Influence of Multiple\nrISC Channels on the Maximum\nEfficiency and Roll-Off of TADF OLEDs

In this work, we look into the detailed photophysical\ncharacterization\nof a multidonor–acceptor (<b>D-A</b>) family of thermally\nactivated delayed fluorescent (TADF) emitters to find correlations\nwith their device performance. Increasing the number of closely packed <b>D</b>s around the <b>A</b> core leads to changes in dihedral\nangles between <b>D</b>s and <b>A,</b> affecting the highest\noccupied molecular orbital (HOMO)/lowest unpccupied molecualar orbital\n(LUMO) separation and impacting the singlet–triplet energy\ngaps. Moreover, <b>D-A</b> dihedral angles change molecular\nconjugation affecting the spread of charge-transfer state energies\nas well as the energy of <b>D</b> local triplet states. The\ncoupling between these triplet states and the dispersion in CT states\nlead to the appearance of multiple rISC channels, a phenomenon that\nis host-dependent, i.e., hosts with different rigidities twist the\ndihedral angles differently. We show that different subsets of rISC\nrates directly impact device performance, where faster rISC leads\nto external quantum efficiencies above 20% while slower rISC rates\nact as parasitic traps, severely affecting device roll-off. This explains\nwhy emitters with excellent peak external quantum efficiencies can\nalso present very poor roll-off.