Dye sensitized solar cells: Energetic considerations and applications

The high concentration of electrolyte ions permeating the nanoporous films of dye sensitized solar cells eliminates all but nanoscopic electric fields in the solution and in the TiO2. The only substantial electric field is expected to occur at the TiO2/solution interface, and it is primarily across this interface that the photopotential drops in operating cells. In the dark, the low conductivity of the TiO2, and the high conductivity of the solution ensure that applied potentials drop over only a small fraction of the nanoporous TiO2 film near the substrate electrode. Therefore, measurements in the dark cannot be directly compared to measurements under illumination because the latter access most or all of the TiO2 film. The sensitizing dye is located partially inside the electrochemical double layer at the TiO2/solution interface and so its redox potential is not fixed relative to either the TiO2 or the solution. If the dye is mostly inside the double layer, its potential will tend to follow that of the TiO2; if it is mostly outside, it will be almost independent of the TiO2. Different photovoltage-limiting kinetic steps can result in these two cases. The narrow absorption spectra of many dyes provides the dye cells with a natural advantage over conventional solar cells in applications such as photoelectrochromic windows and power windows.