TiO₂/ZnO Inverse Opal Composite And Its Application In Solar Cells

Solar energy is one of the most convenient and efficient clean energy sources,furthermore,which is inexhaustible.Solar cells are the most direct way to convert solar energy into other forms of energy.As the third generation of new solar cells,dye-sensitized/quantum dot-sensitized solar cells（DSSCs/QDSSCs）have the virtue of simple preparation process,low cost,high theoretical photoelectric conversion efficiency,and low requirements on environmental temperature and experimental conditions,which have attracted extensive attention.In this thesis,a novel TiO₂/ZnO opal composite was prepared.ZnO nanords was coated on the surface of TiO₂ or ZnO inverse opal and used to fabricate the photoanode of QDSSCs,which greatly improved the photoelectric conversion efficiency of the cells.The main work can be summarized as follow:（1）Preparation of highly ordered titanium dioxide/Zinc oxide inverse opal（TiO₂IO/ZnO IO）TiO₂ and ZnO thin films were grown on the polystyrene sphere template by hydrolytic bath method.After annealing at high temperature,highly ordered inverse opal structure formed.The regular pores with larger surface areais expected to favour the deposition of cadmium sulfide（CdS）and cadmium selenide（CdSe）quantum dots.（2）Preparation of the composite of ZnO nanorods and ZnO/ TiO₂ inverse opalZnO nanorods（ZnO NR）were prepared by hydrothermal method.The combination of rod-like ZnO and honeycomb inverse opal structure provides larger contact area for CdS and CdSe quantum dots,which is beneficial for efficient deposition.In addition,ZnO NR coating on IO film is proven to possess lower interfacial resistance and faster electron transmission.（3）Characterization and optimization of TiO₂/ZnO inverse opal composite photoanodeIn the TiO₂/ZnO/CdS/CdSe photoanode,the conduction band and valence band between quantum dots and oxide semiconductor nanomaterials are arranged in a cascade-structure,which is expected to boost the electron transport and reduce the recombination of electron-hole pairs.In the QDSSCs with TiO₂ IO/CdS/CdSe and ZnO IO/CdS/CdSe photoanodes,the photoelectric conversion efficiency（PCE）reaches 6.65%and 6.07% respectively.However,the solar cells based on ZnO NR@ZnO IO/CdS/CdSe and ZnO NR@TiO₂ IO photoanodes achieve PCE of 7.32% and 8.18%,respectively,which are about 21%,23% and 34% higher than those of pure IO photoanodes based devices.Thus the TiO₂/ZnO inverse opal composite has a positiveeffect on the performance of QDSSCs.Besides,detailed description the photoelectric characteristics and surface morphology of solar cells by Scanning electron microscopy（SEM）,UV-visible spectrophotometer,X-ray diffraction（XRD）,electrochemical impedance test（EIS）,IPCE and Tafel polarization curve were carried out.The improved device performance was attributed to enhanced light absorption,and improved charge transfer and reaction rate at the oxide/QD/electrolyte interface.Subsequently,after the interface modification of zinc sulfide（Zn S）and ammonium fluoride（NH4F）on top of TiO₂/ZnO composite,higher PCE efficiencyof 11.23% was reached,which is the highest in the same type of QDSSCs using CdS/CdSe sensitizer.