Synthesis and characterization of a novel blue electroluminescent polymer constituted of alternating carbazole and aromatic oxadiazole units

Article abstract of DOI:10.1039/a903215k

A novel copolymer poly[N-(2-ethylhexyl)-carbazole-3,6-diyl-1'',3'',4''- oxadiazole-2''',5'''-dioctyloxy-1''', 4'''-phenylene-1'''',3'''',4''''- oxadiazole-2'''',5''''-diyl] (PCOPO) containing an electron rich carbazole moiety and an electron deficient aro

Full text of DOI:10.1039/a903215k

View Article Online / Journal Homepage / Table of Contents for this issue
Synthesis and characterization of a novel blue electroluminescent
polymer constituted of alternating carbazole and aromatic oxadiazole
units
Hong Meng,a Zhi-Kuan Chen,a Xiao-Ling Liu,b Yee-Hing Lai,b Soo-Jin Chuaa and
Wei Huang*a
a Institute of Materials Research and Engineering (IMRE), National University of Singapore, 10
Kent Ridge Crescent, Singapore 119260. E-mail: wei-huang=imre.org.sg
b Department of Chemistry, National University of Singapore, 10 Kent Ridge Crescent, Singapore
119260
Received 22nd April 1999, Accepted 12th May 1999
A novel copolymer poly[N-(2@-ethylhexyl)-carbazole-3,6-diyl-1A,3A,4A-oxadiazole-2A,5A-diyl-2Ó,5Ó-dioctyloxy-1Ó,
4Ó-phenylene-16,36,46-oxadiazole-26,56-diyl] (PCOPO) containing an electron rich carbazole moiety and
an electron deÐcient aromatic oxadiazole unit is synthesized by a polycondensation reaction. The structure of
the polymer is conÐrmed by FT-IR, NMR, and elemental analysis. PCOPO can be partially dissolved in THF,
CHCl , xylene, and DMSO while it can be completely dissolved in chloroform with a small amount of
3
triÑuoroacetic acid (TFA). The optical and electronic properties of the polymer are investigated by UV-Vis
absorption and photoluminescence spectroscopy as well as cyclic voltammetry. The results show that the
polymer Ðlms emit greenish-blue light (475 nm). The bandgap energy of the polymer is estimated by the optical
method (2.82 eV) and the electrochemical measurement (2.94 eV), respectively. Both p-doping and n-doping
processes are observed in cyclic voltammetric investigation. The HOMO and LUMO energies of the polymer
are estimated to be 5.60 and 2.66 eV, respectively. The photophysical and electronic properties as well as the
preliminary electroluminescent device result of the polymer demonstrate that it may be a promising candidate
material for the fabrication of an efficient polymer light-emitting device.
that for hole injection, which leads to charge capture near the
Introduction
metal contact. This results in poor quantum efficiency. To
overcome this problem, two approaches have been developed.
One is to use the low work function metals such as Ca or Mg
as the cathode so as to improve the ability of electron injec-
tion. The other is to introduce the electron transporting and
hole blocking layer to provide an intermediate area for
charges to capture each other. However, there are some tech-
nical problems associated with making the device utilizing
these above two approaches. Alternatively, polymers with
well-matched HOMO and LUMO energy levels with the
respective electrode energy barrier heights will lead to high
efficiency for light-emitting device application. Combining a
typical hole injection moiety and an electron injection unit
into the same polymer backbone might meet this requirement.
Our previous work, based on combined thiophene and oxa-
diazole units as functional materials for blue light emission
LED devices, has demonstrated good results.12h14 In this
paper, we present the work of synthesis and characterization
of a new polymer, poly[N-(2@-ethylhexyl)-carbazole-3,6-diyl-
1A,3A,4A-oxadiazole-2A,5A-diyl-2Ó,5Ó-dioctyloxy-1Ó,4Ó-phenyl-
ene-16,36,46-oxadiazole-26,56-diyl] (PCOPO) (Scheme 1),
in which the oxadiazole moiety as the electron injection and
transporting unit is incorporated into the carbazole main
chain to form a pÈn diblock structure. Based on this concept,
it is expected that the designed polymer will possess good
charge injection and transporting properties for both holes
and electrons. The work described here reinforces our pre-
vious idea towards the design of pÈn diblock copolymers for
promising optoelectronic applications.
In the past few years, many e†orts have been devoted to the
design and synthesis of new electroluminescent (EL) polymers
for the fabrication of blue polymer light-emitting diodes
(PLEDs) with high efficiency.1h3 Blue light generation from
emissive polymers is much more promising for display tech-
nology because inorganic semiconductors such as GaN, SiC,
and ZnSe are proven so far to be difficult for large-area Ñat
panel applications. Several polymer systems have been devel-
oped for blue light emission. Among them, polyÑuorenes
(PFs),4 polycarbazoles (PCs),5 and polythiophenes (PTs)6 as
well as polyoxadiazoles (POs)7 are some examples. Carbazole-
based conducting polymers have been paid increasing atten-
tion recently because of their interesting optical properties and
strong hole-transporting ability in the optoelectronic
devices.8,9 On the other hand, oxadiazole-based polymers
have played an important role in the fabrication of multilayer
LED devices as good electron-transporting materials.10,11
Both carbazole and oxadiazole based luminescent polymers
demonstrate quite di†erent charge injection and transporting
abilities for holes and electrons in PLED devices.
It is reported that the obstacle to improving the electrolu-
minescence efficiency of LED devices is mainly due to the
imbalance of the charge injection rates from opposite contacts
into the emission layer. Consequently high quantum effi-
ciencies are difficult to obtain using either carbazole or oxa-
diazole based polymers alone. For most of the
electroluminescent polymers investigated so far, the barrier for
electron injection from the metal contact is much bigger than
Phys. Chem. Chem. Phys., 1999, 1, 3123È3127
3123

View Article Online
2,5-Dioctyloxylterephthalic dihydrazide (I). 7.2 g (0.015 mol)
of diethyl 2,5-dioctyloxyterephthalate was added to a solution
of 10 ml of hydrazine monohydrate (99%) in 60 ml of
CH OH. The reaction mixture was reÑuxed for 24 h. Then the
3
mixture was cooled and Ðltered to give a white precipitate.
The precipitate was recrystallized from ethanol and dried in
vacuum at 60 ¡C to yield 6.4 g (95%) white crystals. Mp 98.0È
99.0 ¡C. MS: m/z 450. 1H NMR (CDCl ) d 9.19 (broad, 2 H,
3
NH), d 7.84 (s, 2 H), 4.19È4.16 (t, J \ 6.7, 4 H), 4.18È4.16 (d, 4
H, NH ), 1.92È1.82 (m, 4 H), 1.48È1.28 (m, 20 H), 0.88È0.86 (t,
2
J \ 6.8, 6 H). 13C NMR (CDCl ) d 165.24, 150.70, 122.97,
3
115.66, 69.80, 31.61, 29.08, 29.02, 28.97, 25.88, 22.50, 13.95.
Analysis calculated for C
12.43. Found: C, 63.78; H, 9.29; N, 12.22.
H
N O : C, 63.97; H, 9.39; N,
24 42
4 4
Scheme 1 Chemical structure for PCOPO.
N-(2º-ethylhexyl)carbazole.15 To the mixture of 16.7 g (0.1
mol) of carbazole dissolved in 120 ml of ethanol was added
11.2 g (0.2 mol) of potassium hydroxide. The mixture was
reÑuxed for 0.5 h, and then 38.6 g (0.2 mol) of 2-ethylhexyl
bromide was added dropwise over a period of 1 h. The
mixture was further reÑuxed for 6 h. After cooling to room
temperature, the mixture was poured into water (250 ml) and
extracted with ether three times (60 ml each), then dried with
anhydrous magnesium sulfate. The solvents were removed by
rotary evaporation and the residue was distilled to remove the
excess 2-ethylhexyl bromide. The residue was puriÐed by
silica-gel column chromatography using hexane : ethyl acetate
(10 : 1) as the eluent. The yield is 81%. MS: m/z 279. 1H NMR
Experimental
Measurements
NMR spectra were collected on a Bruker ACF 300 spectro-
meter with chloroform-d as solvent and tetramethylsilane as
internal standard. FT-IR spectra were recorded on a Bio-Rad
FTS 165 spectrometer by dispersing samples in KBr disks.
UV-Vis and Ñuorescence spectra were obtained on a Shi-
madzu UV 3101PC UV-Vis-NIR spectrophotometer and a
Perkin Elmer LS 50B luminescence spectrometer with a xenon
lamp as light source, respectively. Elemental analyses were
performed on a Perkin-Elmer 240C elemental analyzer for C,
H, N, and S determinations. Cyclic voltammetry was per-
formed using an EG&G Model 273A potentiostat/galvanostat
under argon atmosphere. All potentials were measured against
a Ag/Ag` (0.1 M in acetonitrile) electrode (0.34 V vs. SCE)
and all of the experimental values in this report were corrected
with respect to SCE. Gel permeation chromatography (GPC)
analysis was conducted with a Perkin Elmer Model 200
HPLC system equipped with PhenogelTM MXL and MXM
columns using polystyrene as standard and THF as eluent.
(CDCl ) d 8.21È8.18 (m, 2 H), 7.57È7.34 (m, 4 H), 7.31È7.29 (m,
3
2 H), 4.23È4.20 (d, J \ 7.2, 2 H), 2.18È2.14 (m, 1 H), 1.51È1.33
(m, 8 H), 1.02È0.94 (m, 6 H). 13C NMR (CDCl ) d 141.07,
3
125.65, 122.97, 120.39, 118.82, 109.08, 47.42, 39.49, 31.15, 28.95,
24.55, 23.20, 14.18, 11.03. Analysis calculated for C
86.02; H, 8.96; N, 5.02. Found: C, 85.79; H, 9.09; N, 5.27.
H
N: C,
20 25
3,6-Bis(N,N-dimethylcarbamoyl)-9-(2º-ethylhexyl)carbazole.16
To a stirred mixture of 9.6 g (0.070 mol) of aluminum chloride
and 30 ml of ethylene chloride under nitrogen was added a
solution of 9.8 g (0.035 mol) of N-(2@-ethylhexyl)carbazole dis-
solved in 30 ml of ethylene chloride. Then a solution of 7.5 g
(0.070 mol) of N,N-dimethylcarbamoyl chloride in 30 ml of
ethylene chloride was added dropwise over 20 min. The
mixture was heated under reÑux for 24 h under nitrogen, then
cooled and poured into 50 ml water and extracted with chlo-
roform three times (20 ml each). The combined organic layer
was washed with water, until the washings were neutral to
litmus paper, and dried over magnesium sulfate. The solvent
was evaporated under reduced pressure and the residue was
puriÐed by silica-gel column chromatography using ethyl
acetate as the eluent. Yield 73%. Mp 55.0È57.0 ¡C. MS: m/z
LED device fabrication
In the fabrication of single layer LED devices, indium tin
oxide-coated (ITO) glass with a resistivity of 250 )~1 was
used as substrate. A uniform Ðlm (thickness about 1200 A) of
the polymer was obtained by spin-coating from the polymer
solution dissolved in triÑuoroacetic acid (TFA)ÈCHCl at a
rate of 5000 rpm for 2 min. The Ðlm was dried in a vacuum
oven at 30 ¡C before the device fabrication. Aluminum was
vapor-deposited (JEOL-400 Vacuum Evaporator) through a
mask as the top electrode at a pressure of around 3 ] 10~4
Torr, yielding a 1200 A layer (Thickness Monitor Model TM-
200R, Maxtek Inc.). An active area of the LED device is about
1 mm2. Electrical contacts were Ðxed using a conductive
epoxy 14G adhesive. Sample processing and handling was
done under ambient atmosphere. Electrical and optical char-
acterizations were also carried out under ambient atmosphere.
CurrentÈvoltage (IÈV ) characteristics were measured with a
Keithley 238 High Current Source Measure unit. Voltages are
given as the potential of the ITO electrode with the aluminum
contact grounded; forward bias denotes ITO contact positive.
3
421. 1H NMR (CDCl ) d 8.18È8.17 (m, 2 H), 7.60È7.56 (m, 2
3
H), 7.40È7.38 (m, 2 H), 4.17È4.16 (d, J \ 7.2, 2 H), 3.12 (s, 12
H), 1.85È1.80 (m, 1 H), 1.37È1.20 (m, 8 H), 0.93È0.81 (m, 6
H). 13C NMR (CDCl ) d 172.32, 141.80, 127.01, 125.63,
3
122.10, 120.00, 108.86, 47.57, 39.24, 30.86, 28.66, 24.26, 22.86,
13.89, 13.85, 10.76. Analysis calculated for C
74.07; H, 8.37; N, 9.97. Found: C, 74.05; H, 8.35; N, 9.33.
H
N O : C,
26 35
3
2
N-(2º-ethylhexyl)carbazole-3,6-dicarboxylic acid.16 A mixture
of 6.0 g (0.014 mol) of 3,6-bis(N,N-dimethylcarbamoyl)-9-(2@-
ethylhexyl)carbazole and 40 ml of 20% ethanolic potassium
hydroxide was reÑuxed for 6 h. The solvent was evaporated
and the residue was poured into water, then the solution was
acidiÐed with concentrated hydrochloric acid. The precipitate
was collected by Ðltration and washed with water. Rec-
rystallization from ethanol gave the white powder product.
Yield 70%. Mp 310È313 ¡C. MS: m/z 367. 1H NMR (DMSO-
Materials
Triethylamine was purchased from Aldrich and was re-
distilled prior to use. Carbazole, 2-ethylhexyl bromide, alu-
minum chloride, N,N-dimethylcarbamoyl chloride, ethylene
chloride, thionyl chloride were used as received (Fluka or
Aldrich chemicals).
d ) d 12.66 (s, br, 2 H), 8.87 (s, 2 H), 8.11È8.09 (d, J \ 8.9, 2 H),
6
The detailed synthetic route to the target polymer is
depicted in Scheme 2.
7.70È7.67 (d, J \ 8.7, 2 H), 4.35È4.32 (d, J \ 7.4, 2 H), 2.01È
1.97 (m, 1 H), 1.35È1.14 (m, 8 H), 0.86È0.72 (m, 6 H). 13C
3124
Phys. Chem. Chem. Phys., 1999, 1, 3123È3127

View Article Online
Scheme 2 Routes of synthesis of monomers and polymers.
NMR (DMSO-d ) d 167.68, 143.54, 127.54, 122.62, 122.12,
and was stirred at this temperature for 3 h. After cooling to
room temperature, the pre-polymer was precipitated in meth-
anol and washed with water and ethanol. Further puriÐcation
of the pre-polymer was carried out by dissolving the polymer
in NMP and precipitated in methanol again. Drying under
vacuum at 60 ¡C for 24 h a†orded a high yield (90%) of a
6
121.80, 109.69, 40.28, 38.46, 29.99, 27.83, 23.50, 22.33, 13.64,
10.53. Analysis calculated for C
N, 3.81. Found: C, 72.27; H, 7.00; N, 4.22.
H
NO : C, 71.91; H, 6.86;
22 25
4
N-(2º-ethylhexyl)carbazole-3,6-dicarbonyl chloride (II).16 A
mixture of 1.0 g (2.72 mmol) of N-(2@-ethylhexyl)carbazole-3,6-
dicarboxylic acid and 15 ml of thionyl chloride under nitrogen
was reÑuxed for 8 h. The excess thionyl chloride was removed
by distillation. The residue solid was puriÐed by rec-
rystallization from hexane and dichloromethane to a†ord a
light yellow powder. Yield 79%. Mp 147.0È149.0 ¡C. MS: m/z
polyhydrazide. 1H NMR (DMSO-d ) d 10.92 (s, 2 H), 10.32 (s,
6
2 H), 8.93 (s, br, 2 H), 8.14 (s, br, 2 H), 7.77 (br, 2 H), 7.53 (br, 2
H), 4.41 (br, 2 H), 4.21 (br, 4 H), 2.05 (br, 1 H), 1.86 (br, 4 H),
1.72È1.23 (m, 28 H), 0.88È0.77 (m, 12 H). Analysis calculated
for (C
69.51; H, 7.56; N, 8.61.
H
N O ) : C, 70.68; H, 8.07; N, 8.96. Found: C,
46 63
5 6 n
404. 1H NMR (CDCl ) d 8.91 (s, 2 H), 8.28È8.26 (d, J \ 8.2, 2
3
H), 7.50È7.47 (d, J \ 8.7, 2 H), 4.26È4.23 (d, J \ 7.4, 2 H),
Preparation of polymer PCOPO.13 0.6 g of polyhydrazide
2.10È1.99 (m, 1 H), 1.44È1.22 (m, 8 H), 0.96È0.83 (m, 6 H). 13C
was dispersed in 20 ml of POCl at room temperature. The
NMR (CDCl ) d 167.72, 143.53, 129.98, 125.71, 125.28, 122.80,
3
3
mixture was reÑuxed for 8 h. After cooling to room tem-
109.83, 48.13, 39.35, 30.83, 28.60, 24.27, 22.82, 13.83, 10.73.
perature, the reaction mixture was poured into water. The
precipitate was collected by Ðltration and washed with water,
ethanol and then ether and Ðnally dried under vacuum at
room temperature to a†ord a greenish-yellow solid. The yield
Analysis calculated for C
3.47, Cl, 17.57. Found: C, 65.00; H, 5.79; N, 3.95; Cl, 17.38.
H
Cl NO : C, 65.35; H, 5.69; N,
22 23
2
2
Preparation of the precursor polymer polyhydrazide.13 To a
stirred solution of monomer I (0.424 mmol as a representative)
in N-methylpyrrolidinone (NMP) (20 ml) containing 0.1 g of
LiCl and two equivalents of pyridine (0.070 ml) or tri-
ethylamine (TEA) (0.120 ml) was added monomer II at room
temperature. The reaction mixture was then heated to 80 ¡C
is 83%. 1H NMR (CDCl : TFA-d \ 20 : 1): d 8.9 (br, 2 H),
3
7.8 (br, 2 H), 7.1 (br, 2 H), 6.9 (br, 2 H), 4.40 (br, 4 H), 3.87 (br,
2 H), 1.83È0.56 (br, 43 H) ppm. Analysis calculated for
(C
H
N O ) : C, 74.26; H, 7.72; N, 9.42. Found: C, 74.01;
46 57
5
4 n
H, 7.31; N, 9.09.
Phys. Chem. Chem. Phys., 1999, 1, 3123È3127
3125

View Article Online
Results and discussion
The polymer is synthesized by the polycondensation method
as reported in our previously published papers.13,14 The di-
substituted aromatic hydrazine reacts with the bischlorocarb-
onyl of N-(2@-ethylhexyl)-carbazole in the presence of pyridine
or triethylamine as a base, which acts as an acid absorbent
reagent to promote the polymerization. N-(2@-ethylhexyl)-car-
bazole was synthesized using a modiÐed method reported by
Lee et al.15 We tried several times by using K CO as the
2
3
base in acetone to synthesize the N-alkylated carbazole,
however, only 60% of yield can be obtained after a long time
of reaction (3 d). By changing the base to KOH with ethanol
as the solvent, we obtained the product with a relatively
higher yield (73%) after only 3 h of reaction. Obviously, using
a strong base in this step can accelerate the alkylation reac-
tion and can also improve the yield. Other materials were suc-
cessfully synthesized according to the literature cited.
Fig. 2 UV-Vis spectra and Ñuorescence spectra of the polymer in
solution and as Ðlms.
The synthesized pre-polymer is light yellow in color and
can be dissolved in DMSO, NMP, and DMF. Cyclo-
dehydration of the polyhydrazide to the Ðnal poly-oxadiazole
is little di†erence in the conformations of the polymer in the
two states.17 However, the emission spectra of the polymer in
solution and as solid Ðlms are quite di†erent. In comparison
with its solution emission peak at 448 nm, the main emission
peak in the solid Ðlms shifted about 25 nm towards longer
wavelength. This may be due to the intrachain and/or inter-
chain mobility of the excitons and excimers generated in the
polymer in the solid state.
was performed by employing POCl as both the solvent and
3
the dehydrating reagent. The Ðnal polymer was obtained as a
greenish-yellow solid. The polymer is partially soluble in
common organic solvents such as chloroform, tetra-
hydrofuran, xylene, and DMSO. However, it is readily dis-
solved in chloroform with a little amount of TFA. The
complete conversion of pre-polymer to Ðnal polyoxadiazole
can be conÐrmed by FT-IR. The FT-IR spectra are shown in
Fig. 1. Both the absorption peak at 1620 cm~1 owing to the
carbonyl group and the broad absorption peak in the range of
3300 cm~1 due to the amide groups of the polyhydrazide dis-
Cyclic voltammetry (CV) was employed to investigate the
redox behavior of the polymer and to estimate the HOMO
and LUMO energy levels of the polymer.17 The polymer Ðlm
on Pt electrode was scanned anodically and cathodically
separately in an acetonitrile solution of n-Bu NClO . Fig. 3
4
4
appeared after treatment with POCl , while a new peak at
depicts the CÈV spectra of both p-doping and n-doping pro-
cesses.
3
1540 cm~1 attributed to the C2N in the oxadiazole ring
clearly appeared in the spectra of the Ðnal polymer. These
results indicate that the cyclodehydration reaction was com-
pleted.14 It is well known that high purity of electrolumines-
cent materials is a crucial factor for good performance in
PLED devices. Impurities existing in the emission and/or
transporting layers will quench the excitons, which will lower
the electroluminescent quantum efficiency. During conversion
of the pre-polymer to the Ðnal polymer, a complete cyclization
ensures the required purity for PLED application and also
clears up quench sites of carbonyl groups in the Ðnal polymer.
The absorption and photoluminescence (PL) spectra of the
polymer both in a solution of chloroform containing a small
amount of TFA and as thin Ðlms which were prepared by
spin-coating the solution on quartz plate, were measured at
room temperature. The spectra are displayed in Fig. 2. In
solution, the polymer gives a main absorption peak at 398 nm
and a shoulder at 418 nm, while the main absorption peak of
the solid Ðlms of the polymer appears at 418 nm with a shoul-
der at 398 nm. The relatively identical absorption spectra of
the polymer in solution and as solid Ðlms indicates that there
During the cathodic scan, the polymer exhibits a reversible
n-doping process. The cathodic peaks appear at [1.90 V
(vs. SCE) with corresponding anodical peaks at [1.78 V (vs.
SCE). The onset potential of the reduction is [1.74 V (vs.
SCE). The reduction potential is comparable to that of 2-(4-
biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole
(PBD)
([1.95 to [1.94 V vs. SCE),14 which is one of the most
widely used electron-transporting/hole-blocking materials.
The onset potentials of n-doping and p-doping processes can
be used to estimate the HOMO and LUMO energy levels of a
conjugated polymer.18 According to the equations reported
by de Leeuw et al.,18 E
LUMO
] 4.4 eV], where Eox
(onset vs. SCE)
are the onset potentials vs. SCE for the oxidation
\ [Ered
] 4.4 eV] and
(onset vs. SCE)
E
\ [Eox
(onset vs. SCE)
and
HOMO
Ered
(onset vs. SCE)
and reduction processes of a polymer. The onset potentials
were determined from the intersection of the two tangents
drawn at the rising current and base line charging current of
the IÈV curves. The LUMO energy of the polymer is thus
Fig. 3 Cyclic voltammogram of PCOPO coated on Pt electrodes in
acetonitrile containing 0.1 M n-Bu NClO at a scan rate of 50 mV
4
4
Fig. 1 FT-IR spectra of PCOPO and its pre-polymer.
s~1.
3126
Phys. Chem. Chem. Phys., 1999, 1, 3123È3127

View Article Online
device fabrication conditions. Further study on the EL
properties of the polymer is still in progress.
Conclusion
A new conjugated polymer constituted of alternating N-(2@-
ethyl hexyl)carbazole and 1,4-bis(1,3,4-oxadiazole-2-yl)-2,5-
dioctyloxybenzene was synthesized and characterized. Photol-
uminescence spectra of the polymer Ðlms indicate that the
polymer is a blue light emitting material. The LUMO and
HOMO energy levels of the polymer are determined by cyclic
voltammetry. A single layer LED device with stable aluminum
metal as negative electrode was successfully fabricated, sug-
gesting that the new polymer has better balanced injection of
electrons and holes in PLEDs compared to MEHÈPPV, and
thus gives an improved EL quantum efficiency.
Fig. 4 CurrentÈvoltage characteristics of the ITO/polymer PCOPO/
Al device.
determined to be 2.66 eV. This value is almost the same as
that of poly[2-methoxy-5-(2@-ethlhexyloxy)-p-phenylenevinyl-
ene (MEHÈPPV) (2.6 eV) reported by Cervini et al.19 It
implies that the polymer may have similar electron-injection
property with MEHÈPPV when it is used as the emitter in
PLEDs. However, the LUMO energy level of 2.66 eV is
smaller than those of poly(cyanoterephthalylidene) (CNÈPPV)
(3.02 eV) and some poly(aromatic oxadiazole)s (2.8È2.9 eV),19
which all exhibit good electron-injection properties.
When we scanned the polymer Ðlms anodically, the
polymer showed a partially reversible anodic peak at 1.69 V
with a cathodic peak at 1.56 V (vs. SCE). The onset potential
was determined to be 1.20 V, so that the HOMO energy level
could be estimated to be 5.60 eV. This value is almost the
same as that of CNÈPPV (5.55 eV), but is bigger than that of
MEHÈPPV (4.87 eV).19 This means that the polymer has
similar hole-injection ability with CNÈPPV when it is used in
PLEDs, but the hole-injection ability is poorer than that of
MEHÈPPV.
In order to increase the quantum efficiency of PLEDs, it is
necessary to balance the injection rates of electrons and holes
into the polymer layers from the opposite contacts. However,
most of the current existing EL polymers, e.g., MEHÈPPV,
the best soluble EL polymer so far, are more favorable for
hole injection when a stable metal, e.g. aluminum, is used as
the cathode. The new blue EL polymer has a similar LUMO
but a higher HOMO energy level when compared to MEHÈ
PPV. Therefore, it could be expected to give an improved EL
quantum efficiency when this polymer is used to fabricate
single-layer PLEDs using stable metals as cathodes.
A light-emitting diode was successfully fabricated with
polymer PCOPO as active material. The stable electrode alu-
minum metal was used as the top electrode. Under a bias in
the forward direction, the single-layer diode begins to emit
visible blue light (observable in a dark room) at about 8 V
with a current density of 1.14 mA cm~2. The currentÈvoltage
(IÈV ) characteristics of the ITO/PCOPO/Al device is dis-
played in Fig. 4, showing a typical diode character. When the
forward bias increases, both the current and the emitting light
intensity increase rapidly after 6 V. The preliminary results
show that the new polymer could be a potential novel active
material to be used in PLEDs. Because the performance and
characterization of the fabricated PLED were measured in
ambient conditions, higher performance PLEDs made out of
this new polymer could be expected under the optimized
References
1
2
3
(a) N. C. Greenham, S. C. Moratti, D. D. C. Bradley, R. H.
Friend and A. B. Holmes, Nature (L ondon), 1993, 365, 628; (b) A.
Kraft, A. C. Grimsdale and A. B. Holmes, Angew. Chem. Int. Ed.
Engl., 1998, 37, 402.
(a) F. Garten, A. Hilberer, F. Cacialli, E. Esselink, Y. vanDam, B.
Schlatmann, R. H. Friend, T. M. Klapwijk and G. Hadziioannou,
Adv. Mater., 1997, 9, 127; (b) B. R. Hsieh, Y. Yu, E. W. Forsythe,
G. M. Schaaf and W. A. Feld, J. Am. Chem. Soc., 1998, 120, 231.
(a) M. Granstrom, M. Berggren and O. Inganas, Science, 1995,
267, 1479; (b) G. Leising, S. Tasch, F. Meghdadi, L. Athouel, G.
Froyer and U. Scherf, Synth. Met., 1996, 81, 185.
M. Ranger, D. Rondeau and M. Leclerc, Macromolecules, 1997,
30, 7686.
J. Kido, K. Hongawa, K. Okuyama and K. Nagai, Appl. Phys.
L ett., 1993, 63, 2627.
M. Berggren, O. Inganas, G. Gustafsson, J. Rasmusson, M. R.
Andersson, T. Hjertberg and O. Wennerstrom, Nature (L ondon),
1994, 372, 444.
W. Huang, H. Meng, W.-L Yu, J. Gao and A. J. Heeger, Adv.
Mater., 1998, 10, 593.
X. T. Tao, Y. D. Zhang, T. Wada, H. Sasabe, H. Suzuki, T. Wata-
nabe and S. Miyata, Adv. Mater., 1998, 10, 226.
V. Rani and K. S. V. Santhanam, J. Solid State Electrochem.,
1998, 2, 99.
4
5
6
7
8
9
10 J. Bettenhausen, M. Greczmiel, M. Jandke and P. Strohriegl,
Synth. Met., 1997, 91, 223.
11 D. E. Fogg, L. H. Radzilowski, B. O. Dabbousi, R. R. Schrock, E.
L. Thomas and M. G. Bawendi, Macromolecules, 1997, 30, 8433.
12 W.-L. Yu, H. Meng, J. Pei, Y.-H. Lai, S. J. Chua and W. Huang,
Chem. Commun., 1998, 18, 1957.
13 (a) W.-L. Yu, H. Meng, J. Pei, W. Huang, Y.-F. Li and A. J.
Heeger, Macromolecules, 1998, 31, 4838; (b). W. Huang, H. Meng,
W.-L. Yu, J. Pei, Z.-K. Chen and Y.-H. Lai, Macromolecules,
1999, 32, 118.
14 (a) W.-L. Yu, H. Meng, J. Pei and W. Huang, J. Am. Chem. Soc.,
1998, 120, 11808; (b) W. Huang, W.-L. Yu, H. Meng, J. Pei and S.
F. Y. Li, Chem. Mater., 1998, 10, 3340.
15 J. H. Lee, J. W. Park and S. K. Choi, Synth. Met., 1997, 88, 31.
16 G. A. Sotzing, J. L. Reddinger, A. R. Katritzky, J. Soloducho, R.
Musgrave and J. R. Reynolds, Chem. Mater., 1997, 9, 1578.
17 T. Benazzi, D. Ades, A. Siove, D. Romero, F. Nuesch and L.
Zuppiroli, J. Chim. Phys. Phys.-Chim. Biol., 1998, 95, 1238.
18 D. M. de Leeuw, M. M. J. Simenon, A. B. Brown and R. E. F.
Einerhand, Synth. Met., 1997, 87, 53.
19 R. Cervini, X. Li, G. W. C. Spencer, A. B. Holmes, S. C. Moratti
and R. H. Friend, Synth. Met., 1997, 84, 359.
Paper 9/03215K
Phys. Chem. Chem. Phys., 1999, 1, 3123È3127
3127