Phase-Selective Synthesis of Cu2ZnSnS4 Nanocrystals through

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Phase-Selective Synthesis of Cu2ZnSnS4 Nanocrystals through Cation Exchange for Photovoltaic Devices Yi-Xiu Wang,†,§ Ming Wei,‡,§ Feng-Jia Fan,†,§ Tao-Tao Zhuang,† Liang Wu,† Shu-Hong Yu,*,† and Chang-Fei Zhu*,‡ †

Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Collaborative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China ‡ CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China S Supporting Information *

ABSTRACT: Cu2ZnSnS4 (CZTS) nanocrystals with two typical structures, i.e., zinc blende (ZB)-derived and wurtzite (WZ) crystal frameworks, have been selectively synthesized via a solution-based route. Initially, Cu2SnS3 (CTS) nanoparticles with two different phases, i.e., zinc blende- and wurtzite-derived, can be prepared with different S sources and temperatures. Afterward, addition of the Zn precursor to the CTS matrix results in the substitutions of Cu and Sn cations, yielding CZTS with desirable phases. This method can be extended to the synthesis of other similar quaternary chalcogenide nanocrystals. The cation exchange method described here provides a convenient approach for fine-tuning the nanocrystal’s cation ratio, which enables us to optimize the solar absorber layer compositions and get a power conversion efficiency of 2.89% in copper-poor and zinc-rich devices. The capability to synthetically access stable phases with controllable morphologies and compositions demonstrates that the developed cation exchange method is powerful as a manufacturing technique for photovoltaic devices.

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crystal cell.18,19 On the other hand, offering proper kinetic control could stabilize the metastable WZ-derived phase of CZTS NCs. Experimentally, Lu et al.20 successfully synthesized CZTS in the WZ phase using a hot-injection route with the assistance of dodecanethiol. Subsequently, several approaches such as noninjection21 and hydrothermal22 techniques have been employed to synthesize hexagonal or orthorhombic WZderived CZTS NCs. Exploration of the synthesis of CZTS NCs in various phases has motivated us to design a convenient and versatile method to prepare CZTS with desirable phases. Theoretically, quaternary chalcogenide semiconductors are derived from cation cross-substitution of ternary I−III−VI2 compounds.23,24 The structure of chalcogenide semiconducting materials can be considered as an intersection of anion and cation sublattices.25−27 Therefore, quaternary chalcogenides such as Cu2ZnSnS4 can potentially be synthesized via the penetration of atoms into ternary matrix while the anion sublattice remains intact. As to the synthesis of CZTS, control of the phase may be achieved through two steps: Cu2SnS3 NCs in the demanded phase serve as a pristine template, and then Zn2+ ions are doped into the matrix to substitute the cations. Additionally, in previous studies on cation exchange, the

INTRODUCTION During the recent decades, the growing global demand for energy has prompted a large number of researchers to draw considerable attention to the preparation of highly efficient and cost-effective solar cells. Copper-based colloidal quaternary semiconductors, such as CuInxGa1−xS(Se)2, have attracted a lot of interest because of its good chemical stability and high efficiency in photoelectric conversion.1 In particular, copper zinc tin sulfide (Cu2ZnSnS4, CZTS) is an ideal candidate for photovoltaic applications as a result of many advantages such as a suitable band gap (1.0−1.5 eV), high optical absorption coefficient, low toxicity, and abundant resources of the elements.2−4 For years, cheap CZTS thin films have achieved an increasing efficiency such as 7.3%,5 8.4%,6 and 12.6%.7 Moreover, the wet-chemical approach for fabricating semiconductor films has been well-developed.8,9 Therefore, the solution-based route10−12 has become a facile choice for preparing CZTS. Recent advances have revealed that Cu2ZnSnS4 adopts two typical structures, i.e., zinc blende (ZB)-derived and wurtzite (WZ)-derived crystal frameworks.13−17 It is well-known that the formation energy contributes mostly to the total energy of a nanocrystal and that nanocrystals prefer to crystallize in the thermodynamically stable phase to lower the total energy. Therefore, traditionally obtained CZTS nanocrystals (NCs) have the ZB-derived structure, which features a tetragonal © XXXX American Chemical Society

Received: April 19, 2014 Revised: September 8, 2014

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Figure 1. (a) Experimental XRD patterns of as-synthesized ZB CZTS and CTS. The simulated XRD patterns of ZB CZTS and cubic CTS (JCPDS 89-2877) are shown below in red. (b) Experimental XRD patterns of as-synthesized WZ CZTS and CTS. The simulated XRD patterns of WZ CZTS and CTS (in red) are also shown. (c) Room-temperature Raman spectra of ZB CZTS and WZ CZTS. (d) UV−vis−NIR spectra of four kinds of NCs. The inset shows the linear extrapolations of plots of (αhν)2 vs photon energy.

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anionic framework of nanocrystals was found to be conserved, which enabled the preservation of the original phase of the reactant nanocrystals.28−30 Because of the facile reaction situation, cation exchange has been identified as an efficient tool for modification of nanocrystals.31−33 With better phase and morphology control, the cation exchange technique may also allow for enhanced control of the composition for the manufacture of photovoltaic thin films. The influence of the composition ratio on CZTSbased thin-film solar cells has been intensively investigated by both theoretical calculations34 and experimental studies.35 It has been proven that high performance is usually obtained with Cupoor and Zn-rich devices.36 Flexible and universal cation exchange brings us renewed excitement to optimize the photovoltaic device performance via control of the cation ratio during the colloidal nanocrystal synthesis stage. Herein we demonstrate a straightforward and effective strategy to synthesize CZTS NCs with two phases. During the synthesis, ZB Cu2SnS3 and WZ Cu2SnS3 NCs were separately synthesized as structure templates. Then we used Zn(S2CNEt2)2 as the Zn2+ source to partially substitute cations of ternary CTS to obtain CZTS with the corresponding structure. The formation mechanism of the nanocrystals hints at the possibility to extend the same synthetic method to other nanocrystals with desired phases. In addition, the composition in the CZTS thin film can be tuned through cation exchange, and a Cu-poor and Zn-rich device exhibited a power conversion efficiency of 2.89%.

EXPERIMENTAL SECTION

Chemicals. Hexane (C6H14, 97%), ethanol (CH3CH2OH, 99.7%), 1-dodecanethiol [CH3(CH2)10CH2SH, 97%], cuprous chloride (CuCl, 99.5%), zinc acetate [Zn(CH3COO)2·2H2O, 99%], stannous chloride (SnCl2·2H2O, 98%), sulfur (S, 99.998%), sodium diethyldithiocarbamate [Na(S2CNEt2)·3H2O, 99.998%], zinc chloride (ZnCl2·2H2O, 98%), ferric chloride (FeCl3, 97%), cobalt chloride (CoCl2·6H2O, 98%), nickel chloride (NiCl2·6H2O, 98%), and cadmium chloride (CdCl2·2.5H2O, 99%) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Oleylamine (C18H37N, 80−90%) and trioctylamine (C24H51N, 80−90%) were purchased from Aladdin Reagent Co. Ltd. (Shanghai, China). All of the chemical reagents were used as received without further purification. Synthesis of Zinc Blende-Derived and Wurtzite-Derived CTS Nanocrystals. To prepare ZB-derived CTS nanocrystals, 0.225 g (2.2 mmol) of CuCl and 0.255 g (1.1 mmol) of SnCl2·2H2O were first dissolved in 10 mL of oleylamine in a three-neck flask by heating to 180 °C at a rate of 10 °C/min. Then 0.108 g (3.3 mmol) of sulfur dissolved in 5 mL of oleylamine was injected into the hot solution. The mixture was heated to 280 °C under an atmosphere of N2 and maintained for 30 min. The black precipitate was collected by centrifugation and washed with hexane and ethanol two times. The obtained ZB-derived CTS was dried in an oven (60 °C in vacuum) for further use. In a typical synthesis of WZ-derived CTS, 0.225 g (2.2 mmol) of CuCl and 0.255 g (1.1 mmol) of SnCl2·2H2O were dissolved in 10 mL of oleylamine and 5 mL of 1-dodecanethiol under an atmosphere of N2. The temperature was increased to 230 °C at a rate of 10 °C/min, and this temperature was maintained for 2 h. Preparation of the Precursor Dithiocarbamate Complexes. The synthesis of Zn(S2CNEt2)2 was based on a previously published procedure.28 ZnCl2·2H2O (1 mmol) was dissolved in 10 mL of H2O, producing a clear blue solution. In a separate beaker, Na(S2CNEt2)· 3H2O (2 mmol) was dissolved in 20 mL of H2O. The Zn2+ solution B

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was then added dropwise to the S2CNEt2− solution under stirring, resulting in the precipitation of the complex. The product was isolated through centrifugation, washed with H2O, and air-dried. Following the above procedure, the complexes M(S2CNEt2)x (M = Zn, Fe, Co, Ni, Cd) were synthesized using MClx (M = Zn, Fe, Co, Ni, Cd) as the Mx+ source. Synthesis of Zinc Blende-Derived and Wurtzite-Derived CMTS (M = Zn, Fe, Co, Ni, Cd) Nanocrystals with Tunable Composition. As-synthesized ZB CTS nanocrystals (0.068 g, 0.2 mmol) were dissolved in 5 mL of 1-dodecanethiol in the presence of 5 mL of trioctylamine in a three-neck flask in air. To prepare CZTS nanocrystals with tunable composition, the specified amount of Zn(S2CNEt2)2 was added to the flask, and the resulting mixture was heated to 250 °C at a rate of 10 °C/min. After 1 h, the corresponding ZB-derived CZTS NCs were obtained by centrifugation and washed with hexane and ethanol two times. For the synthesis of WZ CZTS NCs, the procedure was the same as for the ZB CZTS NCs. Similar to the synthesis of CZTS, CMTS (M = Zn, Fe, Co, Ni, Cd) NCs were prepared from M(S2CNEt2)x (M = Zn, Fe, Co, Ni, Cd) and CTS under the same reaction conditions. Characterization. The structure and composition of the ZB CZTS and WZ CZTS were confirmed by powder X-ray diffraction (XRD) patterns, transmission electron microscopy (TEM), high-resolution TEM (HRTEM), Raman spectroscopy, X-ray photoeletron spectroscopy (XPS), and inductively coupled plasma atomic emission spectroscopy (ICP-AES). XRD patterns were recorded on a PW1710 instrument with Cu Kα radiation (λ = 0.15406 nm). The operation voltage and current were kept at 40 kV and 400 mA, respectively. Nanocrystals dispersed in hexane were drop-cast on carbon-supported Mo grids for TEM and HRTEM observations, which were performed on a JEOL-2010F transmission electron microscope with an acceleration voltage of 200 kV. The ICP-AES measurements utilized an Atomscan Advantage instrument (Thermo Jarrell Ash Corporation). The precision value (as percent relative standard deviation) is 3%. XPS was performed on an ESCALAB 250 instrument using a source of Al Kα radiation. To prepare ICP samples in solution, we employed concentrated nitric acid (14.5 mol/L) to digest the products and then diluted it to 0.3−0.6 mol/L. Scanning TEM energy-dispersive spectroscopy (STEM-EDS) element mapping was carried out on a JEOL-2010F microscope equipped with an Inca Oxford energy-dispersive spectrometer, and Mo grids were used for STEM-EDS mapping. Scanning electron microscopy (SEM) images were carried out on a Zeiss Supra 40 scanning electron microscope at an acceleration voltage of 5 kV. UV−vis spectra were recorded on a Shimadzu UV-240 spectrophotometer scanning from 400 to 1600 nm at room temperature. Raman spectroscopy measurements were carried out with a Horiba Jobin Yvon LabRam high-resolution Raman spectrometer equipped with an excitation wavelength of 514 nm. Field-emission SEM (FE-SEM, FEI, Sirion 200) was used to investigate the cross-sectional morphology of the CZTSSe solar cell devices. Active area efficiencies were determined by the mechanical scratching method. The current density−voltage (J−V) measurements on the solar cells were performed under 1.5 AM (1000 W/m2) irradiation with an Oriel Sol3A solar simulator.

intensities match well with the simulated patterns (see Table S1 in the Supporting Information), suggesting that the CZTS nanocrystals adopt a ZB-derived cubic crystal structure. As a benefit from the pure-phase precursor (cubic CTS), the obtained CZTS inherits the anion framework well. The two kinds of nanocrystals show similar XRD patterns in Figure 1a, indicating that the phase is stable during the synthetic procedure. WZ CZTS exhibits the same result in Figure 1b. The XRD patterns of WZ CTS and CZTS are simulated by theoretical lattice parameters that are consistent with the previous literature.20,22,37 The XRD patterns of the two resulting WZ-derived NCs match well with the simulated patterns, and the strongest diffraction peaks of the two structures are almost in the same position. In short, the arrangements of atoms in CZTS (ZB or WZ) are basically similar to those in the matrix (ZB or WZ CTS), implying that CZTS in the desired phase can be synthesized from CTS with the corresponding anion lattice. Further analysis of the phase purity was assisted by roomtemperature Raman spectra. While the ZB ZnS (JCPDS no. 650309) and WZ ZnS (JCPDS no. 89-2942) show XRD patterns similar to those of ZB-derived CZTS and WZ-derived CZTS, respectively, the most intense Raman peaks of the two kinds of nanocrystals are distinct. As shown in Figure 1c, the ZB-derived CZTS and WZ-derived CZTS present their strongest Raman peaks at 333 cm−1, which is close to the previous report.14,38 It is noticeable that the main peak excludes ZnS (351 and 274 cm−1),39,40 SnS2 (315 cm−1),41 and Cu3SnS4 (318, 348, and 295 cm−1).42 However, a large pedestal below the main peak seems to indicate the minor presence of a distinct phase. This result comes from the possible gradient composition and the dangling bonds of surface atoms. Accordingly, the Raman spectra of the two kinds of CTS NCs are shown in Figure S2. Figure 1d illustrates the UV−vis−NIR spectra of the obtained nanocrystals stably dispersed in hexane. The band-gap energies of the nanocrystals were determined by extrapolating the linear regions of the plots of (αhν)2 versus photon energy (hν). With this approximation, the values for CTS and CZTS were found to be 1.0 and 1.5 eV, respectively, which is consistent with the values in the previous literature.10,22,43−46 The value obtained for CZTS approaches the optimum value for solar photovoltaic conversion.47 The reaction scheme is shown in Scheme 1. The two phases of CZTS are synthesized by penetration of Zn2+ into the Scheme 1. Scheme of the Cation Exchange Reaction

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RESULTS AND DISCUSSION Figure 1a shows the XRD patterns of the as-synthesized ZB CTS and CZTS (the molar ratio of the added CTS and Zn(S2CNEt2)2 was 1:1). It is noticed that the experimental XRD patterns of ZB CTS can be indexed to the standard pattern of cubic CTS (JCPDS no. 89-2877). The resulting diffraction pattern of ZB CZTS does not match accurately with the standard pattern of kesterite CZTS (JCPDS no. 26-0575) (see Figure S1 in the Supporting Information). A diffraction pattern was simulated from the ZB ZnS crystal structure. In this structure, sulfur anions are closely packed and Cu(I), Zn(II), and Sn(IV) cations randomly occupy half of the interstices of the sulfur anions. The experimentally observed diffraction

framework of CTS. Because CTS NCs are prepared at high temperature with innate structure and composition, the obtained CZTS nanocrystals inherit the crystallographic information of the template. While this reaction introduces C

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only one variable, it is predictable that the reaction process can be easily controlled. The STEM-EDS element maps of fields of nanocrystals (Figure 2a,c) confirm that Cu, Zn, Sn, and S are distributed

Figure 2. (a, c) STEM-EDS element maps of (a) ZB CZTS and (c) WZ CZTS. (b, d) EDS line-scan analyses of the major element distributions of (b) ZB CZTS and (d) WZ CZTS across the NCs.

among both the ZB and WZ nanocrystals, but there are some observable distributions within single nanocrystals. With the assistance of EDS line scans (Figure 2b,d), we can conclude that Zn penetration into the matrix starts from the surface because there are more Zn atoms and fewer Cu and Sn atoms on the surface. XPS spectra (Figure S3) also show the Zn enrichment on the surface. We noticed that there are more even composition variations in the WZ nanocrystals (Figure 2c,d). The TEM image in Figure 3a shows that the obtained ZB CTS NCs are slightly polydispersed, with an average diameter of 15 ± 5 nm (the results for WZ NCs are shown in Figure 3c). In comparison, the final product ZB CZTS (Figure 3e) adopts the analogous morphology and size. This result may also be a response to the stable anion crystal framework. The HRTEM images of the two kinds of nanoparticles in Figure 3 show their good crystallinity. The lattice spacing of 0.31 nm in Figure 3b,f is in accordance with the (111) planes of ZB-derived CTS and CZTS. Comparably, the lattice spacings of 0.33 nm shown in Figure 3d,h are ascribed to the (100) planes of WZ-derived CTS and CZTS. The HRTEM images again confirm the phase inheritance relationship between the parent crystals (ZB or WZ CTS) and the product crystals (ZB or WZ CZTS). During the synthesis, CTS and Zn(S2CNEt2)2 complexes were dissolved in organic solvents at a suitable temperature, resulting in favorable solubility and mobility of the ions. Because of the reaction on nanoscale, either the high reaction zone or agreeable solvation of the parent cations drove the process of cation exchange. The extraction of copper and tin cations can be explained by the fact that two Cu+ cations plus one Sn4+ cation have the same amount of charge as three Zn2+ cations. Additionally, Cu and Zn are close in ionic radius, and the Zn substitution of Cu has been shown to be energetically favorable in quaternary sulfides.48−50 Next, contrastive experiments were designed to further investigate the reaction mechanism. We selected the ZBderived structure as an example (the results for the WZ-derived structure are shown in Figures S4 and S5). The element composition of the ZB CTS was confirmed by ICP-AES, with the typical ZB CTS NCs showing an average Cu/Sn/S ratio of approximately 2:1:3, which is close to the stoichiometric ratio. First, we employed different amounts of Zn2+ for cation exchange with a certain amount of CTS (the Zn2+/CTS molar ratio ranged from 0.5:1 to 1.5:1). The average chemical

Figure 3. (a, c, e, g) TEM and (b, d, f, h) HRTEM images of (a, b) ZB CTS, (c, d) WZ CTS, (e, f) ZB CZTS, and (g, h) WZ CZTS. The dimensions of the ZB-derived and WZ-derived nanocrystals fall in the ranges 15 ± 5 nm and 10 ± 5 nm, respectively.

compositions of the products were studied by ICP-AES (Table 1, samples 1−5), where we confirmed that the composition of Table 1. Chemical Compositions of Different Samples Determined by ICP-AESa sample

CTS:M(C5H10NS2)x

product

1 2 3 4 5 6 7 8 9

CTS:Zn(C5H10NS2)2 = 1:0.5 CTS:Zn(C5H10NS2)2 = 1:0.75 CTS:Zn(C5H10NS2)2 = 1:1 CTS:Zn(C5H10NS2)2 = 1:1.25 CTS:Zn(C5H10NS2)2 = 1:1.5 CTS:Fe(C5H10NS2)3 = 1:1 CTS:Co(C5H10NS2)2 = 1:1 CTS:Ni(C5H10NS2)2 = 1:1 CTS:Cd(C5H10NS2)4 = 1:1

Cu2.71Zn0.64SnS4.01 Cu2.45Zn0.87SnS3.97 Cu2.11Zn0.95SnS3.77 Cu1.67Zn1.17SnS3.87 Cu1.42Zn1.30SnS3.82 Cu2.38Fe1.58SnS3.56 Cu1.94Co1.23SnS3.76 Cu2.38Ni0.67SnS3.88 Cu2.11Cd0.87SnS3.86

a Samples 1−5 were synthesized from CTS with different amounts of Zn(C5H10NS2)2. Samples 6−9 were synthesized from CTS with Fe(C 5 H 10 NS 2 ) 3 , Co(C 5 H 10 NS 2 ) 2 , Ni(C 5 H 10 NS 2 ) 2 , and Cd(C5H10NS2)2, respectively.

Zn in the final CZTS increased with the increase in the amount of Zn2+ cation and that the contents of Cu and Sn in the products relatively decreased. The results clearly verified that the CZTS NCs were obtained by partial cation exchange. In this facile reaction, the controllable cation molar ratio gives the possibility of preparing the proper composition of CZTS inks for fabricating thin films. Besides, the TEM images (Figure S6) D

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synthetically provides us with a flexible strategy to tune the band gaps of multiple compounds through partial cation exchange. To verify the photovoltaic application of the obtained CZTS and prove the flexibility and universality of our method, we selected samples 3 and 4 in Table 1 to fabricate thin films to evaluate the power conversion efficiency. Since the dispersibility of WZ CZTS is not good enough to make uniform films, we did not fabricate devices from WZ CZTS nanocrystals. With regard to sample 3, ICP-AES shows an average of Cu/Zn/Sn/S ratio of approximately stoichiometric 2:1:1:4. By contrast, we can calculate that the Cu/(Zn + Sn) ratio of sample 4 is 0.77 and the Zn/Sn is 1.17, which is close to the optimal proportion34−36 for CZTS thin-film absorbers. The device fabrication processes for these two samples were the same and included the following steps: The obtained CZTS NCs were dissolved in hexanethiol (200 mg/mL) with stirring for 2 days, forming relatively well dispersed concentrated inks. The CZTS absorber was deposited onto a molybdenum-coated glass substrate by spin-coating of the precursor solution multiple times with intermediate steps of 300 °C in an atmosphere of air. The as-prepared films were then annealed under Se vapor at 500 °C for 20 min inside a sealed quartz tube (containing the sample and elemental Se pellets) at a heating rate of about 50 K/min. The compositions of the prepared absorbers are shown in Figures S9 and S10. The ∼50 nm cadmium sulfide (CdS) layer was then deposited onto the CZTSSe layer by chemical bath deposition.51 Radiofrequency (RF) magnetron-sputtered iZnO (∼50 nm) and RF magnetron-sputtered Al:ZnO (AZO) were deposited as window layers. Finally, a patterned Al grid was deposited by the thermal evaporation method on the top of the device as the top contact, yielding glass/Mo/CZTSSe/ CdS/ZnO/AZO/Al solar cells, which are shown in Figure 6a,b. The first device fabricated with sample 3 had a mechanical scribing area of approximately 0.35 cm2, which exhibited an open-circuit voltage (Voc) of 300 mV, a short-circuit current density (Jsc) of 17.45 mA·cm−2, a fill factor (FF) of 30.97%, and a power conversion efficiency (η) of 1.61%. The device with optimized absorber (fabricated with sample 4) had a mechanical scribing area of 0.7 cm2 and exhibited Voc = 272 mV, Jsc = 30.28 mA·cm−2, FF = 35.18%, and η = 2.89%. The results are shown in Figure 6c. The improved performance of the CZTSSe thin-film solar cell device fabricated with sample 4 resulted from the optimized composition of the absorber, suggesting that this controllable and flexible method will benefit the development of applications. While our CZTS devices showed mediocre efficiencies, continued optimization of absorber fabrication and interface engineering is underway.

show that the size distributions of samples 1−5 are in good agreement with each other. The phases of the five samples are consistent with the phase of the precursor through partial cation exchange (Figure S7). Second, other cations (Fe3+, Co2+, Ni2+, and Cd2+) successfully underwent cation exchange in the CTS matrix framework. With regard to samples 6−9 in Table 1, we found that the compositions and molar ratios of the products were different. It is possible that the radii of these cations and ionic bonds are distinct. The sizes of the four kinds of nanoparticles are decided by the precursor, which is similar to CTS (Figure 4). This result illustrates that the four samples also have

Figure 4. TEM images of (a) Cu 2 . 3 8 Fe 1 . 5 8 SnS 3 . 5 6 , (b) Cu1.94Co1.23SnS3.76, (c) Cu2.38Ni0.67SnS3.88, and (d) Cu2.11Cd0.87SnS3.86.

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CONCLUSION ZB-derived and WZ-derived CZTS can be selectively synthesized through partial cation exchange from ternary to quaternary sulfides. During this exchange process, the obtained nanocrystals preserve well the crystallographic structure of the original ternary nanocrystals. Through appropriate tuning of the cation ratio during cation exchange, we can easily fabricate absorber layers of photovoltaic devices with desired compositions. Our preliminary results have shown that copper-poor and zinc-rich devices show higher power conversion efficiencies than stoichiometric devices, indicating that the method developed here is a promising approach to optimize the photovoltaic devices. The present synthetic methodology can also be extended to the synthesis of other multinary

Figure 5. XRD patterns of samples 6−9.

retained the inherent anion framework. Figure 5 compares the XRD patterns of CMTS (M = Fe, Co, Ni, Cd) with that of the precursor. It is concluded that the obtained CMTS NCs are mixed-cation alloys rather than phase-segregated mixtures of CTS and MSx. After M is incorporated into the crystal structure, shifts to smaller angles can also be detected in the XRD patterns. The optical properties of the four kinds of nanocrystals were investigated by UV−vis−NIR spectroscopy. It is obvious that the absorption coefficients of the four samples in Figure S8 are different, suggesting that the four kinds of nanocrystals exhibit disparate band gaps. This fact also E

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and 2014CB931800), the National Natural Science Foundation of China (Grants 21431006, 91022032, 91227103, 21061160492, and J1030412), and the Chinese Academy of Sciences (Grant KJZD-EW-M01-1).

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Figure 6. (a, b) SEM images of solar cells fabricated with samples 3 and 4, respectively. (c) Power conversion efficiency results for the two devices.

compounds with desired phases, morphologies, and tunable compositions.

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ASSOCIATED CONTENT

S Supporting Information *

Details about the actual crystal structure used for these simulations, TEM images and XRD patterns of experimental results obtained for WZ-derived CZTS, TEM images of samples 1−5, UV−vis−NIR spectra of samples 6−9, and Raman spectra and experimental XRD patterns of the prepared absorbers after selenization. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions §

Y.-X.W., M.W., and F.-J.F. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of China (Grants 2010CB934700, 2013CB933900, F

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