Sol-gel and microwave processing of titanium dioxide for dye-sensitised solar cells

Dye-sensitised solar cells are a prom1smg alternative to the photovoltaic devices that are currently commercially available, because they are cheaper and simpler to produce. A key component of these solar cells is a nanostructured, porous thin film of titanium dioxide (Ti0 2). The primary aim of this thesis is to investigate the production of nanostructured Ti02 films by sol-gel processing, the effect of microwave heating on nanocrystalline Ti02 and, in particular, the effect of microwave heating on the performance of dye-sensitised solar cells. Microwave heating of ceramic materials has generally been found to reduce the sintering temperature and time that is required, as well as to produce uniform sintering and restrict particle growth. Therefore, the application of microwave heating to Ti0 2 films may be beneficial in the production of dye-sensitised solar cells, as it may reduce the time and energy required to produce the solar cells. Microwave heating of Ti02 films is a complex process and its use introduces challenges to the heat treatment of Ti02 films on conducting glass substrates. The electrically conductive layer on the glass substrates was found to crack easily during microwave heating, as a result of thermal stresses that develop within the conductive layer due to inhomogeneous heating. The addition of a Ti02 layer reduces these thermal stresses by providing a path through which heat is rapidly transported, due to the relatively high thermal conductivity and low heat capacity of Ti02, resulting in suppression of the cracking. In general, problems related to cracking of materials during microwave heating could be reduced by addition of a layer of any material with a relatively high thermal conductivity. The application of thin, compact Ti02 films, known as blocking layers, to the substrate of the working electrode of dye-sensitised solar cells by a simple process involving sol-gel synthesis and dip coating increased the solar cell efficiency. This increase in efficiency was largely due to an increase in the short-circuit current, resulting from the prevention of the back transfer of electrons from the conducting layer on the glass substrate to the electrolyte. The microwave heat treatment of these blocking layers at low temperatures (100°C) gave much better solar cell performance than conventional heating at the same temperature. The high solar cell efficiency was attributed to good sintering of the blocking layer and good interfacial contact between the conductive glass substrate and the blocking layer and between the blocking layer and the nanostructured layer. Combining Ti02 nanoparticles of two different sizes (one produced by a sol­ gel process and the other a commercially available Ti02 powder) resulted in high surface area Ti02 films that produced high short-circuit currents, when they were used in dye-sensitised solar cells. The application of microwave heating to these films was not found to cause any significant change in the solar cell performance compared with conventional heating.