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 photocurrent, 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.
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Hill et al. (2017) studied this question.
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