermoelectric Figure Of Merit Research Articles

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.

Electron-hole dichotomy and enhancement of the thermoelectric power factor by electron-hole-asymmetric relaxation time: A model study on a two-valley system with strong intervalley scattering

Electron-hole dichotomy and enhancement of the thermoelectric power factor by electron-hole-asymmetric relaxation time: A model study on a two-valley system with strong intervalley scattering

Largely Enhanced Thermoelectric Power Factor of Flexible Cu2-xS Film by Doping Mn.

Copper-sulfide-based materials have attracted noteworthy attention as thermoelectric materials due to rich elemental reserves, non-toxicity, low thermal conductivity, and adjustable electrical properties. However, research on the flexible thermoelectrics of copper sulfide has not yet been reported. In this work, we developed a facile method to prepare flexible Mn-doped Cu2-xS films on nylon membranes. First, nano to submicron powders with nominal compositions of Cu2-xMnyS (y = 0, 0.01, 0.03, 0.05, 0.07) were synthesized by a hydrothermal method. Then, the powders were vacuum-filtrated on nylon membranes and finally hot-pressed. Phase composition and microstructure analysis revealed that the films contained both Cu2S and Cu1.96S, and the size of the grains was ~20-300 nm. By Mn doping, there was an increase in carrier concentration and mobility, and ultimately, the electrical properties of Cu2-xS were improved. Eventually, the Cu2-xMn0.05S film showed a maximum power factor of 113.3 μW m-1 K-2 and good flexibility at room temperature. Moreover, an assembled four-leg flexible thermoelectric generator produced a maximum power of 249.48 nW (corresponding power density ~1.23 W m-2) at a temperature difference of 30.1 K, and had good potential for powering low-power-consumption wearable electronics.

Machine learning based models to investigate the thermoelectric performance of carbon nanotube-polyaniline nanocomposites

Thermoelectric materials have been widely recognized as a simple approach for harnessing green energy by converting thermal gradients into electrical energy. However, the intricate and conflicting interplay between electrical conductivity (σ), Seebeck coefficient (S), and thermal conductivity (k) in known materials present a challenge for improving their thermoelectric conversion efficiency. To overcome this challenge, various data-driven machine-learning techniques such as correlation matrix (CM), multiple linear regression (MLR), polynomial regression (PR), principal component analysis (PCA), and artificial neural network (ANN) have been utilized to identify the impact of different structural and compositional factors on the thermoelectric performance in carbon nanotube (CNT)-polyaniline (PANI) based nanocomposites. Our findings suggest that the thermoelectric figure of merit, (ZT=S2σκT) of these nanocomposites can be positively influenced by proper choice of doping agent of PANI and by using SWCNT, since these two parameters influence the thermoelectric outputs in the desired manner. The other input variables have conflicting impacts on the thermoelectric performance, although optimal solutions for maximized performance can be extracted. The investigation provides valuable insights about designing a PANI-CNT nanocomposite system with superior thermoelectric performance.

Enhanced thermoelectric properties of Mm-filled p-type (Fe, Co)Sb3 skutterudites via Co/Fe ratio regulation and extra Sb compensation

The band degeneracy thus thermoelectric performance of (Fe, Co)Sb3 based p-type skutterudites can be well modulated by adjusting Co/Fe ratio. Combining systematic experiments and density function theory (DFT) calculations, we revealed in this work that the variation of Co/Fe ratio could result in complicated and mutually coupled effects on the individual thermoelectric parameters (band degeneracy, band gap, Fermi level, filling limit of fillers, phonon spectrum, etc.) and that using Co/Fe ratio to improve the overall thermoelectric figure of merit is nontrivial. We eventually found that the optimal Co/Fe ratio for Mm (Misch metal) filled (Fe, Co)Sb3 lies at 1.3:2.7, which is somewhat off the Co/Fe = 1:3 point where the largest valence band degeneracy appears. Extra Sb was then introduced to compensate Sb volatilization for improved electrical transport performance. An overall peak figure of merit ZTmax of 1.06 was eventually achieved in the sample Mm0.8Fe2.7Co1.3Sb12.60, which is among the highest ones for all reported (Fe, Co)Sb3 based p-type skutterudites so far.

The thermoelectric properties of novel two-dimensional semiconductor ZrNBr alloyed with Fluorine: A DFT+U survey

In this survey, the thermoelectric properties of novel two-dimensional transition metal halide (ZrNBr) alloyed with Fluorine (F) in the replacement of Br atoms (with x = 0.5) have been studied. The density functional theory along with Hubbard’s potential has been used to study the density of states, the Seebeck coefficient, electrical conductivity, the electronic contribution of thermal conductivity, and the thermoelectric power factor. The dynamic and structural stability of the compounds has been proven by cohesive energy and phonon calculation results. By considering Hubbard’s potential of 3 eV the energy band gap of the electronic density of states has been enlarged to values of 2.73, and 2.7 eV for pure ZrNBr, and ZrNBr0.5F0.5, respectively. Moreover, the largest values of the thermoelectric power factor at room temperature (300 K) and without considering Hubbard’s potential were 29.12 × 1016 and 101 × 1016μW/m K2 s and at the negative chemical potential for pure ZrNBr, and ZrNBr0.5F0.5, respectively. While considering Hubbard’s potential, the largest peaks at 300 K were 26.42 × 1016μW/m K2 s (at positive chemical potential) and 41.41 × 1016μW/m K2 s (at negative chemical potential), for pure ZrNBr, and ZrNBr0.5F0.5, respectively. These values are in the ranges of the results for materials which are potential candidates for thermoelectric applications.

Thermoelectric properties of Ga-doped InSb alloys

Internal structure and thermoelectric properties of In1-xGaxSb alloys, prepared by melting synthesis at 1123 K, were systematically studied in this paper. Crystal structure was investigated by XRD analysis. All samples exhibited crystalline structure of single InSb phase. Transport and electrical properties were studied against temperature elevating. Electrical conductivity was determined via the standard four probe method. The thermal behavior of the electrical conductivity over the temperature range between 273 K and 473 K showed semiconducting behavior. Seebeck coefficient measurements were conducted via differential method in the temperature range 273–473 K. The measurements of Seebeck coefficient confirmed the electrical conductivity behavior of the samples. The maximum obtained Seebeck coefficient value |S| of the parent InSb compound was observed at 224 μV/K, obtained at 273 K. The alloy has been doped with Ga in order to enhance thermoelectric properties. Results showed that introducing of Ga into In sites leads to significant increase in the Seebeck coefficient up to 330.4 μV/K. Power factor was estimated from the measurements of the electrical conductivity and the Seebeck coefficient. Interesting enhancement in the thermoelectric power factor was observed with Ga-doping. The maximum power factor was obtained for the most doped sample exhibiting 2.3 μWm−1 K−2 at 273 K. The thermal conductivity was obviously reduced by substituting In with Ga. The dimensionless figure of merit was estimated showing an enhancement with the Ga addition. The maximum recorded ZT was 0.017 observed at 403 K.

Post-selenization induced structural phase transition and enhanced thermoelectric properties in AgBiSe2 alloy thin films

Thermoelectric materials have the potential to harness waste heat for energy purposes, and there is considerable scientific and technological interest in managing their thermal properties. BiAgSe2, a narrow bandgap semiconductor, is a strong candidate for use as a thermoelectric material due to its remarkably low thermal conductivity. The compound of AgBiSe2 crystallizes in the hexagonal symmetry of the grown sample at room temperature and transitions to a cubic phase, increasing the post selenization duration at 773K. The granular surface morphology has been observed and slightly decreases with increases the more post selenization treatment. The enhancement of the Seebeck coefficient value from 126 to 177 μV/K due to the phase transition, grain refinement and post-selenization treatment. The improvement of charge carrier concentration and electrical conductivity is due to improving the crystallinity and diffusion of Se atoms in the AgBiSe2 alloy thin film with post-selenization time. The maximum thermoelectric power factor (5.16 μWcm−1K−2) of stable cubic AgBiSe2 alloy thin film has been achieved at room temperature. However, the stable cubic phase of the AgBiSe2 alloy thin film exhibits excellent thermoelectric behaviour, making it a promising material for applications in thermoelectric devices.

The Role of Electronic Bandstructure Shape in Improving the Thermoelectric Power Factor of Complex Materials

The Role of Electronic Bandstructure Shape in Improving the Thermoelectric Power Factor of Complex Materials

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.

Janus breathing-Kagome lattice Nb3TeX7 (X = Cl, Br and I) with low thermal conductivity and high thermoelectric figure of merit

Kagome lattices have attracted much attention because of their potential in superconductivity and topological applications. In this work, we proposed three two-dimensional Janus breathing-Kagome lattices, Nb3TeCl7, Nb3TeBr7 and Nb3TeI7, and further investigated their stability, electronic and thermoelectric (TE) transport performance. We illustrated that the three materials all possess high stability, and low cleavage energy of 0.21–0.22 J/m2. Besides, they exhibit common features of Kagome lattices, such as fat bands and van Hove singularities near band-edges, which lead to high Seebeck coefficient (0.37–0.57 mV/K) and TE power factor of (2.79 − 3.71 mW/K2m). Further, owing to their asymmetric Janus structures, the strong coupling between acoustic and optical phonons, as well as low frequency for lying-phonons, the three lattices also possess low lattice thermal conductivity of 1.06–2.76 W/mK. Ultimately, they deliver promising TE figure of merit of {0.28, 0.39, 0.43} at 300 K, and increase further to {1.09, 1.39, 1.40} at 700 K, demonstrating breathing-Kagome lattice Nb3TeX7 are suitable candidates in TE application.

Simultaneous high thermoelectric and photocatalytic performance towards single-layer ZnX2S4 (X = Al, Ga, In)

Thermoelectric generation and photocatalytic water splitting to produce hydrogen are key measures to solve energy shortage and environmental pollution. In this work, we proposed three unexplored 2D materials, ZnAl2S4, ZnGa2S4 and ZnIn2S4, and further investigated their stability, thermoelectric and photocatalytic water splitting performance. We revealed the three single-layers possess low cleavage energies of 0.23–0.28 J/m2, and simultaneously show high mechanical, thermal and dynamic stability. Besides, the single-layers are indirect semiconductors with band-gaps of 2.62/2.15/1.93 eV, and deliver thermoelectric power factors of 3.03/4.06/5.92 mW/K2m at 300 K. Also, due to the high nonlinear phonon dispersion and strong acoustic-optical interactions, they have high phonon scattering rates, as well as low lattice thermal conductivities of 1.11–3.06 W/mK. As a result, their thermoelectric figure of merit can reach 0.12/0.11/0.10 at 300 K, and increases to 0.77/0.69/0.66 at 700 K. Moreover, they also have suitable REDOX band-edges, which can drive photocatalytic water splitting to produce hydrogen and oxygen, and show high absorption coefficients of ∼ 105 cm−1 from visible to ultraviolet.

Structural and thermoelectric properties of MoSe2/CNT nanocomposites

Two-dimensional (2D) materials have emerged as a focus of research on energy conversion in recent years owing to their high thermal and thermoelectric efficiency. The authors of this study investigate the impact of carbon nanotubes (CNTs) on the structural, morphological, and thermoelectric properties of molybdenum diselenide (MoSe2). X-ray diffraction (XRD), Raman spectroscopy, Energy dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analyses were used to characterize the nanocomposites. The results showed that the layers of MoSe2 surrounded the CNTs and carrier transport occurred from MoSe2 to CNTs. The conductivity of the nanocomposite samples was significantly enhanced by incorporating the CNTs, which are known as mobility boosters. The thermoelectric power factor was 3.6 times higher than that of the pristine sample at ambient temperature. However, the random orientation of the CNTs in the nanocomposite at temperatures above 370 K led to high scattering at the interface that hindered the flow of current and reduced the mean free path of the carriers. Moreover, disordered grain boundaries and defects within the CNTs contributed to increased scattering at higher temperatures, and increased the power factor of the pristine MoSe2 by only 1.25 times. The results of this study demonstrated the potential of MoSe2/CNT nanocomposites for use in thermoelectric applications.

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

Improved thermoelectric power factor of multilayered poly(3,4-ethylenedioxythiophene) polystyrene sulfonate and Cu2Se thin films

Cu2Se is a promising thermoelectric material due to its superionic behavior with superior transport properties. Here we report a facile method of spin coating for the fabrication of multilayers of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and Cu2Se thermoelectric materials for mid-temperature range applications. The synthesis of multilayered thin films with a unique organic-inorganic hybrid framework could be used in flexible thermoelectric devices. The detailed investigation of PEDOT:PSS and Cu2Se multilayered thin films reveals interaction between organic and inorganic material as inferred by AFM and FTIR. The electronic transport properties were investigated for all specimens, with the highest PF of 20.1 µW/m. K2 at 450 K was achieved from the bilayer stacking of Cu2Se and PEDOT:PSS, which is about 82 % higher as compared to that of pure PEDOT:PSS at the same temperature. These improved transport properties are the combined effect of energy filtering and interface effects. The proposed strategy opens up an avenue for research on chalcogenide based composite materials with an organic framework for flexible thermoelectric device applications.

The Effect of Width-Mismatch of Modulated Nanowaveguides on the Thermoelectric Efficiency.

Width-modulated nanowaveguides are promising for thermoelectric efficiency enhancement because electron and phonon transport properties can be geometrically tuned for improved performance. The shape of the modulation profile drastically affects the transport properties. Optimization of the width modulation for simultaneous maximum thermoelectric transport and minimum thermal transport is challenging because of the interconnected electron and phonon transport properties. We addressed this problem by analysing the effect of each characteristic dimension of a single rectangular modulation unit on electron and phonon transport. We identified distinct behaviours for electrons and phonons. We reveal that whereas phonon thermal conductance decreases with increasing width-mismatch, the electron thermoelectric power factor shows a non-monotonic dependence. It is pointed out that optimal width-mismatch that maximizes thermoelectric efficiency is mainly determined by electron transport and should be identified by maximizing the thermoelectric power. Our work points to a new strategy of optimizing geometry-modulated metamaterials for maximum thermoelectric efficiency.

Enhanced Thermoelectric Performance in the SrTi0.85Nb0.15O3 Oxide Nanocomposite with Fe2O3-Functionalized Graphene.

Doped SrTiO3 is considered one of the potential thermoelectric (TE) candidates but its TE figure of merit, ZT needs to be improved for practical application of electricity generation from high-grade waste-heat. In the present work, enhanced TE performance has been realized for SrTi0.85Nb0.15O3 (STN) perovskite adopting the strategy of composite formation with Fe2O3-functionalized graphene (FGR). We have achieved a maximum electrical conductivity of 1.4 × 105 S m-1 for 1 wt % FGR added to STN, which is around 1185% larger than that of pristine STN. The presence of FGR in the STN matrix acts as a mobility booster of electrons, overcoming the effect of Anderson localization of electrons, which impedes the electron transport in STN. This is evident from the order of magnitude increase in weighted mobility of STN after FGR addition. Furthermore, the incorporation of FGR causes about a 34% decrease in the lattice thermal conductivity. The Debye-Callaway model demonstrates that the phonon-phonon Umklapp scattering is primarily responsible for reduced thermal conductivity. The presence of FGR sheets along the grain boundaries of STN, Fe2O3 nanoparticles, and lattice imperfections gives rise to the glass-like temperature-independent phonon mean-free-path, especially above Debye temperature. The maximum ZT ∼ 0.57 has been obtained at 947 K for the 1 wt % FGR sample, which is around 420% higher than that of pristine STN. Furthermore, we have fabricated a prototype of a four-legged n-type TE module, demonstrating one of the highest power outputs of 18 mW among reported oxide thermoelectrics.

Assessing thermoelectric performance of quasi 0D carbon and polyaniline nanocomposites using machine learning

Thermoelectric materials have been widely recognized as a simple approach to harness green energy by converting thermal gradients into electrical energy. However, the intricate interplay between electrical conductivity, Seebeck coefficient, and thermal conductivity in thermoelectric materials presents a challenge to improving their efficiency. Traditional experimental methods and calculation methods have troublesome steps and long cycles for predicting new thermoelectric materials. In this work we present materials informatics-based approach, where statistical and machine learning models like correlation matrix, multiple linear regression, principal component analysis and artificial neural network were employed to find the relationship between features and thermoelectric performance. Furthermore, artificial neural network was used to analyze the roles of several compositional and microstructural features along with temperature on electrical conductivity, thermal conductivity, Seebeck coefficient and thermoelectric figure of merit (ZT) for PANI and quasi 0D carbon-based composites.

Thermoelectric Properties of HfSe2 Monolayer

In this work detailed investigation of thermoelectric properties of HfSe2 monolayer is investigated employing Boltzmann transport formalism. We present numerical calculations of thermoelectric properties namely, electrical conductivity, σ, electronic thermal conductivity, Ke , diffusion thermopower, Sd and thermoelectric figure of merit, ZT , considering the scattering of electrons from acoustic phonons and charged impurities. We find that HfSe2 monolayer possess extremely low electronic thermal conductivity and relatively high thermopower (~ 100µV/K). Our calculations show that thermoelectric figure of merit is more than one at higher temperatures thus making HfSe2 monolayer suitable candidate for thermoelectric applications.

Significant Enhancement of Thermoelectric Performance in Bi0.5 Sb1.5 Te3 Thin Film via Ferroelectric Polarization Engineering.

The Bi0.5 Sb1.5 Te3 (BST) thin film shows great promise in harvesting low-grade heat energy due to its excellent thermoelectric performance at room temperature. In order to further enhance its thermoelectric performance, specifically the power factor and output power, new approaches are highly desirable beyond the common "composition-structure-performance" paradigm. This study introduces ferroelectric polarization engineering as a novel strategy to achieve these goals. A Pb(Zr0.52 Ti0.48 )O3 /Bi0.5 Sb1.5 Te3 (PZT/BST) hybrid film is fabricated via magnetron sputtering. Density functional theory calculationsdemonstrate PZT polarization's influence on charge redistribution and interlayer charge transfer at the PZT/BST interface, facilitating adjustable carrier transport behavior and power factor of the BST film. As a result, a 26.7% enhancement of the power factor, from unpolarized 12.0 to 15.2 µWcm-1 K-2 , is reached by 2kV out-of-plane downward polarization of PZT. Furthermore, a five-leg generator constructed using this PZT/BST hybrid film exhibits a maximum output power density of 13.06Wm-2 at ΔT = 39 K, which is 20.8% higher than that of the unpolarized one (10.81Wm-2 ). The research presents a new approach to enhance thermoelectric thin films' power factor and output performance by introducing ferroelectric polarization engineering.