Sustainable energy production is critical for meeting growing worldwide energy demands in a carbon-neutral manner. One area of research interest to meet this is the development of dye-sensitized solar cells (DSSCs). The operative components of a DSSC consist of: a photosensitive dye adsorbed onto a TiO2 semiconductor, a counter electrode, and a redox mediator. Photosystem I (PSI) is a naturally occurring photoelectrochemical reaction center which converts photons to chemical potential energy during photosynthesis and has shown the ability to act as the photosensitizer in biohybrid DSSCs. The functionality and efficiency of the device are dependent on a number of factors, including the band-gap of the dye, conduction band of the semiconductor, and the midpoint potential of the mediator. Although the I-/I3- redox mediator couple is the most commonly used in DSSCs, it is corrosive to proteins, has a midpoint potential similar to PSI, and absorbs light in the visible spectrum making it unfavorable for a biohybrid devices. Herein, we report on the development of biologically compatible bi-pyridine based cobalt redox mediator complexes for use in PSI-based photovoltaic devices. These Co (II/III) complexes are not corrosive and offer more negative midpoint potentials, resulting in greater driving forces for the regeneration of PSI available for photoexcitation. A series of complexes bearing electron donating functional groups have been prepared and their compatibility tested with PSI. We investigated a number of organic/aqueous solvent conditions to satisfy both mediator and PSI solubility requirements for electrochemical activity. Electron transfer assays have shown that these cobalt complexes are capable of directly reducing photooxidized PSI+. We also report on the effects of a mixed organic solvent-aqueous phase on PSI+ reduction kinetics, slowing direct reduction rates approximately 8-38 depending on the donor, with implication for dye regeneration rates in biohybrid photovoltaic devices based on electrolyte choice. Currently, the synthesis of more water-soluble cobalt complexes bearing polyethyleneglycol (PEG) groups is underway. Both bipyridine and phenanthroline based cobalt complexes with PEG groups will be tested for their electron mediator capabilities with PSI. (Supported by grants from the CSUB Start-up funds, NSF, NIH and Army Research Laboratory.)