Combination of Carrier Concentration Regulation and High Band

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Combination of Carrier Concentration Regulation and High Band Degeneracy for Enhanced Thermoelectric Performance of Cu3SbSe4 Dan Zhang, Junyou Yang, Qinghui Jiang, Zhiwei Zhou, Xin Li, Jiwu Xin, Abdul Basit, Yangyang Ren, Xu He, Weijing Chu, and Jingdi Hou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08121 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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ACS Applied Materials & Interfaces

Combination of Carrier Concentration Regulation and High Band Degeneracy for Enhanced Thermoelectric Performance of Cu3SbSe4 Dan Zhang, Junyou Yang*, Qinghui Jiang*, Zhiwei Zhou, Xin Li, Jiwu Xin, Abdul Basit, Yangyang Ren, Xu He, Weijing Chu, Jingdi Hou

State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science & Technology, Wuhan 430074, PR China.

ABSTRACT The effect of Al-, Ga- and In-doping on the thermoelectric (TE) properties of Cu3SbSe4 has been comparatively studied on the basis of theoretical prediction and experimental validation. It is found that tiny Al/Ga/In substitution leads to a great enhancement of electrical conductivity with high carrier concentration and also large Seebeck coefficient due to the preserved high band degeneracy and thereby a remarkably high power factor. Ultimately, coupled with the depressed lattice thermal conductivity, all three elements (Al/Ga/In) substituted samples have obtained a highly improved thermoelectric performance with respect to un-doped Cu3SbSe4. In compared with the samples at the same Al/In doping level, the slight Ga-doped sample presents better TE performance over the wide temperature range, and the Cu3Sb0.995Ga0.005Se4 sample presents a record high ZT value of 0.9 among single doped Cu3SbSe4 at 623K, which is about 80% higher than that of pristine Cu3SbSe4. This work offers an alternative approach to boot the TE properties of Cu3SbSe4 by selecting efficient dopant to weaken the coupling between electrical conductivity and Seebeck coefficient.

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KEYWORDS: Cu3SbSe4; Al/Ga/In doping; carrier concentration; high band degeneracy; decoupling; thermoelectric properties

1. INTRODUCTION The R&D for alternative energy technologies have been continually promoted by the increasing global energy consumptions and resulted environmental issues.1,

2

Thermoelectric (TE) materials have drawn extensive attention as a key solution in recent years because they can realize the direct conversion between heat and electricity. The conversion efficiency of a TE device is decided by the material figure-of-merit ZT = S2σT/κ, where S, σ, κ and T are the Seebeck coefficient, electrical conductivity, thermal conductivity (Comprising the lattice contribution κL and the electronic part κe) and the absolute temperature, respectively. Most of the state-of-the-art TE materials that have high ZT value more or less include some toxic, rare, or expensive elements which potentially

hamper

their

commercialization

process.3-9

Therefore,

Cu-based

semiconductors have attracted great interests recently as a cheap and Pb-free robust TE candidate.10-12 Owing to advantages of large carrier effective mass (~1.1me) and small band-gap (~0.29eV),13 stannite semiconductor Cu3SbSe4 is stepping into sight as a cheap and Pb-free Cu-based promising TE material. Many efforts have been made to substitute Ge or Sn for Sb to improve its intrinsically poor electrical conductivity,13-17 however, less improvement has been obtained in TE performance mainly due to the simultaneously great diminution of Seebeck coefficient when doping with Ge or Sn, and the separation


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of the degenerate valence bands is the main cause for the rapid decrease of Seebeck coefficient in the heavy Ge/Sn-doped Cu3SbSe4 (Figure S1).18,

19

Therefore, it is a

challenge to optimize the interrelated electrical conductivity and Seebeck coefficient simultaneously for high performance Cu3SbSe4 TE material. Herein, we systematically explored the doping effect of M (M= Al, Ga, In) on TE properties of Cu3SbSe4 by means of theoretical prediction and experimental validation. After tiny efficient Al/Ga/In substitution, the electrical resistivity decreases obviously while the Seebeck coefficient stays high because almost no destruction to the high degeneracy of valence band,20 and the lattice thermal conductivity also decreases by the introduced point defects scattering to phonons. As a result, a greatly enhanced ZT value has been obtained over the wide temperature range for all the Cu3Sb1-xMxSe4 (M=Al, Ga, and In) samples, and the maximum ZT value approaches 0.9 at 623K in Cu3Sb0.995Ga0.005Se4, as well as we know, it is the highest value ever reported for single doped Cu3SbSe4-based TE materials at 623K.

2 EXPERIMENTAL SECTION 2.1 Synthesis. Cu3Sb1-xMxSe4 (x= 0, 0.005, 0.01, 0.015, 0.02; M= Al, Ga, In) samples were synthesized via high temperature melting, ball milling and hot-pressing method. Stoichiometric mixtures of elemental powders with high purity were firstly weighted and sealed in evacuated silica tubes, and heated at 1173K for 12 h, then cooled to 773 K in the furnace and quenched. The quenched ingots were directly annealed at 573K for 2 days to make them more homogeneous. After that, they were pulverized, sieved and ball



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milled. In the end, the obtained powders were hot-pressed at 673 K for 1h under 120 MPa. 2.2 Characterization and Measurement. The phase structure was characterized by X-ray diffraction (XRD) on a step scan mode with Cu Kα radiation (PANalytical, Almelo, Netherlands). The microstructure of the samples was analyzed by field-emission scanning electron microscopy (FESEM, NanoSEM 450 ) equipped with energy-dispersive X-ray spectroscopy (EDS). Transmission electron microscopy (TEM) observation was performed on a JEM-2100 microscope operated with an acceleration voltage of 200 kV. The simultaneous measurement of Seebeck coefficient and electrical resistivity was carried out on a commercial measurement system (Namicro-III, Wuhan, China) from room temperature to 623K. The thermal conductivity (κ) was calculated as the product of material density determined by the Archimedes’ method, thermal diffusivity measured by a laser flash method (LFA 427, Netzsch Instruments, Germany) and specific heat derived by differential scanning calorimeter (Diamond DSC, PerkinElmer Instruments, USA). Carrier concentration and mobility of the sample were measured on a Hall effect measurement system (HMS 5500, Ecopia, South Korea) under constant magnetic fields of ± 0.5 T. 2.3 Theory Calculation. The first-principles calculations have been carried out on the Vienna ab initio simulation package (VASP) using the Perdew-Burke-Ernzerhof (PBE) 21

generalized gradient approximation (GGA) and projector augmented wave (PAW) 22

pseudopotentials. Hubbard-like term (+U) was also introduced in order to deal with the localization of d-electrons on Cu atoms.23-25 A supercell model of 2×2×1 tetragonal unit


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cell was selected within our computation resources, a plane-wave cutoff energy of 400 eV, 6×6×6 Monkhorst-Pack k-point mesh and an energy conversion threshold of 1.0×10–5eV per atom were used for calculations.

3 RESULTS AND DISCUSSION 3.1 Electronic Structure Calculation and Analysis

Figure 1. Band structure of pristine (a) and doped Cu3SbSe4: (b) AlSb; (c) GaSb; (d) InSb.

As Al/Ga/In possesses one valence electron less than Ge/Sn, its substitution for Sb probably provide more p-type carriers in comparison to that of Ge/Sn. Hence, the electronic structure of Al-, Ga- and In-doped Cu3SbSe4 were calculated and the results were shown in Figure 1-2. As shown in Figure 1a, the undoped Cu3SbSe4 presents three-fold degenerate bands at Gamma point, which results in the intrinsically large Seebeck coefficient.24 It is shown that Al, Ga and In tend to occupy Sb site in Cu3SbSe4 due to the smallest calculated forming energy than those of other sites (Cu & Se).25, 26



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The Fermi level moves down-ward into valence band on doping with Al/Ga/In (Figure1b-d), thus resulting in higher hole concentration and the enhanced electrical conductivity in Cu3SbSe4. However, the above degenerate valence bands diverge in heavy doped Cu3SbSe4 which will certainly lead to the diminution of Seebeck coefficient, and the similar split of degenerate bands has also been reported in other heavy doped Cu3SbSe4.17 Thus, selecting highly efficient dopants to greatly lower the substitution concentration should be a feasible attempt to improve the electrical conductivity while maintain the large Seebeck coefficient with little divergence of degenerate bands. Moreover, our theoretical calculation results (Figure 1-2 and Figure S1) also indicate that Al, Ga and In dopants are more efficient in contributing carriers in Cu3SbSe4 with respect to the widely used Ge/Sn elements, because they shift the Fermi level deeper into the valence band even at the same doping level. Therefore, Al/Ga/In substitution is implemented to enhance the electrical properties and thereby improve the TE properties of Cu3SbSe4 in the following sections.

Figure 2. The change of DOS after doping with Al/Ga/In/Ge/Sn.


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3.2 Effect of Ga-doping content on the Electrical and Thermal Transport Properties of Cu3SbSe4

Figure 3. Powder XRD patterns of Cu3Sb1-xGaxSe4 samples (a), a representative Rietveld refinement result using X-ray diffraction data from matrix (b), the results of lattice parameters (c), and room temperature carrier mobility µ & carrier concentration nh as a function of the Ga-doping content (d).

Considering the smallest difference in electronegativity and moderate ionic radius difference between Ga (χ(Ga) = 1.81, r(Ga3+) = 0.47Ǻ) and Sb (χ(Sb) = 2.05, r(Sb5+)= 0.6Ǻ) in compared with Al (χ(Al) = 1.68, r(Al3+)= 0.39Ǻ) and In (χ(In) = 1.78, r(In3+)= 0.62Ǻ), Ga probably has higher solid solubility in Cu3SbSe4,27 thus Ga-doping is firstly studied. Figure 3a shows the powder XRD pattern of Ga-doped Cu3SbSe4 samples. One can observe that all the diffraction peaks match very well with Cu3SbSe4 (No. 01-085-0003) and no visible impurity phase appears within detection limit, revealing all the Ga-substituted samples are single phase Cu3SbSe4. The calculated lattice parameters from Rietveld refinement on Fullprof software28 (Figure 3c) decrease when Ga enters



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into Sb site because the smaller ionic radius of Ga than Sb and the resulted shrinkage of unit cell. Noticeably, they level off when x(Ga) exceeds 0.015, indicating that the solution limit of Ga in Cu3SbSe4 is around 0.015. Moreover, Figure 3d shows that Ga can contribute approximately 1.5 holes per atom at low doping content, and the carrier concentration (nh) saturates when x exceeds the solution limit. At the same time, the carrier mobility (µ) decreases after Ga-doping due to the extra ionized impurity scattering.

Figure 4. FESEM images of x(Ga) = 0.01 (a) and the corresponding elemental EDS mapping (b-e), the enlarged image (f) of selected region in (a), and the TEM image (g) with an inserted high-resolution transmission electron microscopy (HRTEM) picture.


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Figure 5. Temperature dependent thermoelectric properties for the Cu3Sb1-xGaxSe4 (x = 0, 0.005, 0.01, 0.015, 0.02) bulk samples: (a) electrical resistivity, (b) Seebeck coefficient, (c) power factor, (d-e) total and lattice thermal conductivities, and (f) ZT value.

The microstructure of the x(Ga) = 0.01 sample is analyzed by FESEM and TEM (Figure 4). The multi-scaled grains in bulk sample are beneficial to suppress lattice thermal conductivity by integrating phonon scattering in wide range (Figure 4a).5, 20 Elements of Cu, Sb, Ga and Se are found to have a homogeneous distribution in the EDS mapping images (Figure 4b-e), which implies Ga has doped into Cu3SbSe4 uniformly without



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obvious second phase aggregate. The nano-scaled particles can be clearly observed in the enlarged SEM and TEM images (Figure 4f-g), which were confirmed as Cu3SbSe4 by HRTEM lattice image. Figure 5 displays the TE properties of Cu3Sb1-xGaxSe4 (x = 0, 0.005, 0.01, 0.015, 0.02) bulk samples. As shown in Figure 5a-b, both the electrical resistivity (ρ) and Seebeck coefficient (S) increase with the rising of temperature after Ga doping, indicating the Ga-doped samples have changed from the intrinsic semiconductor of pristine Cu3SbSe4 into a degenerate semiconductor and this consists well with the results of first-principles calculations (Figure 1c). The ρ and S are decreased as the fraction of Ga increases mainly due to the increased carrier concentration (Figure 3d), and the reduction in ρ for the doped samples is very remarkable as compared with un-doped Cu3SbSe4 due to the efficient p-type doping of Ga, for example, it decreases from 195.88µΩ.m (x = 0) to 58.54µΩ.m (x = 0.005) only with tiny amount of Ga substitution. As a consequence, the power factor has greatly improved in the wide temperature region (Figure 5c) for the Ga-doped samples and the maximum one is up to 1312.73µWm-1K-2, however, its enhancement starts to decline when x(Ga) is above 0.005 mainly due to the diminution of S from the split of the degenerate bands at higher doping concentration (Figure 1c). As described in Figure 5d, the thermal conductivity (κ) decreases slightly after Ga-doping, and the lattice thermal conductivity (κL) obtained by subtracting the carrier contribution κe from κ was also presented in Figure 5e, where κe is estimated using the Wiedemann-Franz law as the way in our previous work,12, 29 one can observe that κL decreases after doping with Ga due to the increased point-defects scattering of phonons,


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and it agrees with the relationship of κL ∝ 1/T, which suggests that the dominant scattering in phonon transport is the acoustic phonon-phonon Umklapp scattering. Finally, a peak ZT value of 0.90 at 623K is achieved for the x(Ga) = 0.15 sample, which is about 80% higher than that of pristine Cu3SbSe4. 3.3 Distinct Impact of Al, Ga and In Substitution on the Thermoelectric Properties of Cu3SbSe4

Figure 6. Powder XRD spectrums of Cu3Sb1-xMxSe4 (M= Al, Ga and In) samples (a) and the results of lattice parameters from Rietveld refinement (b).



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Figure 7. Temperature dependent thermoelectric properties for the Cu3Sb1-xMxSe4 (x = 0.005, 0.01;M = Al, Ga, In) bulk samples: (a) electrical resistivity, (b) Seebeck coefficient, (c) power factor (PF), (d) change of Seebeck coefficient with Hall carrier concentration at room temperature (i. e. Pisarenko curve), (e) total and lattice thermal conductivities, and (f) ZT value. The total thermal conductivities are displayed in the inserted figure of 7(e).

As the solid solubility of Ga is very low and high Ga doping density can quickly diminish the Seebeck coefficient in Cu3SbSe4, the substitution concentration of other same group elements (Al and In) is designed below 0.01. Figure 6a displays the powder XRD pattern of the bulk Al-, Ga- and In-doped Cu3SbSe4 samples. All appeared


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diffraction peaks exhibit good match with the Cu3SbSe4 (No. 01-085-0003) and no visible impurity phase can be observed within detection limit, indicating all the bulk Al/Ga/In-doped Cu3SbSe4 samples are single phase. The refined lattice parameters are also shown in Figure 6b. The smallest change of lattice parameters in Al-doped samples shows the effective Al doped into the matrix lattice is lowest, thus, Al has lowest solid solubility in Cu3SbSe4 mainly due to the largest ionic radius and electronegativity difference between Sb; Element In should also has lower solid solubility than that of Ga as the increase in In-doping concentration from 0.005 to 0.01 makes tiny change in lattice parameters, which can be ascribed to the relatively larger electronegativity difference between In-Sb as comparison to that of Ga-Sb. Therefore, their solid solution limit in Cu3SbSe4 are in sequence of Ga > In > Al.

Figure 8. The ZT values at 623K for the present Cu3Sb0.995A0.005Se4 (A= Al, Ga, In) and other single doped Cu3SbSe4.

The TE properties of Al- Ga- and In-doped Cu3SbSe4 are compared and presented in Figure 7. One can see that all doped samples have become degenerate semiconductor



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(Figure 7a-b) and agrees well with our theoretical calculations (Figure 1b-d). The values of ρ and S in doped samples are in order of Al > In > Ga, which consists with their corresponding solid solubility in Cu3SbSe4 and the resultant carrier concentration. The greatly decreased ρ and relatively large S for all doped samples enable the remarkable enhancement in power factor (Figure 7c). The S versus nh curve in Figure 7d clearly gives us an insight into the advantage of the highly efficient Al/Ga/In substitution compared with the widely used Ge/Sn doping: 0.5 percent of Al/Ga/In substitution can provide comparable or even higher carrier concentration than that of one percent Ge-doping and also has little deviation from the Pisarenko curve due to the nearly non-changed band structure.30,

31

The preserved high degeneracy for the x = 0.005

samples, which can be indirectly reflected from the Pisarenko curve, is beneficial to large Seebeck coefficient. At the same time, the high doping concentration, such as x(Ga)= 0.015/0.02; x(Ge)= 0.01/0.02 and x(Sn)= 0.025/0.05, make the S deteriorate rapidly with obvious deviation from the Pisarenko curve due to severe split of the degenerate bands at VBM (Figure 1 and S1).32 In addition, the calculated κL decreases after doping with Al/Ga/In because of the increased point-defects scattering to phonons and thus κ also has a slight decline. As a consequence, the ZT values of Al/Ga/In substituted samples obtain great improvement in wide temperature range with respect to un-doped matrix, and the maximum one increases from 0.83 for Cu3Sb0.995Al0.005Se4 to 0.88 for Cu3Sb0.995In0.005Se4 and then to 0.90 for Cu3Sb0.995Ga0.005Se4 sample. As compared in Figure 8, to the best of our knowledge, the peak ZT value of 0.9 is the highest value for single doped Cu3SbSe4 at 623K in literature13,14,16,17,33 and even higher


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than the maximum ZT value at 650K of Ge-S co-doped Cu3SbSe4.15 Moreover, considering the high thermal conductivity in Al/Ga/In doped Cu3SbSe4, there is still plenty of room to further boot the TE performance of this promising Pb- & Te-free TE material by phonon engineering, such as nano-structuring,34 introduction of nano-sized second phase.35-37

CONCLUSION In summary, the detailed effect of Al-, Ga- and In- doping on the TE properties of Cu3SbSe4 has been comparatively studied by combining theory and experiment. We demonstrate in this intrinsically high band degeneracy Cu3SbSe4 compound that tiny Al/Ga/In substitution (x= 0.005) leads to an enhanced electrical conductivity with high carrier concentration, a large Seebeck coefficient with preserved high band degeneracy and thereby a remarkably high power factor.

As compared the samples at the same

Al/In doping level, slight Ga-doped sample has better TE performance in wide temperature range mainly due to the high electrical conductivity from the high solid solubility in Cu3SbSe4, and the achieved maximum ZT values of 0.9 at 623K is about 80% higher than that of pristine Cu3SbSe4, and is also the reported highest ZT value for single doped Cu3SbSe4 at the same temperature in literature.

ASSOCIATED CONTENT Supporting Information Band structure of the doped Cu3SbSe4: (a) GeSb, (b) SnSb. This material is available free of charge via the Internet at http://pubs.acs.org.



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AUTHOR INFORMATION Corresponding Author *Email: [email protected] *Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is co-financed by National Natural Science Foundation of China (Grant NO. 51572098 and 51632006), National Basic Research Program of China (Grant NO. 2013CB632500), Natural Science Foundation of Hubei province (Grant NO. 2015CFB432), Open Fund of State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology (NO. 2016-KF-5). The technical assistance from the analytical and testing center of HUST is likewise gratefully acknowledged.

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Regulation

of

Electrical

and

Thermal

Transport

Properties

to

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Table of Contents Graphic 33x13mm (300 x 300 DPI)


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Combination of Carrier Concentration Regulation and High Band

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