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A green approach towards deposition of CdS thin films by an electrochemical technique

Anup Mondal 1Utpal Madhu 2Nil Ratan Bandyopadhyay 2

1. Bengal Engineering and Science University Department of Chemistry, Howrah 711103, India
2. Bengal Engineering and Science University School of Materials Science and Engineering, Howrah 711103, India

Abstract

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 S2O32- 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 S2O32- ions simultaneously get reduced to S (S2O32- + 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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Related papers

Presentation: Oral at E-MRS Fall Meeting 2007, Symposium B, by Anup Mondal
See On-line Journal of E-MRS Fall Meeting 2007

Submitted: 2007-05-14 13:29
Revised:   2009-06-07 00:44