Preparation Of Modified Nano TiO₂ And The Performance Of The Assembled Dye-sensitized Solar Cells

Dye-sensitized solar cell(DSSC) is considered to be a new type of the most promising photovoltaic cell for its low cost, no pollution for environment and simple preparation process. Considering the advantages of nontoxicity, chemical stablility, and relatively low price, TiO2 has been extensively used in DSSCs, photocatalysts, sensors, water splitting, and lithium ion batteries. The photoanode of TiO2 is the most important part of dye-sensitized solar cell, which should meet the following conditions: high specific surface area, fast electron transport rate and low recombination of electron-hole pairs. Therefore, nano TiO2 is needed to be further modified to obtain the higher photoelectric conversion efficiency.In this report, it is composed mainly of the preparation of the composited TiO2-graphene, TiO2 nanotube array and nitrogen-doped TiO2 and the improvement of the photovoltaic performance of dye-sensitized solar cells. The main experiments and results are as follows:(1) Titanium oxysulfate(TiOSO4) and graphene were used as titanium source and supporter respectively to synthesize anatase TiO2-graphene(TiO2-G) composite. Crystal structure, morphology, and composition of TiO2-G were investigated by X-ray diffraction(XRD), filed emission scanning electron microscope(FESEM), transmission electron microscope(TEM), and thermogravimetric analysis(TGA). Both TiO2-G and blank TiO2 powders exhibited spindle-shaped structure with the long axis along [001]. Compared to unsupported TiO2, TiO2 nanoparticles uniformly formed on graphene surface. When fabricated into dye sensitized solar cells with TiO2-G as the photoanode, photoelectrical conversion efficiency of TiO2-G(2.30%) was much higher than that of blank TiO2(0.89%) prepared at the same conditions. Moreover, high sintering temperature enhanced photoelectrical performance of the composite. When the temperature was increased from 450 ?C to 600 ?C, the conversion efficiency was improved from 1.50% to 2.60%. The findings above demonstrate that TiO2-G composite would have great potential for applications in dye-sensitized solar cells.(2) In this study, densely aligned TiO2 nanotube array films were grown directly on transparent conductive fluorine-doped tin oxide(FTO) substrates by two steps hydrothermal method. Crystal structure and morphology of TiO2 nanotubes were investigated by XRD, FESEM and TEM. According to FESEM, it could be known that when etching temperature was increased from 120 oC to 200 oC, the Ti O2 nanorods was first etched into nanotubes and then broke up into nanorods. In addition, dye sensitized solar cells were fabricated with TiO2 nanotubes as the photoanode, and the highest photoelectrical conversion efficiency(η) of 3.20% was obtained from the DSSC with TiO2 etched at 140 oC. It was speculated that TiO2 nanotubes with larger specific surface area showed ability of higher dye loading and light harvesting abilities. It can be seen that the morphology of titanium dioxide have certain effect on the photoelectrical properties of dye-sensitized solar cells. Furthermore, the calcination treatment made the photoelectric properties of the etched TiO2 nanorods significantly improved, and the conversion efficiency first increased and then decreased. The highest η of 4.74% could be obtained by the cell assembled with TiO2 nanotubes electrode sintered at 400 °C for 3 h.(3) Nitrogen doped rutile TiO2 nanorod arrays were prepared on transparent conductive fluorine-doped tin oxide(FTO) substrates by a facile hydrothermal process, using ammonia solution as nitrogen source and butyl titanate as titanium source. The characterization XPS denoted that only a small amount of nitrogen from reactant was added into TiO2, and formed the O-Ti-N structure. With the doping concentration increased, the length of nanorod increased sharply. Meanwhile, the photoelectrical conversion efficiency of the assembled dye sensitized solar cells first increased and then decreased, with the highest value of 2.88%. When sintered at 450 ?C for 3 h, the photoelectrical conversion efficiency was increased to about 3.76%. From XPS, it could be that the sintering treatment introduced the nitrogen element on the surface of the samples known to add into TiO2. Above all, the moderate nitrogen doped into sintered TiO2 will increase the photoelectrical conversion efficiency. In order to evaluate the all solid state solar cell, the common liquid electrolyte was also used for comparison, and the conversion efficiency of the latter(5.74%) was much higher than that of the former. Therefore, more work focused on the all solid state solar cell was still needed to reduce the interfacial resistance and accelerate the photogenerated charge transfer and finally improve the photoelectrical conversion efficiency.After changing the morphology of TiO2 and doping it with nonmetal, the photoelectric conversion efficiency of dye-sensitized solar cells can be significantly improved. Considering the modification of TiO2 nanorods, reactant concentration, reaction temperature and sintering treatment all have some effects on the diameter, length and the crystall degree. With the optimization of modified methods, the photoelectric conversion efficiency of DSSC could be improved obviously. In order to obtain a higher conversion efficiency, to select a suitable preparation and processing programme is still the focus of our further work.