Rigidification with indolocarbazole and molecular orbitals regulation by peripheral donation towards pure green polycyclo-heteraborin MR-TADF scaffolds for stable narrowband OLEDs

Multi-resonance boron-nitrogen-containing thermally activated delayed fluorescence (MR-TADF) emitters are well known for their high photoluminescence quantum yield (PLQY) and tuneable optoelectronic properties in organic light-emitting diodes (OLEDs). However, developing pure green emitters with narrow full-width at half-maximum (FWHM) reaching towards the Broadcast Service Television 2020 (BT2020) and National Television System Committee (NTSC) standards is still challenging. Herein, we demonstrated two highly symmetrical green MR-TADF scaffolds, namely, BpIC-DPA and BpIC-Cz, based on the rigidification and regulation of molecular orbitals (MO) engineering strategies. Both materials contain curvilinear indolocarbazole (pIC) donor, which enhances the rigidity of the whole MR-TADF skeletons. Furthermore, the MOs were regulated using the peripheral donors (such as tert-butyl substituted diphenylamine and carbazole moieties). Notably, both materials showed narrow FWHM below 25 nm and high PLQY in pure green emission. Consequently, the fabricated OLEDs based on BpIC-Cz showed maximum external quantum efficiency (EQE_max) of 25.7 % with alleviated efficiency roll-off characteristics (EQE_{1000 nits} = 24.3 %). Mainly, the corresponding CIE coordinates reach almost towards BT2020 and NTSC green-colour industrial standards. Furthermore, the BpIC-Cz-based device resulted in a significantly longer device-lifetime (LT₉₀) of 291 h at the initial luminance of 1000 nits, indicating their high stability.