Influence of sensitizers on charge recombination in dye-sensitized solar cells

The objectives of this study are to elucidate the effect of dye structures on the charge recombination in dye-sensitized solar cells (DSCs) and to propose the design guide for new sensitizers. The electron lifetime was measured for DSCs with dyes whose structures were systematically varied. It was found that the observed trends in the measured lifetime were explained by three factors : (1) blocking effect, (2) partial charges, and (3) dispersion forces. The blocking effect was controlled by the molecular size of the dyes and the density of adsorbed dyes. The partial charges mean how much charges are localized on particular atoms, e.g., oxygen and metal atoms. The dispersion force is determined by polarizability of sensitizers. Previously the effect of the dispersion force was not evident. This was because the effect was smaller than the others. Under the conditions that the effects of blocking effect and partial charges were little, the effect of dispersion force appeared. Based on the results, design guide for the dyes to retard recombination is to increase the adsorption density, to have little partial charges or leave those far from the surface of the TiO2, and to add obstacle moiety to the framework of the sensitizers. It was also effective to employ bulky redox couples to enhance the blocking effect.