Improving 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 (𝚫ES1Tn) offers insight into clarifying the mechanism of efficiency roll-off of organic light-emitting diodes (OLEDs). However, experimental detection of the 𝚫ES1Tn is challenging due to Kasha’s rule. Here, we report two emitters, PhCz-O-DiKTa and PhCz-DiKTa, showing multi-resonant thermally activated delayed fluorescence (MR-TADF). By modulating the conjugation between the MR-TADF DiKTa emissive center and donor substituent, emission directly from the T2 state was for the first time observed in MR-TADF emitters. Single crystal and reduced density gradient (RDG) analyses reveal that the origin of the reduced observed concentration-quenching results from weak CH and slipped  stacking interactions, which suppress nonradiative transitions. Theoretical and photophysical investigations reveal that the 𝚫ES1T2 difference influences the reverse intersystem crossing (RISC) rate. The OLEDs employing PhCz-O-DiKTa and PhCz-DiKTa as emitters show maximum external quantum efficiencies (EQEmax) 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.