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

Conventional dye-sensitized solar cells function efficiently only with a single redox couple, I-/I-2, because of the unusually slow kinetics for I-2 reduction on SnO2 and TiO2 surfaces. When faster redox couples such as ferrocene/ferrocenium are employed, the rapid interfacial recombination of photoinjected electrons with the oxidized half of the redox couple eliminates the photovoltaic effect. To make use of other, perhaps more appropriate, redox couples in these cells, the interfacial recombination processes must be understood and controlled. Charge recombination at the SnO2/solution interface is clearly distinguishable from recombination at the nanoporous TiO2/solution interface. Dark current measurements probe mainly the former reaction, although the latter may be the dominant recombination mechanism under illumination. We introduce two methods for passivating the interfaces that decrease the recombination rates by orders of magnitude. One method involves electropolymerization of an insulating film of poly(phenylene oxide-co-2-allylphenylene oxide) on the solvent-exposed parts of the SnO2 surface. The other involves treating the dye-sensitized film with reactive methyltrichlorosilane 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 cells, and they may facilitate the development of a viable solid-state version of these cells.