High‐Performance Hot‐Exciton OLEDs via Fully Harvesting Triplet Excited States from Both the Exciplex Co‐Host and the TBRb Emitter

The high‐level reverse intersystem crossing (HL‐RISC, T(2) → S(1)) process from triplet to singlet exciton, namely the “hot exciton” channel, has recently been demonstrated in the traditional fluorescent emitter of TBRb. Although it is a potential pathway to improve the utilization of non‐radiative triplet exciton energy, highly efficient fluorescent organic light emitting diodes (FOLEDs) based on this “hot exciton” channel have not been developed. Herein, high‐efficiency and low‐efficiency roll‐off FOLEDs are achieved through doping TBRb molecules into an energy‐level matched exciplex co‐host. Combining the low‐level RISC (LL‐RISC, EX(3) → EX(1)) process in the exciplex co‐host with the HL‐RISC process of hot excitons in TBRb to fully harvest the triplet energy, a record‐high external quantum efficiency (EQE) of 20.4% is obtained via a proper Dexter energy transfer of triplet excitons, realizing the efficiency breakthrough from fully fluorescent material‐based OLEDs with TBRb as an end emitter. Furthermore, the fingerprint Magneto‐electroluminescence (MEL) as a sensitive measuring tool is employed to visualize the “hot exciton” channel in TBRb, which also directly verifies the effective energy confinement and the full utilization of hot excitons. Obviously, this work paves a promising way for further fabricating high‐efficiency TBRb‐based FOLEDs for lighting and flat‐panel display applications.