Hot-Injection Synthesis of Cu-Doped Cu2ZnSnSe4 Nanocrystals to

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Hot-Injection Synthesis of Cu-Doped Cu2ZnSnSe4 Nanocrystals to Reach Thermoelectric zT of 0.70 at 450 °C Dongsheng Chen, Yan Zhao, Yani Chen, Biao Wang, Yuanyuan Wang, Jun Zhou, and Ziqi Liang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 25 Oct 2015 Downloaded from http://pubs.acs.org on October 26, 2015

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ACS Applied Materials & Interfaces

Hot-Injection Synthesis of Cu-Doped Cu2ZnSnSe4 Nanocrystals to Reach Thermoelectric zT of 0.70 at 450 °C Dongsheng Chen,†,ǁ Yan Zhao,†,⊥ Yani Chen,† Biao Wang, ‡Yuanyuan Wang,§ Jun Zhou,*,‡ and Ziqi Liang*,† †

Department of Materials Science, Fudan University, Shanghai 200433, China

‡

Center for Phononics and Thermal Energy Science, School of Physics Science and

Engineering, Tongji University, Shanghai 200092, China ⊥

Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210,

China §

School of Environmental and Materials Engineering, Shanghai Second Polytechnic

University, Shanghai 201209, China ǁ

College of Mathematics and Physics, Shanghai University of Electric Power, Shanghai

200090, China *Corresponding author: [email protected], [email protected]

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ABSTRACT: As a new class of potential mid-range temperature thermoelectric materials, quaternary chalcogenides like Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) suffer from low electrical conductivity due to insufficient doping. In this work, Cu-doped CZTSe nanocrystals consisting of polygon-like nanoparticles are synthesized with sufficient Cu doping contents. The hot-injection synthetic method, rather than the traditional one-pot method, in combination with the hot-pressing method is employed to produce the CZTSe nanocrystals. In Cu-doped CZTSe nanocrystals, the electrical conductivity is enhanced by substitution of Zn2+ with Cu+, which introduces additional holes as charge carriers. Meanwhile, the existence of boundaries between nanoparticles in as-synthesized CZTSe nanocrystals collectively results in intensive phonon-boundary scatterings, which remarkably reduce the lattice thermal conductivity. As a result, an average thermoelectric figure of merit of 0.70 is obtained at 450 °C, which is significantly larger than that of the state-of-the-art quaternary chalcogenides thermoelectric materials. The theoretical calculations from the Boltzmann transport equations and the modified effective medium approximation are in good agreement with the experimental data. KEYWORDS: Cu2ZnSnSe4, hot-injection, nanocrystals, thermoelectric materials, figure of merit

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Quaternary chalcogenides such as Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) are not only promising materials for solar cells,1,2 but also potential thermoelectric (TE) materials.3,4 The energy utilizations in both solar cells and TE devices are eco-friendly. In particular, the applications of TE devices have been widely recognized as a highly promising transformative technology to transfer waste heat from automobile exhaust systems or excess solar thermal flux directly into electrical power.5−8 The energy conversion efficiency of TE materials is characterized by a dimensionless figure of merit zT = S2σT/κ, where T is the absolute temperature, S is the Seebeck coefficient, σ is the electrical conductivity, and κ is the thermal conductivity which consists of electronic thermal conductivity (κe) and lattice thermal conductivity (κL). Simultaneous increase of σ and reduction of κ are preferred for the zT enhancement. By far, the most common TE materials include BiSbTe-based materials (zT = ~0.8−1.1) near room temperature,9 PbTe-based materials (zT = ~1) in mid-range temperature regime (500−900 K),10 and SiGe alloys (zT = ~1.2) in high-temperature regime.11,12 All these materials however contain elements such as Bi, Pb, Te, and Ge which are expensive or toxic. Meanwhile, most conventional TE materials are narrow band gap semiconductors in which a bipolar effect may reduce the TE efficiency.13 To overcome these disadvantages, the TE properties of quaternary chalcogenidesCZTS, CZTSe, Cu2ZnGeSe4, Cu2CdSnSe4 and Cu2FeSnS4have been studied.14−23 Most of these chalcogenides (except for Cu2ZnGeSe4 and Cu2CdSnSe4) are nontoxic and abundant. Moreover, their band gaps are wide enough to avoid the bipolar effect. Their low thermal conductivities due to the complexity of the crystallographic structures enable those quaternary chalcogenides to be a potentially 3



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outstanding class of TE materials. The classical quaternary chalcogenideCZTSwhen used in TE devices, has a major drawback that the electrical conductivity is too low. Cu-doping has been thus employed as a promising way to enhance the electrical conductivity by introducing additional charge carriers. The best zT value of 0.36 was obtained for CZTS bulk materials at 427 °C through Cu-doping by using a solid-state preparation method.23 Besides the Cu-doping, another method of enhancing the electrical conductivity is replacing sulfur atom by the heavy-atom Se since the intrinsic carrier concentration of CZTSe is much higher than that of CZTS. As a result, the maximum zT values of un-doped and Cu-doped CZTSe were 0.19 and 0.86 obtained at 527 °C, respectively.22 More recently, Yu et al. used a one-pot chemical synthetic route to produce non-stoichiometric CZTSe nanocrystals, which showed a maximum zT value of 0.44 at 450 °C.15 Owing to its low reaction temperature and short reaction time, this wet synthetic route is favored than the solid-state preparation method.23 Moreover, Wu et al. reported a hot-injection solution method of producing CZTS nanocrystals, which was more stable than those made in the one-pot synthetic route.14 A unique advantage of such hot-injection method lies in its enabling rapid nucleation and growth of monodispersed nanocrystals.24 However, the obtained peak zT value in thus made CZTS nanocrystals was only 0.14 at 427 °C. Herein, we report the application of hot-injection method for synthesizing Cu-doped CZTSe nanocrystals for TE applications. We investigated the effects of Cu-doping contents on the TE properties of as-synthesized CZTSe nanocrystals. The substitution of Zn2+ with Cu+ introduces additional holes which results in an increase of electrical conductivity. The 4


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boundaries between nanoparticles created in these hot-injection synthesized CZTSe nanocrystals, lead to intensive phonon-boundary scatterings, which cause a remarkable reduction of the lattice thermal conductivity. As a result, increase of electrical conductivity and decrease of lattice thermal conductivity were simultaneously achieved in these Cu-doped CZTSe nanocrystals, in comparison to the un-doped CZTSe counterparts. We thus obtained a maximum zT value of 0.70 ± 0.05 at 450 °C, which is among the highest zT value in the existing CZTS and CZTSe based TEs. CZTSe nanocrystals were synthesized by a hot-injection method as schematically illustrated in Figure 1. The details are described in the Experimental Section. Firstly, two batches of precursor solutions of (1) SeO2 in 1-octadecene (1-ODE) and (2) a mixture of CuCl, Zn(CH3COO)2 and SnCl2·2H2O in oleylamine (OLA) were prepared at 80 °C and 70 °C, respectively. Next, SeO2 solution was injected into the mixture solution at 100 °C under nitrogen atmosphere and then elevated to 280 °C. The reaction was then maintained for 30 min and cooled down to room temperature. The resulting CZTSe nanocrystals exhibits a wide optical band gap of ~1.45 eV from UV-vis absorption measurement. Finally, the brown-black products were obtained by precipitation with adding a mixture of ethanol and hexane and then centrifugation. The obtained CZTSe solids were subsequently hot pressed at 400 °C and 60 MPa for 30 min.

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Figure 1. Hot-injection synthesis of CZTSe nanocrystals for TE applications. (a) Schematic of the experimental procedure and (b) the overall chemical reaction. The crystalline structures of these CZTSe nanocrystals were characterized by X-ray diffraction (XRD) measurements. Figure 2 shows the XRD patterns for the Cu-doped samples, Cu2+xZn1-xSnSe4, with two different Cu-doping contents (x = 0.05 and 0.1). We find the characteristic peaks located at 27.1, 45.0, 53.4, 65.6, and 72.3°, respectively, corresponding to the reflections from (200), (−331), (331), (262), and (−531) crystal planes, which agrees well with the previous report.15 After Cu doping, the lattice parameters of a and c decrease, which may be attributed to the substitution of Zn2+ by smaller-sized Cu+.23 Moreover, the formation of Cu2−xSe is found in Cu2.1Zn0.9SnSe4 sample, similar to the report by Zeier et al.20 The Cu2-xSe impurity phases along with the grain boundaries created in the nanocrystals notably enhance the phonon-boundary scatterings and thus significantly reduces the lattice thermal conductivity.

Figure 2. XRD patterns of Cu2+xZn1-xSnSe4 nanocrystals with various Cu-doping contents, x = 0, 0.05, and 0.1. The morphology of the obtained CZTSe nanocrystals before (Figure 3a−c) and after (Figure 3d−f) hot-pressing was intensively examined with both field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) equipped 6


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with energy dispersive X-ray spectroscopy (EDX). Figure 3a shows the SEM graph of surface-clean Cu2.1Zn0.9SnSe4 nanocrystals with polygon-like shape and sizes varying from 20 nm to 50 nm. Note that the grain packing of as-synthesized CZTSe nanocrystals is very dense due to the small size fluctuation in the hot-injection synthesis, which is known to generate monodispersed nanocrystals. Thus, rich grain boundaries are formed in the obtained CZTSe nanocrystals. In addition, the sizes of un-doped and Cu-doped CZTSe nanocrystals are shown in Figure S1. TEM imaging was used to further characterize the microstructures of these samples, as shown in Figure 3b and c. The low-magnification TEM graph (Figure 3b) shows the nanocrystals with different sizes and shapes. Moreover, the selected area electron diffraction (SAED) image (Figure 3c) exhibits the diffraction rings corresponding to the (200), (−331), (331), (262), and (−531) directions of CZTSe, respectively. The d-spacing values obtained for all these SAED diffraction rings agree in well with those of CZTSe. The appearance of multiple diffraction rings might be caused by the random orientation of the polycrystallites. To reveal the morphological influences of hot-pressing, the morphology of the fracture and polished surfaces of hot-pressed Cu2.1Zn0.9SnSe4 nanocrystals are shown in Figure 3d−f. Figure 3d shows the SEM image of the fracture surfaces. Comparing with those before hot-pressing, the samples after hot-pressing exhibit the larger sizes of particles (up to 80 nm) due to the merging of small-sized particles under high temperature and pressure. Grain boundaries between these large-sized nanoparticles are further inspected by TEM studies. Figure 3e displays the TEM graph of the polished surfaces (displayed in the inset), showing the polygon-like nanoparticles with varying sizes from 80−100 nm. Rich grain boundaries 7



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between different nanoparticles are observed and some of them are highlighted by red dashed lines. The lattice fringes as marked in red dashed circle in Figure 3f are determined to be 0.33 nm, corresponding to the inter-planar distance of the (200) plane of CZTSe. In particular, some obvious dislocationsa line defectare found and marked in red dashed lines. The existence of dislocations can further lower the lattice thermal conductivity. Finally, the compositions of both un-doped, Cu-doped CZTSe nanocrystals are characterized by EDX analysis (Figure 3g and h), giving the local compositions of Cu : Zn : Sn : Se = 2.2 : 0.3 : 1 : 3.4 and 2.4 : 0.5 : 1 : 3.7. These measured composition results significantly deviate from the feeding ratio of Cu : Zn : Sn : Se = 2 : 1 : 1 : 4 and 2.1 : 0.9 : 1 : 4 during the synthesis process. Therefore, as-synthesized CZTSe nanocrystals are highly inhomogeneous. The possible reason is that the Zn sources showed a much lower reactivity than both Cu and Sn sources.15 After hot-pressing, local compositions of nanocrystals are Cu : Zn : Sn : Se = 2.2 : 0.4 : 1 : 3.3 (Figure 3i). It is obvious that the compositions of Cu and Se are lower after hot-pressing.

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Figure 3. Morphologies of Cu2.1Zn0.9SnSe4 nanocrystals. 1) Before hot-pressing: (a) SEM image, (b) low-magnification TEM graph and (c) its image of lattice fringes. 2) After hot-pressing: (d) SEM image of fracture surface, (e) low-magnification TEM graph of polished surface which is displayed in the inset, and (f) high-resolution TEM graph of polished surface. 3) EDX spectra of (g) un-doped and (h) Cu-doped nanocrystals before hot-pressing, and (i) Cu-doped nanocrystals after hot-pressing, showing the composition differences. We examined TE properties of these Cu-doped CZTSe nanocrystals in comparison to those un-doped analogues. The electrical conductivity and the Seebeck coefficient in CZTSe nanocrystals were measured simultaneously by the steady-state four-probe method in the range of temperature of 25−450 °C. These experimental data were fitted by using the following Boltzmann transport equations (BTE):25

e2  ∂f  σ = 2 ∫ k 3τ ( E ) − 0 dE 3π m *  ∂E 

(1a) 9



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

1 eT

∂f 0 

∫ k τ ( E )( E − µ ) − ∂E  dE 3

S=

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

∂f 0 

∫ k τ ( E ) − ∂E  dE 3

(1b)

Here, e is the charge of hole, m* is the effective mass, k is the wave vector, µ is the chemical 2 2 potential, E = h k /(2m*) is the kinetic energy of electron where ħ is the Plank constant, −1 and f 0 = {exp[(E − µ ) /(k BT )] + 1} is the Fermi-Dirac distribution where kB is the Boltzmann

constant. The energy dependent relaxation time τ(E) is calculated by using the Mathiessien's rule.26 All the possible scattering mechanisms such as the electron-acoustic phonon scattering, the

electron-optical phonon

scattering, the electron-impurity scattering, and the

electron-boundary scatterings were considered in our calculations.27,28 Figure 4a shows the temperature dependence of electrical conductivity, σ, in CZTSe nanocrystals with different Cu-doping contents (x = 0, 0.05, and 0.1). The dots in the figure are the experimental data and the curves with the same color are the theoretical fitting from BTE calculations with reasonable parameters as shown in Table S1 in the Supporting Information. The carrier densities used in our calculations are determined by the Hall measurement which are shown in Figure S2. The calculation results are in good agreement with the experimental data. As for different samples, σ increases from 42 S/cm in un-doped sample to 78.8 S/cm in Cu2.05Zn0.95SnSe4 nanocrystals and further to 103.1 S/cm in Cu2.1Zn0.9SnSe4 nanocrystals at 450 °C, which are smaller than those of previously reported undoped CZTSe at the given temperatures.15 This phenomenon implies that Cu doping along with possible inhomogeneous local composition of Se or Sn result in a dramatic increasing of carrier density and σ. The reason is that the substitution of Zn2+ with Cu+ introduces holes as charge carriers in the system.29 We also find that σ in Cu-doped CZTSe nanocrystals 10


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decreases gradually with increasing temperature because the relaxation time of the electron-phonon scattering in eq 1a is inversely proportional to the temperature. In contrast, σ in un-doped CZTSe nanocrystals increases with increasing temperature. The opposite temperature dependence between un-doped and Cu-doped CZTSe nanocrystals was also found in another report.23 Figure 4b shows the temperature dependence of the Seebeck coefficient, S, in CZTSe nanocrystal with different Cu-doping contents. The sign of S is always positive for all samples, indicating that the charge carriers are holes and the samples are p-type. The value of S monotonically increased with increasing temperature since more holes are distributed to high energy states which leads to a higher average entropy (average energy divided by T) of holes. The measured maximum value of S reaches 297.9 µV/K for un-doped sample at 450 °C, which is larger than those of the undoped CZTSe in previous reports.15 The S values in Cu-doped nanocrystals are 245.1 µV/K in Cu2.05Zn0.95SnSe4 nanocrystals and 239.8 µV/K in Cu2.1Zn0.9SnSe4 nanocrystal at 450 °C, respectively. Both of them are smaller than those of un-doped nanocrystals because of larger carrier concentration upon Cu doping. Larger carrier concentration is expected to result in lower Seebeck coefficient.

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Figure 4. Temperature dependence of (a) electrical conductivity, (b) Seebeck coefficient, (c) thermal conductivity (solid lines) and lattice thermal conductivity (dashed lines), and (d) zT in CZTSe nanocrystals with different Cu-doping contents. The dots with error bar are experimental data and the curves are from theoretical calculations. Figure 4c shows the temperature dependence of the total thermal conductivities, κ, and the lattice thermal conductivities, κL, in CZTSe nanocrystals with different Cu-doping contents. The measurements are repeated three times to obtain average data points. The errors, mainly determined by the random errors, are plotted to show the uncertainty of the measurement. The theoretically calculated κ and κL are also given in the plots. With increasing temperature, both κ and κL are reduced due to stronger phonon-phonon scattering. The dots represent the measured κ, while the solid curves are the calculated κ which consists of κL calculated from the modified effective medium approximation30 and κe calculated from the BTE shown in eq 2. The calculated κL is also shown separately in 12


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dotted curves. The κL of our samples is 0.85 and 0.56 W m−1 K−1 at room temperature and 450 °C, respectively, both of which are much smaller than the values reported previously for CZTSe (ca. 1.3 and 0.8 W m−1 K−1 at room temperature and 450 °C, respectively). The calculated κL of all the nanocrystals are very low which can be attributed to the increased phonon-boundary scatterings at grain boundaries.14,21 With increasing Cu-doping contents, the κL at 450 °C drops from 0.55 in Cu2ZnSnSe4 to 0.49 W/(mK) in Cu2.05Zn0.95SnSe4 and further down to 0.46 W/(mK) in Cu2.1Zn0.9SnSe4. Notably, such extremely low κL values are even smaller than that in the state-of-the-art PbTe TE materials.10 This is mainly ascribed to the phonon-boundary scattering and the lattice distortion because excess Cu dopants occupy the Zn positions in the lattice.23 2   3  ∂f 0      ∫ k τ ( E )( E − µ ) − ∂E dE   ∂f 0  1  3      2 κe = dE − ∫ k τ ( E )( E − µ )  −  2 3Tπ m *   ∂f 0   ∂E  3  k τ ( E ) − dE   ∫   ∂E    

(2)

Owing to the improved electrical conductivities and reduced thermal conductivities, the maximum zT value, which uses the averaged thermal conductivity in three-time measurements, reaches 0.70 at 450 °C for the nominal composition of Cu2.1Zn0.9SnSe4 sample. The corresponding uncertainty of the obtained zT value is ± 0.05 due to the errors of thermal conductivity. Even higher zT values are anticipated to achieve at higher temperatures due to the rising trend of zT with temperature in these Cu-doped CZTSe samples. However, the Cu2ZnSnSe4 samples become unstable around 450 °C and then begin to decompose at ~610 °C.23 In conclusion, we have employed a hot-injection synthetic method of producing 13



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Cu-doped CZTSe nanocrystals consists of nanoparticles for mid-range temperature TE applications. In Cu-doped Cu2+xZn1-xSnSe nanocrystals, the electrical conductivity was enhanced by substitution of Zn2+ with Cu+, which introduces additional holes as charge carriers. The increased grain boundaries between nanoparticles generated in the hot-injection synthesis are combined to substantially enhance phonon-boundary scatterings, which remarkably reduce the lattice thermal conductivity. As a result, a maximum zT value of 0.70 ± 0.05 was obtained at 450 °C, which is larger than the state-of-the-art value of Cu doped CZTSe bulk materials prepared using solid state synthesis (i.e., 0.64 at 450 °C).22

ASSOCIATED CONTENT Supporting Information Experimental section, Figure S1 and 2, fitting parameters used for calculation in the Boltzmann equations (Table S1) and the Cp measurements certified in Netzsch application laboratory. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] *Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is sponsored by Shanghai Pujiang Program (2013) under grant No. 13PJ1400500 14


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(Z. L.). J. Z. acknowledges the support from the program for New Century Excellent Talents in Universities (Grant No. NCET-13-0431) and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (Grant No. TP2014012).

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ToC Image

The hot-injection method is employed to synthesize the Cu-doped CZTSe nanocrystals. A slight increase of electrical conductivity and a significant decrease of lattice thermal conductivity were simultaneously achieved, leading to a maximum zT value of 0.70.

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Hot-Injection Synthesis of Cu-Doped Cu2ZnSnSe4 Nanocrystals to

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