High-Performance Narrowband Green OLEDs Enabled by Indolocarbazole Isomer Engineering and Steric Shielding.

The development of emitters complying with the BT.2020 standard is crucial for ultra-high-definition (UHD) displays. However, achieving green multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters with both high efficiency and high color purity remains challenging. Here, two narrowband green MR-TADF emitters, Mes-b-ICzBN and Mes-c-ICzBN, are developed via a molecular engineering strategy combining ICz isomer engineering and peripheral steric shielding. Both emitters fuse a rigid indolocarbazole (ICz) fragment and incorporate bulky mesitylene groups, with the ICz fragment extending the MR framework and limiting structural relaxation, and the mesitylene groups minimizing aggregation-induced spectral broadening. Notably, isomer engineering of the ICz moiety enables fine-tuning of the emission peak, yielding narrowband green emission closer to the BT.2020 green standard. Consequently, organic light-emitting diodes (OLEDs) based on Mes-b-ICzBN and Mes-c-ICzBN exhibit CIE coordinates of (0.25, 0.70) and (0.22, 0.71), full widths at half maximum (FWHMs) of 26 nm and 27 nm, and maximum external quantum efficiencies (EQEmax) of 33.3% and 30.6%, respectively. Furthermore, by integrating these OLEDs with backplane drivers to construct active-matrix (AM) OLED display panels, a high-dynamic-range pico-projector prototype is successfully demonstrated. This synergistic molecular engineering strategy shows great potential for high-color-purity green MR-TADF emitters in UHD displays.