Sonochemical Method for Preparation of Copper Indium Sulfide Nanoparticles and their Application for Solar Cell

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1 Send Orders for Reprints to Combinatorial Chemistry & High Throughput Screening, 2014, 17, Sonochemical Method for Preparation of Copper Indium Sulfide Nanoparticles and their Application for Solar Cell Omid Amiri, Masoud Salavati-Niasari *, Mohammad Sabet and Davood Ghanbari Institute of Nano Science and Nano Technology, University of Kashan, Kashan, I.R. Iran Abstract: In this paper, CuInS 2 (CIS) nanoparticles were synthesized successfully via a new copper precursor [bis(acetylacetonato)copper(ii)], [Cu(acac) 2 ]; at room temperature by ultrasonic method. The effect of sulfur source, solvent, and reaction time was investigated on product morphology and particle size. A series of analyses was performed to characterize the CuInS 2 microsphere including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. CuInS 2 nanoparticles were prepared and coated on FTO. Later, the coated FTO was sintered so that a compact and dense CuInS 2 film was produced and measured for photovoltaic characteristics such as V oc, J sc and FF. Keywords: CuInS2, semiconductors, solar cells, ultrasonic. 1. INTRODUCTION New renewable and pure sources of energy have grabbed special attention for their environmental concerns due to the increase in energy consumption. The ternary I-III-VI 2 chalcopyrite which is imperative in solar energy materials and particularly in the semiconductor CuInS 2, shows industrial potential in photovoltaics because of its high absorption coefficient and direct band gap of 1.52 ev which lies in the optimum range for solar energy conversion [1, 2]. Compared to Cu(In, Ga)Se 2 solar cells, CIS solar cells are considerable options because of their potential to obtain a higher open circuit voltage and lower environmental impacts that they produce. Some of these methods are affected by many factors, namely, long reaction time, high pressure, high temperature, expensive surface materials (surfactant) and so on. Among several non-vacuum techniques, nanoparticle-based ink printing is considerable, given the convenience for large-scale production and sufficient utilization of materials [3, 4]. Three principal methods have been reported for preparation of nanoparticles related to CIS films. In the first method, CIS particles are directly synthesized by mechanical alloying or sol-gel method. In the case of sol-gel however, the process leads to porous CIS aggregates which are readily oxidized [5]. As for mechanical alloying, large distribution of particle sizes is a serious problem in their preparation [6]. In the second one, metal precursors such as metal oxides or metal salts are first prepared. Behind reduction and solemnization/sulfurization, separate particles are sintered together to form incorporated films. However, since metaloxide impurities may remain in the films and produce shrinks and cracks, an extra reduction process must be carried out [7]. In the third method, fine Cu-In alloy *Address correspondence to this author at the Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box , I.R. Iran; Tel: ; Fax: ; salavati@kashanu.ac.ir particles are produced via a top-down physical method (gas atomization or mechanical alloying) [8, 9]. Ultrasonic method is a facile route that operates under ambient conditions. Formation, growth and implosive collapse of bubbles in a liquid owing to ultrasonic sound waves radiation produce localized hot spots that can drive many chemical reactions [10]. Ultrasonic phased array techniques have been applied to ultrasonic nondestructive evaluation (NDE) in industries such as nuclear, aerospace and steel industry [11]. The sonochemical method has proved to be a good method to obtain new materials and nanomaterials [12-14]. The chemical effects of ultrasonic irradiation arise from acoustic cavitations; in other words, the creation, growth and rupture of bubbles in a liquid medium result in an immediate high temperature and pressure pulse [15, 16]. In this work, copper indium sulfide nanostructures were synthesized successfully using conventional and sonochemical precipitation technique. The effect of ultrasound parameters on crystal size and morphology of the synthesized CuInS 2 has been studied to figure out the advantage of ultrasound (input ultrasonic energy) in the synthesis of CuInS EXPERIMENTAL SECTION 2.1. Physical Measurements and Materials All the chemicals reagents used in our experiments were of analytical grade and were used as received without further purification. A multiwave ultrasonic generator (Sonicator 3000; Bandeline, MS 72, Germany), equipped with a converter/transducer and titanium oscillator (horn), 12.5 mm in diameter, operating at 20 khz with a maximum power output of 600 W, was used for the ultrasonic irradiation. The ultrasonic generator automatically adjusted the power level. The wave amplitude in each experiment was adjusted as needed. XRD patterns were recorded by a Rigaku D-max C III, X-ray diffract meter using Ni filtered Cu Ka radiation. Scanning electron microscopy (SEM) images were obtained on Philips XL-30ESEM equipped with an energy dispersive /14 $ Bentham Science Publishers

2 184 Combinatorial Chemistry & High Throughput Screening, 2014, Vol. 17, No. 2 Amiri et al. X-ray spectroscopy. Room temperature photoluminescence (PL) was studied on an F-4500 fluorescence spectrophotometer Synthesis of CuInS 2 Nanocrystals Nine hundred mg (0.1 mmol) of [bis(acetylacetonato) copper(ii)] ([Cu(acac) 2 ]), 20 mg of InCl 3, 5.2 mg of sodium dodecyl sulfate (SDS), and 135 mg of thioacetamide (TAA) were dissolved in 30 ml propylene glycol (PG), all in 4 separate beacher. The Indium chloride solution was then added drop-wise to the copper acetylacetonate solution under constant stirring using a magnetic stirrer (mixture A) for 10 minutes. After that, SDS solution was added drop-wise to mixture A under constant stirring using a magnetic stirrer (mixture B) for 10 minutes. Then the whole mixture was added drop-wise to TAA solution under constant stirring using a magnetic stirrer for 10 minutes. Finally, the mixture was irradiated with ultrasonic (60W) for 30 minutes in pulse mode (2 min on and 1 min off). Also the reaction carried out at the room temperature. Checked for yield and characterized by XRD, FT-IR, PL, and SEM analysis. More details of the experiment are given in Table Preparation of Nanocrystalline CuInS 2 Electrodes To prepare the CuInS 2 electrodes, FTO glass used as current collector (Solar 4 mm thickness, pilkington, tec 7, Japan) which was primarily cleaned in a detergent solution using an ultrasonic bath for 10 minutes, then rinsed with water and ethanol. In the end, CIS electrode was prepared by doctor balding of CuInS 2 paste on FTO (resistivity: 14 X/h, pilkington, tec 7, Japan). The electrode configuration was a transparent layer of nanocrystalline CuInS 2 with a mean size of 24.5 nm and a scattering layer microcrystalline CuInS 2. The prepared electrodes were sintered at 350 C for 30 minutes. CdS were also deposited on the CuInS 2 film by traditional SCBD method [17]. In brief, CuInS 2 film was dipped into an ethanol solution containing 0.25 M of Cd (NO 3 ) 2 for 20 s. Later, it was rinsed with ethanol, dipped for another 20 s into a 0.25 M of ethanol solution of CH 4 N 2 S and rinsed again with ethanol. The two-step dipping procedure was considered as one cycle. In addition, this sequential coating was repeated for several cycles. 3. RESULTS AND DISCUSSION XRD analysis, which is the most applied technique to identify the crystallinity of structures, was employed to investigate the prepared sample. The X-ray diffraction spectrum of sample number 3, after annealing at 350 C, is shown in Fig. (1). The phases of CuInS 2 were Tetragonal with lattice parameters a = nm, b = nm, and c = As can be seen from Fig. (1), the sharp peaks suggest that the sample was highly crystallized. All possible peaks of copper Indium sulfide were observed, which indicate the polycrystalline nature of the product. Bragg's reflections for CuInS 2 nanoparticles were observed in XRD pattern at 2θ value of C, C, C, and 55 C representing (101), (112) and (103) planes of Tetragonal structure of copper Indium sulfide with the space group of I- 42d (JCPDS No ) (Fig. 1). From XRD data, Fig. 1, the crystallite size (D c ) of the prepared CuInS 2, sample no. 3, was calculated to 24.4 nm by using the Debey-Scherrer equation [18], D c = D C = Kλ βc0sθ where β is the breadth of the observed diffraction line at its half intensity maximum, K is the so-called shape factor which usually takes a value of about 0.9, and λ is the wavelength of X-ray source used in XRD (0.154 nm). The average size of the particles of sample no. 3 was 24.5 nm, which is to some extent, in agreement with those observed in SEM images. The adsorbed Cu 2+ and In 3+ ions on the Table 1. The Reaction Conditions of CuInS 2 Synthesized from [Cu(acac) 2 ], InCl 3 and Several Sulfur Source Sample No. Surfactant Solvent Sulfur Source Time (min) Power (W) *Propylen glycol. Glycerol. 1 SDS PG* TSC SDS PG Tu SDS PG TAA SDS PG L-Cyst SDS PG TGA SDS PG Na 2S 2O SDS PG CS SDS PG Tu SDS PG Tu SDS H 2O Tu SDS Gly Tu SDS PG Tu SDS PG Tu 30 60

3 Preparation of Copper Indium Sulfide Nanoparticles Combinatorial Chemistry & High Throughput Screening, 2014, Vol. 17, No substrate reacted with the S -2 ions in an ion precursor solution and formed the sulfide compound of In 2 S 3 and CuS according to Eq. (1) and (2). The reaction for the formation of stoichiometric CuInS 2 during annealing process is given in Eq. (3) [19]. Fig. (1). The X-ray diffraction pattern of CuInS 2 nanoparticle (sample no. 3). Cu 2+ + S 2- CuS (1) 2In S 2- In 2 S 3 (2) 2CuS + In 2 S 3 2CuInS 2 + S (3) Fig. (2) shows the photoluminescence spectra of CuInS 2 nanoparticles after being annealed for 2 h at 350 C under Ar atmosphere disperse in ethanol at room temperature. The spectrum also shows the photoluminescence emission at nm in solution. The photoluminescence efficiency of CuInS 2 nanoparticles at an excitation wavelength of 390 nm was found to be 99.1%. Furthermore, emissions were observed at 2.35 ev. Fig. (2). Photoluminescence spectra of CuInS 2 disperse in ethanol at room temperature. The product size and morphology were characterized under different conditions by scanning electron microscopy (SEM). To investigate the effect of sonication time on the morphology of the products, the reaction was carried out for 20 min (sample no. 8), 30 min (sample no. 2) and 40 min (sample no. 9) (Fig. 3a-c). When reaction time was chosen as 20 min small nanoparticles were obtained (Fig. 3a). By increasing the reaction time to 30 min smaller nanoparticles were achieved (Fig. 3b). Finally by choosing the reaction time to 40 min (Fig. 3c) aggregated particles were obtained that due to high energy of nanoparticles surfaces. In other words by increasing the time from 30 min to 40 min, the nanoparticles obtained in 30 min aggregated together and lump-like structure was achieved. Fig. (4) shows the effect of ultrasonic power on product size and morphology of nanostructures. When the power was 50 w, the particle size was about 100 nm (Fig. 4a). By increasing the power, the produced bubbles had more energy, and the products were agglomerated (Fig. 4b). Further increase in the power led to the production of uniform particles with small size of about 60 nm. Thus, optimal power was recorded at 70 W (Fig. 3b). In our experiment to investigate the effect of sulfide source on the product morphology, CuInS 2 was synthesized by sodium sulfite (Na 2 S 2 O 3 ), thiourea (Tu), thioglycolic acid (TGA), thioacetamide (TAA), thiosemicarbazide (TSC), L-cysteine (L-Cyst) and carbon disulfide (CS 2 ) as the sulfur source. The release order of sulfur was observed from maximum to minimum for TSC, Tu, TAA, L-Cyst, TGA, Na 2 S 2 O 3, and CS2 respectively. SEM observations of the CuInS 2 nanostructure with different sulfur sources are shown in Fig. (5). When TSC was used (Fig. 5a) as sulfur source, CuS and In 2 S 3 were formed quickly, and rate of creation of cores was more than growth, due to the fact that sulfur can be released quickly [20]. Thus the particles size become small and agglomeration takes place. By changing sulfur source to Tu the sulfur was released slower than the TSC, but the rate of creation of cores was still more than growth. So particle size was small, but the particles were not coherent, as depicted in Fig. (3b) when TAA was used as sulfur source, the release rate of sulfur was lower than before. Therefore the rate of growing was greater and the particles were bigger than before (Fig. 5b). By changing the sulfur source to L-cyst, the rate of creation of cores and growth became optimum and the obtained particles were in the same size of about 40 nm (Fig. 5c). Fig. (5d) shows SEM image of CuInS 2 that TGA was used as sulfur source; the rate of growth was greater than the rate of creation of cores so the particles became bigger. When Na 2 S 2 O 3 was used as sulfur source the resulted particles were bigger than before (Fig. 5e). When CS 2 was used as sulfur source, we didnot obtain any products; the CS 2 released the sulfur very slowly. For investigating the effect of solvent on the product size and morphology, three solvents were used with different surface tension: (a) (water) with a low surface tension, (b) (PG) with a medium surface tension and (c) (Glycerol) with a high surface tension. Fig (6) shows SEM image of CuInS 2 that water was used as a solvent. When water is used as solvent, aggregation of nanoparticles to specific value is possible [21]. With changing the solvent to Propylene glycol the product was formed from small nanoparticles and the particles were more uniform than before (Fig. 3b), because solvent bubbles had more energy than water. With changing the solvent to glycerin, we didnot obtain any product due to the fact that solvent had high surface tension. So this power was not able to create the solvent bubbles and the reaction could not proceed. Fig. (7) shows TEM image of sample No. 2. As can be seen in this figure, the product was composed from nanoparticles ranging from 30 to 40 nm in diameter that is in good agreement with other results. In this work CuInS 2 paste of nanoparticles and thin film with doctor balding technique was prepared (Fig. 8).

4 186 Combinatorial Chemistry & High Throughput Screening, 2014, Vol. 17, No. 2 Amiri et al. Fig. (3). SEM images of the as-synthesized CuInS 2 nanostructures for: (a) 20 min (sample no. 8), (b) 30 min (sample no. 2) and (c) 40 min (sample no. 9). Fig. (4). SEM images of the CuInS 2 nanostructures at different power: (a) 50 W (sample no. 12) and (b) 60 W (sample no. 13).

5 Preparation of Copper Indium Sulfide Nanoparticles Combinatorial Chemistry & High Throughput Screening, 2014, Vol. 17, No Fig. (5). SEM images of the CuInS 2 nanostructure with different sulfur source: (a) TSC, (b) TAA, (c) L-Cyst, (d) TGA and (e) Na 2 S 2 O 3.

6 188 Combinatorial Chemistry & High Throughput Screening, 2014, Vol. 17, No. 2 Amiri et al. Fig. (6). SEM images of CuInS2 (sample no. 10). J-V curves give key parameters to explain a solar cell characteristic. These parameters are: short-circuit current (jsc) and the open-circuit voltage (VOC). Another important parameter for solar cells is fill factor (FF) that describes the quality of the diode Equation, [22]: FF = vmax * Jmax voc * Jsc various methods such as ILGAR, ligand exchange, solvothermal, spray pyrolysis, solvothermal and sputtering from different precursors. In comparison with similar other works that are illustrated in Table 2, we used low cost, fast and simple sonochemical method. Herein, to the best of our knowledge, for the first time this inorganic precursor has been used for the preparation of CuInS2 by sonochemical method. This film was exposed under illumination with 100 mw/cm2, and measurement Voc, Jsc and FF. representative JV curve of a CuInS2 nanoparticles device was shown in Fig. (9). Fig. (8). SEM images of thin film CuInS2 4. CONCLUSIONS Fig. (7). TEM image of sample No. 2. Nanoparticles of CuInS2 have recently been grown by Table 2. In summary, CuInS2 nanoparticles with the tetragonal structure type were synthesized by a sonochemical method. This method brings forward a broad idea to synthesize other rare-earth compounds with various morphologies and novel properties. The XRD, PL, TEM and SEM were used to Different Approach for Synthesis of CuInS2 Nanostructures Method Cu Source In Source S Source Size Morphology Ref. 1 ILGAR CuCl InCl3 H2 S nm Particle [23] 2 Ligand exchange CuCl InCl3 S powder 12 nm Particle [24] 3 Spray pyrolysis CuCl2 InCl3 CH4N2S 1 µm Thin film [25] 4 Solvothermal CuCl2.2H2O In(NO3)3.9H2O CH4N2S µm Cubic-like [26] 5 Sputtering Cu In H2 S nm Particle [27]

7 Preparation of Copper Indium Sulfide Nanoparticles Combinatorial Chemistry & High Throughput Screening, 2014, Vol. 17, No characterize the products. The effects of some parameters such as sulfur source, solvent, time and power sonication on the product size and morphology were also investigated. The CuInS 2 thin film was deposited on the glass substrates by doctor blading and its feature in the solar cell such as V oc, J sc and FF was investigated. Fig. (9). A representative I-V curve of a CuInS 2 nanoparticles (sample no. 2). CONFLICT OF INTEREST The authors declare that they do not have any conflict of interest. ACKNOWLEDGEMENTS Authors thank the graduate council of University of Kashan for providing financial support to undertake this work. REFERENCES [1] Siemer, K.; Klaer, J.; Luck, I.; Bruns, J.; Klenk, R.; Bräunig, D. Efficient CuInS 2 solar cells from a rapid thermal process (RTP). Solar Energy Materials and Solar Cells, 2001, 67, [2] Seeger, S.; Ellmer, K. Reactive magnetron sputtering of CuInS2 absorbers for thin film solar cells: Problems and prospects. Thin Solid Films, 2009, 517, [3] Peng, S.; Cheng, F.; Liang, J.; Tao, Z.; Chen, J. Facile solutioncontrolled growth of CuInS 2 thin films on FTO and TiO 2/FTO glass substrates for photovoltaic application. Journal of Alloys and Compounds, 2009, 481, [4] Wang, Z.; Mo, X.; Li, J.; Sun, D.; Chen, G. Low-temperature synthesis and characterization of the single chalcopyrite phase CuInS 2 compound by vacuum sintering method. Journal of Alloys and Compounds, 2009, 487, L1-L4. [5] Lee, S-Y.; Kim, K-H.; Park, B-O.; Oxidation effect on densification of CuInS 2 absorber layer by paste coating. Thin Solid Films, 2008, 516, [6] Wada, T.; Kinoshita, H. Preparation of CuIn(S,Se) 2 by mechanochemical process. Thin Solid Films, 2005, , [7] Kaelin, M.; Rudmann, D.; Tiwari, A.N. Low cost processing of CIGS thin film solar cells. Solar Energy, 2004, 77, [8] Chen, G.; Wang, L.; Sheng, X.; Liu, H.; Pi, X.; Yang, D. Chemical synthesis of Cu(In) metal inks to prepare CuInS2 thin films and solar cells. Journal of Alloys and Compounds, 2010, 507, [9] Başol, B.M. Low cost techniques for the preparation of Cu(In,Ga)(Se,S) 2 absorber layers. Thin Solid Films, 2000, 361, [10] Suslick, K.S.; Choe, S.B.; Cichowlas, A.A.; Grinstaff, M.W. Sonochemical synthesis of amorphous iron. Nature, 1991, 353, [11] Mahaut, S.; Godefroit, J-L.; Roy, O.; Cattiaux, G. Application of phased array techniques to coarse grain components inspection. Ultrasonics, 2004, 42, [12] Shen, G.; Chen, D.; Tang, K.; Fang, Z.; Sheng, J.; Qian, Y. Polyolmediated synthesis of porous nanocrystalline CuInS2 foam. Journal of Crystal Growth, 2003, 254, [13] Han, S.; Kong, M.; Guo, Y.; Wang, M. Synthesis of copper indium sulfide nanoparticles by solvothermal method. Materials Letters, 2009, 63, [14] Yu, C.; Fan, Q.; Xie, Y.; Chen, J.; shu, Q.; Yu, J.C. Sonochemical fabrication of novel square-shaped F doped TiO 2 nanocrystals with enhanced performance in photocatalytic degradation of phenol. Journal of Hazardous Materials, 2012, , [15] Prasad, K.; Pinjari, D.V.; Pandit, A.B.; Mhaske, S.T. Phase transformation of nanostructured titanium dioxide from anatase-torutile via combined ultrasound assisted sol-gel technique. Ultrasonics Sonochemistry, 2010, 17, [16] Prasad, K.; Pinjari, D.V.; Pandit, A.B.; Mhaske, S.T. Synthesis of titanium dioxide by ultrasound assisted sol-gel technique: Effect of amplitude (power density) variation. Ultrasonics Sonochemistry, 2010, 17, [17] Nairn, J.J.; Shapiro, P.J.; Twamley, B.; Pounds, T.; Von Wandruszka, R.; Rick Fletcher, T.; Williams, M.; Wang, C.; Norton, M.G. Preparation of ultrafine chalcopyrite nanoparticles via the photochemical decomposition of molecular single-source precursors. Nano Letters, 2006, 6, [18] Pol, V.G.; Palchik, O.; Gedanken, A.; Felner, I. Synthesis of europium oxide nanorods by ultrasound irradiation. Journal of Physical Chemistry B., 2002, 106, [19] Long, F.; Wang, W-M.; Tao, H-c.; Jia, T-k.; Li, X-M.; Zou, Z-g.; et al. Solvothermal synthesis, nanocrystal print and photoelectrochemical properties of CuInS 2 thin film. Materials Letters, 2010, 64, [20] Katoh, R.; Tasaka, Y.; Sekreta, E.; Yumura, M.; Ikazaki, F.; Kakudate, Y.; Fujiwara, S. Sonochemical production of a carbon nanotube. Ultrasonics Sonochemistry, 1999, 6, [21] Hollingsworth, J.A.; Banger, K.K.; Jin, M.H.C.; Harris, J.D.; Cowen, J.E.; Bohannan, E.W.; Switzer, J. A.; Buhro, W.; Hepp, A. F. Single source precursors for fabrication of I-III-VI 2 thin-film solar cells via spray CVD. Thin Solid Films, 2003, [22] Hamadanian, M.; Gravand, A.; Jabbari, V. High performance dyesensitized solar cells (DSSCs) achieved via electrophoretic technique by optimizing of photoelectrode properties. Materials Science in Semiconductor Processing, 2013, 16, Received: February 21, 2013 Revised: August 4, 2013 Accepted: August 20, 2013