Fabrication and charge-transfer characteristics of CdS QDs sensitized vertically grown flower-like ZnO solar cells with CdSe cosensitizers

This paper proposes the concept of using bisensitizers (CdS and CdSe) to improve the light-harvesting capabilities and charge transfer in ZnO photoelectrodes. The novel flower-like ZnO nanoarrays were grown by chemical bath deposition and they are sensitized with CdS quantum dots (QDs) and a CdSe overlayer. We observed a strong quenching of band-edge emission following the growth of QDs on ZnO, which is evidence for the occurrence of charge transfer between the QDs and the ZnO. A possible charge-transfer mechanism is proposed and discussed on the basis of energy-band alignment. Bisensitized ZnO electrodes resulted in a higher photoconversion efficiency (η = 1.08%) than that of single CdS QD-sensitized solar cells (η = 0.52%). The high photocurrent and improved power conversion are attributed to more charge-carrier generation facilitated by an extended spectral response and charge injection between the QDs and TiO ₂. Schematic representation of CdS and CdSe bisensitized flower-like ZnO solar cell structure investigated in this study.