Polyol-mediated synthesis of copper indium sulphide by solvothermal process

Materials Chemistry and Physics 94 (2005) 434–437

Polyol-mediated synthesis of copper indium sulphide by solvothermal process S. Gorai, S. Chaudhuri ∗ Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700032, India Received 4 November 2004; received in revised form 25 April 2005; accepted 18 May 2005

Abstract A simple polyol-mediated solvothermal method has been proposed to synthesize copper indium sulphide. XRD studies reveal that the products are well crystallized. SEM indicates rod-like (with different aspect ratio) and star-shaped flake-like morphology of the products. The products are also characterized by optical studies and compositional analysis (XRF). XRF results show the formation of stoichiometric and non-stoichiometric copper indium sulphides depending on the reaction conditions. © 2005 Elsevier B.V. All rights reserved. PACS: 61.50.Nw; 81.10.Aj Keywords: Crystal morphology; Solvothermal process; Sulphides; Semiconducting compounds

1. Introduction Among different semiconductor absorber layers for fabrication of solar cells, chalcopyrite semiconducting materials represented by Cu(In,Ga)(S,Se)2 received much attention. Highly efficient solar cells were fabricated with CuInSe2 , CuIn1−x Gax Se2 and CuInS2 . Solar cells fabricated with CuInS2 thin film exhibited efficiency of ∼16% on an area of 1 cm2 [1], whereas the highest reported efficiency in its quarternary Cu(InGa)Se2 based solar cell is around 19% [2]. Consideration of the “Green technology” in recent years attracted researchers to use CuInS2 as a suitable absorber material replacing toxic selenides. Further, CuInS2 has a band gap of 1.5 eV, which is very near to the optimum band gap (1.4 eV) for solar cells. Development of a low cost technique for the preparation of the non-toxic CuInS2 absorber layers will be very effective to replace selenides. In addition to solar cells, CuInS2 is also useful as non-linear optical material [3]. Various methods were proposed for fabrication of CuInS2 including spray pyrolysis [4], rf sputtering [5,6], sequential

∗

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deposition of Cu2 S and In2 S3 [7], sulphurization [5,6,8], electrodeposition [9], coevaporation [10–12] and chemical processes [13–16]. In recent years, several morphologies of CuInS2 have been reported, such as nanoparticles [17,18], nanorods [19,20], nanotubes [21], foam-like nanocrystallites [22] and porous CuInS2 microspheres [23]. Previously, we prepared bulk stoichiometric CuInS2 by wet chemical method [13,14] which was a pH-controlled method. In the present work, we propose a convenient solvothermal method to synthesize CuInS2 flakes/rods with different aspect ratio.

2. Experimental Copper(II) chloride dihydrate (99%, E. Merk), indium(III) chloride and thiourea (Spectrochem, India) were used to prepare copper indium sulphide. Indium chloride was prepared from indium metal (Plasma Materials, USA, 99.999%) and hydrochloric acid (International Chemicals, L.R.). Two types of molar ratio combinations of the precursor solutions such as (a) Cu:In:S = 1:1:3.5 and (b) Cu:In:S = 1:1.25:3.5 were put into a Teflon-lined stainless steel autoclave. The autoclave was 80% filled with different solvents such as polyethylene

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glycol-300 (PEG-300) and polyethylene glycol-600 (PEG600). It was maintained at 200 ◦ C for 12 h and then air-cooled to room temperature. The black precipitate was filtered and washed with ethanol and distilled water. Finally, the precipitate was dried at 80 ◦ C for 3 h in vacuum. The products synthesized from 1:1 molar solutions of copper and indium salts containing PEG-300 and PEG-600 solvents are named as samples I and II, respectively. Similarly, the products obtained from indium-rich solutions containing PEG-300 and PEG-600 solvents are named as samples III and IV, respectively. The samples were characterized by XRD (Seifert, 3000P) using monochromatic Cu K␣ radiation (Ni filter). Morphologies of the samples were investigated by SEM (Hitachi, S-2300). The powder samples were uniformly dispersed in ethanol for deposition on soda lime glass substrates (by screen printing). The reflectance spectra of the films were recorded by a spectrophotometer (Hitachi U-3410).

3. Results and discussion The products were characterized by XRD measurements. All the XRD spectra, shown in Fig. 1, match well with the standard JCPDS (27-0159) pattern of CuInS2 . The strong and sharp reflection peaks (at 2θ = 27.9◦ , 32.3◦ , 46.2◦ , 55.0◦ ) suggest that the synthesized products are well crystallized. When these samples were scanned with very slow rate (inset of Fig. 1), in addition to the above peaks of CuInS2 some lower intensity peaks of CuS (marked by *) were also observed for samples I, II and IV. It may be concluded that slight amount CuS was formed in case of Cu-rich samples (samples I, II and IV). In order to determine chemical compositions of the molecule, XRF analysis was done (Table 1). The results show that the products, synthesized from 1:1 molar solution of copper and indium using PEG-300 as solvent, had Cu1.27 In0.61 S2.1 phase while for PEG-600 solvent Cu1.5 In0.6 S1.87 phase was formed. This indicates that at this condition slightly Cu-rich copper indium sulphide was formed. Products obtained from indium-rich solutions containing PEG-300 solvent show nearly stoichiometric phase of Cu1.02 In0.96 S2.01 and from this solution using PEG-600 as the solvent Cu1.1 In0.7 S2.1 was obtained. Fig. 2 shows the reflectance–wavelength spectra of the synthesized powders prepared from indium-rich and equimolar solutions of copper and indium salt with PEG-300 and PEG-600 solvents. The band gaps of the materials were Table 1 XRF analysis of the products Sample no.

Cu (at%)

In (at%)

S (at%)

I II III IV

31.86 37.88 25.57 29.87

15.41 15.21 23.98 17.33

52.72 46.90 50.25 52.78

Fig. 1. XRD patterns for copper indium sulphide powders synthesized at 200 ◦ C by using different PEG solvents. Asterisk (*) indicates peak of CuS.

determined from the differential reflectance spectra [24], which are shown in Fig. 3. The band gaps of the samples obtained from these spectra are shown in Table 2. It may be observed that the values of band gap of sample III, which was in perfect stoichiometry were close to the bulk value (1.5 eV) of CuInS2 . Other samples being slightly Cu-rich (as seen from Table 1) had lower band gaps which are as expected. The morphology of the product was investigated by SEM (Fig. 4). Fig. 4a shows the SEM of sample I, which indicates, star-shaped flake-like morphology. The average width and thickness of one separate flake are ∼0.26 ␮m and ∼75 nm, respectively. Fig. 4b–d shows the SEM pictures of samples II–IV, which indicate rod-like structure having different aspect ratio (length/diameter). For samples II–IV, these ratios were 8, 10 and 6, respectively.

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Fig. 2. Optical reflectance (R%) vs. wavelength (λ) traces for copper indium sulphide powders.

Fig. 3. Plot of [dR/dλ] vs. λ for copper indium sulphide powders.

Present work shows that polyethylene glycol is a good capping agent, as well as a good dispersive medium for the precursor materials, which may play an important role in the nucleation and growth of copper indium sulphide with

different morphologies. In the literature, it has been reported that many metal ions can form complexes with thiourea in alcohols [25,26]. In our case, it is believed that firstly Cu–thiourea and In–thiourea complexes were formed in

Table 2 Variation of stoichiometry, morphology and band gap of copper indium sulphide with the change of solvent Sample

Solvent

Molar ratio of Cu:In precursor

Morphology

Band gap (eV)

I. Cu1.27 In0.61 S2.1 II. Cu1.5 In0.6 S1.87 III. Cu1.02 In0.96 S2.01 IV. Cu1.1 In0.7 S2.1

PEG-300 PEG-600 PEG-300 PEG-600

1:1 1:1 1:1.25 1:1.25

Star-shaped flakes Rods Rods Rods

1.48 1.46 1.50 1.49

Fig. 4. SEM micrographs of copper indium sulphide powders: (a) sample I, (b) sample II, (c) sample III and (d) sample IV.

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polyethylene glycol. The formation of the above two species prevents the formation of copper sulphide and indium sulphide because of the free ions, Cu2+ , Cu+ , In3+ , S2− in the solution. At elevated temperature and pressure a major part of Cu2+ was reduced to Cu+ [27]. Polyethylene glycol and thiourea also help in the reduction of Cu2+ to Cu+ . At our reaction temperature, thermal decompositions of the above two complex species help to produce copper indium sulphide with flake- and rod-like morphologies. The above complexes are relatively stable as they are capped by the solvent polyethylene glycol with hydroxyl groups. So, at the decomposition stage, the process will proceed more slowly and produce a smaller number of nuclei in the solution than direct ion exchange reaction. At the same time, the boiling glycol helps to mix these newly formed nuclei homogeneously, which may result into the oriented growth of copper indium sulphide. Although, the main idea of the proposed mechanism has been reported in the literature [28], the exact interaction between polymer molecule and the metal–thiourea complexes to control the morphology of the product is still vague and needs in-depth understanding.

4. Conclusion Nearly stoichiometric copper indium sulphide with rodand flake-like morphologies were prepared via simple solvothermal route using polyethylene glycol as the solvent. The solvent used here helped to control the morphology of the material. It is evident from the present work that this interesting polymer solvent assisted fabrication method may be suitable to prepare similar important multinary chalcogenides with special morphologies.

Acknowledgements We are thankful to Prof. Pushan Ayyub and Mr. Nilesh Kulkarni, Tata Institute of Fundamental Research, Mumbai, India, for helping us to record the X-ray fluorescence spectra. The authors also like to thank Mr. K.K. Das of this institute (Indian Association for the Cultivation of Science, Kolkata, India) for recording the SEM pictures.

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