Kinetic Analysis of Photochemical Upconversion by Triplet-Triplet Annihilation: Beyond Any Spin Statistical Limit

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.