Thermally cross-linkable hyperbranched polymers containing triphenylamine moieties: Synthesis, curing and application in light-emitting diodes

Article abstract of DOI:10.1016/j.polymer.2010.08.001

This paper demonstrates synthesis of hyperbranched polymers (HTP and HTPOCH₃), containing triphenylamine moieties in main chain and thermally cross-linkable periphery or terminal vinyl groups, and application as hole-transporting layer (HTL) in

Full text of DOI:10.1016/j.polymer.2010.08.001

Polymer 51 (2010) 4484e4492
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Thermally cross-linkable hyperbranched polymers containing triphenylamine
moieties: Synthesis, curing and application in light-emitting diodes
Juin-Meng Yu, Yun Chen*
Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 May 2010
Received in revised form
This paper demonstrates synthesis of hyperbranched polymers (HTP and HTPOCH ), containing tri-
phenylamine moieties in main chain and thermally cross-linkable periphery or terminal vinyl groups,
and application as hole-transporting layer (HTL) in multilayer light-emitting diodes. Absorption and
photoluminescence (PL) spectroscopy, cyclic voltammetry (CV) and differential scanning calorimetry
3
24 July 2010
Accepted 3 August 2010
Available online 7 August 2010
(
DSC) were employed to investigate their photophysical, electrochemical properties and thermal curing
behaviors, respectively. The hyperbranched HTP and HTPOCH
3
were readily cross-linked by heating scan,
) of
ꢀ
with the exothermic peaks being at 221 and 210 C respectively. The glass-transition temperatures (T
g
Keywords:
Hyperbranched polymers
Triphenylamine
ꢀ
ꢀ
the hyperbranched polymers were higher than 140 C after thermal cross-linking at 210 C for 30 min.
Multilayer light-emitting diodes (ITO/PEDOT:PSS/HTL/MEH-PPV/Ca/Al), using HTP and HTPOCH as HTL,
3
Thermally cross-linkable
were readily fabricated by successive spin-coating. The performance of MEH-PPV device (maximum
2
luminance: 9310 cd/m , luminance efficiency: 0.26 cd/A) was effectively enhanced by inserting the
2
2
thermally cross-linked HTP or HTPOCH
.33 cd/A). This indicates that these thermally cross-linkable hyperbranched HTP and HTPOCH
suitable for the fabrication of multilayer PLEDs using solution processes.
3
as HTL (HTP: 12610 cd/m , 0.32 cd/A; HTPOCH
3
: 14060 cd/m ,
0
3
are very
Ó 2010 Elsevier Ltd. All rights reserved.
1
. Introduction
linear counterparts, hyperbranched polymers possess higher
concentration of reactive vinyl groups, leading to densely cross-
linked structures after thermal curing. Highly cross-linked struc-
tures usually result in better solvent resistance and thermal
stability. Solvent resistant characteristic is essential in the fabrica-
tion of multiple-layer polymeric light-emitting diodes (PLEDs),
since it will prevent solvent erosion during subsequent spin-
coating processes. Accordingly, solvent resistance is a prerequisite
for hole-transporting polymers applied in PLEDs.
Polymeric light-emitting diodes have been viewed as major
next-generation flat panel displays due to its low power con-
sumption and large area display via spin-coating or inject-printing
techniques [14e17]. To obtain high performance PLEDs necessitates
efficient and balanced charges injection/transport from cathode
and anode, respectively [18]. The multiple-layer EL devices are an
alternative to attain efficient EL emission. But during the fabrication
of a multiple-layer device the deposited layer may be dissolved by
the subsequent spin-coating solution [19]. To avoid this undesirable
dissolution, cross-linkable precursors with some specific functional
groups become necessary for the device fabrication [18e21].
During EL device fabrication hole-transporting layer (HTL) is
usually the first one deposited upon hole-injection PEDOT:PSS layer
Hyperbranched polymers have gained much attention recently
due to attractive advantages derived from their unique structures
[1]. For example, hyperbranched structure has been introduced to
conjugated polymers to suppress the formation of undesirable
aggregate and excimer which generally deteriorates the perfor-
mance of polymeric light-emitting diode (PLED) [2e6]. Hyper-
branched polymers possess many advantages over linear
counterparts, such as lower intrinsic viscosity, improved solubility
and reduced intermolecular interaction [7]. Moreover, stable and
homogeneous amorphous film is readily obtainable owing to
reduced inter-chain cohesive force [8]. This amorphous and
homogeneous morphology is critical for the fabrication of efficient
multilayer light-emitting diodes [7e10]. Moreover, the terminal
functional groups of a hyperbranched polymer can be readily
changed by controlling the stoichiometry of feed monomers. For
instance, the terminal of a hyperbranched polymers are reactive
vinyl groups when prepared by the Heck coupling reaction using an
excess of di- or trivinyl aromatic monomer [11e13]. Comparing to
[
22]. Triphenylamine-based derivatives are the most commonly
*
Corresponding author. Tel.: þ886 2085843; fax: þ886 2344496.
E-mail address: yunchen@mail.ncku.edu.tw (Y. Chen).
used hole-transporting material due to their high hole mobility and
0
032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.08.001

J.-M. Yu, Y. Chen / Polymer 51 (2010) 4484e4492
4485
ionization potential (high HOMO energy level). In general, high
HOMO level is beneficial to hole-injection and transport [23,24]. Jen
and coworkers synthesized numerous thermally cross-linkable
hole-transporting materials derived from triphenylamine for
solution processed polymer light-emitting diodes [18,25e28].
However, most polymer HTLs have been focused on linear structure
with little attention paid to hyperbranched-based ones [29]. As
mentioned above, the unique properties of hyperbranched poly-
mers would result in EL devices with improved stability and effi-
ciency over those of linear counterparts [24]. Reynolds and
made up of a polymer-coated glassy carbon as the working electrode,
an Ag/AgCl electrode as the reference electrode, and a platinum wire
as the auxiliary electrode. The electrodes were immersed in aceto-
4 4
nitrile containing 0.1 M (n-Bu) NClO as electrolyte. The energy levels
were calculated using the ferrocene (FOC) value of ꢁ4.8 eV with
respect to vacuum level, which is defined as zero [38]. Surface
morphology and root-mean-square (rms) roughness of deposited
polymer films were measured using an atomic force microscope
(AFM) equipped with a Veeco/Digital Instrument Scanning Probe
Microscope (tapping mode) and a Nanoscope IIIa controller. The root-
mean-square (rms) roughness of a deposited polymer film was
estimated by averaging the rms roughness of at least two different
spots.
2
coworkers synthesized an AB -type monomer from triphenyl-
amine, from which cross-linked hyperbranched polymer were
obtained via the Heck reaction [24]. A PLED device using the
hyperbranched polymer as HTL and MEH-PPV as emitting layer
4
2
exhibited a maximum luminance of about 10 cd/m .
3
2.2. Synthesis of hyperbranched polymers (HTP and HTPOCH )
We have successfully developed tri-functional fluorene deriva-
tives to be applied in the preparation of hyperbranched copoly-
fluorenes [30e33]. As part of our effort in the preparation of
hyperbranched copolyfluorenes for optoelectronic applications, we
have further synthesized new thermally cross-linkable hyper-
branched polymers from 2,4,7-trivinyl-9,9-dihexylfluorene (3) and
triphenylamine-derived dibromo monomers (4, 5) via the Heck
reaction. The introduction of fluorene moiety into polymer main
chain leads to not only good thermal and chemical stability but also
good charge mobility [34e36]. Moreover, when applied as HTL in
multilayer devices using MEH-PPV as emitting layer, the opto-
electronic performances were greatly improved, with maximum
Hyperbranched polymers containing reactive vinyl groups were
synthesized from dibromo compound (4 or 5) and an excess of
trivinylfluorene (3) by the Heck coupling reaction. For example,
a mixture of monomer 4 (0.12 g, 0.3 mmol), monomer 3 (0.13 g,
2 2 3
0.33 mmol), Pd(OAc) , tri-o-tolyphophine, tributylamine, K CO ,
7 mL of DMF, 10 mL of toluene and 0.5 mL of water was stirred at
ꢀ
100 C for 46 min under nitrogen atmosphere. The solution was
poured into a large amount of methanol and the appeared precipi-
tate was collected by filtration and dried in vacuo at room temper-
ature for 1 day to afford HTP (Yield: 56%). HTPOCH was prepared by
3
analogous procedures, exceptusing monomer 5 instead of monomer
4 (Yield: 45%).
4
2
luminance higher than 10 cd/m . These results demonstrate that
the hyperbranched polymers are promising hole-transporting
materials for the fabrication of multilayer PLEDs.
1
ꢀ
HTP: H NMR (400 MHz, CDCl3, TMS, 25 C):
d
7.81e7.03 (m,
),
), 1.99e1.95 (m,
). C NMR (400 MHz,
ArH), 6.84e6.76 (m, 2H, ¼ CH-Ar), 5.83e5.78 (m, 3H, ¼ CH
2
5
.50e5.48 (m, 1H, ¼ CH
2
), 5.29e5.25 (m, 2H, ¼ CH
2
13
2
. Materials and methods
4H, -CH
CDCl
2
-),1.25e0.63 (m, 22H, -CH
2
- and -CH
3
ꢀ
3
, TMS, 25 C):
d
152.1, 147.3, 146.9, 137.2, 135.7, 132.0, 129.3,
2
.1. Materials and characterization
127.6, 127.3, 125.3, 124.7, 123.9, 123.3, 120.3, 119.4, 116.6, 54.3, 40.78,
1.4, 29.7, 23.6, 22.5, 13.9. Anal. Found (%) for HTP: C, 88.8; H, 7.9; N,
1.9.
HTPOCH
(m, ArH), 6.87e6.81 (m, 2H, ¼ CH-Ar), 5.83e5.78 (m, 3H, ¼ CH
), 3.83e3.78 (m,
3
2
,4,7-Tri(bromomethyl)-9,9-dihexylfluorene (1), 2,4,7-tri[methy-
lene (triphenylphosphonium bromide)]-9,9-dihexylfluorene (2),
,4,7-trivinyl-9,9-dihexylfluorene (3) and 4-methoxy-N,N-bis(4-
1
ꢀ
3
: H NMR (400 MHz, CDCl
3
, TMS, 25 C):
d
7.79e7.01
),
2
2
bromophenyl)aniline (5) were synthesized according to the proce-
dures reported previously [30,37]. The synthetic procedures and
characterization data are described in the Supporting Information.
5.50e5.48 (m, 1H, ¼ CH
2
), 5.29e5.21 (m, 2H, ¼ CH
2
3H, Ar-OCH ), 1.99e1.95 (m, 4H, -CH
3
2
-), 1.11e0.63 (m, 22H, -CH
2
-
1
3
ꢀ
and -CH
3
). C NMR (400 MHz, CDCl
3
, TMS, 25 C):
d
156.5, 152.1,
0
Tri-(o-tolyl)phosphine, tributylamine and 4,4 -dibromotriphenyl-
147.2,140.1,137.2,136.6, 135.7,127.5,127.2,123.9, 123.3, 122.8, 120.2,
119.4, 116.6, 114.8, 55.5, 54.3, 40.7, 31.4, 29.7, 23.6, 22.5, 13.9. Anal.
Found (%) for HTPOCH : C, 87.4; H, 8.0; N, 1.8.
3
amine (4) were procured from Acros Co. and used as received.
Palladium (II) acetate was purchased from Wako Co. and used
without further purification. N,N-Dimethylformamide (DMF) and
other reagents were used without further purification except
specifically notified. All synthesized compounds were identified by
2.3. Fabrication of light-emitting diodes
1
13
1
13
H NMR, C NMR, and elemental analysis (EA). The H and C NMR
Multiple-layer light-emitting diodes (ITO/PEDOT:PSS/HTP or
spectra were recorded on a Bruker AMX-400 MHz FT-NMR, and the
chemical shifts are reported in ppm using tetramethylsilane (TMS) as
an internal standard. The molecular weights and molecular weight
distributions of polymers were determined by a gel permeation
chromatograph (GPC) using THF as eluent, using monodisperse
polystyrene standards as calibration standards. The inherent
3 3
HTPOCH /MEH-PPV/Ca/Al) employing HTP or HTPOCH as hole-
transporting layer were fabricated to investigate their optoelec-
tronic characteristics. The device without the hole-transporting
layer (ITO/PEDOT:PSS/MEH-PPV/Ca/Al) was also fabricated for
comparative study. The ITO-coated substrate glasses were washed
successively in ultrasonic baths of neutraler reiniger/de-ionized
water (1:3 v/v) mixture, de-ionized water, acetone and 2-prop-
anol, followed with cleaning in a UV-ozone chamber. A thick hole-
injection layer of PEDOT:PSS was spin-coated on top of the cleaned
ꢁ1
viscosities were measured at concentration of 0.05 g dL in CHCl
3
at
ꢀ
3
0 C using an Ubbelohde viscometer. The elemental analysis was
carried out on a Heraus CHN-Rapid elemental analyzer. The thermal
curing behaviors and thermal transitional properties of the hyper-
branched polymers were recorded using a differential scanning
ꢀ
ITO glass and annealed at 150 C for 15 min in a dust-free atmo-
sphere. Upon the hole-injecting PEDOT:PSS layer was spin-coated
(4000 rpm) with a hole-transporting layer from a hyperbranched
ꢀ
calorimeter (DSC), Mettler DSC 1, with a heating rate of 20 C/min.
Absorption and photoluminescence (PL) spectra were measured on
a Jasco V-550 spectrophotometer and a Hitachi F-4500 spectrofluo-
rometer, respectively. Electrochemical properties were investigated
with a voltammetric analyzer (model CV-50W from Bioanalytical
Systems, Inc.) equipped with a three-electrode cell. The cell was
polymer (HTP or HTPOCH
3
) solution in chlorobenzene (1 mg/mL).
ꢀ
The hole-transporting layer was cured at 210 C for 30 min under
nitrogen atmosphere; then it was spin-coated (3000 rpm) with
MEH-PPV solution (10 mg/mL in chlorobenzene) to deposit
emitting layer. Polymer solutions were filtered through a syringe

4
486
J.-M. Yu, Y. Chen / Polymer 51 (2010) 4484e4492
Br
t-BuOk/DMF
CH (CH2) Br
(
CHO)n
HBr
Br
3
5
Br
1
+
Br-
P Ph
3
NaOH(aq)
PPh3
DMF
-BrPh
+P
3
3
7wt% HCHO(aq)
-BrPh
+P
3
3
2
Scheme 1. Synthetic routes of branch monomer (3).
Br
Br
N
R
+
4
5
: R = H
: R = OCH3
H
3
e
c
k
r
e
a
c
t
i
o
n
R
R
N
N
N
R
HTP : R = H
HTPOCH3 : R = OCH3
o
2
10 C; 30 min
under nitrogen
Cured HTP
Cured HTPOCH
3
3
Scheme 2. Synthesis and thermal cross-linking of hyperbranched polymers (HTP and HTPOCH ).

J.-M. Yu, Y. Chen / Polymer 51 (2010) 4484e4492
4487
Table 1
3. Result and discussion
3
Polymerization results and thermal properties of HTP and HTPOCH .
(104)a
M
(104)a PDIa
h
inh (dL/g)
b
6
H (J/g)c
T
( C)
ꢀ
c
Polymer
M
n
w
g
3.1. Synthesis and characterization of hyperbranched polymers
(
HTP and HTPOCH
3
)
HTP
HTPOCH
0.7
0.3
1.5
0.8
2.1 0.82
2.6 0.47
42.9
50.5
109.2 (149.6)
96.8 (141.7)
3
a
Dibromo monomer 5 was synthesized from 4-methoxy-
Determined by gel permeation chromatography using polystyrene as standard.
b
c
ꢀ
Inherent viscosity: measured at a concentration of 0.05 g/dL CHCl
3
at 30 C.
N,N-diphenylaniline and N-bromosuccinimide, following the
procedures reported in literature [37]. The preparation procedure
of branch monomer 2,4,7-trivinyl-9,9-dihexylfluorene (3) had been
reported in our previous study (Scheme 1) [30]. Hyperbranched
hole-transporting polymers containing triphenylamine (HTP and
Determined by differential scanning calorimeter (DSC) with a heating rate of
ꢀ
2
0
C/min in the first heating scan. The values in the parentheses are the T
g
s of the
cured polymers (obtained from the second scan).
filter (0.2
m
m pore size) before the spin-coating. Finally, a thick
HTPOCH
3
) were synthesized via the Heck coupling reaction using
2
approach (Scheme 2). To increase the amount of reactive
layer of Ca and Al were successively deposited as cathode via
A
2
þ BB’
ꢁ
6
vacuum deposition under 1 ꢂ 10
Torr. The luminance-bias,
vinyl groups available for following thermal curing, an excess of
monomer 3 was employed for the polymerization. However, the
polymerization led to final gelation even at this stoichiometric
imbalance due to the presence of tri-functional monomer 3. In
order to avoid the cross-linking of the polymers, the polymeriza-
tion was interrupted before the gellation. This early interruption of
polymerization gives rise to low polymer yields (45e56%). The
current density-bias, and EL spectral characteristics of the devices
were recorded using a combination of Keithley power source
(
model 2400) and Ocean Optics usb2000 fluorescence spectro-
photometer. Device fabrication was done in ambient conditions,
with the following performance tests conducted in a glove-box
filled with nitrogen.
Fig. 1. 1H NMR of HTP (a) and HTPOCH
caused by trace CHCl ).
3
3 *
(b) dissolved in CDCl . The chemical shifts are reported in ppm using tetramethylsilane (TMS) as an internal standard ( refers to the signal
3

4
488
J.-M. Yu, Y. Chen / Polymer 51 (2010) 4484e4492
ꢀ
3
measured in CHCl (0.05 g/dL) at 30 C were 0.82 and 0.47 dL/g,
respectively, which were lower than that of linear poly(9,9-dihex-
ylfluorene) (4.7 dL/g). The relative low inherent viscosities of HTP
HTP
and HTPOCH
polymers (HTP and HTPOCH
organic solvent such as chloroform and toluene. Fig. 1 shows the H
NMR spectra of the hyperbranched HTP and HTPOCH , in which
3
support their branched structure [40]. The resulting
HTPOCH3
) are readily soluble in common
Second scan
3
1
Tg
o
3
2
21 C
the peaks located at 0.6e1.3 ppm and 2.0 ppm has been assigned to
protons of pendant hexyl groups. The characteristic peaks at
5.2e6.0 ppm confirms the existence of periphery vinyl groups, from
which thermally cross-linkable characteristic can be expected for
the hyperbranched polymers.
5
0
100
150
200
250
o
Temperature ( C)
First scan
o
Tg
210 C
3.2. Thermal curing and morphology study of the hyperbranched
polymers
5
0
100
150
200
250
o
3
Since the hyperbranched polymers (HTP, HTPOCH ) contain
reactive vinyl groups in periphery and terminal units, they will
form as three-dimensional structure by suitable thermal curing.
Temperature ( C)
3
Fig. 2. DSC traces of HTP and HTPOCH . First run was used to observe the cross-linking
reaction heat of vinyl groups. The second run was employed to examine completeness
of the curing reaction and to determine the glass-transition temperature (inset). (Scan
rate: 20 C/min).
The curing behaviors of the HTP and HTPOCH
by differential scanning calorimetry (DSC) as shown in Fig. 2.
During the first heating scan, the glass-transition temperatures (T
3
were investigated
ꢀ
g
)
ꢀ ꢀ
3
of the HTP and HTPOCH are observed at 109.2 C and 96.8 C, with
ꢀ
ꢀ
weight-average molecular weights (M
w
) of HTP and HTPOCH
3
, as
exotherm peaks being at 221 C and 210 C, respectively. The
exothermic heat is attributable to cross-linking reaction of the vinyl
groups. After the first heating cycle, the glass-transition tempera-
tures (T ) of the HTP and HTPOCH are raised significantly to be
determined by gel permeation chromatography using mono-
4
4
disperse polystyrene as standard, was 1.5 ꢂ 10 and 0.8 ꢂ 10
(
Table 1), with the polydispersity indexes (PDI) being 2.1 and 2.6,
g
3
ꢀ
ꢀ
respectively. The large PDI values are attributable to growth rate
difference between larger molecules and small ones. Because the
growth rate is proportional to functionality, larger molecules grow
faster than smaller ones that result in broadened molecular weight
149.6 C and 141.7 C (inset), substantiating their thermal cross-
linking reaction during the first scan. No obvious melting endo-
ꢀ
thermic is observed in the DSC traces up to 250 C. Accordingly, the
hyperbranched polymers are basically amorphous materials with
distribution [39]. The inherent viscosities of HTP and HTPOCH
3
high T s, which can reduce the tendency of crystallization and
g
ꢀ
Fig. 3. The AFM images of (a) cured HTP and (b) cured HTPOCH
Scan size: 5 ꢂ 5 m).
3
films. The HTP and HTPOCH
3
were coated on ITO glass and cured at 210 C for 30 min under nitrogen atmosphere.
(
m

J.-M. Yu, Y. Chen / Polymer 51 (2010) 4484e4492
4489
Table 2
during cross-linking often leads to microcrack formation in their
cured products. This may also happen in the thermal curing of
3
Photophysical properties of HTP and HTPOCH .
Polymer UVevis
l
max
PL
(nm)
l
max solution UVevis
l
max
PL
(nm)
l
max film
3
HTP and HTPOCH that increases undesirable surface roughness.
a
a
solution (nm)
film (nm)
Atomic force microscopy (AFM) was employed to observe their
surface morphology and to estimate the root-mean-square (rms)
roughness of pristine and cured polymer films. Fig. 3 shows the
418, 341sb
423, 340sb
3
478
488
424, 345s
431, 344s
b
b
483
494
HTP
HTPOCH
a
ꢁ5
1
ꢂ 10 M in CHCl
3
.
AFM images of cured HTP and HTPOCH
3
films. Both HTP and
b
The s means wavelength of shoulder.
HTPOCH
exhibit homogeneous films after thermal curing at
3
ꢀ
210 C for 30 min, no defect such crack or pinhole is observed
in their AFM images. Moreover, the root-mean-square (rms)
roughness of pristine HTP and HTPOCH films are 0.85 nm and
maintain long-term morphological stability under device opera-
3
tion. To examine the appropriate curing time, their films were
subjected to 210 C to observe the exothermic behaviors for 80 min.
As shown in the DSC traces (Fig. S1 in Supporting Information), the
exothermic curing reaction is almost complete within 30 min.
0.95 nm respectively. To fabricate efficient PLED device requires
highly homogeneous film of each deposited polymers. Hyper-
branched polymers can improve film morphology because of
their highly branched and globular structure, which decreases the
inter-chain cohesive force to result in more stable amorphous
film [8e10,41]. Furthermore, the root-mean-square (rms) rough-
ness of films can be decreased slightly to be 0.77 nm and 0.89 nm
after the thermal curing. This means that the thermal curing
raises not only solvent resistance but also smoothness of their
film surface.
ꢀ
Therefore, the curing conditions of HTP and HTPOCH
following investigations were set at 210 C for 30 min under
nitrogen atmosphere.
3
in the
ꢀ
Successfully fabrication of PLED devices requires homoge-
neous and defect-free polymer layer to prevent reduced life-time
and deteriorated performance. Volume shrinkage of prepolymers
0.25
0.20
0.15
0.10
0.05
0.00
a
b
0.15
0.10
0.05
0.00
HTP
Cured HTP
HTP spin-coated
with chlorobenzene
Cured HTP spin-coated
with chlorobenzene
300
400
500
600
300
400
500
600
Wavelength (nm)
0
.25
.20
.15
.10
.05
.00
0.25
0.20
0.15
0.10
0.05
0.00
c
d
HTPOCH3
HTPOCH3
Cured HTPOCH
Cured HTPOCH
spin-coated with
chlorobenzene
3
0
3
spin-coated with
chlorobenzene
0
0
0
0
3
00
400
500
600
300
400
500
600
Wavelength (nm)
ꢀ
3
before and after spin-coated with chlorobenzene: (a) and (c) are pristine films, (b) and (d) are thermally cured films (210 C for
Fig. 4. Absorption spectra of HTP and HTPOCH
0 min under nitrogen atmosphere).
3

4
490
J.-M. Yu, Y. Chen / Polymer 51 (2010) 4484e4492
Table 3
Table 4
a
Electrochemical properties of the HTP and HTPOCH
3
.
Optoelectronic properties of the light-emitting diodes .
opt
(eV)d
E
ox vs. FOC (V)a
E
HOMO (eV)b
E
LUMO (eV)c
E
Hole-
Turn-on
Maximum
Luminance
(cd/m )
Luminance
CIE 1931
(x, y)
transporting Voltageb
Efficiency
HTP
HTPOCH
0.37(0.83)
0.27(0.73)
ꢁ5.17
ꢁ5.07
ꢁ2.60
ꢁ2.44
2.57
2.53
2
c
layer
(V)
(cd/A)
3
Non
HTP
HTPOCH
3.1
2.7
2.7
9310
12610
14060
0.26
0.32
0.33
(0.55, 0.43)
(0.56, 0.42)
(0.57, 0.42)
a
E
E
E
FOC ¼ 0.46V vs. Ag/AgCl.
b
c
HOMO ¼ ꢁe(Eox, FOC þ 4.8 V).
opt
3
LUMO ¼ ꢁe(EHOMO þ E
g
).
d
opt
a
Band-gaps estimated from onset absorption (
l
onset): E
g
¼ 1240/
l
onset
.
Device structure: ITO/PEDOT/Hole-transporting layer/MEH-PPV/Ca/Al.
Arbitrarily defined as the voltage required for a luminance of 10 cd/m .
The luminance efficiency at 1000 cd/m .
b
c
2
2
3
.3. Photophysical properties of HTP and HTPOCH
3
Table 2 summarizes the characteristic optical data of HTP and
HTPOCH in CHCl and as thin films spin-coated on quartz plate,
with their absorption and PL spectra being shown in Fig. S2 (Sup-
porting Information). The HTP and HTPOCH in CHCl show
a absorption maxima at 418 nm and 423 nm with shoulders at
HTPOCH
3
are beneficial to the fabrication of multilayer PLED
3
3
devices via inkjet-printing and other coating methods.
3
3
3.4. Electrochemical properties
*
3
41 nm and 340 nm, which can be attributed to the
p
e
p
transition
*
Cyclic voltammetry (CV) has been constantly applied and
considered as an effective tool to investigate electrochemical prop-
erties of conjugated polymers. HTP- or HTPOCH -coated ITO glasses
were used as the working electrode, immersed in anhydrous
acetonitrile containing 0.1 M tetra-n-butylammonium perchlorate
of the main chain and the n-
p transition of the triphenylamine
segments, respectively. In film state the absorption peaks red-shift
slightly to 424 and 431 nm. The PL emission maxima in CHCl locate
at 478 and 488 nm for HTP and HTPOCH respectively, which shift
bathochromically to 483 and 494 nm in film state. The red-shifts for
HTP (5 nm) and HTPOCH (6 nm) in going from solution to film
3
3
3
4 4
(Bu NClO ) as electrolyte, to conduct the CV measurements. Char-
3
acteristic electrochemical data are summarized in Table 3. The
highest occupied molecular orbital (HOMO) is estimated by using
the equation EHOMO ¼ ꢁ(Eox þ 4.8) eV, where Eox is the onset
oxidation potential relative to the ferrocene/ferrocenium couple.
state are close to those of absorption (6 nm, 8 nm). This suggests
that the undesirable inter-chain interactions are effectively pre-
vented by the hyperbranched structures of the polymers.
3
Both HTP and HTPOCH are thermally cross-linkable polymers
which are potentially applicable in the fabrication of multilayer
PLED devices. However, highly solvent resistant is a prerequisite for
multilayer PLEDs fabricated by solution processes. To examine the
16000
solvent resistance of the HTP and HTPOCH
3
before and after
a
1
4000
thermal curing, a good solvent (chlorobenzene) was spin-coated on
the top of the polymer films and then their absorption spectral
variations were investigates. As shown in Fig. 4, the absorption
No HTL
12000
With HTP
With HTPOCH
1
0000
3
3
spectra of the pristine HTP and HTPOCH films disappear almost
completely after chlorobenzene spin-coating, indicating that the
films are dissolved out during the coating process. However, the
absorption spectral intensity of the thermally cured HTP and
8000
6
000
3
HTPOCH remains almost unchanged after the spin-coating. Their
4000
high solvent resistance is attributable to significantly increased
cross-linking density after the thermal curing. The HTP and
2
000
0
HTPOCH
3
also reveal good resistant to toluene as shown in Fig. S3.
films are transparent films which are
0
2
4
6
8
10
The cured HTP and HTPOCH
3
Voltage (V)
highly resistant toward organic solvent. Therefore, the solution-
processable and thermally cross-linkable characteristics of HTP and
0
.5
b
-
2.44
0.4
-
2.6
-
2.7
-
2.9
Ca
0
0
0
0
.3
.2
.1
.0
HTP
No HTL
MEH-PPV
With HTP
ITO
4.8
With HTPOCH
3
-
4.3
Al
HTPOCH
3
PEDOT
0
200
400
600
2
800
-
5.02
-
5.17 -5.07
-
-
5.2
Current density (mA/cm )
Fig. 6. (a) Brightness versus voltage and (b) luminance efficiency versus current
density characteristics of multilayer PLEDs with or without hole-transporting HTP or
Fig. 5. The band diagrams of HTP, HTPOCH
3
, PEDOT:PSS and electrodes.
3 3
HTPOCH layer. Device structure: ITO/PEDOT:PSS/(HTP or HTPOCH )/MEH-PPV/Ca/Al.

J.-M. Yu, Y. Chen / Polymer 51 (2010) 4484e4492
4491
The lowest unoccupied molecular orbital (LUMO) is calculated by
multilayer PLEDs seems contributed by a combination of several
factors. Firstly, hole transport is facilitated due to high HOMO levels
opt
LUMO ¼ ꢁe(EHOMO þ E
opt
E
g
), where the optical band-gap (E
estimated from onset absorption. The onset oxidation potentials of
HTP and HTPOCH are 0.83 V and 0.73 V respectively. The much
lower oxidation potential of HTPOCH is ascribed to the additional
electron-donating methoxy groups at triphenylamine units. The
HOMO level of HTP and HTPOCH
are ꢁ5.17 and ꢁ5.07 eV and the
LUMO levels are ꢁ2.6 and ꢁ2.44 eV respectively. The band diagrams
of HTP, HTPOCH , PEDOT:PSS and work functions of electrodes are
shown in Fig. 5. Clearly, the HOMO levels of HTP (ꢁ5.17 eV) and
HTPOCH
g
) is
of the hyperbranched HTP (ꢁ5.17 eV) and HTPOCH
3
(ꢁ5.07 eV).
3
Secondly, the thermally cross-linked HTP or HTPOCH
3
acts as
3
a buffer layer not only to smooth surface but also to prevent
PEDOT:PSS chain from penetrating into the emitting layer to
quench the emission [18,42]. Finally, the hyperbranched HTP and
3
HTPOCH
to increase the possibility of recombination in emitting layer (MEH-
PPV). Consequently, HTPOCH -based device shows higher
3
also serve as electron-blocking layer, especially the latter,
3
3
2
3
(ꢁ5.07 eV) are close to that of ITO/PEDOT:PSS (ꢁ5.2 eV),
maximum luminance (14060 cd/m ) than HTP-based one
2
facilitating the injection and transport of holes. Accordingly, the
hyperbranched polymers are potential hole-injection or hole-
transporting materials.
(12610 cd/m ) due to its raised LUMO level. Furthermore, the
energy levels of the hyperbranched polymers can be readily tuned
by replacing the triphenylamine moieties with other hole-
transporting groups. Accordingly, thermally cross-linkable hyper-
branched polymers containing hole-transporting moieties are
promising candidates for the fabrication of efficient multilayer
PLEDs.
3.5. Optoelectronic properties of light-emitting diodes using HTP
and HTPOCH as hole-transporting layers
3
To demonstrate the application of the hyperbranched polymers
HTP and HTPOCH ) as hole-transporting layer, multiple-layer
PLEDs with a configuration of ITO/PEDOT:PSS/HTP or HTPOCH
(
3
4. Conclusions
3
/
MEH-PPV/Ca (50 nm)/Al (100 nm) were fabricated to investigate
their optoelectronic characteristics. The hyperbranched polymers
were spin-coated on top of ITO glass deposited with PEDOT:PSS,
Two hyperbranched polymers (HTP and HTPOCH ) containing
triphenylamine moieties have been successfully synthesized via
Heck reaction. The resulted hyperbranched polymers contain
thermally reactive vinyl groups in periphery or terminal units. The
3
ꢀ
followed by thermal treatment at 210 C for 30 min under nitrogen
atmosphere to obtain transparent and insoluble films. The device
without hole-transporting layer (ITO/PEDOT:PSS/MEH-PPV/Ca/Al)
was also fabricated simultaneously for comparative study. Fig. 6
shows current density versus bias and luminance versus bias
relationships of the EL devices, with the characteristic data
summarized in Table 4. The main emission peaks are situated
around 581 nm and 624 nm for all devices (Fig. 7), mainly
hyperbranched HTP and HTPOCH were thermally cross-linked to
3
form as transparent films. The HTP and HTPOCH demonstrated
3
ꢀ
ꢀ
exothermic peaks at 221 C and 210 C, respectively, due to thermal
cross-linking of the vinyl groups. The glass-transition temperatures
(T ) were 149.6 C and 141.7 C after thermal curing at 210 C for
ꢀ
ꢀ
ꢀ
g
30 min. Moreover, both HTP and HTPOCH demonstrated homo-
3
geneous film morphology after thermal curing, with the root-
contributed from MEH-PPV. Furthermore, the luminance efficiency
mean-square (rms) roughness being 0.77 nm and 0.89 nm. The HTP
and HTPOCH3 were used as HTL to fabricate multilayer light-
emitting diodes (ITO/PEDOT:PSS/HTL/MEH-PPV/Ca/Al) by succes-
sive spin-coating process. The maximum luminances of the devices
2
(
at 1000 cd/m ) and maximum luminance of MEH-PPV devices
3
with HTP or HTPOCH as hole-transporting layer devices are
superior to those without hole-transporting layer. For instance, the
2
2
maximum luminance (luminance efficiency at 1000 cd/m ) of the
with HTL were 12610 and 14060 cd/m , with luminance efficiencies
2
2
devices are 12610 (0.32 cd/A) and 14060 cd/m (0.33 cd/A)
(at 1000 cd/m ) being 0.32 and 0.33 cd/A for HTP and HTPOCH
3
2
respectively, while it is 9310 cd/m (0.26 cd/A) for that without
respectively, which were superior to those of MEH-PPV device
2
hole-transporting layer. The luminance of HTP- or HTPOCH
3
-based
without HTL (9310 cd/m , 0.26 cd/A). Current results demonstrate
devices is improved when compared to the hyperbranched HTL
reported by Reynolds [24]. The performance enhancement of the
that the thermally cross-linkable hyperbranched polymers are
promising hole-transporting materials applicable for the fabrica-
tion of multilayer PLEDs. Furthermore, their hole-transporting
ability can be readily adjusted by replacing triphenylamine moiety
with other hole-transporting groups.
1
1
0
0
0
0
0
.2
.0
.8
.6
.4
.2
.0
Acknowledgment
MEH-PPV
With HTP
With HTPOCH3
The authors thank the National Science Council of Taiwan for
financial aid through project NSC 98-2221-E-006-003 MY3.
Appendix. Supplementary material
The supporting information associated with this article can be
found in the on-line version at doi:10.1016/j.polymer.2010.08.001.
References
[
[
1] Lo S-C, Burn PL. Chem Rev 2007;107(4):1097e116.
2] He Q-Y, Lai W-Y, Ma Z, Chen D-Y, Huang W. Eur Polym
500
550
600
650
700
750
800
J 2008;44
(
10):3169e76.
3] Tsai L-R, Chen Y. Macromolecules 2007;40(9):2984e92.
4] Tsai L-R, Chen Y. J Polym Sci Part A Polym Chem 2007;45(19):4465e76.
Wavelength (nm)
[
[
Fig. 7. EL spectra of multilayer device with or without thermally cured HTP or
HTPOCH as hole-transporting layer. Device configuration: ITO/PEDOT:PSS/(hole-
transporting layer)/MEH-PPV/Ca(50 nm)/Al(100 nm).
[5] Tsai L-R, Lee C-W, Chen Y. J Polym Sci Part A Polym Chem 2008;46
3
(17):5945e58.
[6] Wu C-W, Lin H-C. Macromolecules 2006;39(21):7232e40.

4
492
J.-M. Yu, Y. Chen / Polymer 51 (2010) 4484e4492
[23] Shirota Y, Kageyama H. Chem Rev 2007;107(4):953e1010.
[
7] Wang PW, Lin YJ, Devadoss C, Bharathi P, Moore JS. Adv Mater 1996;8
3):237e41.
8] Peng Q, Yan L, Chen D, Wang F, Wang P, Zou D. J Polym Sci Part A Polym Chem
007;45(22):5296e307.
9] Wang H, Sun Y, Qi Z, Kong F, Ha Y, Yin S, et al. Macromolecules 2008;41
10):3537e42.
10] Wen G-A, Xin Y, Zhu X-R, Zeng W-J, Zhu R, Feng J-C, et al. Polymer 2007;48
7):1824e9.
(
[24] Paul GK, Mwaura J, Argun AA, Taranekar P, Reynolds JR. Macromolecules
2006;39(23):7789e92.
[25] Cheng Y-J, Liu MS, Zhang Y, Niu Y, Huang F, Ka J-W, et al. Chem Mater
2007;20(2):413e22.
[26] Liu S, Jiang X, Ma H, Liu MS, Jen AKY. Macromolecules 2000;33(10):
3514e7.
[27] Shao Y, Gong X, Heeger AJ, Liu M, Jen AKY. Adv Mater 2009;21(19):1972e5.
[28] Jiang XZ, Liu S, Liu MS, Herguth P, Jen AKY, Fong H, et al. Adv Funct Mater
2002;12(11e12):745e51.
[
[
2
(
[
(
[
[
11] Lim S-J, Seok DY, An B-K, Jung SD, Park SY. Macromolecules 2005;39(1):9e11.
12] Klarner G, Lee JI, Lee VY, Chan E, Chen JP, Nelson A, et al. Chem Mater 1999;11
(
7):1800e5.
13] Sun H, Liu Z, Hu Y, Wang L, Ma D, Jing X, et al. J Polym Sci Part A Polym Chem
004;42(9):2124e9.
[29] Satoh N, Cho J-S, Higuchi M, Yamamoto K. J Am Chem Soc 2003;125
(27):8104e5.
[
[
2
[30] Yu J-M, Chen Y. Macromolecules 2009;42(21):8052e61.
[31] Tsai L-R, Chen Y. J Polym Sci Part A Polym Chem 2007;45(23):5541e51.
[32] Tsai L-R, Chen Y. J Polym Sci Part A Polym Chem 2008;46(1):70e84.
[33] Tsai L-R, Chen Y. Macromolecules 2008;41(14):5098e106.
[34] Redecker M, Bradley DDC, Inbasekaran M, Woo EP. Appl Phys Lett 1998;73
(11):1565e7.
[35] Babel A, Jenekhe SA. Macromolecules 2003;36(20):7759e64.
[36] Redecker M, Bradley DDC, Inbasekaran M, Woo EP. Appl Phys Lett 1999;74
(10):1400e2.
[37] Tan Z, Tang R, Xi F, Li Y. Polym Adv Technol 2007;18(12):963e70.
[38] Liu Y, Liu MS, Jen AKY. Acta Polym 1999;50(2e3):105e8.
[39] Radke W, Litvinenko G, Müller AHE. Macromolecules 1998;31(2):239e48.
[40] Gao C, Yan D. Prog Polym Sci 2004;29(3):183e275.
[41] Robinson MR, Wang S, Bazan GC, Cao Y. Adv Mater 2000;12(22):1701e4.
[42] Niu YH, Liu MS, Ka JW, Bardeker J, Zin MT, Schofield R, et al. Adv Mater
2007;19(2):300e4.
14] Burroughes JH, Bradley DDC, Brown AR, Marks RN, Mackay K, Friend RH, et al.
Nature 1990;347(6293):539e41.
15] Akcelrud L. Prog Polym Sci 2003;28(6):875e962.
16] Hsiao C-C, Hsiao A-E, Chen S-A. Adv Mater 2008;20(10):1982e8.
17] Chen S-A, Lu H-H, Huang C-W. Adv Polym Sci 2008:49e84.
18] Huang F, Cheng Y-J, Zhang Y, Liu MS, Jen AKY. J Mater Chem 2008;18
[
[
[
[
(
38):4495e509.
19] Lin C-Y, Chen Y-M, Chen H-F, Fang F-C, Lin Y-C, Hung W-Y, et al. Org Electron
009;10(1):181e8.
20] Park JH, Yun C, Park MH, Do Y, Yoo S, Lee MH. Macromolecules 2009;42
18):6840e3.
21] Lim B, Hwang J-T, Kim JY, Ghim J, Vak D, Noh Y-Y, et al. Org Lett 2006;8
21):4703e6.
22] Liu MS, Niu Y-H, Ka J-W, Yip H-L, Huang F, Luo J, et al. Macromolecules
008;41(24):9570e80.
[
[
[
[
2
(
(
2