Influence of processing conditions on the structural properties of electrodeposited CdS films

Abstract

Cadmium sulphide (CdS) films were electrodeposited from an aqueous electrolytic solution onto indium tin oxide and fluorine doped tin oxide substrates, commonly used in photovoltaic applications. The CdS films were deposited for between 1 minute and 4 hours at temperatures ranging from 30°C to 90°C and a deposition potential of 600 mV with respect to a Ag/AgCI reference electrode. Some films were annealed in an argon atmosphere at temperatures of 300°C, 400°C and 460°C for 15 minutes and 30 minutes with and without CdCh2 treatment. The cleaning efficiency of the substrates by several solvents was determined using atomic force microscopy (AFM), surface roughness and scanning electron microscopy (SEM). It was found that the cleaning of the substrates could be monitored by the RMS roughness of the surfaces, and that the efficiency of the cleaning methods was substrate dependent. The microstructure, composition and optical properties of the films were determined using SEM, transmission electron microscopy, transmission electron diffraction, AFM, x-ray photoelectron spectroscopy (XPS), transmittance and Raman spectroscopy. All films, including those formed at the lowest deposition temperature of 30°C, were crystalline with the hexagonal (wurtzite) structure. At deposition temperatures ≤50°C the crystallites were extremely small as evidenced by broad diffraction rings and Raman peaks. At higher deposition temperatures (≥60°C) the deposition rate increased and the crystalline quality improved significantly. At the very early stages of deposition the deposits consisted of isolated, three-dimensional particles. Rapid lateral growth of these particles dominated the initial stages of deposition. Field emission SEM and AFM showed that ostensibly single particles consisted of agglomerates of elongated and plate-like grains with no preferred orientation. The final films had a columnar structure with good contact between the substrate and the film. XPS results showed that there was no significant compositional change in the deposition temperature range used and the films were all slightly cadmium rich. Carbon, oxygen and chlorine were detected mainly on the surface of the films. After annealing, the composition of the CdS films were close to stoichiometric once the surface layer had been removed. There was an improvement in the crystallinity of the films after annealing.