Studying the influence of axial substituents on hyperfluorescence in organic light-emitting diodes using boron subphthalocyanines as fluorescent emitters

Abstract

Boron subphthalocyanines (BsubPcs) are a class of macrocycles that are often stable and straightforward to synthesize. Their optical properties such as high extinction coefficient and strong fluorescence have led to their incorporation into optoelectronic devices including OLEDs. However, the best demonstrated OLEDs using BsubPc emitters were unable to convert triplet excitons generated through applied bias into light. To address this, we fabricated OLEDs incorporating an assistant dopant into the emissive layer, 2-phenyl-4′-carbazole-9H-thioxanthen-9-one-10,10-dioxide (TXO-PhCz), capable of thermally converting triplet excitons into singlets, which are then transferred to the BsubPc emitter, a process referred to as hyperfluorescence. We also varied the axial substituent attached to the BsubPc core to determine its effect on the resulting performance. We found that the use of the TADF assistant dopant increased current efficiency up to 200% while maintaining more than 90% emission from the BsubPc in most cases. The best BsubPcs had short alkoxy axial groups, achieving an average current efficiency of 1.34 cd A⁻¹ across a luminance range of up to ∼10³ cd m⁻² using tert-butoxy BsubPc. These devices are the most efficient, and pure color OLEDs demonstrated using BsubPcs as the primary emitter at reasonable luminance outputs. These findings highlight the promise of BsubPc emitters in high-performance OLEDs when paired with a TADF assistant dopant, offering a viable route to efficient and color-pure devices.