Cobalt terpyridine complexes could rival the classical triiodide/iodide redox couple as efficient alternative redox couples for dye-sensitized solar cells. Cobalt bis(2,2',6',2 ''-terpyridine) complexes enable systematic tuning of the redox potential by variation of the substituents on the terpyridine ligand to optimize the open circuit voltage and the overall photovoltaic performance of the device. A [Co-II(Cl-terpy)(2)](TFSI)(2)/[Co-III(Cl-terpy)(2)](TFSI)(3) based electrolyte in combination with the Y123 donor-pi-acceptor dye yielded a solar to electric power conversion efficiency (PCE) of 8.7% under standard air mass 1.5 global (AM 1.5G) simulated sunlight. Transient photocurrent and photovoltage decay as well as electrochemical impedance spectroscopy was used to rationalize the differences in the photovoltaic parameters of devices with iodide/triiodide and cobalt complex based redox electrolytes.