Sb Codoping for the Tuning Carrier Concentration and

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Cu/Sb Codoping for Tuning Carrier Concentration and Thermoelectric Performance of GeTe-Based Alloys with Ultralow Lattice Thermal Conductivity Luo Yue,† Teng Fang,‡ Shuqi Zheng,*,† Wenlin Cui,† Yue Wu,† Siyi Chang,† Lijun Wang,†,§ Pengpeng Bai,⊥ and Huaizhou Zhao¶ †

State Key Laboratory of Heavy Oil Processing, College of Science, China University of Petroleum, Beijing 102249, PR China School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China § Materials Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia ⊥ Department of Mechanical Engineering, State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, PR China ¶ Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, PR China

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S Supporting Information *

ABSTRACT: Pristine GeTe shows promising thermoelectric performance but is limited by the high carrier concentration (nH) from Ge vacancies and thermal conductivity. Herein, Cu/Sb was chosen as codopants to suppress the high nH and to decrease thermal conductivity. In this condition, a promising zT of ∼1.62 under 773 K was acquired in the Ge0.85Te(CuSb)0.075 system proposed in this paper/work. Results show that as the dopant concentration increases, the power factor rises due to the reduction of the nH to ∼1 × 1020 cm−3. Apart from this, the total thermal conductivity also declines from ∼7.4 W m−1 K−1 to ∼1.59 W m−1 K−1 originating from an ultralow lattice thermal conductivity, in which the multiscatter mechanism from grain boundaries and point defect disperses the frequency phonons differently. The findings in this paper combine thermal and electronic strategies and lay the foundation to develop Pb-free thermoelectric materials. KEYWORDS: thermoelectric materials, Cu/Sb codoped GeTe, Seebeck coefficient, zT value, carrier concentration, ultralow lattice thermal conductivity, multiscatter mechanism in power generation24 and solar energy conversion (photovoltaic-thermoelectric).25 However, lead telluride (PbTe) cannot be widely used due to the toxicity of Pb.26 Considering the similarity in band structures, germanium telluride (GeTe) as a Pb-free thermoelectric (TE) material shows a promising thermoelectric performance.27−29 GeTe materials attract increasing attention in the fields of thermoelectricity and ferroelectricity.30,31 A phase transition of GeTe materials takes place from rhombohedral structure at room temperature to cubic structure at high temperature of approximately 700 K.32 The high thermoelectric performance at room temperature is due to its unique conduction and high-degeneracy Σ bands. The high-temperature structure resembles those in other IV

1. INTRODUCTION Thermoelectric materials convert heat into electricity.1−3 Thermoelectric material is a clean and reliable technology to convert energy under special circumstances with the advantages of less moving parts and noise pollution. The performance of thermoelectric materials is evaluated by a dimensionless figure of the merit zT = (S2σ/κtot) T, where S, σ, κtot, and T represent the Seebeck coefficient, the electrical conductivity, total thermal conductivity, and the absolute temperature.4 Two strategies have been enhanced the zT value. One method is to boost power factor by introducing resonant state doping, band engineering, phase transition, and charger carrier scattering modification.5−9 The other method is to reduce the κ value by introducing point defects such as substitutions, nanostructuring, hierarchical architecturing, and interstitials.10−17 Thermoelectric materials with excellent thermoelectric properties (such as lead telluride,18,19 germanium telluride,20 and bismuth telluride21−23) have a wide range of applications17 © XXXX American Chemical Society

Received: December 19, 2018 Accepted: March 25, 2019 Published: March 25, 2019 A

DOI: 10.1021/acsaem.8b02213 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Energy Materials

Figure 1. (a) XRD patterns of the codopedGeTe samples; (b) lattice parameters and Hall measurement results of the carrier concentration at 300 K.

2. MATERIALS AND METHODS

monotellurides, in which an energy separation exists between the heavy and light bands.33 Considering the intrinsically high Ge vacancies, pure GeTe shows a high carrier concentration,33 thereby leading to a high σ value but large total thermal conductivity under room temperature and hence resulting in a lower thermoelectric performance. Previous studies focused on increasing the zT value of GeTe, and therefore, TE performance was improved by decreasing the value of nH. Pei et al.33 first reported the optimal carrier concentration of cubic phase GeTe (∼1−3 × 1020 cm−3) and obtained a striking improvement in the zT of ∼1.7 through Pb and Sb doping at the Ge position by tuning nH and alloying GeTe with Se to decrease the thermal conductivity.34,35 Yang et al.36 synthesized Ge1−xInxTe to achieve an excellent zT of ∼1.3 under ∼628 K by introducing the resonance level, and Hong et al.37 acquired zT of ∼2.4 at ∼650 K by In/Sb codoped to optimize carrier concentration. Investigations of Pei et al.38 showed that Pb/Bi codoping can realize the converging of the slight symmetry and simultaneously decline the lattice thermal conductivity with an ideal zT of 2.4 at 600 K. Meanwhile, Pei et al. doped Pb and Sb to achieve a superior thermoelectric properties because of tuning carrier concentration and regulating lattice distorting.39 Zheng et al.9 synthesized Ge1−−‑yMnxSbyTe compounds by changing the phase transition temperature and tuning nH (∼6 × 1020 cm−3), thereby showing an excellent zT of ∼1.61 under 823 K. These studies above indicate that optimizing carrier concentration (1∼3 × 1020 cm−3) could effectively improve the thermoelectric properties of GeTe-based materials. Chen et al. and Pei et al. introduced Cd tuning band convergence and introduced Bi to optimized carrier concentration realizing an excellent thermoelectric performance.40,41 Pei et al. found the substitution of three Ge2+ by two Sb3+ leaves one Ge vacancy could reduce the hole concentration and lattice thermal conductivity.42 Chen et al. tuned carrier concentration by Sb doping with Zn-alloying induced band alignment to realize an extraordinary thermoelectric.43 This paper studies thermal and electric schemes to improve the GeTe thermoelectric performance. A series of codoped Ge1−2xTe(CuSb)x (x = 0.025, 0.05, 0.075, 0.10) compounds were prepared through the traditional solid reaction. The synergistic effect of Cu/Sb codoped on GeTe was also investigated, that is, Cu doping tuned the nH, and valence band convergence was achieved by Sb doping in the GeTe. Apart from this, the role of Cu/Sb ions in the electrical and multiscatter effects caused by the point defect and grain boundaries was also revealed.

All pellets with the composition Ge1−2xTe(CuSb)x were stored in the high-vacuum (∼10−4 Pa) evacuated quartz tube on high-purity constituent elements including Ge (99.99%), Te (99.999%), Cu (99.99%), and Sb (99.99%). The mixture was heated at 723 and 1273 K for 12 and 6 h, respectively. After maintaining at 1273 K for 6 h, all pellets were quenched into water and annealed for 3 days. Then the ingots were crushed and ground in an agate mortar to obtain fine powders. Then the powder was enclosed into the die and compacted by spark plasma sintering (SPS, SPS-211Lx, Fuji Electronic Industrial Co., Ltd.) at 773 K for 5 min, thereby resulting in dense pellets with a diameter of 12.7 mm. The phase structure was analyzed by room-temperature X-ray diffraction (XRD) with Cu Kα (λ = 1.5406 Å) radiation on a Bruker D8 diffractometer ranging from 20° to 80°. The lattice parameters of all compounds were calculated through Rietveld refinement, and the microscopic morphology was measured by field emission scanning electron microscopy (SEM, Hitachi SU-8010) and transmission electron microscopy (TEM, FEI, Tecnai G2 F20). Measurements of σ and S values of all pellets were performed by a Namicto-III S/electric resistance measuring system (Namicro-3L, Wuhan Joule Yacht Science & Technology Co., Ltd.) in a He atmosphere from 323−773 K. Thermal diffusivity (D) was directly tested by a laser flash diffusivity method, and the κtot value was calculated by κtot = DCpd. The specific heat capacity (Cp) was measured by DSC STA 409PC Netzsch (Figure S1), and d was the density determined by the Archimedes principle. The roomtemperature Hall coefficient was tested through Ecopia HMS-3000 by using the four-contact probe method in a magnetic field (±0.55 T). Density of states (DOS) and defect formation energy were performed via the density functional theory (DFT) method by using the Vienna ab initio simulation package (VASP).44 The projector augmented wave (PAW) method45 and Perdew−Burke−Ernzerhof46 with generalized gradient approximations (GGA-PBE) were served as the exchange−correlation energy. To perform self-consistent calculations of the plane wave cutoff energy (500 eV) and the electronic convergence criterion (10−5 eV), Monkhorst−Pack method with 7 × 7 × 7 k-mesh was used. Spin−orbit coupling effect was considered with the magnetic moment of 0.6 (z axis).

3. RESULTS AND DISCUSSION 3.1. Phase Composition. Figure 1a indicates the XRD patterns of the Ge1−2xTe(CuSb)x (x = 0, 0.025, 0.050, 0.075, and 0.100) bulks. All the diffraction peaks are able to match the standard PDF card (JCPDS card no. 47−1079) and index on the pure GeTe phase. Different ionic size between Cu1+, Sb3+, and Ge2+ could cause distortion of lattice parameters. The lattice parameters expand linearly with increasing Cu/Sb content and subsequently remain stable when the Cu/Sb content is higher than ∼75% (Figure 1b). Therefore, a left-shift B


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Figure 2. SEM images of the free fractured surface of the Ge1−2xTe(CuSb)x bulks; (a) Ge0.95Te(CuSb)0.025, (b) Ge0.90Te(CuSb)0.05, (c) Ge0.85Te(CuSb)0.75, and (d) Ge0.80Te(CuSb)0.1. (e) Polished surface of Ge0.80Te(CuSb)0.1 pellet and (f) EDS mapping of the main elements of panel e.

of the diffraction peaks for the Cu/Sb codoped samples can be seen in the inset in Figure 1a. The existence of a small part of Ge impurity phases is present in Ge1−2xTe(CuSb)x (x = 0.025, 0.050) compounds, which is due to the reduced solubility of Ge after Cu\Sb codoping. Similar problems have been encountered in Gelbstein’s and Biswas’s studies.47−49 Meanwhile, the intensity of the peaks gradually shrinks, and the peaks of (024) and (220) gradually merge, which indicate the presence of β-GeTe. The results demonstrate a structural transition from rhombohedral to cubic upon Cu/Sb doping. 3.2. Microstructure Characterization. To further determine the phase composition and illustrate the microstructure, the fracture SEM and EDS analyses were conducted in Figure 2. The grain size of all Cu/Sb codoped pellets is approximately 1.5−5 μm. As the doping content increases, the grain size considerably decreases and forms a lamellar structure. The lamellar structure could facilitate the scattering of various frequency phonons to reduce the κlat (discussed in the following parts). Figure 2e shows the microscopic morphology of the sintered Ge0.80Te(CuSb)0.1 pellet after polishing, and the EDS maps exhibit a homogeneous distribution of the elements in Figure 2f. To further clarify the special structure, TEM images of Ge0.8Te(CuSb)0.1 are presented in Figure 3. Figure 3a and b are the typical low-magnification TEM and high resolution transmission electron microscope (HRTEM) images of the edge section with a herringbone structure, respectively.48 The two lattice planes (202) and (220) with the interplanar distances of 0.234 and 0.154 nm are clearly observed from the GeTe rhombohedral structure. The inset is a fast Fourier transformed diffraction pattern, showing a pseudocubic structure for lack of spot splitting.49 The reversed FFT image of the white square area in Figure 3b is displayed in Figure 3c, from which the grain boundaries are clearly shown. The grain boundaries can scatter long-wavelength phonons50 together with point defect scattering by Cu/Sb codoping, which is beneficial to decreasing the κlat.

Figure 3. (a) Low-magnification TEM images of the Ge0.80Te(CuSb)0.10 powder sample. (b) HRTEM image shows grain boundaries and (c) reversed FFT images from the marked square area.

3.3. Electrical Transport Properties. Figure 4a depicts the temperature-dependent σ of the Ge 1−2x Te(CuSb)x samples. The σ values of all pellets initially decrease and then increase when temperature goes up, thus exhibiting a partially degenerated semiconductor behavior. Compared with pristine GeTe sample, the σ values of all the Cu/Sb-doped samples are obviously lower, which is due to the significant decrease in the nH. As the Cu/Sb content increases, the σ value has a significant reduction trend. It is obvious that at 300 K, the σ value drops from 7000 S cm−1 in pristine GeTe to 540 S cm−1 in Ge0.80Te(CuSb)0.10. The reduction in σ results from the significant decrease of carrier concentration (nH, from 8.47 × 1020to 1.87 × 1020 cm−3), which is attributed to the Sb ions obtained the trivalent states and the Cu ions obtained the monovalent states (which will be discussed later). Figure 4b depicts the plot of the temperature-dependent S. All Ge1−2xTe(CuSb)x shows typical p-type semiconductors verified by the positive S and indicate a degenerated semiconducting behavior with an evident slope in the phase transition region and an increase in the S value with increasing Cu/Sb contents. At room temperature, the S values for C


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Figure 4. Electrical thermoelectric characteristics of Ge1−2xTe(CuSb)x (a) electrical conductivity, (b) Seebeck coefficient, and (c) power factor.

alloys (Pisarenko relationship) are shown in Figure 6.33−37,51−55 The two-valence band structure (unique domi-

pristine, Ge0.90Te(CuSb)0.05 and Ge0.85Te(CuSb)0.075 are 27, 89, and 111 μV K−1 due to the reduction in nH and the increase in DOS near Fermi level, as shown in Figure 5. This

Figure 6. Room-temperature Hall carrier concentration-dependent (a) Seebeck coefficient (Pisarenko relationship) and (b) Hall mobility in comparison with literature data on doped GeTe.

Figure 5. Computed density of states (DOS) for (a) codoped and pristine room-temperature R-GeTe and (b) codoped and pristine high-temperature C-GeTe.

nant heavy Σ band) with acoustic scattering enables the prediction of the nH-dependent S, thereby indicating a rigid band approximation. Thereby, the optimal nH of the GeTe ranges from 1 × 1020 cm−3 to 3 × 1020 cm−3. The codoped systems significantly increase the S value at the cost of reducing the Hall nH, indicating that the materials with Cu/Sb codoping are the most promising for the thermoelectric performance. For instance, the S value of the Cu/Sb codoped pellets is higher than that of Sb- or Bi-doped materials with the same nH. Considering the high dopant concentration, codoping samples show a decreased mobility at room temperature, as shown in Figure 6b. Given the redistribution of nH from L and Σ bands and the increased density of ionized impurities with increasing contents, the mobility of the codoped GeTe materials is significantly reduced. On the condition of the same doping content of Ge1−xSbxTe, the nH values of Ge0.95Sb0.05Te and Ge0.90Te(CuSb)0.05 are ∼3.99 × 1020 and ∼2.96 × 1020 cm−3, respectively. Thus, Cu and Sb are considerably effective in reducing the nH because Sb ions obtain the trivalent states and Cu ions obtained the monovalent states. The electrical fundamentals of Cu\Sb in reducing nH is unclear but can be estimated by forming pairs of Ge2+-Ge2+substituted by the Cu1+-Sb3+ units.56,57 The reduction of nH indicates elevated formation energy of the Ge vacancies, as proven by DFT. The point defect formation energies in GeTe could be calculated as follows:39

result is mainly attributed to the higher d-partial DOS of Cu (Figure S2). Two peaks could be found in the temperaturedependent S among our samples. The first peak is caused by the redistribution of the nH from the Σ and L bands.33 Given the high mobility and small DOS of the effective mass derived from L band,8 both the σ and S are influenced. With the increase of content, the structure of GeTe gradually changes from rhombohedral to cubic, which influences the band structure. Compared with α-GeTe, the L band of β-GeTe is higher. Because of the difference in the band structure, the carrier concentration changes lead to the increase of Seebeck coefficient. As the doping content increases, it is more characteristic of β-GeTe (which has been discussed in XRD experiments), and thereby the first peak shifts left.33 The second peak is attributed to the rearrangement in the nH because of the intrinsic carriers formed by bipolar diffusion. Considering the increased contribution of minority carriers and the activation energy at high temperature, bipolar diffusion exhibits an adverse effect on S. The temperature-dependent plots of the power factors (S2σ) are revealed in Figure 4c. The power factors first increase and then decrease with the elevated contents. The pristine has the power factor of ∼5.5 μW cm−1 K−2 at 300 K, which goes up to a maximum of ∼39.34 μW cm−1 K−2 at 723 K. After codoped with Cu/Sb, the power factors increase due to the optimum nH (1 × 1020 cm−3 to 3 × 1020 cm−3, discussed later). In addition, it is shown that the thermoelectric properties are isotropic in the GeTe-based materials (Figure S3). To elucidate the relationship between the nH and electrical properties, the S versus nH plots of the Cu/Sb codopedGeTe

Ef (VGe) = Etot (VGe) + E(Ge bulk ) − Etot

(1)

Where Etot and Etot(VGe) represent the total energies of the supercell before and after the introduction of an additional Ge vacancy, respectively; E(Gebulk) is the energy of a single Ge D


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(CuSb)0.10 decreases to ∼1.59 W m−1 K−1 at room temperature. The lowest κtot value of ∼1.21 W m−1 K−1 is obtained in Ge0.80Te(CuSb)0.10 at 523 K. The D, L, and κlat are indicated in Figure 8b−d, respectively. The calculated κlat by deducting the electronic contribution (κele) from κtot calculated to form the Wiedemann−Franz law shown as follows:

atom in its ground state bulk phase. As shown in Figure 7, the Ge vacancy of pristine GeTe exhibits the lowest formation

κele = LσT

(2)

where L, σ, and T represent the Lorenz number, the electrical conductivity, and the absolute temperature, respectively. The Lorenz number is calculated using the following equation:58 É ÄÅ Å −|S| ÑÑÑ ÑÑ L = 1.5 + expÅÅÅÅ ÅÅÇ 116 ÑÑÑÖ (3) The debasement of κele results from the decreased σ value (Figure 4b), whereas the decreased κlat caused by the enhanced phonon scattering. The κlat value decreases to ∼0.19 W m−1 K−1, even lower than the theoretical minimum. Two main reasons can explain the reduction in the κlat value in the Cu/Sb codoped samples. First, the Cu/Sb codopants could cause possible mass fluctuations and increase the point defects, thereby enhancing the phonon scattering to the long-frequency (short wavelength) phonons.59−62 Second, the low κlat value is ascribed to the scattering of the long-wavelength phonons from grain boundaries in the Cu/Sb codoping GeTe compounds, as shown in Figure 3. The multiscatter mechanism can scatter different phonon wavelengths and thereby significantly reduce the κlat value. To study the role of each phonon scattering source on decreasing κlat, the phonon transport was investigated applying the Debye−Callaway model based on numerical calculation. In this calculation, the different phonon scatterings of Umklapp processes (U), nanoprecipitates (NP), planar vacancies (PV), grain boundaries (B), electron−phonon scattering (E), and point defects (PD) were considered. The spectral lattice thermal conductivity (κs) is shown in Figure 9, taking into

Figure 7. Formation energy of Ge and Te vacancies in pristine GeTe (red) and that of Ge vacancy in Ge25CuTe27, Ge25SbTe27, and (GeTe)1−x(CuSb)x alloys (gray).

energy, indicating Ge vanacy is the main source of concern carriers in pristine compounds. The Ge vacancy of the point defect formation is higher compared with Cu and Sb singly doped samples from 0.884 to 3.362 and 1.3873 eV, thereby resulting in hard forming hole carrier, respectively. As Cu/Sb codoped contents rise, the point defect formation energy gradually increases, resulting in a decreased nH. This results are consistent with that of Hall carrier measurement. Thus, the high point defect formation energy by Cu/Sb codoping compared with a single dopant indicates that Cu has a positive effect on GeTe-based alloys. 3.4. Thermal Transport Properties. The temperaturedependent κtot of Ge1−2xTe(CuSb)x (x = 0, 0.025, 0.05, 0.075, and 0.1) compounds is described in Figure 8a. The κtot value was negatively correlated with temperature. The κtot value of pristine GeTe is ∼7.6 W m−1 K−1, while that of Ge0.80Te-

Figure 9. Calculated spectral lattice thermal conductivity (κs) taking account into various phonon scattering mechanisms including U, E, B, and PD.

account of phonon scattering with various mechanisms including U, E, B, and PD. The integration of κs with respect to the phonon frequency (ω) equals to κlat. As can be seen, grain boundaries and point defects cause quite strong phonon scatterings, accounting for the obtained low κlat. On the basis of eqs S1−S15 and the regarding parameters in Table S1, the κs for our sample by considering various phonon scatterings has been calculated. On the basis of the discussions above, it is safe to conclude that the multiscale scattering enhances the scattering of phonons with wide frequencies leading to the significant reduction in κlat for the samples.

Figure 8. Thermal thermoelectric characteristics of Ge1−2xTe(CuSb)x between 300 and 780 K. (a) Total thermal conductivity, (b) thermal diffusivity, (c) Lorenz number, and (d) lattice thermal conductivity. E


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Figure 10. (a) Temperature-dependent zT values. (b) Average zT values (300−773 K) for Ge1−2xTe(CuSb)x.

3.5. Figure of Merits. Figure 10 indicates the temperaturedependent zT values of the codoped Ge1−2xTe(CuSb)x (x = 0, 0.025, 0.05, 0.075, and 0.10). Given the simultaneous elevation in the thermopower and the decline in the thermal transport, significantly increased zT value compared with that of the pristine GeTe can be obtained. With the increasing dopant concentrations, the zT values of all codoped samples first increase and then decrease when the temperature continuously goes up. This trend is mainly caused by bipolar diffusion from the intrinsic thermal excitation at high temperature. The maximum zT value is 1.62 at 773 K for Ge0.85Te(CuSb)0.075 system. The excellent zT value is mainly attributed to the cowork forming the promotion of electric properties and reduction of the thermal properties. The average zT value determines the efficiency of the thermoelectric device in practical applications for the wide working temperature. The results compare the average zT values of several GeTe-based alloys from 300 to 773 K. The maximum zTave is 0.85 in Ge0.85Te(CuSb)0.075, which is higher than those of (Ge 0 . 8 0 Pb 0 . 2 0 ) 0 . 9 0 Mn 0 . 1 0 Te (0.65), 6 3 Ge0.90Sb0.10Te (0.70),10 Ge0.95Mn0.05Te (0.70),64 Ge0.90Bi0.10Te (0.71),33 and Ge0.98In0.02Te (0.75).36

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86 010 89733200. Fax: +86 010 89733973. ORCID

Shuqi Zheng: 0000-0003-1549-8000 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of China (No. 51871240) and the National Postdoctoral Program for Innovative Talents (No. BX201700132). Calculations were carried out in Shenzhen Cloud Computing Center, National Supercomputing Center, Shenzhen, China (Shenzhen Cloud Computing Center). The authors are grateful for helpful discussions with Xiangli Wen and Bingwei Luo.

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4. CONCLUSIONS A series of Cu/Sb codoping GeTe compounds were prepared, which suppressed the intrinsically high nH and distorted the DOS near the Fermi level, thereby resulting in a promising improvement of power factors. The electrical fundamentals of the Cu/Sb forming pairs of Ge2+−Ge2+ substituted by the Cu1+-Sb3+ units were investigated as well. Results show that an increase in power factor from ∼30 μW cm−1 K−2 to ∼39.34 μW cm−1 K−2. Cu/Sb codoping could introduce alloy and grain boundary scatters. This phenomenon is beneficial to scattering various phonon spectra and thereby result in a decline in the speed of sound, an extremely low thermal conductivity, and an extremely low κlat. Consequently, the Ge0.85Te(CuSb)0.075 samples exhibit an excellent zT value of ∼1.62 under 773 K.

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c-GeTe; calculation for phonon scattering sources by using Debye-Callaway model (PDF)

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* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b02213. Measured Cp and anisotropy on transport properties; computed PDOS near top of Fermi level for r-GeTe and F


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