Fluorene-Framework Engineering Enables Narrowband Red Multi-Resonance Emitters for High-Efficiency With Negligible...

Red multi-resonance (MR) emitters are highly desirable for high-definition OLED displays, yet achieving narrowband emission, high efficiency, and suppressed efficiency roll-off simultaneously remains a formidable challenge. Herein, we introduce a C-C bond-linked fluorene-framework engineering that modulates the 3D environment around the BNO core through σ-decoupled sp³ node at C9 position of a fluorenol unit. Two emitters, SF-BNO, and PF-BNO, sharing a π-system, but differing in the ring-closure state of the two aryl arms at C9, provide a controlled test of decoupling strategy. Both emitters exhibited narrowband red emission at 618 nm with a full-width half-maximum (FWHM) of 34 nm, while PF-BNO achieved a high photoluminescence quantum yield (PLQY) of 98%. When implemented in exciplex-assisted phosphorescence-sensitized fluorescence OLEDs using SF-BNO and PF-BNO as final emitters, the devices delivered maximum external quantum efficiencies (EQE_max) of 29.51% and 31.87%, respectively, and retained to ultra-low roll-off ratios of 0.3% and 1.1%, respectively. Furthermore, optimized top-emission (TE) PF-BNO OLED achieved a current efficiency of 61.03 cd A^-1. Microcavity-mediated spectral narrowing produced saturated red emission at 620 nm with an ultra-narrow 23 nm FWHM and CIE coordinates of (0.68, 0.31), approaching BT.2020 while maintaining high efficiency and reduced roll-off, confirming the effectiveness of this strategy.