Electro-deposition of Cadmium Zinc Sulphide at High Cadmium Ion Concentration, Low Zinc Ion Concentration, High Temperature and Low pH

Abstract

Thin films are nanoscale materials which are widely used for solar cells and other optoelectronic devices. Cd(1-x)ZnxS (cadmium zinc sulphide) is formed by incorporating zinc ions to CdS (cadmium sulphide). Cd(1-x)ZnxS is a n-type semiconductor material which has a wider band gap than that of n-type CdS. Therefore, Cd(1-x)ZnxS can be used as a window material when application required low absorption of light and n-type semiconductor properties. Cd(1-x)ZnxS has been electro-deposited by varying cadmium ion concentration, zinc ion concentration, pH, deposition temperature and deposition time. Results reported here were based on the depositions conditions; 0.1 mol dm-3 cadmium ion concentration, 0.01 mol dm-3 zinc concentration, 2.45 - 2.50 pH and 50 °C deposition temperature. Electro-deposition experiments were carried out by Gamry ―series G 300‖ potentiostat while, working electrode was fluorine doped tin oxide/glass substrate, reference electrode was Ag/AgCl electrode and counter electrode was a semi-spherical graphite rod. The deposition voltage was identified from the cyclic voltammograms and shapes of the deposition current vs time plots. Electrodeposition reported in here was carried out at under-deposition voltages. The best values for electro-deposition parameters; voltage, pH, temperature and time were identified by observing their influence on the band gap values of the thin films deposited and the open circuit voltages of photo-electrochemical cell consisting of 0.1 mol dm-3 sodium thiosulphate electrolyte and the thin film semiconductor. A band gap range of 2.5 eV – 2.6 eV was obtained for Cd(1-x)ZnxS layer which is higher than the band gap of CdS. The open circuit voltage varied from -48 mV to -190 mV during optimization of voltage, pH, temperature and time. An X-ray diffraction spectrum has shown that Cd(1-x)ZnxS layer has a single hexagonal crystal phase. The crystal parameter, a = 4.1264 Å and it was lower than the standard CdS (a = 4.1364 Å). The results indicate that Cd(1-x)ZnxS thin films can be produced under the given conditions as a window layer for thin film solar cells in order to harvest more light and hence to improve the efficiency.