Intramolecular FRET Cascades Enable Small Efficiency Roll-off in Solution-Processed OLEDs

ABSTRACT Multiresonant thermally activated delayed fluorescence (MR‐TADF) emitters hold significant promise for use in organic light‐emitting diodes (OLEDs) as they deliver bright, narrowband emission. However, their typically moderate reverse intersystem crossing rate constants ( k RISC ) result in triplet exciton accumulation that feeds undesired annihilation processes, resulting in pronounced efficiency roll‐off, an issue amplified in solution‐processed OLEDs. Here, we report three MR‐TADF emitters, TRZ‐SpAc‐DBN , Ph‐SpAc‐DBN , and DOBNA‐SpAc‐DBN , that efficiently manage exciton kinetics using an intramolecular FRET design that combines fast k RISC donor‐acceptor (D‐A) TADF cores with highly emissive peripheral MR‐TADF units. The three compounds show narrowband emission (FWHM = 24 nm) and have high photoluminescence quantum yields (up to ∼100%) in both solution and doped films in mCP, with k RISC values exceeding 1 × 10 5 s −1 . When employed as terminal emitters in solution‐processed hyperfluorescent OLEDs with 5 t BuCzTRZ as the sensitizer, devices showed high maximum external quantum efficiencies ( EQE max of up to 20.7%) and remarkably low efficiency roll‐off (below 2.5% at EQE 1000 ). Notably, devices with TRZ‐SpAc‐DBN achieved an extraordinary EQE 10,000 of 13.6%, corresponding to only 34% efficiency roll‐off. These findings highlight the effectiveness of combining a FRET‐enabled MR‐TADF emitter design with hyperfluorescent device architectures to realize high‐efficiency and exceptionally low efficiency roll‐off solution‐processed OLEDs.