Overview Researchers are investigating cheaper sources of solar energy by replacing silicon with titanium dioxide. These Gratzel cells, coated with light absorbing dye, are very efficient and economical to produce and have potential for use in poorer countries, where the reduced cost will help make solar energy more competitive. Background Dr Alison Walker, a theoretical physicist at the University of Bath Department of Physics, is exploring the possibilities of eliminating the use of silicon in solar cells through the use of Gratzel cells – work motivated by experiments carried out by a group led by Professor Laurence Peter at of the Bath Department of Chemistry. Gratzel Cells and How They Work Gratzel cells are named after their inventor, the Swiss scientist Michael Gratzel. Instead of silicon, they rely on titanium dioxide (TiO2) – a cheap and widely available material used in everything from paints to coffee whiteners. In Gratzel cells particles of TiO2, coated with a dye that absorbs at a wide range of wavelengths given off by sunlight, are placed between two electrodes in an electrolyte solution containing iodine ions. The cells generate electricity when the energy captured by the dye makes the electrons in the dye molecules jump from one orbital to another. The electrons then jump onto the TiO2 particles and diffuse towards one electrode, while the iodine ions carry electrons from the other electron to regenerate the dye. Advantages of Gratzel Cells over Conventional Solar Cells In physics terms, Gratzel cells offer very high efficiencies and the economics are promising because they are based on TiO2, a cheap and widely available material. “The nice thing about Gratzel cells,” comments Dr Walker, “is that you could imagine them being used in the poorest countries.” Modelling Gratzel Cells Dr Walker and her colleagues are using Monte Carlo simulations – a type of mathematical modelling based on random numbers – to help them better understand the way Gratzel cells work. The mathematical models are making it possible for the group to explore the effects of using different grain sizes and electrolytes in Gratzel cells and helping them to pinpoint where the main losses of electricity occur. Their ultimate aim is to find ways to cut the cost of Gratzel cells by making them more efficient. “Solar cells are much too expensive at the moment to compete with other forms of power generation,” explains Dr Walker. “The basic need is to make solar cells that generate power at reasonable cost.” | 
