Excellent Thermoelectric Materials Research Articles

Anti-Wilson mechanism for adjusting bandgap and electronic transport property of a half-Heusler material LiCdP

Using first-principles calculations, we have found that LiCdP, an existing half-Heusler material, exhibits an anti-Wilson mechanism for adjusting the bandgap. Specifically, instead of widening according to the conventional Wilson mechanism, the bandgap shows a significant decrease in response to lattice strain, eventually closing at 5.02% tensile strain. This anti-Wilson mechanism is attributed to the weakened repulsion between the 3s and 5s orbitals of the P and Cd atoms, respectively, as observed by the analysis of the neighbouring atomic orbital coupling. In addition, we have found that the ZT value, which measures the thermoelectric efficiency of this material, can reach 1.33 at a temperature of T = 1200 K when lattice strain is induced by thermal expansion. This result suggests that LiCdP is an excellent thermoelectric material in a high operating temperature range. From the point of view of actual applications, such an interesting tunability of the bandgap of LiCdP provides a novel alternative for designing electronic or optoelectronic devices in a controllable way.

Research of the impact of uniaxial tension on the thermoelectric properties of NbCoX (X=Ge, Sn, Pb) compounds

Half-Heusler materials have been confirmed as excellent thermoelectric materials. To simulate the stress conditions in engineering applications, we adopt uniaxial tensile strain to assess the impact of lattice changes on NbCoX (X=Ge, Sn, Pb) compound's thermoelectric performance. The research reveals that when NbCoX compounds are stretched by 1.5 %, the material almost does not decrease the power factor but significantly reduces thermal conductivity, thereby enhancing the ZT value. At a carrier concentration of 1E22 cm−3 and a temperature of 873 K, the predicted ZT values for p-type NbCoSn compound reach 0.359 and 0.354 after stretching by 1.5 % in the x-y and z directions, respectively. Compared to the previous original values, this improvement is approximately 1.5 times, highlighting the significant role of tensile strain in enhancing thermoelectric performance. This may be attributed to changes in compound lattice symmetry induced by strain, leading to increased defects and interfaces, which complicate phonon transport paths, resulting in a decrease in the average phonon mean free path and consequently lowering thermal conductivity. Additionally, with the increase in atomic number of Ge, Sn, and Pb, their thermoelectric figure of merit also increases continuously, with ZT values remaining higher under stretching than compression. Such research provides a new avenue for designing and optimizing high-performance thermoelectric materials and offers insights into strain engineering for future exploration of 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.

Unexpected thermal transport properties of MgSiO3 monolayer at extreme conditions

The thermal transport properties of mantle minerals are of paramount importance to understand the thermal evolution processes of the Earth. Here, we perform extensively structural searches of two-dimensional MgSiO3 monolayer by CALYPSO method and first-principles calculations. A stable MgSiO3 monolayer with Pmm2 symmetry is uncovered, which possesses a wide indirect band gap of 4.39 eV. The calculations indicate the lattice thermal conductivities of MgSiO3 monolayer are 49.86 W (mK)−1 and 9.09 W (mK)−1 in x and y directions at room temperature. Our findings suggest that MgSiO3 monolayer is an excellent low-dimensional thermoelectric material with high ZT value of 4.58 from n-type doping in the y direction at 2000 K. The unexpected anisotropic thermal transport of MgSiO3 monolayer is due to the puckered crystal structure and the asymmetric phonon dispersion as well as the distinct electron states around the Fermi level. These results offer a detailed description of structural and thermal transport properties of MgSiO3 monolayer at extreme conditions.

Enhanced thermoelectric performance of Cu2SnSe3 by synergic effects via cobalt-doping

Diamond-like Cu2SnSe3 has drawn intensive attention as an excellent environmentally-friendly p-type thermoelectric material. In this study, the effects of Co-doping at Sn site on its thermoelectric properties were investigated. Results revealed an effective increase in carrier concentration and enhancement of density-of-states (DOS) effective mass m* due to the role of Co as acceptor and probably the contribution of its d states in the valence bands. Consequently, a remarkable power factor of up to 9.55 μW cm−1 K−2 has been achieved at 823 K. In addition, lattice thermal conductivity was strongly suppressed due to the intensified phonon scattering and softening of bonding as revealed by the sound velocity analysis. Ultimately, a maximal ZT of 0.97 at 823 K and an average ZT of ∼ 0.7 have been achieved in Cu2Sn0.88Co0.12Se3, almost double that of the pristine one.

Ultralow lattice thermal conductivity and excellent thermoelectric performance of monolayer CdGaInS4: a first-principles investigation.

Monolayer CdGaInS4 is an excellent optoelectronic material, but its thermoelectric properties remain unexplored. Through first-principles studies, we investigate the thermoelectric transport properties of monolayer CdGaInS4. The results show that the degenerate weakly dispersive valence band results in an ultrahigh Seebeck coefficient, and the small parabolic electron pockets lead to good electron mobility and conductivity. The ultralow lattice thermal conductivity is attributed to the complete decoupling and softening of the low frequency out-of-plane mode and the strong bonding anharmonicity, giving rise to significant phonon scattering. These results provide good physical descriptors for the search for and theoretical design of excellent two-dimensional thermoelectric materials, and motivate relative measurements in monolayer CdGaInS4 and its applications as a promising thermoelectric material.

High power factor and ultra-low lattice thermal conductivity in Sn1−xWxTe alloys via interstitial defects modulation

SnTe, a potential substitute for excellent medium-temperature thermoelectric (TE) material PbTe, has recently attracted much attention in TE energy conversion. Herein, we demonstrate a rare and effective strategy to synergistically optimize the electrical and thermal transport properties of SnTe by introducing Tungsten(W) interstitial defects. Concretely, the formation energy of intrinsic Sn vacancy defects is reduced by introducing W-interstitial defects, thus increasing the hole carrier concentration and overall electrical conductivity. Meanwhile, the increase of band degeneracy (band flattening and valence band convergence) markedly enhances the Seebeck coefficient, which exhibits the high average power factor of ∼17.79 μWcm−1K−2. Moreover, the lattice thermal conductivity is significantly reduced to 0.46 Wm−1K−1, which arises from the enhancement of phonon scattering caused by various processes including dislocations, crystal defects and multiscale nanoprecipitation. These combined effects lead to a maximum ZT value of ∼0.79 at 823 K for Sn0.97W0.03Te, especially the average ZT value remarkably increased to 0.26. Conclusively, unlike conventional work to optimize electrical transport performance (reducing hole carrier concentration), this work provides an innovative strategy to significantly improve the electrical properties of SnTe through the modulation of W-interstitial defects.

Rattling-like behavior and band convergence induced ultra-low lattice thermal conductivity in MgAl2Te4 monolayer

Inspired by the excellent stability exhibited by experimentally synthesized two-dimensional (2D) MoSi2N4 layered material, the thermal and electronic transport, and thermoelectric (TE) properties of MgAl2Te4 monolayer are systematically investigated using the First-principles calculations and Boltzmann transport theory. The mechanical stability, dynamic stability, and thermal stability (900 K) of the MgAl2Te4 monolayer are demonstrated, respectively. The MgAl2Te4 monolayer exhibits a bandgap of 1.35 eV using the HSE06 functional in combination with spin-orbit coupling (SOC) effect. Band convergence in the valence band is favorable to improve the thermoelectric properties. The rattling thermal damping effect caused by the weak bonding of MgTe bonds in MgAl2Te4 monolayer leads to ultra-low lattice thermal conductivity (0.95/0.38 W/(m·K)@300 K along the x-/y-direction), which is further demonstrated by the phonon group velocities, phonon relaxation time, Grüneisen parameters, and scattering mechanisms. The optimal zT of 3.28 at 900 K is achieved for the p-type MgAl2Te4 monolayer, showing the great promising prospect for the excellent p-type thermoelectric material. Our current work not only reveals the underlying mechanisms responsible for the excellent TE properties, but also elaborates on the promising thermoelectric application of MgAl2Te4 monolayer material at high temperature.

Layer-dependent excellent thermoelectric materials: from monolayer to trilayer tellurium based on DFT calculation.

Monoelemental two-dimensional (2D) materials, which are superior to binary and ternary 2D materials, currently attract remarkable interest due to their fascinating properties. Though the thermal and thermoelectric (TE) transport properties of tellurium have been studied in recent years, there is little research about the thermal and TE properties of multilayer tellurium with interlayer interaction force. Herein, the layer modulation of the phonon transport and TE performance of monolayer, bilayer, and trilayer tellurium is investigated by first-principles calcuations. First, it was found that thermal conductivity as a function of layer numbers possesses a robust, unusually non-monotonic behavior. Moreover, the anisotropy of the thermal transport properties of tellurium is weakened with the increase in the number of layers. By phonon-level systematic analysis, we found that the variation of phonon transport under the layer of increment was determined by increasing the phonon velocity in specific phonon modes. Then, the TE transport properties showed that the maximum figure of merit (ZT) reaches 6.3 (p-type) along the armchair direction at 700K for the monolayer and 6.6 (p-type) along the zigzag direction at 700K for the bilayer, suggesting that the TE properties of the monolayer are highly anisotropic. This study reveals that monolayer and bilayer tellurium have tremendous opportunities as candidates in TE applications. Moreover, further increasing the layer number to 3 hinders the improvement of TE performance for 2D tellurium.

Strong Electron–Phonon and Phonon–Phonon Interactions Lead to High Thermoelectric Performances in Lead Phosphorene via Symmetry Breaking

AbstractUnderstanding the full electron–phonon and phonon–phonon interaction effects in semiconductors is crucial for tuning electronic and phonon transport properties with the purpose to the enhancement of thermoelectric performances. In this work, based on the detailed investigations on 2D lead phosphorene (PbP) by using the first‐principle calculations, it is demonstrated that, in , , and PbP, the electron–phonon interactions are significantly stronger than conventional group‐IV monolayers such as silicene by five orders in magnitude, and intravalley scatterings dominate over the intervalley scattering due to less degenerate band extrema near Fermi level. Based on the selection‐rule analysis, it is found that the symmetry breaking in the janus γ phase compared to the higher‐symmetry α phase allows fewer symmetry operations, leading to more allowed scattering channels for electrons and phonons, subsequently suppressing the electron and phonon transport in PbP. Due to the low Debye temperatures, and strong four‐phonon interactions, the lattice thermal conductivities at room temperature in these monolayers are sharply decreased. Finally, it is found that the maximum thermoelectric figure of merits for these three monolayers reach 0.90/0.24/1.25 at room temperature, respectively. This work not only paves the way for the promising applications of PbP monolayers in thermoelectric devices, but also suggests that symmetry breaking and strong high‐order phonon–phonon interactions are crucial to identifying excellent thermoelectric materials.

The Potential of Arsenic‐based Zintl Phases as Thermoelectric Materials: Structure & Thermoelectric Properties

AbstractZintl phase thermoelectric materials have generated tremendous interest due to possessing structural features conducive to high thermoelectric performances. On the other hand, both arsenic and arsenic‐based compounds have become attractive in electronics due to having interesting properties like narrow bandgap, tunable carrier concentration, and non‐centrosymmetric structures. The structure of arsenic compounds plays a telling role in determining their efficiency as thermoelectric materials. They also show the scope to be doped as both p‐ and n‐type conduction providing exciting new materials with applications as a full module. These attributes make them appealing as thermoelectric materials for further research. This short review is an overview of the different structures of arsenic‐based Zintl ternary materials that have potential to be excellent thermoelectric materials.

The synergy of Sb doping and vacancy equilibrium charge improves the thermoelectric properties of GeMnTe2

GeMnTe2 is considered to be an excellent thermoelectric material because it is cheaper and easier to obtain than GeTe, has a higher carrier concentration than MnTe, and its cubic structure is not prone to phase transition and is relatively stable. The aim of this paper is to optimize the carrier concentration of GeMnTe2 and reduce the thermal conductivity by Charge Balance Vacancy Engineering, thus giving the material higher thermoelectric properties. In this paper, GeMnTe2 has obtained better thermoelectric properties by Charge Balanced Vacancy Engineering. Sb is introduced into GeMnTe2 as an effective dopant, which leads to an increase in the density of states near the Fermi level. In the heat, when Sb doping concentration was 0.1 and the actual content of Ge was 0.85, the highest power factor was 13.72 μWcm−1K−2, which was 17.70% and was head and shoulders above the pure. The ZT value was 0.67 and the maximum value of Ge0.97MnSb0.02Te2 sample was 0.8 at the pure phase condition of 770 K, which was about 19.94% higher than that of GeMnTe2, which proved that Charge Balance Vacancy Engineering is an effective technique to improve the thermoelectric performance of GeMnTe2 thermoelectric devices.

Structural, optical and thermoelectric properties of (Al and Zn) doped Co9S8-NPs synthesized via co-precipitation method

In the previous decade nanostructure materials have emerged as the necessary ingredient in electronic application and enhancement of device performance. Cobalt sulfide is one such transition metal sulfide which has provoked a lot of interest in the field of research. The present study was related to pure, (Al and Zn) doped cobalt sulfide NPs have been fabricated using facile co-precipitation method. The cubic structure has conformed to XRD analysis and crystallite size of pure and doped Co9S8 NPs in the range of 11 to 29 nm and with W-H method varies from 29 to 51 nm. The SEM micrograph shows that the agglomeration of the particles in the form of clusters. To conform the vibrational modes of (Al and Zn) doped Co9S8-NPs was done with Raman spectroscopy analysis. The absorbance of prepared materials was investigated with UV–VIS and it was also observed that the band gap decreased from 3.6 to 1.94 eV with doping agents. In case of IV analysis shows that the resistivity decreased and conductivity increased of Al and Zn doped Co9S8 NPs. Furthermore, the sharp increase in potential difference at lower temperature indicates that the doped material is an excellent thermoelectric material for energy storage devices.

Large Improvement of Thermoelectric Performance by Magnetism in Co-Based Full-Heusler Alloys.

Full-Heusler alloys (fHAs) exhibit high mechanical strength with earth-abundant elements, but their metallic properties tend to display small electron diffusion thermopower, limiting potential applications as excellent thermoelectric (TE) materials. Here, it is demonstrated that the Co-based fHAs Co2 XAl (X = Ti, V, Nb) exhibit relatively high thermoelectric performance due to spin and charge coupling. Thermopower contributions from different magnetic mechanisms, including spin fluctuation and magnon drag are extracted. A significant contribution to thermopower from magnetism compared to that from electron diffusion is demonstrated. In Co2 TiAl, the contribution to thermopower from spin fluctuation is higher than that from electron diffusion, resulting in an increment of 280µWm-1 K-2 in the power factor value. Interestingly, the thermopower contribution from magnon drag can reach up to -47µVK-1 , which is over 2400% larger than the electron diffusion thermopower. The power factor of Co2 TiAl can reach 4000µWm-1 K-2 which is comparable to that of conventional semiconducting TE materials. Moreover, the corresponding figure of merit zT can reach ≈0.1 at room temperature, which is significantly larger than that of traditional metallic materials. The work shows a promising unconventional way to create and optimize TE materials by introducing magnetism.

Strong anharmonicity-assisted low lattice thermal conductivities and high thermoelectric performance in double-anion Mo2 AB 2 (A = S, Se, Te; B = Cl, Br, I) semiconductors

The thermoelectric properties of layered Mo2 AB 2 (A = S, Se, Te; B = Cl, Br, I) materials are systematically investigated by first-principles approach. Soft transverse acoustic modes and direct Mo d–Mo d couplings give rise to strong anharmonicities and low lattice thermal conductivities. The double anions with distinctly different electronegativities of Mo2 AB 2 monolayers can reduce the correlation between electron transport and phonon scattering, and further benefit much to their good thermoelectric properties. Thermoelectric properties of these Mo2 AB 2 monolayers exhibit obvious anisotropies due to the direction-dependent chemical bondings and transport properties. Furthermore, their thermoelectric properties strongly depend on carrier type (n-type or p-type), carrier concentration and temperature. It is found that n-type Mo2 AB 2 monolayers can be excellent thermoelectric materials with high electric conductivity, σ, and figures of merit, ZT. Choosing the types of A and B anions of Mo2 AB 2 is an effective strategy to optimize their thermoelectric performance. These results provide rigorous understanding on thermoelectric properties of double-anions compounds and important guidance for achieving high thermoelectric performance in multi-anion compounds.

Realizing high thermoelectric performance for p-type SiGe in medium temperature region via TaC compositing

SiGe is recognised as an excellent thermoelectric material with superior mechanical properties and thermal stability in regions with high temperatures. This study explores a novel strategy for co-regulating thermoelectric transport parameters to achieve high thermoelectric properties of p-type SiGe in the mid-temperature region by incorporating nano-TaC into SiGe combined ball milling with spark plasma sintering. By optimizing the amount of TaC in the SiGe matrix, the power factors were significantly increased due to the modulation doping effect based on the work function matching of SiGe with TaC. Simultaneously, the ensemble effect of the nanostructure leads to a significant decrease in thermal conductivity. Thus, a high ZT of 1.06 was accomplished at 873 K, which is 64 % higher than that of typical radioisotope thermoelectric generator. Our research offers a novel strategy for expanding and enhancing the thermoelectric properties of SiGe materials in the medium temperature range.

Enhanced Anharmonicity by Forming Low-Symmetry Off-Center Phase: The Case of Two-Dimensional Group-IB Chalcogenides.

Enhanced anharmonicity is required to achieve many interesting phenomena in thermoelectricity, superconductivity, ferroelectricity, etc. Here, we propose a novel mechanism for enhancing anharmonicity by forming the low-symmetry off-center ground state, such as the s(II) phase, in two-dimensional AIB2X chalcogenides (AIB = Cu, Ag and Au; X = S, Se, and Te). In this system, the in-plane rotational phonon mode introduces a much stronger anharmonicity in the distorted s(II) phase than in the nondistorted s(I) phase. We show that the stabilities of the s(I) and s(II) phases arise from the ionicity and the ionic size; for example, the low ionicity and the small ionic size favor the s(II) phase. We further demonstrate that the anharmonicity can be tuned by controlling the strain-induced s(II)-to-s(I) phase transition, which explains the anomalous lattice thermal conductivity. Our work relates anharmonicity to symmetry-breaking structural distortion and widens the ways to design excellent thermoelectric materials.

Charge Balanced Vacancy Engineering to Enhance the Thermoelectric Properties of GeMnTe2

As a derivative of GeTe and MnTe, GeMnTe2 exhibits a cubic structure with no phase transition. Compared to GeTe, it is considerably cheaper and has a significantly high carrier concentration compared to MnTe, making it an excellent thermoelectric (TE) material with promising applications. Herein, a charge‐balanced vacancy engineering strategy is employed, resulting in a significant improvement in the power factor of GeMnTe2. Furthermore, the density functional theory calculation shows that the doping of Bi and the absence of Ge synergically increase the density of states near the Fermi level. Power factors of ≈13.7 μW cm−1 K−2 are achieved at 8% Bi content, a 22% improvement compared to the undoped system, while a final maximum ZT value of about 1.12 and an average ZT value of 0.72 are achieved. This indicates that charge‐balance vacancy engineering is an effective optimization method for the GeMnTe2 system, which can effectively improve the TE performance.

High thermoelectric performance of intrinsic few-layers T-HfSe2

Due to their distinct electrical, mechanical, thermal, and optoelectronic properties, two-dimensional layered materials have drawn a lot of research and interest. This study employs first-principles calculations along with Boltzmann transport theory to precisely predict the thermoelectric properties of few-layer intrinsic (2 L, 3 L, and 4 L) T -HfSe 2 . The increase in layers causes band degeneracy, which greatly improves the Seebeck coefficient and power factor, we obtain an amazingly high power factor of 1.2 × 10 13 cm −3 at room temperature. Meanwhile, the lattice thermal conductivity of few-layer intrinsic T -HfSe 2 is found to be lower than that of monolayer caused by van der Waals interactions. Ultimately, a maximum p-type ZT value of ~0.53 can be reached at 300 K, which is close to the ZT value of ~0.6 for the latest work GeTe-based TE materials. Moreover, we obtai n n -type ZT of ∼1.3, which is amazingly high and close to Pb-doped SnSe ~1.2. The results indicate that layered HfSe 2 is quite an excellent thermoelectric material at room temperature. In summary, this work demonstrates the tremendous advantages of few-layered HfSe 2 in thermoelectric applications.

Enhanced High-Temperature Thermoelectric Performance by Strain Engineering in BiOCl

Semiconductor BiOCl has a layered structure with ultralow lattice thermal conductivity [Q.D. Gibson et al., Science 373, 1017--1022 (2021)] and has potential applications in the field of thermoelectric materials. In the present study, the thermoelectric properties of BiOCl crystals are accurately predicted using the first-principles calculation combined with Boltzmann transport theory. The dimensionless figure of merit ($ZT$) of $p$-type BiOCl is found to be 0.2 at room temperature, reaching 1.1 at 800 K. In addition, applying in-plane biaxial tensile strain ${ϵ}_{xy}$ can lead to a further increase in the value of $ZT$ to 1.9 at 800 K. This means that $p$ BiOCl is an excellent high-temperature thermoelectric material. And this is due to the fact that biaxial strain drastically reduces the lattice thermal conductivity and the shift of the valence band toward the Fermi level can optimize the carrier concentration. Thus, the present work paves a way for the design of adjustable high-temperature thermoelectric materials.