Effect of cadmium precursor on the physico-chemical properties of electrochemically-grown CdS thin films for optoelectronic devices application: a comparative study

Thin film CdS materials for possible optoelectronic devices application have been synthesized by the electrochemical deposition method, using two different cadmium salts and sodium thiosulphate as precursors. In order to keep the synthesis process as simple as possible, no additional chemicals were added as complexing agents or buffers, and a two-electrode set-up was used. The qualities of the resulting films from the different cadmium precursors were compared by characterizing them for their physico-chemical properties using state-of-the-art glancing incidence X-ray diffraction, UV–Vis spectrophotometry, scanning electron microscopy and energy dispersive X-ray spectroscopy (EDX). The results obtained show significant influence of cadmium precursor on the structural, optical and chemical properties of the films. Whereas CdCl₂ precursor produced CdS with purely hexagonal structure in both as-grown and heat-treated conditions, Cd(CH₃COO)₂ produced CdS with purely cubic crystal structure, with films from both precursors showing clear difference in physical appearance. Energy bandgap values estimated for the films are in the range (2.49–2.57) eV and (2.22–2.23) eV in as-deposited form, with CdCl₂ and Cd(CH₃COO)₂ precursors respectively. After annealing, these values become 2.42 eV with CdCl₂ and (2.30–2.31) eV with Cd(CH₃COO)₂. Although CdS from both precursors show similar surface morphology, EDX results reveal that CdS films from CdCl₂ precursor are more stoichiometric with Cd/S atomic ratios of (0.90–0.93) compared to films from Cd(CH₃COO)₂ precursor with Cd/S values of (0.87–0.88).