Systematic Study to Choose Appropriate Materials Combination for Green Hyperfluorescent Organic Light-Emitting Diodes

Hyperfluorescent organic light-emitting diodes (HF-OLEDs) combine a thermally activated delayed fluorescence (TADF) molecule with a terminal emitter and have achieved extensive progress as they can fully harness the advantages of both materials. However, the energy differences between the materials and their impact on the efficiency roll-off of the devices need to be investigated to further understand the energy transfer process in detail. In this context, a systematic study was done by combining two efficient TADF materials, 4BPy-mDTC and 3BPy-mDTC, as assistant dopants with various terminal emitters. The resulting HF-OLEDs showed an external quantum efficiency (EQE) above 20%, yet devices experienced roll-off at high luminance when the lowest excited energy level differences between the TADF assistant dopant and terminal emitter are either small or large. 4BPy-mDTC has a small energy difference with Ir(ppy)3 and C545T, showing reasonable performances but high roll-off at high brightness. However, the performance was boosted in the case of 3BPy-mDTC with maximum EQEs of 28 and 23% for Ir(ppy)(3) and C545T, respectively. The device with Ir(ppy)(3) could retain 87%, and C545T retained 67% of their maximum EQE at 500 cd m(-2). Additionally, 3BPy-mDTC showed reduced performance for DTA-AN due to a larger energy difference, while it worked well with 4BPy-mDTC. The results suggest that the energy differences between the materials are vital in improving the device performance and roll-off characteristics by maintaining an optimum energy gap. This work provides insight into developing highly efficient hyperfluorescent OLEDs by modulating the energy difference between the materials.