Rational Molecular Design of Efficient Yellow-Red Dendrimer TADF for Solution-Processed OLEDs: A Combined Effect of Substitution Position and Strength of the Donors

The development of high-performance solution-processed red organic light-emitting diodes remains a challenge, particularly in terms of maintaining efficiency at high luminance. Here, we designed and synthesized four novel orange-red solution-processable TADF dendrimers, 2GCzBP, 2DPACzBP, 2FBP2GCz and 2FBP2DPACz. We systematically investigated the effect of substitution position and strength of donors on the opto-electronic properties. The reverse intersystem crossing rate constant (kRISC) of the emitters having donors substituted at positions 11 and 12 of the dibenzo[a,c]phenazine (BP) is more than 10-times faster than that of compounds substituted having donors substituted at positions 3 and 6. Compound 2DPACzBP, containing stronger donors than 2GCzBP, exhibits a red-shifted emission and smaller singlet−triplet energy splitting, EST, of 0.01 eV. The solution-processed OLED with 10 wt% 2DPACzBP doped in mCP emitted at 640 nm and showed a maximum external quantum efficiency (EQEmax) of 7.8%, which was effectively maintained out to a luminance of 1000 cd m−2. Such a device performance at relevant display luminance is among the highest for solution-processed red TADF OLEDs. The efficiency of the devices was improved significantly by using 4CzIPN as an assistant dopant in a hyperfluorescence (HF) configuration, where the 2DPACzBP HF device shows an EQEmax of 20.0% at EL of 605 nm and remains high at 11.8% at a luminance of 1000 cd m−2, which makes this device one of the highest efficiency orange-to-red HF SP-OLEDs to date.