On the Adherence of Chemically Deposited CdS Films to Common Inorganic Substrates: Effect of Cd2+ Concentration in Solution, Substrate Surface Chemistry, and Reaction Temperature

The bad adherence to the substrate of chemical-solution-deposited chalcogenide films in alkaline solution is a current technological problem. However, this issue is belittled since is commonly solved by employing pre-sensitized substrates or by empirically changing the deposition parameters until finding the proper chemical formulation. For these reasons, the issue of the adherence has not been previously studied. The present research aims to properly discuss the issue of adherence by taking as example CdS. As a substrate, we used float glass sheets to take advantage of the surface duality of this interesting flat glass (different tin content). It was found that both the surface chemical composition of the substrate and the concentration of Cd2+ in the reaction solution play important roles in the adherence of the CdS thin films; an important effect of temperature was also found. The results were also consistent for indium tin oxide and silicon wafer surfaces. We propose a general surface reaction scheme which considers both the surface reactive sites and the hidroxo-cadmium-thiourea complexes of the reaction solution, as well as the formation of a surface intermediate entity which dissociates into a CdS molecule bonded to the surface and byproducts. The adherence depends on the quantity of surface intermediate entities formed, and therefore, the conditions that promote their formation contribute to the adherence: high reagent concentration, high-reactive surface sites, and low reaction temperature.