Adsorption and Cation-Exchange Behavior of Zinc Sulfide on Mesoporous TiO₂ Film and Its Applications to Solar Cells

Zinc sulfide (ZnS) was deposited onto the surface of mesoporous TiO₂ film by a typical successive ionic layer adsorption and reaction (SILAR) process. By inducing a spontaneous cation exchange between ZnS and a target cation (Pb²⁺, Cu²⁺, Ag⁺, or Bi³⁺) dissolved in a chemical bath when they are in contact, it was demonstrated successfully that white translucent ZnS on the substrate could be changed to new brown-colored metal chalcogenides and the amount of ZnS deposited originally by different conditions could be compared in a qualitative way with the degree of color change. By utilizing this simple but effective process, the evolution of a well-known ZnS passivation layer prepared from different chemical baths in quantum dot (QD)-sensitized solar cells could be tracked visually by checking the degree of color change of TiO₂/ZnS electrodes after the induced specific cation exchange. When applied to representative CdS QD-sensitized solar cells, it was revealed clearly how the different degrees and rates of ZnS deposition could affect the overall power conversion efficiency while finding an optimized passivation layer over TiO₂/CdS electrode. An acetate anion-coupled Zn²⁺ source was observed to give a much faster deposition of a ZnS passivation layer than a nitrate anion one because of its higher pH-induced more-favorable adsorption of Zn²⁺ on the surface of TiO₂. As another useful application of the ZnS-based cation exchange, as-deposited ZnS was used as a template for preparing a more complex metal chalcogenide onto a mesoporous TiO₂ film. The ZnS-derived Sb₂S₃-sensitized electrode showed a promising initial result of over 1.0%