High Thermoelectric Figure Of Merit Research Articles

Understanding the origin of the high thermoelectric figure of merit of Zintl-phase KCaBi.

Herein, we have investigated the unexplored thermoelectric properties of Zintl-phase KCaBi using first-principles calculation and the solution of the Boltzmann transport equation. KCaBi shows intrinsically very low lattice thermal conductivities (κl) along the (x(y), and z)-directions of (1.78, 0.68) and (1.15, 0.43) W m-1 K-1 at 300 and 800 K, respectively. Along with the effect of very low κl, the high figure of merit (ZT) for p-type KCaBi results from the high Seebeck coefficients (S) and optimal electrical conductivities (σ), which originate from the high and steep total density of state (TDOS) at the valence band edge and the less dispersed multi-valley nature of the valence band edge in the band structure. On the other hand, large ZT for n-type KCaBi results from moderate S and high σ caused by the sloped TDOS at the conduction band edge and the highly dispersed nature of the conduction band edge in the band structure, and very low values of κl. The highest ZT of KCaBi that we obtained at 800 K along the (x(y), and z)-directions was (1.83, 0.80) for the p-type case at a hole concentration of 1021 cm-3 and (1.36, 1.22) for the n-type case at electron concentration 7 × 1018 cm-3. Our study demonstrates that both p-type and n-type KCaBi have the potential to be promising thermoelectric materials.

Realizing high average zT in GeTe through band modulation and suppressing Ge vacancies

GeTe is regarded as an excellent thermoelectric material, while its intrinsically high hole concentration impedes its further enhancement in thermoelectric performance. In this work, we introduce a two-step strategy by Bi and Y co-doping and Pb alloying to optimize the thermoelectric properties of the GeTe compound. Aliovalent Bi doping on the Ge site is found to effectively reduce the hole concentration, and a small substitution of Bi by Y atoms can further increase the effective mass, resulting in a high thermoelectric figure of merit (zT) of ∼ 1.92 at 723 K. Alloying Pb with high concentration elaborately decreases the carrier concentration to around 1.09 × 1020 cm−3, leading to an enhancement of Seebeck coefficient. In addition, Pb alloying maintains stable carrier mobility, contributing to a high power factor. PbTe alloying induces a strong point defect scattering and strain field fluctuation, leading to a low lattice thermal conductivity. Benefiting from the synergistically optimized effects of Bi and Y co-doping and Pb alloying, a peak zT of ∼ 2.26 at 723 K and an average zT of 1.64 over 300–773 K are realized in Ge0.8Pb0.15Bi0.04Y0.01Te.

Enhanced thermoelectric figure of merit in graphene nanoribbons by creating a distortion and transition-metal doping

Thermoelectric (TE) materials have the exceptional ability to directly convert waste heat into electricity and theoretical studies-based guidance is an effective way to investigate high-performance nanostructured TE materials throughout a range of temperatures. Herein, we studied the influence of distortion and transition metal (TM) atoms (V, Cr, Co and Pt) on the electronic and TE properties of armchair graphene nanoribbons (AGNRs) at various temperatures. The results showed that TM-doping and distortion might significantly alter the electronic structure of AGNRs. The band gap and valence band convergence are reduced as a result of the synergistic effect. A higher TE figure of merit results from a combination of decreased heat conductivity and increased electrical conductivity brought about by the electronic structural change. This study proposes that by doping and distortion-induced TE characteristics engineering of AGNR, high-performance nanostructured TE materials may be produced that have a wide range of TE applications.

Syntheses, crystal structures, and physical properties of non-centrosymmetric MgIn2Te4 and MnIn2Te4

In this study, we prepared two ternary tellurides, MgIn2Te4 and MnIn2Te4 (single crystals and bulk polycrystalline phases), by directly fusing the elements at high temperatures using the sealed tube synthesis method. Single crystal X-ray diffraction studies confirmed the non-centrosymmetric tetragonal structures (space group: I4‾ 2m) of the MgIn2Te4 and MnIn2Te4 phases with two formula units (Z = 2). Interestingly, the refined unit cell lengths of a = b = 6.2149(5) Å and c = 12.4240(19) Å for the MgIn2Te4 are slightly longer than those of the MnIn2Te4 structure (a = b = 6.1901(3) Å and c = 12.3877(7) Å). The asymmetric unit of these two MIn2Te4 (M = Mg and Mn) isostructural compounds contains three crystallographically independent sites: two disordered metal sites (M1/In1, M2/In2) and one Te1 site. Each metal atom in these structures is tetrahedrally coordinated with four Te1 atoms. Optical absorption studies for polycrystalline MgIn2Te4 and MnIn2Te4 samples showed bandgaps of 1.1(2) eV and 0.9(2) eV, respectively, consistent with these powders' black color. Seebeck studies confirm that holes are the majority of charge carriers for the MgIn2Te4 sample. The thermal conductivity (ktot) studies of the samples reveal ultralow values at 773 K: 0.37 Wm−1K−1 for the MgIn2Te4 and 0.44 Wm−1K−1 for the MnIn2Te4. Magnetic studies on the polycrystalline MnIn2Te4 show a paramagnetic behavior with a magnetic moment of 5.73 μB. Our theoretical studies indicate the possibility of achieving a high thermoelectric figure of merit (zT > 1.5) by optimizing the hole concentration in the MgIn2Te4 sample.

Compromise Design of Resonant Levels in GeTe‐Based Alloys with Enhanced Thermoelectric Performance

AbstractResonant levels (RLs) are expected to increase the density of states (DOS) abruptly, thereby increasing the Seebeck coefficient for a high thermoelectric figure of merit (ZT). However, the negative effects of RLs on reducing the carrier mobility and band gap have rarely been explored. In this work, the pros and cons of resonant‐state doping of group IIIA elements (In, Ga) in GeTe‐based alloys are studied by experimental transport properties and density‐functional‐theory calculations. RLs effects of Fermi level pinning and increased effective mass are strong in Ge0.9‐xInxSb0.1Te but weak in Ge0.9‐y(In0.5Ga0.5)ySb0.1Te, which shows dependence on the hump DOS relative to the background, location of RLs, and energy width of RLs. Using the In‐Ga co‐doping strategy to make a compromise between the DOS, carrier mobility, and bipolar transport, combined with beneficial alloying effects of Pb and Sb, a peak ZT≈ 2.1 at 773 K and a high average ZT≈ 1.43 within 300–773 K can be obtained in Ge0.78In0.005Ga0.005Pb0.1Sb0.07Te. The corresponding single‐leg device shows a remarkable energy conversion efficiency of 13.7% at a temperature difference of 451 K. This work suggests that RLs are not always beneficial for improving ZT, and the compromise design of RLs should be carefully considered to advance ZT.

Chemical Composition Modulation Realizing Remarkable Improvement of Thermoelectric Performance in CuInTe2-Based Alloy.

CuInTe2 (CIT) is one of the typical ternary chalcogenides known for its characteristic mixed polyanionic/polycationic site defects, making it a subject of continuous interest in the field of thermoelectrics. In this work, we propose a chemical composition modulation strategy for CIT by alloying GeTe and then introducing a copper deficiency (denoted by VCu). This strategy aims to unpin its Fermi level (Fr) and shift Fr into the valence band (VB) while simultaneously enabling coupling between the optical and acoustic phonon, thereby providing an extra phonon scattering path at low frequencies. The simultaneous composition regulations not only enhance the carrier concentration (nH) to 1019-1020 cm-3 but also significantly reduce the lattice thermal conductivity (κL) to ∼0.48 W m-1 K-1, thus effectively realizing electro-acoustic coordination in the present material. As a consequence, the thermoelectric (TE) performance is remarkably improved with the highest TE figure of merit (ZT) of 1.51 at ∼838 K. This value ranks at a higher level among CIT-based materials, which showcases the great significance of chemical composition modulation.

Hydrogenation driven ultra-low lattice thermal conductivity in β 12 borophene

Borophene gathered large interest owing to its polymorphism and intriguing properties such as Dirac point, inherent metallicity, etc but oxidation limits its capabilities. Hydrogenated borophene was recently synthesised experimentally to harness its applications. Motivated by experimental work, in this paper, using first-principles calculations and Boltzmann transport theory, we study the freestanding β 12 borophene nanosheet doped and functionalised with hydrogen (H), lithium (Li), beryllium (Be), and carbon (C) atoms at different β 12 lattice sites. Among all possible configurations, we screen two stable candidates, pristine and hydrogenated β 12 borophene nanosheets. Both nanosheets possess dynamic and mechanical stability while the hydrogenated sheet has different anisotropic metallicity compared to pristine sheet leading to enhancement in brittle behaviour. Electronic structure calculations reveal that both nanosheets host Dirac cones (DCs), while hydrogenation leads to shift and enhancement in tilt of the DCs. Further hydrogenation leads to the appearance of additional Fermi pockets in the Fermi surface. Transport calculations reveals that the lattice thermal conductivity changes from 12.51 to 0.22 W m−1 K−1 (along armchair direction) and from 4.42 to 0.07 W m−1 K−1 (along zigzag direction) upon hydrogenation at room temperature (300 K), demonstrating a large reduction by two orders of magnitude. Such reduction is mainly attributed to decreased phonon mean free path and relaxation time along with the enhanced phonon scattering rates stemming from high frequency phonon flat modes in hydrogenated nanosheet. Comparatively larger weighted phase space leads to increased anharmonic scattering in hydrogenated nanosheet contributing to ultra-low lattice thermal conductivity. Consequently, hydrogenated β 12 nanosheet exhibits a comparatively higher thermoelectric figure of merit (∼0.75) at room temperature along armchair direction. Our study demonstrates the effects of functionalisation on transport properties of freestanding β 12 borophene nanosheets which can be utilised to enhance the thermoelectric performance in two-dimensional (2D) systems and expand the applications of boron-based 2D materials.

Thermoelectric Materials Based on Lead Telluride and Prospects for their Practical Application

Lead telluride (PbTe) is considered one of the most promising materials in thermoelectrics due to its unique thermoelectric properties. This semiconductor exhibits a high thermoelectric figure of merit (ZT) in certain temperature ranges, making it highly effective for converting heat energy into electricity. Additionally, PbTe is characterized by stability and low thermal conductivity, which further enhances the efficiency of thermoelectric devices. Another advantage of using PbTe is its relative affordability and high availability of raw materials. This makes it attractive for manufacturing mass thermoelectric devices such as thermoelectric modules for automobiles, industrial thermoelectric generators, heat recirculation, and others. The paper provides a review of works and an analysis of general approaches to semiconductor thermoelectric materials, including lead telluride.

Computational approach to enhance thermoelectric performance of Ag2Se by S and Te substitutions.

Designing an n-type thermoelectric material with a high thermoelectric figure of merit at near room temperature is extremely challenging. Generally, pristine Ag2Se reveals unusually low thermal conductivity along with a high electrical conductivity and Seebeck coefficient, which leads to high thermoelectric performance (n-type) at room temperature. Herein, we report a pseudo-ternary phase (Ag2Se0.5Te0.25S0.25) that exhibits significantly high thermoelectric performance (zT ∼ 2.1) even at 400 K. First-principles calculation reveals that the Rashba type of spin-dependent band spitting, which originates due to sulfur and tellurium substitution, helps to improve the thermopower magnitude. We also show that the intrinsic carrier mobility is not only controlled by the carrier effective mass but is substantially limited by longitudinal acoustic and optical phonon modes, which is an extension of the deformation potential theory. Locally off-center sulfur atoms, together with the increase in configurational entropy via substitution of Te and S atoms in Ag2Se, lead to a drastic reduction in the lattice thermal conductivity (klat ∼ 0.34 W m-1 K-1 at 400 K). The Rashba effect coupled with the configurational entropy synergistically results in a high thermoelectric figure of merit in the n-type thermoelectric material working in the near-room-temperature regime.

Computational prediction of high thermoelectric performance in As2Se3 by engineering out-of-equilibrium defects.

We employed first-principles calculations to investigate the thermoelectric transport properties of the compound As2Se3. Early experiments and calculations have indicated that these properties are controlled by a kind of native defect called antisites. Our calculations using the linearized Boltzmann transport equation within the relaxation time approximation show good agreement with the experiments for defect concentrations of the order of 1019 cm-3. Based on our total energy calculations, we estimated the equilibrium concentration of antisite defects to be about 1014 cm-3. These results suggest that the large concentration of defects in the experiments is due to kinetic and/or off-stoichiometry effects and in principle it could be lowered, yielding relaxation times similar to those found in other chalcogenide compounds. In this case, for relaxation time higher than 10 fs, we obtained high thermoelectric figures of merit of 3 for the p-type material and 2 for the n-type one.

Entropy engineering enabled atomically dispersed Cu doping leading to an exceptionally high thermoelectric figure of merit in n-type lead chalcogenides

Entropy engineering fully dissolved extra Cu atoms into the interstitial lattice sites of lead chalcogenides, yielding high charge carrier mobility and ZT values in a wide temperature range.

A van der Waals p-n heterostructure of GaSe/SnS2: a high thermoelectric figure of merit and strong anisotropy.

In recent years, van der Waals heterostructures (vdWHs) with controllable and peculiar properties have attracted extensive attention in the fields of electronics, optoelectronics, spintronics and electrochemistry. However, vdWHs with good thermoelectric performance are few due to the complex coupling of thermoelectric coefficients. Here, we employ density functional theory and Boltzmann's transport equation to explore the thermoelectric properties of the p-n vdWH of GaSe/SnS2, which has been experimentally observed to exhibit high performance as an optoelectronic device. We reveal that GaSe/SnS2 possesses strong anisotropy in terms of electronic transport resulting from the anisotropic carrier relaxation time. The longer carrier relaxation time in the y-direction for n-type induces a high power factor of 0.084 W m-1 K-2 at 300 K, while it is only 0.0087 W m-1 K-2) in the x-direction. The strong coupling of low-mid frequency phonon branches and the relatively weak Sn-S bond-induced anharmonicity hinder the phonon transport, which results in the lattice thermal conductivity of GaSe/SnS2 (14.61 and 15.43 W m-1 K-1 along the x- and y-directions at 300 K) being much smaller than the average value of GaSe and SnS2 (43.44 W m-1 K-1 at 300 K). The optimal thermoelectric figure of merit at 700 K for GaSe/SnS2 reaches 2.99, which is significantly higher than those of the constituents of GaSe (0.58) and SnS2 (1.04). The present work highlights the potential thermoelectric applications and the understanding of the thermoelectric transport mechanism for the recently synthesized p-n vdWH of GaSe/SnS2 with a high thermoelectric figure of merit and strong anisotropy.

Control of spin on ferromagnetism and thermoelectric properties of K2GeMnX6 (X = Cl, Br, I) halide perovskites: emerging candidates for semiconductor spintronics and thermoelectric applications

The Figure showcases high thermoelectric figure of merit (zT) values at 300 K for K2GeMnCl6, K2GeMnBr6, and K2GeMnI6 (1.01, 1.00 and 0.99), highlighting their significant potential for renewable energy applications.

Effect of Ag-addition on the thermoelectric properties of Cu12Sb4S13 tetrahedrite

Tetrahedrite, Cu12Sb4S13, an eco-friendly thermoelectric material with earth-abundant and low-cost constituents, has garnered global interest. This study investigated the thermoelectric characteristics of Ag-added tetrahedrites, which were prepared through solid-state synthesis and subsequently hot-pressed. The minimal temperature dependence of the transport coefficient indicated the presence of strong electron–phonon coupling in the samples. It was observed that Ag addition in tetrahedrite could successfully scatter acoustic phonons, reducing the lattice thermal conductivity while minimally affecting the power factor. The enhanced anharmonicity induced by Ag addition is the primary cause of reduced lattice thermal conductivity. Raman spectroscopy data showed that Ag addition could weaken the Sb–S bond, further supporting the previous argument. Consequently, the lattice thermal conductivity was lowered to ∼0.27 W m−1 K−1 and obtained for the composition Ag0.025 added Cu11.975Sb4S13. A relatively high power factor of ∼1.3 mW m−1 K−2 was obtained for the same composition. Owing to the lowest total thermal conductivity ∼1.09 W m−1 K−1, the sample with composition Ag0.025 added Cu11.975Sb4S13 showed the highest thermoelectric figure of merit of 0.87 at 738 K.

Electronic structure, optical properties and high thermoelectric figure of merit in iron phthalocyanine polymer: FP-LAPW comparative studies

The comparative study of optoelectronic and thermoelectric properties of Iron phthalocyanine polymer, FePc, has been performed using the full-potential linearized augmented plane wave (FP-LAPW) method. The results show that the band gap of FePc is 2.039 eV with two deep trap bands. Optical spectra such as the real and imaginary parts of the dielectric function, reflectivity, electron energy loss spectroscopy, refractive index, extinction, and absorption coefficients are calculated and the main peaks are compared with the available experimental data. Calculated optical spectra are in good agreement with the experimental data of diethylamine and triethylamine polymers. The maximum values of 2190.2 μVK−1 and 1.02 are obtained for the Seebeck coefficient and the figure of merit ZT at 300 K for the chemical potentials near the Fermi level, respectively. The results show that FePc polymer is a good candidate for optoelectronic device applications.

First-principles study of the structural, mechanical, dynamical, and transport properties of Cs2NaInX6[X = Br,I] for thermoelectric applications

Thermoelectricity generation techniques utilized for conversion of waste heat into electric power are currently gaining prominence. It is necessary that for improved efficiency, the materials used for such applications should have a high thermoelectric figure of merit (ZT) and appropriate stability. In this study, the properties of Cs2NaInX6 double perovskites (DPs) were examined using a first-principles approach to ascertain their suitability as thermoelectric materials. The findings revealed that the materials are mechanically stable according to the Born stability criteria for cubic crystals. They were also observed to be anisotropic given isotropic values of 0.702 and 0.662 for Cs2NaInBr6 and Cs2NaInI6 DPs respectively. Phonon calculations indicate soft modes at the Brillouin zone boundary and center (Γ) for Cs2NaInI6 DP, which suggests ferroelectric and anti-ferroelectric distortions. Similarly for Cs2NaInBr6 DP, the soft modes were exclusively found at the BZ center (Γ), suggesting ferroelectric distortion. The transport coefficient properties calculated are used for the determination of the ZT values. The calculated values of ZT at ambient temperature are of the order 1.0 for all the DPs which is high enough for thermoelectric applications suggesting that they may be potential thermoelectric materials

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.

Photoemission Study of the Thermoelectric Group IV‐VI van der Waals Crystals (GeS, SnS, and SnSe)

AbstractGroup IV‐VI van der Waals crystals (MX, where M = Ge, Sn, and X = S, Se) are receiving increasing attention as semiconducting thermoelectric materials with nontoxic, earth‐abundant composition. Among them, SnSe is considered the most promising as it exhibits a remarkably high thermoelectric figure of merit (ZT), initially attributed to its low lattice thermal conductivity. However, it has been shown that the electronic band structure plays an equally important role in thermoelectric performance. A certain band shape, described as a “pudding mold” and characteristic for all MXs, has been predicted to significantly improve ZT by combining good electrical conductivity with high Seebeck coefficient. This subtle feature is explored experimentally for GeS, SnS, and SnSe by means of angle‐resolved photoemission spectroscopy. The technique also allows for the determination of the effective mass and Fermi level position of as‐grown undoped crystals. The findings are supported by ab initio calculations of the electronic band structure. The results greatly contribute to the general understanding of the valence band dispersion of MXs and reinforce their potential as high‐performance thermoelectric materials, additionally giving prospects for designing systems consisting of van der Waals heterostructures.

Growth of high carrier mobility 2-Dimensional hexagonal Bi2Te3-xSex nanoplatelets and its thermoelectric performance studies

Morphologically controlled growth of selenium-doped 2-Dimensional (2D) Bi2Te3-xSex nanoplatelets have been optimized by solvothermal synthesis using polyvinyl-pyrro-lidone (PVP) surfactant from the pre-optimized growth conditions of 2D-Bi2Te3 hexagonal nanoplatelet. An optimization was carried out using the Se-doping of x = 0.3 under variable growth durations of 4 h, 8 h, and 12 h. X-ray powder diffraction studies confirmed the complete crystallization with a pure phase of 2D-Bi2Te3-xSex at an extended duration of 12 h synthesis. Crystallization with defined hexagonal morphology was confirmed by FESEM analysis. Selenium doping into the 2D-Bi2Te3-xSex lattice from x = 0.1 to x = 0.4 indicates a step-by-step shift in the 2θ value of the (015) predominant peak towards higher 2θ values. Raman analysis revealed the alteration of the appearance of A1u2 to A1g2 upon the lattice inclusion of “Se” in the place of “Te”. Hall measurements on the consolidated pellet of 2D-Bi2Te3-xSex resulted in the highest electron mobility of 1675 cm2V−1s−1 with carrier concentration of 2.9 × 1018 cm−3 for Se-doping x = 0.2. Thermal conductivity analysis in comparison with the total thermal conductivity and the calculated electronic thermal conductivity revealed the lowest lattice thermal conductivity of 0.17 Wm−1K−1 for the Se-doping x = 0.2 after the correction for structural anisotropy factor (f). The highest thermoelectric figure of merit of 0.74 was achieved in this course of work.

Enhancing power generation sustainability of thermoelectric pillars by suppressing diffusion at Bi-Sb-Te/Sn electrode interface using crystalline Co-P coatings

Bismuth telluride-based thermoelectric (TE) devices were widely used in waste heat recovery due to their high thermoelectric figure of merit (ZT), but the severe diffusion between Sn-based solder and TE pillar generated porous and brittle Sn-Te intermetallic compound (IMC), which seriously restricted the reliable service of devices. Ni-based coating was used but the thick Ni-Te IMC also impaired the properties of TE pillars. In this work, the crystalline Co-P coating was inserted between the p-type TE material Bi0.5Sb1.5Te3 and the solder by electrodeposition, and the power generation sustainability of the TE pillar were significantly improved, and the mechanism was revealed. The microstructural characterization proved that the Co-P coating could effectively resist the diffusion of active Sn, Bi, Sb, Cu atoms. Co-P coating would provide support for the structural integrity and performance sustainability of TE pillars. It was worth noting that an ultrathin CoTe2 IMC layer was formed between the Co-P coating and the Bi-Sb-Te pillar, which was much thinner than the Ni-Te IMC layer reported in the literature. This indicated that Co-P only consumed little thermoelectric elements, but also a good metallurgical bond was formed between the Co-P and the Bi-Sb-Te. After aging at 423 K for 150 h, the maximum output power and average Seebeck coefficient of the TE pillar without coating dropped significantly by 74% and 34%, respectively, and the internal resistance rose by 67%. Remarkably, the maximum output power and average Seebeck coefficient of the TE pillar protected by the Co-P coating only decreased by 14% and 5%, respectively, and the internal resistance only increased by 6%.