A model for the analysis of interference reflection data of nanoporous TiO2 dye-sensitized solar cells (DSSC) is presented. It is shown that the interference reflection technique [Turrion, M.; Macht, B.; Tributsch, H.; Salvador, P. J. Phys. Chem. B 2001, 105, 9732] allows one to monitor very precisely the evolution of the depletion layer of the fluor-doped SnO2 (FTO) conducting substrate at the DSSC back contact with the position of the Fermi level. The model shows that this technique features a much larger sensitivity than the capacitance-voltage Mott-Schottky method for determining both the flatband potential (U-0) and the Helmholtz layer capacitance (CH) at the FTO/electrolyte interface, under working conditions of the DSSC. Under illumination the band-bending in the FTO substrate is controlled by two factors: (a) the position of Fermi level (FTO bulk potential), which is determined by the accumulation of photogenerated electrons in the conduction band of the nanostructured TiO2 film, and (b) the values of U-0 and C-H, which determine the potential distribution at the FTO/electrolyte interface. It is suggested that both parameters U-0 and C-H determine a constraint on the maximum photovoltage attainable by the DSSC, as the conduction band edge of the FTO at the FTO/TiO2 interface cannot be higher than that of the TiO2, otherwise the transfer of dye photoinjected electrons from the TiO2 to the FTO would be hindered. According to our analysis, it can be concluded that the theoretical maximum photovoltage cannot be higher than 0.8 V, a value never surpassed experimentally with untreated DSSC.