Relative energetics at the semiconductor sensitizing dye electrolyte interface

Chemical oxidation, spectro-electrochemical reduction, and potential-dependent photoluminescence were employed to investigate changes in the redox potential of eight different adsorbed sensitizing dyes relative to the potentials of the semiconductor and the electrolyte solution. The redox potentials of the investigated dyes were pH-independent in solution but developed a pH dependence that varied between 21 and 53 mV per pH unit upon adsorption to an oxide surface. Electrochemical experiments show that the apparent redox potential of the dye could also be influenced by an external bias applied to the electrode. The adsorption-induced potential changes were found to depend on both the dye structure and the electrolyte composition. A proposed model that considers the position of the specifically adsorbed dye relative to the ionic double layer at the electrode/electrolyte interface explains these results qualitatively. When the dye is mostly inside the double layer, its potential will tend to follow changes in the semiconductor potential; when it is mostly outside, its potential will be almost independent of the semiconductor. The fact that the potential of the sensitizing dye is not fixed relative to either the semiconductor or the electrolyte solution has important implications for the understanding and optimization of dye-sensitized cells.