Strategy for efficiency roll-off mitigation in hyper-fluorescence OLEDs via bulky substituents in TADF emitter

Achieving high efficiency at high operating brightness is critical for organic light-emitting diodes (OLEDs). Effective exciton utilization in OLED materials is essential to achieve this goal. Thermally activated delayed fluorescence (TADF) facilitates exciton transitions from triplet to singlet states via reverse intersystem crossing (RISC), enabling high efficiency in TADF materials. However, rotation and vibration of molecules in donor-acceptor type TADF materials lead to a broad full width at half maximum (FWHM). To address these issues, a hyper-fluorescence (HF) mechanism was developed, which combines high efficiency and narrow FWHM through Förster resonance energy transfer (FRET) by employing a suitable fluorescent dopant with narrow FWHM as the final emitter. Nevertheless, the HF mechanism is limited by triplet-triplet annihilation (TTA), which is influenced by intermolecular distance in TADF materials, resulting in efficiency roll-off at high luminance. Therefore, in this study, 2,4,6-tris(2-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-1,3,5-triazine (TtBCz-Trz), a well-known TADF material with bulky substituents, was utilized as the sensitizer to suppress undesirable interactions caused by intermolecular proximity. For comparative device performance evaluation, 2,4,6-tris(2-(9H-carbazol-9-yl)phenyl)-1,3,5-triazine (TCz-Trz) was also employed as a sensitizer. Additionally, DtBuCzB–p-tBuPh was synthesized and utilized as the final dopant in an HF device. A maximum external quantum efficiency (EQE) of 28.6 % was achieved in the HF device fabricated using TtBCz-Trz with bulky tert-butyl groups as the TADF sensitizer. At a high brightness of 1000 nits, this device maintained an EQE of 21.8 % with an efficiency roll-off of 23.7 %. This roll-off was 16 % lower than that of the device fabricated using TCz-Trz without bulky substituents as the TADF sensitizer. These results confirm that incorporating bulky substituents in the TADF sensitizer significantly reduces TTA, mitigating efficiency roll-off issues. The findings of this study will contribute to advancing OLED technology, making it more feasible to produce high-performance, energy-efficient, and long-lasting devices for a wide range of applications.