Optimization of the CdS quantum dot sensitized solar cells with ZnS passivation layer

Quantum dot sensitized solar cell (QDSC) is assembled with CdS/ZnS cosensitized TiO₂ photoanode, Pt counter electrode and the polysulfide electrolyte. Since the conduction band of ZnS is higher than that of CdS, ZnS can suppress reversed transformation of electrons and improve the efficiency of electron collection as the passivation layer. The morphology and composition of photoanodes are characterized by XRD, SEM, AFM, EDS and XPS analysis. Results show that CdS and ZnS QDs are covered on the surface of TiO₂ porous photoanode successfully to degrade the surface roughness and TiO₂ crystal structure has not changed with the introduction of QDs. The photoelectric property of assembled QDSC is analyzed by EIS and J–V curves. The charge recombination at photoanode/electrolyte interface is less likely to occur due to enhanced charge transfer resistance after coating ZnS, leading to a higher power conversion efficiency (PCE) of cells. However, PCE of cell decreases when excessive ZnS QDs are introduced. The photoelectric property of cells sensitized with CdS and ZnS QDs in different cycles is compared and the effect of ZnS incorporated amount on photoelectric property of QDSC is discussed emphatically. It is found that cells sensitized with CdS in seven cycles and ZnS QDs in five cycles exhibit the best photoelectric performance and PCE of which is much higher than that of bare CdS sensitized cell.