Samsung Advanced Institute of Technology

Achieving narrow-bandwidth emission and high external quantum efficiency (EQE) simultaneously has been a challenge for next-generation blue-emitting organic light-emitting diodes (OLEDs). In this study, we developed novel multiple-resonance thermally-activated delayed fluorescence (MR-TADF) emitters by fusing an indolocarbazole unit with two carbazole skeletons using para-oriented nitrogen atoms. The resulting rigid and planar π-system without electron-accepting atoms exhibits pure blue photoluminescence at 461 nm, reaching a 100％ quantum yield with a full-width-at-half-maximum (FWHM) of 16 nm. Higher-level quantum chemistry calculations confirmed an multiple-resonance (MR) effect within the extended π-conjugation and an enhanced triplet-to-singlet crossover (10⁴ s^？1) through a reduced energy gap (ΔE_ST) coupled with large spin-vibronic coupling mediated by low-lying triplet excited states. An OLED fabricated using the MR-TADF emitter with CIE color coordinates of (0.12, 0.16) exhibits a record high EQE of 30.9％ and a small FWHM of 23 nm. With further optimization of the device structure, we achieved a high EQE of 33.8％ without additional outcoupling enhancements owing to the near-perfect horizontal alignment of the emitting dipoles.