Self-assembled bilayers offer a promising strategy to directly harness photon upconversion via triplet-triplet annihilation (TTA-UC) and increase maximum theoretical solar cell efficiencies from 33% to >43%. Here we demonstrate that the choice of redox mediator in these solar cells has a profound influence on both the light harvesting and TTA-UC efficiency. Devices with Co-II/III(phen)(3) as the redox mediator produced the highest photocurrent yet generated from TTA-UC (0.158 mA cm(-2)) under 1 sun. Despite generating less photo current, Co-II/III(pz-py-pz)(2) devices achieved maximum TTA-UC efficiency at excitation intensities well below solar irradiance (0.8 mW cm(-2)), which is on par with the lowest value yet reported for any TTA-UC system. The large variation in performance with respect to mediator is attributed to triplet excited-state quenching via (1) energy transfer or paramagnetic quenching by the Co-II species and (2) excited-state electron transfer to Co-III species.