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Advances in Environment-Friendly SnTe Thermoelectrics Wen Li, Yixuan Wu, Siqi Lin, Zhiwei Chen, Juan Li, Xinyue Zhang, Linglang Zheng, and Yanzhong Pei ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00658 • Publication Date (Web): 06 Sep 2017 Downloaded from http://pubs.acs.org on September 8, 2017
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Advances in Environment-Friendly SnTe Thermoelectrics Wen Li, Yixuan Wu, Siqi Lin, Zhiwei Chen, Juan Li, Xinyue Zhang, Linglang Zheng, Yanzhong Pei* Interdisciplinary Materials Research Center, School of Materials Science and Engineering, Tongji Univ., 4800 Caoan Rd., Shanghai 201804, China
ABSTRACT PbTe, has been leading the advancements in the field of thermoelectricity, due 1.8 Band convergence to its capability for demonstrating and integrating various new concepts. However, Resonant states Interstitial defects 1.5 toxicity of Pb is always concerned for terrestrial applications, which inspires a Nanostructures Convergence+Interstitial great advancement to be achieved very recently in its alternative analogue SnTe. Convergence+nano 1.2 Convergence+Resonant+Nano Challenges making SnTe in p-type as thermoelectrically efficient as PbTe, rely on a reduction of its carrier concentration, valence band offset and lattice thermal 0.9 conductivity. Utilization of newly developed concepts including both band and 0.6 defect engineering, amazingly increases the thermoelectric figure of merit, zT, from 0.4 up to 1.6 while remaining a nontoxic composition. The corresponding 0.3 SnTe conceptual route diagram is surveyed and future considerations on composition, 0.0 crystal structure and microstructure for further advancements are discussed in this 300 400 500 600 700 800 900 review. Concepts discussed here not only have promoted SnTe as a highly efficient T (K) environment-friendly thermoelectric material but also guided the advancements in many other thermoelectrics. Developing a sustainable and environmental-friendly always be a concern for terrestrial applications in a large-scale, energy source is a crucial themes in this century. Without unfortunately. As an environment-friendly alternative for PbTe, SnTe has hazardous emissions or moving parts, thermoelectric, enabling recently attracted increasing attentions due to the same crystal direct conversion between heat and electricity, attracts structure and similar band structure as compared to those of extensive attentions as a clean energy technology for solving PbTe (Figure 1). The biggest differences between the environmental issue. Large-scale applications need highly thermoelectric SnTe and PbTe rely on: (a) SnTe inherently has efficient thermoelectric materials, which is characterized by the a carrier concentration much higher than that needed due to dimensionless figure of merit zT=σS2T/(κE+κL), where σ, S, T, κE and κL are the electrical conductivity, the Seebeck existence of Sn vacancies7; (b) the energy offset (∆EL-Σ∼0.3eV8) coefficient, the absolute temperature, the electronic and lattice between L and Σ valence bands in SnTe is too large to ensure a components of the thermal conductivity, respectively. sufficient contribution to thermoelectric transport from the Early strategies for enhancing zT mainly focus on low-lying but high valley-degeneracy band (Σ band with a Nv of 12); (c) SnTe has a much higher lattice thermal conductivity minimizing κL, the only one independent parameter determining zT. Introduction of point defects, dislocations and (κL∼3 W/m-K) than that of PbTe (∼2 W/m-K) at room nanostructures for phonon scattering, has been widely utilized temperature, presumably resulting from its lighter constituent to effectively reduce κL and thus to enhance zT. However, κL in atomic mass. These inferiorities in SnTe limit the many thermoelectric materials has been approaching the thermoelectric figure of merit (zT) in pristine SnTe to be only amorphous limit1, motivating the development of electronic 0.4. Greatly inspired by the successful strategies for strategies for further zT enhancements. zT-enhancements realized in PbTe, existing efforts, focusing on Strong coupling effect among S, σ and κe, had led early carrier concentration optimization7, 9, valence band offset10-19 strategy for electronic performance enhancement to rely on and lattice thermal conductivity reductions6, 20-23 have recently carrier concentration (nH) optimization through chemical led to great enhancements in zT of SnTe as well. doping for nearly the past half a century. Until very recently, PbTe SnTe novel electronic strategies have been developed through engineering the band structure, which are typified by band convergence2/nestification3 and resonant states4. These approaches successfully decouple the strong correlation at Eg=0.18 eV Eg=0.3 eV some degree, enabling effective improvements in electronic ∆EL-Σ=0.3 eV ∆EL-Σ=0.05∼ ∼0.1 eV performance and thus in zT. L L Σ Σ Band Utilization of the strategies mentioned above, significantly Band Band Band enhanced zT has been frequently realized in historical PbTe thermoelectrics. This is largely enabled due to the existence of Figure 1. A schematic diagram of band structures for PbTe and L and Σ valence bands with a small energy offset, SnTe at room temperature. ∆EL-Σ~0.15eV (Figure 1). Both L and Σ valence bands have a In this review, the above mentioned successful strategies high band degeneracy (Nv, 4 for L and 12 for Σ), which can be through band and defect engineering, for enhancing zT of SnTe, engineered to align/converge within a few kBT for a high are summarized. In brief, the carrier concentration can be effective Nv of 12∼16. This leads to a significant enhancement reduced by excess of Sn for cation vacancy compensation and in zT2. Alternatively, the introduction of resonant states has also chemical doping by donors such as halogens and trivalent found to increase the electronic performance and therefore zT4. impurities; the resonant states can be introduced by doping In addition to band engineering, minimizing κL through with group IIIA elements while the L-Σ valence band offset can nanostructures1, dislocations5 and alloying defects6 for an be effective reduced by substituting Sn with divalent elements effective phonon scattering, leads to a further enhancement in such as alkaline-earths and transition metals; the reduction in zT of 2 or higher in p-PbTe5. However, the toxicity of Pb can the lattice thermal conductivity can be achieved by additional zT
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Since both power factor (PF=S2σ) and zT can only be maximized in a narrow energy range of Fermi level (a small range of carrier concentration), optimization of carrier concentration for realizing the maximal zT of SnTe is usually achieved by self- and impurity-doping. In more details and due to the existence of high centration cation vacancies, the Hall carrier concentration (nH) in stoichiometric SnTe is about 1020 cm-3 or higher, which is significantly higher than that is needed (1018~1019 cm-3, depending on temperature) for maximizing zT (Figure 2). Doping with Sb/Bi on Sn site or I on the Te site enables a reduction in nH7, 9. Alternatively, excess of
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The two-valence-band structure in SnTe is very similar to that in PbTe, leading to an expectation of similar enhancements in zT when the two valence bands can be engineered to converge effectively. The reason for a zT-enhancement by a large number of transporting bands can be simplified as an increase in electrical conductivity without explicitly reducing the Seebeck coefficient2. This concept has mostly been demonstrated in PbTe thermoelectrics for a well improved zT 2, 5, 31, 32 . Sharing with PbTe the similarities of chemical properties, crystal structure as well as band structure, SnTe is therefore motivated to be thermoelectrically enhanced through band convergence. Although an nH of >1020 cm-3 could involve the contribution from the low-lying heavy valence band (Σ) of SnTe to the transport properties, these nH are overwhelmingly higher than the need for optimized zT of the first valence band (L), leading an overall zT to be very low in SnTe when nH is high (Figure 2). An effective approach is to involve the contributions of both bands well aligned or at least with an energy offset (∆EL-Σ) a few kBT, which is not well fulfilled in pristine SnTe. Therefore, substitution of Sn with divalent elements such as alkaline-earths and transition metals opens possibilities for a reduced ∆EL-Σ and thus an increased zT in SnTe, which is very similar to the case in PbTe alloys 2, 5, 31, 32.
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Band engineering approaches including band convergence and resonant states for improving the electronic thermoelectric SnTe
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Where mI*, Edef, Cl, kB and ħ is the inertial effective mass, deformation potential coefficient, combined elastic moduli, Boltzmann constant and reduced Planck constant. It is therefore a high Nv but a low mI*, Edef and κL are desired for a high quality factor B. Although low mI* and Edef are proven to be beneficial to zT in PbTe28 and PbSe29, respectively, an effective reduction on either of these two parameters still remains challenging in a given material. Existing efforts so far for enhancing B factor and therefore zT mostly rely on the increase of Nv by band engineering and the reduction of κL by defect engineering. It is important to note that an optimal carrier concentration is always required to realize the maximum zT, since a high B factor does not guarantee a high zT to be obtained when the carrier concentration (n) deviates much from its optimum.
Sn leads to a compensation of cation vacancies, which reduces the hole concentration as well16. Limited by the solubility of the dopants in SnTe, the lowest nH reported so far is about 4×1019 cm-3, which is achieved by I-doping and close to the optimum at T>700 K. An optimization of carrier concentration realizes a slightly enhanced zT up to 0.6 at 773 K7, as shown in Figure 2b. Although iodine is found to be effective for reducing nH, other dopants with an even higher doping effectiveness are still desired, because further compositional complexity for converged bands and/or desired microstructures to ensure a high zT may reduce the dopant solubility. Nevertheless, known dopants effectively enable a broad range of carrier concentrations (1019~1021 cm-3) to better understand the transport properties based on a two-band model (Kane band and parabolic band approximations for L and Σ valence bands, respectively)7. The model predictions on nH-dependent Seebeck coefficient and zT (curves in Figure 2) fit well with the experimental results. An observed peak Seebeck coefficient at nH~6×1020 cm3 can be attributed to the contribution of the low-lying Σ band. The multiple-band structure offers possibilities for electronic performance enhancements, particularly when the constituent branches of bands can be engineered to be aligned or at least to have an energy offset (∆EL-Σ) within a few kBT2, 30.
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phonon scattering through point defects and/or nanostructures. In fact, synergic effects of both band and defect engineering, successfully lead to a zT as high as 1.6 to date24 in SnTe, promoting it as a highly efficient thermoelectric material with environment-friendly compositions. Perspectives on a design of composition, crystal structure and microstructure for further advancements in thermoelectric SnTe are proposed as well. The strategies discussed here, are believed to be equally applicable to both existing and new thermoelectrics for enhancements. Considering the charge transport in a parabolic band with acoustic scattering, as which most of thermoelectrics can be approximated25, thermoelectric quality factor (B) characterizes the highest possible zT26, providing the material can be doped to locate its optimal carrier concentration [nopt~(md*T)1.5 with md* as the density of state mass]27:
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Figure 4. Room temperature Hall Carrier concentration dependent Seebeck coefficient for various SnTe alloys with converged valence bands11, 13, 14, 16, 35, resonant states12, 23, 36, or other effects6, 21, 37 with a comparison to pristine SnTe7 (a) and temperature dependent zT for single-phased alloys12, 13, 16, 23, 35, 36 and composites11, 14-16, 21 of SnTe (b).
The sufficient ∆EL-Σ-reduction, enabled by divalent impurity substitutions, effectively involve the contribution of the low-lying Σ valence band to the transport properties, which enhances the Seebeck coefficient at a given carrier concentration as compared with that of pristine SnTe (Figure 4a). As an alternative understanding to band convergence, the reduction in energy between L and Σ bands leads to more conducting chanels for charge carriers, enabling a much higher electrical conductivity at a given reduced Fermi energy (i.e. a given Seebeck coefficient) and thus a higher power factor and zT (Figure 4b). Successful experimental demonstrations in SnTe are typified by a heavy substitution of Sn with Ca11, Mg13, Mn35, Cd16and Hg14. Similar Seebeck coefficient enhancements is obtained by resonant doping as well, the effect of which is firstly demonstrated in Tl-doped PbTe4, because the introduced resonant states locally distort the band structure in the vicinity of Fermi level for a high density of state mass. Such an effect has also been observed in SnTe with In-doping36. Studies afterwards in SnTe alloying with InTe36, 38, In2Te323 and AgInTe212 all achieve a room temperature Seebeck coefficient locating well above the Pisarenko curve, indicating the In-induced resonant states are indeed responsible (Figure 4a). DFT calculations show that Ag-doping reduces the L-Σ band offset of SnTe, therefore, the Seebeck coefficient enhancement
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The above mentioned zT-enhancements in SnTe alloys partially originate from the reduced lattice thermal conductivity (κL) resulting from the compositional complexity, which is important for this material because of its intrinsic high κL as compared to PbTe, PbSe or SnSe. Numerous efforts focus on the reduction of κL for enhancing the zT of SnTe through phonon scattering by zero dimensional (0D point defects6, 12-16, 35, 39 ) and two dimensional (2D boundary interfaces of nanostructures11, 15, 16, 20, 21, 23, 36, 37, 40) defects. The existence of 0D and 2D defects shortens the mean free path of high- and low-frequency phonons, respectively. Therefore an effectively reduced κL, which has been widely demonstrated in IV-VI semiconductors and many other thermoelectrics. Successful reduction in κL for SnTe thermoelectrics at high temperatures maximizing zT, through above mentioned defect engineering is surveyed in Figure 5. In the form of solid solution with divalent monotellurides/monoselenide, where the κL-reduction is dominated by substitutional point defects, a minimal κL of 0.8 W/m-K is obtained. This is realized in SnTe with 9% MgTe13 alloying, where both the mass and size contrasts between host and guest atoms are large. It is interesting to note the even lower κL can be achieved in SnTe alloying with Cu2 Te6 and In2Te323, where respectively exists interstitials and vacancies in additional to substitutions. Both interstitials and vacancies, in principle, enable maximal mass and strain fluctuations as compared to substitutions, therefore a more effective κL-reduction can be expected at a comparable total concentration of point defects41.
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Based on density functional theory (DFT), calculated energy change in L-Σ valence band offset due to various substitutions of Sn by divalent impurities14, 33, 35 is surveyed in Figure 3. At a given substitution concentration of 1/27 on Sn site, Mg, Mn, Cd, Hg and Zn enable an effective convergence of L and Σ valence bands, many of which has been demonstrated experimentally13, 14, 16, 33, 35. It is therefore expected that sufficiently converged L and Σ valence bands can be obtained in SnTe with divalent impurities having either a high solubility (∼10% or higher, such as Ca, Mg and Mn) or a strong effect on reducing ∆EL-Σ (such as Cd and Hg). Although Zn is theoretically shown effective for converging L and Σ valence bands, experimentally its solubility is found to be too low to ensure a sufficient reduction in ∆EL-Σ of SnTe. It should be noted that cation-substitution may simultaneously increase the band gap (Eg) of SnTe, ensuring a weak influence due to thermally excited minority carriers at high temperatures that high zT can be achieved.
in SnTe-AgInTe2 alloys might partially come from the valence band convergence12. Nevertheless, resonant doping successfully increases zT in SnTe alloys with In-doping (Figure 4b). It should be noted that a slightly enhanced Seebeck coefficient is also observed in SnTe with substitutions of Sb and Ga on Sn site21, 37. Being different from SnTe alloys with converged valence bands and/or resonant states, the enhancement of Seebeck coefficient in Sn1-xSbxTe37 and Sn1.03-xGaxTe21 is attributed to an energy filtering effect induced by nanostructures and an involvement of a new low-lying impurity valence bands introduced by Ga, respectively. Due to the electronic performance enhancements through either band convergence or resonant states mentioned above, peak zT of 1.335 and 1.136 are respectively achieved in single-phased SnTe alloys (Figure 4b), which are much higher than that of pristine SnTe (zT∼0.4).
SnTe-AgInTe2
Figure 3. The calculated band gap and the L-Σ valence band offset (∆EL-Σ) for SnTe and Sn26MTe27 (M=Ca, Sr, Ba, Mg, Mn, Cd, Zn, Hg) alloys11, 13, 16, 33-35.
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Figure 5 Comparison on the measured lowest lattice thermal 6, 12-16, 35, 39 11, conductivity (κL) for SnTe solid solutions and composites 15, 16, 20, 21, 23, 36, 37, 40 . The black line shows the amorphous limit of
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The above mentioned strategies, carrier concentration optimization and band engineering focus on enhancement of electronic performance while defect engineering focuses on minimization of lattice thermal conductivity. Since these are in principle independent to each other for the overall zT-enhancements of SnTe, it is therefore reasonable to believe that a synergy of multiple effects would lead to an even greater enhancement of zT in SnTe thermoelectrics (Figure 4b). The synergic effect of both MnTe-alloying for band convergence/point defect scattering and iodine-doping for carrier concentration optimization realizes a peak zT up to 1.335. Similar effects result in an increased zT of 1.05 in SnTe-AgI system43. Using a self-doping strategy with an excess of Sn for carrier concentration optimization, Sn1.03Te alloying with CdTe realizes converged bands and substitutional defects for an enhancement of zT up to unity16. A further introduction of insoluble CdS/ZnS nano-precipitates enables an additional reduction in κL (Figure 5), and thus an even higher zT of ∼1.3 is achieved16. Similarly, Bi-doping for carrier concentration optimization, HgTe-alloying for band convergence and over-solubility HgTe for nano-precipitates synergically enable a peak zT of 1.3514. Moreover, alloying with CaTe simultaneously introduces multiple effects of carrier concentration optimization, high concentration point defects/nanostructures as well as band convergence, therefore synergically realizes a peak zT of 1.3511. Successful examples also include SnTe with In-substitution on Sn site, where the solvents are either InTe36 or In2Te312, 23. Soluble In impurities create resonant states for an enhanced Seebeck coefficient (Figure 4a). In addition to phonon scattering due to alloy defects in the matrix phase, a further κL-reduction is realized by nanostructures through ball milling in SnTe-InTe36 (Figure 5). As a result, the synergic effect of both resonant states and phonon scattering by multi-dimensional defects, leads to an enhancement of a peak zT up to 1.136. Moreover, the synergic effects of band convergence, resonant states and nano-precipitates in SnTe increase not only the peak zT up to 1.4, but also the average zT up to 0.8 in Sn0.97Cd0.015In0.015Te-3%CdS15.
Te (C u
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Figure 6. TEM images and corresponding FFT patterns for Sn0.96Sb0.04Te (a, b) and Sn0.85Sb0.15Te (c, d)37.
2
The co-existence of both interstitial (Cu) and substitutional (CuSn) defects in SnTe-Cu2Te alloys is evident from the comparably low formation energies of these two types of defects by DFT calculations6. With the help of an alloy scattering model involving these two types of point defects, the experimentally observed κL-reduction can be understood and is dominated by interstitials for an effective phonon scattering6. Actually, alloying with Cu2Te leads to the largest κL-reduction in SnTe to date, and the κL as low as 0.5 W/m-K approaches its amorphous limit (0.4 W/m-K) 6 according to the Debye-Cahill model42. Importantly, alloying SnTe with Cu2Te shows a negligible effect on the band structure and electronic transport properties of SnTe (Figure 4a), successfully leading a purely thermal approach for enhancing zT up to unity in a single phased solid solution6. This further enables SnTe-Cu2Te alloy as an excellent starting material for further zT-improvements through an integration with other approaches (particularly electronic ones such as band convergence). Phonon scattering due to vacancies should in principle offer a similar reduction in κL as that due to interstitials, but alloying with In2Te3 realizes a slightly less κL-reduction (Figure 5) because of the lower solvent solubility and thus a lower vacancy concentration23. When the content of a solvent is beyond its solubility at equilibrium, it can precipitate out to form fine microstructures under proper synthesis conditions. Such a fine microstructure may produce a high-density boundary interfaces for scattering of phonon with long wavelengths, in addition to that by point defects in the solid solution matrix. Both nano precipitates and nanostructuring are widely utilized to further reduce κL of SnTe, as shown in Figure 5. Successful examples include SnTe-InTe36, SnTe-HgTe17, SnTe-CdTe-CdS16, SnTe-InTe-CdTe-CdS15, SnTe-SrTe20 and SnTe-GaTe21, SnTe-SbTe37, SnTe-GdTe40 systems, where the fine microstructures are obtained by ball milling36, adding insoluble solvents15, 16 or by thermodynamic precipitation14, 20, 21. Taking SnTe-SbTe37 as an example, a high concentration of Sb (≥4%) leads to nanoprecipitates, as shown in Figure 6, for an effective phonon scattering. The obtained further κL-reduction from the parent alloy materials, all contribute to the high zT realized in these composites. The lowest κL of 0.5 W/m-K achieved in SnTe composites reported so far, approaches the amorphous limit (0.4 W/m-K) as well. Although the κL is largely reduced by various mechanisms such as point defects and nanostructuring mentioned above, majority of high-zT SnTe materials still show a κL much higher than its amorphous limit. This implies the existence of available room for further reducing κL targeting an even higher performance in SnTe thermoelectrics.
SnT e (C uT 2 e) 0
SnTe estimated by the Debye-Cahill model42.
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Figure 7. Temperature dependent zT for SnTe, SnTe(Cu2Te)0.05 and Sn0.91Mn0.14Te(Cu2Te)0.05.
As pointed out above, alloying with Cu2Te offers a purely reduction in κL without inducing any observable effects on electronic transport properties6. While alloying with
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MgTe/MnTe enables effectively converged valence bands due to the high solubilities. Both effects can be realized in the form of alloy, which motivates the work of synergic effect of interstitial point defects and band convergence for zT-maximization of SnTe10, 24. It is found that the existence of MnTe reduces the solubility of Cu2Te at low temperatures, which leads a reduction in κL to be more effective at high temperatures. As a result of a κL as low as ∼0.5 W/m-K and effective converged L and Σ valence bands, peak zT values of 1.6 (Figure 7) and 1.4 are respectively achieved in SnTe-Cu2Te solid solutions with MnTe-24 and MgTe-alloying10. A zT of 1.6 is actually a record among known p-type thermoelectrics other than IV-VI semiconductors24. This high zT and nontoxic composition promote SnTe as a promising environment-friendly alternative for p-PbTe. Considerations on composition, crystal structure and microstructure for future advancements in SnTe Electronically, L and Σ valence bands have different locations in reciprocal space, which correspond to different directions in the real space. Therefore, the required strain for tuning the band energy individually is highly directional. This is unfortunately impossible in cubic SnTe, because an isotropic strain is only allowed through lattice expansion or contraction. This leads to a directional lattice distortion as a new degree of freedom to approach the convergence of valence bands of SnTe. Since the [111] direction corresponds to the L band, a lattice distortion along this direction through such as alloying, yields a rhombohedral structure as its low-temperature phase (T
