Low Roll‐Off Deep‐Blue Hyperfluorescence OLEDs With Sterically Engineered Emitter

ABSTRACT Deep‐blue hyperfluorescence (HF) OLEDs are indispensable for achieving BT.2020 color standards, yet their practical application is hindered by severe efficiency roll‐off at high luminance. Herein, we present a sterically engineered multi‐resonance (MR) emitter, pICz‐2TP, to address this challenge via a z‐axis steric engineering strategy. This unique molecular architecture selectively promotes long‐range Förster resonance energy transfer while effectively inhibiting short‐range Dexter energy transfer. Incorporating terphenyl (TP) moieties onto the pICz core endows pICz‐2TP with ultrapure deep‐blue emission, a high photoluminescence quantum yield of 89%, and a rapid radiative decay rate on the order of 10 8 s −1 . The HF OLED utilizing pICz‐2TP as terminal emitter and m4TCzPhBN as sensitizer exhibits outstanding device performance, featuring narrow‐band emission at 441 nm with a full‐width at half‐maximum of 23 nm, CIE coordinates of (0.15, 0.05), and an excellent peak external quantum efficiency (EQE) of 28.0%. Notably, the EQE remains as high as 16.6% at 1,000 cd m − 2 , demonstrating a remarkable suppression of efficiency roll‐off and positioning the device among the best‐performing deep‐blue OLEDs. This work showcases z‐axis steric engineering as a powerful and generalizable strategy for modulating exciton dynamics, toward low‐roll‐off high‐performance deep‐blue HF OLEDs that meet next‐generation color standards.