Realization of H-Type Aggregation in Rubrene-Doped OLEDs and Its Induced Enhancement of Delayed Fluorescence

The molecular aggregation effect on the photophysical properties of many organic semiconductors has been well studied, but this behavior of rubrene molecules is still vague in rubrene-doped organic light-emitting diodes. Surprisingly, via recording temperature-dependent electroluminescence and photoluminescence spectra, an intriguing H-type aggregation of rubrene molecules is realized from rubrene-doped devices and films only when 4,4',4 ''-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine (m-MTDATA) or 4,4',4 ''-tris(N-(2-naphthyl)-N-phenyl-amino)triphenylamine (2TNATA) is selected as the host; otherwise, the J-type aggregation of rubrene molecules is solely observed. We propose that this H-type aggregation is caused by the increased steric hindrance between adjacent rubrene molecules because the unique molecular structures of 12 freely rotating chemical bonds in m-MTDATA or 2TNATA are suppressed in rubrene-doped solid-state films. More importantly, we find that the H-type aggregation of rubrene molecules induces a nearly 3-fold increase in the fluorescence of rubrene-doped devices at 20 K The increased fluorescence can be mainly attributed to the intensified triplet-triplet annihilation (TTA) process because the parallel alignment of transition dipole moments in H-aggregation can facilitate the TTA process via enlarging the TTA reaction cross-section. Thus, our findings deepen the understanding of molecular interactions in aggregation systems and pave the way for achieving highly efficient devices operated especially in a low-temperature environment such as outer space.