Hierarchical ZnO microspheres photoelectrodes assembled with Zn chalcogenide passivation layer for high efficiency quantum dot sensitized solar cells

The power conversion efficiency of quantum dot sensitized solar cells with ZnO as photoelctrode is relatively low, largely due to high charge recombination rate at the ZnO surface. This paper reports high-efficiency hierarchical ZnO microspheres photoelectrodes with appropriate Zn chalcogenide passivation layers (ZnO@ZnS and ZnO@ZnSe) were constructed for reducing charge recombination of the resultant QDSCs. They were fabricated via a facile chemical solution route without any template or high temperature condition followed with controlled ion exchange. ZnS or ZnSe serves as passivation layer in CdS/CdSe QDSCs and their influences on the charge recombination as well as the light absorption were investigated. It was found that the ZnS or ZnSe passivation layer can effectively reduce charge recombination and significantly enhance the fill factor and open circuit voltage of the resulting QDSCs. Both of ZnS and ZnSe passivation layers enhanced the overall performance of the resultant QDSCs, which generated high power conversion efficiency of 5.13% and 5.20%, present 52% and 55% enhancement compared with 3.36% for the solar cell with hierarchical ZnO microspheres photoelectrode. ZnSe appears more favoring the deposition of QDs and enhancing the light absorbance than ZnS, resulting in an increased photocurrent density.