Energy‐Level Ladder Engineering for High‐Efficiency Pure Green Organic Light‐Emitting Diodes with Suppressed Efficiency Roll‐Off

ABSTRACT Multiple resonance (MR) emitters with excellent color purity hold promise for high‐performance organic light‐emitting diodes (OLEDs), yet their practical applications face severe efficiency roll‐off at high luminance due to inefficient triplet exciton utilization inherent in themselves. Herein, an energy‐level ladder engineering strategy is presented to address this challenge. By introducing an electron‐rich fragment into the MR framework, a locally excited (LE) intermediate energy level (T 2 ) is constructed that facilitates rapid spin‐flipping with reduced thermal activation energies. The synthesized emitter, tCzBNNOp, with pure green emission at 520 nm (CIE: (0.20, 0.72)), exhibits a remarkably high reverse intersystem crossing rate ( k RISC = 1.45 × 10 6 s −1 ). Theoretical calculations and experimental data jointly reveal that the LE T 2 state with large spin orbital coupling and a small energy difference between S 1 , creating an extra RISC channel. In the corresponding non‐sensitized OLED devices, tCzBNNOp achieves a maximum external quantum efficiency (EQE max ) of 30.3% with an efficiency roll‐off of only 12.5% at 1000 cd m −2 , outperforming its structural analog in roll‐off suppression. Moreover, the device retains an EQE of 16.6% at 5000 cd m −2 and demonstrated better operational stability. This work establishes energy‐level ladder engineering as a powerful approach for developing high‐efficiency MR‐based OLEDs with suppressed efficiency roll‐off.