Shown in Figure 9.5 is a schematic of a potentially low-cost tandem device that achieves direct cleavage of water into hydrogen and oxygen by visible light. This is based on two photosystems in series (tandem), with electron flows as shown in the figure. The top cell (exposed to light) is a thin film of tungsten trioxide that absorbs the blue portion ofthe solar spectrum. The valence band holes decompose water directly to oxygen. The conduction band electrons are fed into the second photosystem, the dye-sensitized nanocrystalline TiO2 cell. This is placed directly under the tungsten trioxide film and captures the green and red part of the solar spectrum. The photoelectrons in the conduction band of the titania reduce water to produce hydrogen gas.
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![Подпись: Figure 9.5 Proposal [112] for inexpensive thin- film tandem cell for water splitting. In its realization, the two cells are stacked, just as in Figure 9.1. The downward sloping arrow in the centerofthisfigure represents the “transparent ohmic contact” connecting the two cells in](/img/1238/image208_2.gif "Possibility of a Mass Production Tandem Cell Water-Splitting Device")
scale of electrochemistry, is about 4.5 eV below the vacuum energy. On the right are shown energy levels for some additional reduction/ oxidation reactions in water solution.
Figure 9.1. The “dye-sensitized TiO2” cellon the right is as described in connection with Figures 6.16-6.18, and the 1.6eV arrow shown here corresponds to the dye absorption energy rather than the titania band gap.
This device perhaps can be produced as a wide-area membrane with water on both sides, generating oxygen on the one side and hydrogen on the other side. Unlike the expensive semiconductor system discussed earlier, the oxide surfaces are inert with respect to water exposure and do not involve single crystals or vacuum processing steps.
9.3.3
