Reducing Efficiency Roll‐Off in Multi‐Resonant Thermally Activated Delayed Fluorescent OLEDs through Modulation of the Energy of the T2State

Abstract

AbstractThe S1state and high‐lying triplet excited states (ΔES1Tn) offer insight into clarifying the mechanism of efficiency roll‐off of organic light‐emitting diodes (OLEDs). However, experimental detection of the ΔES1Tnis challenging due to Kasha's rule. Here, two emitters,PhCz‐O‐DiKTaandPhCz‐DiKTa, showing multi‐resonant thermally activated delayed fluorescence (MR‐TADF) are reported. By modulating the conjugation between the MR‐TADFDiKTaemissive center and donor substituent, emission directly from the T2state 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 ΔES1T2difference influences the reverse intersystem crossing rate. The OLEDs employingPhCz‐O‐DiKTaandPhCz‐DiKTaas 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 S1and T2states.