Reducing efficiency roll-off in multi-resonant thermally activated delayed fluorescent OLEDs through modulation of the energy of the T₂ state

The S₁ state and high-lying triplet excited states (ΔE_S1Tn) offer insight into clarifying the mechanism of efficiency roll-off of organic light-emitting diodes (OLEDs). However, experimental detection of the ΔE_S1Tn is challenging due to Kasha's rule. Here, two emitters, PhCz-O-DiKTa and PhCz-DiKTa, showing multi-resonant thermally activated delayed fluorescence (MR-TADF) are reported. By modulating the conjugation between the MR-TADF DiKTa emissive center and donor substituent, emission directly from the T₂ state is for the first time observed in MR-TADF emitters. Single crystal and reduced density gradient analyses reveal the origin of the reduced observed concentration-quenching results from weak CH···π and slipped π···π stacking interactions, which suppress nonradiative transitions. Theoretical and photophysical investigations reveal that the ΔE_S1T2 difference influences the reverse intersystem crossing rate. The OLEDs employing PhCz-O-DiKTa and PhCz-DiKTa as emitters show maximum external quantum efficiencies (EQE_max) of over 20%, but very different efficiency roll-off behavior (54.5% vs 13.6% at 100 cd m⁻²). Thus, this design provides a possible solution to mitigating device efficiency roll-off by designing MR-TADF emitters with degenerate S₁ and T₂ states.