Intramolecular FRET cascades enable small efficiency roll-off in solution-processed OLEDs

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 kRISC 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 × 10⁵ s⁻¹. When employed as terminal emitters in solution-processed hyperfluorescent OLEDs with 5tBuCzTRZ 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 EQE₁₀₀₀). 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.