Characterisation of chemical bath deposited CdS thin films on different substrates using electrolyte contacts

Abstract

Aqueous electrolyte contacts have been used to characterize CdS thin films grown using chemical bath deposition (CBD) on 4 different fluorine-doped tin oxide (FTO) coated conducting glass substrates (identified by the supplier Pilkington Group Limited as TEC-15, TEC-7, NSG low-E, and NSG Solar). The porosity of CdS layers prepared on different substrates was evaluated by voltammetry using the oxidation of hexacyanoferrate (II) to probe residual electrochemical activity arising from the underlying conducting substrate. Using a simple model, it can be concluded that thin films deposited on NSG substrates at 85 °C are probably nanoporous with a porosity of up to ~ 20% of the coverage of the CdS surface. The porosity appears to depend on the nature of the substrate rather than the physical parameters of the deposition. In the case of non-porous films deposited on TEC substrates, analysis of the impedance gave values of the space charge capacitance from which the doping density and flat band potential were derived. The doping densities for CdS were found to be in the order of 10 17 cm −3–10 20 cm −3 depending on the substrate. The transmission spectra, recorded in the UV–visible range, reveal a relatively high transmission coefficient in the obtained films. The transmittance data further indicate that the optical band gap is closely related to the deposition conditions. Micro-Raman analysis showed structural similarity and differences between porous and non-porous thin films.