The High Performance of Quantum Dot Sensitized Solar Cells Co-Sensitized with Mixed-Joint CdS and ZnS Quantum Dots

Abstract

We propose a facile way to prepare CdS and ZnS quantum dots (QDs) in photoanode oxide films in quantum dot sensitized solar cells (QDSCs). CdS QDs as sunlight harvester and ZnS as passivation layer are deposited in mesoporous RGO/TiO2 (reduced graphene oxide and TiO2 composite) films alternately through successive ionic layer adsorption and reaction (SILAR) to engineer the mixed-joint CdS/ZnS QDs. For comparison, another CdS and ZnS QDs are prepared with depositing all CdS firstly and then ZnS QDs in mesoporous RGO/TiO2 films. The power conversion efficiency (PCE) of mixed-joint CdS/ZnS based cell samples improve greatly, up to 6.37%, almost two times higher than that of bare CdS based quantum dot sensitized solar cells (2.54%), and also higher than that of CdS/ZnS based QDSCs (4.12%) obviously. Since the conduction band of ZnS (bandgap, 3.6ev) is higher than that of CdS (bandgap, 2.25ev), ZnS quantum dots can suppress reversed transport of photo-excited electrons and improve the efficiency of electron collection and meanwhile, they also impede hole transfer from the valance band of CdS to electrolytes. It seems to be an optimal way to solve this contradiction when mixed-joint CdS/ZnS are used to cosensitize QDSCs due to their characteristic architecture. The mechanism of effect of ZnS QDs on the photovoltaic performance of QDSCs is investigated combined sensitized photoanode microstructural characterization and electrochemical impedance spectroscopy (EIS) and J-V curve analysis of cell samples.