Discovery of Hot Excitons from the Conventional Blue Fluorescent TBPe Emitter and their Microscopic Evolution Channels Mediated by Host Materials

Although high‐efficiency blue fluorescent TBPe‐based organic light‐emitting diodes (OLEDs) are reported, the microscopic mechanisms of excited states in TBPe emitters remain vague. Here, using fingerprint magneto‐electroluminescence probing tools, hot excitons and their high‐level reverse inter‐system crossing (HL‐RISC, S 1 ←T 2 ) and triplet fusion (T 2 F, T 2 +T 2 →S n →S 1 ) in TBPe are discovered for the first time, which is also consistently demonstrated by their transient‐electroluminescence. Interestingly, all their rich microscopic physical behaviors can be reasonably explained within the framework of excited state dynamics involved in these TBPe‐based OLEDs. The carrier mobility, polarity, and triplet energy of host materials can significantly affect the number of generated T 2,TBPe hot‐excitons and thus the probabilities of HL‐RISC and T 2 F channels, which are further modulated by exothermic low‐level Dexter energy transfer (DET) or endothermic high‐level DET channels of cold or hot excitons. Moreover, utilizing the HL‐RISC channel of hot excitons in TBPe dopants with the conventional low‐level RISC channel of an exciplex co‐host, a relatively high external quantum efficiency of 7.09% is achieved from blue fluorescent TBPe‐doped OLEDs. Thus, this work renews the physical understanding of the microscopic evolution channels of excited states in TBPe emitters and lays a solid foundation for further designing high‐efficiency blue fluorescent OLEDs.