Synthesis of bipolar charge transporting block copolymers and characterization for organic light-emitting diode

Article abstract of DOI:10.1002/pola.23853

A series of hole and electron transporting random and block copolymers consisting of triphenylamine moiety as a hole transporting unit and oxadiazole moiety as an electron transporting unit have been prepared via a nitroxide mediated radical polymerization. Oxadiazole monomers with t-butyl or trifluoromethyl groups, 2 and 7, respectively, were used for copolymerization. Photoluminescent measurements of polymers revealed that the formation of the exciplex between triphenylamine and oxadiazole units tends to occur in the order of random copolymers, block copolymers, and polymer blends, implying phase-separated morphologies in block or blend systems. The polymers were applied for OLED devices, and we found that the morphology in the polymer layer critically affected device performance. The block copolymer comprising hole and electron transporting units with the composition of 14/86 showed the highest external quantum efficiency over 10%.

Full text of DOI:10.1002/pola.23853

Synthesis of Bipolar Charge Transporting Block Copolymers and
Characterization for Organic Light-Emitting Diode
KOUSUKE TSUCHIYA,1 HIDEMASA KASUGA,1 AKIRA KAWAKAMI,2 HIDEO TAKA,2 HIROSHI KITA,2 KENJI OGINO1
1Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology,
2-24-16 Nakacho, Koganei-shi, Tokyo 184-8588, Japan
2Display Technology R&D Laboratories, Konica Minolta Technology Center Inc., 2970 Ishikawa-machi, Hachioji-shi,
Tokyo 192-8505, Japan
Received 18 August 2009; accepted 21 November 2009
DOI: 10.1002/pola.23853
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: A series of hole and electron transporting random and
block copolymers consisting of triphenylamine moiety as a hole
transporting unit and oxadiazole moiety as an electron transport-
ing unit have been prepared via a nitroxide mediated radical poly-
merization. Oxadiazole monomers with t-butyl or trifluoromethyl
groups, 2 and 7, respectively, were used for copolymerization.
Photoluminescent measurements of polymers revealed that the
formation of the exciplex between triphenylamine and oxadiazole
units tends to occur in the order of random copolymers, block
copolymers, and polymer blends, implying phase-separated mor-
phologies in block or blend systems. The polymers were applied
for OLED devices, and we found that the morphology in the poly-
mer layer critically affected device performance. The block copoly-
mer comprising hole and electron transporting units with the
composition of 14/86 showed the highest external quantum effi-
C
V
ciency over 10%. 2010 Wiley Periodicals, Inc. J Polym Sci Part A:
Polym Chem 48: 1461–1468, 2010
KEYWORDS: block copolymers; light-emitting diodes; living radi-
cal polymerization; oxadiazole; triphenylamine
INTRODUCTION As an early strike on organic electrolumi-
nescent (EL) materials reported by Tang and Vanslyke was
brought out, EL materials have attracted much attention and
have been developed for a flat organic light-emitting diode
(OLED) display.1 The OLED display has a number of advan-
tages over conventional displays such as high brightness,
high contrast, low-voltage operation, large viewing angle,
and extremely thin device.2 Hole and electron transporting
organic materials, typically low-molecular compounds, con-
struct multilayered structure between electrodes that results
in a high quantum efficiency in the recombination of positive
and negative charges. However, low-molecular weight com-
pounds have to be subject to chemical vapor deposition to
build up organic layers, which hamper mass production of a
large area display because of cost issue. In practical point of
view, polymeric OLED materials are amenable for wet proc-
esses such as spin coating that can overcome this problem.
The host-guest systems using Ir complex as a phosphores-
cent dopant have accomplished high EQE for OLED devices.3
Polymeric materials in which phosphorescent guests can be
stably dispersed are good candidates for host matrix in the
system. Many types of polymers comprising hole and/or
electron transporting units have been reported as host poly-
mers.4–8 Aggregation of phosphorescent dopants should be
avoided because of high concentration quenching. Therefore,
the polymers with phosphors attached as pendant groups,
which prevent the quenching as well as facilitate effective
energy transfer from polymer backbone to the phosphor,
have been reported by some researchers.6–8
Nevertheless, of all the merits to use polymeric materials for
OLED, polymers suffer from difficulty in fabrication of multi-
layered structure that affords an efficient recombination of
charges. Recent devices uses water-dispersed poly(3,4-ethyl-
enedioxythiophene) (PEDOT) as a hole injection layer, and
another polymer layer was consequently laminated by spin
coating. However, the polymer layer was unable to accumu-
late because the spin coating process using organic solvents
affects the polymer layer beneath except the hydrophilic
PEDOT layer. Although the use of water-soluble polymers
enables to construct many functional layers by spin coating
The increase of external quantum efficiency (EQE) is a key
concern for organic materials to be applied to OLED devices,
but the internal quantum efficiency of luminescent materials
theoretically saturates 25% resulting in lower EQE. Heavy
metal complexes with a strong spin-orbital coupling such as
Ir complex emit phosphorescence effectively because inter-
system crossing from singlet to triplet excited states fol-
lowed by radiative transition to ground states is very fast.
technique,
approach to circumvent this problem is preparing
a
few examples have been reported.9,10 An
Additional Supporting Information may be found in the online version of this article Correspondence to: K. Ogino (E-mail: kogino@cc.tuat.ac.jp)
C
V
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 1461–1468 (2010) 2010 Wiley Periodicals, Inc.
BIPOLAR CHARGE TRANSPORTING BLOCK COPOLYMERS FOR OLED, TSUCHIYA ET AL.
1461

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
multifunctional polymers in which hole transporting, elec-
tron transporting, and emitting abilities are included by
means of random or block copolymerizations.11–13
added dropwise, the mixture was stirred at 0 ꢀC for 3 h.
Additional stirring was continued at room temperature for 5
h. The resulting mixture was poured into water, and the pre-
cipitation was filtered. The crude product was dispersed in
small amount of THF, filtered, and washed with water. The
white solid was obtained after drying under vacuum. The
Block copolymers assemble into micro- or nano-phase sepa-
rated structures with various domain shapes such as lamella,
cylinder, or sphere. Exploiting nanostructures of block
copolymers with appropriate designs can improve perform-
ance of applications due to allocation of functionality to each
domain.14,15 Fluorine containing segments in block copoly-
mers were well known as surface modifiers that gather to-
ward the interface between air and polymers, and change
the surface to hydrophobic16,17 or antifouling.18,19 In this ar-
ticle, we prepared a series of block copolymers consisting of
triarylamine and oxadiazole monomers as hole and electron
transporting units, respectively. Trifluoromethyl groups were
introduced into an oxadiazole monomer to create a pseudo-
layered structure by dragging electron transporting segments
into the surface during film formation. The light-emitting
properties of the block copolymers were examined and were
compared with those of random copolymers and polymer
blends with the similar monomer compositions.
ꢀ
yield was 25.2 g (61.1%). Mp: 110.7 C (DSC).
1H NMR (500 MHz, DMSO-d6, d, ppm): 11.12 (s, 1H), 10.91
(s, 1H), 8.57 (s, 2H), 8.41 (s, 1H), 8.10 (d, 2H), 8.04 (d, 2H),
3.89 (s, 3H). 13C NMR (125 MHz, DMSO-d6, d, ppm): 165.65,
165.12, 162.97, 136.37, 134.49, 132.60, 130.85 (q, J ¼ 34
Hz), 129.44, 128.31, 128.01, 125.85, 123.06 (q, J ¼ 272 Hz),
52.51. HRMS (m/z): calcd for C18H12F6N2O4, 435.0780;
found, 435.0604 [M þ H]þ.
Synthesis of 2-[3,5-Bis(trifluoromethyl)phenyl]-5-(4-
methoxycarbonylphenyl)-1,3,4-oxadiazole (5)
To a two-necked flask equipped with a stopcock and a con-
denser were added 4 (6.51 g, 15.0 mmol) and phosphoryl
chloride (60 mL) under nitrogen atmosphere, and the mix-
ꢀ
ture was stirred at 100 C for 12 h. Phosphoryl chloride was
removed by vacuum distillation, and water was added to
wash the resulting mixture. After vacuum filtration and dry-
ing, the crude compound was recrystallized from toluene to
give aꢀ white crystal. The yield was 4.53 g (72.6%). Mp:
113.3 C (DSC).
EXPERIMENTAL
Materials
Toluene was distilled over calcium hydride and stored under
nitrogen. Tetrahydrofuran (THF), diethyl ether, and diglyme
(diethyleneglycol dimethyl ether) were used after the distil-
lation over sodium and benzophenone. 3-Vinyltriphenylamine
(1),20 4-vinylbenzyl 4-[5-(4-t-butylphenyl)-1,3,4-oxadiazole-
2-yl]benzoate (2),21 and N-(1-phenylethoxy)-N-(2-methyl-1-
phenylpropyl)-t-butylamine (8)22 were synthesized as
reported in literatures. The other reagents were used as
received.
1H NMR (500 MHz, CDCl3, d, ppm): 8.60 (s, 2H), 8.26 (d,
2H), 8.22 (d, 2H), 8.08 (s, 1H), 3.98 (s, 3H). 13C NMR (125
MHz, CDCl3, d, ppm): 166.12, 164.95, 162.83, 133.60, 133.19
(q, J ¼ 35 Hz), 130.56, 127.29, 127.10, 127.01, 125.90,
125.44, 122.83 (q, J ¼ 273 Hz), 52.72. HRMS (m/z): calcd
for C18H10F6N2O3, 417.0674; found, 417.0698 [M þ H]þ.
Synthesis of 4-[5-(3,5-Bis(trifluoromethyl)phenyl)-1,3,4-
oxadiazole-2-yl]benzoic Acid (6)
Synthesis of 3,5-Bis(trifluoromethyl)
benzoylhydrazine (3)
To the solution of sodium hydroxide (0.870 g, 15.5 mmol) in
small amount of water was added the dispersion of 5 (3.23
g, 7.75 mmol) in THF (30 mL) and ethanol (90 mL), and the
To a three-necked flask equipped with a stopcock and a con-
denser were added methyl 3,5-bis(trifluoromethyl)benzoate
(18.5 g, 9.60 mmol) and methanol (12 mL) under nitrogen
atmosphere and stirred for 30 min. After hydrazine monohy-
drate (3.00 mL, 62.0 mmol) and methanol (12 mL) were
added to the solution, the mixture was stirred at 40 ꢀC for
16 h. The solution was concentrated and washed with water.
After drying a white solid was obtained. The yield was
ꢀ
mixture was stirred at 70 C for 3 h. After water was added
to the mixture at room temperature, insoluble solid was
removed by vacuum filtration. To the filtrate was added 0.5
N HCl aq. until pH of the solution became 2. The precipita-
tion was filtered and washed with water three times. After
drying, slightly pinkish soꢀlid was obtained. The yield was
2.20 g (67.7%). Mp: 136.7 C (DSC).
ꢀ
15.1 g (81.6%). Mp: 135.9 C (DSC).
1H NMR (500 MHz, DMSO-d6, d, ppm): 10.27 (s, 1H), 8.45 (s,
2H), 8.23 (s, 1H), 4.66 (s, 2H). 13C NMR (125 MHz, DMSO-d6,
d, ppm): 162.59, 135.53, 130.90 (q, J ¼ 33 Hz), 127.77,
124.67, 123.13 (q, J ¼ 272 Hz). HRMS (m/z): calcd for
C9H6F6N2O, 273.0463; found, 273.0528 [M þ H]þ.
1H NMR (500 MHz, DMSO-d6, d, ppm): 11.10 (s, 1H), 8.70 (s,
2H), 8.40 (s, 1H), 8.32 (d, 2H), 8.13 (d, 2H). 13C NMR (125
MHz, CDCl3, d, ppm): 167.62, 165.19, 163.34, 136.90, 134.83,
132.07 (q, J ¼ 33 Hz), 131.06, 128.31, 128.23, 127.41,
126.39, 123.76 (q,
J
¼
272). HRMS (m/z): calcd for
C
17H8F6N2O3, 403.0517; found, 403.0530 [M þ H]þ.
Synthesis of 1-[3,5-Bis(trifluoromethyl)benzoyl]-2-(4-
methoxycarbonylbenzoyl)hydrazine (4)
To a two-necked flask equipped with a stopcock and an addi-
tion funnel were added 3 (41.2 g, 150 mmol) and pyridine
(250 mL) and cooled down to 0 ꢀC under nitrogen atmos-
phere. After the solution of terephthalic acid monomethyl
ester chloride (29.9 g, 151 mmol) in pyridine (250 mL) was
Synthesis of 4-Vinylbenzyl 4-[5-(3,5-Bis(trifluoromethyl)
phenyl)-1,3,4-oxadiazole-2-yl]benzoate (7)
To a two-necked flask equipped with a stopcock and a con-
denser were added 6 (2.19 g, 5.24 mmol), sodium bicarbon-
ate (0.661 g, 7.87 mmol), and dimethylformamide (30 mL)
1462
INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
SCHEME 1 Hole/electron trans-
porting monomers and nitroxy-
amine initiator.
under nitrogen atmosphere, and the mixture was stirred for
1 h. 4-Chloromethylstyrene (1.60 g, 10.5 mmol) was added
to the solution, followed by stirring at 60 ꢀC for 15 h. The
mixture was poured into water and extracted with chloro-
form. 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 to give a white solid. The yield was 1.68 g
(62.0%).
in methanol. The recovered polymer was dissolved in THF
and reprecipitated in methanol/acetone (1/1). The precipi-
tated polymer was collected and dried.
Device Fabrication
Before preparation of device, a glass slide with indium tin
oxide (ITO) patterns was washed by an alkaline cleaner
under sonication and rinsed with deionized water. The sub-
strate was subsequently washed by 2-propanol under sonica-
tion, rinsed with clean 2-propanol, and dried with nitrogen.
1H NMR (500 MHz, CDCl3, d, ppm): 8.60 (s, 2H), 8.26 (s, 4H),
8.08 (s, 1H), 7.44 (m, 4H), 6.73 (dd, 1H), 5.78 (d, 1H), 5.40
(s, 2H), 5.29 (d, 1H). 13C NMR (125 MHz, CDCl3, d, ppm):
165.43, 164.88, 162.84, 138.01, 136.37, 135.11, 133.59,
133.17 (q, J ¼ 33 Hz), 130.64, 128.81, 127.33, 127.11,
127.08, 126.63, 125.89, 125.49, 122.84 (q, J ¼ 273 Hz),
Poly(3,4-ethylenedioxythiophene):poly(styrene
sulfonate)
(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 lm of membrane filter, followed by
annealing at 200 ꢀC for 1 h. Polymer layer doped with Ir
complex (6 wt %) was laminated on PEDOT:PSS by spin-
coating at 300 rpm for 30 s from 1,1,2-trichloroethane solu-
tion (3 mg/mL) filtered by 0.2 lm of membrane 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 of thickness was deposited by thermal
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, respec-
tively. Finally, the device was set into a metal can with bar-
ium oxide as a drying agent under nitrogen atmosphere and
passivated with an epoxy resin (XNR 5570-B1, Nagase Chem-
teX Corporation) irradiated by UV light.
114.69, 67.22. HRMS (m/z): calcd for
C26H16F6N2O3,
519.1143; found, 519.1228 [M þ H]þ.
General Procedure for Synthesis of Random Copolymers
and Homopolymers
To a glass tube equipped with a stopcock were added initia-
tor 8 and monomers in the stated ratio under nitrogen
atmosphere. After diglyme 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 mix-
ture was cooled down to room temperature and poured in
methanol. The precipitated polymer was collected and dried.
General Procedure for Synthesis of Block Copolymers
To a glass tube equipped with a stopcock were added macro-
initiator and second monomer in the stated ratio under
nitrogen atmosphere. After diglyme was added, the mixture
was degassed by freeze–thaw cycles. The tube was sealed
Measurements
1H and 13C NMR spectra were obtained on
a JEOL
ALPHA500 instrument at 500 and 125 MHz, respectively.
Deutrated chloroform was used as a solvent with tetra-
methylsilane as an internal standard. Number- and weight-
ꢀ
under nitrogen and heated at 130 C for the stated time. The
mixture was cooled down to room temperature and poured
BIPOLAR CHARGE TRANSPORTING BLOCK COPOLYMERS FOR OLED, TSUCHIYA ET AL.
1463

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 Random Copolymerizations of Various Monomersa
PDIb
Conv. (%)
Mol Ratioc (M1/M2)
b
b
Polymer
ETM (M1/I)
HTM (M2/I)
Mn (kDa)
Mw (kDa)
P(1-r-2)-1
P(1-r-2)-2
P(1-r-7)-1
P(1-r-7)-2
P(1-r-7)-3
a
2 (8)
1 (48)
1 (18)
1 (60)
1 (60)
1 (32)
9.1
5.3
12.7
7.1
1.39
1.35
1.35
1.34
1.47
87
73
83
80
77
12/88
34/66
15/85
7/93
2 (10)
7 (10)
7 (4)
13.2
4.7
17.9
6.3
7 (8)
6.4
9.4
17/83
Polymerizations were carried out in diglyme (0.4 g/mL) at 130 ꢀC for 24 h.
Determined by GPC eluted with chloroform using polystyrene standards.
Estimated from 1H NMR spectra.
b
c
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 1.0 mL/min and calibrated by standard polysty-
rene samples. Differential scanning calorimetry analyses
were performed on a Rigaku DSC-8230 under a nitrogen
bone to localize toward surface of polymer thin film during
coating process. Various types of copolymer, diblock copoly-
mers or random copolymers comprising hole and electron
transporting units, were synthesized to investigate the corre-
lation between performance of polymer EL device and mor-
phology in polymer thin films. The results of random copoly-
merizations are summarized in Table 1. Alkoxyamine 8
shown in Scheme 1 was used as an initiator for nitroxide
mediated radical polymerization because of its excellent abil-
ity to control on polydispersity.20 The random copolymers
with different compositions and molecular weights are
obtained in good yields. From 1H NMR estimation, the com-
positions of all the random copolymers were found to be
almost similar to monomer feed ratios.
ꢀ
atmosphere at heating and cooling rate of 10 C/min. UV–vis
and photoluminescent (PL) analyses were conducted on a
JASCO V-570 spectrophotometer and a JASCO FP-6500 spec-
trofluorometer, respectively.
RESULTS AND DISCUSSION
Polymer Synthesis
As a hole transporting monomer (HTM), 3-vinyltriphenyl-
amine (1) was prepared by coupling of diphenylamine and
3-bromostyrene using palladium catalyst as shown in
Scheme 1. In the meantime, oxadiazole derivatives with t-
butyl or trifluoromethyl moieties, 2 or 7 respectively, were
selected for electron transporting monomers (ETM). The
preparation of 7 was conducted by the same procedure as
that of 2 reported in literature.21 Trifluoromethyl group
were introduced into the end of oxadiazole pendant group,
which enable electron transporting parts in polymer back-
Homopolymerization of each monomer was carried out to
prepare macroinitiators for block copolymerizations. The
results are shown in Table 2. Homopolymers of 1 with nar-
row polydispersity index (PDI) are obtained for P(1)-1 and
P(1)-2 but in low conversions below 30%. Elongation of
reaction time does not improve the conversion with slight
increase in PDI (P(1)-3) even in bulk or highly concentrated
conditions. Moderate conversions over 50% as well as nar-
row PDIs between 1.15 and 1.26 are achieved for
TABLE 2 Homopolymerizations of Various Monomersa
e
e
Polymer
Monomer (M1/I)
Time (h)
Mn (kDa)
Mw (kDa)
PDIe
Conv. (%)
P(1)-1b
P(1)-2b
P(1)-3c
P(2)-1
P(2)-2
P(2)-3
P(2)-4
P(2)-5
P(7)-1d
P(7)-2d
P(7)-3d
a
1 (55)
1 (8)
2
4.6
2.4
6.7
7.3
3.4
7.2
4.5
3.7
6.0
4.0
3.3
d
5.1
2.6
8.8
8.7
4.2
8.6
5.4
4.3
7.6
5.0
3.8
1.12
1.07
1.31
1.20
1.22
1.20
1.19
1.15
1.26
1.23
1.16
10
29
32
83
45
77
51
51
51
55
51
1.5
20
18
4
1 (50)
2 (35)
2 (6)
2 (16)
2 (8)
15
4
2 (6)
4
7 (7)
12
12
12
7 (3.5)
7 (3.5)
Polymerizations were carried out in diglyme (0.2 g/mL) at 130 ꢀC.
0.4 g/mL.
b
c
e
Bulk conditions.
0.8 g/mL.
Determined by GPC eluted with chloroform using polystyrene
standards.
1464
INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
TABLE 3 Preparation of Block Copolymers Using Various Macroinitiatorsa
PDIc
Yield (%)
Mol Ratiod (M1/M2)
c
c
Polymer
Initiator
Monomer (M2/I)
Time (h)
Mn (kDa)
Mw (kDa)
P(1)-b-P(2)-1b
P(1)-b-P(2)-2b
P(2)-b-P(1)-1
P(2)-b-P(1)-2
P(2)-b-P(1)-3
P(2)-b-P(1)-4
P(7)-b-P(1)-1
P(7)-b-P(1)-2
a
P(1)-1
P(1)-1
P(2)-2
P(2)-2
P(2)-4
P(2)-5
P(7)-2
P(7)-3
2 (35)
2 (35)
1 (45)
1 (30)
1 (30)
1 (12)
1 (12)
1 (12)
18
72
24
24
24
24
24
24
6.4
10
10
7.5
5.6
5.6
9.3
5.9
c
13
2.00
1.97
1.28
1.26
1.33
1.22
1.43
1.22
50
43
41
41
25
51
42
28
42/58
44/56
6/94
20
13
9.4
7.4
6.8
13
14/86
40/60
33/67
6/94
7.2
20/80
Polymerizations were carried out in diglyme (0.8 g/mL) at 130 ꢀC.
0.4 g/mL.
Determined by GPC eluted with chloroform using polystyrene standards.
Estimated from 1H NMR spectra.
b
d
polymerizations of 2 and 7 (P(2) and P(7)). Furthermore,
longer reaction time can increase the molecular weight and
the conversion (P(2)-3), which proves that living radical
polymerization proceeded reasonably. These homopolymers
were used as macroinitiators to build up block copolymers
and as components in polymer blend systems.
FIGURE 2 PL spectra of (a) macroinitiators, (b) polymers with
the composition 2/1 ¼ 14/86, and (c) polymers with the compo-
sition 7/1 ¼ 6/94 in thin film states.
FIGURE 1 GPC profiles of (a) P(1)-b-P(2)-1 and P(1)-1, and (b)
P(2)-b-P(1)-2 and P(2)-2.
BIPOLAR CHARGE TRANSPORTING BLOCK COPOLYMERS FOR OLED, TSUCHIYA ET AL.
1465

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 4 Device Performances for Various Polymer Systems at 2.72 mA/cm2
Mol ratio
ETM/HTM
PL kmax
Voltage
(V)
Luminance
(Cd/m2)
Current
Power
Polymer
(nm)
EQE (%)
Efficiency (Cd/A)
Efficiency (Lm/W)
P(2)-b-P(1)-2
14/86
12/88
14/86
33/67
34/66
33/67
6/94
480
480
381
483
483
383
516
516
367
523
523
371
7.9
7.8
8.2
7.3
7.2
9.0
8.4
8.2
7.1
7.7
7.0
6.7
946
490
327
485
495
786
258
354
169
206
269
178
10.1
5.2
3.5
5.0
5.1
8.3
2.9
3.9
1.8
2.2
2.9
1.8
34.8
18.0
12.0
17.8
18.2
28.9
9.5
13.9
7.3
4.6
7.6
8.0
10.1
3.6
5.0
2.7
3.1
4.4
3.1
P(1-r-2)-1
P(1)-3/P(2)-5 (blend 1)
P(2)-b-P(1)-4
P(1-r-2)-2
P(1)-3/P(2)-5 (blend 2)
P(7)-b-P(1)-1
P(1-r-7)-2
7/93
13.0
6.2
P(1)-3/P(7)-3 (blend 3)
P(7)-b-P(1)-2
6/94
20/80
17/83
20/80
7.6
P(1-r-7)-3
9.9
P(1)-3/P(7)-3 (blend 4)
6.5
Diblock copolymers consisting of hole and electron trans-
porting segments were prepared by propagation of second
block from the macroinitiators as shown in Table 3. Molecu-
lar weights increase for all the block copolymers; however,
block copolymers initiated from P(1)-1 shows broaden PDIs.
Poor solubility of poly(3-vinyltriphenylamine) caused deacti-
vation of propagation terminal and thermal polymerization.
On the other hand, block copolymers initiated from electron
transporting macroinitiators (P(2)-b-P(1) and P(7)-b-P(1))
are obtained with narrower PDIs. Although living radical
polymerizations for P(2)-b-P(1) and P(7)-b-P(1) series
were controlled, there remained small amount of macroini-
tiators. Solvent fractionation by reprecipitation can separate
pure block copolymers with PDIs below 1.4 from the crude
product containing unreacted macroinitiators (less than
10%). The yields listed in Table 3 are the recovery yields af-
ter the purification. Figure 1 shows GPC profiles of block
copolymers (P(1)-b-P(2)-1, P(2)-b-P(1)-2) and correspond-
ing macroinitiators (P(1)-1, P(2)-2). Bimodal distribution is
observed in the profile of P(1)-b-P(2)-1, indicating that
undesired thermal radical polymerization occurred in addi-
tion to living radical propagation to give a broad PDI (2.00).
In contrast, the profile of P(2)-b-P(1)-2 reveals that the PDI
is nearly maintained after polymerization of second block
with a shift toward high molecular weight. All the composi-
tions of block copolymers were estimated from 1H NMR
spectra by comparing the signal assignable to methylene pro-
tons of oxadiazole units with all the signals of aromatic pro-
tons (see Supporting Information).
kmax are 480 and 516 nm for the polymer possessing 2 and
7 ETM units, respectively. These bathochromic shifts were
caused by emission from an exciplex of HTM and ETM units.
The tendency can be clearly observed that the PL intensity
of emission from exciplex for random copolymers is strong-
est among polymers with same composition and weakest is
that for polymer blends. Because block copolymers and poly-
mer blends takes a phase separated morphology in these
films, the probability to generate an exciplex at the interface
between HTM and ETM decreases compared to that of ran-
dom polymers.
Evaluation for OLED Devices
To demonstrate the effect of morphology in the polymer
layer on OLED device performance, we evaluated light-emit-
ting properties of devices based on random copolymers,
block copolymers, and polymer blends. The devices with 6
wt % of tris(2-phenylpyridine) iridium(III) [Ir(ppy)3] as a
dopant were constructed as follows: ITO/PEDOT:PSS (30
nm)/Polymer:Ir(ppy)3 (30 nm)/BCP (50 nm)/LiF (0.5 nm)/
Al (100 nm). Table 4 summarized the device performances
at the same current density of 2.72 mA/cm2 using various
polymers. All the devices showed a phosphorescent emission
with a peak top at 515 nm derived from Ir(ppy)3. Device
performances for random copolymer, block copolymer, and
polymer blend differed from each other in spite of the same
composition. This is attributed to morphologies in the poly-
mer layer that affects on the recombination of hole and elec-
tron charges. Furthermore, in the case of our selection for
the combination of hole and electron transporting mono-
mers, higher contents of hole transporting units show higher
EQEs.
Photoluminescence Properties
To investigate the light-emitting properties of the polymers
in a film state, photoluminescence measurements were car-
ried out for the polymers. Figure 2 shows PL spectra excited
at the maximum wavelengths (kmax) in each UV absorption
spectrum (see Supporting Information). Although homopoly-
mers give the peak tops from 371 to 384 nm (a), a strong
emission appears around 500 nm for the block and random
copolymers but slightly for polymer blends. The values of
In comparison for device performances among the series of
polymers consisting of 1 and 2 units as shown in Figure 3,
block copolymer system for the ratio of ETM/HTM ¼ 14/86
(P(2)-b-P(1)-2) gives the highest EQE. On the other hand,
the polymer blend P(1)-3/P(2)-5 ¼ 33/67 (blend 2) shows
unreasonably a high EQE. This is explained by the fact that
1466
INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
FIGURE 3 Device performances on (a)
I-V, (b) luminance versus voltage, (c)
luminance versus current density, and
(d) EQE versus current density charac-
teristics for the polymers consisting of
1 and 2.
PL kmax appeared around 370 nm for polymer blends, which
is effective for the energy transfer to Ir(ppy)3 absorption
derived from metal to ligand charge transfer (MLCT).
Because only blend 2 exhibited an excellent smooth film, the
other polymer blends showed lower EQEs.
decrease compared with t-butyl counterparts. The highest
occupied molecular orbital (HOMO) level of oxadiazole units
containing trifluoromethyl groups lowered, as confirmed by
red shifts of kmax in PL spectra. As the gap between the
HOMO levels of 7 and BCP (6.7 eV) become smaller, holes
cannot be trapped at the BCP layer causing a decrease in
EQEs. The devices based on the block copolymers and espe-
cially polymer blends lower EQEs compared with the ran-
dom copolymers, which implies that self assembled
OLED performances using a series of fluorinated polymers
are shown in Figure 4. Unfortunately, EQEs for all the poly-
mers (random copolymers, block copolymers, and blends)
FIGURE 4 Device performances on (a)
I-V, (b) luminance versus voltage, (c)
luminance versus current density, and
(d) EQE versus current density charac-
teristics for the polymers consisting of
1 and 7.
BIPOLAR CHARGE TRANSPORTING BLOCK COPOLYMERS FOR OLED, TSUCHIYA ET AL.
1467

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
5 Vaeth, K. M.; Tang, C. W. J Appl Phys 2002, 92, 3447–3453.
structures with fluorinated segments gathering to the surface
were built up in block copolymers and polymer blends
layers.
6 Chen, X. W.; Liao, J. L.; Liang, Y. M.; Ahmed, M. O.; Tseng,
H. E.; Chen, S. A. J Am Chem Soc 2003, 125, 636–637.
7 Jiang, J. X.; Jiang, C. Y.; Yang, W.; Zhen, H. G.; Huang, F.;
CONCLUSIONS
Cao, Y. Macromolecules 2005, 38, 4072–4080.
We prepared a series of hole and electron transporting ran-
dom and block copolymers consisting of triphenylamine moi-
ety as hole transporting unit and oxadiazole moiety as elec-
tron transporting unit via nitroxide mediated radical
polymerization. Oxadiazole monomers with t-butyl or tri-
fluoromethyl groups, 2 and 7 respectively, were used for
copolymerization. The compositions of all the polymers were
consistent with monomer feed ratios. PL measurements of
polymers revealed that the formation of exciplex between
triphenylamine and oxadiazole units tends to occur in the
order of random copolymers, block copolymers, and polymer
blends, implying phase-separated morphologies in block or
blend systems. The polymers were applied for OLED devices,
and we found that the morphology in the polymer layer crit-
ically affected device performance. The block copolymer
comprising 1 and 2 units with the composition of 2/1 ¼
14/86 showed high EQE over 10%. On the other hand,
although the device performances of polymers containing 7
units differ from one another due to morphological change,
fluorinated block copolymers showed low quantum efficien-
cies because of the lack of hole blocking property in the BCP
layer. There should be a correlation between the morphology
in polymer layers and device performances, and further
effort to discover high effective light-emitting materials com-
ponents for block copolymer systems is essential.
8 Schulz, G. L.; Chen, X. W.; Chen, S. A.; Holdcroft, S. Macro-
molecules 2006, 39, 9157–9165.
9 Wang, L.; Liang, B.; Huang, F.; Peng, J. B.; Cao, Y. Appl Phys
Lett 2006, 89, 151115.
10 Huang, F.; Niu, Y. H.; Zhang, Y.; Ka, J. W.; Liu, M. S.; Jen,
A. K. Y. Adv Mater 2007, 19, 2010–2014.
11 Furuta, P. T.; Deng, L.; Garon, S.; Thompson, M. E.; Frechet,
J. M. J. J Am Chem Soc 2004, 126, 15388–15389.
12 Suzuki, M.; Tokito, S.; Sato, F.; Igarashi, T.; Kondo, K.;
Koyama, T.; Yamaguchi, T. Appl Phys Lett 2005, 86, 103507.
13 Ma, B.; Kim, B. J.; Deng, L.; Poulsen, D. A.; Thompson, M.
E.; Frechet, J. M. J. Macromolecules 2007, 40, 8156–8161.
14 Park, C.; Yoon, J.; Thomas, E. L. Polymer 2003, 44,
6725–6760.
15 Developments in Block Copolymer Science and Technology;
Hamley, I. W., Ed.; Wiley: Chichester, UK, 2004.
16 Borkar, S.; Jankova, K.; Siesler, H. W.; Hvilsted, S. Macro-
molecules 2004, 37, 788–794.
17 Yokoyama, H.; Sugiyama, K. Langmuir 2004, 20,
10001–10006.
18 Krishnan, S.; Wang, N.; Ober, C. K.; Finlay, J. A.; Callow, M.
E.; Callow, J. A.; Hexemer, A.; Sohn, K. E.; Kramer, E. J.;
Fischer, D. A. Biomacromolecules 2006, 7, 1449–1462.
REFERENCES AND NOTES
19 Krishnan, S.; Ward, R. J.; Hexemer, A.; Sohn, K. E.; Lee, K.
L.; Angert, E. R.; Fischer, D. A.; Kramer, E. J.; Ober, C. K. Lang-
muir 2006, 22, 11255–11266.
1 Tang, C. W.; Vanslyke, S. A. Appl Phys Lett 1987, 51,
913–915.
20 Deng, L.; Furuta, P. T.; Garon, S.; Li, J.; Kavulak, D.; Thomp-
2 Akcelrud, L. Prog Polym Sci 2003, 28, 875–962.
son, M. E.; Frechet, J. M. J. Chem Mater 2006, 18, 386–395.
3 Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.;
Lee, H. E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson,
M. E. J Am Chem Soc 2001, 123, 4304–4312.
21 Sato, H.; Sakaki, Y.; Ogino, K.; Ito, Y. Polym Adv Technol
1997, 8, 454–458.
4 Lee, C. L.; Lee, K. B.; Kim, J. J. Appl Phys Lett 2000, 77,
22 Benoit, D.; Chaplinski, V.; Braslau, R.; Hawker, C. J. J Am
2280–2282.
Chem Soc 1999, 121, 3904–3920.