We propose TiO₂ electrodes consisting of two-dye-stained-TiO₂ layer and three dimensional porous Ti electrodes as shown in Figure 1 to absorb light with wider wavelength.
The cell consists of two-dye stained-TiO₂ layers (Two-layer-cell)), where upper and bottom layers are stained with dyes having longer and shorter wavelength respectively. It was difficult to stain the TiO₂ layer selectively with two dyes respectively by using a mere dipping process. We found that Two-layer-cell can be prepared by staining the substrate under pressurized CO₂ atmosphere. It was proved that the two-layer-cell has advantage over cells consisting of randomly-stained-TiO₂ layer (Cocktail type cell) in terms of high efficiency. In addition, we report advantages of the dye-staining-process under pressurized CO₂ process which enables extremely quick dye-uptake and inhibits inconvenient dye aggregation and makes it possible to passivate surface states of TiO₂ effectively which was observed by thermally stimulated current. 10.4% efficiency is reported.
In order to collect the electrons in TiO₂ layers effectively, the TiO₂ layer was covered with porous Ti electrodes. When the Ti electrode thickness became thick (150nn), the electrolyte diffusion was disturbed at the sacrifice of the increase in the conductivity of the Ti layer. Thick and porous Ti electron collecting layer fabricated by our new process solved the problem. In addition, it is reported that high efficiency FTO-free dye sensitized solar cells were fabricated by using the porous Ti electrode and the efficiency reach 7.4%.