Influence of Multiple rISC Channels on the Maximum Efficiency and Roll-Off of TADF OLEDs

Abstract

In this work, we look into the detailed photophysical characterization of a multidonor-acceptor (D-A) family of thermally activated delayed fluorescent (TADF) emitters to find correlations with their device performance. Increasing the number of closely packed Ds around the A core leads to changes in dihedral angles between Ds and A, affecting the highest occupied molecular orbital (HOMO)/lowest unpccupied molecualar orbital (LUMO) separation and impacting the singlet-triplet energy gaps. Moreover, D-A dihedral angles change molecular conjugation affecting the spread of charge-transfer state energies as well as the energy of D local triplet states. The coupling between these triplet states and the dispersion in CT states lead to the appearance of multiple rISC channels, a phenomenon that is host-dependent, i.e., hosts with different rigidities twist the dihedral angles differently. We show that different subsets of rISC rates directly impact device performance, where faster rISC leads to external quantum efficiencies above 20% while slower rISC rates act as parasitic traps, severely affecting device roll-off. This explains why emitters with excellent peak external quantum efficiencies can also present very poor roll-off.