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.