Efficient Sky-Blue Narrow-Band Thermally Activated Delayed Fluorescence Emitters Integrating Charge-Transfer and Multiple-Resonance Character Based on Indolocarbazole

Organic emitters with narrow-band emission are of great significance for realizing high-definition organic light-emitting diodes (OLEDs). The multiple-resonance thermally activated delayed fluorescence (MR-TADF) is constrained by slow reverse intersystem crossing (RISC). Herein, we proposed a strategy to adjust their excited state natures by linking MR units with an indolocarbazole (ICz) donor. The designed emitters, namely, ICz-BN and tICZ-BN, showed the advantages of MR-TADF with a narrow full-width at half-maximum (FWHM) along with a minimal triplet-singlet energy gap (ΔEST). The fast radiative decay (>10⁷ s⁻¹) and large RISC rate constants (kRISC > 10⁵ s⁻¹) were realized. The maximum external quantum efficiency (EQEmax) of 28.5% and 24.7% were achieved along with the FWHM of 36 and 34 nm and emission peaks of 491 and 489 nm. The tICz-BN-based device exhibited good resistance to concentration quenching. This strategy provides a feasible approach for designing high-efficiency narrow-band sky-blue TADF emitters.