Yet to Be Challenged: TiO2 as the Photo-Anode Material in Dye-Sensitized Solar Cells

Utilization of renewable sources can reduce the impact of increasing global energy demand on the rate of depletion of fossil fuels. One of the most studied and implemented routes to meet this energy demand is to harvest solar energy. Among solar-energy harvesting devices, dye-sensitized solar cells have been recognized as some of the cheapest and most environment friendly technologies, since they do not require high purity of starting materials or advanced fabrication techniques. A dye-sensitized solar cell is composed of a working electrode, in which the light absorbing sensitizer is chemisorbed onto the surface of a wide bandgap semiconductor; a redox electrolyte, that is placed in between two electrodes and functions to regenerate the sensitizer; and a counter electrode, which is a catalyst which accelerates a redox reaction with the electrolyte. Titanium dioxide (anatase phase) is the most widely used semiconductor material in dye-sensitized solar cells due to its low cost, chemical stability and optical properties. In this chapter, the literature on optimizing TiO2 as a semiconductor material for n-type DSCs is reviewed. The evolution of TiO2 nanostructures and techniques such as doping, composite preparation and surface modification are elaborated on. These methods have enhanced both the chemical and physical properties of TiO2 nanostructures. Moreover, despite good overall performance, rapid recombination kinetics are a major disadvantage inherent in TiO2. Thus, research has been carried out to substitute TiO2 with alternative semiconductors. In view of this, other potential competitors for photo-anode material are reviewed and assessed. Finally, the prospects of an ideal semiconductor material for dye-sensitized solar cells is discussed.