Abstract
The development of highly efficient and deep blue emitters satisfying the color specification of the commercial products has been a challenging hurdle in the organic light-emitting diodes (OLEDs). Here, deep blue OLEDs with a narrow emission spectrum with good color stability and spin-vibronic coupling assisted thermally activated delayed fluorescence are reported using a novel multi-resonance (MR) emitter built on a pure organic-based molecular platform of fused indolo[3,2,1-jk]carbazole structure. Two emitters derived from 2,5,11,14-tetrakis(1,1-dimethylethyl)indolo[3,2,1-jk]indolo[1',2',3':1,7]indolo[3,2-b]carbazole (tBisICz) core are synthesized as the MR type thermally activated delayed fluorescence emitters realizing a very narrow emission spectrum with a full-width-at-half-maximum (FWHM) of 16 nm with suppressed broadening at high doping concentration. The tBisICz core is substituted with a diphenylamine or 9-phenylcarbazole blocking group to manage the intermolecular interaction for high efficiency and narrow emission. The deep blue OLEDs achieve high external quantum efficiency (EQE) of 24.9%, small FWHM of 19 nm, and deep blue color coordinate of (0.16, 0.04) with good color stability with increase in doping concentration. To the authors' knowledge, the EQE in this work is one of the highest values reported for the deep blue OLEDs that achieve the BT.2020 standard.