Fluorene‐Framework Engineering Enables Narrowband Red Multi‐Resonance Emitters for High‐Efficiency With Negligible Roll‐Off Toward BT.2020

Abstract

ABSTRACT 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 (EQEmax) 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.