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.

Band Structure and Phonon Transport Engineering Realizing Remarkable Improvement in Thermoelectric Performance of Cu2SnSe4 Incorporated with In2Te3.

Cu2SnSe4 (CTS) ternary chalcogenides have potential applications in thermoelectrics for they crystallize in a high-symmetry cubic structure and consist of earth-abundant and eco-friendly elements. However, the pristine CTS does not have optimal thermoelectric (TE) performance (ZT = 0.35 at ∼700 K), so further investigation is required in this regard. In this work, we propose an incorporation of In2Te3 with a defect zinc-blende cubic structure into CTS, aiming to regulate the electronic and phonon transport mechanism simultaneously. The first-principles calculation reveals that the element In favors the residing at a vacancy site as an interstitial atom while Te at the Se site, which leads to band convergence and degeneracy, respectively. As a result, the electrical property improves with a 22% increase in the power factor (PF), and at the same time, the lattice thermal conductivity (κL) reduces to 0.31 W K-1 m-1 at 718 K. Synergistic engineering realizes a remarkable improvement in TE performance with the highest figure of merit (ZT) of 0.92 at 718 K. This value is ∼3 times that of the pristine CTS and stands among the highest in the Cu2SnSe4 family so far, which proves that the incorporation of In2Te3 into CTS is a good proposal.

Improved Thermoelectric Performance of p-Type Argyrodite Cu8GeSe6 via the Simultaneous Engineering of the Electronic and Phonon Transports.

Guided by the concept of "phonon-liquid electron-crystal", many n-type argyrodite compounds have been developed as candidates for thermoelectric (TE) materials. In recent years, the p-type Cu8GeSe6 (CGS) compound has attracted some attention in TEs due to the presence of very strong atomic vibrational arharmonicity inside the sublattice, which is caused by the weak bonding between Cu ions and [GeSe6]8-. However, its TE performance is still poor, with a ZT value of only 0.2 at 623 K. Therefore, in this work, we propose to engineer both the electronic and phonon transports in CGS by incorporating the species In2Te3. This strategy tunes the carrier concentration and at the same time increases the phonon scattering on the point defects (InGe, Ininterstitial, and TeSe) and randomly distributed tetrahedra ([InSe4]5- and [GeTeSe3]4-). As a result, the phase transformation at 329 K in CGS is eliminated, and the peak ZT value is enhanced from 0.27 for CGS to ∼0.92 for (Cu8SnSe6)0.9(In2Te3)0.1 at 774 K; this thus proves that the incorporation of In2Te3 in CGS is an effective way of regulating its TE performance.

Improved Thermoelectric Performance of P-type SnTe through Synergistic Engineering of Electronic and Phonon Transports.

SnTe has been regarded as a potential alternative to PbTe in thermoelectrics because of its environmentally friendly features. However, it is a challenge to optimize its thermoelectric (TE) performance as it has an inherent high hole concentration (nH∼2 × 1020 cm-3) and low mobility (µH∼18 cm2 V-1 s-1) at room temperature (RT), arising from a high intrinsic Sn vacancy concentration and large energy separation between its light and heavy valence bands. Therefore, its TE figure of merit is only 0.38 at ∼900 K. Herein, both the electronic and phonon transports of SnTe were engineered by alloying species Ag0.5Bi0.5Se and ZnO in succession, thus increasing the Seebeck coefficient and, at the same time, reducing the thermal conductivity. As a result, the TE performance improves significantly with the peak ZT value of ∼1.2 at ∼870 K for the sample (SnGe0.03Te)0.9(Ag0.5Bi0.5Se)0.1 + 1.0 wt % ZnO. This result proves that synergistic engineering of the electronic and phonon transports in SnTe is a good approach to improve its TE performance.

Synergistic Optimization of the Electronic and Phonon Transports of N-Type Argyrodite Ag8Sn1-xGaxSe6 (x = 0-0.6) through Entropy Engineering.

The argyrodite compound, Ag8SnSe6 (ATS), which is one of the promising thermoelectric (TE) candidates, is receiving growing attention in thermoelectrics recently. However, its TE performance is still low and phases are unstable as the temperature varies. In this work, inspired by entropy engineering, we eliminate the ß/γ phase transformation at ∼355 K via alloying Ga, thus extending its high-temperature cubic phase from 320 to 730 K. In the meantime, the power factor (PF) enhances by 10% and lattice thermal conductivity (κL) reduces by 40% at 723 K. As a result, the ZT value is boosted to ∼1.15 for Ag8Sn0.5Ga0.5Se6, which stands high among the ATS systems. This proves that the entropy engineering is an effective approach to extend the high-temperature range for the cubic γ-phase and improve its TE performance simultaneously.

Remarkable Improvement of Thermoelectric Performance in Ga and Te Cointroduced Cu3SnS4.

Ternary sulfide Cu3SnS4 (CTS) receives growing interest in photocatalytic and gas sensing applications; however, limited attention has been paid to the application in thermoelectrics in virtue of its intrinsic high carrier concentration. In this work, a high figure of merit of Ga (ZT) and Te cointroduced CTS with the composition of (Cu3SnS4)1-x(Ga2Te3)x (x = 0.105) has been realized via synergistic optimization of the electronic and thermal transport properties. The incorporation of Ga into CTS results in a downshift of both the conduction and valence bands, which effectively promotes the active hybridization of Sn 5s and S 3p orbitals near the Fermi level (EF) and optimizes the carrier concentration. In the meantime, the lattice thermal conductivity (κL) generally decreases on account of the local internal distortion induced by Ga(Te) substitution at the Cu(S) site. Moreover, the phonon transport is greatly suppressed above ∼725 K attributed to the melting of the second-phase Te on the grain boundaries. Consequently, the highest ZT value of ∼0.96 is obtained at 798 K. This value is ∼3.6 times that of the pristine CTS and ranks the highest in the CTS system to date.

N-type thermoelectric Ag8SnSe6 with extremely low lattice thermal conductivity by replacing Ag with Cu.

Argyrodite family compounds inherently possess low lattice thermal conductivity (κ L) due to the liquid-like behavior of cations and the intimate interplay among mobile ions. Hence, they have become the focus of discussion in thermoelectrics recently. However, the major bottleneck for further improvement of their thermoelectric (TE) performance is their low carrier concentration. In this work, we take an advantage of the unique structure of Ag8SnSe6, in an attempt to further reduce the lattice part (κ L) while at the same time improve their electrical property. The results show that the κ L value reduces from 0.17 W K-1 m-1 to 0.12 W K-1 m-1 when Ag is substituted for Cu through induced point defects and lattice distortion and that the power factor (PF) increases from 4.1 µW cm-1 K-2 to 4.4 µW cm-1 K-2 at 645 K after enhancing the Seebeck coefficients. Finally, the maximum ZT value of ∼0.85 is attainted for Ag7.95Cu0.05SnSe6 at 645 K, an increase by a factor of 1.3 compared to that of the pristine Ag8SnSe6. This result demonstrates that the replacement of Ag by Cu in Ag8SnSe6 is an effective way to improve its TE performance.

Cation vacancy related crystal structure and bandgap and their effects on the thermoelectric performance of Cu-ternary systems Cu3+δIn5Te9 (δ = 0-0.175).

In this work the crystal structure and bandgap in the Cu3+δIn5Te9 material system were engineered through modifying the copper vacancy concentration (Vc). The results reveal that the crystal distortion parameter (ψ) increases as the Vc value decreases, which plays a fundamental role in enhancing the phonon scattering, thereby reducing the lattice part (κL) to the minimum value 0.21 W K-1 m-1 at ∼830 K. Although the electrical properties degrade due to the reduced Hall carrier concentration (nH) caused by the widened bandgap (Eg) as the Vc value increases, the mobility (µ) increases. As a consequence, the thermoelectric performance remarkably improves with a highest ZT value of ∼1.0 for the sample Cu3+δIn5Te9 (δ = 0.1). This value doubles that of the pristine Cu3In5Te9. The work gives insight into the potential phonon scattering in the distorted crystal structure in Cu-ternary systems and sheds some light on the design of high performance thermoelectric materials.

Silver vacancy concentration engineering leading to the ultralow lattice thermal conductivity and improved thermoelectric performance of Ag1-xInTe2.

AgInTe2 compound has not received enough recognition in thermoelectrics, possibly due to the fact that the presence of Te vacancy (VTe) and antisite defect of In at Ag site (InAg) degrades its electrical conductivity. In this work, we prepared the Ag1-xInTe2 compounds with substoichiometric amounts of Ag and observed an ultralow lattice thermal conductivity (κL = 0.1 Wm-1K-1) for the sample at x = 0.15 and 814 K. This leads to more than 2-fold enhancement in the ZT value (ZT = 0.62) compared to the pristine AgInTe2. In addition, we have traced the origin of the untralow κL using the Callaway model. The results attained in this work suggest that the engineering of the silver vacancy (VAg) concentration is still an effective way to manipulate the thermoelectric performance of AgInTe2, realized by the increased point defects and modified crystal structure distortion as the VAg concentration increases.

Manipulating Localized Vibrations of Interstitial Te for Ultra-High Thermoelectric Efficiency in p-Type Cu-In-Te Systems.

Thermoelectric materials are of imperative need on account of the worldwide energy crisis. However, their efficiency is limited by the interplay of high electrical and lower thermal conductivities, that is, the figure of merit (ZT). Owing to their unique crystal structures, Cu-In-Te-based chalcogenides are suitable for both and thus have attracted much attention recently as potential thermoelectrics. Here we explore a newly developed Cu-In-Te derivative compound Cu3.52In4.16Te8. With a proper adjustment of Cu2Te doping, this material shows an ultralow lattice thermal conductivity (κL) (0.3 WK-1m-1) and, consequently, a figure of merit (ZT) as high as 1.65(±0.15) at 815 K: the highest value reported for p-type Cu-In-Te to date. The reduction in κL is directly related to the alteration of local symmetry around the interstitial Te, resulting in an effectively optimized phonon transport through localized "rattling" of the same. Although the Hall carrier concentration reduces upon Cu2Te addition due to the unpinning of the Fermi level (EFermi) toward the conduction band minimum, the power factor remains stable. The knowledge depicted here not only demonstrates the potential of Cu3.52In4.16Te8-based alloys as a promising TE, but also provides guidelines for developing further high-performance thermoelectric materials by enhancing the electronic conductivity.

Co-regulation of the copper vacancy concentration and point defects leading to the enhanced thermoelectric performance of Cu3In5Te9-based chalcogenides.

Copper vacancy concentration (V c) in ternary Cu-In-Te chalcogenides is an important factor to engineer carrier concentration (n H) and thermoelectric performance. However, it is not sufficient to regulate the phonon scattering in the Cu3In5Te9-based chalcogenides. In this work we manipulate the V c value and point defects simultaneously through addition of Cu along with Ga substitution for In in Cu3In5Te9, and thereby increase the carrier concentration and reduce the lattice thermal conductivity. This strategy finally enables us to achieve ∼60% enhancement of the TE figure of merit (ZT) at V c = 0.078 compared with the pristine Cu3In5Te9. It is also used as guidance to achieve the high TE performance of the ternary chalcogenides.

Improved thermoelectric performance of solid solution Cu4Sn7.5S16 through isoelectronic substitution of Se for S.

Cu-Sn-S family of compounds have been considered as very competitive thermoelectric candidates in recent years due to their abundance and eco-friendliness. The first-principles calculation reveals that the density of states (DOS) increases in the vicinity of the Fermi level (Ef) upon an incorporation of Se in the Cu4Sn7.5S16-xSe x (x = 0-2.0) system, which indicates the occurrence of resonant states. Besides, the formation of Cu(Sn)-Se network upon the occupation of Se in S site reduces the Debye temperature from 395 K for Cu4Sn7S16 (x = 0) to 180.8 K for Cu4Sn7.5S16-xSe x (x = 1.0). Although the point defects mainly impact the phonon scattering, an electron-phonon interaction also bears significance in the increase in phonon scattering and the further reducion of lattice thermal conductivity at high temperatures. As a consequence, the resultant TE figure of merit (ZT) reaches 0.5 at 873 K, which is 25% higher compared to 0.4 for Cu4Sn7.5S16.

Increased effective mass and carrier concentration responsible for the improved thermoelectric performance of the nominal compound Cu2Ga4Te7 with Sb substitution for Cu.

Although the ternary chalcopyrite compound Cu2Ga4Te7 has relatively high thermal conductivity and electrical resistivity, it has a high carrier concentration, thus making it a good thermoelectric candidate. In this work we substitute Sb for Cu in this compound, aiming at engineering both the electrical and thermal properties. Rietveld refinement revealed that the nominal compounds Cu2-x Sb x Ga4Te7 (x = 0-0.1) crystallize with the crystal structure of CuGaTe2 with the real compositions deviating from those of their nominal ones. Besides, Sb resides in Cu sites, which increases both the effective mass and the Hall carrier concentration. Therefore, the Seebeck coefficient increases at high temperatures, and the lattice thermal conductivity reduces due to increased phonon scattering from point defects and electron-phonon interactions. As a consequence, the thermoelectric (TE) performance improves with the highest TE figure of merit (ZT) of 0.58 at 803 K. This value is about 0.21 higher than that of the pristine Cu2Ga4Te7.

Improvement of thermoelectric performance of copper-deficient compounds Cu2.5+δ In4.5Te8 (δ = 0-0.15) due to a degenerate impurity band and ultralow lattice thermal conductivity.

Cu-In-Te ternary chalcogenides have unique crystal and band structures; hence they have received much attention in thermoelectrics. In this work we have observed an enhancement in Hall carrier concentration (n H) and ultralow lattice thermal conductivity (κ L) when Cu was added to ternary Cu2.5+δ In4.5Te8 (δ = 0-0.15) compounds. The enhancement in n H is attributed to a degenerate impurity band at the G point in the valence band maximum (VBM), while the extremely low κ L results from the increased lattice disorder. We thus obtained the minimum κ L value of only 0.23 W K-1 m-1 in the sample at δ = 0.1 and 820 K, which is in good agreement with the calculation using the Callaway model. The highest thermoelectric figure of merit ZT is 0.84 for the material at δ = 0.1, which is about 0.38 higher than that of the pristine Cu2.5In4.5Te8.

Unequal bonding in Ag-CuIn3Se5-based solid solutions responsible for reduction in lattice thermal conductivity and improvement in thermoelectric performance.

Owing to their unique crystal and band structures, in thermoelectrics increasing attention has recently been paid to compounds of the ternary I-III-VI chalcopyrite family. In this work, unequal bonding between cation and anion pairs in Cu1-y Ag y In3Se4.9Te0.1 solid solutions, which can be effectively used to disturb phonon transport, has been proposed. The unequal bonding, which is represented by the difference of bond lengths Δd, Δd = d (Cu-Se) - d (In-Se) and anion position displacement from its equilibrium position Δu = u - 0.25, is created by the isoelectronic substitution of Ag for Cu. At y = 0.2 both the Δd and Δu values reach their maxima, resulting in a remarkable reduction in lattice thermal conductivity (κ L) and an improvement in TE performance. However, as the y value increase to 0.3 both Δd and Δu values decrease, causing the κ L value to increase and the ZT value to decrease from 0.5 to 0.24 at 930 K. Accordingly, unequal bonding might be an alternative way to improve the TE performance of ternary chalcopyrites.

Enhanced thermoelectric performance of a chalcopyrite compound CuIn3Se5-xTex (x = 0~0.5) through crystal structure engineering.

In this work the chalcopyrite CuIn3Se5-xTex (x = 0~0.5) with space group through isoelectronic substitution of Te for Se have been prepared, and the crystal structure dilation has been observed with increasing Te content. This substitution allows the anion position displacement ∆u = 0.25-u to be zero at x ≈ 0.15. However, the material at x = 0.1 (∆u = 0.15 × 10-3), which is the critical Te content, presents the best thermoelectric (TE) performance with dimensionless figure of merit ZT = 0.4 at 930 K. As x value increases from 0.1, the quality factor B, which informs about how large a ZT can be expected for any given material, decreases, and the TE performance degrades gradually due to the reduction in nH and enhancement in κL. Combining with the ZTs from several chalcopyrite compounds, it is believable that the best thermoelectric performance can be achieved at a certain ∆u value (∆u ≠ 0) for a specific space group if their crystal structures can be engineered.

Engineering Band Structure via the Site Preference of Pb(2+) in the In(+) Site for Enhanced Thermoelectric Performance of In6Se7.

Although binary In-Se based alloys have in recent years gained interest as thermoelectric (TE) candidates, little attention has been paid to In6Se7-based compounds. Substituting Pb in In6Se7, preference for Pb(2+) in the In(+) site has been observed, allowing Fermi level (Fr) shift toward the conduction band, where the localized state conduction becomes dominant. Consequently, the Hall carrier concentration (nH) has been significantly enhanced with the highest nH value being about 2-3 orders of magnitude higher than that of the Pb-free sample. Meanwhile, the lattice thermal conductivity (κL) tends to be reduced as the nH value increases, owing to an increased phonon scattering on carriers. As a result, a significantly enhanced TE performance has been achieved with the highest TE figure of merit (ZT) of 0.4 at ∼850 K. This ZT value is 27 times that of intrinsic In6Se7 (ZT = 0.015 at 640 K), which proves a successful band structure engineering through site preference of Pb in In6Se7.

Lattice defects and thermoelectric properties: the case of p-type CuInTe2 chalcopyrite on introduction of zinc.

I-III-VI2 chalcopyrites have unique inherent crystal structure defects, and hence are potential candidates for thermoelectric materials. Here, we identified mixed polyanionic/polycationic site defects (ZnIn(-), VCu(-), InCu(2+) and/or ZnCu(+)) upon Zn substitution for either Cu or In or both in CuInTe2, with the ZnIn(-) species originating from the preference of Zn for the cation 4b site. Because of the mutual reactions among these charged defects, Zn substitution in CuInTe2 alters the basic conducting mechanism, and simultaneously changes the lattice structure. The alteration of the lattice structure can be embodied in an increased anion position displacement (u) or a reduced bond length difference (Δd) between d(Cu-Te)4a and d(In-Te)4b with increasing Zn content. Because of this, the lattice distortion is diminished and the lattice thermal conductivity (κL) is enhanced. The material with simultaneous Zn substitution for both Cu and In had a low κL, thereby we attained the highest ZT value of 0.69 at 737 K, which is 1.65 times that of Zn-free CuInTe2.