Post-annealing of CdS/ZnS-assembled TiO₂ films for photoelectrochemical solar cells

CdS/ZnS-assembled TiO₂ films are annealed at 350 °C in an inert Ar ambient before and after deposition of the ZnS overlayer and are denoted as the types I and II, respectively. As-grown and the annealed CdS-quantum-dot (QD)-assembled TiO₂ samples without a ZnS overlayer were also prepared for comparison. Annealing of CdS-QD-assembled TiO₂ without ZnS significantly reduced the electron lifetime due to the coalescence of CdS QDs on the TiO₂ surface. The electron lifetime of the annealed CdS-QD-assembled TiO₂ was recovered because of ZnS overlayer due to its being an intermediate layer and to the energy barrier effects at the TiO₂/electrolyte and the CdS QD/electrolyte interfaces. The resultant photoelectrochemical solar cell (PEC) with the type I film exhibited better energy conversion efficiency than the PECs without the ZnS. The cell performance of the PEC with the type II film was further improved, as compared to that with the type I film. This can be attributed to the additional effect (improved interfacial contact at the CdS/ZnS interface) of the postannealing after the formation of the ZnS overlayer.