Article pubs.acs.org/cm
Versatile and Low-Toxic Solution Approach to Binary, Ternary, and Quaternary Metal Sulfide Thin Films and Its Application in Cu2ZnSn(S,Se)4 Solar Cells Qingwen Tian, Gang Wang, Wangen Zhao, Yanyan Chen, Yanchun Yang, Lijian Huang, and Daocheng Pan* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun, Jilin 130022, China S Supporting Information *
ABSTRACT: We developed a versatile and environmentally friendly solution approach for the fabrication of a variety of metal sulfide nanocrystal thin films. Metal oxides, metal hydroxides, metal chlorides, metal acetates, and metal acetylacetonates can be used as the starting materials and dissolved in thioglycolic acid and ethanolamine, forming many types of metal−organic precursor solutions. High quality CdS, SnS, CuInS2, CuSbS2, Cu2ZnSnS4, Cu(In0.7Ga0.3)S2, and luminescent Ag-doped ZnxCd1−xS nanocrystal thin films have been successfully prepared by spin-coating their corresponding metal precursor solutions. Cu2ZnSn(S,Se)4 thin film solar cell with a power conversion efficiency of 6.83% has been realized by this versatile method.
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INTRODUCTION
Metal sulfide nanocrystal thin films, such as CuS, Cu2S, ZnS, CdS, SnS, FeS2, Sb2S3, PbS, MoS2, CuAlS2, Cu2BaS2, CuSbS2, KSb5S8, CuInS2, Cu2ZnSnS4 etc., have been widely used in transparent conductive electrodes, low-emissivity windows, phase change materials, thin film transistors, quantum dot light-emitting diodes, and thin film solar cells.1−20 Thus, the fabrication of metal sulfide nanocrystal thin films has attracted considerable attention in the past two decades. These metal sulfide nanocrystal thin films are mainly deposited by selfassembling or coating their nanocrystal solutions, which usually require the complex nanocrystal synthesis and tedious postpurification.13−17 Besides, these nanocrystal thin films can also be directly deposited by chemical bath deposition, spray pyrolysis, electrodeposition, and molecular precursor-based solution approaches.21−29 The molecular precursor-based solution approach without the need of complex nanocrystal synthesis is particularly suitable for low-cost and high-speed deposition of metal sulfide nanocrystal thin films. Recently, Mitzi and co-workers developed a general hydrazine-based solution approach to fabricate many types of metal chalcogenide thin films, including Sn(SSe)2, In2Se3, KSb5S8, Cu(InGa)(S,Se)2 (CIGSSe), Cu2ZnSn(S,Se)4 (CZTSSe), etc.30−34 However, hydrazine is a highly toxic and explosive solvent, and it is undesirable in green chemistry. Therefore, developing a low-toxic and general solution method for the direct fabrication of a variety of metal sulfide nanocrystal thin films is still of great significance. © 2014 American Chemical Society
EXPERIMENTAL SECTION
Chemicals. Tin (II) oxide (SnO, 99.9%), cadmium hydroxide (Cd(OH)2, AR), antimony (III) oxide (Sb2O3, 99.99%), manganese(II) oxide (MnO, 99.99%), bismuth (III) oxide (Bi2O3, 99.99%), silver (I) oxide (98%), boron (III) oxide (B2O3, 99.99%), ethanolamine (AR), 2-methoxyethanol (AR), thioglycolic acid (HSCH2COOH, 99%), cadmium sulfate (CdSO4, 99%), thiourea (NH2CSNH2, 99%), and selenium powder (99.9%) were purchased from Aladdin. Copper (I) oxide (Cu2O, 99.9%), copper (II) chloride (CuCl2, 99%), copper (II) acetate (Cu(Ac)2, 99.99%), copper (II) acetylacetonate (Cu(acac)2, 99.99%), gallium acetylacetonate (Ga(acac)3, 99.99%), iron (II) acetylacetonate (Fe(acac)2, 99.95%), nickel (II) acetylacetonate (Ni(acac)2, 95%), zinc (II) oxide (ZnO, 99.99%), and indium hydroxide (In(OH)3, 99.99%) were purchased from Sigma-Aldrich. Copper (II) oxide (CuO, 99.99%) was obtained from ZhongNuo Advanced Material (Beijing) Technology Company, and ammonium hydroxide (NH4OH, 25%) was purchased from Beijing Chemical Works. All chemicals were used as received without any further purification. Preparation of a Variety of Molecular Precursor Solutions. First, 4.0 mL of 2-methoxyethanol, 1.2 mL of thioglycolic acid, and 2.0 mL of ethanolamine were mixed in a 25 mL conical flask with magnetic stirring at room temperature. The same solution was prepared in 17 flasks. Afterward, CuO (0.159 g, 2 mmol), Ag2O (0.0232 g, 0.1 mmol), ZnO (0.163 g, 2 mmol), SnO (0.269 g, 2 mmol), In(OH)3 (0.332 g, 2 mmol), Ga(acac)3 (0.734 g, 2 mmol), Cd(OH)2 (0.293 g, 2 mmol), B2O3 (0.070 g, 1 mmol), Sb2O3 (0.292 g, 1 mmol), Bi2O3 (0.466 g, 1 mmol), MnO (0.142 g, 2 mmol), Received: January 21, 2014 Revised: April 24, 2014 Published: April 25, 2014 3098
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Figure 1. (top) Dissolution mechanism of metal hydroxides in thioglycolic acid and ethanolamine; (bottom) digital pictures of a variety of metal− organic precursor solutions as well as as-prepared CdS, CuInS2, Cu2ZnSnS4, SnS, CuSbS2, and Cu(In0.7Ga0.3)S2 nanocrystal thin films on glass substrates; (samples A, B, and C) directly fabricated Ag-doped ZnxCd1−xS luminescent quantum dot thin films with a Zn/Cd ratio of 5:1, 3:1, and 1:1, respectively. Fe(acac)2 (0.508 g, 2 mmol), Ni(acac)2 (0.514 g, 2 mmol), Cu2O (0.143 g, 1 mmol), CuCl2 (0.341 g, 2 mmol), Cu(Ac)2 (0.399 g, 2 mmol), and Cu(acac)2 (0.524 g, 2 mmol) were loaded into the flasks, respectively. Subsequently, the flask was placed on a 65 °C hot plate until all of the solid was dissolved. Finally, the precursor solution was centrifuged at 12 000 rpm for 5 min. Fabrication of Metal Chalcogenide Thin Films. (a) CdS, SnS, CuInS2, CuSbS2, and Cu(In0.7Ga0.3)S2 nanocrystal thin films: these metal sulfide thin films were fabricated by spin-coating the corresponding metal precursor solutions on a glass substrate at 3000 rpm for 20 s in a nitrogen-filled glovebox, followed by a sintering process on a 300 °C hot plate for 3 min. (b) Ag-doped ZnxCd1−xS luminescent quantum dot thin films: Ag, Zn, and Cd precursor solutions were mixed and spun on a glass substrate. Then, the films were placed on a 210 °C hot plate for 15 s. The Ag doping concentration is 1.0%, and the molar ratio of Zn/Cd is 5:1, 3:1, and 1:1, respectively. (c) CZTS thin film: (i) Preparation of CZTS precursor solution. CZTS precursor solution was formed by dissolving CuO (1.76 mmol), ZnO (1.2 mmol), and SnO (1.0 mmol) into 4 mL of 2methoxyethanol, 1.2 mL of thioglycolic acid, and 2 mL of monoethanolamine. A clear and light-yellow solution was obtained after about 30 min on a 65 °C hot plate under magnetic stirring. Subsequently, the precursor solution was centrifuged at 12 000 rpm for 5 min. These procedures were performed in the open air. The CZTS precursor solution obtained by this way was so stable that it can be used in a few months. (ii) Preparation and selenization of CZTS nanocrystal thin film. CZTS precursor thin film was deposited by spincoating the precursor solution on a molybdenum coated soda lime glass substrate (20 × 20 × 1.1 mm) at 3000 rpm for 20 s in a nitrogenfilled glovebox (H2O and O2 levels maintained below 1 ppm). The film was then sintered on a 320 °C hot plate for 2 min to remove the organic residues, forming a CZTS nanocrystal thin film. This spincoating/sintering step was repeated seven times to obtain a ∼2.0 μm thick CZTS thin film. Next, the selenization of CZTS nanocrystal thin film was performed in a graphite box containing 0.4 g of selenium powder at 540 °C for 15 min in a rapid thermal processing (RTP) furnace (MTI, OTF-1200X-4-RTP) under nitrogen flow (40 mL/ min). Fabrication of CZTSSe Photovoltaic Devices. CZTSSe solar cells were fabricated according to the conventional structure of glass/ Mo/CZTSSe/CdS/i-ZnO/ITO/Al. First, a 60 nm thick CdS buffer
layer was deposited onto CZTSSe/Mo/glass substrate by a chemical bath deposition (CBD) method.27−29 Next, 70 nm thick i-ZnO and 250 nm thick indium tin oxide (ITO) thin films were deposited by RFsputtering and DC-sputtering on the CdS layer, respectively. Finally, the Al grid electrodes were thermally evaporated through a metal shadow mask as a current collector. A solar cell device with an active area of 0.368 cm2 (approximately 90% of total device area) was separated by mechanical scribing, yielding four standard glass/Mo/ CZTSSe/CdS/i-ZnO/ITO/Al solar cells on the same substrate. No antireflection layer was applied. Characterizations. The X-ray diffraction (XRD) patterns were recorded using a Bruker D8 X-ray diffractometer. The scanning electron microscope (SEM) images were taken on a Hitachi S-4800 equipped with an energy dispersive X-ray (EDX) analyzer (Bruker AXS XFlash detector 4010). The sample for EDX measurement was prepared on a glass slide. The thickness of the thin film was measured by a step profiler (AMBIOS, XP-100). J−V curves were measured with a Keithley 2400 source meter and an Abet Class AAB solar simulator (Sun 2000) by a homemade probe station.27 The light intensity was calibrated to 100 mW/cm2 using a Newport optical power meter (model 842-PE). The external quantum efficiency curve was measured using a Zolix SCS100 QE system equipped with a 150 W xenon light source and a lock-in amplifier. An atomic force microscope (AFM) image was taken on a Bruker Dimension Icon. An 1H-nuclear magnetic resonance (NMR) spectrum was recorded on a Bruker Avance 300. Thermogravimetric analysis (TGA) was performed by a STA 449F3 of NETZSCH. Elemental analysis of C, H, N, and S was conducted on Elementar Analysensysteme GmbH. A photoluminescence (PL) spectrum was measured on a Hitachi F-7000 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. X-ray photoelectron spectroscopy (XPS) was measured by an ESCALABMKII 250 XPS (VG Co., U.K.) using an Al Kα X-ray source. Characterization of Zn Precursor (Zn(SCH2 COO −H 3N+ CH2 CH2OH) 2). First, the Zn precursor was precipitated by the addition of 20 mL of chloroform and then was centrifuged at 12 000 rpm for 5 min. Next, the precipitation was redissolved in 4 mL of acetone. The purification procedure was repeated two more times. The solid was dried in an oven at 80 °C for 48 h. Finally, the product was characterized by 1H NMR, TGA, and elemental analysis (C, H, N, and S). 1H NMR (400 MHz, d6-DMSO): δ (ppm) 7.83 (broad, 3H, H3N+), 5.20 (broad, 1H, OH), 3.57 (triplet, 2H, CH2O), 2.94 (singlet, 2H, SCH2), 2.85 (triplet, 2H, N+CH2). 3099
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Since ZnS powder was found after TGA measurement, the calculated weight loss of Zn precursor (1 − MW(ZnS)/MW(C8H20O6N2S2Zn)) was 73.65%, which is very close to the measurement result of TGA (73.03%). Elemental analysis (wt %): calculated for C8H20O6N2S2Zn, 25.96 C, 5.41 H, 7.57 N, 17.31 S. Found: 25.86 C, 5.03 H, 6.29 N, 17.01 S.
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RESULTS AND DISCUSSION Here, we proposed a low-toxic and general solution approach to fabricate many types of metal sulfide nanocrystal thin films. Metal oxides and metal hydroxides were used as the starting materials and dissolved in thioglycolic acid (HSCH2COOH) and ethanolamine (H2NCH2CH2OH). Note that thioglycolic acid is widely used in in vivo bioimaging as a capping agent due to its low toxicity.35,36 Figure 1 top shows the dissolution mechanism of metal hydroxides in thioglycolic acid and ethanolamine. The chemical structure of metal precursor was confirmed by 1 H NMR (see Figure S1, Supporting Information), TGA, and elemental analysis. It is worth mentioning that metal oxides and metal hydroxides cannot be dissolved in thioglycolic acid in the absence of organic amines. Under basic conditions, organic thiols have a greater affinity with metal ions,37 thereby metal oxides and metal hydroxides are more easily dissolved. It was found that carboxyl group in thioglycolic acid plays a minor role in dissolving metal oxides because ethyl thioglycolate (HSCH2COOCH2CH3) and ethanedithiol (HSCH2CH2SH) can also be used to dissolve metal oxides (see Figure S2, Supporting Information). Apart from metal oxides and metal hydroxides, metal chlorides, metal acetates, and metal acetylacetonates can also be used to prepare metal−organic precursor solutions, as shown in Figure 1. As a result, Cu, Zn, Sn, In, Ga, Cd, B, Sb, Bi, Mn, Fe, and Ni precursor solutions were prepared by dissolving their corresponding metal precursors, which provide a general approach to deposit a variety of metal sulfide nanocrystal thin films by a spin-coating method, followed by a sintering process at high temperatures. Note that thioglycolic acid was also used as the sulfur source by thermal decomposition. Pinhole-free and crack-free binary CdS and SnS, ternary CuInS2 and CuSbS2, and quaternary Cu(InGa)S2 and Cu2ZnSnS4 nanocrystal thin films have been successfully fabricated by this general method (see Figures 1 and S3, Supporting Information). Here, 2methoxyethanol rather than water was chosen as the solvent to dilute metal precursor solutions for depositing metal sulfide thin films due to a better wetting ability on a molybdenumcoated substrate though these metal precursors are dissolved in water. Figure 2a shows a series of XRD patterns of as-prepared CdS, CuInS 2, Cu 2ZnSnS 4, SnS, CuSbS 2, and Cu(In 0.7 Ga 0.3)S 2 nanocrystal thin films. It was observed that CdS nanocrystal thin film has a zincblende structure, whereas CuInS2, Cu2ZnSnS4, and Cu(In0.7Ga0.3)S2 nanocrystal thin films possess a typical wurtzite structure. Recently, zincblende and wurtzite multinary Cu-based nanocrystals have been extensively reported in the literature.38−43 All the metal ions occupy the same position in the wurtzite unit cell. Additionally, CuSbS2 nanocrystal thin film has a chalcostibite structure (JCPDS, No. 44-1417), but SnS thin film shows a mixed zincblende/ orthorhombic structure.44 To further confirm these nanocrystal thin films, their chemical compositions were determined by EDX spectra and are listed in Figure 2b. It was found that their real compositions are very close to the ideal stoichiometric values. Figure S4, Supporting Information, shows the
Figure 2. XRD patterns (a) and EDX spectra (b) of as-prepared CdS, CuInS2, CuZnSnS4, SnS, CuSbS2, and Cu(In0.7Ga0.3)S2 nanocrystal thin films.
absorption spectra of as-fabricated CdS, SnS, CuInS2, and Cu2ZnSnS4 thin films, and their calculated band gaps are 2.52, 1.71, 1.48, and 1.53 eV, respectively. Surprisingly, luminescent Ag-doped ZnxCd1−xS quantum dot thin films can also be directly fabricated by coating the mixed Ag, Zn, and Cd precursor solution. PL spectra of Ag-doped ZnxCd1−xS quantum dot thin films were shown in Figure S5, Supporting Information. A tunable PL emission between green and red was obtained by changing the Zn/Cd ratio 5:1 to 1:1, as shown in Figures 1 and S5, Supporting Information. Moreover, the chemical compositions and the morphology of Ag-doped ZnxCd1−xS quantum dot thin films were characterized by EDX spectra and AFM, as presented in Figures S6 and S7, Supporting Information. These luminescent quantum dot thin films without the involvement of the complicated quantum dot synthesis have a high potential in the field of quantum dot-based light-emitting diodes.45 Recently, several groups have exerted great efforts to develop some molecular precursor-based solution approaches and successfully used them in CZTSSe thin film solar cells.29,34,46−51 These approaches can be divided into three systems, i.e., hydrazine-based solution approach,34,46−48 metal salt/thiourea-based solution approach,49−51 and dithiocarbamate-based solution approach.29 The most successful molecular precursor-based solution approach in terms of solar cell efficiency is the hydrazine-based deposition approach reported by Mitzi and co-workers,34,46,47 then Yang’s group expanded this approach.48 In this article, our versatile approach was adopted to fabricate Cu2ZnSn(S,Se)4 thin film solar cells. First, a ∼2 μm thick Cu2ZnSnS4 nanocrystal thin film was obtained by repeating the spin-coating process 7 times. Then, the selenization was conducted in elemental Se vapor at 540 °C for 15 min, forming a large-grained Cu2ZnSn(S,Se)4 absorber layer. Generally, large crystal grains are highly desired for the high performance CZTSSe solar cells. Figure 3a,c shows the topview images of as-prepared CZTS and selenized CZTSSe thin films. After selenization, CZTSSe thin film exhibits a compact and densely packed morphology with a grain size larger than 1 3100
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Figure 3. SEM top-view images of as-prepared CZTS (a) and selenized CZTSSe (c) thin films; SEM cross-sectional images of asprepared CZTS nanocrystal thin film (b) and a completed glass/Mo/ CZTSSe/CdS/i-ZnO/ITO/Al solar cell device (d).
μm. The crystal growth of the CZTSSe thin film was further confirmed by XRD since the XRD peaks become narrow and sharp after selenization, indicating that the selenized film is highly crystalline (see Figure S8, Supporting Information). Interestingly, the metastable wurtzite structure of as-prepared CZTS thin film was converted into the thermodynamically stable kesterite structure after selenization, which can be further confirmed by a Raman spectrum of CZTSSe thin film, which is shown in Figure S9, Supporting Information. It was observed that the selenized CZTSSe sample exhibits an obvious bilayer structure consisting of a densely large-grained top layer approximately 0.7 μm and a carbon-rich small-grained bottom layer about 1.4 μm, as shown in Figure 3d. According to some reports, a highly conductive carbon layer seems to have no significant effect on the performance of the CZTSSe solar cells.14 In addition, EDX analysis revealed that the selenized CZTSSe thin film has a Cu/(Zn + Sn) ratio of 0.87, Zn/Sn ratio of 1.03, and Se/(S + Se) ratio of 0.89, confirming that a desired Cu-poor and Zn-rich CZTSSe absorber layer was obtained (see Figure S10, Supporting Information). The valence state of Cu ions in CZTSSe thin film was confirmed by the XPS spectrum, as shown in Figure S11, Supporting Information. From the XPS results, it can be seen that the Cu 2p peaks located at 931.5 and 951.3 eV with a splitting value of 19.8 eV (Figure S11a, Supporting Information) correspond to Cu+ rather than Cu2+. Figure 4a displays the dark and light J−V curves of a typical CZTSSe solar cell. The as-fabricated CZTSSe device exhibits a power conversion efficiency (PCE) of 6.83% with a high short circuit current density (Jsc) of 31.54 mA/cm2, an acceptable open circuit voltage (Voc) of 0.424 V, and a low fill factor (FF) of 51.07%. To further investigate Jsc, the external quantum efficiency (EQE) spectrum was measured and shown in Figure 4b. The calculated Jsc from the EQE curve was 31.62 mA/cm2, very close to the value obtained by light J−V curve. The band gap of CZTSSe was determined to be 1.11 eV by a plot of [E × ln(1 − EQE)]2 vs E, which is somewhat lower than optimal 1.15 eV for highly efficient CIGSSe and CZTSSe solar cells.52,46 Our PCE was mainly limited by a low FF. According to the analysis of dark J−V curve (Figure S12, Supporting Information), the series resistance (Rs), shunt resistance (Rsh), diode quality factor (A), and reverse saturation current
Figure 4. J−V curves of the CZTSSe solar cell measured in the dark and under AM 1.5 G illumination; inset, a digital photograph of CZTSSe solar cells. (b) EQE spectrum of the CZTSSe solar cell; inset, the band gap was determined by the [E × ln(1 − EQE)]2 vs E curve.
density (Jo) was calculated to be 2.4 Ω·cm2, 2326 Ω·cm2, 1.6, and 8.5 × 10−4 mA/cm2, respectively. Our Rs is significantly higher than those of high performance CZTSSe solar cells in the literature.46 This is probably due to a thick small grained layer between the absorber layer and back contact layer. Thereby, our PCE could be further enhanced by eliminating the small grained layer under a more harsh selenization condition. Furthermore, Voc should be increased by improving the hole concentration of CZTSSe absorber layer via incorporating a small amount of sodium.53 The short-term stability of the CZTSSe solar cell device was evaluated in the dark. The as-fabricated CZTSSe solar cell was stored in air without encapsulation and was measured under standard conditions every 5 days. Figure 5a presents the stability of CZTSSe solar cell device for 60 days, and a slight decrease in PCE was observed. Moreover, our CZTSSe solar cells exhibit excellent repeatability, and more than 80% of CZTSSe devices have a PCE of over 6%, as shown in Figure 5b.
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CONCLUSIONS In summary, a versatile and low-toxic solution method has been developed to prepare many types of metal sulfide precursor solutions, including Cu, Ag, Zn, Sn, In, Cd, Ga, B, Sb, Bi, Mn, Fe, Ni, etc. A variety of metallic compounds, such as metal oxides, metal hydroxides, metal chlorides, metal acetates, and metal acetylacetonates, can be used as the starting materials and dissolved in the mixed thiol/amine solution to prepare metal− organic precursor solutions. High quality CdS, SnS, CuInS2, CuSbS2, Cu(InGa)S2, Cu2ZnSnS4, and luminescent Ag-doped ZnxCd1−xS nanocrystal thin films have been successfully fabricated by spin-coating their corresponding metal precursor solutions. These metal sulfide nanocrystal thin films have many high potential applications in thin film transistors, photo3101
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ACKNOWLEDGMENTS We are grateful to Prof. Jun Liu for the helpful discussion on NMR data. This study was finically supported by the National Natural Science Foundation of China (No: 91333108; 51302258; 51172229; and 51202241).
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Figure 5. (a) Stability of CZTSSe solar cell stored in air for 60 days; (b) efficiency distribution of as-fabricated 60 CZTSSe solar cell devices.
detectors, nanocrystal solar cells, and quantum dot light emitting diodes. By this low-toxic solution approach, a PCE of 6.83% has been achieved for Cu2ZnSn(S,Se)4 solar cells. In addition, this versatile solution approach should be extended to fabricate other Cu-based thin film solar cells, such as Cu(MxIn1−x)(S,Se)2 (M = B3+, Al3+, Ga3+), Cu(SbxBi1−x)(S,Se)2, and Cu2MSn(S,Se)4 (M = Cd2+, Fe2+, Co2+, Mn2+, Ni2+). Besides, these metal precursor solutions can also be used to synthesize various types of water-soluble metal sulfide nanocrystals. These studies are currently underway in our laboratory.
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ASSOCIATED CONTENT
S Supporting Information *
NMR spectrum of Zn precursor; SEM images of as-prepared SnS, CdS, CuInS2, and CuSbS2 thin films; the absorption spectra of as-fabricated CdS, SnS, CuInS2, and Cu2ZnSnS4 thin films; PL spectra of Ag-doped and undoped ZnxCd1−xS quantum dot thin films; EDS spectra, chemical compositions, and AFM image of Ag-doped ZnxCd1−xS quantum dot thin films; Raman and XPS spectra of CZTSSe thin film; XRD patterns and chemical compositions of CZTS and CZTSSe thin films; the analysis of dark J−V curve. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*(D.P.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 3102
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Chemistry of Materials
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dx.doi.org/10.1021/cm5002412 | Chem. Mater. 2014, 26, 3098−3103
