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

Multiresonant thermally activated delayed fluorescence (MR-TADF) compounds hold significant promise as emitters for organic light-emitting diodes (OLEDs) as they usually show bright, nar-rowband emission, which can be tuned based on their structure. However, their typically moder-ate reverse intersystem crossing rate constants (kRISC) result in triplet exciton accumulation in the device that then feeds undesired exciton annihilation processes, resulting in pronounced effi-ciency roll-off, an issue amplified in solution-processed OLEDs due to morphological and structural imperfections. Here, we report a family of three MR-TADF emitters, TRZ-SpAc-DBN, Ph-SpAc-DBN, and DOBNA-SpAc-DBN, that efficiently manage exciton kinetics using an in-tramolecular FRET design that combines a high kRISC donor-acceptor (D-A) TADF core with highly emissive peripheral MR-TADF units. All three emitters exhibit narrowband emission (FWHM = 24 nm) and high photoluminescence quantum yields (ΦPL up to ~100%) in both solu-tion and doped films in mCP, with kRISC values exceeding 1 × 105 s 1. When employed as termi-nal emitters in solution-processed hyperfluorescent OLEDs with 5tBuCzTRZ as the sensitizer, the devices showed high maximum external quantum efficiencies (EQEmax of up to 20.7%) and remarkably low efficiency roll-off (below 2.5% at EQE1000). Notably, the device with TRZ-SpAc-DBN achieved a record-high EQE10,000 of 13.6% corresponding to only a 34% efficiency roll-off at this luminance. 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.