Preparation Of Three-dimensional Hierarchical ZnO Nanocrystalline Thin Film And Its Application In Quantum Dot Sensitized Solar Cells

Energy is one of the necessary conditions for the development of human society.Meanwhile,the clean energy has always been the goal pursued by all countries.However,coal and petroleum are the main energy source in China.The large consumption of coal and petroleum has brought serious environment problems.In order to improve environmental safety and solve the problems of traditional energy supply security,global energy production and utilization is transferred to the efficient,convenient,clean and sustainable energy source,such as solar energy,wind energy,tidal energy,geothermal energy and biomass energy.Solar energy is attracting more and more attention because of its incomparable advantage,such as inexhaustibility and cleanness.Quantum dot solar cells are the third generation of solar cells which are the solar cells using quantum dots as sensitizers.Quantum dot solar cells possess many advantages,such as easy preparation,high efficiency and low costs.Nanocrystal semiconductor materials are usually used as electron transport layers,for instance,TiO₂,ZnO and SnO₂,etc.Among them,zinc oxide（ZnO）is a wide band gap semiconductor material.The forbidden band gap is 3.37 eV.ZnO has a suitable band position,good electronic transmission characteristics,strong morphology controllability and low cost,which is a promising solar cell anode material.In this article,ZnO nanostructures with wurtzite structure are used as photoanodes.The nanosheet-structured ZnO grew on the FTO substrate perpendicularly by electrodeposition.Then,the ZnO nanorods were perpendicularly grown on the ZnO nanosheets using hydrothermal method.The prepared three-dimensional hierarchical ZnO films were co-sensitized using CdS and CdSe quantum dots and assembled with PbS counter electrode to prepare QDSSCs.The photoelectric conversion efficiency of the assembled solar cells were investigated in detail.The main research contents and research results of this project are summarized as follows:（1）The ZnO nanosheets were prepared by electrodeposition.The temperature of the electrolyte was controlled using an oil bath and the ZnO nanosheets were deposited using a three-electrode system.The conditions of electrodeposition,such as the temperature of the electrolyte,the concentration of the precursor and the electrodeposition voltages,were investigated in detail.ZnO nanosheets were perpendicularly grown on the FTO substrate.The morphology and microstructure of the obtained samples were characterized by SEM.ZnO nanosheets films with 8μm could be electrodeposited at-1.1V（vs.Ag/AgCl reference electrode）at 70°C.The deposition time is 0.5 h.The ZnO nanosheets films were also characterized by XRD.（2）ZnO Nanorods were vertically grown on the surface of ZnO nanosheets using hydrothermal method.The morphology of ZnO nanorods-nanosheets was regulated through the controlling of the concentration of seed layer precursors,（NH₄）₂TiF₆ treatment of ZnO nanosheets and the hydrothermal reaction times.The nanorods adhered to each other when ZnO nanosheets were not treated with（NH₄）₂TiF₆.The length of ZnO nanorods is ca.200 nm.Thin uniform ZnO NRs were obtained when ZnO nanosheets were treated with（NH₄）₂TiF₆.The length of the ZnO NRs was approximately 2μm.（3）The morphology of the as-prepared ZnO NS-NR films was affected by several factors,such as the TiO₂ intermediate layer,the concentration of Zn（CH₃COO）₂ in the seed precursor solution,and the hydrothermal reaction time.ZnO NRs were vertically grown on the ZnO NSs under specific conditions.Such ZnO NS-NR films exhibited considerably higher light-scattering ability compared with that of the ZnO NS films.Quantum dot（QD）-sensitized solar cells were assembled using the CdS/CdSe QDs co-sensitized ZnO NS-NR films as the photoanodes.The growth of the ZnO NRs led to the improvement in the short-circuit current density from 12.4 mA cm^-2 to 15.1 mA cm^-2.A high power conversion efficiency of 6.0%was obtained with（NH₄）₂TiF₆ treatment for 4 min.