Expanded multiple-resonance structure for highly efficient narrowband deep-blue organic light-emitting diodes

Excellent color purity and high external quantum efficiency (EQE) are major requirements in the development of deep-blue organic light-emitting diodes (OLEDs). To achieve this, multiple-resonance (MR)–thermally activated delayed fluorescence (TADF) emitters have been considered as promising options. Herein, we suggest a novel expanded MR design strategy to fabricate deep-blue MR–TADF emitters derived from a fused indolo[3,2,1-jk]carbazole framework. The expanded MR structure managed a triplet excited state for the accelerated spin–vibronic coupling-assisted reverse intersystem crossing and increased the emission dipole orientation while maintaining the high efficiency and deep-blue emission color. The rigid and planar structure of the MR core yielded a small full-width at half-maximum (FWHM; less than 16 nm), high photoluminescence quantum yield (over 97%), and high horizontal emitting dipole orientation (over 90%), and facilitated a second-order spin–vibronic coupling-assisted triplet-to-singlet spin crossover. The fabricated MR–TADF OLEDs recorded a high EQE of 24.3% and FWHM of 21 nm at a CIE_y of 0.044, thereby satisfying the BT.2020 blue standard. Additionally, further optimized device architecture provided an EQE of 26.8%.