posted on 2021-11-08, 12:03authored byJaber Saghaei, Tanja Leitner, Van Thi Ngoc Mai, Chandana Sampath
Kumara Ranasinghe, Paul L. Burn, Ian R. Gentle, Almantas Pivrikas, Paul E. Shaw
Exciplex
or charge transfer blends can be emissive in a similar
manner to thermally activated delayed fluorescent materials but without
the requirement for covalent linkages. Exciplex blends have been mainly
used in organic light-emitting diodes fabricated using thermal evaporation,
which is in part due to the poor film forming properties of evaporable
materials when processed from solution. In this paper, we report the
synthesis of three oxadiazole derivatives designed to have good solubility
and compare the properties with commercial 1,3-bis[2-(4-t-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD-7). The acceptor materials
have 4-(2-ethylhexyloxy)phenyl or 9,9-di-n-alkylfluoren-2-yl
(where the alkyl groups are either n-propyl or n-hexyl) end groups as opposed to the 4-t-butylphenyl moieties of OXD-7. Neat films of the synthesized oxadiazole
derivatives were found to have electron mobilities an order of magnitude
higher than that of OXD-7. The oxadiazole derivatives were found to
be miscible with the hole transporting 4,4′,4″-tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA) with charge transfer
emission from solution-processed films. The 4-(2-ethylhexyloxy)phenyl-oxadiazole
derivative exhibited the best overall performance when blended with
m-MTDATA, with the solution-processed films having a photoluminescence
quantum yield of around 21 ± 3% and simple bilayer OLEDs having
a maximum external quantum efficiency of 5.3 ± 0.3%.