Dye-sensitized solar cells are a promising alternative to traditional inorganic semiconductor- based solar cells. Here we report an open-circuit voltage of over 1,000 mV in mesoscopic dye- sensitized solar cells incorporating a molecularly engineered cobalt complex as redox mediator. Cobalt complexes have negligible absorption in the visible region of the solar spectrum, and their redox properties can be tuned in a controlled fashion by selecting suitable donor/acceptor substituents on the ligand. This approach offers an attractive alternate to the traditional I3−/I− redox shuttle used in dye-sensitized solar cells. A cobalt complex using tridendate ligands [Co(bpy-pz)2]3+/2+(PF6)3/2 as redox mediator in combination with a cyclopenta- dithiophene-bridged donor-acceptor dye (Y123), adsorbed on Tio2, yielded a power conversion efficiency of over 10% at 100 mW cm − 2. This result indicates that the molecularly engineered cobalt redox shuttle is a legitimate alternative to the commonly used I3 − /I − redox shuttle.