| Abstract
| - 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 [Turrión, M.; Macht, B.; Tributsch, H.;Salvador, P. J. Phys. Chem. B2001, 105, 9732] allows one to monitor very precisely the evolution of thedepletion layer of the fluor-doped SnO2 (FTO) conducting substrate at the DSSC back contact with the positionof 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 (U0) and the Helmholtz layercapacitance (CH) at the FTO/electrolyte interface, under working conditions of the DSSC. Under illuminationthe band-bending in the FTO substrate is controlled by two factors: (a) the position of Fermi level (FTObulk potential), which is determined by the accumulation of photogenerated electrons in the conduction bandof the nanostructured TiO2 film, and (b) the values of U0 and CH, which determine the potential distributionat the FTO/electrolyte interface. It is suggested that both parameters U0 and CH determine a constraint on themaximum photovoltage attainable by the DSSC, as the conduction band edge of the FTO at the FTO/TiO2interface cannot be higher than that of the TiO2, otherwise the transfer of dye photoinjected electrons fromthe TiO2 to the FTO would be hindered. According to our analysis, it can be concluded that the theoreticalmaximum photovoltage cannot be higher than 0.8 V, a value never surpassed experimentally with untreatedDSSC.
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