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

ABSTRACT 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 (EQE max ) 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.