White electroluminescent single-polymer achieved by incorporating three polyfluorene blue arms into a star-shaped orange core

Article abstract of DOI:10.1002/pola.26061

A series of star-like dopant/host single-polymer systems with a D-A type star-shaped orange core and three blue polyfluorene arms were designed and synthesized. Through tuning the doping concentration of the orange core and thermal annealing treatment of white polymer light-emitting diodes based on them, highly efficient white electroluminescence has been achieved. A typical single-layer device (ITO/PEDOT:PSS/polymer/Ca/Al) realized pure white emission with a luminous efficiency of 16.62 cd A^-1, an external quantum efficiency of 6.28% and CIE coordinates of (0.33, 0.36) for S-WP-002TPB3 containing 0.02 mol % orange core. The high efficiency of the devices could be mainly attributed to the suppressed concentration quenching of the dopant units, more efficient energy transfer from polymer host to orange dopant and thermal annealing-induced α-phase polyfluorene (PF) self-dopant in amorphous PF host.

Full text of DOI:10.1002/pola.26061

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White Electroluminescent Single-Polymer Achieved by Incorporating
Three Polyfluorene Blue Arms into a Star-Shaped Orange Core
Lei Chen,1,2 Pengcheng Li,1,2 Hui Tong,1 Zhiyuan Xie,1 Lixiang Wang,1
Xiabin Jing,1 Fosong Wang1
1State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, People’s Republic of China
2Graduate School of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China
Correspondence to: L. X. Wang (E-mail: lixiang@ciac.jl.cn) or H. Tong (E-mail: chemtonghui@ciac.jl.cn)
Received 12 January 2012; accepted 12 March 2012; published online
DOI: 10.1002/pola.26061
ABSTRACT: A series of star-like dopant/host single-polymer sys-
tems with a D-A type star-shaped orange core and three blue
polyfluorene arms were designed and synthesized. Through
tuning the doping concentration of the orange core and ther-
mal annealing treatment of white polymer light-emitting
diodes based on them, highly efficient white electrolumines-
cence has been achieved. A typical single-layer device (ITO/
PEDOT:PSS/polymer/Ca/Al) realized pure white emission with a
luminous efficiency of 16.62 cd Aꢀ1, an external quantum effi-
ciency of 6.28% and CIE coordinates of (0.33, 0.36) for S-WP-
002TPB3 containing 0.02 mol % orange core. The high effi-
ciency of the devices could be mainly attributed to the sup-
pressed concentration quenching of the dopant units, more
efficient energy transfer from polymer host to orange dopant
and thermal annealing-induced a-phase polyfluorene (PF) self-
C
V
dopant in amorphous PF host. 2012 Wiley Periodicals, Inc. J
Polym Sci Part A: Polym Chem 000: 000–000, 2012
KEYWORDS: benzothiadiazole; branched; conjugated polymers;
light-emitting diodes; white electroluminescence
A
ꢀ1. Nevertheless, the disadvantage of intrinsic phase sepa-
INTRODUCTION White
organic
light-emitting
diodes
(WOLEDs) have attracted widely research interest,1 as their
potential applications in full-color flat displays, back light of
liquid crystal displays, and solid-state illumination sources.
Compared to WOLEDs based on small-molecule emitters,
white polymer light-emitting diodes (WPLEDs) show huge
advantages in low cost, large area, and flexible displays, as
they could be easily fabricated using solution-manufacturing
process. After nearly two decades’ effort, several strategies
have been presented to achieve WPLEDs, such as multiple
emissive layer device systems,2 single emissive layer poly-
mer-blend systems,3 and single-polymer systems.4 Although
the interfacial mixing of different layers and serious bias-de-
pendent electroluminescent (EL) spectra limit the applica-
tions of multilayer-device systems, WPLEDs based on poly-
mer-blend systems have achieved great progress in device
efficiency. Among them, for fluorescent material systems,
Yang group reported two-color simultaneous WPLEDs with a
ration of different chromophores in polymer-blend systems
affects the device efficiency and life-time for long-term oper-
ation. This problem could be avoided through covalently
attaching chromophores to polymer host to develop single-
polymer systems. At the early stage, WPLEDs based on sin-
gle-polymer systems were mainly demonstrated with blue-
light emission from blue dopant and orange-light emission
from its aggregation/excimer/electromer.4 However, this kind
of white electroluminescence usually show low efficiency, as
the formation of aggregate/excimer/electromer induces to
low radiative decay rates of excitons.
Our group has proposed a new strategy to develop two- or
three-color white light emitting single-polymers by incorporat-
ing an orange emitter or both a green fluorophore and a red
fluorophore to a blue light emitting polymer host.9 Compared
to corresponding polymer-blend system, WPLEDs based on
these single-polymer systems show higher EL efficiency and
more stable bias-independent EL spectra.10 Besides, the intrin-
sic phase separation could be avoided due to the chemical mo-
lecular dispersion feature, so that longer device operation life-
time is expectable. By enhancing the fluorescent quantum effi-
ciency (/f) of the dopant unit,11 covalently attaching the dop-
ant to the side chain of the blue host,12 introducing blue
luminous efficiency (LE) of 17.9 cd Aꢀ1 5 Cao et al.6 illus-
,
trated a three-color WPLED with a LE of 14.0 cd Aꢀ1. Using
phosphorescent dyes, several groups have realized a LE
more than 30.0 cd Aꢀ1 for two-color phosphorescent
WPLEDs.7 The most efficient three-color phosphorescent
WPLEDs was reported by Cao et al.8 with a LE of 24.3 cd
C
V
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CHART 1 Chemical structure of TPB3, Mon-TPB3 and S-WP-xTPB3.
dopants to improve the efficiency of the blue species,13 the EL
efficiencies of the devices based on single-polymers were suc-
cessively improved to 12.8 cd Aꢀ1 and 8.6 cd Aꢀ1 for single-
layer two- and three-color WPLEDs. The LE of them could be
further improved to 16.9 and 15.4 cd Aꢀ1, respectively, by
introduction of additional alcohol soluble electron-injection
layers.14 Recently, these approaches also have been adopted in
several single-polymer systems reported by other groups. For
instance, Cao et al.,15 Shu et al.,16 Shim et al.,17 Hsu et al.,18
Chen et al.,19 and Chow et al.20 have succeeded in realizing
WPLEDs based on single-polymer systems. Through covalently
incorporating fluorescent green emitting 2,1,3-benzothiadiazole
(BT) units and a phosphorescent red emitting osmium complex
into the backbone of PF, Shu et al.21 developed a white electro-
recently, we reported a series of star-like single-polymers
through incorporating six PF arms in a star-shaped orange
core
(1,3,5-tris(4-(7-(4-(N,N-diphenylamino)phenyl)-2,1,3-
benzothiadiazole-4-)phenyl)benzene,TPB6).27 The concentra-
tion quenching of the orange dopants in these polymers
could be completely suppressed in the doping range. The
single-layer devices based on these polymers realized highly
efficient white electroluminescence with an external quan-
tum efficiency (EQE) of 6.36% and a LE of 18.01 cd Aꢀ1
.
However, the TPB6 unit is difficult to synthesize and purify
in large amount due to its relatively poor solubility, which
greatly limits its further application.
In this article, by introduction of three methyl groups to
TPB6 unit, we designed a star-shaped orange core (TPB3,
see in Chart 1) with improved solubility. TPB3 and its corre-
sponding A3 type brominated comonomer could be obtained
much easier compared with TPB6 and its A6 type comono-
mer. Moreover, the single-polymers achieved by incorporat-
ing three PF arms into TPB3 orange core also show high EL
efficiency. We ascribed the high efficiency to the following
three aspects: (1) By introduction of a star-shaped orange
core to build star-like polymers with three blue PF branching
arms, the concentration quenching effect of the orange dop-
ant units could thus be efficiently suppressed. (2) Blue-
shifted absorption band of the orange dopant compared to
previously used linear orange dopant11 would lead to supe-
phosphorescent single-polymer with a LE of 10.7 cd Aꢀ1
,
ranked as one of the highest values reported so far for white
emitting single-polymers by simultaneously using both singlet
and triplet excitons. In general, WPLEDs based on single-poly-
mer systems show suppressed intrinsic phase separation and
bias-independent EL spectra, but lower efficiency compared
with those devices based on polymer-blend systems containing
phosphorescent dyes. Thus, we have to develop new white EL
single-polymer materials with improved efficiency.
In our previous work, most of the dopants were linear D-A
type molecules. They severely suffer from concentration
quenching22 due to the strong intermolecular interactions in
solid state, which will lead to low EL efficiency.23 Thus, we
believe that the inhibition of the concentration quenching of
dopant units will be beneficial for improving EL efficiency of
WPLEDs based on single-polymer systems with ‘‘dopant/
host’’ feature. Star-shaped molecules and polymers have
recently attracted widely research interests due to their
unique molecular structures and optoelectronic properties.24
Especially, their highly branched and globular features will
be helpful for eliminating the intermolecular interaction and
concentration quenching. For example, Huang et al.25 have
reported a set of fluorene-based six-armed nanostar macro-
€
rior Forster energy transfer from PF host to the dopant and
the appropriate orange emission wavelength with almost the
strongest human eye response was beneficial for improving
the brightness, LE, and power efficiency (PE) of the polymer
light-emitting diodes (PLEDs). (3) Formation of a-phase PF
self-dopant in amorphous PF host through thermal annealing
treatment of the devices could greatly enhance the blue
emission part. As a result, white electroluminescence with si-
multaneous blue emission (436/460 nm) and orange emis-
sion (560 nm) was obtained. The best single-layer device of
the star-like polymers achieved a high LE of 16.62 cd Aꢀ1
and an EQE of 6.28% with Commission Internationale de
L’Eclairage (CIE) coordinates of (0.33, 0.36). The LE and EQE
molecules. They show greatly improved solid-state
/
f
(around 0.90) compared to that of linear PF (0.55).26 Very
2
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SCHEME 1 Synthetic routes of star-shaped orange dopant and polymers.
value of the three-arm polymer are comparable to those of
six-arm polymers but almost twice that of the linear orange
molecule doped polymers.11,23b
Mon-TPB3. Star-shaped polymers were synthesized by Suzuki
polycondensation with an AB type 9,9-dioctylfluorene mono-
mer and A3 type orange dopant monomer. The doping con-
centration of TPB3 was controlled to be 0.01, 0.02, and
0.03% to tune the relative intensity of the blue emission and
orange emission. The polymers are recorded as S-WP-xTPB3,
where x denotes the percentage content of TPB3.
RESULTS AND DISCUSSION
Design and Synthesis
Scheme 1 illustrates the chemical structure of orange dye and
polymers. We designed a (DA)3D’ type orange dye because it
is beneficial for reducing their concentration quenching effect
in solid state mainly due to the intermolecular dipole–dipole
interactions. First, compared to corresponding D-A type mole-
cules, D-A-D’ type molecules possess a pair of antiparallel
dipoles, thus the dipole–dipole interaction of them is
weaker.22,28 Second, star-shaped molecular architecture could
further suppress the intermolecular interaction of the dop-
ants.24 We selected 2,1,3-benzothiadiazole unit as the electron
acceptor, as its derivatives are of high Uf.28 (4-Methyl-phenyl)-
diphenyl-amine and 1,3,5-triphenylbenzene (TPB) were chosen
to be the peripheral electron-donating group and central unit,
respectively. As a result, we obtained a star-shaped orange
model compounds (MCs) with high Uf, named as TPB3. PF
was used as the branching arms because of its blue emission,
high Uf in solid state, and good charge-carrier transport prop-
erties.29 By incorporating three PF blue arms into the star-
shaped orange core, we obtained a star-like single-polymer
system. More branched arms are also favorable for weakening
the concentration quenching effect of the dopant.
Properties of Orange Dopant
The photophysical, electrochemical, and thermal properties
of TPB3 were studied, their corresponding values are listed
in Table 1. The absorption and photoluminescent (PL) spec-
tra in dilute toluene solution of TPB3 are shown in Figure 1.
The absorption (kabs) and emission (kem) peaks of TPB3 are
at 452 and 572 nm, respectively. The absorption spectrum of
the linear orange dopant we previously used, 4,7-bis(4-(N-
phenyl-N-(4-methylphenyl)amino)phenyl)-2,1-3-benzothiadia-
zole11 (MTPABT, kabs ¼ 465 nm) was also supplied. We can
see that the absorption spectrum of TPB3 overlaps with the
PL spectrum of PF better than that of MTPABT, favoring
€
more efficient Forster energy transfer from PF host to TPB3
in PL and EL process.
Figure 2 shows the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO)
energy levels of TPB3 and PF. Both of the HOMO/LUMO
energy levels of TPB3 are located between the HOMO/LUMO
energy levels of PF, indicating effective charge trapping in
the EL process. TPB3 also exists good thermal properties
with a high glass transition temperature (Tg) of 162 ꢁC and
thermal decomposition temperature (Td) of 605 ꢁC, which
further guarantee their thermal stability in EL devices.
The synthetic routes of the orange MC (TPB3), monomer
and polymers are also shown in Scheme 1. Bromination of
TPB3 with n-(C4H9)4NBr3, we afforded its A3 type monomer,
TABLE 1 Photophysical, Electrochemical, and Thermal Properties of TPB3
k
abs/ nm
kemi / nm
Uf
Eonset oxd/ V
E
HOMO/ eV
Eonset red/ V
ꢀ1.22
E
LUMO/ eV
Eg/ eV
Tg/ꢁC
Td/ꢁC
TPB3
452
572
0.79
0.99
ꢀ5.33
ꢀ3.12
2.21
162
605
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FIGURE 1 Absorption spectra of TPB3, MTPABT, and PL spec-
FIGURE 3 Absorption and PL spectra of the polymers in film.
trum of TPB3 in dilute toluene solution.
EL Performances of the Polymers
To investigate the EL properties of the polymers, single-layer
devices [ITO/PEDOT:PSS (40 nm)/polymer (100 nm)/Ca (10
nm)/Al (100 nm)] were fabricated. Here, ITO, PEDOT and PSS
respectively stands for indium tin oxide, poly(3,4-ethylenediox-
ythiophene) and poly(styrenesulfonate). The EL spectra of the
devices of these polymers at 7.0 V are shown in Figure 4. The
corresponding emission maxima and CIE coordinates are out-
lined in Table 2. It can be seen that the EL spectra of S-WP-
xTPB3 also show two simultaneous blue and orange emission
bands with peaks at 424/440 nm and 560 nm, respectively.
However, comparing PL (Fig. 3) and EL (Fig. 4) spectra, we
note that the relative intensity of orange peaks in the EL spec-
tra is significantly higher than that of the PL spectra of the
same polymer composition. It seems that, in EL process, direct
charge trapping might also play an important role besides the
Photophysical Properties of the Polymers
Figure 3 shows the absorption spectra of S-WP-003TPB3
and PL spectra of all the polymers in solid films. Actually, all
these polymers exhibit similar absorption spectra to PF with
an absorption peak around at 393 nm, ascribed to the p–p*
transition of the PF backbones. The absorption of orange
dopant at about 452 nm could not be observed because of
its extremely low content. The PL spectra of all these poly-
mers in solid films with an excitation wavelength of 380 nm
show two emission peaks at around 436 and 560 nm.
According to our previous studies,9–13 the two distinguish-
able PL peaks of the star-like polymers can be attributed to
the individual emissions originating from the PF backbone
and the orange dopant units. The orange emission band
€
comes from the Forster energy transfer from PF host to
€
Forster energy transfer process from PF host to orange dop-
TPB3 dopants due to the spectra overlap between the emis-
sion spectrum of PF and absorption spectrum of TPB3 (Fig.
1). The relative intensity of the orange emission increases
successively as the amount of orange dopant units in the
polymer increases because of the higher extent energy
transfer.
ants, resulting in enhanced orange emission.
The EL performances of the devices of S-WP-xTPB3 are also
demonstrated in Table 2. The LE–current density curves of
FIGURE 2 HOMO/LUMO energy levels of TPB3 and PF.
FIGURE 4 EL spectra of the pristine polymers at 7.0 V.
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TABLE 2 EL Performance of the Pristine Device of S-WP-xTPB3
Von (V)
LE (cd Aꢀ1
)
PE (lm Wꢀ1
)
Brightness (cd mꢀ2
)
EQE (%)
kmax (nm)
CIE/ (x, y)
S-WP-001TPB3
S-WP-002TPB3
S-WP-003TPB3
4.5
5.0
5.0
9.61
12.96
15.27
6.04
6.51
9.48
9139
12270
15470
3.21
4.29
4.93
424/448/560
424/448/560
424/448/560
(0.41, 0.48)
(0.42, 0.49)
(0.45, 0.51)
the PLEDs are illustrated in Figure 5. In contrast to our pre-
vious reports based on linear dopants,11,23b the EL efficiency
based on the TPB3-containing polymers increase succes-
sively with doping concentration. The EL performance is
improved from a LE of 9.61 cd Aꢀ1, a maximum brightness
of 9139 cd mꢀ2 and a maximum EQE of 3.21% for S-WP-
001TPB3 to a LE of 15.27 cd Aꢀ1, a maximum brightness of
15,470 cd mꢀ2 and a maximum EQE of 4.93% for S-WP-
003TPB3. As a result, we realized much more efficient elec-
troluminescence from star-like single-polymer systems com-
pared with linear orange molecule doped polymers. Three
considerable factors might contribute to it. First, star-shaped
dopant units molecular design strategy is helpful for reduc-
ing the dipole–dipole molecular interaction of the dopant
units. Second, more PF arms might provide better shield to
the dopant cores. Thus, the concentration quenching effect of
the dopant units could be further suppressed. Third, com-
pared to MTPABT units, more effective energy transfer from
PF host to TPB3 leads to higher EL efficiency of PLEDs (Fig.
1). We found that when the content of TPB3 changed from
0.01 to 0.03 mol %, the turn-on voltage of the device
increased a little, probably due to the charge-trapping effect
of TPB3 unit.30
the annealed devices are shown in Figure 6. We observed
that the relative intensity of blue emission of the annealed
device is reinforced significantly with red-shifted peak emis-
sion at 436/460 nm. As a result, white electroluminescence
has been realized for the annealed device of S-WP-002TPB3,
with CIE coordinates of (0.33, 0.36).
The absorption spectra of annealed films of these polymer
and pristine film of S-WP-003TPB3 are shown in Figure 7a.
A new absorption band around 422 nm can be observed due
to the formation of a-phase PF.14b Based on the relative in-
tensity of this new band, more than 20% a-phase PF was
formed.14b The a-phase PF induced the enhanced and red-
shifted blue emission band in PL spectrum of S-WP-003TPB3
(Fig. 7b), as well as the similar aforementioned phenomena
in EL spectra.
The EL performances of the annealed devices of the poly-
mers are listed in Table 3. The EL efficiencies of all the
annealed PLEDs are higher than corresponding pristine ones,
especially for the EQE values, which improved around 30%.
The device of S-WP-002TPB3 realized a comparably high LE
of 16.62 cd Aꢀ1 (Fig. 8) at a current density of 22.56 mA
cmꢀ2 with a brightness of 3717 cd mꢀ2, which is almost
twice that of MTPABT-doped linear polymers. The optimizing
mechanism is mainly due to the following four factors. First,
the formation of crystalline a-phase PF favors balanced hole
and electron transport to improve the light-emitting efficien-
cy.14a Second, the generated a-phase PF acts as an electron
trap site and self-dopant.31 It allows efficient charge trapping
and energy transfer to occur. Thus, the charge recombination
Although the LE efficiencies of the pristine devices are rela-
tively high, they do not realize white electroluminescence
due to the excessive energy transfer from PF host to TPB3
dopant. To obtain white electroluminescence, the devices
were thermal annealed at 120 ꢁC for 0.5 h in vacuum box
before the Ca/Al cathode was evaporated. The EL spectra of
FIGURE 5 Luminous efficiency–current density curves of the
pristine devices.
FIGURE 6 EL spectra of the annealed polymers at 7.0 V.
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FIGURE 7 Absorption spectra (a) and PL spectra of annealed polymers in film state.
efficiency and the radiative decay efficiency of polymer sys-
tems are improved. Third, red-shifted blue emission with
stronger human eye response benefits for higher LE and PE.
Fourth, the content of a-phase PF in polymer is so high
(around 20%) that it actually serves as an assistant-dopant32
for orange dopant as well, which leads to the cascade energy
transfer33 from amorphous PF through a-phase PF to orange
dopants. As a result, more efficient energy transfer from
blue host to orange dopant and superior device efficiency
were achieved. The turn-on voltage of the annealed devices
increases about 1.0 V compared to the pristine ones because
of the charge-trapping effect of a-phase PF.
with silica column chromatography (eluent: petroleum ether/
CH2Cl2 2:1) and recrystallized in petroleum ether/CH2Cl2
mixed solvents to afford a yellow solid. Yield: 1.55 g (68.3%).
1H NMR (CDCl3, 300 MHz) d (ppm): 7.92 (d, 1H), 7.81 (d, 2H),
7.56 (d, 1H), 7.30 (t, 2H), 7.20–7.09 (m, 8H), 7.07 (t, 3H), 2.37
(s, 3H). MALDI-TOF (Matrix-assisted laser desorption/ ioniza-
tion-time of flight mass spectrometry) (m/z): 471.0 [Mþ].
1,3,5-Tris(4-(7-(4-(N-phenyl-N-(4-methylphenyl)amino)-
phenyl)-2,1,3-benzothiadiazole-4-)phenyl)benzene (TPB3)
A mixture of 1,3,5-tris(4-(boronic acid)phenyl)benzene (3)
(0.18 g, 0.40 mmol), 4-(4-(N-phenyl-N-(4-methylphenyl)ami-
no)phenyl)-7-bromo-2,1,3-benzothiadiazole (5) (0.62 g, 1.32
mmol), Pd(PPh3)4 (0.02 g, 0.01 mmol), 2 M aqueous K2CO3
(10 mL), Aliquat 336 (0.10 g), and toluene (30 mL) was
EXPERIMENTAL
Materials
ꢁ
4,7-Dibromo-2,1,3-benzothiadiazole34 (1), tributyl(4-(N-phe-
nyl-N-(4-methylphenyl)amino)phenyl)stannane11 (2), 1,3,5-
tris(4-(boronic acid)phenyl)benzene27 (3), 2-bromo-7-(trime-
thyleneborate)-9,9-dioctylfluorene23b (4) were synthesized as
literatures.
heated to 100 C and stirred in dark for 12 h. After workup,
the mixture was extracted with CHCl3, washed with water
and dried with anhydrous Na2SO4. The residue was purified
by column chromatography (eluent: petroleum ether/CHCl3
1:1) and recrystallized twice in petroleum ether/CHCl3
mixed solvents to obtain a yellow solid. Yield: 0.36 g
(60.1%). 1H NMR (CDCl3, 300 MHz) d (ppm): 8.14 (d, 6H),
8.01 (s, 3H), 7.93 (d, 6H), 7.91–7.85 m, 9H), 7.79 (d, 3H),
7.29 (t, 6H), 7.23–7.16 (m, 12H), 7.15–7.09 (m, 12H), 7.05
(t, 3H), 2.35 (s, 9H). 13C NMR (CDCl3, 75 MHz) d (ppm):
154.50, 154.36, 148.05, 147.04, 144.65, 142.31, 141.20,
137.10, 134.21, 133.18, 132.49, 132.45, 131.32, 130.50,
130.31, 130.02, 128.42, 127.85, 127.61, 125.92, 125.70,
125.57, 122.99, 115.36, 21.16. Anal. Calcd. For C99H69N9S3:
C, 80.30; H, 4.70; N, 8.51; Found: C, 79.92; H, 4.25; N, 8.78.
MALDI-TOF (m/z): 1479.5 [Mþ].
4-(4-(N-Phenyl-N-(4-methylphenyl)amino)phenyl)-7-
bromo-2,1,3-benzothiadiazole (5)
A mixture of 4,7-dibromo-2,1,3-benzothiadiazole (1) (2.85 g,
9.71 mmol), tributyl(4-(N-phenyl-N-(4-methylphenyl)amino)-
phenyl)stannane (2) (2.67 g, 4.86 mmol), Pd(PPh3)4 (0.04 mg,
0.04 mmol), and toluene (40 mL) was kept stirred under ar-
ꢁ
gon in the dark at 100 C for 24 h. After cooling, the solution
was stirred in saturated aqueous potassium fluoride for 3 h,
then extracted with dichloromethane, washed with water and
dried with anhydrous Na2SO4. The crude product was purified
TABLE 3 EL performance of the annealed device of S-WP-xTPB3
Von (V)
LE (cd Aꢀ1
)
PE (lm Wꢀ1
)
Brightness (cd mꢀ2
)
EQE (%)
kmax (nm)
CIE (x, y)
S-WP-001TPB3
S-WP-002TPB3
S-WP-003TPB3
5.5
5.8
6.0
11.05
16.62
17.86
6.03
7.73
7.55
9624
15100
16890
4.52
6.28
6.49
436/460/560
436/460/560
436/460/560
(0.29, 0.30)
(0.33, 0.36)
(0.39, 0.45)
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acetone for 24 h. The reprecipitation procedure in dichloro-
methane-methanol was then repeated several times. The
final product was obtained after drying in vacuum with a
yield of 60–70%.
PF: light yellow fiber. Mn ¼ 7.43 ꢂ 104, PDI ¼ 2.25
S-WP-001TPB3: orange fiber. Mn ¼ 8.21 ꢂ 104, PDI ¼ 2.28
S-WP-002TPB3: orange fiber. Mn ¼ 7.95 ꢂ 104, PDI ¼ 2.32
S-WP-003TPB3: orange fiber. Mn ¼ 7.28 ꢂ 104, PDI ¼ 2.34
As previous single-polymers reported in our group, all these
polymers exhibits similar 1H NMR and elemental analysis
results as those of PF due to the low content of orange dop-
ant units: 1H NMR (CDCl3, 300 MHz) d (ppm): 7.87 (d, 2H),
7.72 (br, 4H), 2.10 (br, 4H), 1.14 (br, 24H), 0.81 (t, 6H). Anal.
Calcd: C, 89.69; H, 10.31. Found: C, 89.08; H, 10.02.
FIGURE 8 Luminous efficiency–current density curves of the
annealed devices of the polymers.
Measurement and Characterization
1H NMR spectra were recorded with a Bruker AV300 NMR
spectrometer. The elemental analyses were performed using
a Bio-Rad elemental analysis system. MALDI-TOF was meas-
ured by a Bruker Daltonics Flexanalysis system. The number-
and weight-average molecular weights of the polymers were
determined by gel permeation chromatography (GPC) using
a Waters 410 instrument with polystyrene as standard and
THF as eluent. UV-Vis absorption spectra were measured by
a Perkin–Elmer Lambda 35 UV/Vis spectrometer. PL spectra
were recorded by a Perkin–Elmer LS50B spectrofluorometer.
The relative fluorescent quantum yields of TPB3 in toluene
solution were measured as literature35 using Nile Red in 1,4-
dioxane (0.68) as the standard compound. Cyclic voltammo-
grams of polymer films on glassy carbon electrodes were
recorded on an EG&G 283 (Princeton Applied Research)
potentiostat/galvanostat system at room temperature in a
solution of n-Bu4NClO4 (0.10 M) in fresh CH2Cl2 at a scan
rate of 100 mV sꢀ1. A Pt wire and an Ag/AgCl electrode
were used as the counter electrode and the reference elec-
trode, respectively. Thermal properties were measured using
a Perkin–Elmer TGA-7 thermogravimetric analyzer and a
Perkin–Elmer DSC-7 differential scanning calorimeter under
1,3,5-Tris(4-(7-(4-(N-(4-bromophenyl)-N-
(4-methylphenyl)amino)phenyl)-2,1,3-
Benzothiadiazole-4-)phenyl)benzene (Mon-TPB3)
To a solution of TPB3 (0.15 g, 0.10 mmol) in fresh chloro-
form, tetrabutylammonium tribromide (0.15 g, 0.31 mmol)
in chloroform solvent (10 mL) was dropwise added. The
mixture was stirred at room temperature for 6 h. Then, it
was washed with aqueous sodium hydrogen sulfate and
brine followed by drying over anhydrous Na2SO4. After filtra-
tion and removal of the solvent, the residue was purified by
flash silica column chromatography (eluent: CHCl3) and
recrystallized twice in petroleum ether/CHCl3 mixed solvents
1
to give a yellow solid. Yield: 0.15 g (87.2%). H NMR (CDCl3,
300 MHz) d (ppm): 8.14 (d, 6H), 8.01 (s, 3H), 7.90 (d, 12H),
7.83 (d, 6H), 7.40 (d, 12H), 7.21 (d, 6H), 7.05 (d, 12H), 2.35
(s, 9H). 13C NMR (CDCl3, 75 MHz) d (ppm): 154.47, 154.34,
148.02, 147.03, 144.64, 142.25, 141.12, 137.05, 134.21,
133.12, 132.50, 132.38, 131.29, 130.52, 130.31, 130.00,
128.40, 127.81, 127.59, 125.93, 125.70, 125.52, 122.96,
115.37, 21.18. Anal. Calcd. For C99H69Br3N9S3: C, 69.23; H,
3.87; N, 7.34; Found: C, 68.94; H, 4.24; N, 7.69. MALDI-TOF
(m/z): 1713.2 [Mþ]
nitrogen with a heating rate of 10 C minꢀ1. The EL spectra
ꢁ
and current–voltage and brightness-voltage characteristics of
the devices were measured in air with a Spectrascan PR650
spectrophotometer in the forward direction and a computer-
controlled Keithley 2400 instrument. The EQE values were
calculated from the brightness, current density, and EL spec-
trum, assuming a Lambertian distribution.
General Procedure of the Polymers
A
mixture of 2-bromo-7-(trimethyleneborate)-9,9-dioctyl-
fluorene (4), Aliquat 336 (0.10 g, 0.25 mmol), Pd(PPh3)4
(0.01 g, 0.01 mmol) under argon was added degassed tolu-
ene solution of orange dopant Mon-TPB3 with corresponding
feed ratios, 2 M aqueous K2CO3 (2 mL) and degassed toluene
(6 mL in total). The resulting mixture was stirred in the Device Fabrication
dark at 90 ꢁC for 48 h, and then sequentially end-capped ITO glass plates were degreased in an ultrasonic acetone sol-
with 0.1 M phenylboronic acid (2 mL) and bromobenzene (1 vent bath and then dried in a heating chamber at 120 ꢁC.
mL), stirring for 12 h for each addition. After cooling, the The PEDOT:PSS was spin-coated on the cleanedꢁITO at 3000
reaction mixture was poured into methanol and filtered. The rpm for 60 s and then baked for 20 min at 120 C to give an
precipitate was collected and dissolved in dichloromethane, approximate thickness of 40 nm. The polymer layer (ꢃ 100
washed with water and dried with anhydrous Na2SO4. After nm) was then spin-coated on to the PEDOT/ITO coated glass
evaporating most of the solvent, the residue was precipitated substrate in fresh toluene solution (around 15 mg mLꢀ1
)
in stirred methanol solvent to give a fiber-like solid. The under ambient atmosphere. Finally, a thin layer of calcium
polymer was further purified by Soxhlet extraction with (10 nm) followed by a layer of aluminum (100 nm) was
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deposited in a vacuum thermal evaporate or through a
shadow mask at a pressure of 3 ꢂ 10ꢀ3–5 ꢂ 10ꢀ3 Pa. The
active area of the diodes was about 12 mm2. For the
annealed device, after emitting polymer layer was spin
coated on the PEDOT:PSS layer, the devices were annealed at
120 ꢁC for 30 min and then cooled for 30 min in vacuum
box before evaporating the Ca/Al cathode.
CONCLUSIONS
4 (a) Tsai, M. L.; Liu, C. Y.; Hsu, M. A.; Chow, T. J. Appl. Phys.
Lett. 2003, 82, 550–552; (b) Lee, Y. Z.; Chen, X. W.; Chen, M. C.;
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In summary, by introduction of star-shaped orange dopant to
building star-like polymers with three PF arms, the problem
of concentration quenching effect in PLEDs based on single-
polymer systems was avoided, and highly efficient PLEDs
have been realized. Furthermore, through optimizing the
devices with thermal annealing treatment to generate self-
doping a-phase PF, we achieved white electroluminescence
with reinforced and red-shifted blue emission, balanced
charge transport, and superior energy transfer from blue
host to orange dopant. As a result, WPLEDs has been
achieved with a high LE of 16.62 cd Aꢀ1, an EQE of 6.28%,
and CIE coordinates of (0.33, 0.36). As we know, it is among
one of the best results for WPLEDs based on fluorescent
polymers.
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ACKNOWLEDGEMENTS
The authors are grateful to the 973 Project (2009CB623601
and 2009CB930603), the Science Fund for Creative Research
Groups (No.20921061), and the National Natural Science Foun-
dation of China (Nos. 51173179, 20904055, and 21074130).
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