Potential Distribution and Photovoltage Origin in Nanostructured TiO₂ Sensitization Solar Cells:  An Interference Reflection Study

A well-defined modulation reflection spectrum due to a multiple interference process is originated in the TiO 2 dye sensitizated solar cell (DSSC). Experimental evidence is shown that the interference process leading to the reflection spectrum takes place at the n-conducting F:SnO 2 (FTO) layer of the FTO/TiO 2 back contact. Moreover, the interference reflectance spectrum is influenced by the applied potential and illumination bias and disappears when FTO is metallized with 10 Å of platinum. These results show that FTO/TiO 2 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 FTO/TiO 2 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 TiO 2 conduction band.