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Apr 26, 2013 - ... Key Laboratory of High Performance Ceramics and Superfine Nanostructure, Shanghai Institute of Ceramics, Chinese Academy of Science...
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Controllable Synthesis of Cu2In2ZnS5 Nano/Microcrystals and Hierarchical Films and Applications in Dye-Sensitized Solar Cells Yufeng Liu,† Fuqiang Huang,*,†,‡ Yian Xie,† Houlei Cui,† Wei Zhao,† Chongyin Yang,† and Ning Dai§ †

CAS Key Laboratory of Materials for Energy Conversion and State Key Laboratory of High Performance Ceramics and Superfine Nanostructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China ‡ Beijing National Laboratory for Molecular Sciences and State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China § National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, People’s Republic of China S Supporting Information *

ABSTRACT: Cu2In2ZnS5 microcrystals with controllable nanostructures are synthesized via a facile solvothermal method. The microcrystals consist of numerous nanorods packed along a preferred crystal orientation. The crystal size of Cu2In2ZnS5 is tunable into the nanoscale. Cu2In2ZnS5 nanocrystals are composed of nanoparticles with an average size of 4.2 nm. Moreover, microcrystals were assembled on a molybdenum substrate to form a Cu2In2ZnS5 thin film with a hierarchical architecture. The hierarchical nanostructure benefits to increase the optical path, decrease the reflection to capture photons effectively, and provide multiple channels for directly transferring charges to the conducting substrate. The hierarchical thin films were exploited as counter electrodes of dye-sensitized solar cells, which enhanced the catalytic activity of the counter electrode. The power conversion efficiency reached up to 6.1%, comparable to that of the dye-sensitized solar cells with a Pt counter electrode.

1. INTRODUCTION The ternary CuInS2 (CIS) semiconductor is an important photovoltaic material for thin-film solar cells due to the appreciated direct band gap (∼1.5 eV) and high absorption coefficient (>105 cm−1).1−6 CIS quantum dots are excellent due to low toxicity compared to Cd-based materials, and high photoluminescence quantum yielded in a CIS/ZnS core/shell structure.7−12 Moreover, CIS semiconductor can tune the band gap by forming Cu2In2ZnS5 (CIZS) solid solution with ZnS.13−16 Solid solution CIZS and CIS/ZnS core/shell structure nanocrystals have been extensively investigated in the fields of photoluminescence and biological imaging,10,11,17,18 but there are few studies on catalytic activity of CIZS compared other metal chalcogenides.19−24 Especially, there is no report on the investigation of CIZS as a counter electrode of dye-sensitized solar cells (DSSCs). The design and synthesis of thin films with special nanostructures as counter electrodes in DSSCs can increase the interaction between the surface of thin films and photons and electrons, which enhances the catalytic activity and the ability of charge collection. Therefore, the photoelectric performance of thin films can be improved by controlling their nanostructures.25−33 Up to date, controllable synthesis for thin films with various nanostructures has attracted extensive attention. Therefore, it is important to explore novel and facile experimental methods to obtain thin films with special nanostructures. In this paper, a solvothermal method is employed to synthesize CIZS nanocrystals and microcrystals with control© XXXX American Chemical Society

lable nanostructures via in situ reaction of a uniform and clear precursor solution, including metal oxide, hydroxide, and thioacetic acid. An attractive feature of this method is favorable to form uniform products without introducing any impurities. More interesting, hierarchical CIZS films with large surface areas are obtained by assembling densely packed CIZS nanorods on a molybdenum substrate via a one-step reaction. The hierarchical film can effectively increase the optical path to capture photons and provide multiple channels to transfer the carriers. Ultimately, CIZS hierarchical films are exploited as counter electrodes of dye-sensitized solar cells (DSSCs) with an efficiency up to 6.1%.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Cu2In2ZnS5 Nanocrystals. A 0.1 mmol portion of Cu2O, 0.2 mmol of In(OH)3, 0.2 mmol of ZnO, and 2 mL of CH3COSH were, in turn, added into a 40 mL vial to form a black mixture under stirring. A 1 mL portion of ammonia aqueous solution was then added drop by drop into the vial to form a clear red solution, followed by adding 20 mL of anhydrous ethanol and 0.8 mmol of polyvinylpyrrolidone (PVP) into the solution under stirring for 5 min. Finally, the mixture was transferred and sealed into a 35 mL Teflon-lined stainless autoclave to keep in an electric oven at 150 °C for 6 h. The product was taken out and poured into 40 mL of acetone, Received: February 26, 2013 Revised: April 24, 2013

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Raman microscope using green laser (λ = 532 nm) excitation. Photocurrent density−voltage (J−V) characteristics were measured using a Keithley model 2440 source meter under AM 1.5 illumination. A 1000 W Oriel solar simulator was used as a light source, and the power of the light was calibrated to 1 sun light intensity by using a NREL-calibrated Si cell (Oriel 91150). Cyclic voltammetry (C−V) measurements were performed using a Pt wire as auxiliary electrode, a Ag/AgCl electrode as reference electrode, and CIZS hierarchical films as working electrode on a CHI 660 electrochemistry workstation with the scanning rate of 50 mV/s.

and centrifuged at 13 000 rmp for 5 min. The precipitates were collected and redispersed in ethanol to form a stable solution. 2.2. Synthesis of Cu2In2ZnS5 Microcrystals. A 0.1 mmol portion of Cu2O, 0.2 mmol of In(OH)3, 0.2 mmol of ZnO, and 2 mL of CH3COSH were, in turn, added into a 40 mL vial to form a black mixture under stirring. A 1 mL portion of ammonia aqueous solution was then added drop by drop into the vial to form a clear red solution under stirring for 5 min. Finally, the mixture was transferred and sealed into a 35 mL Teflon-lined stainless autoclave to keep in an electric oven at 150 °C for 6 h. The product was taken out and poured into 40 mL of acetone and centrifuged at 13 000 rmp for 5 min. The precipitates were collected and redispersed in ethanol to form a stable solution. 2.3. Synthesis of Cu2In2ZnS5 Hierarchical Films. A 0.4 mmol portion of Cu2O, 0.8 mmol of In(OH)3, 0.8 mmol of ZnO, and 4 mL of CH3COSH were, in turn, added into a 40 mL vial to form a black mixture under stirring. A 2 mL portion of ammonia aqueous solution was then added to form a clear red solution, followed by adding 20 mL of anhydrous ethanol into the solution. The mixture was transferred into a 35 mL Teflon-lined stainless autoclave. Finally, one piece of molybdenum on soda lime glass (1 cm × 2 cm) was put into the Teflon-lined autoclave and sealed into a heating furnace at 150 °C for 6 h. The thin film was then blown dry with nitrogen after cleaned to remove the contamination on the surface of the thin film. Finally, the Cu2In2ZnS5 thin film was annealed under a Se atmosphere at 500 °C for 20 min. 2.4. Fabrication of Dye-Sensitized Solar Cells. First, 12 μm thick transparent films of 20 nm sized TiO2 particles were screen-printed on FTO substrates. The electrodes were sintered in dry air at 450 °C for 30 min. The electrodes were immersed in 40 mM TiCl4 aqueous solution at 70 °C for 30 min. After being sintered in dry air at 450 °C for 30 min again, the electrodes were immersed into a 0.3 mM solution of ruthenium dye N719 in anhydrous ethanol overnight. Furthermore, Pt sputtered on the FTO substrate and Cu2In2ZnS5 hierarchical films on the molybdenum substrate were used as counter electrodes. The electrolyte used consisted of 0.1 M LiI, 0.05 M I2, 0.3 M 1,2-dimethyl-3-propylimidazolium iodine, and 0.5 M tert-butylpyridine in 3methoxypropionitrile. Finally, the photoanode and the counter electrode were assembled and clipped in a sandwich-type arrangement with the electrolyte solution placed in between. The active area was 0.25 cm2. 2.5. Characterization. Field emission scanning electron microscopy (FESEM) images were acquired using an FEI Sirion 200 with an energy-dispersive X-ray (EDS) analysis. Low-and high-resolution transmission electron microscopy (TEM) images were taken on a JEOL JEM-2100F at an accelerating voltage of 200 kV. TEM samples were prepared by dip-casting CIZS nanocrystals dispersed in ethanol onto carbon-coated copper TEM grids. X-ray diffraction (XRD) patterns was collected on a Bruker D8 Focus X-ray diffractometer equipped with a monochromatized source of Cu Kα radiation (λ = 0.15406 nm) at 1.6 kW (40 kV, 40 mA). The pattern was recorded in a slow-scanning mode with 2θ from 10° to 80° with a scan rate of 6°/min. UV−vis absorbance spectra were recorded on a Hitachi U-3010 spectrophotometer with a scanning velocity of 240 nm/min. X-ray photoelectron spectroscopy (XPS) data were obtained using an ESCALAB 250 X-ray photoelectron spectrometer for surface analysis. Raman spectroscopy was performed on a Renishaw inVia

3. RESULTS AND DISCUSSION Two CIZS products with different sizes and nanostructures were obtained controllably by adding the surfactant into the reaction vessel. The CIZS nanocrystals and microcrystals were characterized by TEM and FESEM shown in Figure 1. As

Figure 1. (a) FESEM of CIZS nanocrystals. (b) TEM of an individual CIZS nanocrystal. (c) Inner morphology of the nanocrystal. (d) FESEM of CIZS microcrystals. (e) FESEM of an individual CIZS microcrystal. (f) Surface morphology of the microcrystal.

shown in Figure 1a,b and Figures S1 and S2a (Supporting Information), monodisperse CIZS nanocrystals have a woollike surface and uniform size with an average diameter of 126.4 nm when there is existing surfactant in the precursor. Moreover, the nanocrystals possess complicated secondary nanostructures, as shown in the TEM of the inner morphology of an individual CIZS nanostal and the HRTEM of nanocrystals (Figure 1c and Figure S2b, Supporting Information). There are numerous small spheral CIZS nanoparticles with an average size of 4.2 nm, which are assembled to form a large CIZS nanostal. However, microcrystals with a different nanostructure were obtained via the same method to synthesize the nanocrystals, except for when there is no surfactant existing in the reaction. The size of microcrystals, which ranges from 1 to 2 μm, is not uniform as the size of nanocrystals (Figure 1d,e and Figure S3, Supporting Information). There is a different secondary nanostructure for the microcrystals, which is made up of a lot of nanorods, as shown in Figure 1f. It is obvious that the surfactant (PVP) plays an important role in the reaction. Two kinds of Cu2In2ZnS5 products with different sizes and controllable nanostructures can be synthesized by adding a surfactant (PVP) in the precursor. The formation process of CIZS nanocrystals and microcrystals with different nanostructures is illustrated in Figure 2. First, a uniform red precursor including Cu, In, Zn, and S was B

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Figure 2. Illustration of the formation process of CIZS nanocrystals and microcrystals.

Figure 3. (a, b) FESEM images of CIZS hierarchical films. (c) TEM of nanorod scratched from the hierarchical thin film. (d) HRTEM image of the nanorod.

obtained by solving Cu2O, In(OH)3, and ZnO in thioacetic acid and ammonia aqueous solution. The uniform precursor avoided introducing extra anions in the reaction, which is favorable to form nano/microcrystals with a pure phase and uniform morphology. The morphology of CIZS products was controlled by adding polyvinylpyrrolidone (PVP) in the precursor solution. The spheral CIZS nuclei were formed by heating the precursor when the surfactant exists. The CIZS nuclei then grew to assemble into a large wool-like CIZS nanocrystal with PVP ligands, as shown in Figure 2. However, when there is no surfactant existing in the precursor, the CIZS nuclei grew along a preferable crystal orientation to form rodlike CIZS nanocrystals because of the inhibition of surfactant in the reaction. Finally, the rodlike CIZS nuclei were assembled into a large microcrystal with a hierarchical nanostructure. Similar to the formation mechanism of microcrystals, a hierarchical thin film was obtained when a molybdenum substrate was placed in Teflon-lined autoclave. For further investigating the properties of hierarchical nanostructures, a CIZS thin film on a molybdenum substrate was prepared via a solvothermal method, which was exploited to synthesize the microcrystals. Similar to the microcrystal, the surface of the thin film was composed of interconnected CIZS nanorods (Figure 3a,b). TEM of scratched CIZS films displays a rodlike morphology, consistent with the result of FESEM images (Figure 3c). The hierarchical nanostructure consisted of nanorods with a length of 100−200 nm and an average diameter of 22.4 nm, which can provide many sites of intense absorbance for the thin films. The hierarchical nanostructure in the thin film is polycrystalline (Figure S4, Supporting Information). The nanorod nucleates along the (111) crystal direction, as shown in Figure 3d. Such a rodlike architecture can endow a large surface area for absorbing more light when applied in an optoelectronic device. The hierachical nanostructure can increase the optical path to capture light efficiently by reflex between the network. In addition, the CIZS hierarchical nanostructure can provide a lot of channels to transfer the carriers produced on their surface to the conducting substrate. The cross section of the hierarchical film shows a clear doublelayer structure. The top layer with a thickness of ∼1.5 μm is a

compact CIZS layer with the sheet resistance of ∼103 ohm above a layer of ∼900 nm molybdenum (Figure S5, Supporting Information). The phase purities of CIZS nanocrystals, microcrystals, and thin films were confirmed by XRD patterns. The typical XRD patterns of CIZS nanocrystals and microcrystals that can be indexed to the cubic structure (JCPDS No. 47 1370) diffraction pattern are shown in Figure 4a. Diffraction peaks of CIZS nanocrystals and microcrystals at 2θ = 28.1, 46.8, and 55.3° correspond to (111), (220), and (131) planes of the tetragonal CIZS structure. The diffraction peaks of CIZS hierarchical films selenized at 2θ = 27.0, 44.6, and 52.8° display a left shift

Figure 4. (a) XRD patterns of CIZS nanocrystals, microcrystals, and hierarchical thin films on Mo substrate. (b) UV−vis absorbance spectroscopy of CIZS hierarchical films on FTO substrate. (c) Raman spectrum of CIZS hierarchical films. (d) EDS analysis of CIZS hierarchical films. C

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compared with that of CIZS nanocrystals and microcrystals because of the increase in lattice parameter with Se replacing S in the Cu2In2ZnS5 matrix after the film was annealed under a Se atmosphere. In addition, the full width at half-maximum of CIZS hierarchical films is smaller than that of CIZS nanocrystals and microcrystals because of their larger grain size and crystallinity. Figure 4b shows UV−vis absorbance spectroscopy of CIZS nanocrystals. It is estimated that the band gap energy of CIZS nanocrystals is ∼1.49 eV from the UV−vis absorbance spectroscopy in Figure 4b. Raman peak of CIZS hierarchical film locates at 295 cm−1, which approaches that of CIS material in the reported results (Figure 4c).34−36 EDS analysis of several areas of CIZS nanocrystals implies an average composition of Cu0.42In0.39Zn0.19S, as depicted in Figure 4d. XPS is used to confirm the composition of the hierarchical thin film and chemical state of four constituent elements in CIZS. Figure 5a−d corresponds to XPS of Cu 2p, In 3d, Zn 2p,

Figure 6. (a) Cyclic voltammograms of CIZS hierarchical thin film on Mo substrate and Pt on FTO as electrode at a scanning rate of 0.5 mV s−1 in the voltage range of −0.4 to 1.2 V vs Ag/AgCl at room temperature. (b) Current density−voltage (J−V) curves of CIZS hierarchical thin film and Pt as electrode.

DSSCs are 6.9% and 6.1%, respectively, which indicates that the CIZS hierarchical films have good catalytic activity (Figure 6b). CIZS hierarchical thin films possess excellent electrocatalytic activity due to their hierarchical nanostructure. However, the electrocatalytic activity of CIZS thin film without a hierarchical nanostructure is ordinary. The electrocatalytic activities of both the hierarchical thin film and some non-Pt counter electrodes are all comparable to the Pt counter electrode in DSSCs.24,25 Linear sweep voltammetry (Figure S6a, Supporting Information) is implemented to investigate the correlation between current density and applied potential of the CIZS electrode in the dark and under illumination (>420 nm). It is obvious that photocurrent density is enhanced under illumination in comparison with dark conditions. Figure S6b (Supporting Information) is the corresponding photocurrent response to on/off light cycling of the CIZS electrode. Evidently, the steady photocurrent generation could be obtained during the chopped light source. More importantly, the flexible method can obtain simultaneously nearly monodisperse nanocrystals and hierarchical films with special nanostructures by reaction of the clear solution of the precursor including Cu2O, In(OH)3, ZnO, thioacetic acid, and ammonia aqueous solution, which avoids introducing extra anions in the products compared to other metal salts. The method provides a novel thought for the synthesis of other metal sulfides in the future.

Figure 5. (a) Cu 2p: bonding energy at 931.7 and 951.6 eV with a peak split of 19.9 eV from Cu(I). (b) In 3d: bonding energy at 445.4 and 452.9 eV with a peak split of 7.5 eV from In(III). (c) Zn 2p: bonding energy at 1022.0 and 1045.0 eV with a peak split of 23.0 eV from Zn(II). (d) S 2p: bonding energy at 161.9 and 163.1 eV from S in sulfide phases in CIZS hierarchical films.

4. CONCLUSIONS In summary, a novel and facile method to synthesize nearly monodisperse CIZS nanocrystals, microcrystals, and hierarchical films is presented by reaction of a uniform solution in this paper. CIZS hierarchical films with large surface areas are formed by interconnecting CIZS nanorods. The hierarchical nanostructure can capture more light to generate the maximum photocarriers, which increases the catalytic activity. CIZS hierarchical films on molybdenum are exploited as a counter electrode material of dye-sensitized solar cells (DSSCs) for the first time. The DSSC with a power conversion efficiency of up to 6.1% displays good catalytic activity of CIZS hierarchical thin films.

and S 2p, respectively. There are two narrow and symmetric peaks at 932.7 and 952.5 eV in Figure 5a, corresponding to Cu (I) with a peak splitting of 19.8 eV. Figure 5b indicates that In 3d has a bonding energy at 445.1 and 452.7 eV with a peak split of 7.6 eV from In(III); Figure 5c indicates that Zn 2p has a bonding energy at 1022.0 and 1045.0 eV with a peak split of 23.0 eV from Zn(II); and Figure 5d is S 2p, which is according to the bonding energy at 161.9 and 163.1 eV from S in sulfide phases in CIZS hierarchical thin film. All of these confirm that the hierarchical products are composed of CIZS. Because the special nanostructure of CIZS films can capture more light and provide numerous transport channels, the catalytic activity of CIZS hierarchical thin film might be enhanced. To prove the conjecture, the cyclic voltammogram of the CIZS electrode is measured (Figure 6a), which displays obvious potential peaks similar to the Pt electrode. The photocurrent−voltage curves of dye-sensitized solar cells with Pt and CIZS hierarchical films on molybdenum as counter electrodes are shown in Figure 6b. The efficiencies of the two

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ASSOCIATED CONTENT

S Supporting Information *

FESEM, TEM, and HRTEM images of CIZS nanocrystals; TEM image and SAED of CIZS microcrystals, FESEM image of cross section of CIZS hierarchical thin film; and linear sweep voltammetry and periodic on/off photocurrent response of D

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CIZS hierarchical thin film. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-21-52411620. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National 863 Program of China (Grant No. 2011AA050505,), NSF of China (Grant Nos. 91122034, 51202274, 51125006, 51102263, 61076062, 21101164), STC of Shanghai (Grant No. 10JC1415800), and Innovation Project of SICCAS (Grant No. Y23ZC6160G).

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