Preparation Of TiO₂ Nanoarrays And Their Application In Dye-sensitized Solar Cells

Titanium dioxide?TiO₂?, due to its stability, unexpensive, nontoxic characteristic and higher refractive index etc excellent physical and chemical properties, is one of most important semiconductor materials and a hot topic of material. Dye sensitized solar cells?DSSCs? based on TiO₂ has attracted great interest in a range of scientific and industrial fields because of its relatively low cost, simple preparation method and high efficiency. The disordered pore structure of the currently used TiO₂ photoanode has been a major factor that limits the further improvement of DSSCs. First, the disordered pore structure induces a short electron diffusion length, which increases the recombination probability of the electrons with redox species. Second, the transport and collection of electrons are restricted by multiple trapping and detrapping events along defects, surface states, and grain boundaries. Third, the currently used TiO₂ photoanode made of nanometer sized particles exhibited high transparency and a weak light-scattering ability in the visible range. Considerable efforts have been devoted to explore alternative photoanode structures for superior carrier collection and light harvesting abilities, including one dimensional?1D? nanowires, nanotube arrays, and hierarchical structures. In this paper, three-dimensional TiO₂ nanobelts film on Ti, hierarchical TiO₂ nanotube films and hierarchical TiO₂ nanowire film on FTO were prepared. The power conversion efficiency of DSSCs with the obove three films were studied.1. Preparation of three-dimensional interwoven structure of Ti O2 film. The three-dimensional?3D? interweaved TiO₂ nanotape networks have been synthesized via a facile hydrothermal method. The X-ray powder diffraction?XRD? pattern indicates that the as-prepared sample is highly crystallized anatase phase. The field emission scanning electron microscopy?FE-SEM? images indicate that the as-prepared sample is highly porous and robust structure due to the formation of 3D-network among the individual TiO₂ nanotape. The TiO₂ photoanode with 3D interweaved network structure was adopted to assemble the DSSCs, and a photoelectric conversion efficiency of 5.05% was achieved.The improved device performance is obtained due to improved light scattering ability, increased connection at the TiO₂/Ti interface, and enhanced amount of dye loading, as compared to the cell based on the TiO₂ nanotube.2. Preparation of TiO₂ nanotube array films on FTO glass substrates. TiO₂ nanowire arrays were obtained by one step hydrothermal reaction, and then hierarchical TiO₂ nanotube arrays were also obtained by second-step hydrothermal reaction. The DSSC assembled by hierarchical TiO₂ nanotube arrays obtained the photoelectric conversion efficiency of 5%. The hierarchical TiO₂ nanotube arrays have hollow tubular structure and the outer surface of the nanotube was covered with nanoparticles. So the specific surface area becomes larger, the absorption capacity of the dye is increased, the photoelectric performance of the DSSC is improved significantly.3. Preparation of hierarchical TiO₂ nanowire array films on FTO glass substrate. A bilayer structure of hierarchical TiO₂ nanowire film was prepared by a two-step solvent thermal reaction. The hierarchical Ti O2 nanowire arrays were formed perpendicularly to FTO substrates as the bottom layer. The top hierarchical TiO₂ nanowires intertwined and coated the bottom TiO₂ nanowire arrays in different directions. The composition and morphology of hierarchical TiO₂ nanowire film were characterized By XRD and SEM. Dye-sensitized solar cell?DSSC? assembled with the hierarchical TiO₂ nanowire film achieved an overall photoelectric conversion efficiency of 4.39%, significantly higher than that derived from smooth TiO₂ nanowire film?2.07%?.