Efficient NIR-to-vis photon upconversion in binary rubrene films deposited by simplified thermal evaporation

Low-power NIR-to-vis upconversion (UC) of incoherent excitation mediated by triplet-triplet annihilation (TTA) has a variety of promising applications, e.g. in photovoltaics; however, these are strongly hampered by low UC efficiency in the solid state. The issue is mainly related to the most efficient rubrene annihilator (emitter) utilized for this spectral range, which experiences severe concentration quenching in films due to singlet fission (SF). Herein, a simplified thermal evaporation technique is introduced to alter the morphology of binary rubrene films without involving a singlet sink aiming to suppress SF and boost UC efficiency. Hot-plate evaporation of rubrene under ambient nitrogen on a Peltier-cooled substrate pre-coated with a sensitizer layer is demonstrated to significantly improve the FL quantum yield and triplet energy transfer after annealing, subsequently ramping UC quantum yield up to (1.2 +/- 0.15)% (out of maximum 50%). This is at least twice as high as that found in any other binary NIR-UC film reported so far. Moreover, we find that the statistical probability (f factor) of producing a singlet from two triplets via TTA in amorphous rubrene films (f = 19.5%) is close to that estimated for rubrene in a solution. This finding infers a maximum UC yield of 1/2 x f approximate to 10% and explains why there are no reports on rubrene UC systems exceeding this value.