A physicochemical analysis on the chemical bath deposition technique (CBD) used for ZnS films deposition is discussed. Based on species present in solution and solubility diagrams, CBD-ZnS films experimental conditions are proposed at 80 • C by keeping fixed the ZnCl 2 and thiourea (SC(NH 2 ) 2 ) concentrations and by using different ones of NH 4 NO 3 and KOH used for the bath, to obtain different pH values. Better conditions were experimentally obtained for synthesizing ZnS thin films with high optical quality and good crystalline orientation. These better conditions produced ZnS films with bandgap energy of 3.73 eV and film thickness about 100 nm. It was observed that the crystalline structure of the deposited films depend strongly on the physicochemical conditions, giving a blenda-cubic structure with (111) as preferential orientation. However, additional diffraction peaks of ZnO and Zn(OH) 2 were observed on films deposited far from the better conditions. The surface roughness of the films ranged between 2.2 to 12.6 nm as obtained from atomic force microscopy images. XPS analysis on films surface confirms that excess of O concentrations affects the crystalline structure and the optical quality. Stoichiometry of the CBD-ZnS films was measured through [S]/[Zn] ratio and was found to be between 0.5 and 0.57.

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ZnS films were deposited onto glass substrates by the chemical bath technique at temperatures from 60 to 90 • C. Zinc chloride, potassium hydroxide, ammonium nitrate, and thiourea were used as chemical components. According to the species distribution diagrams, the concentration of the chemical components were maintained constant except for thiourea whose concentration is required to decrease when the bath temperature increases, in order to maintain constant the Zn(O H) 2− 4 /H S − ions concentration. To investigate the kinetic of growth of the ZnS films, their thicknesses were measured as a function of deposition time and bath temperature. The activation energy value estimated for the growing process was E a = 44.9 kJ/mol which is typical for chemical reactions. The mean value of the bandgap energy of the films was 3.67 eV with optical transmittances up to 80% for all bath temperatures. The rms-roughness of the deposited films was observed to decrease when the bath temperature increases. Results of X-ray diffraction analysis on deposited films reveal a cubic (111) 25 report uniformity and smoother morphology properties of CBD-ZnS films for improving solar cells devices. Investigations of some physical properties on the CBD-ZnS films were done by Gümüs et al. 26 and Ben-Nasr et al. 27 Gümüs et al. discuss the polycrystalline structure, electrical resistivity, carrier density, and the bandgap energy of the ZnS films as a function of the deposition parameters (deposition time, bath temperature, and concentrations of the chemical reagents). Ben-Nasr et al, study the effect of the pH and the annealing process on the structural, morphological and electronic properties of the ZnS films. Additionally, the effect of the bath temperature and concentrations of the chemical components on the growth rate, crystalline structure and optical properties of the CBD-ZnS films were investigated by Sundara. 28 An interesting work for understanding the process to control the deposition of ZnS and CdS materials and their chemical differences were reported by O'Brien et al. 29 As can be seen from the literature, CBD-ZnS films have recently received more attention due to their attractive properties that can be controlled during the deposition process under determined experimental conditions. In the CBD technique, deposition of metal-chalcogenide occurs in the surface of substrate immersed in an aqueous solution containing both the metallic and the chalcogenide ions. The well-known deposition mechanism to describe the chemical deposition is via the solubility z E-mail: isjuv.gp@gmail.com product K sp (for ZnS at 25 o C, K sp = 3 × 10 −25 ). 30 Thus, the material deposition occurs when the ionic product of both metallic and chalcogenide ions exceeds the solubility product K sp . They are chemically attached in the aqueous medium and precipitate on the substrate surface producing the metal-chalcogenide film. For the CBD technique is common to require, besides the metallic and non-metallic ions, complementary chemical reagents used as comp...

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Abstract. Zinc sulphide thin films were deposited on Corning glass substrates by the chemical bath deposition technique at different temperatures. The influence of the bath temperature and deposition time on the morphological and optical properties of the ZnS films are herein investigated. ZnS films were deposited by changing the bath-temperature from 50 ºC to 90 ºC, and deposition times from 60 to 160 min. Thin and transparent films were obtained with thicknesses from 10 to 90 nm with the increment of the bath temperature, meanwhile the band gap energy E g values diminishes from 4.15 to 3.4 eV. The quality of the ZnS film surfaces was also influenced by increasing the bath temperature, as showed by the reduced grain size and the increase of roughness, obtained from atomic force microscopy images. ZnS films of good optical quality were obtained at 90 ºC with a mean value of E g =3.56± 0.03 eV. IntroductionDue to its suitable optical properties, zinc sulphide (ZnS) emerges as an attractive semiconductor material for electronic and solar applications. ZnS thin films are commonly used as optical filter due to its appropriate band gap energy value of E g = 3.68 eV at room temperature [1][2][3]. They can also be used as buffer layer or as antireflective coating [4] for improving the efficiency of thin film solar cells. Efficiencies above 18 % have been reported for ZnS/Cu(In,Ga)Se 2 (CIGS) thin film solar cells [5]. Cadmium sulphide (CdS) has been typically used as buffer layer on CIGS thin solar cells to protect the Cu(In,Ga)Se 2 junction from possible damage during the zinc oxide (ZnO) sputtering deposition, which would cause unwanted modifications to the surface of the CIGS absorber. The efficiency of the CdS/CIGS solar cells drops at short wavelengths due to optical absorption losses from the CdS layer. The solar absorption can be improved by replacing the CdS film with other semiconducting material with a wider energy band gap. Because of its appropriate optical characteristics, ZnS arises as a good candidate for being used as a buffer layer in solar cell applications. The ZnS higher band gap energy (3.68 eV) [6] compared to that one of the CdS (2.42 eV) makes the ZnS wavelength range of transparency wider (above 330 nm) than the CdS one (above 520 nm) [8], increasing then the amount of sunlight on the photovoltaic conversion process. In addition, replacing CdS with ZnS is environmentally attractive by its non-toxicity. The effects of the chemical bath temperature on the ZnS rate of growth and of annealing temperature on the structural properties ZnS films have been studied [9,10]. In this work, ZnS thin films were deposited on glass substrates by the chemical bath deposition (CBD) technique varying the bath temperature and the deposition time, in order to study the influence of such parameters on the ZnS morphological and their optical properties.