Synthesis of charge transporting block copolymers containing 2,7-dimethoxycarbazole units for light emitting device

Article abstract of DOI:10.1016/j.polymer.2009.12.024

The block copolymers consisting of 2,7-dimethoxycarbazole- and oxadiazole-containing segments as hole and electron transporting units, respectively, were synthesized by NMRP manner. OLED devices were fabricated using block copolymer, random copolymer, and polymer blend for matrix of the emitting layer with Ir(ppy)₃ as a phosphorescent dopant in order to investigate morphological effect on the performance. From the finding that the block copolymer system overwhelmed the others in EQE, we assumed that a morphology with dimethoxycarbazole units assembled to the surface of PEDOT:PSS played a considerable role for effective recombination of charges as well as sufficient charge injection into the emitting layer.

Full text of DOI:10.1016/j.polymer.2009.12.024

Polymer 51 (2010) 616–622
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis of charge transporting block copolymers containing
2,7-dimethoxycarbazole units for light emitting device
Kousuke Tsuchiya a, Kota Sakaguchi a, Hidemasa Kasuga a, Akira Kawakami b, Hideo Taka b,
,
Hiroshi Kita b, Kenji Ogino a
*
a Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei-shi, Tokyo 184-8588, Japan
b Display Technology R&D Laboratories, Konica Minolta Technology center inc., 2970 Ishikawa-machi, Hachioji-shi, Tokyo 192-8505, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The block copolymers consisting of 2,7-dimethoxycarbazole- and oxadiazole-containing segments as
hole and electron transporting units, respectively, were synthesized by NMRP manner. OLED devices
were fabricated using block copolymer, random copolymer, and polymer blend for matrix of the emitting
layer with Ir(ppy)3 as a phosphorescent dopant in order to investigate morphological effect on the
performance. From the finding that the block copolymer system overwhelmed the others in EQE, we
assumed that a morphology with dimethoxycarbazole units assembled to the surface of PEDOT:PSS
played a considerable role for effective recombination of charges as well as sufficient charge injection
into the emitting layer.
Received 26 August 2009
Received in revised form
18 December 2009
Accepted 21 December 2009
Available online 4 January 2010
Keywords:
Block copolymer
Polymer light emitting diode
Nitroxide mediated radical polymerization
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
layer, resulting in high EQE compared to the random copolymer
systems without such a structure [11–13].
Numerous efforts on investigation for light emitting materials
have been done for decades, thus the market for organic light
emitting diode (OLED) applications has been getting larger and
larger [1]. Polymeric light emitting materials have been attracted
much attention because of good suitability for easy processing such
as spin-coating and ink-jetting techniques. Nowadays, phospho-
rescent system dispersing a dopant such as Ir complex in polymer
matrix prevails over other polymeric devices in external quantum
efficiency (EQE) [2–6], but still remains behind compared with
multi-layer organic small molecular devices fabricated by chemical
vapor deposition. Single-layer polymer OLED device created by wet
process can be an excellent candidate for mass production of
devices, if the EQE is improved.
Copolymer or polymer blend systems consisting of several
functional units such as hole and electron transporting components
have been studied by many researchers [7–10]. In recent works, it
was demonstrated that the device performance based on bipolar
charge transporting block copolymers was significantly influenced
by phase-separated morphology in the polymer layer, and that the
nanophase separation of block copolymers facilitated charges
recombination due to lowering charge transport in the emitting
In our previous work, block copolymers comprising triphenyl-
amine and fluorinated oxadiazole units as hole and electron
transport segments, respectively, were prepared to construct
pseudo-layered structure by assembling oxadiazole segment into
the air surface, which we assumed that achieve sufficient charge
recombination as seen in multi-layered organic devices [14].
However, introduction of trifluoromethyl groups made the highest
occupied molecular orbital (HOMO) level lower, which hamper
effective charges recombination in the emitting layer because of
poor hole confinement in spite of hole blocking layer. Here, we
describe the synthesis and device evaluation of novel block
copolymers including 2,7-dimethoxycarbazole units as hole
transport component. Methoxy groups are expected to assist the
self-assembly of hole transporting segment near poly(3,4-ethyl-
enedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) layer by
hydrogen bonding with sulfonate groups [15,16].
2. Experimental section
2.1. Materials
Xylene and o-dichlorobenzene were distilled over calcium
hydride and stored under nitrogen. Tetrahydrofuran (THF) and
diethyl ether were used as distilled over sodium and
* Corresponding author. Tel./fax: þ81 42 388 7404.
E-mail address: kogino@cc.tuat.ac.jp (K. Ogino).
0032-3861/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2009.12.024

K. Tsuchiya et al. / Polymer 51 (2010) 616–622
617
benzophenone. As an initiator, N-(1-phenylethoxy)-N-(2-methyl-
1-phenylpropyl)-t-butylamine (10) was prepared according to
literature [17]. The other reagents were used as received.
2.4. Synthesis of 2,7-dimethoxycarbazole (3)
To two-necked flask equipped with
a
a
stopcock and
a condenser were added 2 (46.5 g, 0.179 mol) and triethyl phosphite
(135 mL, 0.788 mol) under nitrogen atmosphere, and the mixture
was refluxed at 170 ꢀC for 4 h. After cooling down to 0 ꢀC, the
resulting precipitation was collected by filtration. The white crystal
was washed with methanol and dried at 100 ꢀC under vacuum. The
2.2. Synthesis of 4-bromo-3-nitroanisole (1)
4-Methoxy-2-nitroaniline (168 g, 1.00 mol) was dissolved in
48% HBr aq. (300 mL), and cooled down to 0 ꢀC. To this was added
the solution of sodium nitrite (80.0 g, 1.16 mol) in water (100 mL)
dropwise, and the solution was stirred at room temperature for
2 h. The resulting dispersion was poured in the solution of cupper
(I) bromide (80.0 g, 0.558 mol) in 48% HBr aq. (80 mL) under
nitrogen atmosphere, and stirred at 80 ꢀC for 5 min. After cooling
down to room temperature, the mixture was extracted with
dichloromethane, and the organic layer was washed with sodium
bicarbonate aq. and saturated brine. The organic layer was dried
with magnesium sulfate and concentrated by rotary evaporator.
The crude product was purified by vacuum distillation
(150–151 ꢀC/7.5 mmHg) affording a yellow solid. The yield was
yield was 28.5 g (70.1%). 1H NMR (DMSO-d6):
d 10.98 (s, 1H), 7.83 (d,
2H), 6.93 (s, 2H), 6.72 (d, 2H), 3.80 (s, 6H).
2.5. Synthesis of 2,7-dimethoxy-N-(4-formylphenyl)carbazole (4)
To two-necked flask equipped with stopcock and
a
a
a condenser were added 3 (27.3 g, 120 mmol), 4-bromobenzalde-
hyde (24.1 g, 130 mmol), palladium (II) acetate (0.270 g, 1.20 mmol),
potassium carbonate (49.8 g, 120 mmol), and dry xylene (150 mL)
under nitrogen atmosphere. Tri-t-butylphosphine (36.0 mL,
3.60 mmol, 0.1 M solution in xylene) was added, and the solution
was stirred at 120 ꢀC under reflux for 6 h. After cooling down to
room temperature, the resulting mixture was extracted with
chloroform and water. The organic layer was dried with magne-
sium sulfate and concentrated. The crude product was purified by
silica gel column chromatography eluted with toluene to afford
a yellow crystal. The yield was 16.7 g (42.0%). 1H NMR (CDCl3):
167 g (72.1%). 1H NMR (CDCl3):
1H), 3.86 (s, 3H).
d 7.59 (d, 1H), 7.37 (d, 1H), 6.99 (dd,
2.3. Synthesis of 2-nitro-4,40-dimethoxybiphenyl (2)
d
10.13 (s, 1H), 8.15 (d, 2H), 7.90 (d, 2H), 7.78 (d, 2H), 6.91 (m, 4H),
To the mixture of 1 (46.0 g, 0.198 mol) and 4-iodoanisole (60.0 g,
0.256 mol) was added cupper powder (30.0 g, 0.472 mol) at 180 ꢀC,
and stirred for 2 h. After heating up to 210 ꢀC, another cupper
powder (15.0 g, 0.236 mol) was added to the reaction mixture and
stirring was continued for 2 h. The resulting mixture was cooling
down to room temperature. After N-methylpyrrolidone (10.0 mL)
was added, the mixture was extracted with acetone using Soxhlet
extractor for 18 h. The solution was concentrated by rotary evap-
orator, and cooled down to 0 ꢀC to recrystallize. The yellow crystal
was collected and dried under vacuum. The yield was 28.0 g
3.85 (s, 6H).
2.6. Synthesis of 2,7-dimethoxy-N-(4-vinylphenyl)carbazole (5)
To
a three-necked flask equipped with a stopcock and
a condenser were added methyltriphenylphosphonium bromide
(25.9 g, 72.4 mmol), dry diethyl ether (48 mL), and dry THF (32 mL)
under nitrogen atmosphere. Lithium diisopropylamide (32 mL,
24.1 mmol, 2.0 M solution in heptane, THF, ethylbenzene) was
added at ꢁ25 ꢀC, and stirred for 1 h. The solution of 4 (8.00 g,
24.1 mmol) in dry THF (80 mL) was added and the mixture was
(54.5%). 1H NMR (CDCl3):
d 7.30 (d, 2H), 7.19 (d, 2H), 7.10 (d,1H), 6.91
(d, 2H), 3.88 (s, 3H), 3.83 (s, 3H).
I
OMe
NaNO2
HBr
CuBr
HBr
NO2
Br
NO2
NO2
Cu
P(OEt)3
MeO
MeO
NH2
MeO
OMe
H2O
H2O
1
2
O
H
O
H
Br
H
N
Pd(OAc)2, P(t-Bu)3
K2CO3
Ph3PCH2Br
LDA
MeO
OMe
N
N
THF, ether
MeO
OMe
Xylene
MeO
OMe
3
4
5
O
O
Cl
O
O O
H2N NH2
methanol
Br
Br
Br
pyridine
H2N NH
EtO
HN NH
7
6
(HO)2B
POCl3
Pd(PPh3)4, K2CO3
THF/H2O
O
N N
8
O
Br
N
N
9
Scheme 1. Synthesis of monomers 5 and 9.

618
K. Tsuchiya et al. / Polymer 51 (2010) 616–622
2.8. Synthesis of 1-(4-t-butylbenzoyl)-2-(4-bromobenzoyl)
hydrazine (7)
N
O
N
O
n
To a two-necked flask equipped with a stopcock and an addi-
tional funnel were added 6 (20.0 g, 93.0 mmol) and pyridine
(250 mL) under nitrogen atmosphere. The solution of t-butylben-
zoyl chloride (18.3 g, 93.0 mmol) in pyridine (100 mL) was added
dropwise at 0 ꢀC, then the mixture was stirred for 3 h. After an
additional stirring was continued for 3 h at room temperature,
water was added to precipitate the product. The white solid was
filtered and washed with water. The yield was 33.1 g (94.8%). 1H
10
9
o-dichlorobenzene
N
N
O
NMR (DMSO-d6):
d 10.41 (s, 2H), 7.86 (d, 4H), 7.72 (d, 2H), 7.52 (d,
2H), 1.31 (s, 9H).
2.9. Synthesis of 2-(4-bromophenyl)-5-(4-t-butylphenyl)-1,3,4-
oxadiazole (8)
N
O
n
m
To
a two-necked flask equipped with a stopcock and
a condenser were added 7 (33.0 g, 87.9 mmol) and phosphorous
oxychloride (200 mL) under nitrogen atmosphere, and stirred at
100 ꢀC for 20 h. After cooling down to room temperature, excess
phosphorous oxychloride was removed under vacuum. Water was
added to the residue, and the precipitate was filtered and washed
with water. The white crystal was obtained by recrystallization
from ethanol. The yield was 20.0 g (63.7%). 1H NMR (DMSO-d6):
5
N
MeO
OMe
o-dichloro-
benzene
N
N
O
d
8.02 (d, 4H), 7.80 (d, 2H), 7.62 (d, 2H), 1.33 (s, 9H).
Scheme 2. Preparation of block copolymer via NMRP.
2.10. Synthesis of 2-(4-t-butylphenyl)-5-(4-vinylbiphenyl-40-yl)-
1,3,4-oxadiazole (9)
stirred at room temperature for 36 h. The resulting mixture was
extracted with chloroform and water, and the organic layer was
dried with magnesium sulfate and concentrated by rotary evapo-
rator. The crude product was purified by silica gel column chro-
matography twice eluted with chloroform, after that with toluene.
Recrystallization from methanol afforded white crystal. The yield
To a two-necked flask equipped with a condenser were added 8
(4.65 g, 13.0 mmol), 4-vinylphenylboronic acid (2.00 g, 13.5 mmol),
tetrakis(triphenylphosphine)palladium(0) (0.312 g, 0.27 mmol),
and dry THF (15.0 mL) under nitrogen. After the solution of
potassium carbonate (3.74 g, 27.1 mmol) in oxygen-free water
(13.5 mL) was added, the mixture was stirred at 70 ꢀC for 24 h. The
resulting mixture was extracted with chloroform, and the organic
layer was dried with magnesium sulfate and concentrated by rotary
evaporator. The crude product was purified by silica gel column
chromatography eluted with chloroform/ethyl acetate (95/5) fol-
lowed by recrystallization from methanol to yield the white crystal.
was 5.51 g (69.2%). 1H NMR (CDCl3):
d 7.89 (d, 2H), 7.65 (d, 2H), 7.51
(d, 2H), 6.89–6.79 (m, 5H), 5.87 (d, 1H), 5.37 (d, 1H), 3.82 (s, 6H).
2.7. Synthesis of 4-bromobenzoic hydrazide (6)
The yield was 2.99 g (60.5%). 1H NMR (CDCl3):
d 8.21 (d, 2H), 8.09 (d,
To
a three-necked flask equipped with a stopcock and
2H), 7.77 (d, 2H), 7.64 (d, 2H), 7.57 (d, 2H), 7.53 (d, 2H), 6.78 (dd,1H),
5.84 (d, 1H), 5.32 (d, 1H), 1.38 (s, 9H).
a
condenser were added ethyl 4-bromobenzoate (25.0 g,
109 mmol), hydrazine monohydrate (40.0 mL, 825 mmol), and
methanol (220 mL) under nitrogen atmosphere, and the mixture
was stirred at 40 ꢀC for 16 h. After methanol was concentrated, the
precipitation was collected by vacuum filtration and washed with
2.11. General procedure for syntheses of macroinitiators (P1–P4)
water. The yield was 20.3 g (86.6%). 1H NMR (DMSO-d6):
d
9.79
To a glass tube equipped with a stopcock were added 9 and 10
(s, 1H), 7.75 (d, 2H), 7.65 (d, 2H), 4.47 (s, 2H).
in the stated ratio under nitrogen atmosphere. After fresh dry
Table 1
NMRP of 5 and 9 in various conditionsa.
PDIb
m:nc
b
b
Polymer
Monomer (M)
Initiator (I)
mol ratio [M]/[I]
Time [h]
Yield [%]
Mn [kDa]
Mw [kDa]
P1
P2
P3
P4
P5
B1
B2
B3
B4
9
9
9
9
5
5
5
5
5
10
10
10
10
10
P1
P2
P2
P3
6
6
6
1.5
2
3
1
6
24
24
24
24
65
59
53
37
38
19
40
51
26
2.5
2.9
3.0
4.6
2.0
2.9
6.1
7.3
5.4
3.1
3.4
3.4
6.1
2.7
3.7
8.0
9.2
8.3
1.25
1.14
1.15
1.33
1.35
1.27
1.30
1.26
1.54
–
–
–
–
25
25
0.87
2
4.5
5
–
42:58
67:33
80:20
84:16
a
Polymerizations were carried out in o-dichlorobenzene (3 mol/L) at 130 ꢀC under nitrogen atmosphere.
Determined by GPC eluted with chloroform using polystyrene standard.
Molar ratio of 5 to 9 estimated from 1H NMR.
b
c

K. Tsuchiya et al. / Polymer 51 (2010) 616–622
619
Table 2
and rinsed with deionized water. The substrate was subsequently
washed by 2-propanol under sonication, rinsed with clean 2-
propanol, and dried with nitrogen. PEDOT:PSS with 30 nm of
thickness was spin-coated on the substrate at 2500 rpm for 30 s
from the dispersion in water filtered by 0.2 mm of membrane filter,
followed by annealing at 200 ꢀC for 1 h. Polymer layer was lami-
nated on PEDOT:PSS by spin-coating at 300 rpm for 30 s from 1,1,2-
Random copolymerizations of 5 and 9a.
Polymer Feed ratio [5]:[9] Yield [%] Mnb [kDa] Mwb [kDa] PDIb m:nc
R1
R2
R3
R4
20:80
50:50
67:33
83:17
39
40
65
28
37
52
12
11
101
122
51
2.71 15:85
2.44 44:56
4.13 60:40
3.34 78:22
37
a
Polymerizations were carried out in o-dichlorobenzene (1 mol/L) at 80 ꢀC for
trichloroethane solution (3 mg/mL) filtered by 0.2 mm of membrane
18 h under nitrogen atmosphere.
filter, and annealed at 120 ꢀC for 1 h under nitrogen atmosphere. On
the polymer, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)
with 50 nm thickness was deposited by thermally evaporation in
carbon pot at 240 ꢀC under vacuum with a rate of 1.5 Å/s. As
a cathode, lithium fluoride with 0.5 nm of thickness followed by
aluminum with 100 nm of thickness was deposited on the organic
layers at a rate of 0.1 Å/s and a rate of 4.5 Å/s using tantalum and
tungsten boats, respectively. Finally, the polymer device was set
into a metal can with barium oxide as a drying agent under
nitrogen atmosphere, and passivated with an epoxy resin (XNR
5570-B1, Nagase ChemteX Corporation) irradiated by UV light. The
typical size of the light emitting area was 4 mm2.
b
Determined by GPC eluted with chloroform using polystyrene standard.
c
Molar ratio of 5 to 9 estimated from 1H NMR.
o-dichlorobenzene was added, the mixture was degassed by
freeze–thaw cycles. The tube was sealed under nitrogen, and
heated at 130 ꢀC for the stated time. The mixture was cooled down
to room temperature, and poured in methanol/acetone (2/1). The
polymer was dissolved in chloroform and reprecipitated in
methanol/acetone (2/1) repeatedly. The precipitated polymer was
collected and dried.
2.12. General procedure for syntheses of block copolymers (B1–B4)
To a glass tube equipped with a stopcock were added macro-
initiator (P1–P4) and 5 in the stated ratio under nitrogen atmo-
sphere. After fresh dry o-dichlorobenzene was added, the mixture
was degassed by freeze–thaw cycles. The tube was sealed under
nitrogen, and heated at 130 ꢀC for the stated time. The mixture was
cooled down to room temperature and poured in acetone. The
polymer was dissolved in chloroform and reprecipitated in
acetone/THF (9/1) repeatedly. The precipitated polymer was
collected and dried.
2.15. Measurements
1H and 13C NMR spectra were obtained on a JEOL ALPHA400
instrument at 400 and 100 MHz, respectively. Deutrated chloro-
form was used as a solvent with tetramethylsilane as an internal
standard. Number- and weight-average molecular weights (Mn and
Mw) were determined by gel permeation chromatography (GPC)
analysis with JASCO RI-2031 and UV-970 detectors eluted with
chloroform at a flow rate of 0.5 mL/min and calibrated by standard
polystyrene samples. Differential scanning calorimetry (DSC)
analyses were performed on a Rigaku DSC-8230 under a nitrogen
atmosphere at heating or cooling rate of 10 ꢀC/min. The cyclic
voltammetry (CV) was measured at room temperature using
2.13. General procedure for syntheses of random copolymers
(R1–R4)
The random copolymers were synthesized from 5 and 9 by
a similar method as described in preparation of macroinitiators. As
an initiator, 1 mol% of 2,20-azobis(isobutyronitrile) (AIBN) was used.
Purification of the polymers was performed by reprecipitation in
methanol several times.
2.14. Device fabrication
Prior to preparation of device, a glass slide with indium tin oxide
(ITO) patterns was washed by an alkaline cleaner under sonication
Fig. 2. Cyclic voltammograms of B4 film for (a) oxidative and (b) reductive waves vs
Ag/AgCl as reference electrode.
Fig. 1. 1H NMR spectrum of B4 measured in CDCl3 at 400 MHz.

620
K. Tsuchiya et al. / Polymer 51 (2010) 616–622
Table 4
a 1000
Device performances based on various polymer systems at 2.7 mA/cm2.
B4
Polymer
m:n
Voltage
[V]
Luminance
[Cd/m2]
EQE
[%]
Power efficiency
[lm/W]
800
R4
blend
R1
R2
R3
B1
B2
B3
B4
R4
15:85
44:56
60:40
42:58
67:33
80:20
84:16
78:22
84:16
7.90
7.04
7.20
8.00
6.20
8.00
6.78
6.83
6.82
307
375
479
371
320
302
567
485
464
3.20
3.98
4.97
3.50
3.39
3.12
5.88
4.98
4.88
4.48
6.15
7.68
5.40
5.95
7.15
9.65
8.20
7.86
600
400
200
0
blend
300
350
400
450
500
550
600
Methoxy groups allow the hole transport segment to assemble in
the neighborhood of PEDOT/PSS because of their hydrogen bonding
with protons of PSS. On the other hand, styryl type monomer
containing oxadiazole unit (9) was synthesized to construct elec-
tron transport segment.
Wavelength (nm)
400
300
200
100
0
b
B4
R4
blend
The block copolymers consisting of 5 and 9 were prepared by
means of nitroxide mediated radical polymerization (NMRP) using
10 as an initiator (Scheme 2) [17]. Table 1 summarized the results
of polymerization in various conditions. Homopolymers of 9 as
macroinitiators with narrow polydispersity index (PDI) ranging
from 1.14 to 1.33 are obtained in moderate yields. Increasing feed
ratio of monomer to initiator ([M]/[I]) slightly broadened PDI for
P4 (1.33). The propagation of second block from macroinitiators
was carried out with various concentrations of 5. Block copoly-
mers with different molar ratios of hole to electron transport
segment are obtained. All the block copolymers were purified by
fractional reprecipitation in acetone/THF (9/1) to remove
unreacted homopolymer. However, PDIs slightly increased
compared with those of macroinitiators, especially for high feed
ratio as is seen for B4 (1.54).
Random copolymers were prepared using AIBN for comparison
with block copolymers in device performance (Table 2). The poly-
mers with various compositions of 5 and 9 are obtained by
changing feed ratios. It is clearly observed that higher amount of
electron transport unit tends to be introduced than that of mono-
mer feed ratios because of high reactivity of 9.
The chemical structures of all the block copolymers were
confirmed by 1H NMR spectra as illustrated in Fig. 1 for B4. Signals
assignable to methoxy group and t-butyl group can be clearly
observed at 3.24 and 1.57 ppm, respectively, which prove that both
hole and electron transport units are assembled in the polymer
structure. The molar ratios of hole to electron transport units in
block copolymers (m:n) were calculated from the integral ratio of
methoxy group to all aromatic protons.
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 3. PL spectra of B4, R4, and polymer blend excited at 313 nm (a) in chloroform
solution and (b) in film.
a typical three electrodes system with a working (carbon wire),
a reference (Ag/AgCl), and a counter electrode (Pt spiral) under
a nitrogen atmosphere at a sweep rate of 10 mV/s. The block
copolymer film was fabricated on the carbon electrode from chlo-
roform solution. A 0.1 M solution of tetra-n-butylammonium
perchlorate in anhydrous acetonitrile was used as an electrolyte.
UV–vis and photoluminescent (PL) analyses were conducted on
a JASCO V-570 spectrophotometer and a JASCO FP-6500 spectro-
fluorometer, respectively.
3. Results and discussion
3.1. Polymer synthesis
As a hole transporting monomer, 2,7-dimethoxy-N- (4-vinyl-
phenyl)carbazole (5) was prepared by 5 steps including the
coupling of 2,7-dimethoxycarbazole (3) and 4-bromobenzaldehyde
followed by the transformation of formyl to vinyl group (Scheme 1).
Table 3
Energy levels of block copolymers.
Material
HOMOa [eV]
LUMOa [eV]
LUMOb [eV]
Bandgapb [eV]
B1
B2
B3
ꢁ5.85
ꢁ5.80
ꢁ5.80
ꢁ5.90
ꢁ6.3
ꢁ3.73
ꢁ3.79
ꢁ3.81
ꢁ3.79
–
2.50
3.35
3.35
3.35
3.35
3.9
ꢁ2.45
ꢁ2.45
ꢁ2.55
ꢁ2.4
B4
PBDc
PVKd
ꢁ5.8
–
–
ꢁ2.2
3.6
3.0
d
Ir(ppy)3
ꢁ5.4
ꢁ2.4
a
Estimated by Eoox and Eroed determined from cyclic voltammetry.
Determined by cutoff wavelength in UV absorption spectrum.
Reference [18].
b
c
Fig. 4. Current density and brightness characteristics vs. applied voltage for the
devices based on B4, R4, and polymer blend systems.
d
Reference [7].

K. Tsuchiya et al. / Polymer 51 (2010) 616–622
621
(30 nm)/polymer:Ir(ppy)3 (30 nm)/BCP (50 nm)/LiF (0.5 nm)/Al
(100 nm) for all the polymer systems. The polymer blend was
prepared by mixing each homopolymers synthesized by NMRP
with similar Mn as the segments of block copolymer. As prelim-
inary experiments, the performances of devices utilizing random
copolymers with different compositions (R1–R4) were examined
as summarized in Table 4. The more the content of hole transport
unit increases, the higher the maximum external quantum effi-
ciency (EQE) of the devices becomes. This result indicates that
the hole injection is a bottleneck for the device performance
using the combination of polymer segments consisted of 5 and 9.
Therefore, the ratio of hole transport units to electron transport
units of around 80:20 was selected for block (B4), random (R4),
and blend copolymers so as to compare the performances at the
same composition. The polymer blend composes of P1 and P5
with the molar ratio of 5:9 ¼ 84:16. Fig. 4 illustrates current
density and brightness characteristics against applied voltage on
the devices. All the polymer systems show similar profiles, and
devices using block copolymer and polymer blend pass slightly
much current due to modified charge injection by phase-sepa-
rated structure in the polymer layer, where carbazole units
probably gather in neighbor of PEDOT:PSS layer. The EQE at
various current densities were determined for all devices as
shown in Fig. 5. It is found that block copolymer system exceeds
other polymer systems in EQE, reaching up to 6.3%. This resulted
from the fact that preventing charges from passing to opposite
electrodes as well as improving the injection of charges into the
phosphorescent layer was possibly achieved by gradated
assembly of hole transport segment next to PEDOT:PSS layer in
the block copolymer system. In the blend system, because the
similar assembled structure would be constructed as the block
copolymer system, high EQE is obtained in smaller current
densities. However, thermodynamically unstable structure in the
polymer blend caused a decrease in EQE with increasing current,
and also resulted in poor life time of device performance (less
than a half hour for the brightness half time).
Fig. 5. External quantum efficiency (EQE) vs. current density for the devices based on
B4, R4, and polymer blend systems.
3.2. Optical and electrical properties of polymers
In order to estimate the energy levels of block copolymer, cyclic
voltammetry in film was measured for both oxidative and reduc-
tive waves. Fig. 2 shows the cyclic voltammogram of B4. Oxidative
and reductive peaks can be clearly observed in both operations,
which is attributed to simultaneous coexistence of hole and
electron transporting moieties in the polymer backbone, namely
carbazole and oxadiazole derivatives. The HOMO levels of block
copolymers were estimated from standard redox potential (Eoox
)
values in the oxidation cycle. The energy levels of block copoly-
mers were summarized in Table 3 with those of 2-(4-biphenylyl)-
5-(4-t-butylphenyl)-1,3,4-oxadiazole (PBD), poly(vinylcarbazole)
(PVK), and fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) as refer-
ences [7,18]. The HOMO and the lowest unoccupied molecular
orbital (LUMO) levels of block copolymers are similar as the
HOMO level of PVK and the LUMO level of PBD, respectively. Since
the HOMO level lies lower than that of Ir(ppy)3, energy transfer
from the excited polymer backbone to phosphorescent dopant can
effectively occurs when the block copolymer is used as matrix for
Ir(ppy)3.
3.4. Conclusion
PL measurements were carried out for B4, R4, and polymer
blend of homopolymers P1 and P5 (5:9 ¼ 84:16) in chloroform
solution (Fig. 3a) and in film (Fig. 3b). In solution, block copoly-
mer and polymer blend show emission peak around 390 nm
derived from carbazole and/or oxadiazole units, whereas only
a weak emission at 466 nm is observed in the spectrum of R4. In
the case of random copolymer, carbazole and oxadiazole units
are settled adjacent to each other, causing an exciplex between
them. In the meantime, no significant difference is observed
among three samples in film state, showing an emission from
exciplex at 445 nm. This finding indicates that, in either block
copolymer or polymer blend film, phase-separated structure is
not macroscopic one but within the size where hole and electron
transport segments can create exciplex because of good misci-
bility of both segments. In addition, effective energy transfer to
phosphorescent dopant Ir(ppy)3 can be expected for all polymers
as the maximum emission wavelength at 445 nm matches
absorption of Ir(ppy)3 attributed to metal to ligand charge
transfer (MLCT).
The block copolymers consisting of carbazole- and oxadiazole-
containing segments as hole and electron transporting units,
respectively, were synthesized by NMRP manner. Methoxy groups
were introduced to the carbazole monomer, which would lead hole
transport segment to the surface of PEDOT:PSS layer by hydrogen
bonding with sulfonic acids. It was found by 1H NMR character-
ization that block copolymers possessing various ratios of hole to
electron transport units were obtained with low polydispersity
index. CV measurement of B4 revealed that the polymer showed
bipolar redox behavior with HOMO and LUMO levels at ꢁ5.93 eV
and ꢁ2.54 eV, respectively, enabling hole and electron to be easily
injected into the polymer layer. Finally, with Ir(ppy)3 as a phos-
phorescent dopant, OLED devices were fabricated using block
copolymer, random copolymer, and polymer blend for matrix of the
emitting layer. From the finding that the block copolymer system
overwhelmed the others in EQE, we assumed that a morphology
with dimethoxycarbazole units assembled to the surface of
PEDOT:PSS played a considerable role for effective recombination
of charges as well as sufficient charge injection into the emitting
layer.
3.3. Device evaluation
In order to make an inspect for morphological effect on the
OLED performance, electrical phosphorescent properties were
studied on the device using B4, R4, and polymer blend doped by
6 wt% of Ir(ppy)3. The devices were fabricated as ITO/PEDOT:PSS
Appendix. Supplementary data
Supplementary data associated with this article can be found in
the online version at doi:10.1016/j.polymer.2009.12.024.

622
K. Tsuchiya et al. / Polymer 51 (2010) 616–622
[10] Xia YJ, Friend RH. Adv Mater 2006;18:1371.
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