Highly efficient light-emitting diodes based on fluorene copolymer consisting of triarylamine units in the main chain and oxadiazole pendent groups

Article abstract of DOI:10.1021/ma051842r

We have synthesized a fluorene-based copolymer, PFA-OXD, which possesses bipolar charge-transporting functionalities, through the incorporation of electron-rich triphenylamine moieties and electron-deficient oxadiazole pendent groups into the polymer back

Full text of DOI:10.1021/ma051842r

9028
Macromolecules 2005, 38, 9028-9036
Highly Efficient Light-Emitting Diodes Based on Fluorene Copolymer
Consisting of Triarylamine Units in the Main Chain and Oxadiazole
Pendent Groups
Fang-Iy Wu, Ping-I Shih, and Ching-Fong Shu*
Department of Applied Chemistry, National Chiao Tung University, 300, Hsinchu, Taiwan
Yung-Liang Tung and Yun Chi*
Department of Chemistry, National Tsing Hua University, 300, Hsinchu, Taiwan
Received August 22, 2005
ABSTRACT: We have synthesized a fluorene-based copolymer, PFA-OXD, which possesses bipolar charge-
transporting functionalities, through the incorporation of electron-rich triphenylamine moieties and
electron-deficient oxadiazole pendent groups into the polymer backbone and onto the C9 position of each
fluorene repeating unit, respectively. The propeller-shaped triphenylamine units in the main chain and
the rigid cardo-type OXD side chains endow this polymer with a very high glass transition temperature
(306 °C) and good thermal stability, without sacrificing its good solution processability. Both in dilute
solution and as a thin film, PFA-OXD exhibits a bright blue emission upon excitation of either the main
chain or oxadiazole side chains; this phenomenon indicates that efficient energy transfer occurs from the
photoexcited pendent groups to the polymer backbone. An organic light-emitting device incorporating
this copolymer as the emitting layer exhibits a voltage-independent and stable blue emission having
color coordinates (0.14, 0.15) at 9 V; the maximum brightness was 7128 cd m-2 and the maximum
luminance efficiency was 2.07 cd A-1. In addition, doping PFA-OXD as a host material with 2.6 wt % of
a red-emitting phosphorescent dye, Os(fppz), led to a high-brightness, highly efficient red electrolumi-
nescent device having CIE coordinates of (0.65, 0.34), a maximum brightness of 28 440 cd m-2, and a
maximum external efficiency of 9.30%.
Introduction
alternative explanation for the formation of the green
emission is that it is caused by keto defects arising from
oxidation at the C9 positions of fluorene units that have
not been fully alkylated.9 If this is the case, purification
of monomers to a high degree prior to their polymeri-
zation is required to avoid the formation of the keto
defects.
Conjugated polymer light-emitting diodes (PLEDs)
have attracted considerable interest because of their
potential applications in flat-panel displays.1,2 Solution
processing of semiconductor polymers has the advantage
of allowing spin-coating and printing methods to be
utilized for the preparation of large-area-display devices.
In addition, PLEDs offer the flexibility of fine-tuning
the luminescent properties of the device through ma-
nipulation of chemical structures. Many conjugated
polymers whose emission wavelengths can be tuned to
span the entire visible spectrum rangesthrough simple
copolymerization of appropriate monomersshave been
synthesized as components of potential polymer-based
LEDs.3 Polymers having large band gaps so that they
emit blue light efficiently are of particular interest
because they can be used either as blue light sources in
full-color displays or as host materials for lower energy
fluorescent or phosphorescent dyes.4
Because of their high photoluminescence (PL) and
electroluminescence (EL) efficiencies, polyfluorenes (PFs)
are promising materials for blue light emitting diodes.5
Nevertheless, there are some drawbacks that continue
to hamper their potential applicability, especially the
undesired green emission that appears upon thermal
annealing or device operation. This low-energy emission
band has been attributed to the formation of excimers
or aggregates in the solid state.6 Several approaches
have been described for the suppression of excimer or
aggregate formation, including the attachment of steri-
cally hindered side groups at the C9 position of fluorene
units7 and the introduction of disorder into the π-con-
jugated system of the rigid-rod backbone of PFs.8 An
Another problem for PFs in PLED applications is
their high ionization potential (5.8 eV);10 i.e., a high-
energy barrier exists for holes to travel into the PFs
from the indium tin oxide (ITO)/poly(3,4-ethylenedioxy-
thiophene) (PEDOT) layer (5.2 eV),11 which results in
higher driving voltages.7a Although blends prepared by
doping PFs with hole-transporting molecules have led
to some improvement in hole injection from the anode,
as well as EL stability,12 physical blending of these
small molecules into polymeric hosts runs the risk of
phase separation occurring over time. The incorporation
of electron-rich tertiary aromatic amines into polyfluo-
rene backbones can lead to copolymers having lower
ionization potentials (5.0-5.3 eV)13 that are better
matched with the work function of the ITO/PEDOT
layer (5.2 eV). Consequently, devices based on these
copolymers exhibit lower operating voltages relative to
those of devices based on the corresponding fluorene
homopolymers.14 Moreover, the hole mobility of such
fluorene-triarylamine copolymers can reach as high as
3 × 10-3 cm2 V-1 s-1 (at a bias field of 2.5 × 105 V cm-1),
which is about 1 order of magnitude higher than that
of poly(9,9-dioctylfluorene) (POF) and is comparable to
that of the widely used hole-transporting molecule N,N′-
diphenyl-N,N′-bis(3-methylphenyl)-1,1-biphenyl-4,4′-di-
amine (TPD).l5
10.1021/ma051842r CCC: $30.25 © 2005 American Chemical Society
Published on Web 09/30/2005

Macromolecules, Vol. 38, No. 22, 2005
LEDs Based on PFA-OXD 9029
demanding pendent groups may also prevent the oc-
currence of π-stacking between polymer chains and
suppress the formation of excimers in the solid state.7
We observed a bright and stable blue electrolumines-
cence from a device prepared using PFA-OXD as the
emitting layer. Furthermore, we also obtained high-
performance red-electrophosphorescent devices display-
ing a high current and luminance when employing PFA-
OXD as the polymeric host and Os(fppz) as an efficient
red phosphor.16
Figure 1. Chemical structure of PF-TPA-OXD, a polyfluorene
possessing both hole- and electron-transporting moieties as
side chains.
Experimental Section
Materials. 4-Butyl-N,N-diphenylaniline,17 monomer 3,7b
Os(fppz),16 and the electron-transporting material 1,3,5-tris-
(N-phenylbenzimidazol-2-yl)benzene (TPBI)18 were prepared
according to reported procedures. The solvents were dried
using standard procedures. All other reagents were used as
received from commercial sources, unless otherwise stated.
Characterization. 1H and 13C NMR spectra were recorded
on Varian UNITY INOVA AS500 (500 MHz) and Bruker-DRX
300 (300 MHz) spectrometers. Mass spectra were obtained
using a JEOL JMS-HX 110 mass spectrometer. Size exclusion
chromatography (SEC) was performed using a Waters chro-
matography unit interfaced with a Waters 410 differential
refractometer; three 5-µm Waters styragel columns (300 × 7.8
mm) were connected in series in order of decreasing pore size
(104, 103, and 102 Å); tetrahydrofuran (THF) was the eluent.
Standard polystyrene samples were used for calibration.
Differential scanning calorimetry (DSC) was performed using
a SEIKO EXSTAR 6000DSC unit at a heating rate of 20 °C
min-1 and a cooling rate of 40 °C min-1. Samples were scanned
from 30 to 370 °C, cooled to 0 °C, and then scanned again from
30 to 370 °C. The glass transition temperatures (Tg) were
determined from the second heating scan. Thermogravimetric
analysis (TGA) was undertaken using a DuPont TGA 2950
instrument. The thermal stabilities of the samples were
determined under nitrogen atmosphere by measuring their
weight loss while heating at a rate of 20 °C min-1. UV-vis
spectra were measured using an HP 8453 diode-array spec-
trophotometer. PL spectra were obtained from a Hitachi
F-4500 luminescence spectrometer. Cyclic voltammetry (CV)
measurements were performed using a BAS 100 B/W electro-
chemical analyzer in anhydrous acetonitrile containing 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) as the
supporting electrolyte at a scan rate of 100 mV s-1. The
potentials were measured against an Ag/Ag+ (0.01 M AgNO3)
reference electrode using ferrocene as the internal standard.
The onset potentials were determined from the intersection
of two tangents drawn at the rising and background currents
of the cyclic voltammogram. Atomic force microscopy measure-
ments were undertaken in the tapping mode using a Digital
Nanoscope IIIa instrument under ambient conditions.
Fabrication of Light-Emitting Devices. LED devices
were fabricated in the structure indium tin oxide (ITO)/poly-
(styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene) (PE-
DOT) (35 nm)/polymer emitting layer (50-70 nm)/TPBI (30
nm)/Mg:Ag (100 nm)/Ag(100 nm). The PEDOT was spin-coated
directly onto the ITO glass and dried at 80 °C for 12 h under
vacuum to improve both hole injection and the substrate’s
smoothness. The light-emitting layer was spin-coated on top
of the PEDOT layer using chlorobenzene as the solvent, and
then the sample was dried for 3 h at 60 °C under vacuum.
Prior to film casting, the polymer solution was filtered through
a Teflon filter (0.45 µm). The TPBI layer, which was used as
an electron-transporting layer that would also block holes and
confine excitons, was grown by thermal sublimation in a
vacuum of 3 × 10-6 Torr.19 Subsequently, the cathode, Mg:Ag
(10:1, 100 nm) alloy, was deposited by coevaporation onto the
TPBI layer; this process was followed by placing an additional
layer of Ag (100 nm) onto the alloy as a protection layer. The
current-voltage-luminance relationships were measured un-
der ambient conditions using a Keithley 2400 source meter
and a Newport 1835C optical meter equipped with an 818ST
silicon photodiode.
Scheme 1. Synthesis of PFA-OXDa
a Reagents: (i) n-BuLi/THF, B(OMe)3; (ii) 2 N HCl; (iii)
pinacol/benzene; (iv) Pd(PPh3)4/K2CO3, toluene/H2O/Aliquat
336.
To achieve high electroluminescent efficiency, it is
necessary to balance the transport of electrons and
holes. Previously, we achieved balanced charge recom-
bination in electroluminescent devices by utilizing
PF-TPA-OXD,7c a fluorene-based copolymer possessing
both hole- and electron-transporting moieties as side
chains (Figure 1). This blue fluorene-based copolymer
is also a very promising candidate for use as a host
material in molecularly doped electrophosphorescence
devices because of its wide energy gaps and large num-
bers of charge-transporting pendent groups, which im-
prove the confinement of triplet excitons in the phos-
phorescent dopants and provide efficient charge trans-
portation, respectively.4h-l In this study, we introduced
electron-deficient oxadiazole (OXD) groups as pendent
units to compensate for the poor electron-transporting
ability of a p-type fluorene-triarylamine copolymer.
Scheme 1 depicts the chemical structure of PFA-OXD,
which possesses bipolar transport properties; we pre-
pared it through Suzuki coupling copolymerization.
Instead of the deep-blue emission of typical PFs that
occurs at 425 nm, the emission maximum of PFA-OXD
is red-shifted to 461 nm; i.e., it is located in the blue
region where the human eye is more sensitive. PFA-
OXD has the attractive feature of possessing a high
content of OXD groupssto raise the electron-transport-
ing capability efficientlyswhile retaining the integrity
of the optical properties of the polymer backbone be-
cause of the interrupted π-conjugation, between the
pendent OXD units and the polymer backbone, caused
by the C9 atom of each substituted fluorene unit. More-
over, we expected that the presence of these sterically

9030 Wu et al.
Macromolecules, Vol. 38, No. 22, 2005
4-Butyl-N,N-bis(4-bromophenyl)aniline (1). A mixture
of 4-butyl-N,N-diphenylaniline (0.93 g, 3.09 mmol) and CuBr2/
Al2O3 (10.0 g)20 in CCl4 (25 mL) was heated at 60 °C for 2 h.
The solid part was removed by filtration and washed with
CCl4. The solvent was evaporated from the combined filtrate
under reduced pressure. The crude product was purified by
column chromatography, eluting with hexane to give a color-
less liquid (0.93 g, 65.5%). 1H NMR (300 MHz, CDCl3): δ 0.93
(t, J ) 7.4 Hz, 3H), 1.31-1.43 (m, 2H), 1.53-1.64 (m, 2H),
2.57 (t, J ) 7.5 Hz, 2H), 6.90 (d, J ) 8.6 Hz, 4H), 6.96 (d, J )
8.3 Hz, 2H), 7.08 (d, J ) 8.3 Hz, 2H), 7.30 (d, J ) 8.6 Hz, 4H).
13C NMR (75 MHz, CDCl3): δ 14.0, 22.4, 33.6, 35.0, 114.9,
124.9, 125.0, 129.5, 132.2, 138.8, 144.4, 146.7. MS: m/z 459
[M]+.
Figure 2. 1H NMR spectrum (500 MHz, CDCl3) of PFA-OXD
in CDCl3. The asterisk indicates the signal arising from CHCl3.
Synthesis of Monomer 2. n-Butyllithium in hexane (2.5
M, 8.0 mL) was added slowly under nitrogen to a stirred
solution of 4-butyl-N,N-bis(4-bromophenyl)aniline (1, 3.70 g,
8.06 mmol) in THF (30 mL) at -78 °C, and then the mixture
was stirred for 1 h at this temperature. Trimethyl borate (2.2
mL, 19.4 mmol) was added at -78 °C, and then the mixture
was warmed slowly to room temperature and stirred for
another 1 h. The reaction mixture was poured into 2 N HCl
(100 mL) and stirred overnight, followed by extraction with
ether and drying (MgSO4). Concentration under reduced
pressure gave the corresponding diboronic acid, which, without
further purification, was reacted with pinacol (1.00 g, 8.47
mmol) in benzene under reflux overnight using a Dean-Stark
trap to effect condensation through the azeotropic removal of
water. The resulting residue was passed through a short
column (silica gel; EtOAc) and purified by recrystallization
from THF/hexane (1:8) to afford 2 (2.18 g, 49.0%). 1H NMR
(300 MHz, CDCl3): δ 0.92 (t, J ) 7.3 Hz, 3H), 1.27-1.39 (m,
26 H), 1.56-1.61 (m, 2H), 2.56 (t, J ) 7.6 Hz, 2H), 7.00 (d, J
) 8.6 Hz, 2H), 7.03 (d, J ) 8.6 Hz, 4H), 7.07 (d, J ) 8.6 Hz,
2H), 7.64 (d, J ) 8.6 Hz, 4H). 13C NMR (75 MHz, CDCl3): δ
14.0, 22.4, 24.8, 33.6, 35.1, 83.6, 122.4, 125.8, 129.3, 135.8,
138.9, 144.5, 150.2. HRMS [M]+: calcd for C34H45B2NO4, m/z
553.3535; found, 553.3536. Anal. Calcd for C34H45B2NO4: C,
73.80; H, 8.20; N, 2.53. Found: C, 73.50; H, 8.36; N, 2.87.
the corresponding boronic acid, which, without further
purification, we converted to the boronic ester 2 upon
reaction with pinacol. We synthesized the alternating
copolymer PFA-OXD containing oxadiazole pendent
groups through Suzuki polymerization of equimolar
amounts of the diboronate monomer 2 and the dibromo
monomer 3, using Pd(PPh3)4 as the catalyst, in a
mixture of toluene and aqueous K2CO3 (2.0 M) in the
presence of Aliquat 336 as a phase transfer reagent.21
1
We used H and 13C NMR spectroscopy to verify the
chemical structure of the copolymer. Figure 2 presents
a representative 1H NMR spectrum of PFA-OXD in
CDCl3. In the aliphatic region, we assign the resonances
at δ 0.91, 1.35, 1.57, and 2.55 to the aliphatic protons
of the n-butyl chains attached to the triphenylamine
units of PFA-OXD, with the intense peak centered at
1.33 ppm representing the methyl group protons of the
tert-butyl substituent on the OXD pendent groups. We
established the chemical shift assignments of the aro-
matic protons from detailed analyses of the 1H and 13
C
NMR spectroscopic data, aided by two-dimensional
1H-1H correlation spectra (1H-1H COSY) and long-
range 1H-13C heteronuclear correlation spectra (HMBC).
In the 13C NMR spectrum, we ascribe the signal at 65.7
ppm, which superimposes with the signal of the C9
carbon atom of monomer 3, to the C9 carbon atom of
the fluorene repeating units in the copolymer. We
employed size exclusion chromatography (SEC), using
THF as the eluent and calibrating against polystyrene
standards, to determine the molecular weight of the
polymer. PFA-OXD possesses a weight-average molec-
ular weight (Mw) of ca. 3.8 × 104 g mol-1 and a
polydispersity index of 2.4.
PFA-OXD. Aqueous K2CO3 (2.0 M, 5.0 mL) and Aliquat 336
(ca. 70 mg) were added to a mixture of monomer 2 (253 mg,
456 µmol) and 3 (400 mg, 456 µmol) in toluene (8.0 mL). The
mixture was degassed; then tetrakis(triphenylphosphine)-
palladium (10 mg, 1.9 mol %) was added in one portion under
N2, and then the solution was heated at 110 °C for 72 h. The
end groups were capped by heating the mixture under reflux
for 12 h with benzeneboronic acid (130 mg, 1.07 mmol) and
then for 12 h with bromobenzene (170 mg, 1.07 mmol). The
reaction mixture was cooled to room temperature and precipi-
tated into a mixture of MeOH and H2O (2:1 v/v, 150 mL). The
crude polymer was collected, washed with excess MeOH,
dissolved in chloroform, and then reprecipitated twice into
MeOH. Finally, the polymer was washed with acetone for 48
h using a Soxhlet apparatus and then dried under vacuum to
We investigated the thermal properties of PFA-OXD
by performing thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) measurements.
As indicated in Figure 3, PFA-OXD exhibits excellent
thermal stability with the onset decomposition temper-
ature at ca. 400 °C followed by a 5% weight loss
occurring at 465 °C. In the DSC measurement, we
observed a distinct glass transition at 306 °C, but
detected no crystallizing or melting peaks at tempera-
tures up to 370 °C (see the inset of Figure 3). On the
other hand, the sample of POF prepared by rapid cooling
from its molten state exhibited, in succession, a glass
transition (Tg) at 67 °C, an exothermic crystallization
peak at 94 °C, and a melting peak at 152 °C.8c It is
evident that the presence of the propeller-like triph-
enylamine (TPA) units in PFA-OXD hinders close pack-
ing of the polymer chains and suppresses their crystal-
lizability.22 Moreover, the introduction of two rigid
oxadiazole groups onto the C9 position of each fluorene
repeating unit provided a three-dimensional cardo
structure to enhance the chain rigidity of PFA-OXD and
1
give PFA-OXD (326 mg, 70.4%). H NMR (500 MHz, CDCl3):
δ 0.88-0.93 (m, 3H), 1.27-1.38 (m, 20H), 1.53-1.61 (m, 2H),
2.53-2.57 (m, 2H), 7.01-7.09 (m, 4H), 7.09-7.14 (m, 4H),
7.39-7.48 (m, 8H), 7.48-7.53 (m, 4H), 7.59 (s, 2H), 7.62-7.65
(m, 2H), 7.82-7.86 (m, 2H), 7.95-8.06 (m, 8H). 13C NMR (125
MHz, CDCl3): δ 14.0, 22.4, 31.1, 33.6, 35.0, 65.7, 120.8, 121.0,
122.7, 123.6, 124.0, 125.2, 126.0, 126.7, 127.1, 127.7, 128.8,
129.3, 134.5, 138.6, 138.7, 140.6, 144.7, 147.2, 149.2, 150.6,
155.3, 164.0, 164.6. Anal. Calcd for C71H61N5O2: C, 83.91; H,
6.05; N, 6.89. Found: C, 82.83; H, 6.14; N, 6.56.
Results and Discussion
Synthesis. Scheme 1 displays the synthetic route we
used to prepare monomer 2 and the fluorene-triphenyl-
amine copolymer (PFA-OXD). Compound 1 was synthe-
sized by bromination of 4-butyl-N,N-diphenylaniline
with alumina-supported copper(II) bromide.20 The lithi-
ation of 1 with n-BuLi, followed by treatment with
trimethyl borate and hydrolysis in aqueous HCl, gave

Macromolecules, Vol. 38, No. 22, 2005
LEDs Based on PFA-OXD 9031
Table 1. Photophysical Properties of PFA-OXD, POF, and
OXD
solutiona
filmb
PL
(nm)
Abs
(nm)
PL
(nm)
Abs
(nm)
Φfc
Φf
PFA-OXD 297, 397 449d 0.72 300, 397 462d 0.43
POF
OXD
391
298
418
347
393
425
0.55e
a Evaluated in THF. b Evaluated in the solid state and prepared
from solutions in chlorobenzene. c Quantum yield (Φf) determined
in THF, relative to 9,10-diphenylanthracence in cyclohexane (0.9),
upon excitation at 365 nm. d Excited at 397 nm. e The thin film
quantum efficiency of POF, as measured in an integrating sphere,
was 0.55 (ref 28).
relative to that observed for POFsthat we ascribe to a
π-π* transition derived from the polymer backbone. An
additional absorption band at 297 nm appears to arise
mainly from the oxadiazole side chains because the
model compound, OXD in THF, also has an absorption
having λmax at 298 nm. Upon excitation at 397 nm of
the fluorene-triphenylamine main chain in THF, the
PL displays an emission band at 449 nm. Although
there is a moderate red shift with respect to that
observed for the signal of POF (418 nm), the emission
remains in the blue region and is closer to the maximum
of a relative photopic luminous efficiency function than
is the characteristic emission of POF.24 We also noted
that, in contrast to POF, which exhibits a vibronic fine
structure, PFA-OXD exhibits a much less resolved PL
spectrum. This loss of resolution reflects the lack of
intrachain ordering that is due to the presence of
propeller-like TPA segments in the polymer main
chain.25 As indicated in Figure 4, there is good spectral
overlap between the emission band of the oxadiazole
pendent units and the absorption band of the conjugated
main chain of PFA-OXD, which meets the requirement
for Fo¨rster energy transfer.26 Accordingly, most of the
excitons formed in the OXD pendent groups upon direct
photoexcitation are likely to migrate to the fluorene-
triphenylamine backbone, from which the emission
occurs. Upon excitation of the OXD side groups at 300
nm, the PL spectrum (the dashed line drawn in Figure
4) is the same as that obtained when exciting the
polymer backbone at 397 nm; in contrast, we detected
no luminescence from the OXD side chains in the deep-
blue region (350-400 nm). The fact that we observed
complete quenching of the OXD emission even in a very
dilute solution (10-6 M) indicates that efficient intramo-
lecular energy transfer takes place in the PFA-OXD
polymer and may contribute significantly to the emis-
sion intensity of the main chain. The PL quantum yield
in THF, upon exciting at 365 nm, was 0.72, measured
relative to 9,10-diphenylanthrancene (Φf ) 0.9) as a
standard.27
Figure 3. TGA thermogram of PFA-OXD recorded at a
heating rate of 20 °C min-1. Inset: DSC traces of PFA-OXD
recorded at a heating rate of 20 °C min-1
.
Figure 4. UV-vis absorption and PL spectra in THF solution
of (a) PFA-OXD (solid and dashed lines, excited at 397 and
300 nm, respectively), (b) POF (excited at 391 nm), and (c) OXD
(excited at 298 nm).
restrict its segmental mobility, which leads to its much
higher value of Tg.7b,c In addition, the value of Tg of PFA-
OXD is higher than that of PF-TPA-OXD (166 °C),
because PF-TPA-OXD possesses additional 9,9-dioctyl-
fluorene units in its main chain and four flexible n-butyl
chains substituted on two triphenylamino pendent
groups.7c Such a high value of Tg, which should prevent
morphological changes from occurring and suppress the
formation of aggregates and excimers upon exposure to
heat, is desirable for polymers that are used as emissive
materials for light-emitting applications. Although PFA-
OXD has only short n-butyl chains functioning as
solubilizing groups on the TPA unit, it is readily soluble
in a broad range of organic solvents, such as chloroform,
chlorobenzene, THF, and N-methyl-2-pyrrolidinone.
Again, we attribute the high solubility of PFA-OXD to
the presence of the propeller-shaped TPA units, which
minimize interchain interactions between polymer
chains.22a,c,23
Photophysical Properties. Figure 4 presents the
UV-vis absorption and PL spectra of dilute solutions
of PFA-OXD; Table 1 summarizes the spectral data. For
the sake of comparison, Figure 4 also provides the
absorption and PL spectra of POF and 2,5-di(4-tert-
butylphenyl)-1,3,4-oxadiazole (OXD).7b,c In THF solu-
tion, PFA-OXD exhibits an absorption having its λmax
at 397 nmsit is red-shifted by only a few nanometers
In comparison with the results we obtained using
dilute solutions, the absorption spectrum of the PFA-
OXD thin film spin-coated from a chlorobenzene solution
onto a quartz plate displays slight broadening, while the
emission signal exhibits a red shift of 13 nm. We
estimated the PL quantum yield of the PFA-OXD film
to be 0.43 by comparing its fluorescence intensity to that
of the POF polymer thin film sample that had been
excited at 380 nm (Φf ) 0.55).28 A key issue for the
applicability of blue-emitting polymers is their spectral
stability. The undesired green emission bands that often
develop in PL and EL spectra of polyfluorenes have been
attributed to physical (excimers or aggregates forma-

9032 Wu et al.
Macromolecules, Vol. 38, No. 22, 2005
Figure 6. Current density-voltage-luminance characteris-
tics of ITO/PEDOT/PFA-OXD/TPBI/Mg:Ag.
Figure 5. UV-vis absorption and PL spectra (excited at 380
nm) of (a) PFA-OXD and (b) POF films, recorded before and
after annealing at 150 °C for 20 h under a nitrogen atmo-
sphere.
Table 2. Performances of Devices with the Structure
ITO/PEDOT/Polymer/TPBI/Mg:Ag
PFA-OXD
PF-TPA-OXDd
turn-on voltagea (V)
voltageb (V)
3.7
4.7
tion) and chemical (keto defects) degradation pro-
cesses.6,9,29 To examine the color stability of PFA-OXD,
we heated the polymer film for 20 h on a hot plate at
150 °C under a nitrogen atmosphere (Figure 5a). For
the sake of comparison, we also prepared and annealed
a thin film of POF under the same conditions. As
displayed in Figure 5b, an additional absorption band
at 423 nm appears for the POF film after annealing;
the PL spectrum of the annealed film displays a moder-
ate red shift with respect to the pristine film, together
with the appearance of a long-wavelength emission band
at 500-600 nm.6 In contrast, the PL spectra of the PFA-
OXD film remained intactsno long-wavelength tail
appearedsafter its thermal treatment. Thus, it is ap-
parent that the thermal stability of PFA-OXD is im-
proved significantly over that of POF as a result of the
incorporation of TPA units into the PF backbone, which
results in a twisted backbone conformation that, con-
sequently, suppresses the π-stacking of polymer chains.
In addition, the presence of the sterically demanding,
rigid OXD pendent groups, which form three-dimen-
sional cardo structures, further restricts close packing
of the polymer chains and reduces the probability of
interchain interactions; thus, the tendency to form
aggregates and excimers in the polymer film upon
thermal treatment seems to be totally suppressed.7,29
The higher value of Tg of PFA-OXD also accounts for
the enhanced spectral stability of the polymer film.
Electrochemical Studies. We employed cyclic vol-
tammetry (CV) to investigate the redox behavior of PFA-
OXD and to estimate the energy levels of its highest
occupied molecular orbital (HOMO) and lowest unoc-
cupied molecular orbital (LUMO). We performed these
electrochemical measurements using the polymer film
coated on a glassy carbon electrode in an electrolyte of
0.1 M TBAPF6 in acetonitrile and with ferrocene as the
internal standard. We attribute the onset potentials at
+0.40 and -2.42 V to the oxidation of the fluorene-
triphenylamine main chains and the reduction of the
OXD side chains, respectively; we estimate the HOMO
and LUMO energy levels of PFA-OXD to be -5.20 and
-2.38 eV, respectively, with regard to the energy level
of ferrocene (4.8 eV below vacuum).30 The high-lying
HOMO levels are in agreement with the data reported
previously for copolymers incorporating triarylamine
units into the polyfluorene main chains.13 Relative to
the HOMO and LUMO of POF (-5.8 and -2.1 eV,
5.9 (7.7)
7.1 (9.7)
295 (1590)
1.46 (1.59)
brightnessb (cd m-2
luminance efficiencyb
)
407 (1966)
2.04 (1.97)
(cd A-1
)
external quantum
1.56 (1.51)
7128 (at 12 V)
2.07
1.07 (1.16)
3769 (at 12.5 V)
1.66
efficiencyb (%)
maximum brightness
(cd m-2
)
maximum luminance
efficiency (cd A-1
maximum external
)
1.59
1.21
quantum efficiency (%)
EL maximum (nm)
CIE coordinates, x and y
461c
426e
0.14 and 0.15c
0.18 and 0.12e
a At 1 cd m-2 b At 20 mA cm-2 and the data in parentheses
.
were taken at 100 mA cm-2
11 V.
.
c At 9 V. d Data from ref 31. e At
respectively),10 the higher HOMO energy level of PFA-
OXD is better matched to the work function of the
ITO/PEDOT electrode (5.2 eV) and, thus, it should
facilitate hole injection into the polymer layer from the
anode. Moreover, the markedly lower LUMO energy
level of PFA-OXD, which originates from the electron-
deficient nature of the OXD substituents, indicates that
there may be an increase in the electron affinity and
an improvement in the electron injection of the polymer.
Electroluminescence Properties of LED De-
vices. To evaluate the potential of PFA-OXD to behave
as a blue emitter in polymer LED applications, we
fabricated devices having the configuration ITO/
PEDOT/polymer/TPBI/Mg:Ag. Figure 6 presents the
current density-voltage-luminance characteristics of
the device employing PFA-OXD as the emitting layer.
Because of the low ionization potential of PFA-OXD,
which matches well the work function of the ITO/
PEDOT electrode, this device turned on at a rather low
voltage (3.7 V, corresponding to 1 cd m-2). With the
facilitation of hole and electron injection, as well as
transport, we obtained the maximum external quantum
efficiency of 1.59% at 37.2 mA cm-2 (6.5 V) with a
brightness of 771 cd m-2; the maximum brightness of
7128 cd m-2 occurred at a current density of 563 mA
cm-2 (12 V). Table 2 summarizes the performance of
the PFA-OXD-based device. For the sake of comparison,
we include the data of the corresponding device based
on PF-TPA-OXD.31 The PFA-OXD electroluminescence
device exhibits much better performance than does the

Macromolecules, Vol. 38, No. 22, 2005
LEDs Based on PFA-OXD 9033
Figure 8. Solid-state PL spectrum of the PFA-OXD film
(excited at 380 nm) and the absorption and PL spectra of
Os(fppz) (excited at 425 nm) recorded in CH2Cl2 solution.
Figure 7. Solid-state PL spectrum of the PFA-OXD film and
EL spectra of the ITO/PEDOT/PFA-OXD/TPBI/Mg:Ag devices,
recorded at various voltages.
PF-TPA-OXD-based device: it has a lower turn-on volt-
age, a higher quantum efficiency (η %), and a brighter
luminance. It is noteworthy that this PFA-OXD-based
device seems to be able to endure a higher current
density and to achieve a higher brightness than do other
devices employing polyfluorene derivatives as emitters
in the same device configuration.7d,e,31 As indicated in
Figure 7, this PFA-OXD-based device exhibits an EL
maximum at ca. 460 nm in the resulting spectrum; this
value is almost identical with that obtained in the
corresponding PL spectrum, which indicates that both
the PL and EL originate from the same radiative decay
process of the singlet exciton and that no exciplexes
formed at the PFA-OXD/TPBI interface. Figure 7 also
demonstrates that the PFA-OXD-based device exhibits
a voltage-independent and stable EL spectrum. Upon
increasing the applied voltages from 7 to 13 V (i.e.,
beyond the voltage required to achieve maximum bright-
ness), we observed no significant change in the EL
spectra; the Commission Internationale de L’Eclairage
(CIE) color coordinates at 7 and 13 V were (0.14, 0.14)
and (0.15, 0.18), respectively, which fall in the pure-
blue region of a CIE chromaticity chart. In contrast, the
polydialkylfluorene-based device exhibited an undesir-
able lower energy emission band between 500 and 600
nm during device operation, turning the pure-blue
emission to a blue-green color, i.e., an unstable emission
color.32
Red Phosphorescence from a Binary Blend. The
preparation of electrophosphorescent LEDs utilizing
polymers as host materials for the emitting layer is
attractive because of the potential for applications in
large-area devices prepared using simple solution pro-
cessing. Efficiently electrophosphorescent polymer-
based devices have been prepared by doping PF or
poly(N-vinylcarbazole) (PVK) with triplet complexes.4e-h
For the purpose of balancing charges, some electron-
transporting materials, such as 2-(4-biphenylyl)-5-(4-
tert-butylphenyl)-1,3,4-oxadiazole (PBD) and 3-(4-bi-
phenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-tri-
azole (TAZ), have also been introduced to compensate
for the poor ability of the host polymers to transport
electrons.33 Furthermore, devices exhibiting a higher
current and brightness, together with lower driving
voltages, can be achieved by co-doping with the hole-
transporting molecule TPD.34
Figure 9. (a) PL (excited at 380 nm) and (b) EL spectra of
blends prepared from PFA-OXD doped with different amounts
of Os(fppz).
red-emitting phosphorescent dye, Os(fppz), to realize
efficient electroluminescence in conjunction with high
current and luminance. The observed lifetime of
Os(fppz) is 0.7 µs, which is considerably shorter than
that of Ir(btp)2(acac) (5.8 µs).35 This shorter exciton
lifetime may suppress both TT and PT annihilation36
and lead to improved device quantum efficiency, even
at a high current density. As depicted in Figure 8, the
PL spectrum of PFA-OXD and the absorption spectrum
of Os(fppz) overlap to a moderate extent, which suggests
that energy transfer may occur from PFA-OXD to
Os(fppz). Figure 9a presents the degree of energy trans-
fer between the PFA-OXD host and Os(fppz) dopant at
doping ratios from 0.9 to 4.4 wt % under photoexcitation
of the host. Upon excitation of PFA-OXD, the PL profile
of the blend contains two emission bands: one centered
at ca. 460 nm, which originates from the residual
emission of the PFA-OXD host, and the other at ca. 620
nm, which corresponds to the radiative decay from the
triplet state of Os(fppz) to the ground state. Even at a
higher doping level (4.4 wt %), the PL still displays a
significant contribution from the PFA-OXD host. In
contrast, the corresponding EL spectra (Figure 9b)
indicate that at a doping concentration of 2.6 wt %, the
dopant emission dominates completely and results in a
red emission having CIE color coordinates of (0.65, 0.34).
In this study, we used PFA-OXD, which possesses
hole-transporting backbones and electron-transporting
side chains, as the polymeric host and doped it with a

9034 Wu et al.
Macromolecules, Vol. 38, No. 22, 2005
Figure 11. Energy level diagram for devices having the
configuration ITO/PEDOT/Os(fppz):PFA-OXD/TPBI/MgAg.
current density of 341 mA cm-2 (18 V). A further
increase in doping concentration to 4.4 wt % resulted
in a slight decrease in the external quantum and lumi-
nance efficiencies, probably as a result of concentration
quenching. We also used atomic force microscopy (AFM)
to investigate the phase morphology in the PFA-OXD-
Os(fppz) blending system. We observe clearly from the
AFM phase image that no phase separation occurred
in the 4.4 wt % Os(fppz) doped PFA-OXD film. The root-
mean-square surface roughnesses of the pure PFA-OXD
film and the 4.4 wt % Os(fppz) doped polymer film were
0.26 and 0.30 nm, respectively. These similar root-mean-
square surface roughnesses for the undoped and doped
PFA-OXD films support that no detectable phase sepa-
ration occurred within the blended PFA-OXD-Os(fppz)
system.4f We infer that the Os complex is dispersed well
in PFA-OXD, probably because the polar moieties in
PFA-OXD provide a stabilizing environment for the
osmium dopant.
Table 3 summarizes the performances of the doped
devices that we investigated in this study. The bright-
ness and efficiency of the Os(fppz)-doped devices that
we describe in this paper are significantly higher than
those we reported previously when employing polyfluo-
rene derivatives containing charge-transporting pendent
groups as hosts.7e,39 Even at a higher current density
of 100 mA cm-2, ca. 90% of the peak efficiency was
sustained, with a bright phosphorescence emitting from
the red-emitting devices. For instance, for the device
doped with 2.6 wt % Os(fppz) at a current density of
100 mA cm-2, we obtained an external quantum ef-
ficiency of 8.13% in conjunction with a high brightness
of 11 036 cd m-2. We attribute the high performance of
these Os(fppz)-doped devices, even at higher current
Figure 10. Plots of (a) current density vs applied voltage and
(b) luminance vs applied voltage for devices prepared from
PFA-OXD doped with different amounts of Os(fppz).
These results suggest that both energy transfer and
direct charge trapping/recombination on the Os(fppz)
guest are responsible for the observed EL.4f,g,37 Parts a
and b, respectively of Figure 10 present the current
density vs voltage (I-V) and luminance vs voltage
(L-V) characteristics of the devices prepared using
different amounts of Os(fppz) in PFA-OXD. It is obvious
that the operating voltages of the doped devices are
higher than those of the nondoped device and that the
I-V characteristics shift to higher voltages upon in-
creasing the doping concentration of Os(fppz) in the
PFA-OXD host. This result also implies that Os(fppz)
moieties serve as charge-trapping sites when dispersed
in the PFA-OXD matrix.4f,g,37 According to the energy
level diagram constructed in Figure 11, the HOMO and
LUMO levels of Os(fppz) are -4.5 and -2.2 eV, respec-
tively,16 which indicate that a hole can potentially be
trapped at the HOMO of Os(fppz) by a barrier depth of
0.7 eV after injection from the ITO/PEDOT electrode,
and can then wait for recombination with an opposite
charge (electron) to form excitons. The holes trapped in
the Os(fppz) sites would build up a positive space-charge
field in the host matrix to impede the further injection
of holes from the anode, in turn leading to higher
operating voltages.33a,38 At a doping concentration of 2.6
wt %, the maximum external quantum efficiency of
9.30% occurs at a current density of 5.2 mA cm-2 (11
V), together with a brightness of 658 cd m-2. The
maximum luminance of 28 440 cd m-2 occurred at a
Table 3. Performances of Devices Having the Structure ITO/PEDOT/Os(fppz):PFA-OXD/TPBI/Mg:Ag
Os(fppz) (wt % in PFA-OXD)
0.9 wt %
2.6 wt %
4.4 wt %
turn-on voltagea (V)
5.8
7.0
7.5
voltageb (V)
10.4 (13.1)
1688 (6825)
8.46 (6.83)
5.94 (4.80)
16 395 (at 16 V)
9.65
12.4 (14.7)
2484 (11036)
12.42 (11.06)
9.13 (8.13)
28 440 (at 18 V)
12.65
13.5 (16.6)
1919 (9173)
9.59 (9.18)
7.17 (6.86)
19 264 (at 20.5 V)
9.71
brightnessb (cd m-2
)
luminance efficiencyb (cd A-1
)
external quantum efficiencyb (%)
maximum brightness (cd m-2
maximum luminance efficiency (cd A-1
)
)
maximum external quantum efficiency (%)
6.78
620
0.44 and 0.28
9.30
623
0.65 and 0.34
7.26
624
0.65 and 0.35
EL maximumc (nm)
CIE coordinates, x and yc
a At 1 cd m-2 b At 20 mA cm-2 and the data in parentheses were taken at 100 mA cm-2 c At 9 V.
. .

Macromolecules, Vol. 38, No. 22, 2005
LEDs Based on PFA-OXD 9035
Y.-H.; Chen, B.; Liu, S.; Yip, H.; Bardecker, J.; Jen, A. K.-Y.;
Kavitha, J.; Chi, Y.; Shu, C.-F.; Tseng, Y.-H.; Chien, C.-H.
Appl. Phys. Lett. 2004, 85, 1619. (l) Niu, Y.-H.; Tung, Y.-L.;
Chi, Y.; Shu, C.-F.; Kim, J. H.; Chen, B.; Luo, J.; Carty, A.
J.; Jen, A. K.-Y. Chem. Mater. 2005, 17, 3532.
density, to the balanced charge injection and transpor-
tation supported by PFA-OXD and the shorter triplet
lifetime of Os(fppz); effective exciton confinement within
the emitting layer, caused by the hole/exciton blocking
(TPBI) layer, could also account for these results.
In conclusion, we have realized a stable blue light
emitting polymer, PFA-OXD, which possesses bipolar
charge-transporting capability, by incorporating two
electron-deficient oxadiazole groups onto the C9 position
of each fluorene unit in the hole-transporting fluorene-
triphenylamine backbone. From its combination of
propeller-like triphenylamine units in the backbones
and rigid cardo-type OXD pendent groups, this polymer
exhibits a very high glass transition temperature (306
°C) without sacrificing its good solution processability.
An EL device based on PFA-OXD displays a stable blue
emission having color coordinates of (0.14, 0.15), a
maximum brightness of 7128 cd m-2, and a maximum
luminance efficiency of 2.07 cd A-1. Moreover, when
using PFA-OXD as the host material doped with 2.6 wt
% Os(fppz) complex, we realized a high brightness and
highly efficient red electroluminescent device having
CIE coordinates of (0.65, 0.34), a maximum brightness
of 28 440 cd m-2, and a maximum external efficiency of
9.30%. Even at a higher current density (100 mA cm-2),
the device maintains a high efficiency (8.13%) with
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Council for financial support. Our special thanks go to
Prof. C.-H. Cheng, Dr. J.-P. Duan, and Dr. H.-T. Shih
for their support and cooperation during the preparation
and characterization of the light-emitting devices.
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