Separating triplet exciton diffusion from triplet-triplet annihilation by the introduction of a mediator.

Triplet-triplet annihilation photon upconversion (TTA-UC) combines the energy of two photons to provide one of higher energy that can be used to drive photochemical or photophysical processes. TTA-UC proceeds at high efficiencies in dilute solution, but in solid state the efficiency drastically reduces. This is because exciton diffusion, compared to molecular diffusion in solid annihilator films, suffers concentration induced quenching, undermining efficient emission. Here, we provide a method to decouple the triplet exciton diffusion and the annihilation processes using an exciton transporting mediator as host. At low exciton densities emission occurs from the annihilator, while at higher exciton intensities TTA and emission from the mediator is observed. The low concentration of the annihilator dopant gives evidence for a hetero-TTA mechanism being active, annihilation occurring between the mediator and an annihilator molecule. Monte-Carlo simulations qualitatively reproduced the experimental results and give a direction for future optimization. This work hence demonstrates successful separation of exciton diffusion from annihilation by the introduction of a triplet mediator host, and with this approach support the development of highly efficient solid-state TTA-UC materials.