A well-defined modulation reflection spectrum due to a multiple interference process is originated in the TiO2 dye sensitizated solar cell (DSSC). Experimental evidence is shown that the interference process leading to the reflection spectrum takes place at the ii-conducting F:SnO2(FTO) layer of the FTO/TiO2 back contact. Moreover, the interference reflectance spectrum is influenced by the applied potential and illumination bias and disappears when FTO is metallized with 10 Angstrom of platinum. These results show that FTO/TiO2 cannot be considered as an ohmic but as a rectifying contact where the FTO behaves as a highly doped n-type semiconductor which absorbs an important part of the equilibrium contact potential in the dark. On the basis of our experimental results a new insight on the role of the dark equilibrium contact potential at the FFO/TiO2 interface in the processes of electric charge separation and photovoltage generation is given. Evidence is shown that the theoretically maximum attainable photovoltage in a DSSC is in one direction limited by the equilibrium redox potential in the dark, and in the other direction by the (light intensity dependent) bottom of the TiO2 conduction band.