Step-Up Thermoelectric Performance Realized in Bi2Te3 Alloyed

Feb 28, 2019 - The successful combination of these two strategies eventually results in a peak figure of ... This value is 2-fold of that in pristine ...
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Step-Up Thermoelectric Performance Realized in Bi2Te3 Alloyed GeTe via Carrier Concentration and Microstructure Modulations Di Wu,†,‡ Lin Xie,‡ Xiaolian Chao,† Zupei Yang,*,† and Jiaqing He*,‡ †

Key Laboratory for Macromolecular Science of Shaanxi Province, Shaanxi Key Laboratory for Advanced Energy Devices, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China ‡ Department of Physics, Southern University of Science and Technology, Shenzhen 518055, China

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S Supporting Information *

ABSTRACT: We report that outstanding thermoelectric performances can be achieved in 7% Bi2Te3 alloyed GeTe when it is simultaneously doped with iodine and treated with a water quenching process. The introduction of moderate iodine can help optimize the electrical properties; moreover, a water quench treatment was found to induce a novel vacancy microstructure, i.e., Ge vacancy “clusters”, which helps to modulate the thermoelectric performance at higher temperatures. The successful combination of these two strategies eventually results in a peak figure of merit ZT ∼ 2.2 at 723 K in water quenched Ge0.93Bi0.07Te1.005I0.03. This value is 2-fold of that in pristine GeTe (∼1.1). Corresponding average ZT between 323 and 773 K is 1.23, in comparison with 0.44 for pristine GeTe. The calculated single-leg device efficiency of water quenched Ge0.93Bi0.07Te1.005I0.03 is ∼11.2% at a temperature gradient from 298 to 773 K, suggesting GeTe is a very promising thermoelectric at intermediate-temperature range. KEYWORDS: thermoelectric, GeTe, microstructure modulation, ferroelectric domain, localized vacancy clusters, device efficiency

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INTRODUCTION As a simple and clean energy conversion technique, the thermoelectric effect is believed to have great potential in power generation and refrigeration.1 The device efficiency is determined by an intrinsic figure of merit ZT of the specific thermoelectric material, which is a temperature dependent value written as ZT = (S2σ/κ)T, where S, σ, and κ are the Seebeck coefficient, electrical conductivity, and thermal conductivity, respectively. Ideal thermoelectric materials shall possess decent electrical transport properties together with low thermal conductivity.2−7 For this reason, semiconductors with proper band gap, other than metals and insulators, are widely believed to exhibit good thermoelectric performance.1,8,9 As a narrow band gap semiconductor, GeTe exhibits p-type conducting behavior due to the intrinsic high-density Ge vacancies.10 The intrinsic hole concentration in pristine GeTe can reach ∼5 × 1021 cm−3 as reported.11,12 Such a high hole concentration in GeTe can surely benefit the electrical conductivity; however, it also leads to a low Seebeck coefficient as well as unnecessarily high electrical thermal conductivity. Tremendous efforts have been made in order to reduce the hole concentration to a moderate level, mostly through cation doping by Pb, Bi, Sb, and Ag, etc.13−16 Notably, the introduction of Bi can play dual roles in modulating the electrical conductions,17 i.e., hole concentration reduction and valence band modulation. Another approach to improve the figure of merit ZT in GeTe is through reducing its lattice thermal conductivity, either by nanoprecipitation14,18,19 or alloying.20−22 © XXXX American Chemical Society

In this work, we demonstrate a strategy to simultaneously realize carrier concentration modulation and lattice thermal conductivity reduction. We first chose an optimized composition of 7% Bi2Te3 alloyed GeTe and then modulated its hole concentration via iodine-doping at the anion site. Bi2Te3 alloying lifted up the peak ZT from 1.1 in pristine GeTe to 1.65, while iodine-doping boosted this value to ∼1.85. The bestperformed iodine-doped sample was further treated with a water quench (WQ) process and exhibited a significantly reduced lattice thermal conductivity due to the Ge vacancy “clusters” as observed under scanning transmission electron microscope (STEM). The localized feature of these vacancy “clusters” not only reduced the effective conduction hole numbers but also greatly enhanced the mid-frequency phonon scattering. As a result, a remarkable high peak ZT ∼ 2.2 at 773 K was achieved in the water quenched sample with nominal composition Ge0.93Te0.07Te1.005I0.03. This value is a 100% enhancement over that of pristine GeTe. Our results provide a successful template for simultaneous modulation of electrical and thermal transport, thus might shed light on future research in GeTe and similar thermoelectric systems.23 Received: January 10, 2019 Accepted: February 28, 2019 Published: February 28, 2019 A

DOI: 10.1021/acsaem.9b00057 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Energy Materials

Figure 1. Temperature dependent (a, b) σ and S, (c, d) κ and calculated κL, (e, f) PF and ZT of furnace cooled GeTe alloyed with varying fractions of Bi2Te3 (x = 0%, 1%, 3%, 5%, 7%, and 9%). The inset of part b shows the calculated Pisarenko curves (single parabolic band model) vs experimental data, indicating an obvious valence band modulation upon Bi2Te3 alloying.

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RESULTS AND DISCUSSION First, in order to find the optimal alloying ratio of Bi2Te3, we systematically measured the electrical and thermal transport performance in furnace cooled (FC) Ge1−xBixTe1+0.5x as x increased from 0 to 9%. Powder X-ray diffraction (XRD) patterns can be found in the Supporting Information. The electrical conductivity decreased monotonically from ∼7200 to ∼1600 S/cm at room temperature as an increasing amount of Bi2Te3 was introduced to GeTe, Figure 1a; meanwhile, the Seebeck coefficient was significantly enhanced from 32.6 at x = 0 to 103.0 μV/K at x = 9%, Figure 1b. The systematic behaviors of electrical conductivity and Seebeck coefficient originate from the dual roles of Bi2Te3, i.e., valence band convergence and carrier concentration tuning, as discussed in our previous work.24 The temperature dependence of electrical conductivity exhibits the highly degenerated semiconducting feature of all our samples. Measurements of thermal conductivity were demonstrated in Figure 1c; it was clearly seen that total thermal conductivity is significantly reduced upon Bi2Te3 alloying. This reduction comes partly from the decrease of electrical conductivity and, more importantly, from the obvious reduction of lattice thermal conductivity as calculated in Figure 1d. As more and more Bi2Te3 was introduced in the GeTe lattice, a

gradual reduction of room temperature lattice thermal conductivity from ∼2.7 to 1.0 W/mK was observed. Considering that Bi2Te3 has completely dissolved into the GeTe lattice, this giant reduction of lattice thermal conductivity, or enhanced phonon scattering, shall certainly be attributed to the increased alloying scattering due to Bi substituting Ge, at least partly. Another possible scattering source might be the induced cation vacancies by Bi2Te3, since each Bi2Te3 can provide a cation vacancy due to its different cation/anion ratio with GeTe matrix. With power factors calculated in Figure 1e, we showed the temperature dependent figure of merit ZT in Figure 1f. As a result of the trade-off between power factor and thermal conductivity, the 7% Bi2Te3 alloyed GeTe exhibits the largest peak ZT value ∼1.65 at 773 K, a 50% enhancement over that of pristine GeTe ∼ 1.1; actually, its figure of ZT is superior to that of pristine GeTe over the whole temperature range. In order to further tune the hole concentration of Ge0.93Bi0.07Te1.035, we added iodine as an n-type dopant. The effect of I-doping on electrical transport was obvious, as shown in Figure 2a,b. Electrical conductivity of Ge0.93Bi0.07Te1.035−xIx was reduced gradually as more iodine substitutes for tellurium, especially in the high-temperature range; meanwhile, Seebeck coefficients of different I-doped samples remained the same B

DOI: 10.1021/acsaem.9b00057 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

Figure 2. Temperature dependent (a, b) σ and S, (c, d) κ and calculated κL, and (e, f) PF and ZT of furnace cooled Ge0.93Te0.07Te1.035 samples doped with x fraction iodine (x = 0%, 1%, 2%, and 3%). Room temperature (g, h) hole concentration p and mobility μ of furnace cooled x fraction of iodinedoped Ge0.93Te0.07Te1.035 with x = 0%, 1%, 2%, and 3%.

below 423 K and quickly diverged thereafter. The peak Seebeck coefficients of I-doped samples (x = 1%, 2%, and 3%) were around 250 μV/K, in comparison to ∼200 μV/K for the sample with x = 0. Hall measurements (Figure 2g,h) showed that iodinedoping indeed reduced the room temperature hole concentration of Ge0.93Bi0.07Te1.035 from 7.3 × 1020 to 4.3 × 1020, 5.2 × 1020, and 5.6 × 1020 cm−3, and that the corresponding carrier mobility increased from 15 cm2/(V s) to 28, 23, and 17 cm2/(V s) as x = 1%, 2%, and 3%, respectively. Due to the trade-off between Seebeck coefficient and electrical conductivity, the overall power factor did not exhibit any enhancement in the entire temperature range from 323 to 773 K, Figure 2e. Nevertheless, the reduced electrical

conductivity, especially at higher temperatures, resulted in a considerable drop of total thermal conductivity κ (Figure 2c), and an overall enhancement of figure of merit ZT as shown in Figure 2f. Peak ZT at 773 K of Ge0.93Bi0.07Te1.035 obtained a slight enhancement from 1.65 to 1.69, 1.85, and 1.68 as the Idoping level increased from 1%, to 2%, to 3%, individually. The peak ZT ∼ 1.85 for Ge0.93Bi0.07Te1.015I0.02 is about 12% higher than that of Ge0.93Bi0.07Te1.035, demonstrating that hole concentration tuning via iodine-doping could also be an effective way to obtain better thermoelectric performance in GeTe. During the investigation, we found that water quenched samples always exhibit better thermoelectric performance than furnace cooled ones, Figure S2. As compared with furnace C

DOI: 10.1021/acsaem.9b00057 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

Figure 3. (a) Classical submicron domain structures under low-magnification TEM, and (b) Ge vacancy “clusters” (pointed out by yellow arrows) in water quenched Ge0.95Bi0.05Te1.025 samples under low-magnification TEM. (c) STEM-HAADF manifestation of the vacancy “cluster” feature and (d) atomic resolution TEM image of a selected vacancy “cluster” with localized characterization. The inset of part d is the corresponding FFT pattern of the selected area.

by tuning I-doping level were shown in Figure 4. Compared with the iodine-free sample, I-doping resulted in higher electrical conductivity at low temperatures and lower electrical conductivity at higher temperatures. Higher electrical conductivity at low temperatures could help maintain the high power factor in that temperature range, while lower electrical conductivity at higher temperatures is favorable for low total thermal conductivity (Figure 4c) and thus high ZT. Seebeck coefficients exhibited a totally opposite trend compared to that of electrical conductivity as expected, shown in Figure 4b. The peak power factor exhibits an obvious shift to lower temperatures upon Idoping, Figure 4e, implying a reduction of hole concentration. It is worth noting that the lattice thermal conductivity was gradually reduced as the I-doping level increased, Figure 4d. This phenomenon was not observed in FC samples as shown in Figure 2. It is thus suspected that the introduction of iodine might have a certain interplay with the localized vacancy “clusters”; i.e., iodine might affect the formation and/or morphology of vacancy “clusters”, thus enhancing the phonon scattering and resulting in the reduction of lattice thermal conductivity. The detailed study of the vacancy morphology due to iodine-doping has exceeded the scope of this work and might be presented as an individual paper in the near future. Finally, we calculated the figure of merit ZT of I-doped WQ Ge0.93Te0.07Te1.035 in Figure 4f. Compared to the iodine-free sample with peak ZT ∼ 1.65 at 773 K, x I-doping lifted the peak ZT to 2.0, 2.0, and 2.2 with x = 1%, 2%, and 3%, respectively. We also calculated the engineering ZT and single-leg device efficiency,29,30 in order to evaluate the device performance of our samples. Before that, we first exhibited the step-up peak ZT

cooled samples, water quenched samples consistently demonstrate lower electrical conductivity and lattice thermal conductivity. To find out the reason, we conducted Hall measurements for a chosen composition of Ge0.95Bi0.05Te1.025; the results are exhibited in Figure S3. It is seen clearly that water quenching considerably reduced the hole concentration as compared with furnace cooling. This explains the higher Seebeck coefficients observed in these water quenched samples; but what mechanism caused the hole concentration reduction in samples that experienced the water quench process? We conducted STEM studies of the water quenched 5% Bi2Te3 alloyed GeTe sample and, surprisingly, found that there are plenty of Ge vacancy “clusters” existing all over the samples, Figure 3. These localized vacancy “clusters” (as the yellow arrows indicate, Figure 3b−d) of a few nanometers provide additional scattering sources for the high-to-intermediate frequency phonons,25−27 and thus can reduce the lattice thermal conductivity. Meanwhile, the localized feature of these Ge vacancies implies that the net holes participating in electrical conduction decrease. This analysis is consistent with the aforementioned Hall measurement results. We also would like to emphasize that the localized vacancy “clusters” were also proposed in pseudolayered Sb2 Te2(GeTe) n systems by Schnerder et al.,28 but the solid evidence (TEM images) was inadequate; moreover, their effects on electrical and thermal transport properties were not discussed. Eventually, we combined the I-doping and water quench strategies together in the selected composition: 7% Bi2Te3 alloyed GeTe. The characterized thermoelectric performances D

DOI: 10.1021/acsaem.9b00057 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

Figure 4. Temperature dependent (a, b) σ and S, (c, d) κ and calculated κL, and (e, f) PF and ZT of water quenched Ge0.93Te0.07Te1.035−xIx samples doped with x fraction of iodine (x = 0%, 1%, 2%, and 3%). In part f, we also presented ZT values of furnace cooled Ge0.93Te0.07Te1.035 samples with x Idoping for comparison. (g) Scheme of step-up max ZT and average ZT achieved in GeTe by gradual Bi2Te3 alloying, I-doping, and water quenching. (h) Calculated engineering ZT and single-leg device efficiency η of furnace cooled GeTe (pristine GeTe) vs optimized Ge0.93Te0.07Te1.005I0.03 with the water quench process.

values and average ZT values upon Bi2Te3 alloying, I-doping, and water quench treatment, Figure 4g. As compared with furnace cooled pristine GeTe whose peak and average ZT values are 1.1 and 0.44, water quenched Ge0.93Te0.07Te1.005I0.03 obtained a 100% enhancement for peak ZT to 2.2 and a 177% improvement for average ZT to 1.23. The thermoelectric enhancement is more astonishing with regard to engineering ZT and single-leg device efficiency. The former was enhanced from 0.22 to 0.82 while the latter was improved from 4.3% to 11.2% at

a temperature gradient from 298 to 775 K, as shown in Figure 4h.

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CONCLUSIONS We manifeste in this work that the thermoelectric performance of Bi2Te3 alloyed GeTe can be independently modulated via iodine-doping and water quench treatment. Iodine-doping can effectively reduce the hole concentration and thus optimize the thermoelectric power factor, while the water quench treatment can induce a novel microstructure, i.e., Ge vacancy “clusters”, E

DOI: 10.1021/acsaem.9b00057 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

(8) Snyder, G. J.; Toberer, E. S. Complex Thermoelectric Materials. Nat. Mater. 2008, 7, 105. (9) Moshwan, R.; Yang, L.; Zou, J.; Chen, Z.-G. Eco-Friendly SnTe Thermoelectric Materials: Progress and Future Challenges. Adv. Funct. Mater. 2017, 27, 1703278. (10) Damon, D. H.; Lubell, M. S.; Mazelsky, R. Nature of the Defects in Germanium Telluride. J. Phys. Chem. Solids 1967, 28, 520. (11) Christakudis, G. C.; Plachkova, S. K.; Avilov, E. S.; Shelimova, L. E.; Abrikosov, N. K. Transport Phenomena of Ferroelectric Solid Solutions (GeTe)1−x((Ag2Te)1−y(Sb2Te3)y)x with y = 0.60, 0 ≤ x ≤ 1 at 300 K. Phys. Status Solidi B 1985, 127, 419. (12) Koren, N. N.; Levchenko, V. I.; Dikareva, V. V.; Kazushchik, A. V. Physical Properties of Germanium Telluride Crystals Grown from Gas Phase. Phys. Status Solidi (a) 1984, 84, K173. (13) Plachkova, S. K. Thermoelectric Power in the System (GeTe)1x(AgSbTe2)x. Phys. Status Solidi (a) 1983, 80, K97. (14) Gelbstein, Y.; Davidow, J.; Girard, S. N.; Chung, D. Y.; Kanatzidis, M. Controlling Metallurgical Phase Separation Reactions of the Ge0.87Pb0.13Te Alloy for High Thermoelectric Performance. Adv. Energy Mater. 2013, 3, 815. (15) Li, J.; Zhang, X.; Lin, S.; Chen, Z.; Pei, Y. Realizing the High Thermoelectric Performance of GeTe by Sb-doping and Se-alloying. Chem. Mater. 2017, 29, 605. (16) Hong, M.; Chen, Z. G.; Yang, L.; Zou, Y. C.; Dargusch, M. S.; Wang, H.; Zou, J. Realizing zT of 2.3 in Ge1‑x‑ySbxInyTe via Reducing the Phase-Transition Temperature and Introducing Resonant Energy Doping. Adv. Mater. 2018, 30, 1705942. (17) Wu, D.; Zhao, L. D.; Hao, S.; Jiang, Q.; Zheng, F.; Doak, J. W.; Wu, H.; Chi, H.; Gelbstein, Y.; Uher, C.; Wolverton, C.; Kanatzidis, M.; He, J. Origin of the High Performance in GeTe-Based Thermoelectric Materials upon Bi2Te3 Doping. J. Am. Chem. Soc. 2014, 136, 11412. (18) Perumal, S.; Bellare, P.; Shenoy, U. S.; Waghmare, U. V.; Biswas, K. Low Thermal Conductivity and High Thermoelectric Performance in Sb and Bi codoped GeTe: Complementary Effect of Band Convergence and Nanostructuring. Chem. Mater. 2017, 29, 10426. (19) Perumal, S.; Roychowdhury, S.; Biswas, K. Reduction of Thermal Conductivity Through Nanostructuring Enhances the Thermoelectric Figure of Merit in Ge1−xBixTe. Inorg. Chem. Front. 2016, 3, 125. (20) Gelbstein, Y. Phase Morphology Effects on the Thermoelectric Properties of Pb0. 25Sn0. 25Ge0. 5Te. Acta Mater. 2013, 61, 1499. (21) Li, J.; Wu, H.; Wu, D.; Wang, C.; Zhang, Z.; Li, Y.; Liu, F.; Ao, W.q.; He, J. Extremely Low Thermal Conductivity in Thermoelectric Ge0.55Pb0.45Te Solid Solutions via Se Substitution. Chem. Mater. 2016, 28, 6367. (22) Samanta, M.; Biswas, K. Low Thermal Conductivity and High Thermoelectric Performance in (GeTe)1−2x(GeSe)x(GeS)x: Competition between Solid Solution and Phase Separation. J. Am. Chem. Soc. 2017, 139, 9382. (23) Liu, W.-S.; Zhang, B.-P.; Li, J.-F.; Zhao, L.-D. Effects of Sb Compensation on Microstructure, Thermoelectric Properties and Point Defect of CoSb3 Compfound. J. Phys. D: Appl. Phys. 2007, 40, 6784. (24) Wu, D.; Pei, Y.; Wang, Z.; Wu, H.; Huang, L.; Zhao, L.-D.; He, J. Significantly Enhanced Thermoelectric Performance in n-type Heterogeneous BiAgSeS Composites. Adv. Funct. Mater. 2014, 24, 7763. (25) He, J.; Blum, I. D.; Wang, H. Q.; Girard, S. N.; Doak, J.; Zhao, L. D.; Zheng, J. C.; Casillas, G.; Wolverton, C.; Jose-Yacaman, M.; Seidman, D. N.; Kanatzidis, M. G.; Dravid, V. P. Morphology Control of Nanostructures: Na-doped PbTe-PbS System. Nano Lett. 2012, 12, 5979. (26) He, J.; Girard, S. N.; Zheng, J. C.; Zhao, L.; Kanatzidis, M. G.; Dravid, V. P. Strong Phonon Scattering by Layer Structured PbSnS(2) in PbTe based Thermoelectric Materials. Adv. Mater. 2012, 24, 4440. (27) Wu, H.; Carrete, J.; Zhang, Z.; Qu, Y.; Shen, X.; Wang, Z.; Zhao, L.-D.; He, J. Strong Enhancement of Phonon Scattering Through Nanoscale Grains in Lead Sulfide Thermoelectrics. NPG Asia Mater. 2014, 6, No. e108.

which is able to modulate hole concentration and phonon scattering simultaneously. By combining iodine-doping and the water quench treatment, we achieved significant improvement in both peak ZT and average ZT, from 1.1 and 0.44 for pristine GeTe to 2.2 and 1.23 for WQ Ge0.93Te0.07Te1.005I0.03. The calculated ∼11.2% single-leg device efficiency (298−775 K) indicates that GeTe shall be an ideal candidate for thermoelectric power generation and cooling at intermediate temperatures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00057. XRD patterns, additional experimental data, and TEM images (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jiaqing He: 0000-0003-3954-6003 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This contribution was financially supported by the National Science Foundation of China (Grants 11504160, 11874194, and 51632005), the Natural Science Foundation of Guangdong Province (Grant 2015A030308001), and the Key Project of Natural Science Foundation of Shaanxi Province (2015JZ011), and partly by the leading talents of Guangdong Province Program (Grant 00201517). J.H. would also like to acknowledge the support by the Science, Technology and Innovation Commission of Shenzhen Municipality (Grants JCYJ201508311142508365, KQTD2016022619565991, and KQCX2015033110182370).

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REFERENCES

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ACS Applied Energy Materials (28) Schneider, M. N.; Biquard, X.; Stiewe, C.; Schroder, T.; Urban, P.; Oeckler, O. From Metastable to Stable Modifications-in situ Laue Diffraction Investigation of Diffusion Processes During the Phase Transitions of (GeTe)nSb2Te3 (6 < n < 15) Crystals. Chem. Commun. 2012, 48, 2192. (29) Kim, H. S.; Liu, W.; Chen, G.; Chu, C. W.; Ren, Z. Relationship between Thermoelectric Figure of Merit and Energy Conversion Efficiency. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 8205. (30) Kim, H. S.; Liu, W.; Ren, Z. Efficiency and Output Power of Thermoelectric Module by Taking into Account Corrected Joule and Thomson Heat. J. Appl. Phys. 2015, 118, 115103.

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DOI: 10.1021/acsaem.9b00057 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX