Preparation And Enhanced Performance Of Dye-sensitized Solar Cells Based On Double-layered TiO₂ Composite Films

Since O'Regan and Gratzel in 1991 introduced nanosrystalline mesoporous titania with high surface area into dye-sensitized solar cells (DSSCs) and achieved an overall light-to-electric energy conversion efficiency of 7.12% at full sunlight (AM1.5), DSSCs have attracted extensive interest due to easy fabrication process, light weight, cheap cost and an increasing public concern about the shortage of energy supply. In the past two decades, intensive research has been devoted to structural design, material fabrication, photovoltaic characterization and mechanism analysis of TiO2 nanoparticle-based solar cells. As far as the film electrode is concerned, the photovoltaic performance of DSSCs is greatly dependent on film electrode configuration and properties of TiO2 nanostructures. Especially, TiO2 hollow spheres (HS) have potential application in DSSCs due to their unique properties in enhancing light absorption by extending the optical path and facilitating the penetration of electrolytes. Furthermore, TiO2 nanotubes are also of great importance in DSSCs due to their unique advantages in fast charge transport, enhanced light scattering, and large internal surface areas. In this thesis, valuable explorations have been carried out on the fabrication of double-layered film electrodes based on TiO2 nanoparticle layer and hollow spheres and synthesis of tubular TiO2 structures, and study of their application in DSSCs. The main points could be summarized as follows:DSSCs were fabricated based on double-layered composite films of TiO2 nanoparticles and hollow spheres. The photoelectric conversion performances of DSSCs based on nanoparticles/nanoparticles (PP), hollow spheres/hollow spheres (HH), hollow spheres/nanoparticles (HP), and nanoparticles/hollow spheres (PH) double-layered films are investigated, and their photo-electric conversion efficiencies are 4.33,4.72,4.93 and 5.28%, respectively. The enhanced performance of TiO2 nanoparticles/hollow spheres double-layered composite film solar cells can be attributed to the combined effect of following factors. The light scattering of overlayer hollow spheres enhances harvesting light of the DSSCs and the underlayer TiO2 nanoparticle layer ensures good electronic contact between film electrode and the F-doped tin oxide (FTO) glass substrate. Furthermore, the high surface areas and pore volume of TiO2 hollow spheres are respectively beneficial to adsorption of dye molecules and transfer of electrolyte solution.We successfully fabricated novel hierarchical titanate tubular structures assembled by nanotubes and nanosheets by a mildly hydrothermal method using titanate glycolate rods as self template and precursor at weak alkaline solution (0.1 M NaOH). The as-prepared products present hierarchical porous structures with two different tubes. One is macro-tubes along the direction of the template and the other is the nanotubes growing on the surfaces of macro-tubes. The formation of macro-tubes can be ascribed to chemically induced self-transformation mechanism or Kirkendall Effect, while the formation of small nanotubes is due to scrolling of titanate nanosheets caused by the combined effect of surface tension of nanosheets and catalysis of H2O2. Double-layered DSSCs composed of a hierarchical tubular TiO2 (HTT) overlayer and a nanoparticle (NP) underlayer and demonstrated an enhanced photovoltaic conversion efficiency of 6.2% with Isc of 14.1 mA m-2, VOC of 730 mV and ff of 0.6. This facile fabrication method of tubular structures should also find its widely potential applications in the preparation of other transition-metal oxides with novel macro-/mesoporous tubular structures. These novel titanate or TiO2 tubular structures will also find potential applications including photocatalysis, catalysis, electrochemistry, adsorbent, biotechnology and so on.