Dye-sensitized photoelectrochemical solar cells made from nanocrystalline films of TiO 2 doped with copper and sensitized with indoline 149 dye are found to have impressively higher efficiencies compared to equivalent cells made from undoped films. The surface concentration of copper atoms on the TiO 2 where this effect is optimized is nearly the same as the concentration of dye molecules on the TiO 2 surface. Copper doping shifts the flat-band potential of TiO 2 in the negative direction, which is favorable for increasing the open-circuit voltage of the cell. It is suggested that in addition to the linkage of the carboxylate ligand of the dye to the TiO 2 surface, moieties in the rhodanine rings of the dye coordinate to the copper atoms on the TiO 2 surface. The coordination of the dye to copper seems to have a positive influence on the efficiency of the cell.

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Nanostructured high band-gap oxide semiconductor ceramics are extensively investigated to understand their unique properties and applications in dye-sensitized solar cells, sensors, supercapacitors and other electronic and electrochemical devices. The unusual properties of these materials originate from large surface-area to bulk-volume ratio, quantum size effects and formation of space charge layers of dimensions comparable to the crystallite size in depletion of carriers. A prototype device demonstrating the unique properties of nanostructured semiconductor ceramics is the dye-sensitized solar cell. The original version of this device is based on TiO 2 and other familiar stable oxides SnO 2 and ZnO yield lower efficiencies owing to faster recombination. SnO 2 admits faster electron transport because of the lower effective electron mass, but the same property enhances recombination. Ultra-thin barrier layers insulating stannates coated over the SnO 2 crystallites by glazing with alkaline earth chlorides increases the efficiency from ∼1 to 6.6%.

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