FIG. 1 Schematic representation of the principle of the dye-sensitized photovoltaic cell to indicate the electron energy level in the different phases. The cell voltage observed under illumination corresponds to the difference, ΔV, between the quasi-Fermi level of TiO₂ under illumination and the electrochemical potential of the electrolyte. The latter is equal to the Nernst potential of the redox couple (R/R^-) used to mediate charge transfer between the electrodes. S, sensitizer; S^*, electronically excited sensitizer; S⁺, oxidized sensitizer.