Studying the Influence of Axial Substituent 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 prepared OLEDs using an assistant dopant in the emissive layer, 2-phenyl-4'-carbazole-9H-thioxanthen-9-one-10,10-dioxide (TXO-PhCz), that can thermally convert triplet-state excitons into singlets and then transfer them 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, with current efficiency of 1.34 cd/A averaged over a luminance range up to about 103 cd/m2 for 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 results demonstrate the potential for this class of fluorescent emitter when combined with a TADF assistant dopant to improve performance.