Impact of Terminal Emitter on Efficiency Rolloff in Hyperfluorescent OLEDs

Abstract

Hyperfluorescent organic light-emitting diodes (HF-OLEDs) offer high efficiency and narrowband emission by combining thermally activated delayed fluorescence (TADF) sensitizers with fluorescent terminal emitters. However, blue HF devices using multiple-resonance (MR)-TADF as terminal emitters, despite their high EQE, still suffer from pronounced efficiency rolloff. In this study, we investigate the critical role of terminal emitters' highest occupied molecular orbital (HOMO) level alignment in HF-OLED performance. Since three blue MR molecules, DBPA-Cz, DABNA-Cz-blue, and DABNA-Cz-green, possessing similar photophysical properties, are highly sensitive to subtle variations in HOMO energy levels, they exhibit diverse device performances, especially efficiency rolloff. Specifically, for DBPA-Cz and DABNA-Cz-green with deeper HOMO levels (-5.48 and -5.51 eV), their corresponding devices achieved a peak EQE of 26.8% and 22.4%, with only 17% and 13% rolloff at 1000 cd/m2, while the device based on DABNA-Cz-blue with shallow HOMO (-5.40 eV) exhibited a lower EQE of 20.4% and a severe rolloff of 53%. Transient electroluminescence measurements confirmed that the shallow HOMO level of DABNA-Cz-blue promotes charge carrier trapping, leading to pronounced efficiency loss at high luminance. These results underscore the importance of precisely aligning energy levels of terminal emitters to suppress efficiency rolloff and further advancing stable blue OLEDs.