Modeling of photovoltage and photocurrent in dye-sensitized titanium dioxide solar cells

By means of two-dimensional simulation calculations, a detailed analysis of the nanocrystalline TiO2 dye sensitized solar cell (DSC) has been performed. A simplified scheme of the nanoporous structure, which is treated as if the TiO2 film is a continuous medium, is used for modeling. On the basis of material parameters, the model permits the determination of steady-state charge-carrier distributions, the calculation of I-V curves under illumination, dark characteristics, and the spectral response of a DSC. The spatial resolution of the model allows for the answer to the question of the spatial distribution of both the electric and the electrochemical potential in the cell. Thus, a deeper insight into the operation mechanism of a DSC is obtained. Nonnegligible drift currents are found. It is shown quantitatively that the electric potential drops mainly at the TCO/TiO2 interface and not at a Helmholtz layer. The role of the dark interfacial electrical potential difference (built-in potential) for the function of a DSC is discussed. It is shown that, contrary to a conventional p-n junction solar cell, higher photovoltages than those of the dark interfacial electrical potential difference can be obtained.