Structurally Engineered Multi-functional TiO₂ Nanotubes For Application In Dye-sensitized Solar Cells

As one of the widely used semiconductor materials, titanium dioxide nanomaterials play an important role on the application of many aspects because of the diversity of its structure. Especially in nowadays society where energy shortage is increasingly intensified and environmental protection is of great urgency, researches related with clean and renewable solar cells attract more and more attention. For these solar cells, the third generation solar cells——dye-sensitised solar cells have always a huge potential of development because of their advantages of low cost and simple preparation technology. Until today, titanium dioxide nanomaterials, as the best applied materials in the photoanodes of dye-sensitised solar cells, always remains the focus of researches and the materials is in the continuous development and improvement.The main function of TiO₂ nanomaterials, which are regarded as photoanode, is to adsorb dye which generates photoelectron and to transmit photoelectron to FTO glass. Compared with structure of nanoparticles, the structure of nanotubes has very obvious advantages in the electronic transmission. Therefore, because of TiO₂ nanotube arrays’ excellent features, as the most widely research photoanode materials, they has received widely high attention. Studies on dye sensitized solar cells’ photoanode mainly revolve around these aspects:（1） The photoanode structure with a larger specific surface area;（2） The higher electronic transmission efficiency;（3） The higher light harvest efficiency, etc. In order to obtain DSSCs with high efficieny, we focused on multi-functional TiO₂ nanotubes by studying their morphology, structure and crystalline state. The specific research works mainly include the following aspects:（1）. Relative to nanoparticles, the TiO₂ nanotube arrays with smaller specific surface area are one kind of shortcomings. Using the water bath which can raises the anodic oxidation reaction temperature, TiO₂ nanotubes with high specific surface area were fabricated. The nanotubes’ length is 16μm length and diameter is 75 nm. And when applied this small diameter nanotube membrances as the photoanode of DSSCs, it increases the dye adsorption amount, greatly promotes efficiency and improves performance of solar cells.（2）. Nanotube membrances with small diameter, due to its smaller grain and being more vulnerable to the influence of the “substrate effect”, could generate rutile in normal annealing temperature, which could produce negative effect on the performance of dye-sensitised solar cells. Therefore, in order to prepare for small nanotube membrances structure with the pure anatase, we annealled nanotube membrances with small diameter at low temperature and peeled them off. Because this method can not only keep the small nanotube membrances morphology, but also avoid the influence of “substrate effect”. Compared with small nanotubes containing rutile and anatase, the efficiency of DSSCs based on small nanotubes with the pure anatase are greatly improved.（3）. Light scattering layer plays an important role in enhancing the efficiency of the dye sensitized solar cells. Using the large diameter nanotube arrays as light scattering layer which anodized at large voltage, we successfully synthesised TiO₂ nanotube arrays which diameter is close to the visible wavelength. Its scattering effect is very outstanding compared with the typical TiO₂ nanotubes with relatively small diameter. Large TiO₂ nanotubes have obvious advantages and the potential of development. After analysis of the large diameter nanotubes’ morphology by using SEM, we made the comparison and analysis of the performance between dye sensitized solar cells using large diameter nanotubes as light scattering layer and the solar cells with no scattering layer. Not only simulation result, but also experiment results, they all proved that the large diameter titanium dioxide nanotube arrays had excellent scattering effect.（4）. Based on the experimental work of large diameter nanotubes, bi-layer TiO₂ nanotubes were prepared with different diameter in bottom and top layer. By annealing the bi-layer nanotube membrances at high temperature and combining nanoparticle layer, the multi-layer photoanode structures could own different function layer: the dye adsorption layer, electronic transport layer and top layer-light scattering layer. And through the optimization analysis of the thickness of each layer, the DSSCs with the multi-layer photoanode structure could obtained the optimal efficiency. Compared with the single-layer photoanode and bi-layer photoanodes, which are just composed of TiO₂ nanoparticles or both nanoparticles and nanotubes, we obtained the highest efficiency of 6.52%.