Polymer light-emitting diodes based on a bipolar transporting luminescent polymer

Article abstract of DOI:10.1039/b210302h

A soluble electroluminescent polymer containing hole-deficient triphenylamine and electron-deficient oxadiazole units in the main chains has been designed and studied. The design is based on the consideration that the triphenylamine group possesses good hole-transporting property and the oxadiazole unit is known to be of electron-transporting character. Because of the good bipolar transporting performance, the brightness and electroluminescent efficiency are significantly improved and the turn-on voltage is reduced compared with a similar polymer without the electron-deficient oxadiazole units in the main chains. For a device with configuration ITO/PEDOT/polymer/CsF/Al, a maximum brightness of 3600 cd m^-2 and a maximum luminescent efficiency of 0.65 cd A^-1 (quantum efficiency of 0.3%) were obtained in the polymer with oxadiazole units, about 15 times brighter and 15 times more efficient than the corresponding polymer without oxadiazole units.

Full text of DOI:10.1039/b210302h

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Polymer light-emitting diodes based on a bipolar transporting
luminescent polymer
Yanguang Zhang, Yufeng Hu, Hongchao Li, Lixiang Wang, Xiabin Jing, Fosong Wang
and Dongge Ma*
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China.
E-mail: mdg1014@ciac.jl.cn; Fax: 0086-431-5262357
Received 21st October 2002, Accepted 30th January 2003
First published as an Advance Article on the web 14th February 2003
A soluble electroluminescent polymer containing hole-deficient triphenylamine and electron-deficient oxadiazole
units in the main chains has been designed and studied. The design is based on the consideration that the
triphenylamine group possesses good hole-transporting property and the oxadiazole unit is known to be of
electron-transporting character. Because of the good bipolar transporting performance, the brightness and
electroluminescent efficiency are significantly improved and the turn-on voltage is reduced compared with a
similar polymer without the electron-deficient oxadiazole units in the main chains. For a device with
configuration ITO/PEDOT/polymer/CsF/Al, a maximum brightness of 3600 cd m22 and a maximum
luminescent efficiency of 0.65 cd A21 (quantum efficiency of 0.3%) were obtained in the polymer with
oxadiazole units, about 15 times brighter and 15 times more efficient than the corresponding polymer without
oxadiazole units.
electroluminescence properties. The design is based on the
Introduction
consideration that the triphenylamine group possesses good
hole-transporting property and the oxadiazole unit is known to
be of electron-transporting character. Because of the good
bipolar transporting performance, the brightness and electro-
luminescent efficiency are significantly improved and the turn-
on voltage is reduced compared with a similar polymer without
the electron-deficient oxadiazole units in the main chains
(polymer II). For the device with configuration of ITO/
PEDOT/polymer/CsF/Al, a maximum brightness of 3600 cd m22
and a maximum luminescent efficiency of 0.65 cd A21 (quan-
tum efficiency of 0.3%) are obtained for polymer I, about 15
times brighter and 15 times more efficient than the correspond-
ing polymer II.
Polymer light-emitting diodes (PLEDs) have been extensively
studied due to their potential applications in large-area and
flexible displays.1,2 However, most of electroluminescent poly-
mers developed so far transport holes much more efficiently
than electrons (except for those modified with an electron-
withdrawing group, such as cyano-substituted poly(phenylene
vinylene) (PPV)3), resulting in the imbalance of the electron
and hole transport and a shift of the charge recombination
region to near the metal electrode, and therefore in lower device
efficiency due to luminescent quenching by the metal electrode.
The effective methods that have been developed so far for
resolving the low efficiency caused by the imbalanced electron
and hole transport in general luminescent polymers are using a
blend of emissive polymer with charge-transporting materials
as an active layer or introducing an electron-transporting layer
(ETL) to fabricate a multilayer device.4,5 However, phase
separation during storage and operation often occur in the
blending devices due to immiscibility of the different molecules,
resulting in a reduction of the device lifetime.6 Additionally,
effective multilayer devices are difficult to fabricate because of
interlayer mixing, which may damage the emissive layer when
spin-coating the ETL layer. Therefore, it is necessary to design
new luminescent polymers not only with better hole-transport-
ing properties, but also with better electron-transporting
capability to allow a fabrication of efficient single-layer devices.
It has been reported that the electron-transporting properties
in luminescent polymers can be enhanced by introduction of
electron-transporting moieties such as oxadiazole groups into
the side chain or the main chain of the polymers.7–17 Indeed,
the device efficiency is significantly improved when using these
bipolar-transporting luminescent polymers as the active
medium compared with the only hole-transporting luminescent
polymers. However, at present, there are only a few bipolar-
transporting luminescent polymers used to fabricate highly
efficient devices.13,14 Here, we report an efficient soluble
electroluminescent polymer containing hole-deficient triphenyl-
amine and electron-deficient oxadiazole units in the main
chains (polymer I), and study its photoluminescence and
Experimental
Materials
The molecular structures of polymers I and II used in this study
are shown in Fig. 1. The synthetic routes to the polymers are
given in Scheme 1. An improved Wittig polycondensation
reaction was utilized to synthesize the polymers. The key of the
synthetic method is using tributylphosphonium salts instead of
triphenylphosphonium salts as polymerization monomers. The
polymers produced by the improved Wittig polycondensation
possess not only high trans-configuration of vinylene (w95%),
but also excellent yields (yield is 88% for polymer I, 90% for
polymer II) and molecular weight compared to conventional
Wittig and Wittig–Horner methods. Gel permeation chroma-
tography (GPC) measurements indicated that polymer I and II
have number-average molecular weights of 11 K and 30 K,
respectively. Both polymers are soluble in common organic
solvents such as chloroform and tetrahydrofuran (THF).
Sythesis of the corresponding intermediate compounds. Com-
pound (1): p-toluoyl chloride (3.1 g, 19.4 mmol) was added
dropwise to a solution of hydrochloride acid hydrazine
(0.925 g, 8.82 mmol), triethylamine (3.74 g, 37 mmol) and
chloroform (50 ml) at room temperature. The resulting mixture
DOI: 10.1039/b210302h
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Fig. 1 Molecular structures of polymers I and II used in this study.
Scheme 1 Synthesis of the monomer and the polymers.
was stirred at 50 uC for 10 h. After cooling, the white solid
collected was washed with water and methanol to obtain 4.01 g
of product (yield 82%, mp ~ 257–259 uC). 1H NMR (DMSO-
d6, ppm): d 10.4 (s, 2H), 7.8 (d, 4H), 7.3 (d, 4H), 2.3 (s, 6H).
Compound (2): A mixture of compound (1) (2.68 g, 10 mmol)
and POCl3 (250 ml) was refluxed under nitrogen for 12 h. The
excess POCl3 was then distilled out and the residue was poured
into water. The crude solid product was collected by filtration
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and purified by recrystallization from ethanol to give
compound (2) as needle-like crystals (1.98 g, 78%, mp ~
164–166 uC). H NMR (CDCl3, ppm): d 8.02 (d, 4H), 7.33 (d,
4H), 2.44 (s, 6H).
Compound (3): A mixture of compound (2) (1.5 g, 6 mmol),
N-bromosuccinimide (NBS) (2.37 g, 18 mmol), benzoyl
peroxide (0.2 g) and CCl4 (150 ml) was refluxed at 90 uC for
6 h. The mixture was filtered and washed with hot water. The
residual solid was recrystallized from tetrahydrofuran (THF)
to give white crystals (1.52 g, 62%, mp ~ 220–222 uC). 1H
NMR (CDCl3, ppm): d 8.25 (d, 4H), 7.82 (d, 4H), 4.94 (s, 6H).
layer of CsF (1 nm) covered with a layer of aluminium (150 nm)
was deposited as the cathode by thermal evaporation at a
pressure of 1026 Torr. The emissive area was 10 mm2.
UV-vis absorption and photoluminescence spectra were
recorded at room temperature on a Shimadzu UV-vis-NIR
3000PC spectrophotometer and a Perkin-Elmer LS 50B
luminescence spectrometer, respectively. The current–voltage
and light–voltage curves were measured using a Keithley 2400/
2000 current/voltage source unit with a calibrated silicon
photodiode. EL spectra were recorded using a Perkin-Elmer LS
50B luminescence spectrometer.
1
Synthesis of the corresponding tributylphosphonium salts. A
mixture of 1,4-dichloro-p-xylene or 2,5-bis(4-bromomethyl-
phenyl)-1,3,4-oxadiazole and tributylphosphonium salt was
refluxed in dimethylformamide (DMF) solution at 150 uC for
10 h. After cooling to room temperature, 500 ml of diethyl
ether was slowly poured into the mixture solution while
stirring. The initial products were washed with anhydrous
diethyl ether and then dried in a vacuum chamber. The white
solid powder was obtained in 60–90% yield.
Results and discussion
Fig. 2 shows the UV-vis absorption spectra of thin films of
polymers I and II. It can be seen that the polymer I exhibits two
absorption peaks, one at 470 nm and the other at 336 nm. The
peak at 336 nm obviously is related to the existence of
oxadiazole units in polymer I. Compared to polymer II, the
absorption peak of polymer I is red shifted by about 25 nm. A
similar red-shifting phenomenon is also observed in the
photoluminescence spectra, as shown in Fig. 3. These results
indicate that the introduction of the oxadiazole units increases
the conjugated length of the polymer, leading to the red-shifted
spectrum. The broad tail at low energy in the absorption
1,4-Dichloro-p-xylene tributylphosphonium salt. Mp ~ 283 uC,
1H NMR (CDCl3, 400 MHz, ppm): d 7.62 (s, 4H), 4.42 (d, J ~
14.8 Hz, 4H), 2.38 (t, 12H), 1.49 (m, 24H), 0.93 (t, 18H).
FT-IR (KBr): 2960, 2932, 2873, 2797, 1514, 1464, 1100,
860 cm21
.
2,5-Bis(4-bromomethylphenyl)-1,3,4-oxadiazole tributylphos-
phonium salt. Mp ~ 113–115 uC, 1H NMR (CDCl3, 400 MHz,
ppm): d 7.74 (d, J ~ 8.0 Hz, 4H), 7.62 (d, J ~ 8.0 Hz, 4H),
4.80 (d, J ~ 16.4 Hz, 4H), 2.44 (m, 12H), 1.52 (m, 24H), 0.89
(t, 18H).
Synthesis of polymers. Equimolar amounts of triphenylamine
aldehyde monomer and 1,4-dichloro-p-xylene tributylphos-
phonium saltor 2,5-bis(4-bromomethylphenyl)-1,3,4-oxadiazole
tributylphosphonium salt were dissolved in chloroform solu-
tion. The mixture solution was stirred for about 30 min, and
was then added dropwise to three mol equivalents of fresh
NaOEt, and stirred. After 5 h, the reaction was stopped by
pouring into distilled water, and the product extracted with
chloroform and washed with distilled water. Finally the
solvents in the product were removed and the product was
deposited in alcohol. The final polymers were collected by
filtration and dried under vacuum. DSC measurements show
that the glass transition temperature of polymer I is w200 uC,
and that of polymer II is 155 uC.
Fig. 2 UV-vis absorption spectra of thin films of polymers I and II.
1
Polymer I: H NMR (CDCl3, 400 MHz, ppm): d 8.11 (m,
4H), 7.62 (m, 4H), 7.41 (m, 4H), 7.05 (m, 10H), 6.81 (d, 2H),
3.92 (m, 2H), 1.71 (m, 2H), 1.54–0.83 (m, 13H). FT-IR (KBr):
3026, 2924, 2853, 1591, 1504, 1416, 1316, 1286, 1240, 1178,
1164, 1104, 1065, 1014, 961, 868, 830 cm21
.
1
Polymer II: H NMR (CDCl3, 400 MHz, ppm): d 7.51 (s,
4H), 7.42 (d, 4H), 7.03 (m, 10H), 6.89 (d, 2H), 3.94 (t, 2H), 1.84
(m, 2H), 1.51–0.88 (m, 13H). FT-IR (KBr): 3024, 2924, 2854,
1593, 1503, 1470, 1315, 1281, 1238, 1177, 957, 826 cm21
.
Device fabrication and measurements
The used devices in this study are single-layer structures. Prior
to polymer layer spin coating, the substrates were degreased in
ultrasonic solvent baths and then dried in a heating chamber
(w120 uC). Then poly(3,4-ethylenedioxythiophene)–poly(styr-
ene sulfonic acid) (PEDOT : PSS) was spun onto ITO at
3000 rpm for 60 s and then baked for 15 min at 120 uC,
attaining an approximate thickness of 50 nm. The polymer (I or
II) layer was then spin coated onto the PEDOT/ITO coated
glass substrate in chloroform solution (10 mg ml21). A polymer
film of 100 nm thickness was obtained. On top of this, a thin
Fig. 3 Photoluminescence spectra of thin films of polymers I and II.
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Fig. 5 Light–current (L–I) and EL efficiency–current (E–I) character-
istics of the devices based on polymers I and II.
Fig. 4 Current–light–voltage (I–L–V) characteristics of the devices
based on polymer I (a) and polymer II (b)
spectrum of polymer I with respect to polymer II may be
related to an excimer between the triphenylamine and
oxadiazole groups.
Fig. 6 EL spectra of polymers I and II.
possesses very good hole-transport character.18 This indicates
that the polymer II transports holes much more efficiently than
electrons. When fabricating single-layer devices, because of the
imbalanced electron and hole transport, resulting in the poor
recombination of the injected electrons and holes, the emissive
region is close to the quenching cathode, leading to low EL
efficiency of the polymer II. The imbalanced electron and hole
transport may be directly established from the current–light–
voltage curves. If the electron and hole transport is not
balanced, the luminance does not follow the current closely, as
shown by polymer II (Fig. 4(b)). For polymer I, however, the
luminance follows the current closely (Fig. 4(a)), which
indicates an efficient balanced transport for both electrons
and holes. As is well known, oxadiazole groups are excellent
electron-transporting units owing to their electron-withdraw-
ing character.19 The introduced oxadiazole groups in polymer I
therefore enhance the electron transport, and thus improve the
transport balance of the electrons and holes. The importance of
this is reflected in the fact that the turn-on voltage of polymer I
is significantly reduced due to the introduction of the
oxadiazole group.
Fig. 4(a) and (b) illustrate the typical current–light–voltage
(I–L–V) characteristics of the devices based on polymers I and
II, respectively. It can be seen that the polymer I-based device
shows a lower turn-on voltage and a higher luminance. The
turn-on voltage is 4 V, and the maximum brightness reaches
3600 cd m22 at 23 V, which is about a factor of 15 brighter than
that of the polymer II-based device with a value of 248 cd m22
at 17 V. The polymer I-based device also shows a higher EL
efficiency of 0.65 cd A21 (EL quantum efficiency of 0.3%)
than the polymer II-based device, as shown in Fig. 5 where the
light–current (L–I) and EL efficiency–current (E–I) character-
istics are shown. Compared to a maximum EL efficiency of
0.042 cd A21 for the polymer II-based device, the EL efficiency
of the device based on polymer I has been enhanced by a factor
of more than 15. The EL efficiency obtained from the bipolar-
transporting polymer I is also the highest value reported so far
in single-layer devices based on luminescent polymers.
Fig. 6 shows the EL spectra of polymers I and II. The EL
spectra are almost identical to the PL spectra, indicating that
the EL and PL result from the same excited state, i.e. radiation
of singlet excitons. It can be seen that the EL of polymer I also
shows a red shift with respect to polymer II, similar to the PL
spectra.
Conclusions
It can be seen that the designed bipolar-transporting
luminescent polymer in this study not only shows higher EL
efficiency and brightness, but also shows a lower turn-on
voltage. It is well known that the triphenylamine group
We have designed and synthesized a new bipolar-transporting
luminescent polymer by an improved Wittig polyconden-
sation reaction. Owing to the introduction of the good
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Acknowledgements
The work was supported by the NSFC (29725410 and
29992530). One of the authors (D. M.) is grateful for the
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