PLED devices containing triphenylamine-derived polyurethanes as hole-transporting layers exhibit high current efficiencies

Article abstract of DOI:10.1039/b715929c

The innovative polyurethane-type polymers from a triphenylamine derivative, [N,N′-bis(4-hydroxyphenyl)-N,N′-diphenylbenzidine] (TPA) and a carbazole derivative, [9-butyl-3,6-bis(4-hydroxyphenyl)carbazole] (Cz) can be linked through isophorone diisocyanate (IPDI) bridges and incorporated as hole-transporting layers in high-performance PLED devices. The TPA-IPDI-Cz type of polyurethane (PU) materials (P1-P5) showed superb hole injection and transport properties based on the results of the hole-only-device study. We prepared the devices in two kinds of configuration: system (1) Indium Tin Oxide (ITO)/PU (20 nm)/{Iridium(iii) bis(2-phenylpyridine), [Ir(ppy)₃] + 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (t-PBD) + polyvinylcarbazole (PVK)] (50 nm)/Mg (10 nm)/Ag (100 nm), for the device with P1 (DP1), the brightness increased to 14000 cd m^-2, the current efficiency rose to 13.4 cd A^-1, and the turn-on voltage was reduced to 21 V (at 100 cd m^-2). In system (2) ITO/PEDOT-PSS (30 nm)/PU (20 nm)/[Ir(ppy)₃ + t-PBD + PVK] (50 nm)/Mg (10 nm)/Ag (100 nm), with PEDOT-PSS as the hole-injection layer, was compared to the standard device (S2) having the configuration ITO/PEDOT-PSS (50 nm)/[Ir(ppy)₃ + t-PBD + PVK] (50 nm)/Mg (10 nm)/Ag (100 nm); the brightness of the double layer device with P5 (DDP5) increased to 12500 cd m^-2 and the current efficiency dramatically rose to 34.7 cd A^-1, compared with values of 6250 cd m^-2 and 21.8 cd A^-1, respectively, for S2. The Royal Society of Chemistry.

Full text of DOI:10.1039/b715929c

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/materials | Journal of Materials Chemistry
PLED devices containing triphenylamine-derived polyurethanes as
hole-transporting layers exhibit high current efficiencies†
ab
Cheng-Hsiu Ku,a Chao-Hui Kuo,a Chih-Yi Chen,a Man-Kit Leungac and Kuo-Huang Hsieh
*
Received 15th October 2007, Accepted 3rd January 2008
First published as an Advance Article on the web 29th January 2008
DOI: 10.1039/b715929c
The innovative polyurethane-type polymers from a triphenylamine derivative, [N,N0-bis(4-
hydroxyphenyl)-N,N0-diphenylbenzidine] (TPA) and a carbazole derivative, [9-butyl-3,6-bis(4-
hydroxyphenyl)carbazole] (Cz) can be linked through isophorone diisocyanate (IPDI) bridges and
incorporated as hole-transporting layers in high-performance PLED devices. The TPA–IPDI–Cz type
of polyurethane (PU) materials (P1–P5) showed superb hole injection and transport properties based
on the results of the hole-only-device study. We prepared the devices in two kinds of configuration:
system (1) Indium Tin Oxide (ITO)/PU (20 nm)/{Iridium(III) bis(2-phenylpyridine), [Ir(ppy)3] + 2-(4-
biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (t-PBD) + polyvinylcarbazole (PVK)] (50 nm)/Mg
(10 nm)/Ag (100 nm), for the device with P1 (DP1), the brightness increased to 14 000 cd mꢀ2, the
current efficiency rose to 13.4 cd Aꢀ1, and the turn-on voltage was reduced to 21 V (at 100 cd mꢀ2). In
system (2) ITO/PEDOT–PSS (30 nm)/PU (20 nm)/[Ir(ppy)3 + t-PBD + PVK] (50 nm)/Mg (10 nm)/Ag
(100 nm), with PEDOT–PSS as the hole-injection layer, was compared to the standard device (S2)
having the configuration ITO/PEDOT–PSS (50 nm)/[Ir(ppy)3 + t-PBD + PVK] (50 nm)/Mg (10 nm)/
Ag (100 nm); the brightness of the double layer device with P5 (DDP5) increased to 12 500 cd mꢀ2 and
the current efficiency dramatically rose to 34.7 cd Aꢀ1, compared with values of 6250 cd mꢀ2 and 21.8
cd Aꢀ1, respectively, for S2.
through a series of energy levels, where each successive energy
Introduction
gap is lower and more readily conquered, thus enhancing the
performance of the device.
In 1990, Friend and coworkers described the first polymeric light-
emitting diode (PLED),1 in which the luminescent poly(p-phenyl-
enevinylene) (PPV) was fabricated by spin-coating a precursor
polymer onto a transparent conducting indium tin oxide (ITO)
anode substrate, thermally converting the precursor into PPV,
and then evaporating an Al film cathode onto the PPV layer.
Iridium(III) bis(2-phenylpyridine) [Ir(ppy)3] was one of the
earliest developed green-light phosphorescent materials. Ir(ppy)3
is commonly employed for its high external quantum efficiency
performance in emitting devices, making them potentially useful
as low-power-consumption displays and good diffusive illumi-
nating light sources.2,3 The excellent light-emitting ability of
Ir(ppy)3 results from effective Fo¨rster energy transfer between
its singlet and triplet states leading to its high external quantum
efficiency.3 The host material is usually Ir(ppy)3 doped with the
hole-transporting polymer polyvinylcarbazole (PVK) or the elec-
The most widely used hole-transporting materials are triaryl-
amine derivatives because of their relatively high mobilities
and low ionization potentials; their hole-transporting rates are
ca. 1ꢁ10ꢀ3–10ꢀ4 cm V sꢀ1 4–6
.
Their ability to be used in the
spin-coating of large-area electroluminescent (EL) devices and
their large range of possible chemical modifications make triaryl-
amines promising hole-transporting materials. Carbazole-based
polymers are also drawing increased attention for use in light-
emitting devices because of their favorable optical properties
and good hole-transporting abilities.7 A good hole-transporting
material must exhibit a high transport rate, high thermal
stability, and the ability to be prepared in well-formed films.
The basic structural requirements for a hole-transporting mate-
rial are (1) a non-planar molecular structure to increase the
geometrical conformation in the molecules in order to inhibit the
formation of p-stacking, (2) functional groups that increase
the volume and molecular weight of the molecule, thus
enhancing its thermal stability, and (3) rigid groups, such as
carbazole moieties, or a combination of intermolecular hydrogen
bonds and non-planar molecules to increase the effective
molecular weight.8 There are many structures that display good
hole-transporting ability,6–17 for example, N,N0-bis(naphthalen-
1-yl)-N,N0-bis(phenyl)benzidine (NPB) is the most widely used
hole-transporting material because of its simple synthesis and
purification. Shirota, Bettenhausen, Tokito, Jandke, Thelakkat
et al.7,11–14 created amorphous triarylamine-based structures
exhibiting relatively high thermal stability and durability.
tron-transporting
2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-
oxadiazole (t-PBD). The energy within this layer is transferred
aInstitute of Polymer Science and Engineering, National Taiwan
University, Taipei, Taiwan, 106
bDepartment of Chemical Engineering, National Taiwan University,
Taipei, Taiwan, 106
cDepartment of Chemistry, National Taiwan University, Taipei, Taiwan,
106
† Electronic supplementary information (ESI) available: Experimental
details and NMR, mass spectral data for compounds 1–6, PU polymer
data collected as UV-Vis, PL, IR spectra and thermo properties as
DSC, TGA. PLED device fabrication details and measurements for
PU polymers. See DOI: 10.1039/b715929c
1296 | J. Mater. Chem., 2008, 18, 1296–1301
This journal is ª The Royal Society of Chemistry 2008

View Article Online
Scheme 1 Synthesis of the diol monomers.
In this present study, we prepared two series of polyurethanes
functionalized with triphenylamine-carbazole derivatives. By
taking advantage of the well-known hole-injecting ability of
triphenylamine systems and the hole-transporting ability of
carbazole systems,18–22 we combined these two components to
form a series of polymers exhibiting hole-injecting and hole-
transporting ability to replace PEDOT–PSS, the most widely
used hole-injection layer (HIL). We chose to synthesize PU
derivatives because their polymerization can be performed under
metal ion-free conditions, which is an important feature because
a low level of metal ion contaminants is an essential requirement
for high-performance electronic polymers from pure diols and
diisocyanates.23 The configurations of the PLED devices follow
and are separated into two systems for comparison. In system
1, a primitive standard device with ITO/Ir(ppy)3 + PVK +
PBD/Mg/Ag (S1), compares PU polymers inserted into ITO
and the emitting layer. In system 2, the devices were fabricated
in order to compare the hole-transporting capability of the
PEDOT and the PU polymers.
the carbazole derivatives (Cz), which are good hole-transporting
materials, according to a previous report.24 We selected the TPA
monomer
2
[N,N0-bis(4-hydroxyphenyl)-N,N0-diphenylbenzi-
dine] and the Cz monomer 6 [9-butyl-3,6-bis(4-hydroxyphenyl)
carbazole] as the basic components to construct PUs that could
act simultaneously as both hole-injecting and -transporting
layers as potential replacements for the most widely used hole-
injecting polymer PEDOT–PSS and possibly improve both the
brightness and current efficiency of the resulting devices. To
systematically evaluate the performance of the polymers, we
synthesized the homopolymer P1 incorporating only the TPA
monomer, copolymers P2–P4 at TPA-to-Cz feeding ratios of
3 : 1, 1 : 1, and 1 : 3, respectively, and the homopolymer P5
incorporating only the Cz monomer.
Monomer synthesis
Scheme 1 displays our synthetic route toward the two diol mono-
mers of the PU, monomers 2 and 6, which played the role of the
hole-transporting moiety in the PU. We synthesized monomer 2
in a two-step process. The reaction of N,N0-diphenylbenzidine
with 4-iodoanisole in 1,2-dichlorobenzene at 190 ꢂC for 36 h
produced N,N0-bis(4-methoxyphenyl)-N,N0-diphenylbenzidine
(1) in 73% yield after purification. Deprotection of 1 using
BBr3 in CH2Cl2 at ꢀ78 ꢂC for 2 h provided 2 in 92% yield after
column chromotagraphy (CH2Cl2–EtOAc, 4 : 1). We synthesized
the Cz monomer 6 in four steps. 9H-Carbazole was treated with
Results and discussion
Design of polymer
Our synthesized polyurethanes were of the general type TPA–
IPDI–Cz, where carbamate units linked the triphenylamine
derivatives (TPA), which are good hole-injecting materials, to
This journal is ª The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 1296–1301 | 1297

View Article Online
Scheme 2 Synthesis of the TPA–IPDI–Cz (PU) copolymers.
Table 1 Thermal properties of polymers P1–P5
NBS while cooling in an ice bath; recrystallization of the product
provided 3,6-dibromo-9H-carbazole (3) in 92% yield. The reac-
tion of 1-bromobutane with 3 in the presence of a phase transfer
agent and base provided 3,6-dibromo-9-butylcarbazole (4) in
65% yield. Suzuki coupling of 4 with 4-MeOC6H4B(OH)2 yielded
9-butyl-3,6-bis(4-methoxyphenyl)carbazole (5). BBr3-Mediated
deprotection of the two terminal methoxy groups of 5 gave the
monomer diol 6 in 70% yield.
Polymer
Mw
Mn
Mw/Mn
Tg/ꢂC
Td/ꢂCa
P1
P2
P3
P4
P5
7500
6000
7000
6200
7400
4800
5200
5500
3600
5200
1.57
1.16
1.26
1.72
1.42
174
175
178
174
165
274
281
285
295
272
a
The value of Td corresponds to the 5% weight-loss temperature in
TGA.
Synthesis and characterization of polymers P1–P5
which would harm the device performance.25 The values of Td for
our PU derivatives (270–295 ꢂC) were all above the device opera-
tion temperature, suggesting that they would be stable under the
conditions of thermal evaporation. Among the polymers P1–P5,
P4 displayed the highest value of Td.
We performed the polymerizations of monomers 2 and 6 in dry
DMF, using commercially available IPDI as the linker unit, to
obtain the TPA–IPDI–Cz copolymers P2–P4 at feeding ratios
of 3 : 1, 1 : 1, and 1 : 3, respectively (Scheme 2). We also synthe-
sized the homopolymers P1 and P5 from 2 and 6, respectively,
for comparison.
We obtained molecular weight data (values of Mn, Mw, and
PDI) for the prepared polyurethanes using gel permeation chro-
matography (GPC), analyzing against polystyrene standards in
DMF. Table 1 lists these data in addition to the polymers. The
values of Mw were stable, ranging from 6000 to 7500, with Mn
values from 3600 to 5500, suggesting narrow PDIs. The thermal
properties of a polymer are strongly correlated to the perfor-
mance of its devices. A high value of Td implies good thermal
stability. When a device is fabricated, the materials are evapo-
rated under high vacuum and at high temperature; thus,
polymeric materials that have high temperature-stability will
improve the life-time of the emitting device. In contrast, a material
that is unstable when heated might undergo morphological
change, deformation, or degradation of its film, potentially
hindering its ability to transport holes and electrons in the device,
Optical and electrochemical properties of polymers P1–P5
Fig. 1 displays the optical properties of polymers P1–P5, including
their UV-Vis absorption spectra; Table 2 lists the peak maxima
from each spectrum. In this series, the homopolymers P1 and
P5 and the copolymers P2–P4 exhibited their absorption peak
maxima in the region 290–305 nm, which we assign to the p–p*
electronic transition contributed by the polymer backbones. We
observed no spectral wavelength shifts in the solution spectra,
indicating that intra-chain interaction between the TPA and Cz
units was minimal. One of the typical peaks of the TPA moiety
at 300 nm overlapped with the p–p* transition signal; the inten-
sity of another peak at 350 nm reduced along with the TPA
content.26–28 The typical carbazole absorption peak at 260 nm
reduced in intensity along with a decrease in the Cz content,
1298 | J. Mater. Chem., 2008, 18, 1296–1301
This journal is ª The Royal Society of Chemistry 2008

View Article Online
Fig. 2 PL spectra of PU polymer films.
Fig. 1 UV-Vis absorption spectra of PU polymers in THF solution.
2.20 eV; thus, their band gap energies were in the range 3.10–
3.27 eV.
consistent with the polymerizations having been successful at
the various TPA-to-Cz feed ratios. Fig. 2 displays the photo-
luminescence (PL) spectra of P1–P5 as solid films. The peaks
all appeared at 420 nm and exhibited similar band widths.
Electroluminescence properties
We prepared devices in two configurations—(1) ITO/PU (20 nm)/
[Ir(ppy)3 + t-PBD + PVK] (50 nm)/Mg (10 nm)/Ag (100 nm); (2)
ITO/PEDOT–PSS (30 nm)/PU (20 nm)/[Ir(ppy)3 + t-PBD +
PVK] (50 nm)/Mg (10 nm)/Ag (100 nm) and compared their
performance to that of the blank devices S1 and S2, respectively.
Electrochemical characteristics
For organic electroluminescent materials, the HOMO and
LUMO levels are closely related to the ability of electrons and
holes to be injected from each electrode, because charge trans-
port is governed by a hopping process involving redox reactions
of the charge transport materials. We determined the redox
properties of our polymers using cyclic voltammetry (CV), scan-
ning the materials reversibly within the voltage range of their
oxidation and reduction potentials. When the applied potential
reached the oxidation (or reduction) potential, a redox reaction
occurred on the surface of the electrode and a redox current
was generated; we determined these onset potentials from the
points at which the curves exhibited an ious change in the CV
character.29 Table 2 lists the estimated HOMO and LUMO levels
of P1–P5, determined by calibrating to the standard potential of
ferrocene (4.8 eV); it also lists the band gap energy (Eg ¼ EHOMO
System (1): ITO/PU/[Ir(ppy)3 +PVK + t-PBD]/Mg/Ag
In system (1), the PU derivatives were spin-coated onto the ITO
glass substrate and then the emitting layer was spin-coated on
the PU surface; finally, Mg and Ag were vacuum deposited
onto the device. A primitive standard device, denoted ‘‘S1’’ and
having the configuration ITO/[Ir(ppy)3 + t-PBD + PVK] (50
nm)/Mg (10 nm)/Ag (100 nm), was prepared without the PU
layer for comparison. The devices, DP1–DP5, were fabricated
with the structure ITO/PU (20 nm)/[Ir(ppy)3 + t-PBD + PVK]
(50 nm)/Mg (10 nm)/Ag (100 nm). Table 3 summarizes the
brightness–voltage and current–efficiency–voltage behavior of
these devices. Luminescence maxima of 14 000, 9970, 11 800,
9190, and 9240 cd mꢀ2 were recorded for devices DP1–DP5,
respectively. The maximum current efficiencies of DP1–DP5
were 13.4, 5.5, 12.0, 3.6, and 6.4 cd Aꢀ1, respectively, under oper-
ating potentials of 22, 18, 22, 19, and 23 V, respectively. Fig. 3
indicates that relative to the standard device S1, the turn-on
– ELUMO) and the band gap was determined by the equation
ꢀ1 30
1241lonset(nm)
,
where lonset is the onset absorption wave-
length. The HOMO energies were all between 5.15 and 5.30
eV, suggesting that these polymers could be expected to have
good hole injection abilities when fabricated into devices. The
LUMO energy values of these polymers were between 1.97 and
Table 2 UV-Vis absorption, electrochemical, and photoluminescence properties of polymers P1–P5 as solid films
Polymer
labsmax/nm
lPLmax/nm
Eg/eV
HOMO/eV
LUMO/eV
P1
P2
P3
P4
P5
315, 370 (305, 365)a
425 (405)b
420 (425)
420 (423)
423 (420)
425 (427)
3.10
3.14
3.18
3.18
3.27
5.30
5.20
5.15
5.20
5.30
2.20
2.06
1.97
2.02
2.03
260, 310, 360 (260, 300, 360)
260, 305, 355 (260, 295, 355)
260, 300, 350 (260, 290, 350)
260, 300 (260, 290)
a
Values in parentheses represent the value of labsmax(nm) of the polymer in 5 mg/400 mL in THF solution. b Values in parentheses represent the value of
lPLmax(nm) of the polymer in 5 mg/20 mL in THF solution.
This journal is ª The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 1296–1301 | 1299

View Article Online
Table 3 Performance of polymers in system (1) devices
in brightness as a result of the hole-injecting and -transporting
abilities of the TPA and Cz units.
EL emission Turn-on voltage Max. brightness Efficiency
at 100 cd mꢀ2/eV cd mꢀ2/eV
Device lmax/nm
(cd Aꢀ1)/eV
System (2): ITO/PEDOT/PU/[Ir(ppy)3 + PVK + t-PBD]/Mg/
Ag
DP1
DP2
DP3
DP4
DP5
S1
513
510
510
510
513
508
14
17
18
14
17
31
14 000/26
9970/26
11 800/26
9190/21
9240/23
296/37
13.4/22
5.5/18
12.0/18
3.6/19
6.4/23
1.1/32
In system (2), PEDOT–PSS was first spin-coated onto the glass
substrate in DDP1–DDP5 and the standard device S2, followed
by the PU hole-injecting and -transporting layer and the emitting
layer; the metals Mg and Ag were then evaporation-deposited
onto the devices. A double hole-transporting standard device
(S2) having the configuration ITO/PEDOT–PSS (30 nm)/
[Ir(ppy)3 + t-PBD + PVK] (50 nm)/Mg (10 nm)/Ag (100 nm)
was prepared for comparison. Table 4 and Fig. 5 and 6 provide
the results of brightness–voltage and current efficiency–voltage
Table 4 Performance of polymers in system (2) devices
EL emission Turn-on voltage Max. brightness Efficiency
(cd Aꢀ1)/eV
Device lmax/nm
at 100 cd mꢀ2/eV cd mꢀ2/eV
DDP1 511
DDP2 506
DDP3 507
DDP4 509
DDP5 509
11
13
14
14
13
12
12 500/22
12 100/25
9900/24
10 500/23
8280/24
6250/22
29.4/16
19.6/15
17.1/15
21.0/15
34.7/17
21.8/15
Fig. 3 Brightness–voltage plots for the devices DP1–DP5 and the
S2
508
standard device S1.
voltages, maximum brightness, and maximum efficiencies of
DP1–DP5 had improved dramatically. For example, the turn-
on voltage of the device incorporating the P1 homopolymer as
the hole-injecting layer was reduced to 14 V from 31 V for S1,
the maximum brightness increased from 296 cd mꢀ2 at 37 V
for S1 to 14 000 cd mꢀ2 at 26 V for DP1, and the maximum
efficiency improved to 13.4 cd Aꢀ1 at 22 V from 1.1 cd Aꢀ1 at 32
V for the standard device. DP1 exhibited the best hole-injecting
performance in terms of brightness because of the high content
of the TPA in the P1 polymer. Fig. 4 presents the current
efficiency plotted as a function of the applied voltage for the
series of TPA–IPDI–Cz copolymers. The use of the PU layer
as both a hole-injecting and -transporting layer on the ITO
anode led to significantly improved EL performance relative to
that of device S1. DP1–DP5 exhibited significant improvements
Fig. 5 Brightness–voltage plots for system (2) devices DDP1–DDP5.
Fig. 4 Current efficiency–voltage plots for devices DP1–DP5 and the
Fig. 6 Current efficiency–voltage plots of system (2) devices DDP1–
standard device S1.
DDP5.
1300 | J. Mater. Chem., 2008, 18, 1296–1301
This journal is ª The Royal Society of Chemistry 2008

View Article Online
measurements of the devices. Luminescence maxima of 12 500,
12 100, 9900, 10 500, and 8200 cd mꢀ2 were recorded for devices
DDP1–DDP5 respectively. The maximum current efficiencies of
the devices were 29.4, 19.6, 17.1, 21.0, and 34.7 cd Aꢀ1 under
operating potentials of 16, 15, 15, 15, and 17 V, respectively.
Relative to that of the standard device S2 (12.9 cd Aꢀ1), the
device DDP5 exhibited a significantly improved current effi-
ciency of 34.7 cd Aꢀ1 as a result of the presence of P5 as the
hole-transporting layer. The DDP1 device also displayed a higher
current efficiency (29.4 cd Aꢀ1 at 16 V) than that of the standard
device S2 (21.8 cd Aꢀ1 at 15 V).
References
1 J. H. Burroughs, D. D. C. Bradley, A. R. Brown, R. N. Marks,
K. Mackay, R. H. Friend, P. L. Burn and A. B. Holmes, Nature,
1990, 347, 539–541.
2 E. A. Plummer, A. Dijken, H. W. Hofstraat, L. D. Cola and
K Brunner, Adv. Funct. Mater., 2005, 15, 281–289.
3 F. C. Chen, S. C. Chang, G. He, S. Pyo, Y. Yang, M. Kurotaki and
J Kido, J. Polym. Sci., Part B, 2003, 41, 2681–2690.
4 B. Chen, C. S. Lee and S. T. Lee, Jpn. J. Appl. Phys., 2000, 39, 1190–
1192.
5 A. Kimoto, J. S. Cho, M. Higuchi and
Macromolecules, 2004, 37, 5531–5537.
6 U. Bach, K. Cloedt, H. Spreitzer and M Gra¨tzel, Adv. Mater., 2000,
12, 1060–1063.
7 Y. Shirota, K. Okumoto and H Inada, Synth. Met., 2000, 111, 387–
391.
8 Y. Shirota, T. Kobata and N Noma, Chem. Lett., 1989, 1145–1148.
9 J. Salbeck, N. Yu, Bauer, F. Weissotel and H Bestgen, Synth. Met.,
1997, 91, 209–211.
10 H. Fujikawa, M. Ishii, S. Tokito and Y Taga, Mater. Res. Soc. Symp.
Proc., 2000, 621, Q3.
11 J. Bettenhausen and P Strohriegel, Adv. Mater., 1996, 8, 507–510.
12 S. Tokito, H. Tanaka, K. Noda, A. Okada and Y Taga, Macromol.
Symp., 1997, 125, 181–185.
K Yamamoto,
The identical green emissions of these devices suggest that they
all functioned with identical light-emitting mechanisms and
emission zones. We assign the EL peak at 511 nm to the emission
from the Ir(ppy)3 + PVK + t-PBD emitting layer. Introduction
of the PU layer did not alter the emission mechanism. The elec-
troluminescence spectral properties of the PLED devices all
retained the same emission wavelength relative to that of the
standard device S2.
The accelerated life-time study was conducted at an operating
current of 100 mA cmꢀ2. The presence of the PU polymers as
hole-transport layers significantly improved the half-life of the
device.
13 M. Jandke, P. Strohriegel, S. Berleb, E. Werner and W Brutting,
¨
Macromolecules, 1998, 31, 6434–6443.
14 M. Thelakkat and H. W. Schmidt, Adv. Mater., 1998, 10, 219–223.
15 U. Bach, D. Lupo, P. Compte, J. E. Moser, F. Weisso¨rtel, J. Salbeck,
H. Spreitzer and M Gra¨tzel, Nature, 1998, 395, 583–585.
16 G. Yu, J. Gao, J. C. Hummelen, F. Wudi and A. J. Heeger, Science,
1995, 270, 1789–1791.
17 M. Gransro¨m, K. Petrisch, A. C. Arias, A. Lux, M. R. Anderson and
H. R. Friend, Nature, 1998, 395, 257–260.
Conclusions
18 M. Sun, J. Li, B. Li, Y. Fu and Z Bo, Macromolecules, 2005, 38, 2651–
2658.
19 S. Jungermann, N. Riegel, D. Muller, K. Meerholz and O Nuyken,
¨
Macromolecules, 2006, 39, 8911–8919.
20 G. K. Paul, J. Mwaura, A. A. Argun, P. Taranckar and R. Reynolds,
Macromolecules, 2006, 39, 7789–7792.
21 C. W. Ko, Y. T. Tao, J. T. Lin and Thomas, Chem. Mater., 2002, 14,
357–361.
22 A. Baba, K. Onishi, W. Knoll and R. C. Advincula, J. Phys. Chem.,
2004, 108, 18949–18955.
23 C. H. Kuo, K. C. Peng, L. C. Kuo, K. H. Yang, J. H. Lee,
M. K. Leung and K. H. Hsieh, Chem. Mater., 2006, 18, 4121–4129.
24 J. Li, C. Ma, J. T. Tang, C. S. Lee and S Lee, Chem. Mater., 2005, 17,
615–619.
25 Z. Peng, Z. Bao and M. E. Galvin, Chem. Mater., 1998, 10, 2086–
2090.
We have developed PU polymers and copolymers incorporating
triphenylamine and carbazole components linked through IPDI
units. Incorporating these PU derivatives as hole-transporting
layers in PLED devices greatly improved the performance of
the devices; in particular, the use of the triphenylamine PU
homopolymer as the hole-transporting layer resulted in
outstanding brightness (12 500 cd mꢀ2 at 22 V) and superior
current efficiency (29.4 cd Aꢀ1 at 16 V) and carbazole derivative
PU polymer, P1, as the hole-transporting layer in DDP5
obtained dramatic high current efficiency (34.7 cd Aꢀ1 at 17 V).
The presence of these PU layers within the devices not only
improved the device performance but also maintained the emis-
sion color at the same wavelength.
26 A. Baba, K. Onishi, W. Knoll and R. C. Advincula, J. Phys. Chem.,
2004, 108, 18949–18955.
27 E. Bellmann, S. E. Shaheen, S. Thayumanavan, S. Barlow,
R. H. Grubbs, S. R. Marder and B Kippelen, Chem. Mater., 1998,
10, 1668–1676.
Acknowledgements
28 S. Liu, X. Jiang, H. Ma, M. S. Liu, Jen and K. Y. Alex,
Macromolecules, 2000, 33, 3514–3517.
29 L. Liao, Y. Pang, L. Ding and F. E. Karaz, Macromolecules, 2004, 37,
3970–3972.
30 R. Tang, Z. Tan, Y. Li and F. Xi, Chem. Mater., 2006, 18, 1053–1061.
This study was supported by the Ministry of Economic Affairs
(grant no. 93-EC-17-A-08-S1-0015), the National Science
Council of Taiwan, and the Advanced Polymer Nanotechnology
Research Center.
This journal is ª The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 1296–1301 | 1301