Large ZT Research Articles

Interplay between metavalent bonds and dopant orbitals enables the design of SnTe thermoelectrics

Engineering the electronic band structures upon doping is crucial to improve the thermoelectric performance of materials. Understanding how dopants influence the electronic states near the Fermi level is thus a prerequisite to precisely tune band structures. Here, we demonstrate that the Sn-s states in SnTe contribute to the density of states at the top of the valence band. This is a consequence of the half-filled p-p σ-bond (metavalent bonding) and its resulting symmetry of the orbital phases at the valence band maximum (L point of the Brillouin zone). This insight provides a recipe for identifying superior dopants. The overlap between the dopant s- and the Te p-state is maximized, if the spatial overlap of both orbitals is maximized and their energetic difference is minimized. This simple design rule has enabled us to screen out Al as a very efficient dopant to enhance the local density of states for SnTe. In conjunction with doping Sb to tune the carrier concentration and alloying with AgBiTe2 to promote band convergence, as well as introducing dislocations to impede phonon propagation, a record-high average ZT of 1.15 between 300 and 873 K and a large ZT of 0.36 at 300 K is achieved in Sn0.8Al0.08Sb0.15Te-4%AgBiTe2.

The thermoelectric properties of the two-dimensional Nowotny-Juza material NaBeX (X = P, As, and Sb)

The two-dimensional β-type Nowotny-Juza phase-filled tetrahedral compound has excellent electrical conductivity, low thermal conductivity and outstanding thermoelectric properties. This study investigated the stability and electronic properties of NaBeX (X = P, As, and Sb) through first-principles calculations. The results demonstrate that these materials are structurally stable and are direct bandgap semiconductors with bandgaps of 0.6542 eV, 0.6549 eV, and 0.6686 eV, respectively. The electrical and thermal transport properties of NaBeX are investigated by combining the deformation potential theory and Boltzmann transport theory. Subsequently, the thermoelectric properties of NaBeX are evaluated at temperatures ranging from 400 to 800 K. The n-type NaBeX materials exhibit large ZT values of 2.48, 2.62, and 1.72. The results of the present study indicate that n-type NaBeX has the potential to be used as a thermoelectric candidate material at temperatures ranging from 400 to 800 K.

Excellent thermoelectric properties and significant anisotropy for filled-tetragonal NaZnAs caused by strong anharmonicity

We have theoretically investigated the transport properties of the filled-tetrahedral NaZnAs using first-principles calculations in combination with self-consistent phonon theory and the Boltzmann transport equation. The results show that the transport behavior of NaZnAs exhibits obvious anisotropy, and the influence of quartic anharmonicity on the lattice thermal conductivity cannot be neglected as shown by the thermal conductivity results obtained from different computational models. Both the increase in scattering rates(SRs) and the decrease in group velocity signify a strong anharmonicity. We further find that the special vibrational modes due to the weak bonding of Zn atoms are the main source of anharmonicity. We find that considering the bubble diagram for the phonon self-energy correction reduces the κL by up to 28.04% at 700 K, which favors the capture of large ZT values. In addition, The ZT value of NaZnAs can reach 3.07 at 700 K, which shows great potential for thermoelectric applications.

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.

First-Principles Analysis on the Optoelectronic, Structural, Elastic and Transport Characteristics of Novel Fluoroperovskites Cs2TlAgF6 for Green Technology

Cs-based perovskites hold immense significance in the field of green technology due to their unique properties, offering promising avenues for efficient, low-cost devices. In this theoretical work, DFT has been employed to extensively scrutinize the physical properties of double fluoroperovskites Cs2TlAgF6. The modified Becke Johnson functional was used to take exchange-correlation effects into consideration accurately. From the calculated value of formation energy, volume optimization curve, Goldsmith tolerance factor and octahedral tilting, the structural stability is demonstrated. The band structure of Cs2TlAgF6 depicts a direct bandgap of 2.21 eV, proving its semiconducting nature. This study also assessed the mechanical properties in detail, showing the ductile character of Cs2TlAgF6. A thorough examination of optical characteristics reveals the potential application in a variety of photovoltaic devices due to its strong absorption in visible region. The transport attributes are accessed through large ZT value and other thermal parameters. With its exceptional heat-to-electricity conversion properties, this material shows promise for applications in thermoelectric devices, offering a sustainable way to generate electricity from waste heat. The larger value 0.788 of ZT depicts that material exhibit sufficient potential for generating energy from waste heat.

Thermoelectric properties of the bismuth oxychalcogenides Bi2SO2, Bi2SeO2 and Bi2TeO2

We present a detailed theoretical study of the thermoelectric properties of the bismuth oxychalcogenides Bi2ChO2 (Ch = S, Se, Te). The electrical transport is modelled using semi-classical Boltzmann transport theory with electronic structures from hybrid density-functional theory, including an approximate model for the electron lifetimes. The lattice thermal conductivity is calculated using first-principles phonon calculations with an explicit treatment of anharmonicity, yielding microscopic insight into how partial replacement of the chalcogen in the bismuth chalcogenides impacts the phonon transport. We find very good agreement between the predicted transport properties and a favourable cancellation of errors that allows for near-quantitative predictions of the thermoelectric figure of merit ZT. Our calculations suggest recent experiments on n-doped Bi2SeO2 have achieved close to the largest ZT possible in bulk materials, whereas the largest reported ZT for Bi2TeO2 could be improved sixfold by optimising the carrier concentration. We also predict that much larger ZT > 2.5, competitive with the benchmark thermoelectric SnSe, could be obtained for Bi2SO2 and Bi2SeO2 with heavy p-type doping. This study demonstrates the predictive power of this modelling approach for studying thermoelectrics and highlights several avenues for improving the performance of the Bi2ChO2.

Tuning the thermoelectric and optoelectronic attributes of lead-free novel fluoroperovskites Cs2BB'F6 (B = Rb, In, Na and B'=Ir, As, Rh): A first-principles investigation

Cesium-based double perovskite materials have generated significant interest because of their fascinating prospects in various optoelectronic and thermoelectric uses. In this manuscript, the physical characteristics of Cs2BB'F6 (B = Rb, In, Na and B' = Ir, As, Rh) are investigated in detail. Volume optimization curves, negative formation energies, tolerance factor and octahedral tilting are used to evaluate the structural stability. Pugh's ratio, Cauchy pressure and Poisson's ratio endorses the ductile character of all studied perovskites. Debye temperature shows that Cs2NaRhF6 is a stiffer material in comparison to Cs2InAsF6 and Cs2RbIrF6. The electronic bandgap and total density of states of Cs2InAsF6, Cs2RbIrF6, and Cs2NaRhF6 reveal a direct band gap of 2.76 eV, 3.78 eV and 3.6 eV, respectively. Furthermore, Kramer-Kronig equations are used to examine the interaction of studied double perovskites with electromagnetic radiation. The optical parameters reveal the absorption in UV region, which show their potential for laser and UV-shields applications. In Addition, thermoelectric characteristics are calculated using the semi-classical Boltzmann theory, extending the investigation up to 800 K. The large ZT value of 0.92 is achieved for Cs2RbIrF6 at 800K, while Cs2InAsF6 and Cs2NaRhF6 exhibit 0.76 and 0.82 at 800K, respectively. Their high ZT values and electrical conductivity enhance their suitability for use in renewable energy devices.

A DFT approach to correlate the physical characteristics of novel chalcopyrites ASbN2(A = Li, Na) for green technology.

Semiconductor chalcopyrite compounds have been a subject of research interest due to their diverse range of physical properties that have captured the attention of scientists. In this ongoing research, we have examined the physical characteristics of LiSbN2 and NaSbN2 chalcopyrites using DFT. The modified Becke-Johnson (mBJ) potential is utilized for the computation of electronic structures. The stability is attained with negative formation energies and optimization curves. A bandgap of 2.60 eV in LiSbN2 and 3.15 eV in NaSbN2 has been achieved, which is further endorsed by the density of states. An in-depth analysis of the optical properties unveils the potential utility of LiSbN2 and NaSbN2 in various photovoltaic devices, attributed to its pronounced absorption in the UV spectrum. The transport characteristics are also assessed through various transport characteristics. The large electrical conductivity and ZT values for both chalcopyrite compounds are attained. Due to their remarkable capability to convert heat into electricity, these materials display potential for use in thermoelectric devices.

A computational insight into the Zintl SZr2N2 and BaAg2S2 phases for optoelectronic thermoelectric applications

The physical characteristics of SZr2N2 and BaAg2S2 Zintl phases have been explored by utilizing the full-potential augmented plane wave (FP-LAPW) method with the combination of modified Becke-Johnson (mBJ) approximation. The negative formation energies and phonon modes with positive frequencies endorsed the stability of both compounds. The electronic band profile reveals a bandgap of 1.18 eV for SZr2N2 and 2.15 eV for BaAg2S2. The optical characteristics are also calculated. The absorption of light for the studied compounds falls in the ultra-violet region, which shows their possibility to be employed as UV-rays absorbers. A large optical conductivity and low energy loss is also seen for both materials on UV region. The efficiency of temperature-dependent thermoelectric properties increases with the temperature rise, as calculated with the BoltzTraP code. The high electrical with low thermal conductivities and maximum Seebeck coefficient is witnessed for both materials. The large ZT values of 0.75 and 0.80 are achieved for SZr2N2 and BaAg2S2, respectively. All calculated transport parameters indicate the opportunity of using the studied compounds for thermoelectric applications.

The narrow band bap double perovskites X2CuInCl6 (X = K, Rb, Cs) for optoelectronics, and thermoelectric applications

The lead free, non-toxic, dynamically stable behavior of double perovskites make them promising materials for thermoelectric and optoelectronic applications. Therefore, the mechanical, optoelectronic, and transport characteristics of X2CuInCl6 (X = K, Rb, Cs) are addressed comprehensively by density functional theory. The phonon dispersion band structures are computed for dynamic stability, and tolerance factor for structural stability. The band structures show narrow band gaps (0.25, 0.42, 0.56) eV which are significantly important for infrared detectors and other optoelectronic devices operating with visible and IR light. The absorption peaks of maximum intensity are noted at 1.5 eV (827 nm), 2.0 eV (620 nm), and 2.5 eV (496 nm) for K, Rb, and Cs based DPs. Furthermore, thermoelectric behavior has been analyzed by the figure of merit, power factor, Seebeck coefficient, electrical to thermal conductivity ratio against chemical potential (−0.1 eV to 0.1 eV) and temperature (100–500 K). The ultralow values of lattice vibration, and large ZT increase their importance for thermoelectric generators.

Understanding the electronic, structural, optical, photovoltaic and thermoelectric properties of Cs2GeSnBr6 by first-principles and SCAPS-1D simulation

The electronic, optical, structural, thermoelectric and photovoltaic properties of Cs2GeSnBr6 are investigated by using density functional theory and SCAPS-1D simulation. There is a lack of imaginary frequency in the phonon dispersion band structure implies that Cs2GeSnBr6 is stable against lattice distortion. Also, Goldsmith tolerance factor (τ), octahedral factor (µ), and convex-hull calculations demonstrated that Cs2GeSnBr6 can be synthesized. In addition, electronic band structure calculation using GGA-PBE+mBJ+SOC shows that Cs2GeSnBr6 has a direct band gap value of 1.01 eV. Further, optical calculations revealed the higher value of absorption coefficient (α) and optical conductivity (σ) of Cs2GeSnBr6. Motivated by this, we have modelled the ITO/ZnO/Cs2GeSnBr6/Au photovoltaic cell which shows a maximum power conversion efficiency of 18.74%. Moreover, the obtained large ZT value Cs2GeSnBr6 which is originating from its good electrical conductivity with poor thermal conductivity.

Fine Tuning of Defects Enables High Carrier Mobility and Enhanced Thermoelectric Performance of n-Type PbTe.

High carrier mobility is critical to improving thermoelectric performance over a broad temperature range. However, traditional doping inevitably deteriorates carrier mobility. Herein, we develop a strategy for fine tuning of defects to improve carrier mobility. To begin, n-type PbTe is created by compensating for the intrinsic Pb vacancy in bare PbTe. Excess Pb2+ reduces vacancy scattering, resulting in a high carrier mobility of ∼3400 cm2 V-1 s-1. Then, excess Ag is introduced to compensate for the remaining intrinsic Pb vacancies. We find that excess Ag exhibits a dynamic doping process with increasing temperatures, increasing both the carrier concentration and carrier mobility throughout a wide temperature range; specifically, an ultrahigh carrier mobility ∼7300 cm2 V-1 s-1 is obtained for Pb1.01Te + 0.002Ag at 300 K. Moreover, the dynamic doping-induced high carrier concentration suppresses the bipolar thermal conductivity at high temperatures. The final step is using iodine to optimize the carrier concentration to ∼1019 cm-3. Ultimately, a maximum ZT value of ∼1.5 and a large average ZT ave value of ∼1.0 at 300-773 K are obtained for Pb1.01Te0.998I0.002 + 0.002Ag. These findings demonstrate that fine tuning of defects with <0.5% impurities can remarkably enhance carrier mobility and improve thermoelectric performance.

Synergistic effect of alloying on thermoelectric properties of two-dimensional PdPQ (Q = S, Se).

Hosts of 2D materials exist, yet few allow compositional and structural tailoring as the MQ2 (M = Mo, W; Q = S, Se) family does, for which various structural superlattices have been synthesized. Using thorough first-principles calculations, we show how bonding hierarchy contributes to the structural resilience of 2D PdPQ and allows for full-range alloying of sulfur and selenium. Within the structural unit of Pd2P2Q2, the covalently-bonded [P2Q2]4- polyanions hold the structure together with their molecular-like P-P bonds while ionically bonded Pd-Qs allow the S/Se substitution. Here, the bonding hierarchy imparts superior electronic and structural features to the PdPQ monolayers. As such, the flat-and-dispersive valence band and the eight degenerate valleys of the conduction band benefit the p-type and n-type thermoelectricity of pristine PdPQ, which can be further enhanced by alloying. The high-entropy alloying synergistically suppresses the lattice heat transport from 75 to 30 W m-1 K-1 and increases the band degeneracy of PdPQ monolayers, resulting in an overall improvement in zT. Combining these features, in a naïve approach, results in a large zT approaching two for both p-type and n-type doping. However, accurate fully-fledged electron-phonon calculations rebut this promise, showing that at high temperatures, the increased electron scattering results in a stagnant power factor in the flat-and-dispersive valence band. Using a realistic first-principles scattering, we finally calculate the thermoelectric efficiency of PdPQ (Q = S, Se) and highlight the importance of an accurate estimation of electron relaxation time for thermoelectric predictions.

Tuning of band gap by variation of halide ions in K2CuSbX6 (X = Cl, Br, I) for solar cells and thermoelectric applications

The double perovskites have obtained remarkable attention in the field of solar cells and renewable energy applications. Here in present article, the optical and transport properties of K2CuSbX6 (X = Cl, Br, I) are illustrated by DFT based simulation WIEN2K codes. The formation energy has been calculated for thermodynamic stability. The indirect band gaps (1.12, 0.80, and 0.43) for (Cl, Br, I) studied DPs halides are reported from band structures which instigate diverse optoelectronic and transport applications. The width of absorption band increases from Cl to I and shifted towards to lower frequency region. Furthermore, thermoelectric parameters are evaluated in the temperature range 200–600K and chemical potential ±0.15 eV. The ultralow values of lattice conductivity and large ZT at 300K increases their significant importance for thermoelectric generators and renewable energy devices.

Thermoelectric transport in two-terminal topological nodal-line semimetals nanowires

Recently discovered topological nodal-line semimetals (TNLSMs) have received considerable research interest due to their rich physical properties and potential applications. TNLSMs have the particular band structure to lead to many novel properties. Here we theoretically study the thermoelectric transport of a two-terminal pristine TNLSM nanowires and TNLSMs p–n–p junctions. The Seebeck coefficients S c and the thermoelectrical figure of merit ZT are calculated based on the Landauer-Büttiker formula combined with the nonequilibrium Green’s function method. In pristine TNLSM nanowires, we discuss the effect of the magnetic fields φ, the disorder D, the on-site energy µ z , and the mass term m on the thermoelectric coefficient and find that the transport gap can lead to a large S c and ZT. When transmission coefficient jumps from one integer plateau to another, S c and ZT show a series of peaks. The peaks of S c and ZT are determined by the jump of the transmission coefficient plateau and are not associated with the plateau itself. For TNLSMs p–n–p junctions, S c and ZT strongly depend on the parameter ξ of potential well. We can get a large ZT by adjusting the parameter ξ and magnetic field φ. In TNLSMs p–n–p junctions, ZT has the large value and is easily regulated. This setup has promising application prospects as a thermoelectric device.

First principles study of the structural stability, lattice dynamics, optical and thermoelectric properties of NaSrX(X = As, Sb and Bi)

First principles methods are utilized in exploring the structural stability, electronic, optical, phonon, thermal properties and electronic transport coefficients of NaSrX materials. Based on the calculated cohesive and formation energies, NaSrX compounds are found to be energetically stable. NaSrX materials are also shown to be direct band gap semiconductors with strong exciton binding energies between ∼0.13 and 0.20 eV. The dynamical and mechanical stabilities of NaSrX materials are further confirmed from their phonon dispersion curves and elastic constants. The calculated lattice thermal conductivities, κL, for NaSrX compounds exhibit anisotropic behaviour with the lowest value of κL found in the a-direction. Moreover, NaSrBi is shown to possess the lowest average value of κL (1.29 Wm−1K−1) as compared to NaSrAs and NaSrSb at 300 K. p-doped NaSrX compounds are shown to record large values of thermoelectric figure of merits. For instance, the highest calculated figure of merit (ZT) for p-doped NaSrAs is estimated to be 1.66 at 1200 K when p = 1020 cm−3 whereas those of NaSrSb and NaSrBi are 2.78 (at 1000 K) and 3.16 (at 900 K) respectively when p = 1019 cm−3. The large ZT values obtained in these calculations indicate that NaSrX compounds are promising candidates for thermoelectric applications.

Enhanced thermoelectric performance of n-type (PbSe)n(Sb2Te3) pseudo-binary via Zn filling and Ag2Se compositing

Lead selenide (PbSe) is considered to be decent candidate replacing lead telluride (PbTe) as intermediate temperature (600–900 K) thermoelectric application due to its lower intrinsic lattice thermal conductivity, higher thermal stability, and larger abundance of Se than Te. To improve the thermoelectric performance of n-type PbSe, we constructed an n-type PbSe-Sb2Se3 pseudo-binary via alloying Sb2Se3 into PbSe matrix to obtain a reduced lattice thermal conductivity at a broad temperature range. Next, we introduced extra Zn into matrix to optimize the electron concentration. Last, we further added excess Ag2Se to modulate the lattice thermal conductivity at temperature< 523 K. Eventually, we obtained an ultralow lattice thermal conductivity of ~ 0.40 W m−1 K−1 and a large peak ZT of ~ 1.36 at 773 K in the composition of (PbSe)40(Sb2Se3)Zn0.1(Ag2Se)0.75; meanwhile, the average ZT between 323 and 773 K is as high as ~ 0.78, revealing a significant improvement over pristine PbSe.

CrTe as a versatile thermoelectromagnetic multi-functional material

Materials with large magnetocaloric effects and high thermoelectric performance are of fundamental interest to all-solid-state refrigeration technology. Herein, we report CrTe-based ferromagnetic semi-metals as an exceptional example showing both desirable magnetic entropy change (−ΔSM) and promising thermoelectric figure of merit (ZT) near room temperature. A proper amount (9 mol. %) of Cr deficiency is found to stabilize the hexagonal structure of CrTe. Cr0.91Te displays a maximum −ΔSM value of ∼2.4 J/Kg·K under an applied magnetic field of 5 T around its Curie temperature (TC) of 332 K and a strikingly large ZT value of 1.2 × 10−2 at 300 K (two to four orders of magnitude larger than any other ferromagnetic compounds reported so far). The exceptional thermoelectric performance in such a semi-metal is believed to originate from the inherently strong electron–phonon–spin interactions: magnon-drag effect mediated Seebeck coefficient enhancement and phonon–spin coupling induced thermal conductivity reduction. The introduction of antiferromagnetic CrSb not only systematically decreases TC of Cr0.91Te down to or even below room temperature but also boosts its thermoelectromagnetic properties. Specifically, Cr0.91Te0.9Sb0.1 obtains a maximum −ΔSM value of ∼2.6 J/Kg·K @ 5 T around TC = 315 K and a ZT value of 2.0 × 10−2 at 300 K, which represent 8% and 75% improvement over pristine Cr0.91Te, respectively. This research highlights the possibility of exploring ferromagnetic semi-metals as advanced thermoelectromagnetic multi-functional materials.

Realizing high thermoelectric performance in GeTe by defect engineering on cation sites

A large ZT of 1.89 is obtained in GeTe with Ti/Bi co-doping and Ge self-doping on cation sites. Doping of 1.5 mol% Ti can enhance the effective mass and quality factor B. The Ge-excess method has a better modulation effect than the Ge-deficient method.

Ultralow lattice thermal conductivity and high thermoelectric performance of penta-Sb2C monolayer: A first principles study

In this study, by utilizing the first-principles calculation coupled with the Boltzmann transport theory, we comprehensively study the thermoelectric (TE) properties of the Sb2C monolayer. The calculated results show that the Sb2C monolayer owns an inherent ultra-low lattice thermal conductivity of 0.88 W m−1 K−1 at 300 K, which originates from small phonon group velocities, large Grüneisen parameters, and short phonon lifetimes. The Sb2C monolayer also exhibits excellent electrical transport properties mainly due to the degeneration of the bottom conduction bands, which increases the Seebeck coefficient of the n-type doped samples and thus yields a larger power factor. Based on the extremely low lattice thermal conductivity and superior electrical transport performance, a large ZT value of 2.71 for the n-type doped Sb2C monolayer at 700 K is obtained. Our results quantify Sb2C monolayers as promising candidates for building outstanding thermoelectric devices.