Solution-Processed Cu2ZnSnS4 Nanocrystal Solar Cells: Efficient

Dec 18, 2013 - Araki , H.; Kubo , Y.; Mikaduki , A.; Jimbo , K.; Maw , W. S.; Katagiri , H.; Yamazaki , M.; Oishi , K.; Takeuchi , A. Preparation of C...
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Solution-Processed Cu2ZnSnS4 Nanocrystal Solar Cells: Efficient Stripping of Surface Insulating Layers Using Alkylating Agents Satoshi Suehiro,† Keisuke Horita,† Kota Kumamoto,† Masayoshi Yuasa,‡ Tooru Tanaka,⊥ Katsuhiko Fujita,§ Kengo Shimanoe,‡ and Tetsuya Kida*,‡ †

Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences, ‡Department of Energy and Material Sciences, and §Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga Koen 6-1, Kasuga, Fukuoka 816-8580, Japan ⊥ Department of Electrical and Electric Engineering, Saga University, Honjo 1, Saga 849-8502, Japan S Supporting Information *

ABSTRACT: Solution-processed photovoltaic (PV) devices based on semiconductor nanocrystals (NCs) such as Cu2ZnSnS4 (CZTS) and CuInS2 (CIS) are attracting much attention for use in next-generation solar cells. However, the performance of NCbased devices is hindered by insulating surface-capping ligands that limit transfer/transport of charged carriers. Here, to remove surface-capping ligands (long-chain fatty amines) from NCs, we use the strong alkylating agent methyl iodide, which converts primary amines to quaternary amines that have low coordinating affinity to the NC surface. X-ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy analyses confirm the successful removal of capping ligands from the CZTS surface after treatment with methyl iodide without changing the crystal structure of CZTS. CZTS and CIS NC-based devices treated with methyl iodide exhibit a reproducible PV response under simulated sunlight. The developed route can potentially enhance the performance of NC-based devices used in a broad range of applications.

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films by electron beam deposition, followed by sulfurization, and achieved an energy conversion efficiency of 0.66%. In general, CZTS thin films are fabricated by vacuum processes such as electron beam deposition6 and thermal sputtering.7 Recently, a high conversion efficiency of above 9.6% for a CZTS film made by a solution-based process has been reported by IBM.8 This success is a clear indication of the effectiveness of a wet-chemical route to fabricate CZTS light-absorber layers. The advantages of wet-chemical routes over conventional vacuum processes are their lower cost and ability to control material composition. To date, a number of chemical routes such as successive ionic layer adsorption and reaction,9 electroplating,10 screen printing,11 and sol−gel12 methods to make CZTS thin films have been reported.

INTRODUCTION Compound semiconductor solar cells based on CdTe or Cu(In1−xGax)Se2 (CIGS) thin films have achieved high-energy conversion efficiencies of near 20%.1,2 The advances in these devices have led to the large-scale production of commercial photovoltaic (PV) modules. Despite their successful development, CdTe and CIGS are not amenable to realize sustainable production of solar cells because of the cost and scarcity of In, Ga, and Te. The toxicity of Cd also limits the widespread use of CdTe-based solar cells in some countries. Therefore, the development of alternative thin film materials that are made of earth-abundant, nontoxic elements is a major challenge. One possible alternative to CdTe and CIGS is thin films of Cu2ZnSnS4 (CZTS), which was first fabricated by Ito and Nakazawa.3 CZTS contains only earth-abundant, nontoxic elements and has almost ideal properties for solar cells such as a high light absorption coefficient and band gap energy of 1.4− 1.6 eV.4,5 Katagiri et al.6 first demonstrated the promising properties of CZTS as a solar cell material. They fabricated thin © 2013 American Chemical Society

Received: August 21, 2013 Revised: November 22, 2013 Published: December 18, 2013 804

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Scheme 1. Ligand Stripping Using Alkylating Agents such as Meerwein’s Reagent ((C2H5)3OBF4) and Methyl Iodine (CH3I)a

a

R′ = CH3 or C2H5, B− = BF4− or I−.

Thin film deposition using semiconductor nanocrystals (NCs) is a promising route to fabricate flexible, printed solar cells.13 Solution-processed semiconductor NC-based solar cells represent an alternative to organic solar cells because of their superior stability over organic materials. To date, solar cells using CdTe,13 PbSe,14 Cu2S,15 and Cu(In1−xGax)Se2 NCs16 have achieved conversion efficiencies in the range of 1−5% even without high-temperature treatment. Currently, CZTS NC-based devices showed an energy conversion efficiency of just 0.23%,17 which is low when compared with the efficiency reported for vacuum-processed thin film solar cells. Because of the large surface-to-volume ratio of NCs, numerous unpassivated surface atoms are present, which may serve as trap states and increase the probability of charge recombination. Another reason for these low efficiencies arises from the presence of surface-coordinating organic ligands that are used in NC synthesis. Coordinating ligands are necessary to synthesize NCs with controlled sizes and make them soluble in organic solvents, but they behave as insulators and block the charge carrier transport between NCs. Ligand exchange with short alkyl chain molecules such as butylamine,18 1,2-ethanedithiol,19 and ethylenediamine20 has been used to produce conductive NC films. Inorganic capping agents such as Sn2S64−, S2−, and HS− are also used for ligand exchange to fabricate electronic devices.21,22 Recently, a different approach has been developed by Rosen et al.,23 who showed that Meerwein’s reagent ((C2H5)3OBF4) is effective at stripping surface ligands such as carboxylates, phosphonates, and amines with long hydrocarbon chains from NCs such as PbSe, CdSe, ITO, and TiO2. Because of the strong alkylating activity of Meerwein’s reagent, once the ligands react with it, they lose their ability to coordinate to the NC surface. Ligand stripping is thought to be more useful than ligand exchange because it is possible to completely remove surface-insulating layers. Here, we demonstrate that oleylamine-capped CZTS NCbased solar cells exhibit a PV response after ligand stripping with alkylating reagents. We found that alkyl iodides such as methyl iodide (CH3I) can efficiently remove capping ligands from the NC surface while retaining its original crystal structure. The reaction shown in Scheme 1 is performed under moderate conditions. We then fabricate solar cells with the structure glass/ITO/ZnO/CdS/CZTS/Au mainly using solution processes. An emphasis of this study is to develop a

feasible, highly reproducible way to fabricate NC-based solar cells.

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EXPERIMENTAL SECTION Chemicals. Copper(II) acetylacetonate (99.99%), tin(IV) bis(acetylacetonate) dichloride (98%), 1,2-hexadecanediol (98%), triethyloxonium tetrafluoroborate (97.0%), and cadmium sulfate (99.99%) were purchased from Sigma-Aldrich. Zn(II) acetylacetonate (ultrapure) was purchased from Kanto Chemical Co., Inc. Sulfur powder, DMF (99.5%), and CH3I (99.5%) were purchased from Kishida Chemicals. Oleylamine (90%) and oleic acid (95%) were purchased from Wako Chemicals. Acetonitrile (99.5%) and 1,2-ethanedithiol (99.0%) were purchased from Tokyo Chemical Industry Co., Ltd. Synthesis of Cu2ZnSnS4 Nanocrystals. CZTS NCs were synthesized by a reaction between the appropriate metal acetylacetonates and sulfur powder in oleylamine, which works as a solvent and coordinating ligand, according to the literature.24,25 It is known that the performance of CZTSbased solar cells is dependent on chemical composition.26 Thus, we synthesized NCs with a copper-poor and zinc-rich composition that is widely used. Typically, Cu (1.3 mmol), Zn (0.9 mmol), and Sn (0.75 mmol) precursors and sulfur (3 mmol) were added to oleylamine (3 mL) in a three-necked flask. The flask was connected to a Schlenk line, and the reaction system was heated at 120 °C under an Ar flow to remove water and oxygen from the system. Then, the temperature was raised to 230 °C and kept there for 30 min. After the reaction, the product was washed several times with a mixture of hexane and isopropyl alcohol. The product was dispersed in toluene to make a coating ink to fabricate films. Synthesis of ZnO Nanocrystals. In a typical synthetic reaction, Zn(II) acetylacetonate (1 mmol), 1,2-hexadecanediol (10 mmol), oleic acid (5 mL), and oleylamine (15 mL) were loaded into a three-necked flask. The mixture was heated at 120 °C for 30 min under an Ar flow. Then, the reaction temperature was raised to 220 °C and maintained there for 2 h to produce ZnO NCs. The NCs were washed several times with a mixture of hexane and isopropyl alcohol and then dispersed in toluene to make a coating ink. The size of ZnO NCs was ∼8 nm, as determined by TEM analysis. Device Fabrication and Testing. Patterned ITO-coated glass substrates (15 Ω/sq.) were purchased from Kintec Company and cleaned with acetone and deionized water under sonication for 5 min before use. A ZnO window layer 805

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(thickness: 200 nm) was deposited on an ITO-coated glass substrate by drop casting using ZnO NC ink. After deposition, the ZnO film was heated to 300 °C for 30 min to remove capping agents from the ZnO NC surface. A CdS buffer layer (thickness: 30 nm) was deposited on the resulting ZnO film by chemical bath deposition. The glass substrate with the ZnO layer was placed on a hot plate heated at 90 °C, and then an aqueous solution containing CdSO4 (0.015 M) and thiourea (1.5 M) was added dropwise onto the substrate. The solution pH was adjusted to 10 by adding NH4OH. The substrate was removed from the hot plate after 3 min and rinsed with deionized water. A CZTS absorber layer (thickness: 150 nm) was deposited on the CdS layer by spin coating using a toluene dispersion containing CZTS NCs. The rotation speed was 2000 rpm, and the NC concentration of the ink was ∼0.23 mg/mL. The film was subjected to ligand stripping as described in the next section. Finally, the film was coated with gold electrodes (thickness: 100 nm) by thermal vacuum evaporation. The solar cell structure was glass/ITO/ZnO/CdS/CZTS/Au. The PV response of the solar cell was measured using a Keithley 2400 source meter under light irradiation from a solar simulator with an AM 1.5 filter (100 mW/cm2). Stripping of Oleylamine Ligands. Oleylamine ligands were removed from the CZTS NC surface with Meerwein’s agent ((C2H5)3OBF4) following a method described in the literature.23 The spin-coated CZTS film was soaked in acetonitrile (10 mL) containing (C2H5)3OBF4 (100 mM) for 5 min. Then, the film was washed with acetonitrile containing DMF (1 M, 10 mL). Treated films were carefully cleaned with hexane. After this reaction, the CZTS NCs were insoluble in hexane, indicating the efficient elimination of surface oleylamine. Oleylamine ligands were also removed with methyl iodide. Typically, the film was placed in acetonitrile (10 mL) containing CH3I (50 mM) for several minutes and then washed with acetonitrile and hexane. These treatments were carried out under oxygen- and water-free conditions at room temperature. Care must be taken when handling methyl iodide because it is toxic and has a high vapor pressure. As a reference, surface ligands were also exchanged with 1,2-ethanedithiol (C2H4(SH)2). Typically, the film was placed in acetonitrile (10 mL) containing 1,2-ethanedithiol (100 mM) for several minutes and then washed with hexane. Material Characterization. The synthesized NCs were analyzed by XRD using Cu Kα radiation (RINT2100; Rigaku) and TEM (JEM-2000EX/T; JEOL). The size of colloid particles was determined by dynamic light scattering (DLS) analysis using a DLS spectrophotometer (Zetasizer Nano ZS; Malvern Instruments). The composition of the CZTS NCs was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES; SPS1700 HVR, Seiko Instruments Inc.). The presence and absence of capping agents were determined by Fourier transform-infrared (FT-IR) spectroscopy (FTIR4100; JASCO). Light absorbance spectra were acquired with a UV−vis−NIR spectrometer (UV-3100; Shimadzu). The surface morphology and microstructure of the CZTS films were analyzed by AFM (BrukerAXS, NanoScope V/Dimension Icon) and TEM (HD-2700, Hitachi), respectively.

Figure 1. (a) DLS size histogram for CZTS NCs dispersed in toluene. Inset is a photograph of the NC colloid solution. (b) TEM image of Cu2ZnSnS4 NCs. Inset is a HAADF-STEM image of the NCs.

synthesized CZTS NCs dispersed in hexane. The average colloid size of NCs was estimated to be 10 nm with a standard deviation of 1.7 nm. Figure 1b shows a representative TEM image of the CZTS NCs. The measured particle size is almost consistent with the DLS results. The image shows good monodispersity of the NCs without aggregation. The inset in this figure shows a high-angle annular dark-field (HAADF)scanning transmission electron microscopy (STEM) image of the NCs. Clear lattice fringes are seen in this image, indicating that the NCs are highly crystalline. ICP-AES results indicated that the NC composition was Cu1.8Zn1.1Sn1S3.8; the composition was successfully tuned to a copper-poor and zinc-rich composition by controlling the starting precursor ratio, but was slightly sulfur deficient. Figure 2 shows FT-IR spectra of CZTS NC films treated with Meerwein’s agent, methyl iodide, and 1,2-ethanedithiol.

Figure 2. FT-IR spectra of thin films of CZTS NCs: (a) as-synthesized and following treatment with (b) 1,2-ethanedithiol, (c) (C2H5)3OBF4, and (d) CH3I.

To obtain these spectra, CZTS NCs were deposited on a Si substrate to evaluate the applicability of alkylating agents for ligand stripping. As-deposited CZTS NC films exhibited diagnostic signals of oleate ligands at around 2900 cm−1 ascribable to symmetric and antisymmetric CH stretches. After treatment with Meerwein’s agent and methyl iodide, these signals almost disappeared. In contrast, no significant reduction in the intensity of CH stretching bands was observed following treatment with 1,2-ethanedithiol, suggesting incomplete ex-

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RESULTS AND DISCUSSION Characterization of CZTS NC Films Made with Alkylating Agents. Figure 1a shows DLS results of as806

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change of surface ligands. These results clearly suggest that oleylamine ligands can be removed by alkylating agents, which convert oleylamine (a primary amine) to the corresponding quaternary amine that has less affinity to coordinate to the NC surface because of the absence of a lone pair of electrons and its bulky structure. It is considered that BF4− and I− weakly coordinate to the NCs, which can be easily washed from the surface together with the resulting quaternary amine using an organic polar solvent such as DMF, as shown in Scheme 1. The conversion of oleylamine to the corresponding quaternary amine by methyl iodide was verified by 1H NMR spectroscopy. Such conversion occurred because of the high alkylating ability of methyl iodide. However, the formation of the corresponding secondary and tertiary amines was also confirmed, suggesting incomplete conversion. Thus, further optimization of the experimental conditions is necessary to completely convert oleylamine to the corresponding quaternary amine. We next studied the crystal structure of CZTS NCs before and after ligand stripping using the alkylating agents Meerwein’s agent and methyl iodide. The XRD pattern (Figure 3b) of CZTS NC was consistent with that of reference CZTS

Figure 4. Raman spectra of CZTS NCs: (a) reference bulk powder, (b) as-synthesized NCs, (c) (C2H5)3OBF4-treated NCs, and (d) CH3I-treated NCs.

crystal structure was not changed by ligand stripping with alkylating agents. Characterization of CZTS NC-Based Solar Cells. We next studied the performance of PV devices with the structure ITO/ZnO/CdS/CZTS/Au. Figure 5a and b shows AFM

Figure 3. XRD patterns of CZTS NCs: (a) reference bulk powder, (b) as-synthesized NCs, (c) (C2H5)3OBF4-treated NCs, and (d) CH3Itreated NCs.

bulk powder (Kojundo Chemical Laboratory Co., Ltd., Japan) (Figure 3a). No change in the XRD pattern was observed after ligand stripping, as shown in Figure 3c and d. The patterns are all consistent with the kesterite phase. In addition, the crystallite size calculated using the Scherrer formula was constant at ∼11 nm even after the ligand-stripping treatment. It is known that the XRD diffraction pattern of CZTS is similar to those of ZnS and Cu2SnS3. Thus, XRD patterns alone are not sufficient to confirm the presence of CZTS and/or impurity phases. To obtain further structural information, we measured the Raman spectra of the NC samples. As illustrated in Figure 4b, a strong Raman peak was observed at 338 cm−1 in the spectrum of the untreated NCs, which agrees well with that of a bulk powder sample (Figure 4a). The Raman spectra were not influenced by ligand stripping, as shown in Figure 4c and d. In combination with the XRD results, we found that the synthesized NCs were CZTS in the kesterite phase and the

Figure 5. (a, b) AFM and (c, d) TEM images of CZTS films deposited on ZnO/CdS layers before (a, c) and after (b, d) ligand striping with methyl iodide.

images of a CZTS film deposited on a ZnO/CdS layer before and after ligand striping with methyl iodide, respectively. The former image reveals that the initial film was composed of aggregated particles with a size of 100−500 nm, which were almost homogeneously packed to form a dense film. The calculated average and root-mean-square (rms) roughness of this film were 65.9 and 83.1 nm, respectively. After treatment with methyl iodide, cracks appeared in the film surface, suggesting that the film shrank after the removal of oleylamine from the NC surfaces. Accordingly, the average and RMS roughness of this film decreased to 45.0 and 58.1 nm, 807

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respectively. To more clearly see the change in the microstructure of the films after ligand stripping, they were observed by TEM. Figure 5c and d depicts TEM images of CZTS films deposited on ZnO/CdS layers before and after ligand striping, respectively. The CZTS films were etched by a focused ion beam technique to make a thin region in the films for TEM observation. In the film before ligand stripping, the NCs were uniformly closely packed (Figure 5c). In contrast, after treatment, the NCs in the film aggregated (Figure 5d). This NC aggregation should be caused by the ligand stripping that allowed intimate contact between the NCs. Note that the crystal aggregation resulted in the formation of some voids in the film. Such NC aggregation in the films might also be responsible for the crack formation observed after ligand stripping in AFM images (see Figure 5b). Figure 6 shows the UV−vis−NIR absorption spectrum of each layer of the PV device, together with a schematic diagram

Figure 7. Current−voltage (I−V) curves of NC-based devices using Cu2ZnSnS4 treated with (a) (C2H5)3OBF4, (b) CH3I, and (c) 1,2ethanedithiol and (d) using CuInS2 treated with CH3I.

glass/Au/CZTS/ZnO/ITO where the CZTS and ZnO/ITO layers were fabricated by spray coating and sputtering, respectively. The differences in the device structure and processing conditions of the CZTS and ZnO layers may be reasons for the lower performance of our devices. Another probable reason is the thin thickness of the absorber CZTS layer (∼150 nm). Increasing the absorber layer thickness should further improve its efficiency. The crack formation in the CZTS film is another problem. Sintering of this film at moderate temperature might be effective to increase its density and efficiency. In contrast, the device treated with 1,2ethanedithiol showed a lower efficiency than those of the devices treated with other alkylating agents, as shown in Figure 7c. This is probably caused by the incomplete ligand exchange of oleylamine, as confirmed by FT-IR analysis (see Figure 2b. To demonstrate the applicability of methyl iodide as a ligandstripping agent to make NC thin films, we also fabricated a CuInS2 (CIS) NC-based PV device following the same procedure. The synthesis conditions of oleylamine-capped CIS NCs and their characterization are described in the Supporting Information (Figure S1, S2, and S3). As indicated in Figure 7d, a CIS NC-based device also showed a PV response under simulated solar irradiation. An efficiency of 0.19% was obtained without any heat treatment. This value is almost comparable to the efficiencies that have been reported using CuInSe2 NCs29,30 and CIS NCs with high-temperature annealing.31 From the above results, it appears that alkylating agents are effective at ligand stripping to help fabricate NC thin films. Ligand stripping at room temperature is beneficial when forming pn junctions because high-temperature treatments induce the interdiffusion of interfacial elements, which degrades junction quality. We also confirmed that treatment with methyl iodide vapor and subsequent washing with DMF removed surface ligands. Such vapor-phase ligand stripping might be applicable for mass production of NC-based semiconductor films. The method reported here should also offer a reproducible way to fabricate other NC-based electrical devices

Figure 6. Schematic diagram of a CZTS NC-based PV device and UV−vis−NIR absorption spectra of (a) ZnO/ITO, (b) CdS/ZnO/ ITO, (C) (C2H5)3OBF4-treated CZTS/CdS/ZnO/ITO, and (d) CH3I-treated CZTS/CdS/ZnO/ITO films.

of the device structure. The absorption onsets of the bottom ZnO and CdS layers were observed at around 400 and 500 nm, respectively. The absorption onset in the UV region for ZnO is beneficial for the upper CZTS layer to absorb visible light. Meerwein’s agent- and methyl iodide-treated CZTS films on CdS/ZnO layers exhibited absorption onsets at around 900 nm, which corresponds to a band gap of 1.4 eV. This value is the almost same as those reported for CZTS NCs17,24,25,27 and vacuum-processed films.26,28 The visible and NIR absorption of the CZTS layers deposited on the CdS/ZnO layer indicates that the solution-processed ZnO layer works as a window layer. The detailed characterization of this solution-processed ZnO window layer will be reported elsewhere. Figure 7 shows representative current−voltage (I−V) characteristics of NC-based PV devices made following NC treatment with (a) Meerwein’s agent, (b) methyl iodide, and (c) 1,2-ethanedithiol. As expected, devices using as-synthesized CZTS NCs showed no PV response because of the surfaceinsulating oleylamine layer. In contrast, the devices treated with Meerwein’s agent and methyl iodide exhibited clear photocurrents and PV responses under AM 1.5 light irradiation (100 mW cm−2). The calculated power conversion efficiency (η) of these devices (Figure 7a and b) was 0.05%, with an open-circuit voltage (VOC) of 350−400 mV, short-circuit current (Jsc) of 0.40−0.44 mA/cm2, and fill factor (FF) of 0.32. However, the efficiency was lower than that of the CZTS NC-based device (0.23%) first reported by Steinhagen et al.17 with a structure of 808

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such as field effect transistors, light-emitting diodes, nonvolatile memories, and photodetectors.

(6) Katagiri, H.; Sasaguchi, N.; Hando, S.; Hoshino, S.; Ohashi, J.; Yokota, T. Preparation and Evaluation of Cu2ZnSnS4 Thin Films by Sulfurization of E−B Evaporated Precursors. Sol. Energy Mater. Sol. Cells 1997, 49, 407−414. (7) Tanaka, T.; Nagatomo, T.; Kawasaki, D.; Nishio, M.; Guo, Q.; Wakahara, A.; Yoshida, A.; Ogawa, H. Preparation of Cu2ZnSnS4 Thin Films by Hybrid Sputtering. J. Phys. Chem. Solids 2005, 66, 1978− 1981. (8) Todorov, T. K.; Reuter, K. B.; Mitzi, D. B. High-Efficiency Solar Cell with Earth-Abundant Liquid-Processed Absorber. Adv. Mater. 2010, 22, E156−E159. (9) Mali, S. S.; Shinde, P. S.; Betty, C. A.; Bhosale, P. N.; Oh, Y. W.; Patil, P. S. Synthesis and Characterization of Cu2ZnSnS4 Thin Films by SIAR Method. J. Phys. Chem. Solids 2012, 73, 735−740. (10) Araki, H.; Kubo, Y.; Mikaduki, A.; Jimbo, K.; Maw, W. S.; Katagiri, H.; Yamazaki, M.; Oishi, K.; Takeuchi, A. Preparation of Cu2ZnSnS4 Thin Films by Sulfurizing Electroplated Precursors. Sol. Energy Mater. Sol. Cells 2009, 93, 996−999. (11) Zhou, Z.; Wang, Y.; Xu, D.; Zhang, Y. Fabrication of Cu2ZnSnS4 Screen Printed Layers for Solar Cells. Sol. Energy Mater. Sol. Cells 2010, 94, 2042−2045. (12) Tanaka, K.; Fukui, Y.; Moritake, N.; Uchiki, H. Chemical Composition Dependence of Morphological and Optical Properties of Cu2ZnSnS4 Thin Films Deposited by Sol−Gel Sulfurization and Cu2ZnSnS4 Thin Film Solar Cell Efficiency. Sol. Energy Mater. Sol. Cells 2011, 95, 838−842. (13) Gur, I.; Fromer, N. A.; Geier, M. L.; Alivisatos, A. P. Air-Stable All-Inorganic Nanocrystal Solar Cells Processed from Solution. Science 2005, 310, 462−465. (14) Luther, J. M.; Law, M.; Beard, M. C.; Song, Q.; Reese, M. O.; Ellingson, R. J.; Nozik, A. J. Schottky Solar Cells Based on Colloidal Nanocrystal Films. Nano Lett. 2008, 8, 3488−3492. (15) Wu, Y.; Wadia, C.; Ma, W.; Sadtler, B.; Alivisatos, A. P. Synthesis and Photovoltaic Application of Copper(I) Sulfide Nanocrystals. Nano Lett. 2008, 8, 2551−2555. (16) Lee, J. H.; Chang, J.; Cha, J.-H.; Lee, Y.; Han, J. E.; Jung, D.-Y.; Choi, E. C.; Hong, B. Large-Scale, Surfactant-Free Solution Syntheses of Cu (In,Ga) (S,Se)2 Nanocrystals for Thin Film Solar Cells. Eur. J. Inorg. Chem. 2011, 647−651. (17) Steinhagen, C.; Panthani, M. G.; Akhavan, V.; Goodfellow, B.; Koo, B.; Korgel, B. A. Synthesis of Cu2ZnSnS4 Nanocrystals for Use in Low-Cost Photovoltaics. J. Am. Chem. Soc. 2009, 131, 12554−12555. (18) Jarosz, M. V.; Porter, V. J.; Fisher, B. R.; Kastner, M. A.; Bawendi, M. G. Photoconductivity Studies of Treated CdSe Quantum Dot Films Exhibiting Increased Exciton Ionization Efficiency. Phys. Rev. B 2004, 70, 195327. (19) Luther, J. M.; Law, M.; Song, Q.; Perkins, C. L.; Beard, M. C.; Nozik, A. J. Structural, Optical, and Electrical Properties of SelfAssembled Films of PbSe Nanocrystals Treated with 1,2-Ethanedithiol. ACS Nano 2008, 2, 271−280. (20) Riha, S. C.; Fredrick, S. J.; Sambur, J. B.; Liu, Y.; Prieto, A. L.; Parkinson, B. A. Photoelectrochemical Characterization of Nanocrystalline Thin-Film Cu2ZnSnS4 Photocathodes. ACS Appl. Mater. Interfaces 2011, 3, 58−66. (21) Kovalenko, M. V.; Scheele, M.; Talapin, D. V. Colloidal Nanocrystals with Molecular Metal Chalcogenide Surface Ligands. Science 2009, 324, 1417−1420. (22) Nag, A.; Kovalenko, M. V.; Lee, J.-S.; Liu, W.; Spokoyny, B.; Talapin, D. V, Metal-Free Inorganic Ligands for Colloidal Nanocrystals: S2−, HS−, Se2−, HSe−, Te2−, HTe−, TeS32−, OH−, and NH2− as Surface Ligands. J. Am. Chem. Soc. 2011, 133, 10612−10620. (23) Rosen, E. L.; Buonsanti, R.; Llordes, A.; Sawvel, A. M.; Milliron, D. J.; Helms, B. A. Exceptionally Mild Reactive Stripping of Native Ligands from Nanocrystal Surfaces by Using Meerwein’s Salt. Angew. Chem., Int. Ed. 2012, 51, 684−689. (24) Guo, Q.; Hillhouse, H. W.; Agrawal, R. Synthesis of Cu2ZnSnS4 Nanocrystal Ink and Its Use for Solar Cells. J. Am. Chem. Soc. 2009, 131, 11672−1167.

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CONCLUSIONS CZTS NCs of ca. 10 nm diameter were synthesized by a onepot method using metal acetylacetonates and sulfur powder in oleylamine at 230 °C. We used alkylating agents such as methyl iodide and Meerwein’s agent to remove oleylamine from CZTS NC-based films that were made by spin coating using a NC ink. XRD, Raman, and FT-IR analyses revealed that capped oleylamine was removed from the CZTS films by the alkylating agents without changing the crystal structure of CZTS. We demonstrated that solution-processed devices with the structure glass/ITO/ZnO/CdS/CZTS/Au that were treated with alkylating agents showed a PV response under simulated sunlight. In contrast, devices not exposed to ligand stripping showed no PV response because of the surface organic insulating layer. These results indicate that ligand stripping using methyl iodide or Meerwein’s agent will be universally applicable to making semiconductor thin film devices from NC inks.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis procedure, TEM image, crystal structure, and UV− vis−NIR absorption spectrum of CuInS2 nanocrystals. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Kyushu Industrial Technology Center and the Yoshida Gakujutsu Shinko Zaidan. T.K. thanks Prof. Paul Alivisatos of Lawrence Berkeley National Laboratory, Dr. Matt Lucas of UC Berkeley, and members of the Alivisatos group for their help with learning techniques to synthesize nanocrystals and fabricate nanocrystal-based photovoltaic devices. T.K. also thanks Prof. Masato Ito and Prof. Satoshi Hata of Kyushu University for helpful discussion about ligand stripping and TEM measurements, respectively. S.S. acknowledges the financial support from the Kato Foundation for Promotion of Science.

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REFERENCES

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