Soluble Electroluminescent Poly(phenylene Vinylene)s
J. Am. Chem. Soc., Vol. 123, No. 10, 2001 2297
(methyl methacrylate) (PMMA),10 (3) an inert polymer such as
poly(methacrylate) (PMA) having side chains comprising high
electronegative heterocyclic moieties,11 and (4) a conjugated or
nonconjugated polymer having high electronegative heterocyclic
moieties such as oxadiazole12a-e, triazole,12f triazine,12g,h and
quinoxaline12i-k on the main chains. However, the multilayer
device always results in an unfavorable increase in turn-on
voltage and requires careful selection of material and solvent
to avoid damage of the emission layer by spin-coating an ETL.
In the latter case involving a blend, a variation in emissive color
with voltage8a and phase separation during storage and operation12d
might occur, especially for that with molecular materials, in
which the generated joule heat during the operation could
accelerate a rate of recrystallization causing a reduction of the
device stability.
A use of low work function metal as the cathode (such as
calcium, magnesium, lithium) of PLEDs is an alternative to
improving the device performance.13 Due to matching the energy
levels of the cathodes with the lowest unoccupied molecular
orbitals (LUMO) of PPVs, amounts of injected electrons
increase, and the brightness and quantum efficiency of PLEDs
are improved. Although the mobility of electrons is higher than
that of holes by 1 order of magnitude in a single MEH-PPV
chain,14 a recent report5c,d demonstrated that the presence of
electron traps in the bulk film leads to a dramatic decrease in
electron mobility. This leads to a reversal of the sequence of
the mobilities, and the bulk film is regarded as trap-free for
hole transport, and the electric behavior of the hole-only device
shows space-charge limited current (SCLC) characteristics.5c
Thus, the introduction of low work function metals still results
in a device with imbalanced charge transport, and the improve-
ment of electroluminescent efficiency is limited. Consequently,
some efforts have been devoted to a molecular design of
fluorescent materials and a modification of device architecture.
For the molecular design aspect, many efforts have been
attempted to develop new emissive materials with improved
electron-transport capability to allow a fabrication of efficient
single-layer devices and for simplifying the fabrication process.
For such purposes, heterocyclic aromatic moieties containing
CdN segments with high electronegative properties have been
incorporated into polymer main chains or side chains. For single-
layer PLEDs, we and others have introduced electron-transport
moieties on side chains, including electronegative moieties
connecting by a short divalent spacer with PPV backbone15 or
directly attaching to the backbone16 (i.e., conjugation with the
main chain) or to a conventional polymer grafted with a
phenylene vinylene luminescent segment11a,17 or on the main
chain having phenylene vinylene units,18a-d phenylene units18e,f
or thiophene units.18g PLEDs fabricated therewith with Al as
cathode have quantum efficiency improved by 1 order of
magnitude as compared with those of corresponding polymers
without the electron-transport moieties. As calcium is used as
the cathode, either there is no improvement, or only slight
improvement is observed. The incorporation of this kind of
moiety into the conjugated main chains can affect the emissive
spectrum;16,18 as this also happened in small organic electrolu-
minescent molecules by the formation of exciplex,19 the role
of the electron-transport moiety on the charge transport property
is complicated and cannot be accessed easily. For devices with
nonconjugated polymers grafted with both a luminescent
segment and a high electronegative moiety as emissive materials,
high operating voltage and poor operating life have resulted.17a
Thus, to understand a way by which molecular design can
provide a new material with a balanced charge transport at the
desired emission color for any cathode material is necessary.
Here, we report a new series of soluble efficient electrolu-
minescent PPV-based copolymers, which provides a balanced
electron- and hole transport at various levels. The quantum
efficiency is significantly improved, and the turn-on voltage is
reduced for the device with aluminum or calcium as the
cathodes. A series of new soluble random phenylene vinylene
copolymers having the two co-repeat units, 2-methoxy 5-(2′-
ethylhexyloxy) phenylene vinylene (MEHPV)20 and 2-(10′-(p-
(5′′-phenyl-1′′,3′′,4′′-oxadiazole-2′′-yl) phenoxy) decanoxy) phen-
ylene vinylene (POPDPV) is synthesized. The design is based
on the consideration that the PPV backbone is a good hole-
transporting electroluminescent material, the branched alkoxy
side group provides solubility and prevention from crystalliza-
tion, and the electron- deficient moiety, 2,5-diphenyl-1,3,4-
oxadiazole-diyl (OXD), with flexible spacer of sufficient length
as the side chain of POPDPV units, provides a highly improved
electron-transporting property due to intrachain interaction and
efficient mixing between OXD moieties with chromophores on
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