An Overlooked Charge-Transfer Interaction in the Interfacial Triplet–Triplet Upconversion Process in Blue Organic Light-Emitting Diodes

Two low-energy triplets can generate one singlet via triplet–triplet upconversion (TTU), and result in an exciton production yield exceeding 25% in conventional fluorescence-based organic light-emitting diodes (OLEDs). In most cases, since such low-energy triplets induce no serious OLED degradation, TTU-OLEDs are the only commercialized blue OLEDs so far. Herein, it is clarified that the charge-transfer (CT) interaction at a hole-transport/emitter-layer interface is an overlooked pathway to enhance TTU yield significantly. First, a small energy offset at the interface enables the formation of a high-energy CT exciton. Second, the lower energy triplet state originated from an anthracene moiety in the emitter layer collects the interfacial triplet CT. Third, due to the high-density interfacial triplets formation, TTU at the interface contributes to the electroluminescence from the emitter layer or blue dopants even at low current density. This finding underlines the important role of the CT interface to exploit the full potential of TTU in pure-blue OLEDs.