Polypyridyl Co complexes with different substituents were applied as redox mediators in p-type dye-sensitized solar cells (p-DSCs), consisting of mesoporous NiO sensitized with a perylenemonoimide-naphthalenediimide (PMI-NDI) dyad. The photocurrent and photovoltages of the devices were found to depend on the steric bulk of the redox species rather than their electrochem. potential. Bulky substituents were found to slow the detrimental charge recombination reactions between holes in the NiO semiconductor and the reduced form of the redox couple. The open-circuit potential (VOC) of each of the devices was superior to the equiv. PMI-NDI-sensitized p-DSCs contg. the triiodide/iodide redox couple.