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Research focus in photon energy upconversion (UC) utilizing organic compounds that can induce triplet-triplet annihilation (TTA) is associated with promising applications in photocatalysis, bioimaging, night vision, memory devices, targeted drug delivery, and many more. TTA-UC is particularly attractive because of its efficient operation under incoherent excitation and at low power densities (~ mW/cm2) such as those provided by the sunlight. This implies novel strategies in photovoltaics to harvest unabsorbed near-infrared (NIR) photons via their conversion to visible light. The best-performing NIR-to-vis UC systems realized so far, both in solution and solid-state, rely on rubrene (Rub) emitter. However, in the solid-state emissive properties of Rub are heavily influenced by the film morphology.
Here, we introduce a straightforward UC film fabrication technique based on thermal evaporation in an inert atmosphere. In this way obtained highly concentrated Rub films display one order of magnitude higher fluorescence quantum yield (ФFL) compared to the films produced by solution-processing or sophisticated thermal deposition in a vacuum. Moreover, we confirm that in these films emitter and sensitizer are well dispersed and intermixed to result in triplet energy transfer (ФTET > 60%). As a result, the UC films demonstrate record-high NIR-to-Vis UC efficiency (ФUC) of 1.2%. The attained ФUC is at least 2-fold higher compared to that of other bicomponent NIR-UC films reported in the literature so far. Moreover, we evaluate Rub potential as a TTA emitter by calculating the statistical probability ( f ) to obtain one singlet from two triplets via TTA. We determine f to be ~19.7 %. The value is found to be similar to the previously estimated one for Rub in a solution (15.5%). This indicates that f of the Rub is not significantly influenced by the environment and is a more intrinsic property.
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