Rational molecular design of efficient yellow-red dendrimer TADF for solution-processed OLEDs: a combined effect of substitution position and strength of the donors

Abstract The development of high-performance solution-processed red organic light-emitting diodes (OLEDs) remains a challenge, particularly in terms of maintaining efficiency at high luminance. Here, we designed and synthesized four novel orange-red thermally activated delayed fluorescence (TADF) dendrimers that are solution-processable: 2GCzBP , 2DPACzBP , 2FBP2GCz and 2FBP2DPACz . We systematically investigated the effect of substitution position and strength of donors on the optoelectronic properties. The reverse intersystem crossing rate constant ( k RISC ) 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, Δ E ST , 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 (EQE max ) of 7.8%, which was effectively maintained out to a luminance of 1,000 cd m −2 . Such a device∙s 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 EQE max of 20.0% at λ EL of 605 nm and remains high at 11.8% at a luminance of 1,000 cd m −2 , which makes this device one of the highest efficiency orange-to-red HF SP-OLEDs to date.