The galvanic technique for CdS thin film deposition has been modified, so that the spent solution hardly contains free toxic Cd²⁺ ions. In electrochemical deposition of CdS, even the spent solution contains sufficient Cd²⁺ ions, disposal of which is a problem. The typical galvanic deposition of CdS involves an electrolyte solution containing Cd²⁺ and S₂O₃^2- ions. Two electrodes immersed in the solution, one TCO (transparent conducting oxide) coated glass, acting as the cathode and a Cd strip, acting as a sacrificial anode, completes the cell. No external bias is applied for deposition; instead, the two electrodes are short circuited externally by a Cu wire, the difference in the electrode potential serves as the deposition potential.

We have shown that CdS thin films can still be deposited without the prior presence of Cd²⁺ ions in solution. It contained only 0.01M Na₂S₂O₃, pH of which was adjusted to around 3. Deposition was carried at 85⁰C with stirring. After constituting the cell, Cd dissociates leading to release of Cd²⁺ ions in solution (Cd = Cd²⁺ + 2e^-, E⁰ = 0.403 V). The two electrons move via the external circuit to the TCO cathode, where the released Cd²⁺ ions again get reduced to Cd (Cd²⁺ + 2e^- = Cd); the S₂O₃^2- ions simultaneously get reduced to S (S₂O₃^2- + 6H⁺ + 4e^- = 2S + 3H₂O, E⁰ = + 0.50 V); the two reduced elements react to produce CdS on the TCO surface.

X-ray data showed the deposited CdS film to be of cubic phase. EDX analysis yielded Cd:S atomic percentage about 1:1.03. SEM image revealed a uniform and compact surface morphology. Optical absorption study gave band gap energy around 2.36 eV. Resistivity of the films was in the range of 10⁵ - 10⁶ Ω.cm.

Hence, the presence of a cadmium solution in the galvanic cell is not necessary for CdS film deposition, the Cd electrode decays to provide the Cd²⁺ ions for film deposition, keeping the solution practically clean of Cd²⁺ ions after the deposition, and thus enabling the process to be green.

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