Synthesis and electro-optical properties of carbazole containing copolymers with different conjugated structures for polymer light-emitting devices

Article abstract of DOI:10.1080/15421400802620030

New copolymers such as Poly(9-(4-(2-ethyl-hexyloxy)-3,5-dimethylphenyl) carbazole-alt-aminonaphtalene) [P1] and Poly(4-(bis-methylbiphenyl-alt-9H- carbazol-9-yl)benzenamine) [P2] with chromophores separated by non-conjugated methylene spacers, and Poly(4-

Full text of DOI:10.1080/15421400802620030

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Synthesis and Electro-Optical
Properties of Carbazole
Containing Copolymers
with Different Conjugated
Structures for Polymer Light-
Emitting Devices
Jin Woo Kim a , Eun Jung Park a , Eon Joo Lyu a
,
Yun-Su Lee b , Younghwan Kwon c & Lee Soon Park a
a Department of Polymer Science , Kyungpook
National University , Daegu, Korea
b Advanced Display Manufacturing Research Center,
Kyungpook National University , Daegu, Korea
c Department of Chemical Engineering , Daegu
University , Gyeongsan, Gyeongbuk, Korea
Published online: 18 Mar 2009.
To cite this article: Jin Woo Kim , Eun Jung Park , Eon Joo Lyu , Yun-Su Lee ,
Younghwan Kwon & Lee Soon Park (2009) Synthesis and Electro-Optical Properties of
Carbazole Containing Copolymers with Different Conjugated Structures for Polymer
Light-Emitting Devices, Molecular Crystals and Liquid Crystals, 499:1, 112/[434]-127/
[449], DOI: 10.1080/15421400802620030
To link to this article: http://dx.doi.org/10.1080/15421400802620030
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Mol. Cryst. Liq. Cryst., Vol. 499, pp. 112=[434]–127=[449], 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802620030
Synthesis and Electro-Optical Properties of Carbazole
Containing Copolymers with Different Conjugated
Structures for Polymer Light-Emitting Devices
Jin Woo Kim1, Eun Jung Park1, Eon Joo Lyu1,
Yun-Su Lee2, Younghwan Kwon3, and Lee Soon Park1
1Department of Polymer Science, Kyungpook National University,
Daegu, Korea
2Advanced Display Manufacturing Research Center, Kyungpook
National University, Daegu, Korea
3Department of Chemical Engineering, Daegu University, Gyeongsan,
Gyeongbuk, Korea
New copolymers such as Poly(9-(4-(2-ethyl-hexyloxy)-3,5-dimethylphenyl)carbazole-
alt-aminonaphtalene) [P1] and Poly(4-(bis-methylbiphenyl-alt-9H-carbazol-9-
yl)benzenamine) [P2] with chromophores separated by non-conjugated methylene
spacers, and Poly(4-(dioctylfluorene-alt-9H-carbazole-9yl)benzenamine) [P3] based
on alternating fluorene and amine units in the polymer backbone were synthesized
by palladium catalyzed polycondensation reaction. Optical properties of these poly-
mers were characterized in terms of UV=visible absorption and photoluminescence
(PL) emission in solution and film state. Maximum PL emission (kmax,PL) of synthe-
sized copolymers were observed in the blue region of 426 nm–460nm in solution
state, but the three polymers all showed large red shift in film state with the green
emission. The electro-optical properties of these polymers in polymer light-emitting
diode (PLED) devices were investigated from the viewpoint of utilizing the new
polymers as emitting layer and hole injection=transport layer. The polymer P3
was found to function both as hole injection and hole transport layer with perfor-
mance better than that of well known PEDOT:PSS hole injection layer material.
Keywords: amine; carbazole; fluorine; non-conjugated spacer; palladium-catalyzed
polycondensation reaction; polymer light-emitting diode
This work was supported by the Advanced Display Manufacturing Research Center
(ADMRC) Program of the Ministry of Commerce, Industry and Energy of Korea.
Address correspondence to Prof. Lee Soon Park, Department of Polymer Science,
Kyungpook National University, Sangyuk-dong, Buk-gu, Daegu 702-701, Korea (ROK).
E-mail: lspark@knu.ac.kr
112=[434]

Carbazole Containing Copolymers for Polymer Light-Emitting Devices
113=[435]
INTRODUCTION
Organic light-emitting diodes (OLEDs) and polymeric light-emitting
diodes (PLEDs) are currently the subject of an intense research effort
due to their promise as devices for new full-color flat panel display and
luminating applications [1,2]. Electroluminescent conjugated poly-
mers have the possibility of uniformly coating various layers in OLED
by low cost solution process in addition to the ability of fine-tuning
optical and electrical properties by varying their structures [3,4].
Recently increased interest has been paid to carbazole- or fluorene-
based polymers for PLED devices. Polymers containing carbazole
moiety have been studied widely because they have a good charge –
transporting property, wide band gap, and blue emission [5]. The
introduction of charge-transporting group into the p-conjugated
polymers has also been the subject of active research [6,7]. Several
polymers containing an arylamine group were synthesized by Suzuki
coupling and Witting-Horner reaction for the improvement of hole-
transport property [8–11].
In this paper, we synthesized some new polymers with aromatic
amine repeat unit and carbazole moiety in a non-conjugated structure
for the blue emitting layer with good charge transporting property. We
also synthesized new conjugated copolymer based on alternating fluor-
ene and arylamine units by using Pd-catalyzed polycondensation reac-
tion for investigating the effects of the incorporation of aromatic
amines into p-conjugated copolymer system. These polymers were
characterized in terms of their UV=visible and photoluminescence
(PL) properties in solution and solid film state. The effect of the poly-
mer structure on the electro-optical property of OLED devices was
examined from the viewpoint of utilizing the new polymers as emitting
layer and hole injection=transport layer.
EXPERIMENTAL
Synthesis of Monomers
9-(4-(2-Ethylhexyloxy)-3,5-dibromomethylphenyl)carbazole
(monomer 1)
Was synthesized according to the previously reported process [12–
14]. As shown in Scheme 1(a), 4-bromo-2,6-dimethylphenol (0.02 mol)
in 50 ml of acetonitrile was added to a suspension of potassium hydrox-
ide (0.06 mol) in 200 ml of acetonitrile which was prepared by vigorous
stirring for 30 min under nitrogen. 2-Ethylhexyl bromide (0.02 mol)
was added to the solution, followed by reflux for 48 hr. Potassium

114=[436]
J. W. Kim et al.
SCHEME 1 Synthesis of the monomers.
hydroxide was filtered out, and the solvent was evaporated, then the
crude product was washed with water and dried with magnesium sul-
fate. The orange color intermediate, 5-bromo-2-(2-ethylhexyloxy)-1,3-
dimethylbenzenewas obtainedby silicacolumn chromatography (eluent:
hexane) Yield: 60%. 1H NMR (CDCl3), d: 7.15 (2H, Ar-H), 3.6 (2H,
-O-CH2-), 2.26 (6H, Ar-CH3), 2.18 (9H, -CH-, -CH2), 0.91 (6H, -CH3).
In the second stop, 5-bromo-2(2-ethylhexyloxyl)-1,3-dimethylben-
zene (112.79 mmol), carbazole (15.25 mmol), K2PO4 (26.72 mmol),
CuI (0.64 mmol) and trans-1,2-cyclohexane diamine (2.54 mmol) were
used to obtain a white solid product, 9-(4-(2-ethylhexyloxyl)-3,5-
dimethyl-phenyl)carbazole according to the Ullmann reaction. Yield:
30%, 1H NMR (CDCl3), d: 7.38 (4H, Ar-H), 7.26 (4H, Ar-H), 7.16
(2H, Ar-H), 3.6 (2H, -O-CH2-), 2.30 (6H, Ar-CH3), 2.18 (9H, -CH-,
-CH2), 0.91 (6H, -CH3).
In the third step
a mixture of 9-(4-(2-ethylhexyloxyl)-3,5-
dimethyl-phenyl)carbazole (3.68 mmol), NBS (8.09 mmol) and BPO
(0.1618 mmol) were added to CCl4 at room temperature. The following
reaction and workup were carried out as reported previously [13,14].
The orange liquid product, N-4-(2-ethylhexyloxyl)-3,5-dibromomethyl-
phenyl)-carbazole was obtained in 50% yield. 1H NMR (CDCl3), d: 7.48

Carbazole Containing Copolymers for Polymer Light-Emitting Devices
115=[437]
(4H, Ar-H), 7.45 (4H, Ar-H), 7.35 (2H, Ar-H), 4.6 (4H, CH2Br-), 3.96
(2H, -O-CH2-), 2.25 (9H, -CH-, -CH2), 0.91 (6H, -CH3).
N-(4-aminophenyl)-carbazole (monomer 2)
A 250 ml, round-bottom flak was equipped with a reflux condenser
and was then flushed with dry nitrogen. 4-Iodonitrobenzene
(40 mmol), carbazole (30.6 mmol) and potassium phosphate (84 mmol)
were added to 150 ml of 1,4-dioxane at room temperature. After stir-
ring for 40 min, CuI (0.8 mmol) and trans-1,2-cyclohexanediamine
(4.8 mmol) were added to the mixture. The reaction mixure was stirred
for 24 hr at refluxed temperature. After cooling down to room tempera-
ture, the resulting mixture was filtered. 1,4-Dioxane was evaporated,
and ethyl acetate was added. The mixture was washed with distilled
water, and dried with MgSO4. N-(4-nitrophenyl)-carbazole was obta-
tined through column chromatography (hexane:ethylacetate ¼ 20:1)
with 20% of yield.1H NMR (CDCl3), d: 8.58 (1H, Ar-H), 8.55 (1H,
Ar-H), 8.26 (1H, Ar-H), 8.23 (1H, Ar-H), 8.01 (1H, Ar-H), 7.98 (1H,
Ar-H), 7.60 (1H, Ar-H), 7.58 (1H, Ar-H), 7.51 ꢀ 7.45 (2H, Ar-H),
7.39 ꢀ 7.33 (2H, Ar-H).
The
synthesized
intermediate,
N-(4-nitrophenyl)-carbazole
(NPHCZ) (6.9 mmol), SnCl2 (42.1 mmol) and EtOH (25 ml) was heated
at 70ꢁC for 4 hr 30 min. After the mixture was cooled to room tempera-
ture, 1 M NaOH solution was added until the mixture became basic.
After extraction with ethyl acetate, the combined organic layers was
washed and treated with MgSO4 and then dried in vacuum. The deep
1
brown product, monomer 2, was obtained with 65% yield. H NMR
(CDCl3), d: 8.15 (1H, Ar-H), 8.12 (1H, Ar-H), 7.42 ꢀ 7.22 (8H, Ar-H),
6.87 (1H, Ar-H), 7.84 (1H, Ar-H), 3.81 (2H, Ar-NH2).
Polymerization
Three new polymers were synthesized by palladium-catalyzed
polycondensation reaction. A mixture of monomer 1 (3.6 mmol) and
1-naphtylamine (3.6 mmol) was dissolved in toluene (40 ml). Catalysts
consisting of NaO-t-Bu (10.8 mmol), Pd2(dba)3(0.09 mmol), P(t-Bu)3
(0.55 mmol) was added to the monomer solution at room temperature
and then stirred at 100ꢁC for 48 hr. After cooling to room temperature,
the mixture was quenched by adding aqueous ammonia (100 ml) and
the product was extracted with chloroform solvent. The organic
fraction was concentrated and reprecipitated from CHCl3=methanol
several times. Then product [P1] was filtered and dried in
vacuum. Poly(4-(bismethylbiphenyl-alt-9H-carbazol-9-yl)benzenamine)
[P2] was obtained from monomer 2 (CYBZ) (4.4 mmol) and 4,40-bis

116=[438]
J. W. Kim et al.
(bromomethyl)biphenyl (4.4 mmol) by same reaction as polymer P1.
Poly(9,9-dioctyl-fluorene-alt-9H-carbazol-9-yl)benzenamine) [P3] obtained
from monomer 2 (CYBZ) (3.8 mmol) and dioctylfluorene (3.8 mmol) by
same reaction as shown in Scheme 2 [15].
Fabrication of PLED
PLEDs with three different configurations (1) ITO=PEDOT:PSS=
polymer P1, P2, or P3 (EML)=BCP=Alq3=LiF=Al, (2) ITO=PEDOT:
PSS=polymer P1, P2, or P3 (HTL)=Alq3=LiF=Al and (3) ITO=polymer
P1, P2, or P3 (HIL)=Alq3=LiF=Al were fabricated by spin coating and
thermal evaporation method by using OLED device fabrication system
(Sunic Plus 200).
Measurements
Infrared spectra were obtained with a JASCO FT-IR 620 spectrometer
and 1H-NMR spectra was recorded on a Bruker 400 NMR spec-
trometer. The average molecular weight (Mw) of the polymers were
evaluated using gel permeation chromatography (GPC, Waters,
SCHEME 2 Synthesis of the polymers.

Carbazole Containing Copolymers for Polymer Light-Emitting Devices
117=[439]
alliance 2000, THF, polystyrene universal calibration) at room
temperature. UV-visible absorption spectra and photoluminescence
spectra of synthesized product were measured on a JASCO V-650
spectrometer and JASCO FP-6500 fluorescence spectrometer, res-
pectively. The ionization potential (IP) of polymers was obtained
using atmospheric photoelectron spectroscopy (Riken Keiki AC-2).
Electroluminescence (EL) spectra and color coordinates were mea-
sured by using Spectroscan PR 650 (Photoresearch Inc.). Current
and luminance vs. voltage profiles were obtained by using a dc
power supply connected with model 8092 A digital multimeter (Hyun
Chang Product Co. Ltd) and luminance meter (Minolta LS-100),
respectively.
RESULTS AND DISCUSSION
Synthesis and Characterization
Two monomers, 9-(4-(2-ethylhexyloxy)-3,5-dibromomethylphenyl)
carbazole [monomer 1] and N-(4-aminophenyl)-carbazole [monomer
2] were synthesized through several steps with a moderate yield as
presented in Scheme 1. The polymers (P1 and P2) with carbazole ring
and aromatic amine containing methylene spacer unit, and polymer
(P3) based on fluorene and aromatic amine unit in the polymer back-
bone were obtained in good yield by using Pd-catalyzed polycondensa-
tion as shown in Scheme 2. FT-IR spectra of polymer P1 and P2
exhibited strong absorption peaks at 1230 cmꢂ1 coming from tertiary
aliphatic amine in addition to the peak at 1315 cmꢂ1 due to tertiary
aromatic amine. Polymer P3 showed only tertiary aromatic amine
absorption peak at 1307 cmꢂ1. The carbazole and methylene groups
in P1 and P2 polymers and fluorene group in P3 were also confirmed
in the FT-IR spectra.
All polymers showed increased solubility in common organic
solvents, due to the presence of long alkyl groups such as 2-ethylhexyl
and octyl groups. They were found to be soluble in chloroform,
tetrachloroethane, toluene and THF. The weight-average molecular
weights (Mw) of synthesized polymers, P1, P2 and P3, determined
by GPC using polystyrene as the standards, were summarized in
Table 1. As shown in Table 1 three polymers have relatively low
molecular weights, which might be caused by steric effect of twisted
conformation between bulky carbazole and aryl groups. Copolymers,
however, were easily spin coated onto the ITO substrate, and were
found to produce transparent and homogeneous thin film.

118=[440]

Carbazole Containing Copolymers for Polymer Light-Emitting Devices
119=[441]
Optical and Electrochemical Properties
Figures 1(a) and (b) exhibit UV=visible absorption and PL emission
spectra of the synthesized polymers in dilute 1,1,2,2-tetrachloroethane
(TCE) solution, respectively. As shown in Figure 1(a), UV=visible
absorption maxima (kmax,UV) of copolymers, P1 and P2, with non-
conjugated spacer appeared at 293, 342 nm and 292, 341 nm, respecti-
vely. On the other hands, the polymer P3 exhibited maximum absorp-
tion peak at 357 nm due to extended conjugation. The red shifted
absorption peak of P3, compared to polymer P1 and P2 also implied
that the band gap of P1 and P2 is larger than that of P3 polymer.
When excited at their kmax,UV, polymers P1, P2, and P3 showed
maximum PL emission peaks (kmax,PL) in the blue region of 460 nm,
436 nm and 426 nm, respectively. Polymer P1 and P2 showed large
Stoke’s shift compared to P3, which may be explained by the increased
transfer in the vibrational energy levels owing to free rotations
through the single bonds provided by the methylene spacers. All of
polymers exhibited longer PL emission peak in the film state than in
solution, as shown in Figures 2 (a) and (b). In general, p-conjugated
polymers exhibit increased full width at high maximum(FWHM) due
to the stacked p-aggregates in solid film compared to the dilute
solution [16]. Such aggregation leads to red-shifted emissions with
low quantum efficiencies. As shown in Figure 2(a), P2 exhibited large
red shift in film state due to excimer cluster or aggregate formation.
This might be originated from less sterically hindered simple repeating
units of P2 , compared to P1 and P3 polymers. The extent of red shift
in PL emission of P3 was a little smaller than P1 and P2 polymers.
This small red shift and single PL peak of P3 may be due to the pres-
ence of bulky fluorene repeat unit in the polymer chain. This may also
led to higher quantum efficiency than other polymers, as shown in
Table 1. The electro-optical and other characterization data of poly-
mers are summarized in Table 1. As shown in Figure 3, HOMO energy
levels of P1, P2, and P3 were estimated to be 5.30, 5.39 and 5.33 eV,
and LUMO energy levels were determined to be 1.98, 2.33 and
2.45 eV, respectively. This indicates that the optical band gap of the
polymer P1 with non-conjugated segment is larger than that of P2
or P3 polymer. It is also noted from Figure 3 that P2 has lower energy
barrier for transporting holes from HIL or ITO than P1 or P3. This
property is discussed in terms of performance of OLED devices.
Electroluminescent Properties
To investigate EL properties of the polymers, we first fabricated
multilayered PLED devices with ITO=PEDOT:PSS=Polymer (P1, P2,

120=[442]
J. W. Kim et al.
FIGURE 1 UV-Vis and PL spectra in TCE solution; (a) P1 and P2 polymer
(b) P3 polymer.

Carbazole Containing Copolymers for Polymer Light-Emitting Devices
121=[443]
FIGURE 2 PL spectra of the polymers with fluorene and aromatic amine in
Solid film state.

122=[444]
J. W. Kim et al.
FIGURE 3 The energy band gap of synthesized polymers.
or P3)=BCP=Alq3=LiF=Al configuration by using the synthesized
polymers as light emitting layer (EML) as shown in Figure 4. When
P1 and P2 were used as EML, the resulting PLED exhibited very
low light emission due to the presence of methylene spacer in the poly-
mer chain which may result in low charge (electron) transport. P3
exhibited a little luminance of 16 cd=m2 at 23 V and maximum EL
peak at 568 nm. This may be owing to the lower LUMO level of P3
compared to polymer P1 and P2, inducing electron injection into the
emitting layer.
Then we investigated the hole injection and transporting property
of the synthesized polymers. Since the HOMO energy levels of the
polymers were very close to that of PEDOT:PSS, well known hole
injection material. We fabricated multilayered PLED devices with
ITO=PEDOT: PSS=polymer (P1, P2, or P3 )=Alq3=LiF=Al configur-
ation as shown in Figure 5. In this PLED devices Alq3 was used as
an emitting layer, PEDOT:PSS as hole injection layer (HIL) and
synthesized polymer as an hole transport layer (HTL). For compari-
son, a typical device without the synthesized polymer (HTL) was
made as a reference. The devices showed EL emission of Alq3 in
the range from 528 to 532 nm, as shown in Figure 5(a). I-V-L profiles
of the PLED devices were also shown in Figure 5(b). The turn on vol-
tages of the devices containing P1, P2, and P3 as HTL were 11 V, 9 V
and 10 V, respectively. The device fabricated with P3 as HTL resulted

Carbazole Containing Copolymers for Polymer Light-Emitting Devices
123=[445]
FIGURE 4 (a) EL spectra of Polymer P1, P2, and P3 (b) IVL characteristics
of Polymer P1, P2, and P3 as a EML material in the multilayered PLEDs.

124=[446]
J. W. Kim et al.
FIGURE 5 (a) EL spectra of Polymer P1, P2, and P3 (b) IVL characteristics
of Polymer P1, P2, and P3 as a HTL interlayer as compared reference devices
in the multilayered PLEDs.

Carbazole Containing Copolymers for Polymer Light-Emitting Devices
125=[447]
FIGURE 6 (a) EL spectra of Polymer P1, P2, and P3 (b) IVL characteristics
of Polymer P1, P2, and P3 as a HIL interlayer without PEDOT=PSS layer in
the multilayered PLEDs.

126=[448]
J. W. Kim et al.
in much better EL performance with 1,074 cd=m2 at 18 V than the
devices containing P1 (130 cd=m2, 20 V) and P2 (466 cd=m2, 21 V)
with non-conjugated segments. The reference PLED with ITO=
PEDOT:PSS=Alq3=LiF=Al configuration exhibited maximum lumi-
nance of 364 cd=m2 at 8 V. Thus it may be concluded that the
combination of PEDOT:PSS(HIL)=polymer P3 (HTL) increased
the recombination of holes and electrons in the Alq3 light emitting
layer with proper energy level and balanced charge carrier
mobilities.
We also fabricated devices with ITO=polymer (P1, P2, or P3)=
Alq3=LiF=Al configuration in order to replace typical HIL material,
PEDOT:PSS, with the synthesized polymers. As shown in Figure 6(a),
Alq3 emission was observed at the usual emission peak. The maximum
luminescence of devices fabricated with P1 or P2 both as hole injection
and transport layer were 99 cd=m2 at 21 V and 488 cd=m2 at 21 V, com-
pared to 364 cd=m2 at 8 V in the PLED device containing only PED-
OT:PSS as HIL=HTL layer. On the other hand, the device fabricated
with P3 as HIL=HTL exhibited 976 cd=m2 of maximum lumine-
scence at 18 V. It was of interest to note that polymer P3 could perform
both as hole injection and hole transport layer and also the com-
bined performance of P3 as HIL=HTL was better than that of PEDOT=
PSS which is usually used as hole injection layer in the PLED
devices.
CONCLUSIONS
New
copolymers
such
as
Poly(9-(4-(2-ethyl-hexyloxy)-3,
5-dimethylphenyl)carbazole-alt-amino naphtalene) [P1] , Poly(4-(bis-
methylbiphenyl-alt-9H-carbazol-9-yl)benzenamine) [P2] and Poly(4-
(dioctylfluorene-alt-9H-carbazole-9yl)benzenamine)[P3]wereobtained
by using Pd-catalyzed polycondensation reaction. The synthesized
copolymers were soluble in common organic solvents and showed good
film forming property. These polymers showed kmax, PL value in the
blue emission region at 460 nm, 436 nm and 426 nm, respectively, in
solution state. The three polymers exhibited large red shift in the film
state by the formation of p-stacking structures. The polymers were not
useful as light emitting layer materials, especially polymer P1 and P2
which had non-conjugated spacers. The synthesized polymers
especially P3 were found to function as both hole injection and hole
transport material and the combined performance of P3 as HIL=HTL
HTL was higher than that of PEDOT:PSS mainly used as HIL in the
PLED devices.

Carbazole Containing Copolymers for Polymer Light-Emitting Devices
127=[449]
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