Available online at ScienceDirect. Energy Procedia 84 (2015 )

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1 Available online at ScienceDirect Energy Procedia 84 (2015 ) E-MRS Spring Meeting 2015 Symposium C - Advanced inorganic materials and structures for photovoltaics Study of CuS Thin Films for Solar Cell Applications Sputtered from Nanoparticles Synthesised by Hydrothermal Route F. Ghribi a, A. Alyamani b, Z. Ben Ayadi a, K. Djessas c, L. EL Mir a,d * a Laboratory of Physics of Materials and Nanomaterials Applied at Environment (LaPhyMNE), Gabes University, Faculty of Sciences in Gabes, Gabes 6072, Tunisia. b National Nanotechnology Research Centre, KACST, Riyadh, Saudi Arabia. c Laboratoire de Mathématiques et Physique des Systèmes (MEPS), Université de Perpignan, 52, avenue Paul Alduy, Perpignan Cedex, France. d Al Imam Mohammad Ibn Saud Islamic University (IMSIU), College of Sciences, Department of Physics, Riyadh 11623, Saudi Arabia. Abstract Copper sulfide nanoparticles with a grain dimension between 10 nm and 50 nm were obtained by a simple chemical process in the first step. The reaction of thiourea with different sources of copper in distilled water, and by adjusting the proportion of reactants was studied. The synthesis of the nanoparticles was made by simple modified hydrothermal route at 90 C for 2 hour followed by annealing at 500 C for 1 hour using different types of Cu-precursors to obtain numerous compositions and structures of copper sulfide Cu x S (Cu 2 S: chalcocite, Cu 1.95 S: djurleite and CuS: covellite) for a fixed copper sulphur molar ratio. These nanopowders, in second step, were grown onto glass substrates by RF magnetron sputtering technique at room temperature. The optical and structural properties of the films were investigated. The obtained CuS films with a thickness of about 300 nm were polycrystalline textured, preferentially oriented with the (101) crystallographic direction. From optical study the films present a band gap of about 2 ev. The obtained layers are promising for photovoltaic applications The Authors. Published by Elsevier by Elsevier Ltd. This Ltd. is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of The European Materials Research Society (E-MRS). Peer-review under responsibility of The European Materials Research Society (E-MRS) Keywords: Copper sulfide; nanoparticles; thin films; Hydrothermal; Sputtering rf-magnetron; Structural and optical properties * Corresponding author. Tel.: 00 (216) ; fax: 00 (216) address: lassaad.elmir@fsg.rnu.tn The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of The European Materials Research Society (E-MRS) doi: /j.egypro

2 198 F. Ghribi et al. / Energy Procedia 84 ( 2015 ) Introduction Copper sulfide is an important chalcogenide semiconductor due to variation in properties depending on chemical composition [1,2]. The factors responsible for the synthesis of nonstoichiometric compositions of Cu x S are various. Gorai et al. show that the solvent played a key role to control the stoichiometry and morphology of the copper sulfide crystals [3]. Gazelbash et al. obtained Cu 1.8 S and CuS by varying the Cu/S ratio from 2:1 to 1:2 by the arrested precipitation method [4]. The effect of Cu-precursor was investigated by Kumar et al. [5]. Cu 1.8 S was the only product when CuCl 2.2H 2 O was dissociated in air as well as in flowing argon in ethylene glycol. A mixture of Cu 1.8 S and CuS was formed from the chloride ion containing precursor when dissociated solvothermally. The use of Cu(NO 3 ) 2.3H 2 O always yielded CuS. CuSO 4.5 H 2 O yielded a mixture of CuS and Cu 1.8 S on dissociation in the presence of air and argon, as well as under solvothermal conditions. In this study, we investigate the deposition of Cu x S thin films by rf-sputtering, using F. Ghribi et al. process [6], where the target was made from nanoparticles synthesized by solvothermal technique. EL Mir et al. [7,8] and [9] have reported that additional to the effect of deposition condition, the effect of ZnO aerogel nanoparticles sol-gel condition (doping and annealing) can enhanced the sputtered thin films properties. Herein the targets were made from nanoparticles synthesized by the hydrothermal route at low temperature. Hydrothermal reaction incorporates soluble copper and sulphur reagents into a solvent to lower the reaction temperature. To avoid the use of toxic H 2 S, most liquid phase reactions employ an alternate sulfur source such as Thiourea, which decomposes to H 2 S at low temperature [10]. Further the influence of Copper precursor on chemical compositions of Cu x S nanoparticle and thin films has been investigated. 2. Experimental details 2.1. Nano-Cu x S particles synthesis In a typical procedure, 20 mmol of CuSO 4.5 H 2 O and 10 mmol of Thiourea SC(NH 2 ) 2 were dissolved, each in 100 ml of water to form clear solutions under constant stirring. Then solutions were mixed slowly in a 500 ml closed round-bottomed flask with aggressive agitation; a blue green precipitate was formed, which was then transferred into a water bath and maintained at 90 C for 3 hour with continuous agitation to prevent the long reaction time used in hydrothermal synthesis. After cooling to room temperature naturally, the obtained blue-black products were filtered, washed with distilled water and absolute ethanol several times and desiccated at ambient temperature. The resulting product was then annealed at 500 C for one hour under Nitrogen atmosphere. The same procedure was repeated for other Cu-precursors (Cu(NO 3 ) 2.3H 2 O and CuC 4 H 6 O 4.H 2 O) to study his effect in the final product Thin films deposition The obtained products were deposited onto glass substrates by RF magnetron sputtering with rf generator operating at MHz, at room temperature. The sputtering chamber was evacuated to a base pressure of Pa before admitting the sputtering argon gas with high purity ( %) without oxygen. The sputtering targets were prepared from the powder of copper sulfide as described in the first step. The 1 mm thick glass substrates, were ultrasonically cleaned in HCl, rinsed in deionised water, then subsequently in ethanol and rinsed again. During the sputtering process, the substrate temperature was set at room temperature and the target-to-substrates distance was 75 mm Characterization The phase and purity of the as-prepared powder and thin films were examined by X-ray powder diffraction (XRD), using a Siemens Bruker X-ray diffractiometer equipped with CuKα radiation (λ= å). Typically θ-2θ spectra were collected between 2θ=20 and 70 in 0.02 steps. The optical transmittance, absorbance and reflectance of the powder and thin film in the UV to near infrared region (200 nm to 1800 nm) were measured at room temperature with Shimadzu UV-VIS/UV-VIS-NIR Scanning Spectrophotometer. Microstructural characterization

3 F. Ghribi et al. / Energy Procedia 84 ( 2015 ) was obtained using transmission electron microscope TEM (JEOL-JEM-100C). 3. Results and discussion 3.1. Structural and compositional study Fig. 1 represents the XRD patterns of samples S1, S2 and S3. With the same molar ratio of Cu:S = 2:1 of starting product, the composition of the resulting product, change with the type of Cu-precursor as shown in table 1. The diffraction pattern of sample S1 corresponds to the Cu 1.95 S phase, which matches with the standard data [NIST Pattern: djurleite, syn, ].The diffraction peaks of sample S2 match well with the standard peaks of Cu 2 S [NIST Pattern: copper sulfide, ]. Peaks of sample S3 indicate development of the CuS phase [NIST Pattern: covellite ]. This change in stoichiometry is explained by the difference of the solubility of each Cu-precursor and the time of reaction which can be short to dissolve all of the reactants. Fig. 1. X-ray diffraction of Cu xs nanoparticles prepared by hydrothermal route from different Cu-precursor. Fig. 2. TEM micrograph of CuS.particles synthesized from copper sulfite as Cu-precursor.

4 200 F. Ghribi et al. / Energy Procedia 84 ( 2015 ) Fig. 3. X-ray diffraction pattern sputtered Cu xs thin films. The size of copper sulphide particles can be estimated by the Debye-Scherrer formula[11]: 0.9 cos D (1) Where β is the broadening of diffraction line measured at half-maximum intensity (radians) and = Å the wavelength of the CuKα X-ray. The average particle size of the product depends on the Cu-precursor as seen in table.1. Fig. 2 Illustrates TEM image of CuS nanoparticles which have an average diameter of 30 nm. This result is confirmed by XRD result. Table 1. Effect of Cu-precursor on grain size, stoichiometry and band gap energy. Sample Cu-precusor Cu:S molar ration Resulting compound Size of nanoparticles (nm) Band gap (ev) S1 Cu(NO 3) 2.3H 2O 2:1 Cu 195S S2 CuC 4H 6O4.H 2O 2:1 Cu 2S S3 CuSO4.5H 2O 2:1 CuS Fig. 3 shows the XRD pattern of 300 nm sputtered copper sulfide thin films. A pure hexagonal CuS film, with highly (1 0 1) preferential orientation, was obtained from RF sputtering of the nanoparticles of sample S3 at ambient temperature. The grain size of CuS thin film was found to be an average of 32 nm Optical study The transmission and reflectance spectra of thin films are shown in Fig.4. The absorbance was calculated from the following relation: R T A 1 (2)

5 F. Ghribi et al. / Energy Procedia 84 ( 2015 ) These spectra reveal that film has high absorption coefficient (10 5.cm -1 ) and the reflectance is small (~0.1) in the wavelength region of nm. However, percentage transmission of CuS thin film vary in such a way that the transmission in the visible region (λ= nm) becomes more and more remarkably peaked around nm, while at the same time a substantial decrease in transmission throughout the NIR region ( nm) is observed and which is characteristic for copper sulphide thin film. The theory of optical absorption gives the relation between the absorption coefficient α and the photon energy hν, for direct allowed transition as: 1 2 h A h E ) (3) ( g Where hν is the photon energy and Eg the optical bandgap. A is the constant which is related to the effective masses associated with the valence and conduction bands. The absorption coefficient α was calculated from the following equation: A 2,303.( ) d (4) Where, d is thickness and A is the absorbance. The experimental values of (αhν) 2 against hν is plotted in Fig. 5. The variation of (αhν) 2 with hν is linear which indicates that the direct transition is present. Extrapolating the straight line portion of the plot of (αhν) 2 against hν to energy axis for zero absorption coefficient gives the optical band gap energy of the sample. It shows the optical bandgaps of CuS, Cu 1.95S, and Cu 2 S are 2.06, 1.95 and 1.88 ev, respectively, which is comparable with the reported values [12]. Fig. 4.(a) Transmission and (b) reflectance spectra of sputtered copper sulphide films Fig. 6 shows the variation of refractive index n and extinction coefficient K with the incident photon energy (hν). The magnitudes of n and k are calculated by using the relationship in equation 5 and 6. The average of refractive index was 4.09, 3.85 and 3.21 for CuS, Cu 1.95 S and Cu 2 S thin films respectively. ( n 1) ( n 1) 2 R (5) 2 and

6 202 F. Ghribi et al. / Energy Procedia 84 ( 2015 ) K (6) Fig. 5. The plot of (αhν) 2 vs hν of the sputtered copper sulphide films 4. Conclusion Figure 6: Refractive index (a) and extinction coefficient (b) of copper sulfide thin films By using different types of Cu-precursor we obtain numerous compositions and structures of copper sulphide between Cu 2 S (chalcocite) and CuS (covellite). The obtained CuS films with a thickness of about 300 nm were polycrystalline textured, preferentially oriented with the (101) crystallographic direction. From optical study the films present a band gap of about 2 ev. The obtained layers are promising for photovoltaic applications particularly as absorber in solar cells.

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