Interfacial recombination processes in dye-sensitized solar cells and methods to passivate the interfaces

Conventional dye-sensitized solar cells function efficiently only with a si ngle redox couple, I-/I-2, because of the unusually slow kinetics for I-2 r eduction on SnO2 and TiO2 surfaces. When faster redox couples such as ferro cene/ferrocenium are employed, the rapid interfacial recombination of photo injected electrons with the oxidized half of the redox couple eliminates th e photovoltaic effect. To make use of other, perhaps more appropriate, redo x couples in these cells, the interfacial recombination processes must be u nderstood and controlled. Charge recombination at the SnO2/solution interfa ce is clearly distinguishable from recombination at the nanoporous TiO2/sol ution interface. Dark current measurements probe mainly the former reaction , although the latter may be the dominant recombination mechanism under ill umination. We introduce two methods for passivating the interfaces that dec rease the recombination rates by orders of magnitude. One method involves e lectropolymerization of an insulating film of poly(phenylene oxide-co-2-all ylphenylene oxide) on the solvent-exposed parts of the SnO2 surface. The ot her involves treating the dye-sensitized film with reactive methyltrichloro silane vapor that forms an insulating film of poly(methylsiloxane) on both the exposed TiO2 and SnO2 surfaces. These methods make it possible for the first time to use kinetically fast redox couples in dye-sensitized solar ce lls, and they may facilitate the development of a viable solid-state versio n of these cells.