Green light-emitting polyfluorenes with improved color purity incorporated with 4,7-diphenyl-2,1,3-benzothiadiazole moieties

Article abstract of DOI:10.1039/b700004a

By incorporating 4,7-diphenyl-2,1,3-benzothiadiazole instead of 2,1,3-benzothiadiazole into the backbone of polyfluorene, we developed a novel series of green light-emitting polymers with much improved color purity. Compared with the state-of-the-art gree

Full text of DOI:10.1039/b700004a

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www.rsc.org/materials | Journal of Materials Chemistry
Green light-emitting polyfluorenes with improved color purity incorporated
with 4,7-diphenyl-2,1,3-benzothiadiazole moieties
Jun Liu, Laju Bu, Jinpeng Dong, Quanguo Zhou, Yanhou Geng, Dongge Ma, Lixiang Wang,* Xiabin Jing
and Fosong Wang
Received 2nd January 2007, Accepted 18th April 2007
First published as an Advance Article on the web 30th April 2007
DOI: 10.1039/b700004a
By incorporating 4,7-diphenyl-2,1,3-benzothiadiazole instead of 2,1,3-benzothiadiazole into the
backbone of polyfluorene, we developed a novel series of green light-emitting polymers with much
improved color purity. Compared with the state-of-the-art green light-emitting polymer,
poly(fluorene-co-benzothiadiazole) (lmax = 537 nm), the resulting polymers (lmax = 521 nm)
showed 10–20 nm blueshifted electroluminescence (EL) spectra and greatly improved color purity
because the insertion of two phenylene units between the 2,1,3-benzothiadiazole unit and the
fluorene unit reduced the effective conjugation length in the vicinity of the 2,1,3-benzothiadiazole
unit. As a result, the resulting polymers emitted pure green light with CIE coordinates of (0.29,
0.63), which are very close to (0.26, 0.65) of standard green emission demanded by the National
Television System Committee (NTSC). Moreover, the insertion of the phenylene unit did not
affect the photoluminescence (PL) and EL efficiencies of the resulting polymers. PL quantum
efficiency in solid films up to 0.82 was demonstrated. Single-layer devices (ITO/PEDOT/polymer/
Ca/Al) of these polymers exhibited a turn-on voltage of 4.2 V, luminous efficiency of 5.96 cd A21
and power efficiency of 2.21 lm W21. High EL efficiencies and good color purities made these
polymers very promising for display applications.
green–yellow with CIE coordinates (0.39, 0.57). Hence, it is
Introduction
still a great challenge for the material chemist to improve
In the past decade, fluorene-based p-conjugated polymers have
the color purity of poly(fluorene-co-benzothiadiazole) while
emerged as the most promising candidates for use as emitting
maintaining its high PL and EL efficiencies. Covion Corp.
materials in polymeric light-emitting diodes (PLEDs) because
has developed a pure green light-emitting polymer with
CIE coordinates of (0.34, 0.59).14,15 Cambridge Display
of their high photoluminescence (PL) and electoluminescence
(EL) efficiencies, good thermal and spectral stability and color
Technology (CDT) has announced a pure green light-emitting
tunability in the full visible range.1–8 Normally, polyfluorene
polymer with CIE coordinates of (0.31, 0.58), however, their
emits blue light with a large bandgap. By incorporating a
narrow bandgap unit into the polyfluorene backbone, the
emission colors of polyfluorene derivatives can be tuned over
the entire visible range.9–32 The most commonly used narrow
bandgap unit is the electron-deficient 2,1,3-benzothiadiazole.
Indeed, poly(fluorene-co-benzothiadiazole) has become the
state-of-the-art green light-emitting polymer for PLEDs
because of its good thermal and electrical stability, high PL
and EL efficiency.9–18
detailed chemical structures and EL performance were
undisclosed.13 Shim and coworkers have reported a pure green
light-emitting polyfluorene derivative with CIE coordinates of
(0.29, 0.63), however, its EL performance was poor.26
In poly(fluorene-co-benzothiadiazole), light is emitted from
the vicinity of the benzothiadiazole unit (see Scheme 1) because
of energy transfer from the wide bandgap fluorene unit to the
narrow bandgap benzothiadiazole vicinity unit.16 Therefore,
its emission color can be fine tuned by modifying the chemical
structure of the benzothiadiazole vicinity unit. In this article,
we report a series of pure green light-emitting copolymers
based on fluorene and 4,7-diphenyl-2,1,3-benzothiadiazole,
aimed at improving the color purity of the state-of-the-art
green light-emitting poly(fluorene-co-benzothiadiazole). As
shown in Scheme 1, in the new polymers, the insertion of the
two phenylene moieties in the vicinity of the benzothiadiazole
unit makes the vicinity of the benzothiadiazole unit a little
more twisted than before. Therefore, the emission spectra in
the vicinity of the benzothiadiazole unit and consequently of
the resulting polymers are slightly blueshifted and pure green
emission is expected. As a result, the resulting polymers emit
pure green light with the emission maximum at 521 nm and
CIE coordinates of (0.29, 0.63), which are very close to the
High efficiency and long lifetime of green PLEDs have been
realized. Thus, the dominant issue for green light-emitting
polymers is the color purity. Since the human eye is very
sensitive to light in the green–yellow region, pure green emis-
sion is difficult to achieve with polymers because EL spectra of
polymers usually have large FWHM values (FWHM: full
width at half maximum).14 Therefore, most green emitters
appear either bluish-green or yellowish-green. For example,
the emission color of poly(fluorene-co-benzothiadiazole) is
State Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry and Graduate School of Chinese Academy
of Sciences, Chinese Academy of Sciences, Changchun 130022,
P. R. China. E-mail: lixiang@ciac.jl.cn; Fax: +86-431-85685653
2832 | J. Mater. Chem., 2007, 17, 2832–2838
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Scheme 1 Schematic illustration of the reason for the slight spectral blueshift resulting from incorporating 4,7-diphenyl-2,1,3-benzothiadiazole
instead of 2,1,3-benzothiadiazole into polyfluorene.
(0.26, 0.65) of standard green emission demanded by the
National Television System Committee (NTSC). Moreover,
the insertion of the two phenylene units does not affect the EL
efficiency of the resulting polymers. Their single-layer devices
(ITO/PEDOT/polymer/Ca/Al) exhibit a luminous efficiency of
C18H10Br2N2S: C, 48.46; H, 2.26; N, 6.28. Found: C, 48.09; H,
2.53; N, 6.14.
General procedure of Suzuki polymerization36
To a mixture of 2,7-dibromo-9,9-dioctylfluorene (4), 2,7-di-
bromo-9,9-bis(trimethyleneborate)fluorene (5), 4,7-dibromo-
2,1,3-benzothiadiazole (1) or 4,7-bis(4-bromophenyl)-2,1,3-
benzothiadiazole (3) with a corresponding feed ratio, Aliquat
336 (0.10 g, 0.25 mmol) and Pd(PPh3)4 (11.0 mg, 0.01 mmol)
under argon was added 2 M aqueous K2CO3 (2.5 mL) and
toluene (7 mL). The mixture was heated to 90 uC and stirred in
the dark for 48 h, followed by being poured into methanol.
The precipitate was collected by filtration and dried and then
dissolved in dichloromethane. The solution was then washed
with water and dried with anhydrous Na2SO4. After removal
of most of the solvent, the residue was poured into methanol to
give polymer fibre. The polymer was purified by a Soxhlet
extraction in acetone for 24 h. The reprecipitation procedure in
dichloromethane/methanol was then repeated several times.
The final product, a yellowish-green fibre, was obtained after
drying in vacuum with a yield of 45–60%.
8.25 cd A21 and a power efficiency of 4.45 lm W21
.
Experimental
Materials
All the reagents and solvents used for the syntheses were
purchased from Aldrich or Acros companies and used without
further purification. 4,7-Dibromo-2,1,3-benzothiadiazole (1),33
9,9-dioctyl-2,7-dibromofluorene (4)34 and 9,9-dioctyl-2,7-bis-
(trimethyleneborate)fluorene (5)34 were prepared following
already published procedures. All reactions were performed
under a dry argon atmosphere.
4,7-Diphenyl-2,1,3-benzothiadiazole (2)35
A mixture of 4,7-dibromo-2,1,3-benzothiadiazole (2.94 g,
10 mmol), phenylboric acid (2.56 g, 21 mmol), potassium
carbonate (5.52 g, 40 mmol), Pd(PPh3)4 (0.094 g, 0.08 mmol),
H2O (20 mL) and toluene (50 mL) was heated at 90 uC for 5 h.
After cooling, the mixture was poured into water and extracted
with dichloromethane. The organic layer was washed with
water and then dried with anhydrous Na2SO4. After the
solvent was evaporated, the crude product was recrystallized
from ethanol to afford the title compound as a yellow solid.
Yield: 2.39 g (83%). 1H NMR (CDCl3, 300 MHz) d (ppm):
7.96 (d, 4H), 7.80 (s, 2H), 7.56 (t, 4H), 7.47 (d, 2H).
PFG5: 4 (0.2468 g, 0.45 mmol), 5 (0.2792 g, 0.5 mmol) and 3
(0.0223 g, 0.05 mmol) were used in the polymerization. 1H
NMR (CDCl3, 300 MHz) d (ppm): 8.11 (br, 0.26H), 7.88 (br,
2.26H), 7.69 (d, 4H), 7.47 (br, 0.21H), 2.11 (br, 3.87H), 1.36
(br, 22.30H), 0.82 (t, 10.73H). Anal. calcd: C, 89.16; H, 10.05;
N, 0.366. Found: C, 88.12; H, 10.07; N, 0.60.
PFG10: 4 (0.2193 g, 0.40 mmol), 5 (0.2792 g, 0.50 mmol) and
1
3 (0.0446 g, 0.10 mmol) were used in the polymerization. H
NMR (CDCl3, 300 MHz) d (ppm): 8.11 (br, 0.48H), 7.83 (br,
2.63H), 7.68 (d, 4H), 7.47 (br, 0.39H), 2.11 (br, 3.88H), 1.36
(br, 22.06H), 0.82 (t, 10.47H). Anal. calcd: C, 88.62; H, 9.79;
N, 0.74. Found: C, 88.14; H, 10.01; N, 0.69.
4,7-Bis(4-bromophenyl)-2,1,3-benzothiadiazole (3)
A solution of bromine (10 mL, 194 mmol) in chloroform
(10 mL) was added dropwise to a mixture of 4,7-diphenyl-
2,1,3-benzothiadiazole (2.39 g, 8.3 mmol), iodine (0.12 g,
0.47 mmol) and chloroform (30 mL). The resulting mixture
was stirred at room temperature for 12 h. After workup, the
solid was filtered off and washed sequentially with aqueous
NaHSO3, water and methanol to afford the crude product.
Recrystallization in toluene gave the title compound as yellow
PFG20: 4 (0.1645 g, 0.30 mmol), 5 (0.2792 g, 0.50 mmol) and
1
3 (0.0892 g, 0.20 mmol) were used in the polymerization. H
NMR (CDCl3, 300 MHz) d (ppm): 8.11 (br, 1.08H), 7.88 (br,
3.52H), 7.69 (d, 4H), 7.47 (br, 0.88H), 2.11 (br, 3.62H), 1.36
(br, 22.28H), 0.82 (t, 10.63H). Anal. calcd: C, 87.49; H, 9.25;
N, 1.52. Found: C, 86.37; H, 8.23; N, 1.63.
PFY10: 4 (0.2193 g, 0.40 mmol), 5 (0.2792 g, 0.5 mmol) and
1
1 (0.0294 g, 0.10 mmol) were used in the polymerization. H
1
needles. Yield: 2.88 g (78%). H NMR (CDCl3, 300 MHz) d
NMR (CDCl3, 300 MHz) d (ppm): 8.09 (br, 0.32H), 8.01 (br,
(ppm): 7.86 (d, 4H), 7.77 (s, 2H), 7.68 (d, 4H). Anal. calcd for
0.43H), 7.93 (br, 0.52H), 7.85 (d, 0.98H), 7.68 (d, 4H), 7.49 (br,
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0.52H), 2.11 (br, 3.75H), 1.36 (br, 22.28H), 0.82 (t, 10.87H).
Anal. calcd: C, 88.36; H, 9.98; N, 0.77. Found: C, 88.17; H
10.23; N, 0.61.
poly(ethylenedioxythiophene) (PEDOT) was spin-coated on
the treated ITO at 3000 rpm for 60 s and then baked for 15 min
at 120 uC to give an approximate thickness of 40 nm. The
polymer layer (approximately 90 nm) was then spin-coated
onto the PEDOT/ITO coated glass substrate at ambient
atmosphere with their solution in chloroform (10 mg ml21).
Finally, a thin layer of calcium (10 nm) followed by a layer
of aluminum (100 nm) was deposited in a vacuum thermal
evaporator through a shadow mask at a pressure of 3 6
1023 Pa. The active area of the diodes was 16 mm2.
Instrument
1H NMR spectra were recorded with a Bruke Avance 300
NMR spectrometer. Elemental analysis was carried out using a
Bio-Rad elemental analysis system. The number- and weight-
average molecular weights (Mn and Mw, respectively) of the
polymers were determined by gel permeation chromotagraphy
(GPC) with a Waters 410 instrument with polystyrene as the
standard and THF as the eluent. UV-Vis absorption spectra
Result and discussion
were measured by
a Perkin-Elmer Lambda 35 UV-Vis
Synthesis and characterization
spectrometer. PL and EL spectra were recorded with a
Perkin-Elmer LS50B spectrofluorometer. Cyclic voltammo-
grams of polymer films on a glassy carbon electrode were
recorded on an EG&G 283 (Princeton Applied Research)
potentiostat/galvanostat system at room temperature in a
solution of nBu4NClO4 (0.10 M) in acetonitrile at a scan rate
of 100 mV s21. A Pt wire and an Ag/AgCl electrode were used
as the counter electrode and the reference electrode, respec-
tively. The current–voltage and brightness–voltage curves of
the devices were measured using a Keithley 2400/2000 current/
voltage source unit calibrated with a silicon photodiode.
As shown in Scheme 2, the key monomer, 4,7-bis(4-bromo-
phenyl)-2,1,3-benzothiadiazole (3), was synthesized by the
bromination of 4,7-diphenyl-2,1,3-benzothiadiazole (2), which
was prepared by the Suzuki coupling of 4,7-dibromo-2,1,3-
benzothiadiazole and phenyl boric acid. The copolymers were
prepared by Suzuki polycondensation of 2,7-dibromo-9,9-
dioctylfluorene (4), 2,7-bis(trimethyleneborate)-9,9-dioctyl-
fluorene (5) and 4,7-di(4-bromophenyl)-2,1,3-benzothiadiazole
(3). The feed ratios of the three comonomers were 45 : 50 : 5,
40 : 50 : 10 and 30 : 50 : 20. The corresponding polymers are
named as PFG5, PFG10 and PFG20, respectively. The actual
4,7-diphenyl-2,1,3-benzothiadiazole unit (DPBT) content in
Device fabrication
1
the copolymers, calculated from their H NMR, is very close
The indium-tin oxide (ITO) glass plates were degreased in an
ultrasonic solvent bath and then dried in a heating chamber at
a temperature of 120 uC. The poly(styrene sulfonic acid) doped
to the feed ratios of 3 as shown in Table 1. For comparison,
we also used 4,7-dibromo-2,1,3-benzothiazole instead of
4,7-di(4-bromophenyl)-2,1,3-benzothiadiazole to carry out
Scheme 2 Chemical structures and synthetic routes of the monomer and copolymers.
2834 | J. Mater. Chem., 2007, 17, 2832–2838
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Table 1 Molecular weight and composition of PFG5, PFG10, PFG20
and PFY10
Polymer
Feed ratioa
Contentb
Mn
Mw
PDIc
PFG5
PFG10
PFG20
PFY10
a
0.05
0.10
0.20
0.10
0.060
0.107
0.215
0.113
15 000
17 500
18 100
20 300
b
35 000
43 100
46 300
49 100
2.33
2.47
2.56
2.42
Feed ratio of the comonomer 3 or 1. Actual benzothiadiazole
unit content calculated from 1H NMR. Polydispersity index.
c
the polymerization and obtained the polymer PFY10 with the
actual 2,1,3-benzothiadiazole unit (BT) content of 11.3 mol%.
The number-average molecular weights of the polymers, as
measured with GPC using polystyrene as standards, vary from
15 000 to 20 300 with the polydispersity index ranging from
2.33 to 2.47 (see Table 1). All these polymers are soluble
in common organic solvents, such as toluene, chloroform and
tetrahydrofuran.
Photophysical properties
The UV-Vis absorption spectra and PL spectra of the
polymers were measured both in solution and in thin films.
As shown in Fig. 1(a) and Fig. 1(b), the band at 379 nm in the
absorption spectra of PFG5, PFG10, and PFG20 is attributed
to the p–p* transition of the fluorene segments and the small
shoulder at about 420 nm is attributed to the narrow bandgap
in the vicinity of the benzothiadiazole unit. In comparison,
the shoulder in the absorption spectrum of PFY10, which
originates from the vicinity of the benzothiadiazole unit, is
located at about 430 nm. It is obvious that the vicinity of the
benzothiadiazole unit in PFG5, PFG10, and PFG20 shows a
more blueshifted absorption maximum than that in PFY10,
Fig. 2 PL spectra of PFG5, PFG10, PFG20 and PFY10 in dilute
solution (a) and in the solid film (b).
indicating that the vicinity of the benzothiadiazole unit in
PFG5, PFG10, and PFG20 has a larger bandgap and less
effective conjugation length than PFY10. The insertion of the
two phenylene moieties makes the vicinity of the benzothia-
diazole unit a little more twisted and reduces its effective
conjugation length (see Scheme 1).
Fig. 2 shows the PL spectra of the polymers in dilute toluene
solution and in thin solid films with the excitation at 380 nm.
The PL spectra of these polymers in toluene [Fig. 2(a)] show
two emission bands. The band in blue region (400–460 nm) is
attributed to the fluorene segments and the other band in the
green–yellow region (470–600nm) is due to emission from the
vicinity of the benzothiadiazole unit. Considering the weak
absorption band and the strong emission band from the vicinty
of the benzothiadiazole unit, we attribute the emission band
from the vicnity of the benzothiadiazole unit to the Fo¨rster
energy transfer from the fluorene segments to the vicinity of
the benzothiadiazole unit.37 The vicinity of the benzothiadia-
zole unit serves as exciton trap, confining the excitons and
giveing rise to green light emission. The PL spectra of these
polymers in solid film (Fig. 2b) show only a green emission
band with no blue emission band. This is due to the more
complete energy transfer from the fluorene segments to the
vicinity of the benzothiadiazole unit in the solid film compared
to in dilute solution. In the former case, polymer chains are
packed together, enabling both intramolecular energy transfer
and intermolecular energy transfer. In contrast, in the latter
one, polymer chains are separated by solvent molecules and
only intramolecular energy transfer is involved.19 The PL
spectrum of PFG10 is blueshifted by 16 nm compared to that
of PFY10 because the insertion of the phenylene units reduces
the effective conjugation length in the vicinity of the
Fig. 1 Absorption spectra of PFG5, PFG10, PFG20 and PFY10 in
dilute solution (a) and in solid film (b).
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Fig. 4 EL spectra of the devices of PFG5, PFG10, PFG20 and
PFY10.
Fig. 3 Cyclic voltagramms of PFG5, PFG10, PFG20 and PFY10.
benzothiadiazole unit in PFG10. In the PL spectra of PFG5,
PFG10, and PFG20, with the increase in the DPBT unit
content, the green emission band is slightly redshifted due
to the interaction (possibly aggregates, excimers or polaron
pairs)38 in the vicinity of the benzothiadiazole unit.
25.78 eV and LUMO of about 22.17 eV. Their HOMO and
LUMO energy levels are very close to the data reported for the
polyfluorene homopolymer,41 indicating that their oxidation
and reduction processes both originate from the fluorene
segments. The signals for the redox behavior of the benzothia-
diazole unit are too weak to be recorded. From the obtained
onset oxidation and reduction potential, we estimate the
polymers’ electrochemical bandgaps to be about 3.56 eV (see
Table 2), which is much larger than the corresponding optical
bandgaps estimated from the onset of optical absorption in
solid film.
The PL quantum efficiencies (WPL) of these polymer films
were measured with an integrated sphere with the excitation at
409 nm according to the literature.39 PFG5, PFG10, PFG20
and PFY10 all exhibit WPL of 0.80 or so. The insertion of
phenylene units does not affect the PL quantum efficiencies of
the resulting polymers.
Electrochemical properties
Electroluminescence properties
The electrochemical properties of the polymers in the solid
films were investigated by cyclic voltammetry in degassed
acetonitrile with nBu4NClO4 (0.10 M) as the electrolyte. Fig. 3
shows the cyclic voltagramms of PFG5, PFG10, PFG20 and
PFY10. The onset potential of the oxidation process and the
reduction process are listed in Table 2. Based on the onset
potential and the formulae40 [EHOMO = 2(Eox + 4.34) eV and
ELUMO = 2(Ered + 4.34) eV], we estimate the HOMO and
LUMO energy levels of the polymers (see Table 2). All four
polymers show similar energy levels with HOMO of about
To investigate the EL properties of the polymers, single-layer
devices based on these polymers were fabricated with the
configuration ITO/PEDOT (40 nm)/polymer (90 nm)/Ca
(10 nm)/Al (100 nm). Their EL spectra are presented in Fig. 4
and the corresponding CIE coordinates are listed in Table 3.
The polymers’ EL spectra are blueshifted by 20 nm in
comparison with the corresponding PL spectra. For PFY10,
the EL spectrum exhibits a maximum at 537 nm with CIE
coordinates of (0.36, 0.57). In comprarison, the EL spectrum
of PFG10 exhibits a maximum at 525 nm with CIE coordinates
Table 2 Photophysical and electrochemical properties of PFG5, PFG10, PFG20 and PFY5
In solution
abs/nm
In solid film
abs/nm
Oxidation
Reduction
Polymer
l
lPL/nm
l
l/nm
WPL
Ega/eV
Eonset/V
EHOMO/eV
Eonset/V
ELUMO/eV
Egb/eV
PFG5
PFG10
PFG20
PFY10
a
379
379
379
373
429, 525
429, 525
525
379
379
379
367
535
535
545
558
0.81
0.83
0.78
0.84
2.56
2.55
2.55
2.49
1.40
1.44
1.46
1.40
b
25.74
25.78
25.80
25.74
22.18
22.15
22.12
22.19
22.16
22.19
22.22
22.15
3.58
3.59
3.58
3.59
429, 544
Bandgap estimated from the onset wavelength of optical absorption in solid film. Bandgap estimated from the onset oxidation and
reduction potential.
Table 3 EL performance of the devices of the polymers
Onset
voltage/V
Maximum
brightness/cd m22
Luminous
efficiency/cd A21
Power
efficiency/lm W21
CIE coordinates
(x, y)
Polymer
lmax/nm
PFG5
4.2
4.1
4.3
4.6
10 260
22 650
13 680
19 025
5.96
8.25
5.78
4.99
2.21
4.45
2.34
1.90
521
525
528
537
(0.29, 0.63)
(0.31, 0.58)
(0.33, 0.59)
(0.36, 0.57)
PFG10
PFG20
PFY10
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of (0.31, 0.58). The insertion of the phenylene units leads to a
blueshift by 12 nm in the EL spectra and changes the emisson
color from green–yellow to saturated green. The insertion of
the phenylene units greatly improves the color purity of the
resulting polymers. In particular, the CIE coordinates of
PFG5, (0.29, 0.63), are very close to (0.26, 0.65) of the
standard green emission demanded by the NTSC.
polyfluorene, we have developed a novel series of green light-
emitting polymers. The insertion of the two phenylene moieties
between the benzothiadiazole unit and the fluorene unit
reduced the effective conjugation length in the vicinity of the
benzothiadiazole unit and consequently led to a slight blueshift
by 10–20 nm in the EL spectra of the resulting polymers. As a
result, the color purity was greatly improved. CIE coordinates
of (0.29, 0.63), which are very close to the (0.26, 0.65) of
standard green emission required by the NTSC, were achieved.
Furthermore, the insertion of the phenylene units led to no
decrease of the PL and EL efficiencies of the resulting
polymers. Non-optimized single-layer devices of these poly-
mers emitted pure green light with a luminous efficiency of
5.96 cd A21 and a power efficiency of 2.21 lm W21. High EL
efficiencies and good color purities indicated these polymers to
be promising candidates for green light-emitting polymers for
display applications.
The EL performance data for the devices are summarized in
Table 3. The device based on PFY10 exhibits a turn-on voltage
of 4.6 V, luminous efficiency of 4.99 cd A21, power efficiency
of 1.90 lm W21 and maximum brightness of 19 025 cd m22. In
contrast, the device based on PFG10 shows a turn-on voltage
of 4.1 V, luminous efficiency of 8.25 cd A21, power efficiency
of 4.45 lm W21 and maximum brightness of 22 650 cd m22
with CIE coordinates of (0.31, 0.58). The insertion of the
phenylene unit leads to no decrease in the EL efficiencies of the
resulting polymers. Fig. 5(a) shows the voltage–current
density–brightness characteristics of the device based on
PFG10. Fig. 5(b) shows the dependence of the luminous
efficiency and power efficiency on the current density of this
device. The device based on PFG5 emits pure green light with a
Acknowledgements
This work is supported by the National Natural Science
Foundation of China (No. 20574067 and No. 50633040),
Science Fund for Creative Research Groups (No. 20621401)
and 973 Project (No. 2002CB613402).
turn-on voltage of 4.2 V, luminous efficiency of 5.96 cd A21
,
power efficiency of 2.21 lm W21 and maximum brightness of
10 260 cd m22 with CIE coordinates of (0.29, 0.63). This
performance is among the best reported for green electro-
luminescent polymers.9–18,24–27 Further improvement of EL
performance can be expected after device optimization.
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