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

Abstract

The bad adherence to the substrate of chemical-solutiondeposited 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 Cd 2 + 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 hidroxocadmium-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.

Chemical Solution Deposition of Al(OH)3‐AlOOH Films: Application for Surface Sensitizing of Rigid and Flexible Substrates to Improve the Adhesion in Coating Technology Areas

“…Examples of high‐reactivity sites in glasses, which are widely used as substrates, are those local environments created by the incorporation of tin atoms in the bottom side (tin‐bath side) of flat glasses when the glass ribbon is floated on molten tin during industrial production of float flat glasses (tin atoms are present in about 5.24 % of the surface atomic population, as was determined in a previous work [1] ). The upper side (atmosphere side) of such glasses, which is exposed to a reducing atmosphere during the float process, shows a very low content of tin atoms (tin atoms are present in 0.67 % of the surface atomic population, as was determined in a previous work [1] ), and therefore, it has a low number of high‐reactivity sites. This is the reason why a CdS thin film shows poor adhesion to the atmosphere side of float glass when the film is deposited from a low Cd 2+ reaction solution at reaction temperatures equal to and above 40 °C.…”

“…In a previous work, [1] we addressed for the first time the issue of adhesion of thin films deposited by a chemical solution technique to certain substrates. We found that, for the case of cadmium sulfide (CdS), which is still a very relevant semiconductor material, the adherence to a given substrate is improved by an increase in the concentration of cadmium salt or by a decrease in the reaction temperature; these parameters are operative factors related to the reaction solution.…”

“…Since reactions carried out with low Cd 2+ content and at a relatively high temperature (between 50 °C and 90 °C) are desired due to the resulting low environmental impact and low working time, [2] respectively, it is therefore very convenient to use tin‐sensitized glasses as substrates, such as the tin‐bath sides of float glasses; this is the only way to achieve adhesion under these reaction conditions [1] . However, the recent market is demanding tin‐free glasses, as it is well known that an annealing process applied to float glasses generates a “bloom” effect on the tin‐bath side, [3–9] which is undesirable in certain glass manufacturing industries.…”

“…Although tin‐free glasses are excellent for many areas, these substrates present adhesion problems in the areas of coating technology, as mentioned above; we observed the same problem in other tin‐free substrates (where tin is not used in the industrial production process), such as silicon wafers, which are widely used as substrates in electronics. Recommendations to solve this adhesion problem, as also mentioned above, are the use of low temperature or the use of reaction solutions with high Cd 2+ content, [1] which directly impact the production costs and the environment. Definitively, these recommendations, especially the last one, are not compatible with the perspectives of green chemistry.…”

“…A green alternative to the previous recommendations is the incorporation of tin atoms to the substrate surface through a sensitization process that often involves tin(II) chloride (SnCl 2 ), [10‐17] which is enhanced by an annealing process at 250 °C [14] . These “artificially” incorporated tin atoms, which, in the case of glasses, are present as tin(IV) oxide (SnO 2 ) [1,18‐20] or as a tin hydroxide compound, or even as an oxide‐hydroxide mixture, greatly improve the adhesion of most chemical‐solution‐deposited thin films. In fact, when applying this sensitization method with SnCl 2 in tin‐free glasses, a good improvement was observed in the adhesion of tin(II) sulfide (SnS) [21] and magnesium hydroxide (Mg(OH) 2 ) [22] thin films obtained by chemical solution deposition.…”