We report a combined experimental and computational investigation to understand the nature of the interactions between cobalt redox mediators and TiO2 surfaces sensitized by :ruthenium and organic dyes, and their impact on. the performance of the corresponding dye-sensitized solar cells (DSSCs). We : focus: on different ruthenium dyes and fully organic dyes, to understand the dramatic loss of efficiency observed for the prototype Ru(II) N719 dye in conjunction with :Cobalt: electrolytes. Both N719- and Z907-based DSSCs showed an increased lifetime in iodine-based electrolyte compared to the cobalt-based redox-shuttle; While the organic D21L6 and D25L6 cycles endowed.With long alkoxy chains, show no significant:Change in the electron: lifetime regardless of employed electrolyte and deliver,a high photovoltaic efficiency of 6:5% with a cobalt electrolyte. Ab initio molecular dynamics simulations show the formation of.a complex between the cobalt electrolyte and the : surface-adsorbed ruthenium dye, which brings the Co(bpy)(3) species' into Contact, with the TiO2 surface. This translates into a high probability of intercepting TiO2 injected electrons by the oxidized Co(bpy)(3). species, lying close the N719-sensitized TiO2 surface. Investigation of the dye regeneration Mechanism by :the. cobalt electrolyte in the Marcus theory framework led, to substantially different reorganization energies for the high-spin (HS) and low:. spin (LS) reaction pathways. Our calculated reorganization energies. for the LS pathways are in excellent agreement with recent data for a series Of cobalt complexes, lending support to the proposed regeneration pathway. we systematically investigate. a series of Co(II)/Co(III) omplexes, to gauge the impact of ligand:substitution,and of metal coordination (tris-bidentate vs:.bis-tridentate) on the HS/LS.energy difference and reorganization energies. Our results allow us to trace structure/property. relations required for further development of cobalt electrolytes for DSSCs;