Zn-porphyrin-sensitized nanocrystalline TiO2 heterojunction photovoltaic cells.

During the last 10 years, with the development of nanocrystalline films of very high surface area, the photosensitization of wide-bandgap semiconductors, such as TiO2, by adsorbed dyes has become more realistic for solar-cell applications. In a porous film consisting of nanometer-sized TiO2 particles, the effective surface area can be enhanced 1000-fold, thus making light absorption efficient even though there is only a monolayer of dye on each nanoparticle. Using ruthenium sensitizers and a nitrile-based electrolyte, the efficiency of nanocrystalline TiO2 solar cells has reached more than 11% at AM 1.5 sunlight. However, the main long-term limitation of these dyesensitized solar cells is the use of liquid electrolytes. A solution to this problem is the replacement of the liquid electrolyte by a solid hole-conducting electrolyte. For this reason, gel-based electrolytes, polymers, and ptype semiconductors have been extensively studied. Bach et al. have demonstrated that the liquid electrolyte can be replaced by an amorphous organic hole-transport material 2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene (spiro-MeOTAD) creating a solid p-type semiconductor/TiO2 heterojunction. This hole-conducting material allows the regeneration of the sensitizers after electron injection due to its efficient hole-transport properties. However, the overall cell conversion efficiency using the ruthenium dye [Ru(H2dcbpy)2(NCS)2] is significantly lower than the value of 11% observed for the corresponding liquid-junction cell. The low efficiency may be due to the lack of intimate contact between the hydrophilic sensitizer and the hydrophobic hole conductor. Another possibility is that there is an insufficient light absorbance resulting from the fact that the thickness of the nanocrystalline TiO2 film on the electrode is much less than that used in the liquid-junction cell. It has been shown for TiO2-bound tetrakis(carboxyphenyl)porphyrins that the efficiency of electron injection into the TiO2 conduction band and the kinetics of electron injection and recombination are indistinguishable from those of rutheni-