Carbazole‐Decorated Organoboron Emitters with Low‐Lying HOMO Levels for Solution‐Processed Narrowband Blue Hyperfluorescence OLED Devices

Abstract

The hyperfluorescence has drawn great attention in achieving efficient narrowband emitting devices based on multiple resonance thermally activated delayed fluorescence (MR‐TADF) emitters. However, achieving efficient solution‐processed pure blue hyperfluorescence devices is still a challenge, due to the unbalanced charge transport and serious exciton quenching caused by that the holes are easily trapped on the high‐lying HOMO (the highest occupied molecular orbital) level of traditional diphenylamine‐decorated emitters. Here, we developed two narrowband blue organoboron emitters with low‐lying HOMO levels by decorating the MR‐TADF core with weakly electron‐donating carbazoles, which could suppress the hole trapping effect by reducing the hole traps between host and MR‐TADF emitter from deep (0.40 eV) to shallow (0.14/0.20 eV) ones for facilitating hole transport and exciton formation, as well as avoiding exciton quenching. And the large dihedral angle between the carbazole and MR‐TADF core makes the carbazole act as a steric hindrance to inhibit molecular aggregation. Accordingly, the optimized solution‐processed pure blue hyperfluorescence devices simultaneously realize record external quantum efficiency of 29.2%, narrowband emission with a full‐width at half‐maximum of 16.6 nm, and pure blue color with CIE coordinates of (0.139, 0.189), which is the best result for the solution‐processed organic light‐emitting diodes based on MR‐TADF emitters.

Elevating the upconversion performance of a multiple resonance thermally activated delayed fluorescence emitter via an embedded azepine approach

“…Based on prior MR molecular designs, some successful strategies have emerged to enhance RISC rates ( k RISC ) and mitigate the efficiency roll-off in OLEDs. For example, extended MR frameworks 18–23 can effectively lower the value of Δ E ST . With incorporating heavy atoms, 24–27 MR emitters enhance the strength of spin–orbit coupling (SOC), accelerating k RISC .…”

“…tert -butyl or carbazole groups) on the peripheral side of a molecule. 22,29–32 The bulky substituents can suppress π–π stacking interactions between planar MR cores, inhibiting the ACQ quenching. However, there is no practical way to simultaneously resolve the two issues of low up-conversion rate and aggregation quenching.…”