Dye-sensitized photoelectrochemical solar cells (DSCs) are one of the most promising alternatives to conventional inorganic devices owing to their potential low production costs and high conversion efficiencies.W In a typical DSC, a mesoporous n-type TiO2 electrode, anchored with an electron-injecting sensitizer, acts as an photoanode for light absorption, and a platinized fluorine-doped tin oxide (FTO) glass as a passive cathode for catalysis of the electrolyte with a redox couple I~-/I_3~-, sandwiched between the two electrodes (n-DSC). In this n-DSC, light-induced electron injection from the excited sensitizer into the conduction band of TiO2 is followed by hole injection into the electrolyte, generating a photovoltage defined by the difference in chemical potential between the TiO2 photoanode and the electrolyte. On the other hand, a p-type semiconductor electrode (such as NiO) can be sensitized by a hole-injection sensitizer and used as a photocathode in combination with a Pt passive anode in a DSC (p-DSC). In this case, an opposite process occurs, with electron injection into the electrolyte and hole injection into the valence band of NiO, resulting in a photovoltage equal to the difference in chemical potential between the NiO photo-cathode and the electrolyte. With the combination of a photoanode and a photocathode in a single device (np-DSC), a tandem DSC has been advanced with a theoretical efficiency limitation well beyond that of a single junction DSC (n-or p-DSC). In this double junction tandem device, both the active electrodes are connected in series, and thus a high photovoltage, namely a sum of the photovoltages obtained in the corresponding p and n devices, can be expected, with the potential to dramatically improve the efficiency.
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