Asymmetrical-Dendronized TADF Emitters for Efficient Non-doped Solution-Processed OLEDs by Eliminating Degenerate Excited States and Creating Solely Thermal Equilibrium Routes

The mechanism of thermally activated delayed fluorescence (TADF) in dendrimers is not clear. We report that fully-conjugated or fully-nonconjugated structures cause unwanted degenerate excited states due to multiple identical dendrons, which limit their TADF efficiency. We have synthesized asymmetrical "half-dendronized" and "half-dendronized-half-encapsulated" emitters. By eliminating degenerate excited states, the triplet locally excited state is >= 0.3 eV above the lowest triplet charge-transfer state, assuring a solely thermal equilibrium route for an effective spin-flip process. The isolated encapsulating tricarbazole unit can protect the TADF unit, reducing nonradiative decay and enhancing TADF performance. Non-doped solution-processed devices reach a high external quantum efficiency (EQE(max)) of 24.0 % (65.9 cd A(-1), 59.2 lm W-1) with CIE coordinates of (0.24, 0.45) with a low efficiency roll-off and EQEs of 23.6 % and 21.3 % at 100 and 500 cd m(-2).