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.

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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A physicochemical analysis was performed on the solution used for synthetizing zinc sulfide (ZnS) films by chemical bath; such solution contained zinc chloride, potassium hydroxide, ammonium nitrate and thiourea as chemical reagents. Solubility curves and species distribution diagrams were used to obtain the best conditions for ZnS deposition in a wide range of bath temperatures between 25 and 90 • C. Solubility curves evidenced the necessity to adjust the thiourea concentration for each bath temperature in order to obtain enough amounts of sulfur ions needed for ZnS formation. The thiourea concentrations were fixed at 0.2 M (90 • C), 0.6 M (60 • C), 1.25 M (40 • C), and 2.25 M (25 • C) to get higher quality films. Characterization results of films deposited at the conditions described above showed the films to have a bandgap energy value between 3.65 and 3.72 eV, stoichiometry of [Zn]/[S] ≈1, and homogeneous and agglomerate-free surfaces, which make them proper for solar cells applications.