ZnS overlayer on in situ chemical bath deposited CdS quantum dot-assembled TiO ₂ films for quantum dot-sensitized solar cells

ZnS overlayers were deposited on the CdS quantum dot (QD)-assembled TiO ₂ films, where the CdS QDs were grown on the TiO ₂ by repeated cycles of the in situ chemical bath deposition (CBD). With increasing the CdS CBD cycles, the CdS QD-assembled TiO ₂ films were transformed from the TiO ₂ film partially covered by small CdS QDs (Type I) to that fully covered by large CdS QDs (Type II). The ZnS overlayers significantly improved the overall energy conversion efficiency of both Types I and II. The ZnS overlayers can act as the intermediate layer and energy barrier at the interfaces. However, the dominant effects of the ZnS overlayers were different for the Types I and II. For Type I, ZnS overlayer dominantly acted as the intermediate layer between the exposed TiO ₂ surface and the electrolyte, leading to the suppressed recombination rate for the TiO ₂/electrolyte and the significantly enhanced charge-collection efficiency. On the contrary, for Type II, it dominantly acted as the efficient energy barrier at the interface between the CdS QDs and the electrolyte, leading to the hindered recombination rate from the large CdS QDs to the electrolyte and thus enhanced electron injection efficiency.