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Upconversion (UC) via triplet−triplet annihilation (TTA) is a promising concept to improve the energy conversion efficiency of solar cells by harvesting photons below the energy threshold. Here, we present a kinetic study of the delayed fluorescence induced by TTA to explore the maximum efficiency of this process. In our model system we find that more than 60% of the triplet molecules that decay by TTA produce emitters in their first excited singlet state, so that the observed TTA effiency exceeds 40% at the point of the highest triplet emitter concentration. This result thoroughly disproves any spin-statistical limitation for the annihilation efficiency and thus has crucial consequences for the applicability of an upconvertor based on TTA, which are discussed.
Cheng et al. (Tue,) studied this question.