Manipulation of Solubility and Interstitial Defects for Improving

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Manipulation of Solubility and Interstitial Defects for Improving Thermoelectric SnTe Alloys Jing Tang, Bo Gao, Siqi Lin, Xiao Wang, Xinyue Zhang, Fen Xiong, Wen Li, Yue Chen, and Yanzhong Pei ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01098 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018

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ACS Energy Letters

Manipulation of Solubility and Interstitial Defects for Improving Thermoelectric SnTe Alloys Jing Tang1, Bo Gao1, Siqi Lin1, Xiao wang1,Xinyue Zhang1, Fen Xiong2, Wen Li1, Yue Chen2 and Yanzhong Pei*,1 Interdisciplinary Materials Research Center, School of Materials Science and Engineering, Tongji Univ., 4800 Caoan Rd., Shanghai 201804, China. 2 Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China

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Abstract:

*Email: [email protected]

Many efforts have recently been put on advancing thermoelectric SnTe as an alternative to PbTe. Effective methods include energy band convergence, nanostructures and substitutional/interstitial defects. Among these strategies, it is shown that CdTe-alloying effectively reduces the energy offset between valence bands for an enhanced thermoelectric figure of merit, zT. However, its solubility of only ~6% might limit this effect on a full zT optimization in SnTe-CdTe alloys. Here we show an effective approach for increasing CdTe solubility up to ~20%, with the help of the existence of 5% GeTe. This leads the valence bands to be further converged for improving electronic performance. In addition, Cu2Te-alloying is used to create interstitial defects for reducing the lattice thermal conductivity. Eventually, the increase of CdTe-solubility and the creation of Cu-interstitials enable a significantly enhanced zT, indicating the importance of solubility manipulation for engineering the band structure in thermoelectric SnTe alloys. Energy crisis has long been a world-wide challenge. Thermoelectrics, which convert heat into electricity without any hazardous emissions or moving parts, have been regarded as a promising solution. However, a large-scale utilization of this technology is still limited by thermoelectric performance of materials, which is determined by the thermoelectric figure of merit, zT=S2T/ (e+L), where S, T, e and L are the electrical conductivity, the Seebeck coefficient, absolute temperature, electronic and lattice components of the thermal conductivity, respectively1. With years of development, the thermoelectric performance has been significantly improved in many materials including half-Heuslers2-6, lead telluride7-11 and germanium telluride12-13 for power generation applications. According to the Wiedemann-Franz law (e=LT), e is related to the Lorenz factor (L) and electronic conductivity (), indicating a highly conductive material tends to have a high thermal conductivity. Moreover, both electrical conductivity () and Seebeck coefficient are in connection with carrier concentration, this leads to a strong coupling among S and e. Therefore, it is challenging to achieve an increase in zT through a simple manipulation of one of these coupled material properties. To maximize zT, many strategies have been proposed either for decoupling these properties for an improvement in electronic performance (power factor S), or for reducing the lattice thermal conductivity by defects. Electronically, band engineering approaches14, including band convergence8, 13-17, band nestification18 and resonant doping19-23, are particularly effectively. These strategies have applied in many materials such as PbTe8-11, zintl phases24, half-Heuslers4-6 and Te18.

Alternatively, in order to decrease the lattice thermal conductivity, successful strategies are typified by nanostructuring11, 19, 25-26, atomic rattling27, lattice anharmonicity28-30, dislocations8, 31-32, point defects33-41, low sound velocity42-43, complex crystal structures43-46 and liquid-like phonons39, 45, 47-48. These strategies are proven to be successful particularly in IV-VI semiconductors such as PbTe. Alloying 1~5% YbTe49 MnTe10, 50-52, MgTe53-54 or CdTe55-56 with PbTe enables a sufficient reduction in energy offset (∆EL) between L and  valance bands, which leads to a significant improvement in thermoelectric performance. Showing the same crystal structure and very similar chemical properties with PbTe thermoelectric, tin telluride (SnTe) is often considered as an alternative to conventional PbTe. In addition, SnTe shows a very similar valence band structure with that of PbTe, and the most distinct difference relies on the larger energy offset between the valance bands (∆EL of ~0.3eV for SnTeversus ~0.15 eV for PbTe at room temperature57). Such a difference is further believed to be the main electronic origin for its much inferior thermoelectric performance in SnTe. Thermally, SnTe has a higher lattice thermal conductivity due to its relatively low atomic mass, as compared with PbTe. Fortunately being similar with the case of PbTe, alloying SnTe with monotellurides such as MnTe35, 58-59, MgTe37, 60-61, CdTe62-64 and CaTe65 are found to be effective as well for reducing the valence bands offset. Considering the much larger band offset in SnTe17, the required concentration of monotellurides for minimizing the valence bands offset is usually much higher. This essentially limits the thermoelectric performance to be fully optimized through this band 1

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engineering approach, because of the limited solubility of these monotellurides in SnTe. In addition, a higher alloying concentration could open possibilities for reducing the lattice thermal conductivity (L) due to phonon scattering by substitutional defects. It is therefore important to increase the solubility of these monotellurides for improving thermoelectric SnTe. In fact, the high thermoelectric performance realized in MnTe-SnTe35, 58 and MgTe-SnTe37, 60 alloys is believed to be largely due to their high solubility (~15% for MnTe and ~12% for MgTe). Comparing to MnTe and MgTe, CdTe is found to have a very similar effect on reducing the valence bands offset62-64, but its solubility is much lower (only ~6%). This might explain the relatively inferior thermoelectric performance in MgTe-CdTe alloys62-64. It is reasonable to expect a further thermoelectric improvement, once the concentration of CdTe in MgTe-CdTe alloys is promoted to be comparable or higher to those of MnTe- and MgTe-SnTe alloys. This motivates the current work to increase solubility of CdTe in SnTe for improving the thermoelectric performance, which is realized by the assistant of 5% GeTe alloying. A similarly effective increase in the solubility of MnTe in SnTe due to the existence of GeTe58 has been found previously. In this work, we confirmed the increased solubility of CdTe in SnTe to ~20% with the help of 5% GeTe-alloying and its effectiveness on convergence of the valence bands. In addition, Cu2Te-alloying is utilized to create Cu-interstitial defects as strong phonon scattering centers, for reducing the lattice thermal conductivity to its amorphous limit. The synergic effects due to both electronic and thermal strategies successfully enable a significantly enhanced thermoelectric performance of SnTe. The details on synthesis, characterization, measurements of materials, band structure calculations and formation energies calculations are given in the supplementary. Excess of Sn (1~6%) is used to tune the carrier concentration in this work. As shown in phase diagram of SnTe-CdTe (Fig. 1a), the solubility of CdTe in SnTe increases from ~6% to ~20%, with the help of 5% GeTe-alloying. The effect of Ge on solubility of CdTe could be understood by the reduced energies (59.2~58.2meV/atom) of mixing after GeTe-alloying. The X-ray diffraction (XRD) patterns for all the samples are shown in Fig. 1b, which indicate the formation of solid solution with a rock-salt structure for Sn0.95-xGe0.05CdxTe when x≤0.21. With a further increase of nominal concentration of CdTe to 24%, CdTe impurities can be observed. The lattice parameter is found to decrease linearly with increasing x at x≤0.21, followed by a saturation at x>0.21 (Fig. 1c). This indicates a solubility of CdTe is promoted to ~20% in SnTe with 5% GeTe-alloying, as compared with that of SnTe without GeTe-alloying. The reduction in lattice parameter in solid solution region can be understood by the smaller atomic size of Cd as compared with that of Sn. As shown in Fig. S1, further alloying with 5% Cu2Te leads to formation of Cu2Te precipitates at room temperature, while these are believed to be dissolved in SnTe alloys at high temperatures33, 37-38. Importantly, additional Cu2Te-alloying and Sn-excess are found to have negligible effects on the solubility of CdTe. It should be noted that tracing amount of Ge-impurities can be observed, indicating the existence of cation vacancies in these p-type materials34.

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Fig. 1. Phase diagram (a) of SnTe-CdTe66-67 showing an increase in solubility of CdTe with the help of 5% GeTe-alloying; XRD patterns (b) and lattice parameter (c) for Sn0.95-xGe0.05CdxTe with a comparison to those of Sn1-xCdxTe62.

A Scanning Electronic Microscopy (SEM) equipped with an Energy Dispersive Spectrometer (EDS) was used to further characterize the phase composition, and the resultant images are shown in Fig. S2 (without Cu2Te-alloying) and Fig. 2 (with Cu2Te-alloying). The existence of tracing amount of Ge and/or Cu2Te impurities sized in a couple of microns is confirmed by EDS analyses. SEM observations further confirm the solubility of CdTe is immune to Cu2Te-alloying and Sn-excess, which is consistent with the XRD results. As shown in Fig. S1, excess of Sn successfully enables a variation of Hall carrier concentration ranging from 4~0.6×1020 cm-3 at room temperature, such a broad range of carrier concentration ensures a full optimization of thermoelectric performance in SnTe68.

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ACS Energy Letters with pristine SnTe33, where the Seebeck coefficient and mobility can be well predicted by a two-band model (grey curves)70, materials heavily alloyed with CdTe show a significant increase in Seebeck coefficient at an expense of reduced Hall mobility, at both room and high temperatures. This is consistent with the above discussion on the convergence of valance bands due to CdTe-alloying. Being similar to literature SnTe-Cu2Te alloys 33, 37-38, this work shows no observable deviation on Hall carrier concentration dependent Seebeek coefficient and Hall mobility, due to 5% Cu2Te-alloying. This indicates that either Cu2Te- or GeTe-alloying in this work introduce negligible modification in valence band structure 33, 37-38.

Fig. 2. SEM images with composition mappings by EDS for Sn0.79Ge0.05Cd0.2Te(Cu2Te)0.05.

A significant increase in CdTe solubility is expected to enable a well converged valence bands for SnTe alloys, which is firstly supported by the temperature dependent Hall coefficient measurements as shown in Fig. S3. A peak in Hall coefficient is observed which is an indication of band convergence due to the strongly temperature dependent redistribution of carrier between two valance bands69.Peak Hall coefficient appears at lower temperatures with the increasing concentrations of CdTe, indicating a reduction in energy offset between the valence bands. This work focuses on alloys with a concentration of 20% CdTe, which is close to its solubility to ensure not only the convergence of valence bands but also the maximal phonon scattering due to Sn/Cd substitutional defects.

Fig. 3. Carrier concentration dependent Seebeck coefficient (a, b) and Hall mobility (c, d) at 300 K (a, c) and 725 K (b, d) for Sn0.95-xGe0.05CdxTe with and without 5% Cu2Te-alloying, with a comparison to literature model predictions (grey curves)70 and literature results for Sn1-xCdxTe62, 64. The red curves are used to guide the eye.

Hall mobility (Fig. S3) for all the materials show nearly unchanged temperature dependence via H~T-1.5, indicating the charge carrier scattering dominated by acoustic phonons. This enables a meaningful discussion on the carrier concentration dependent Seebeck coefficient and mobility (Fig. 3) without involving the effect of scattering. Comparing

Fig. 4. Calculated band structures of SnTe-CdTe alloys.

The convergence of valence bands due to a heavy CdTe-alloying is confirmed by the band structure calculations, as shown in Fig.4. It is seen that the energy offset between valence bands monotonically decreases with increasing CdTe concentration. In addition, the band gap is found to increase as well. All these band features are beneficial electronically for high thermoelectric particularly at high temperatures. Temperature dependent resistivity, Seebeck coefficient, thermal conductivity and zT for Sn0.75+Ge0.05Cd0.2Te and Sn0.95-xGe0.05CdxTe(Cu2Te)0.05 are shown in Fig. S4 and S5, respectively. The beneficial effect due to CdTe-alloying for thermoelectric performance can be seen in both series of materials, while the following discussion focuses on Sn0.75+Ge0.05Cd0.2Te(Cu2Te)0.05 with thermoelectric properties shown in Fig. 5. The high thermoelectric performance is guaranteed by CdTe-alloying for converged valence bands, Sn-excess for optimized carrier concentration, and Cu2Te-alloying for decreased lattice thermal conductivity. In more details, Seebeck coefficient and resistivity show an increase with increasing temperature (Fig. 5a), indicating a degenerated conduction in these materials. Total thermal conductivity and its lattice component are shown in Fig. 5b, where the lattice thermal conductivity (L) is estimated via Wiedemann-Franz law (e=LT/) from total thermal conductivity. The Lorenz factor (L) is determined by a single parabolic band (SPB) model with acoustic scattering. It can be seen that Cu2Te-alloying enables an effective reduction in L particularly at high temperatures, which is consistent with literature results33, 37-38 that sufficiently high concentration interstitial defects in SnTe alloy phase can only be enabled at high temperatures. This argument is further confirmed by the nearly unchanged sound velocities due to alloying as shown in Figure S6, which excludes the possible effect of lattice softening. According to the Debye-Cahill model71, the minimal lattice thermal conductivity (Lmin) can be estimated


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to be about 0.4 Wm-1K-1 for SnTe33. The lowest L achieved in this work is found to approach such a theoretical minimum, actually.

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This work is supported by the National Natural Science Foundation of China (Grant No. 11474219 and 51772215), the National Key Research and Development Program of China (2018YFB0703600), Fundamental Research Funds for Science and Technology Innovation Plan of Shanghai (18JC1414600), the Fok Ying Tung Education Foundation (Grant No. 20170072210001) and “Shu Guang” Project Supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation. FX and YC are grateful for the financial support from RGC under project numbers 27202516 and 17200017 and the research computing facilities offered by ITS, HKU. References

Fig. 5. Temperature dependent Seebeck coefficient and resistivity (a), total and lattice thermal conductivity (b), thermoelectric figure of merit (c) for Sn0.75+Ge0.05Cd0.2Te(Cu2Te)0.05 with a comparison to SnCd0.03Te-2%CdS64 and peak thermoelectric figure of merit (d) with a comparison to literature results of SnTe-based materials33, 62, 64..

Originating from the effects due to band convergence, and interstitial defects, the thermoelectric performance of SnTe in the form of alloys is successfully improved to 1.3 in this work (Fig. 5c). In addition, the high performance is found to be highly reproducible (Fig. 5c and S7), as confirmed by both heating and cooling measurements on properties as well as XRD resutls (Fig. S7) Evolutionarily, GeTe-assisted promotion of CdTe solubility enables well converged valence bands and Cu2Te takes the responsibility for reducing lattice thermal conductivity, both of which contribute to the zT-enhancement as shown in Fig. 5d. This work successfully increases the solubility of CdTe from ~6% to ~20% in SnTe, which is enabled by a 5% GeTe-alloying. The resultant higher CdTe concentration in SnTe leads to more converged valence bands for improving the electronic performance. In addition, a further Cu2Te-alloying effectively decreases the lattice thermal conductivity to the amorphous limit. The synergy of both electronic and thermal approaches, enable an enhancement in thermoelectric figure of merit of ~1.3 to be achieved in SnTe alloys. This work demonstrates the importance of solubility and defect engineering for improving SnTe thermoelectrics.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Details on materials and methods, XRD patterns, XRD Rietveld refinements, SEM images with composition mappings, sound velocities. Temperature-dependent electronic transport properties, thermal conductivity and zT Acknowledgement

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ACS Energy Letters

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Manipulation of Solubility and Interstitial Defects for Improving

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