Observation of Vibrational Phosphorescence Peaks at Room Temperature and Their Impacts on Triplet–Triplet Annihilation

Abstract Triplet–triplet annihilation (TTA) is a feasible approach for utilizing electrically generated triplet excitons in organic light‐emitting diodes. However, low TTA efficiency is often observed in solids. Owing to the optically inaccessible nature of triplets, the main loss routes of the TTA process remain unknown. In this work, three vibrational emission peaks of the first excited triplet state ( T 1 ) are identified in the sensitized room‐temperature phosphorescence of TTA molecules. These peaks provide strong evidence of the loss channels of TTA. It is found that the monomolecular recombination of T 1 represents an energy‐loss channel that combines the transient photoluminescence of both upconversion fluorescence and phosphorescence. Moreover, the study demonstrates that low‐energy vibrational T 1 acts as an energy‐loss channel with only slight contributions to TTA. This work presents constructive insights into the improvement of TTA efficiency in solid films.