High-Performance Sky-Blue Hyperfluorescence Organic Light-Emitting Diodes Enabled by Efficient Thermally Activated Delayed Fluorescence Sensitizers with Suppressed Efficiency Roll-Off.

High-performance blue emitters remain a major bottleneck for next-generation organic light-emitting diode (OLED) displays. Although multiresonance thermally activated delayed fluorescence (MR-TADF) materials offer exceptional color purity, their practical use is often limited by slow reverse intersystem crossing (RISC), leading to triplet accumulation, efficiency roll-off, and device degradation. Here, we report three aggregation-induced emission TADF emitters (23PalCBP, 23PclCBP, and 32PclCBP) constructed from indolocarbazole donors and a benzophenone acceptor. Among them, 32PclCBP exhibits a very small singlet-triplet energy gap (ΔEST = 0.01 eV) and a high photoluminescence quantum yield (PLQY) of 0.88 in doped films, affording OLEDs with a maximum external quantum efficiency (EQEmax) of 29.3%. When employed as sensitizers in hyperfluorescence (HF) devices with the MR-TADF emitter BNMIPAPh, both sensitized systems deliver narrowband blue emission with a full width at half-maximum of 27 nm. The 32PclCBP-sensitized device achieves a balanced performance with an EQEmax of 28.6% and a substantially reduced efficiency roll-off of 22.7% at 1000 cd m-2, together with modestly improved operational stability relative to the nonsensitized MR-TADF reference. In comparison, the 23PalCBP-sensitized device affords a higher EQEmax of 30.9%, albeit with a larger efficiency roll-off of 31.1%. These results demonstrate that the synergistic optimization of multiple photophysical parameters, including high PLQY, low nonradiative decay, small ΔEST, and moderate RISC kinetics, enables efficient, high-color-purity, and low-roll-off sky-blue HF OLEDs.