White Electroluminescence with Simultaneous Three-Color Emission from a Four-Armed Star-Shaped Single-Polymer System

Article abstract of DOI:10.1002/cjoc.201500308

A four-armed star-shaped single-polymer system with 4,7-bis(5-(4-(9H-carbazol-9-yl)phenyl)-4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (FTBT) as a red emissive core, polyfluorene (PF) as blue emissive arms and 1,3-benzo thiadiazole (BT) as green emissive dopants was designed and synthesized, in which red, green, and blue (RGB) emission balance can be achieved by adjusting the doping concentration of FTBT and BT discreetly. A typical single-emissive-layer device (ITO/PEDOT:PSS/polymer/TPBI/LiF/Al) was studied and discussed, realizing a pure and stable white emission with a luminous efficiency (LE) of 1.59 cd·A^-1 and CIE coordinates of (0.31, 0.34). The high-color-quality white electroluminescence of the devices could be mainly attributed to the suppressed intermolecular interactions, and partial energy transfer from the blue PF arms to the red and green dopants. A four-armed star-shaped single-polymer system with simultaneous red, green, and blue (RGB) emission was designed and synthesized, which realized pure and stable white emission with a luminous efficiency (LE) of 1.59 cd·A^-1 and CIE coordinates of (0.31, 0.34) in a typical single-emissive-layer device.

Full text of DOI:10.1002/cjoc.201500308

FULL PAPER
DOI: 10.1002/cjoc.201500308
White Electroluminescence with Simultaneous Three-Color
Emission from a Four-Armed Star-Shaped
Single-Polymer System
a
a
a
,a,b
a
Yuanda Jiu, Jianyun Wang, Chengfang Liu, Wenyong Lai,* Lingling Zhao,
a
a
a
,a
a,b
Xiangchun Li, Yi Jiang, Weidong Xu, Xinwen Zhang,* and Wei Huang
a
Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials,
Jiangsu National Synergetic Innovation Center for Advanced Materials,
Nanjing University of Posts & Telecommunications, Nanjing, Jiangsu 210023, China
Key Laboratory of Flexible Electronics & Institute of Advanced Materials, Jiangsu National Synergetic Innovation
Center for Advanced Materials, Nanjing Tech University, Nanjing, Jiangsu 211816, China
b
A four-armed star-shaped single-polymer system with 4,7-bis(5-(4-(9H-carbazol-9-yl)phenyl)-4-hexylthio-
phen-2-yl)benzo[c][1,2,5]thiadiazole (FTBT) as a red emissive core, polyfluorene (PF) as blue emissive arms and
,3-benzo thiadiazole (BT) as green emissive dopants was designed and synthesized, in which red, green, and blue
RGB) emission balance can be achieved by adjusting the doping concentration of FTBT and BT discreetly. A typ-
ical single-emissive-layer device (ITO/PEDOT:PSS/polymer/TPBI/LiF/Al) was studied and discussed, realizing a
1
(
−
1
pure and stable white emission with a luminous efficiency (LE) of 1.59 cd•A and CIE coordinates of (0.31, 0.34).
The high-color-quality white electroluminescence of the devices could be mainly attributed to the suppressed in-
termolecular interactions, and partial energy transfer from the blue PF arms to the red and green dopants.
Keywords white electroluminescence, hyperbranched polymers, multi-armed structures, Förster resonance energy
transfer, organic light-emitting diodes
Introduction
and orange emission from the aggregation/excimer/ex-
ciplex, manifesting low efficiency. Later on, a strategy
of covalently attaching various emissive components to
the polymer host has been proposed to improve the effi-
ciency. The intrinsic phase separation could thus be
suppressed because of the single-polymer chemical ar-
chitectures, leading to better operational stability.
White polymer light-emitting diodes (WPLEDs)
have received particular attention due to their potential
applications in low-cost, large-area and flexible devices
[
1-4]
via solution-processed fabrication,
which are ex-
pected to become a complementary or competitive can-
didate to compete with other light sources such as fluo-
rescent tubes and inorganic light-emitting diodes in the
lighting markets in the near future. A conventional ap-
proach to obtain white emission is to use physical
blends in a combination with red, green and blue emis-
Recently, intense efforts have been focused on ex-
ploring this methodology to realize white emission. For
[
20]
example, Woo and co-workers
synthesized a linear
polymer with red chromophore 4,7-bis(2-thienyl)-2,1,3-
benzothiadiazole (DBT), a green dye 2,1,3-benzothia-
diazole (BT) and a blue backbone of tetraoctylin-
denofluorene (IF), emitting white light with maximum
[
5-7]
sive species,
such as polymer-polymer blend sys-
[
8,9]
tems,
polymer-organometallic-complex blend sys-
and polymer-small molecule blend sys-
Despite promising efficiencies, this strategy
[
10,11]
−1
tems,
current efficiency of 0.36 cd•A and CIE coordinates of
[
12,13]
[21]
tems.
(0.34, 0.32). Wang et al.
reported a PF doped with
may suffer from the challenge of phase separation and
bias-dependent electroluminescent (EL) spectra.
quinacridone, showing white emission with CIE coor-
dinates of (0.27, 0.35). Despite significant progress, it is
still a great challenge to achieve highly efficient pure
and stable white EL. The unsaturated white light emis-
sion from the linear polymer systems may mainly orig-
inate from the strong intermolecular interactions in the
solid state due to the π-π* stacking of the linear rigid
[14,15]
To
resolve these issues, single-polymer systems have been
investigated as a prospective strategy to eliminate the
phase separation and obtain bias-independent EL spec-
[16-19]
tra.
At the beginning, this kind of white emission
was achieved by blue emission from the blue dopant
*
E-mail: iamwylai@njupt.edu.cn, iamxwzhang@njupt.edu.cn
Received April 15, 2015; accepted May 24, 2015; published online June 29, 2015.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201500308 or from the author.
Chin. J. Chem. 2015, 33, 873—880
© 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
873

Jiu et al.
FULL PAPER
conjugated polymer backbones, leading to low EL qual-
ity. In this context, star-shaped single-polymer systems
have attracted particular interests because of their un-
filtration and evaporation of the solvent, the residue was
purified by silica column chromatography to give the
title compound as a red solid. Yield: 0.51 g (80%). H
1
[22-25]
usual structural and optoelectronic properties.
Hy-
3
NMR (CDCl , 400 MHz) δ: 8.19－8.27 (m, 4H), 7.19－
perbranched and globular features are expected to elim-
inate intermolecular interactions in order to achieve
high-color-quality and stable white EL. For instance,
8.13 (m, 4H), 7.76－7.80 (dd, J＝5.9, 9.9 Hz, 4H),
7.59－7.66 (m, 4H), 7.51－7.56 (t, J＝9.0 Hz, 4H),
7.34－7.39 (dt, J＝4.9, 8.3 Hz, 4H), 2.85－2.88 (m, 4H),
1.72－1.82 (m, 4H), 1.34－1.44 (m, 12H), 0.89 (s, 6H);
[26]
Wang et al.
have reported star-like single-polymers
1
3
through incorporating six PF arms onto a star-shaped
orange core (1,3,5-tris(4-(7-(4-(N,N-diphenylamino)phe-
nyl)-2,1,3-benzothiadiazole-4-)phenyl)benzene, TPB6),
demonstrating an external quantum efficiency (EQE) of
3
C NMR (CDCl , 100 MHz) δ: 153.96, 152.29, 141.04,
140.54, 139.76, 139.69, 139.37, 134.30, 133.57, 132.31,
130.88, 130.79, 130.69, 129.53, 129.50, 128.73, 127.08,
127.03, 126.97, 124.15, 124.11, 123.34, 123.32, 120.62,
120.59, 120.57, 120.55, 113.38, 113.37, 113.29, 112.98,
111.58, 111.53, 31.94, 31.69, 31.42, 29.71, 29.67, 29.37,
6
.36%. However, the difficulty lies in the relatively poor
solubility of the TPB6 unit.
In our previous contributions, a novel series of
monodisperse conjugated starburst macromolecules has
4 4 3
22.70, 22.64, 14.13. Anal. calcd for C62H50Br N S : C
58.78, H 3.98, N 4.42, S 7.59; found C 58.34, H 3.84, N
[27-35]
been developed.
These monodisperse multibranched
4.49, S 7.19. MALDI-TOF (m/z): Anal. calcd for
＋
molecules combine significant advantages of well-de-
fined chemical structures, good solution processibility,
high purity, suppressed intermolecular interactions and
excellent optoelectronic properties. Highly efficient blue
and red emission have been demonstrated. Thus, there
are some possibilities that we could build up a unique
platform to achieve white EL by means of adjusting the
central core and conjugated arms to construct multi-
armed structures.
62 4 4 3
C H50Br N S : 1262.0; found: 1264.3 [M ].
Synthesis of 2,2'-(9,9-dihexyl-9H-fluorene-2,7-diyl)-
bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (2)
A solution of 2,7-dibromo-9,9-dioctylfluorene (1)
(
15.54 g, 28.3 mmol) in dry THF (250 mL) at −78 ℃
was added to n-BuLi (17.7 mL). After stirring for 1 h,
trimethlyborate (4.2 mL, 37.1 mmol) was added and the
mixture was kept stirred for another 24 h, followed by
－
1
In this contribution, we reported a set of four-armed
star-like single-polymer system by introducing 4,7-bis-
the addition of 2 mol•L hydrochloric acid (100 mL).
The mixture was extracted with ether and the combined
extracts were evaporated to give a white solid, 7-bromo-
9,9-dioctylfluoren-2-yl boric acid. The boric acid was
mixed with 1,3-propanediol (2.5 mL, 34.5 mmol) and
toluene (150 mL), and the resulting mixture was re-
fluxed overnight. Evaporation of the solvent under re-
duced pressure gave the crude product, which was re-
crystallized in ethanol twice to afford the title com-
(
5-(4-(9H-carbazol-9-yl)phenyl)-4-hexylthiophen-2-yl)-
benzo[c][1,2,5]thiadiazole (FTBT) as a red emissive
core, polyfluorene (PF) as blue emissive arms and
1
,3-benzo thiadiazole (BT) as green emissive dopants
into a star-shaped architectures (Scheme 1). By select-
[36,37]
ing the proper concentration of the dopants,
partial
Förster resonance energy transfer (FRET) and charge
trapping from the blue arms to green and red dopants
are accomplished to ensure white EL, exhibiting simul-
taneous blue emission (λmax＝430/450 nm), green emis-
sion (λmax＝519 nm), and red emission (λmax＝571 nm).
A single-emissive-layer device based on this four-armed
single-polymer achieved pure and stable white light
with Commission Internationale d’Eclairage (CIE) co-
ordinates of (0.31, 0.34) and a luminous efficiency (LE)
1
pound as a white solid. Yield: 5.81 g (37%). H NMR
(CDCl , 400 MHz) δ: 7.80－7.82 (d, J＝8.1 Hz, 2H),
3
7.71－7.75 (m, 4H), 1.98－2.02 (m, 4H), 1.39 (s, 24H),
0.99－1.10 (m, 12H), 0.73－0.76 (t, J＝7.1 Hz, 6H),
13
3
0.50－0.56 (dd, J＝7.3, 15.5 Hz, 4H); C NMR (CDCl ,
100 MHz) δ: 150.47, 143.93, 133.66, 128.92, 119.38,
83.72, 55.19, 40.10, 31.45, 29.63, 24.95, 23.57, 22.58,
14.02.
−1
of 1.59 cd•A at 5.8 V.
General procedures for synthesis of the polymers
To a mixture of 2,7-dibromo-9,9-dihexyl-9H-fluo-
rene (1) (quantities given below), 2,2'-(9,9-dihexyl-
Experimental
Synthesis of 4,7-bis(5-(4-(3,6-dibromo-9H-carbazol-
9
H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolane) (2), and Pd(PPh (11.5 mg, 0.01 mmol) un-
der N was added a drop of Aliquat 336, 2 mol•L
9
-yl)phenyl)-4-hexylthiophen-2-yl)benzo[c][1,2,5]-
3 4
)
thia-diazole (FTBT4Br)
－1
2
N-Bromosuccinimide (NBS) (0.39 g, 2.25 mmol)
was added to a solution of 4,7-bis(5-(4-(9H-carbazol-
aqueous potassium carbonate (2.5 mL), and degassed
toluene (5 mL). Solutions of 4,7-bis(5-(4-(3,6-dibromo-
9H-carbazol-9-yl)phenyl)-4-hexylthiophen-2-yl)benzo-
9-yl)phenyl)-4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadi-
azole (FTBT; 0.47 g, 0.5 mmol) in CH Cl (20 mL).
The resulting mixture was kept stirred at room tem-
perature for 6 h. After workup, the mixture was poured
2
2
[c][1,2,5]thiadiazole (FTBT4Br) and solutions of 4,7-di-
bromobenzo[c][1,2,5]thiadiazole (4) were also added.
The mixture was stirred at 90 ℃ for 48 h and then
poured into methanol. The precipitate was collected by
using filtration, dried, and then dissolved in dichloro-
into aqueous NaHSO
with water and then dried with anhydrous Na
3
. The organic layer was washed
SO . After
2
4
874
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Chin. J. Chem. 2015, 33, 873—880

White Electroluminescence with Simultaneous Three-Color Emission
methane. The solution was washed with water and dried
Device fabrication and characterization
2 4
over anhydrous Na SO . After most of the solvent had
OLEDs were fabricated on pre-patterned indium-tin
oxide (ITO) with the sheet resistance of 10－20 Ω/sq.
The substrate was ultrasonic cleaned with acetone, de-
tergent, deionized water, and 2-propanol. Oxygen plas-
ma treatment was made for 4 min as the final step just
before the film coating. Onto the ITO glass was
spin-coated a layer of polyethylenedioxythiophene-
polystyrene sulfonic acid (PEDOT:PSS) film with
thickness of 40 nm from its aqueous dispersion.
PEDOT:PSS film was dried at 80 ℃ for 3 h in the
vacuum oven. The solution of FTBT-RxGy polymers in
chloroform was prepared under nitrogen atmosphere
and spin-coated onto the PEDOT:PSS layer. The typical
thickness of the emitting layer was 80 nm. Then a thin
layer of TPBI as an electron injection cathode and the
subsequent 30 nm thick LiF/aluminum protection layers
were thermally deposited by vacuum evaporation
been removed, the residue was poured into stirred
methanol to give a fiber-like solid. The polymer was
further purified by extracting with acetone for 24 h. The
reprecipitation procedure in THF/methanol was then
repeated several times. The final product, a light orange-
colored fiber, was obtained after drying in a vacuum
with a yield of 80%－90%.
PF: 1 (0.25 g, 0.50 mmol) and 2 (0.30 g, 0.50 mmol)
were used in the polymerization. Gel-permeation chro-
4
matography (GPC): M
n
＝2.47×10 , polydispersity in-
dex (PDI)＝2.09 (polystyrene as standard).
PFB: 1 (0.25 g, 0.50 mmol) and 2 (0.30 g, 0.50
−
4
－1
mmol), BT2Br (0.40 mL 5×10 mol•L solution in
−4
toluene, 2×10 mmol) were used in the polymerization.
Gel-permeation chromatography (GPC): M ＝1.19×
0 , polydispersity index (PDI)＝1.96 (polystyrene as
standard).
FTBT-R5G8: 1 (0.25 g, 0.50 mmol), 2 (0.30 g, 0.50
n
4
1
−4
through a mask at a base pressure below 2×10 Pa.
The cathode area was defined as the active area of the
device. The typical active area of the devices in this
−
4
−1
mmol), BT2Br (0.80 mL 5×10 mol•L solution in
−
4
2
toluene, 4×10
5
mmol) and FTBT4Br (2.50 mL
×10 mol•L solution in toluene, 1.25×10 mmol)
study was 0.15 cm . The spin coating process of the EL
−4
−1
−3
layer and the device performance tests were carried out
within a glovebox under nitrogen atmosphere. The lu-
minance of the device was measured with a calibrated
photodiode. External quantum efficiency was verified
by the measurement of the integrating sphere, and lu-
minance was calibrated after the encapsulation of de-
vices with UV-curing epoxy and thin cover glasses. The
Commission Internationale d’Eclairage (CIE) coordi-
nates have been calculated using 1931 observer pa-
rameters.
4
n
were used in the polymerization. GPC: M ＝6.35×10 ,
PDI＝1.69.
FTBT-R5G12: 1 (0.25 g, 0.50 mmol), 2 (0.30 g, 0.50
−
4
−1
mmol), BT2Br (1.20 mL 5×10 mol•L solution in
−4
toluene, 6×10
5
mmol) and FTBT4Br (2.50 mL
×10 mol•L solution in toluene, 1.25×10 mmol)
were used in the polymerization. GPC: M
−4
−1
−3
4
n
＝3.34×10 ,
PDI＝2.00.
FTBT-R5G15: 1 (0.25 g, 0.50 mmol), 2 (0.30 g, 0.50
−
4
−1
mmol), BT2Br (1.50 mL 5×10 mol•L solution in
−4
toluene, 8×10
5
mmol) and FTBT4Br (2.50 mL
×10 mol•L solution in toluene, 1.25×10 mmol)
were used in the polymerization. GPC: M
PDI＝1.65.
Results and Discussion
−4
−1
−3
4
n
＝5.46×10 ,
The chemical structures of the polymers are depicted
in Scheme 1. FTBT was selected as the core structure
due to its efficient red emission and good solubility in
common solvents because of the alkyl chains on the
thiophene units that facilitate the subsequent synthesis
and purification. PF was utilized as the branching arms
because of its high energy blue emission, high fluores-
cence quantum yields in the solid state, and good charge
transport properties. BT units were employed as the
green dopants with the aim to modulate the balance of
RGB emission to achieve saturated white light emission.
The synthesis of FTBT is detailed in Supporting Infor-
mation. Bromination of FTBT with NBS afforded
FTBT-R2G4: 1 (0.25 g, 0.50 mmol), 2 (0.300 g, 0.50
−
4
−1
mmol), BT2Br (0.40 mL 5×10 mol•L solution in
−4
toluene, 1×10
5
mmol) and FTBT4Br (1.00 mL
×10 mol•L solution in toluene, 7.50×10 mmol)
were used in the polymerization. GPC: M
−4
－1
−4
4
n
＝3.77×10 ,
PDI＝1.71.
FTBT-R4G4: 1 (0.25 g, 0.50 mmol), 2 (0.30 g, 0.50
−
4
−1
mmol), BT2Br (0.40 mL 5×10 mol•L solution in
−4
toluene, 2×10
5
mmol) and FTBT4Br (2.00 mL
×10 mol•L solution in toluene, 7.50×10 mmol)
were used in the polymerization. GPC: M ＝4.54×10 ,
n
−4
−1
−4
4
PDI＝1.77.
4
A (arm-four)-type monomer FTBT4Br. Star-shaped
polymers were synthesized by Suzuki polycondensation
with 9,9-dioctylfluorene monomer, BT monomer and a
1
All these polymers showed similar H NMR spectra
and elemental analysis results. For example, PF: H
1
NMR (CDCl
7
1
3
, 400 MHz) δ: 7.85 (d, J＝7.8 Hz, 2H),
.72－7.65 (m, 4H), 7.53－7.29 (m, 1H), 2.13 (s, 4H),
.57 (s, 1H), 1.14 (s, 16H), 0.87－0.73 (m, 12H). Anal.
4
A (arm-four)-type red dopant monomer FTBT4Br. After
polymerization, these polymers were end-capped with
phenyl groups. Since the polymer was prepared by pol-
ycondensation, the length of the arms in the star-shaped
polymers was uniform. In order to obtain the blue, green
and red emission balance, the doping concentration of
FTBT and BT was adjusted to be (0.05%, 0.08%),
calcd: C 89.76, H 10.24; found C 89.41, H 9.87. The
contents of green- and red-light-emitting dopant units in
these polymers were too low to be detected using H
1
NMR and elemental analysis.
Chin. J. Chem. 2015, 33, 873—880
© 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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875

Jiu et al.
FULL PAPER
(
(
0.05%, 0.12%), (0.05%, 0.15%), (0.02%, 0.04%), and
0.04%, 0.04%) to modulate the corresponding intensity
Table 1 Thermal, electrochemical and photophysical properties
of FTBT
of blue, green and red emission. The polymers are de-
noted as FTBT-RxGy, where x identifies the contents of
FTBT as red fluorescents and y means the contents of
BT as green fluorescents. All the polymers are readily
soluble in common organic solvents, such as toluene,
chloroform, terahydrofuran (THF), etc.
Sample T /℃ T /℃ EHOMO/eV ELUMO/eV E /eV λ /nm λ /nm
g
d
g
abs
em
FTBT 134 410
−5.55
−2.81 2.74 342/503 632
gravimetric analysis (TGA) and differential scanning
calorimetry (DSC). FTBT is thermally stable with 5%
The thermal properties are investigated by thermo-
d
weight loss temperature (T ) at 410 ℃. Glass transition
Scheme 1 Schematic illustration, chemical structures and synthetic routes of the polymers
Br
Br
S
S
NBS
N
N
S
S
N
N
DCM
R
N
N
R
R
N
N
R
S
S
FTBT
x
Br
R
Br
FTBT4Br
S
R
R
N
N
R
S
N N
O
O
n-BuLi, THF
Br2
Br
Br
B
B
Br
Br
O
O
DMF
O
1
B
O
2
4
O
3
BT
y
Br
Br
Br
S
S
N
N
R
N
N
R
S
Br
FTBT4Br
n
Toluene 1, 2, 4
CO
Pd(PPh3)4
R
n
K
2
3
R
R
m
S
N
N
R
m
N
S
N
S
S
N
N
R
N
N
R
N
S
S
N
m
N
R
R
PFB
FTBT-RxGy
y = 0.0004
x = 0.0005 y = 0.0008
x=0.0005 y = 0.0012
N S
m
x = 0.0005 y = 0.0015
x = 0.0002 y = 0.0004
x = 0.0004 y = 0.0004
R
n
R
n
6 13
R = n-C H
S
R
R
N
N
R
R
m
n
m
PF
PFB
876
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Chin. J. Chem. 2015, 33, 873—880

White Electroluminescence with Simultaneous Three-Color Emission
temperature (T
g
) was observed at 134 ℃. The resulting
PF arms but also the red emission from the cores and
green emission from the dopants (Figure 1c). The green
emission can be ascribed to the partial FRET from the
PF arms to the green BT units as mentioned above.
Cyclic voltammetry (CV) is employed to estimate
the lowest unoccupied molecular orbital (LUMO) and
highest occupied molecular orbital (HOMO) energy
levels of the model compound. As shown in Figure S1c,
cyclic voltammogram of FTBT shows two oxidation
peaks with the onset potentials at 0.77 and 0.85 V (vs.
SCE), respectively. The first peak is attributed to the
9-phenyl-9H-carbazole unit and the second one is as-
cribed to the benzothiadiazole backbone. According to
the first onset oxidation potential and the empirical
formula, EHOMO＝−(Eox＋4.8) [eV], the HOMO energy
level of FTBT is estimated to be −5.55 eV. The LUMO
energy level is thus calculated to be −2.81 eV. The ab-
sorption spectra of FTBT and emission spectra of FTBT,
BT, PF and PFB in solid films, are shown in Figure S1d.
In terms of the obvious overlap of the emission spectra
of PF, BT and the absorption spectra of the core struc-
ture FTBT, FRET from the blue emissive fluorene seg-
ments to the red emissive FTBT core units and green
emissive BT units could be achieved.
excellent thermal stability is believed to be able to im-
prove the morphological stability and reduce the possi-
bility of phase separation upon heating.
The absorption spectra of the polymers in THF are
shown in Figure 1a. All the polymers exhibit a strong
absorption band at 390 nm, which is attributed to the
fluorene segments. The absorption of red dopants at
about 503/342 nm is negligible because of the low dop-
ing concentration. PL spectra of the polymers recorded
in dilute THF (Figure 1b) at room temperature are simi-
lar to that of PF, originating from the π-π* transition of
the PF arms. Meanwhile, PL spectra in films of the
polymers consist of not only the blue emission from the
To investigate the EL properties of the resulting
polymers, single-emissive-layer devices with configura-
tion of indium-tinoxide(ITO)/poly(3,4-ethylenedioxy-
thiophene):poly(styrenesulfonate) (PEDOT:PSS) (40
nm)/polymer (80 nm)/1,3,5-tris(1-phenyl-1H-benzimi-
dazol-2-yl)benzene (TPBI) (30 nm)/LiF (1 nm)/Al (80
nm) were fabricated. The devices were thermally an-
nealed at 120 ℃ for 10 min in a vacuum box before
the LiF/Al cathode was evaporated. The EL spectra are
shown in Figure 2a. The corresponding CIE coordinates
are summarized in Table 2. All the EL spectra show
simultaneous blue, green and red emission bands with
peaks at 425/446 nm, 517 nm, and 566 nm, respectively.
In order to achieve balance of blue, green and red emis-
sion, the doping concentration of FTBT and BT was
adjusted carefully. By increasing the content of BT do-
pant to 0.04 mol% and the content of FTBT core to 0.04
mol%, the EL spectrum of FTBT-R4G4 exhibits simul-
taneous blue, red and green emission with comparable
intensity. The CIE coordinates calculated from the EL
spectra of FTBT-R4G4 are (0.31, 0.34), which are very
close to the value of standard white emission (0.33,
0
.33). When the FTBT content further decreases to 0.02
mol%, the EL spectrum of FTBT-R2G4 exhibits negli-
gible red emission from FTBT unit, dominant green
emission from BT segments and dominant blue emis-
sion from fluorene segments. The CIE coordinates cal-
culated from the EL spectra of FTBT-R2G4 are (0.28,
0
0
.34). When the content of FTBT core unit is fixed as
.05 mol%, and the content of BT dye unit increases to
Figure 1 (a) Normalized absorption and (b) PL spectra of
FTBT-RxGy recorded in dilute THF at room temperature; (c)
normalized PL spectra of FTBT-RxGy in films. PL spectra were
obtained upon excitation at the absorption maxima.
0.08 mol%, 0.12 mol%, and 0.15 mol%, the green emis-
sion bands of FTBT-R5G8, FTBT-R5G12 and FTBT-
R5G15 increase much higher than those of FTBT-R
2G4 and FTBT-R4G4. Meanwhile, the EL spectra of
Chin. J. Chem. 2015, 33, 873—880
© 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
877

Jiu et al.
Table 2 EL performance of FTBT-RxGy WPLEDs
FULL PAPER
FTBT-R5G8, FTBT-R5G12 and FTBT-R5G15 exhibit
dominant green emission from the BT unit and rela-
tively low blue emission from fluorene segments. The
CIE coordinates calculated from the EL spectra of
FTBT-R5G8, FTBT-R5G12 and FTBT-R5G15 are (0.34,
−1
−2
Polymer Von/V CEmax/(cd•A ) Lmax/(cd•m ) EQE/% CIE (x, y)
R5G8
7.5
2.74
2.23
1.56
1.32
1.59
0.69
2630
3360
2700
1250
1675
1566
1.10 (0.34, 0.40)
0.84 (0.33, 0.41)
0.58 (0.30, 0.40)
0.57 (0.28, 0.34)
0.70 (0.31, 0.34)
0.29 (0.26, 0.25)
R5G12 7.0
R5G15 6.0
0
.40), (0.33, 0.41), and (0.30, 0.40), respectively. With
the increment of FTBT and BT contents, the intensity of
the red and green emission increases and the intensity of
the blue emission decreases in the EL spectra, suggest-
ing that a complete FRET from the fluorene segments to
the FTBT unit and the BT dye emerges in this process.
Significantly, its EL spectral shape of the device based
on FTBT-R4G4 keeps almost unchanged with increas-
ing the bias voltage from 7 to 10 V (Figure 2b).
R2G4
R4G4
5.0
5.0
blended 5.0
Figure 3 LUMO and HOMO energy levels of FTBT and PF.
The EL performances of the devices of FTBT-RxGy
are also demonstrated in Table 2. The brightness-voltage
characteristics, current efficiency-density characteristics
and EQE-voltage characteristics of the WPLEDs are
illustrated in Figure 4. The CIE coordinates calculated
from the EL spectra of FTBT-R4G4 are (0.31, 0.34),
which are very close to the values of standard white
emission (0.33, 0.33). When the BT content increases,
the blue emission band in the EL spectra becomes very
weak in the case of FTBT-R5G8. When it comes to
FTBT-R5G15, the blue emission becomes negligible
under the same circumstance. By increasing the FTBT
content and decreasing the BT content, the blue, green
and red emission bands in the EL spectra become bal-
anced in the case of FTBT-R4G4 and a red emission
band in the EL spectra becomes negligible in the case of
FTBT-R2G4. This phenomenon can be explained by the
fact that more complete FRET and charge trapping from
the blue emissive fluorene segments to the red emissive
FTBT unit and the green emissive BT dye have oc-
curred in the process.
Figure 2 (a) EL spectra of the devices based on FTBT-RxGy at
7
V. (b) EL spectra of the device based on FTBT-R4G4 under
various bias voltages.
On the basis of the results, the EL spectrum of
FTBT-R4G4 exhibits a balance of blue (425/446 nm
from the fluorene segments), green (517 nm from the
green BT segments) and red (566 nm from the red
FTBT core) emission. The red and green emission bands
of FTBT-R4G4 are much stronger in the EL spectra
compared to those in PL spectra, resulting from the
charge-trapping effect of the red FTBT unit in the EL
process. As shown in Figure 3, the LUMO and HOMO
energy levels of FTBT lie between those of PF. In addi-
tion to the FRET process from the PF host to red and
green dopants, direct charge trapping may also play an
important role in enhancing the red emission during the
EL process.
For comparison, we have physically blended 0.04
mol% FTBT into PFB and fabricated a device with the
blend as the emissive layer. The EL spectra exhibit bias
dependence, and the CIE coordinates of devices based
on the blended layer change from (0.26, 0.25) to (0.36,
0.41) with the driving voltage increasing from 5 to 12 V.
The corresponding device is realized with a luminous
−1
efficiency of 0.69 cd•A , a power efficiency of 0.31
−1
−2
lm•W , a maximum brightness of 1566 cd•m and a
maximum EQE of 0.29% (Figure S2, Table 2), which
are all inferior to those of a star-shaped single-polymer
system. These results imply that covalently attaching
red and green chromophores into a blue host has played
a key role in bias-independent EL spectra of WPLEDs
878
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© 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2015, 33, 873—880

White Electroluminescence with Simultaneous Three-Color Emission
−
1
based on star-shaped single-polymer systems.
sult, a LE of 1.59 cd•A and a maximum brightness of
1
−2
675 cd•m are achieved for FTBT-R4G4. In addition,
the CIE coordinates calculated from the EL spectra of
FTBT-R4G4 are (0.31, 0.34), which are very close to
the values of standard white emission (0.33, 0.33). Fur-
ther optimization regarding the materials and the device
structures is in progress.
Acknowledgement
The authors acknowledge financial support from the
National Key Basic Research Program of China (973
Program, 2014CB648300), the National Natural Science
Foundation of China (21422402, 20904024, 51173081,
6
1136003), the Natural Science Foundation of Jiangsu
Province (BK20140060, BK20130037, BM2012010),
Program for New Century Excellent Talents in Univer-
sity (NCET-13-0872), Specialized Research Fund for
the Doctoral Program of Higher Education
(
(
20133223110008), the Ministry of Education of China
IRT1148), the Synergetic Innovation Center for Or-
ganic Electronics and Information Displays, the Priority
Academic Program Development of Jiangsu Higher
Education Institutions (PAPD), the Six Talent Plan
(
2012XCL035) and Qing Lan Project of Jiangsu Prov-
ince.
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880
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© 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2015, 33, 873—880