We propose a passive factor, autocatalytic activity of some semiconductors (SnO2, WO3), for the loss of efficiency in dye-sensitized solar cells. On one hand, in the photoanode, SnO2 (or WO3) functions as a semiconductor to collect the photoelectrons that are generated by the sensitizer. On the other hand, SnO2 (or WO3) can also work as a catalyst to catalyze the recombination between the photoelectrons and the I3- ions, which results in a large current leakage, a low open-circuit voltage value, and a low efficiency. Some oxides, including, but not limited to, SnO2 and WO3, which have catalytic activity for I3- redn. or other oxidized species of the electrolyte are not suitable for use as semiconductor for the photoanode. In addn. to the semiconductors, phthalocyanine dyes also have catalytic activity for the recombination.