Highly efficient blue-light-emitting diodes from polyfluorene containing bipolar pendant groups

Article abstract of DOI:10.1021/ma030123e

A highly efficient blue-light-emitting copolymer with bulky hole-transporting triphenylamine (TPA) and electron-transporting oxadiazole (OXD) pendant groups at the C-9 position of fluorene was synthesized. The results from photoluminescence and electrochemical measurements reveal that both the side chains and the polyfluorene main chain retain their own electronic characteristics in the copolymer. It shows a pure blue emission with no aggregates or excimers formed even after being annealed at 150°C under nitrogen for 20 h. In addition, it also demonstrates improved charge injection and balanced charge transport in electroluminescence. The maximum external quantum efficiency of a single-layer device using this copolymer as the emitting layer is 1.21% (at a brightness of 354 cd/m² with driving voltage of 7.6 V). The maximum luminance of the device reaches 4080 cd/m² at a bias of 12.0 V and a current density of 640 mA/cm².

Full text of DOI:10.1021/ma030123e

6698
Macromolecules 2003, 36, 6698-6703
Articles
Highly Efficient Blue-Light-Emitting Diodes from Polyfluorene
Containing Bipolar Pendant Groups
Ch in g-F on g Sh u ,*,† Ra ja sek h a r Dod d a , a n d F a n g-Iy Wu
Department of Applied Chemistry, National Chiao Tung University,
Hsin-Chu, Taiwan 30035, R. O. C.
Mich elle S. Liu a n d Alex K.-Y. J en *
Department of Materials Science and Engineering, Box 352120, University of Washington,
Seattle, Washington 98195-2120
Received February 19, 2003; Revised Manuscript Received May 26, 2003
ABSTRACT: A highly efficient blue-light-emitting copolymer with bulky hole-transporting triphenylamine
(TPA) and electron-transporting oxadiazole (OXD) pendant groups at the C-9 position of fluorene was
synthesized. The results from photoluminescence and electrochemical measurements reveal that both
the side chains and the polyfluorene main chain retain their own electronic characteristics in the
copolymer. It shows a pure blue emission with no aggregates or excimers formed even after being annealed
at 150 °C under nitrogen for 20 h. In addition, it also demonstrates improved charge injection and balanced
charge transport in electroluminescence. The maximum external quantum efficiency of a single-layer
device using this copolymer as the emitting layer is 1.21% (at a brightness of 354 cd/m2 with driving
voltage of 7.6 V). The maximum luminance of the device reaches 4080 cd/m2 at a bias of 12.0 V and a
current density of 640 mA/cm2.
In tr od u ction
oxadiazole (OXD) moieties with their phenyl end group
directly attached to the C-9 carbon in every alternating
fluorene unit.15 The electron-deficient OXD derivatives
were introduced to improve both electron injection and
transport. The results from the photoluminescence (PL)
measurements of the isothermally heated PF-OXD thin
film (150 °C for 20 h) show that the commonly observed
aggregate/excimer formation in polyfluorene was effec-
tively suppressed in this polymer due to its 3-D struc-
ture. The LED device using this polymer also shows a
low turn-on voltage, high brightness, and efficiency due
to a more balanced charge recombination in the polymer
emissive layer. However, the ionization potential for PF-
OXD is 5.76 eV, which is very similar to that of POF
(5.8 eV). This means that there is a significant energy
barrier for hole injection from the conducting polymer,
poly(ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS)-modified ITO (∼5.2 eV), to PF-OXD, and
it needs to be improved in order to further enhance the
device performance.
Here we report the synthesis, characterization, and
device performance of a new statistical copolymer PF-
TPA-OXD containing both electron-transporting OXD
moieties and hole-transporting triphenylamine (TPA)
units that are functionalized at the C-9 position of
fluorene. These bipolar substituents provide both func-
tions of suppressing aggregation and improving charge
injection simultaneously.
The development of blue-light-emitting polymers has
been the subject of intense academic and industrial
research due to their potential for full-color flat panel
displays. They can be used either as the active layer in
polymer light-emitting diodes (PLEDs) or as the host
material for generating other colors through energy
transfer to lower energy fluorophores.1-4 Polyfluorenes
(PF), with their high thermal stability and exceptionally
high solution and solid-state fluorescence quantum
yields, are very promising candidates for blue LEDs.4-8
Recently, a fluorene homopolymer, poly(9,9-di-n-octyl-
fluorene) (POF), has been demonstrated as an effective
blue emitter.9 However, this type of polyfluorene tends
to form aggregates and/or eximers during device fabri-
cation and operation, leading to both a red-shifted
emission and lower efficiency.10 This can be overcome
by introducing bulky substituents at the C-9 position
of the fluorene.11-15 Such structural features could
minimize the close chain packing of molecules in solid
state. In addition, the saturated sp3 carbon (C-9) on the
fluorene ring can also effectively block the conjugation
between the side chains and the polymer backbone. As
a result, the pure blue emission from the PF main chain
can be preserved.
More recently, we reported the synthesis of a fluorene-
based alternating copolymer, PF-OXD, which contains
Exp er im en ta l Section
† Dedicated to Professor Kwang-Ting Liu on the occasion of his
65th birthday.
* To whom correspondence should be addressed.
Ma t er ia ls. 2,7-Dibromofluorenone (1),16 4,4′-dibutyltri-
phenylamine (2),17 oxadiazole monomer (4),15 and 2,7-bis-
10.1021/ma030123e CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/09/2003

Macromolecules, Vol. 36, No. 18, 2003
Blue-Light-Emitting Diodes from Polyfluorene 6699
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluo-
rene (5)18 were synthesized according to literature procedures.
9,9-Bis(4-d i(4-b u t ylp h en yl)a m in op h en yl)-2,7-d ib r o-
m oflu or en e (3). To a mixture of 2,7-dibromofluorenone (315
mg, 930 µmol) and 4,4′-dibutyltriphenylamine (1.0 g, 2.8 mmol)
was added methanesulfonic acid (60 µL, 0.93 mmol). The
reaction mixture was then heated at 140 °C under nitrogen
for 12 h. The cooled mixture was diluted with dichloromethane
and washed with aqueous sodium carbonate. The organic
phase was dried over MgSO4, and the solvent was evaporated.
The crude product was purified by column chromatography,
eluting with hexane/ethyl acetate (8:2), followed by recrystal-
lization from acetone to afford 3 (0.50 g, 52%) as white crystals.
1H NMR (300 MHz, CDCl3): δ 0.91 (12 H, t, J ) 7.4 Hz), 1.34
(8 H, m), 1.56 (8 H, m), 2.54 (8 H, t, J ) 7.7 Hz), 6.84 (4 H, d,
J ) 8.7 Hz), 6.94 (4 H, d, J ) 8.7 Hz), 6.97 (8 H, d, J ) 8.4
Hz), 7.03 (8 H, d, J ) 8.4 Hz), 7.44 (2 H, dd, J ) 8.1, 1.5 Hz),
7.50 (2 H, d, J ) 1.5 Hz), 7.54 (2 H, d, J ) 8.1 Hz). 13C NMR
(75 MHz, CDCl3): δ 153.7, 147.1, 145.2, 137.9, 137.7, 136.6,
130.7, 129.4, 129.1, 128.5, 124.8, 121.7, 121.6, 121.4, 64.6, 35.0,
33.6, 22.4, 14.0. Anal. Calcd for C65H66Br2N2: C, 75.43; H, 6.43;
N, 2.71. Found: C, 75.41; H, 6.56; N, 2.25.
Sch em e 1
P F -TP A-OXD. To a solution of 3 (161 mg, 156 µmol), 4 (137
mg, 156 µmol), and 5 (200.0 mg, 312 µmol) in toluene (4.0 mL)
were added aqueous potassium carbonate (2.0 M, 4.0 mL) and
aliquate (20 mg). The above solution was degassed, and
tetrakis(triphenylphosphine)palladium (10 mg, 5.5 mol %) was
added in one portion under a nitrogen atmosphere. The
solution was refluxed under nitrogen for 3 days. The end
groups were capped by refluxing for 12 h each with phenyl-
boronic acid (40 mg, 0.33 mmol) and bromobenzene (52 mg,
0.33 mmol). After this period, the mixture was cooled and
poured into a mixture of methanol and water (150 mL, 7:3
v/v). The crude polymer was filtered, washed with excess
methanol, and dried. The polymer was dissolved in CHCl3 (2.0
mL), filtered, and precipitated into methanol (150 mL). The
precipitate was collected, washed with acetone for 24 h using
a Soxhlet apparatus, and dried under vacuum to give PF-TPA-
OXD (270 mg, 73%). 1H NMR (300 MHz, CDCl3): δ 0.69-0.75
(20 H, m), 0.89 (12 H, t, J ) 7.5 Hz), 1.02-1.19 (40 H, m),
1.24-1.40 (26 H, m), 1.57 (8 H, m), 2.04 (8 H, m), 2.54 (8 H,
m), 6.89-7.16 (24 H, m), 7.51-7.84 (30 H, m), 7.93-8.11 (1
0H, m). 13C NMR (75 MHz, CDCl3): δ 164.8, 164.1, 155.4,
152.9, 151.9, 151.8, 150.9, 149.3, 146.8, 145.4, 141.9, 141.1,
140.4, 140.3, 139.8, 139.1, 138.9, 138.6, 137.6, 129.1, 129.0,
128.9, 127.7, 127.4, 127.3, 126.8, 126.3, 126.1, 124.7, 123.0,
121.9, 121.4, 121.1, 120.9, 120.4, 120.1. 65.9, 64.8, 55.4, 40.4,
35.2, 35.1, 33.7, 31.8, 31.2, 30.0, 29.2, 23.9, 22.6, 22.4, 14.1,
14.0. Anal. Calcd for C172H186N6O2: C, 87.19; H, 7.91; N, 3.55.
Found: C, 86.27; H, 7.73; N, 3.11.
ylanthracene (ca. 5 × 10-6 M solution in cyclohexane, Φf )
0.9).19 The solid-state fluorescence yields were determined by
comparing the fluorescence of the respective polymer films (λexc
) 375 nm) with the film of 9,10-diphenylanthracene in poly-
(methyl methacrylate) (10-3 M) on quartz substrates. Cyclic
voltammetry measurements of the polymer films were per-
formed on a BAS 100 B/W electrochemical analyzer in aceto-
nitrile with 0.1 M tetrabutylammonium hexafluorophosphate
(TBAPF6) as the supporting electrolyte at a scan rate of 50
mV/s. The potentials were measured against an Ag/Ag+ (0.01
M AgNO3) reference electrode with ferrocene as the internal
standard. The onset potentials were determined from the
intersection of two tangents drawn at the rising current and
background current of the cyclic voltammogram.
Device F a br ica tion a n d Testin g. The devices were
fabricated on ITO substrates that had been ultrasonicated
sequentially in detergent, methanol, 2-propanol, and acetone
and had been treated with O2 plasma for 10 min before use.
All the evaporation of the metal electrodes was carried out in
a vacuum evaporator inside an argon atmosphere drybox. A
hole-injecting layer, PEDOT (Bayer Corp.), was spin-coated
at a spin rate of 4000 rpm from its water solution (1.3 wt %)
onto the ITO substrates and cured at 160 °C for 10 min under
nitrogen. Then a layer of copolymer was spin-coated from its
toluene solution (2 wt %) at 1600 rpm. The thickness of the
films was measured on a Sloan Dektak 3030 surface profilo-
meter. The thickness of PEDOT was about 30 nm, and the
thickness of the polymer layer was around 60 nm. A layer of
30 nm thick calcium (Ca) cathode was then vacuum-deposited
at below 1 × 10-6 Torr through a mask, and another protecting
layer of 120 nm thick silver (Ag) was vacuum-deposited. The
device testing was carried out in air at room temperature.
Current-voltage characteristics were measured on a Hewlett-
Packard 4155B semiconductor parameter analyzer. The power
of EL emission was measured using a Newport 2835-C
multifunction optical meter. Photometric units (cd/m2) were
calculated using the forward output power and the EL spectra
of the devices, assuming Lambertian distribution of the EL
emission.20
Ch a r a cter iza tion . 1H and 13C NMR spectra were recorded
on a Varian Unity 300 MHz or a Bruker-DRX 300 MHz
spectrometer. Mass spectra were obtained on a J EOL J MS-
SX/SX 102A mass spectrometer. Size exclusion chromatogra-
phy (SEC) was carried out on a Waters chromatography unit
interfaced to a Waters 410 differential refractometer. Three 5
µm Waters styragel columns (300 × 7.8 mm) connected in
series in the decreasing order of pore size (104, 103, and 102 Å)
were used with THF as eluent, and standard polystyrene
samples were used for calibration. Differential scanning cal-
orimetry (DSC) was performed on a SEIKO EXSTAR 6000
DSC unit using a heating rate of 10 °C min-1 and a cooling
rate of 30 °C min-1. Samples were scanned from 30 to 350 °C
and then cooled to 30 °C and scanned for second time from 30
to 350 °C. Glass transition temperatures (Tg) were determined
from the second heating scan. Thermogravimetric analysis
(TGA) was made on a Du Pont TGA 2950 instrument. The
thermal stability of the samples was determined under
nitrogen by measuring weight loss while heating at a rate of
20 °C min-1. UV-vis spectra were measured with a HP 8453
diode array spectrophotometer. Photoluminescence spectra
were obtained on a Hitachi F-4500 luminescence spectrometer.
The PL quantum yield (Φf) in toluene solution was measured
by excitation of the respective polymer solutions at 365 nm
and compared with the solution emission of the 9,10-diphen-
Resu lts a n d Discu ssion
P olym er Syn th esis a n d Ch a r a cter iza tion . As
shown in Scheme 1, the TPA monomer 3 was synthe-
sized by an acid-catalyzed condensation reaction be-
tween 2,7-dibromofluorenone (1)16 and 4,4′-dibutyltri-
phenylamine (2).17 This method resembles the similar
procedure that was used recently by Mullen et al. to
introduce triphenylamine groups onto the C-9 position
of fluorene.12 The OXD monomer 4 was prepared as

6700 Shu et al.
Macromolecules, Vol. 36, No. 18, 2003
Sch em e 2
reported previously.15 The statistical polyfluorene co-
polymer PF-TPA-OXD was synthesized from a Suzuki
coupling reaction between the dibromides 3 and 4, and
the diboronate 5,18 with a mole ratio of 1.0:1.0:2.0
(Scheme 2). The copolymerization was carried out using
Pd(PPh3)4 as the catalyst in a mixture of toluene and
aqueous K2CO3 (2.0 M) in the presence of aliquate 336
as a phase transfer reagent. The structure of the
1
obtained polymer was confirmed by H and 13C NMR
spectroscopy. On the basis of the integrations of the
aliphatic protons, the composition of monomers 3, 4, and
5 present in this copolymer was estimated as 1.00:1.04:
1.98, which matched very well with the feed ratio. In
the 13C NMR spectrum, there are three signals at δ 64.8,
65.9, and 55.4, which correspond to the C-9 carbons of
the three different fluorene units in PF-TPA-OXD.
These signals are superimposed with the C-9 carbon
signals of monomers 3, 4, and 5.
The polyfluorene copolymer, PF-TPA-OXD, is readily
soluble in common organic solvents, such as toluene,
chloroform, THF, etc. The molecular weights of the
polymer were determined by size exclusion chromatog-
raphy (SEC) analysis, using THF as the eluent, and
calibrated against polystyrene standards. It possesses
a number-average molecular weight (Mn) of approxi-
mately 1.1 × 104, with a polydispersity index of 2.2. The
thermal properties of PF-TPA-OXD were investigated
by differential scanning calorimetry (DSC) and ther-
mogravimetric analysis (TGA). A distinct glass transi-
tion temperature (Tg) was observed at 166 °C in Figure
1. This relatively high Tg is essential for polymers used
as emissive materials for light-emitting applications.21
As revealed by TGA, the polymer also exhibits good
thermal stability with 5% weight loss occurring at
440 °C.
F igu r e 1. Differential scanning calorimetry (DSC) data of PF-
TPA-OXD (heating rate, 10 °C/min) under nitrogen.
Op tica l P r op er ties. The absorption and PL spectra
of PF-TPA-OXD in diluted solution and in solid state
are shown in Figure 2. PF-TPA-OXD in THF solution
exhibits an absorption with a λmax at 389 nm due to a
π-π* transition derived from the conjugated polyfluo-
rene backbone. An additional absorption band around
300 nm is originated from the combined absorption from
the TPA and OXD pendant groups since the triaryl-
amine 2 and 2,5-di(4-tert-butylphenyl)-1,3,4-oxadiazole
(OXD), in THF, have an absorption with λmax at 302 and
298 nm, respectively. It is worth noting that no absorp-
tion bands are observed in the longer wavelength region

Macromolecules, Vol. 36, No. 18, 2003
Blue-Light-Emitting Diodes from Polyfluorene 6701
F igu r e 2. UV-vis absorption and PL (excited at 390 nm)
spectra of PF-TPA-OXD in THF solution and in solid state;
also included are the absorption and PL spectra of the PF-
TPA-OXD film after annealing at 150 °C for 20 h under
nitrogen (the intensities are relative to those of a fresh film).
F igu r e 3. UV-vis absorption, PL (excited at 300 nm), and
excitation (monitored emission at 450 nm) spectra of a PF-
TPA-OXD film (a); UV-vis absorption and PL (excited at 300
nm) spectra of compounds 1 (b) and 6 (c) in THF solution.
(400-600 nm), which would correspond to the charge-
transfer transition from the electron-rich TPA to the
electron-deficient OXD moieties. Upon excitation of the
polyfluorene main chain at 390 nm, the emission
spectrum displays a vibronic fine structure with two
sharp bands at 420 and 444 nm. The PL spectrum is
nearly identical to that obtained from POF in THF (λmax
) 418, 442 nm). This result suggests that the incorpora-
tion of TPA and OXD groups onto fluorene units via the
C-9 carbon does not perturb the main chain conjugation.
The fluorescence quantum yield (ΦFL) of PF-TPA-OXD
in toluene solution (λexc ) 365 nm) was estimated to be
0.95, using 9,10-diphenylanthrancene (Φf ) 0.9) as the
standard.19 In comparison to dilute solutions, the emis-
sion spectrum of a film spin-coated on a quartz substrate
shows a slightly bathochromic shift, with maxima at 426
and 450 nm. The PL quantum yield of the film was
measured to be 0.42 using the same standard in poly-
(methyl methacrylate).
Figure 2 also shows the absorption and PL spectra of
a PF-TPA-OXD film thermally annealed at 150 °C under
a nitrogen atmosphere for 20 h. Both spectra remain
almost unchanged after the thermal treatment. In
contrast, previous reports showed that the annealing
of a POF film, in which each repeating fluorene unit
contains two flexible n-octyl chains at C-9, results in
not only a bathochromical shift in PL wavelength and
a significant reduction in emission intensity but also the
appearance of an additional emission band between 500
and 600 nm.22,23 The absence of any low-energy emission
band after thermal annealing in PF-TPA-OXD could be
attributed to the attachment of bulky TPA and OXD
moieties onto the C-9 position of the fluorene units. The
resulting 3-D rigid structure may prevent π-stacking
between polymer chains and suppress the formation of
aggregates/excimers in the solid state. The higher Tg of
the PF-TPA-OXD also accounts for the better thermal
stability of the polymer film.
polyfluorene backbone.24 Upon irradiation at 300 nm,
attributed to the TPA and OXD moieties, the PF-TPA-
OXD film exhibits a blue fluorescence around 440 nm,
which is the same as that under excitation of the PF
backbone at 390 nm (Figure 2). There is no lumines-
cence from the side chains can be detected at around
360 nm. In addition, the excitation spectrum of PF-TPA-
OXD, monitored at 450 nm, is a perfectly superimposed
image of the absorption spectrum. These results reveal
that the energy transfer from the excited pendant
groups to the PF backbone is very efficient and contrib-
utes significantly to the emission intensity of the main
chain.
Red ox P r op er ties. Cyclic voltammetry (CV) of the
polymer film coated on a glassy carbon electrode was
performed in an electrolyte of 0.1 M tetrabutylammo-
nium hexafluorophosphate (TBAPF6) in acetonitrile
using ferrocene as the internal standard. During the
anodic scan, a reversible oxidation derived from the TPA
moiety was observed at 0.61 V (E°′) with the onset
potential at 0.50 V followed by a quasi-reversible
oxidation with a peak potential at 1.10 V, which can be
assigned as the oxidation of the PF backbone. In the
cathodic sweep, a quasi-reversible reduction derived
from the OXD pendant group exhibited an onset poten-
tial at -2.26 V, while the reduction of the main chain
is irreversible and the peak potential occurred at -2.92
V (Figure 4). On the basis of the onset potentials of the
oxidation and reduction, the HOMO and LUMO energy
levels of PF-TPA-OXD were estimated to be at -5.30
and -2.54 eV, respectively, with regard to the energy
level of the ferrocene reference (4.8 eV below the vacuum
level).25 The high-lying HOMO and low-lying LUMO
levels of PF-TPA-OXD may originate from the electron-
rich and electron-deficient nature of the TPA and OXD
moieties, respectively. The HOMO energy level of this
polymer is similar to what has been reported for a TPA-
substituted PF (-5.34 eV).12 The LUMO level is close
to those reported for a OXD-substituted PF (-2.47 eV)15
and for 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxa-
diazole (PDB) (-2.4 eV).26 The electrochemical mea-
surements reveal that both the side chains and the main
chain do retain their own electronic characteristics in
As shown in Figure 3, there are reasonable spectral
overlaps between the emission bands of compounds 2
(λmax ) 366 nm), OXD (λmax ) 347, 361 nm), and the
absorption band of the conjugated main chain of PF-
TPA-OXD, indicating that there may be energy transfer
from the excited TPA and OXD side chains to the

6702 Shu et al.
Macromolecules, Vol. 36, No. 18, 2003
F igu r e 6. Current density-voltage-brightness (J -V-B)
characteristics of the LED device.
EL spectra may be due to the formation of emissive
fluorenone defects as a result of the electrooxidative
degradation of dioctylfluorene units.28 The rather low
turn-on voltage of 4.4 V (defined as the voltage required
to give a luminance of 1 cd/m2) and low operation
voltages suggest that TPA and OXD side chains function
well for charge injection. The maximum external quan-
tum efficiency of this device is 1.21% (at a brightness
of 354 cd/m2 with driving voltage of 7.6 V), which is more
than 2 times higher than that of the previously reported
device with PF-OXD as the emitting layer. The maxi-
mum luminance for bright blue emission as depicted in
Figure 6 is found to be 4080 cd/m2 (at a bias of 12.0 V
and a current density of 640 mA/cm2). The efficiencies
at this luminance are 0.19 lm/W and 0.63 cd/A, respec-
tively. The higher efficiency and brightness achieved in
PF-TPA-OXD-based devices are due to better charge
injection and more efficient charge recombination.
F igu r e 4. Cyclic voltammogram of PF-TPA-OXD film coated
on a glassy carbon electrode.
Con clu sion s
In conclusion, a highly efficient blue emitter, PF-TPA-
OXD, with bipolar pendant groups at the C-9 position
of fluorene was synthesized. The incorporation of elec-
tron-rich TPA and electron-deficient OXD groups leads
to an increase in both hole and electron affinities. The
polymer shows both a pure blue emission without the
formation of aggregates/excimers and a balanced hole-
and electron-injection/transport compared to those from
the fluorene homopolymer (POF) and copolymer (PF-
OXD), which has only the electron-deficient OXD as side
chains.
F igu r e 5. EL spectra measured for the ITO/PEDOT/PF-TPA-
OXD/Ca/Ag device at different voltages.
the copolymer. These results from CV indicate that the
incorporation of electron-rich TPA and electron-deficient
OXD groups leads to an increase in both hole and
electron affinities and an improvement in charge injec-
tion of the polymer.
E lect r ica l P r op er t ies. The hole- and electron-
transporting properties of this polymer were investi-
gated using a “hole-only” or an “electron-only” configu-
ration that contains a polymer layer sandwiched between
ITO and Au and Al and Ca electrodes, respectively.27
The J -E characteristics of these devices showed that
PF-TPA-OXD possesses lower hole and electron conduc-
tion than those of POF. This may be due to that bulky
substituents like TPA and OXD prevents polymer chain
from packing together. As a result, it lowers the charge
mobility.
Ack n ow led gm en t. C.-F. Shu thanks the National
Science Council of the Republic of China for financial
support. A. J en thanks the Boeing-J ohnson Foundation
for financial support. M. Liu thanks support from the
J oint Institute for Nanoscience funded by the Pacific
Northwest National Laboratory (operated by Battelle
for the U.S. Department of Energy) and the University
of Washington.
Refer en ces a n d Notes
Electr olu m in escen ce. A single-layer LED device
with the configuration of ITO/PEDOT:PSS/PF-TPA-
OXD/Ca/Ag was fabricated. The electroluminescence
(EL) spectra show that the emission falls in the blue
region with the CIE coordinates of (0.193, 0.141) (Figure
5). The new lower energy emission tail appeared in the
(1) Kido, J .; Hongawa, K.; Okuyama, K.; Nagai, K. Appl. Phys.
Lett. 1994, 64, 815.
(2) Kido, J .; Shionoya, H.; Nagai, K. Appl. Phys. Lett. 1995, 67,
2281.
(3) McGehee, M. D.; Bergstedt, T.; Zhang, C.; Saab, A. P.;
O’Regan, M. B.; Bazan, G. C.; Srdanov, V. I.; Heeger, A. J .
Adv. Mater. 1999, 11, 1349.

Macromolecules, Vol. 36, No. 18, 2003
Blue-Light-Emitting Diodes from Polyfluorene 6703
(4) Chen, F.-C.; Yang, Y.; Thompson, M. E.; Kido, J . Appl. Phys.
Lett. 2002, 80, 2308.
(5) Pei, Q.; Yang, Y. J . Am. Chem. Soc. 1996, 118, 7416.
(6) Leclerc, M. J . Polym. Sci., Part A: Polym. Chem. 2001, 39,
2867.
(7) Neher, D. Macromol. Rapid Commun. 2001, 22, 1365.
(8) Becker, S.; Ego, C.; Grimsdale, A. C.; List, E. J . W.; Marsitzky,
D.; Pogantsch, A.; Setayesh, S.; Leising, G.; Mu¨llen, K. Synth.
Met. 2002, 125, 73.
(9) Grice, A. W.; Bradley, D. D. C.; Bernius, M. T.; Inbasekaran,
M.; Wu, W. W.; Woo, E. P. Appl. Phys. Lett. 1998, 73, 629.
(10) Kla¨ner, G.; Davey, M. H.; Chen, W. D.; Scott, J . C.; Miller,
R. D. Adv. Mater. 1998, 10, 993.
(11) Setayesh, S.; Grimsdale, A. C.; Weil, T.; Enkelmann, V.;
Mu¨llen, K.; Meghdadi, F.; List, E. J . W.; Leising, G. J . Am.
Chem. Soc. 2001, 123, 946.
(12) Ego, C.; Grimsdale, A. C.; Uckert, F.; Yu, G.; Srdanov, G.;
Mu¨llen, K. Adv. Mater. 2002, 14, 809.
(13) Pogantsch, A.; Wenzl, F. P.; List, E. J . W.; Leising, G.;
Grimsdale, A. C.; Mu¨llen, K. Adv. Mater. 2002, 14, 1061.
(14) Marsitzky, D.; Vestberg, R.; Blainey, P.; Tang, B. T.; Hawker,
C. J .; Carter, K. R. J . Am. Chem. Soc. 2001, 123, 6965.
(15) Wu, F.-I.; Reddy, D. S.; Shu, C.-F.; Liu, M. S.; J en, A. K.-Y.
Chem. Mater. 2003, 15, 269.
(16) Ranger, M.; Leclerc, M. Macromolecules 1999, 32, 3306.
(17) Thayumanavan, S.; Barlow, S.; Marder, S. R. Chem. Mater.
1997, 9, 3231.
(18) Ranger, M.; Rondeau, D.; Leclerc, M. Macromolecules 1997,
30, 7686.
(19) Eaton, D. Pure Appl. Chem. 1998, 60, 1107.
(20) Greenhan, N. C.; Friend, R. H.; Bradley, D. D. C. Adv. Mater.
1994, 6, 491.
(21) Tokito, S.; Tanaka, H.; Noda, K.; Okada, A.; Taga, Y. Appl.
Phys. Lett. 1997, 70, 1929.
(22) Lee, J . I.; Klaerner, G.; Miller, R. D. Synth. Met. 1999, 101,
126.
(23) Wu, F.-I.; Dodda, R.; Reddy, D. S.; Shu, C.-F. J . Mater. Chem.
2002, 12, 2893.
(24) Fo¨rster, T. Discuss. Faraday Soc. 1959, 27, 7.
(25) Pommerehne, J .; Vestweber, H.; Guss, W.; Mahrt, R. F.;
Ba¨ssler, H.; Porsch, M.; Daub, J . Adv. Mater. 1995, 7, 551.
(26) J anietz, S.; Wedel, A. Adv. Mater. 1997, 9, 403.
(27) Parker, I. D. J . Appl. Phys. 1994, 75, 1656.
(28) Scherf, U.; List, E. J . W. Adv. Mater. 2002, 14, 477.
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