Synthesis and optoelectronic properties of thermally cross-linkable hole-transporting poly(fluorene-co-triphenylamine)

Article abstract of DOI:10.1016/j.polymer.2010.10.065

This paper describes the synthesis of a new thermally cross-linkable hole-transporting poly(fluorene-co-triphenylamine) (PFTV) by Suzuki coupling reaction and its application in polymer light-emitting diodes (PLEDs). The characteristics of PFTV were analy

Full text of DOI:10.1016/j.polymer.2010.10.065

Polymer 52 (2011) 77e85
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and optoelectronic properties of thermally cross-linkable
hole-transporting poly(fluorene-co-triphenylamine)
*
Wen-Fen Su, Yun Chen
Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
This paper describes the synthesis of a new thermally cross-linkable hole-transporting poly(fluorene-co-
triphenylamine) (PFTV) by Suzuki coupling reaction and its application in polymer light-emitting diodes
(PLEDs). The characteristics of PFTV were analyzed by 1H NMR, differential scanning calorimetry, optical
spectroscopy, cyclic voltammetry, and atomic force microscopy. Its HOMO level lies between those of
PEDOT:PSS and poly(9,9-dioctylfluorene), forming a stepwise energy ladder to facilitate hole-injection.
Multilayer device with thermally cross-linked PFTV as hole-transporting layer (ITO/PEDOT:PSS/HTL/PFO/
LiF/Ca/Al) was readily fabricated by successive spin-coating processes, its maximum luminance efficiency
(2.27 cd/A) was significantly higher than that without PFTV layer (0.50 cd/A). In addition, the PFTV was
successfully applied as host for red-emitting Ir(piq)2acac to obtain a device with moderate performance
(5300 cd/m2 and 2.64 cd/A). The PFTV is a promising hole-transporting material for the fabrication of
multilayer PLEDs by wet processes as well as a potential host for phosphorescent PLEDs.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 14 September 2010
Received in revised form
26 October 2010
Accepted 31 October 2010
Available online 18 November 2010
Keywords:
Thermally cross-linkable
Hole-transporting
Triphenylamine
1. Introduction
usually adopted to improve hole-injection from anode [12e18]. For
the second strategy, multilayer devices are required and fabricated
Polymer light-emitting diodes (PLEDs) have attracted much
attention, since the first discovery of electroluminescene of poly
(p-phenylenevinylene) (PPV) in 1990, due to their potential applica-
tions in large area flat panel displays and solid-state lighting [1e5].
Organic materials in PLEDs have some advantages such as potentially
low cost, facile processing by spin-coating and ink-jet printing
methods. A lot of conjugated polymers have been extensively inves-
tigated, such as poly(p-phenylenevinylene) (PPV) [1], polyfluorene
(PF) [6] and their derivatives. PF and its derivatives are promising
blue-light-emitting materials widely used in PLEDs because of its
excellentthermalandchemicalstabilityandhighphotoluminescence
(PL) quantum yield [7e9]. However, the lower highest occupied
molecularorbital(HOMO) level ofPF(EHOMO ¼ ꢀ5.8 eV)creates a high
hole-injection barrier, which lead to imbalance in charges injection
[10,11]. This characteristic ultimately results in low efficiency
obtainable for its polymeric light-emitting diodes.
by adding an extra hole-transporting layer (HTL) to reduce the
hole-injection barrier from anode [19e24]. Several polymers have
been reported as suitable materials for HTL between poly(styr-
enesulphonate):poly(3,4-ethylenedioxythiophene) (PEDOT:PSS)
and emitting layer (EML), such as poly(N-vinylcarbazole) (PVK)
and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,40-(N-(4-sec-butyl-
phenyl))diphenylamine)] (TFB) [23e26]. The hole-transporting
layer (HTL) effectively improves the device efficiency of multilayer
PLEDs. However, for multilayer polymer devices the HTL layer may
be dissolved or destroyed by the solution of emitting layer during
subsequent spin-coating. Therefore, the homogeneity and actual
thickness of the HTL cannot be controlled at will. This will lead to
poor reproducibility in the fabrication of multilayer devices by
solution processes. Accordingly, solvent resistance of the HTL layer
should be high enough to prevent the dissolution during subse-
quent coating. An effective way to increase the solvent resistance
of a polymer is forming as a three dimensional network structure,
which can be attained by thermal or photo-initiated cross-linking
reaction [27e36]. Moreover, the solvent resistance increases with
the increase of cross-linking density. Therefore, the HTL applicable
in multilayer PLEDs fabricated by solution-processes should
possess not only a proper energy level lying between anode and
emitting layer to facilitate hole-injection but also high solvent
resistance to avoid being dissolved during subsequent spin-
coating of the emitting layer.
To solve this problem, in general two strategies have been
adopted, one is through appropriate design of chemical structure
and the other is via adequate modification of device structure. For
the first strategy, the incorporation of hole-transporting moieties
on a main or side chain, such as carbazole or triphenylamine, is
* Corresponding author. Tel.: þ886 6 2085843; fax: þ886 6 2344496.
E-mail address: yunchen@mail.ncku.edu.tw (Y. Chen).
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.10.065

78
W.-F. Su, Y. Chen / Polymer 52 (2011) 77e85
In the present study we synthesized a new thermally cross-
linkable hole-transporting poly(fluorene-co-triphenylamine)
[41,42]. An atomic force microscope (AFM), equipped with a Veeco/
Digital Instrument Scanning Probe Microscope (tapping mode) and
a Nanoscope IIIa controller, was used to examine surface
morphology and to estimate thickness and root-mean-square (rms)
roughness of deposited films. The film thickness of hole-trans-
porting and emitting layers was measured by surface profiler,
(PFTV) by Suzuki coupling reaction, incorporated with pendent
styryl groups which is thermally reactive. The PFTV is composed of
hole-transporting triphenylamine groups and fluorenes substituted
with nonsymmetric and bulky aromatic groups at C-9 position. This
molecular design is expected to enhance thermal and chemically
stability, good film-forming properties and device efficiency.
Moreover, fabricating multilayer PLEDs with PFTV as hole-trans-
porting layer will be highly reproducible due to its thermally cross-
linking ability. The multilayer device with cross-linked PFTV as HTL
shows significantly enhanced emission efficiency than those with
uncross-linked PFTV as HTL or without the PFTV layer. Further-
more, the PFTV was effectively applied as host for red-emitting Ir
(piq)2acac, indicating that it acts both as hole-transporting and host
materials in PLEDs. These results demonstrate that the new PFTV is
a promising hole-transporting and host material for optoelectronic
devices.
a-step 500.
2.2. Synthesis of monomer (3) and copolymer (PFTV)
2.2.1. Synthesis of N,N-bis(4-bromophenyl)-p-(4-vinylphenyl)
aniline (3)
Tri(4-bromophenyl)amine (2, 4.82 g, 10 mmol), p-vinyl-
phenylboronic acid (1.15 g, 5 mmol) and (PPh3)4Pd(0) (0.144 g,
0.13 mmol) were dissolved in a mixture consisting of tetrahydro-
furan (THF: 20 mL), aqueous solution of 2 M K3PO4 (11 mL). The
mixture was first purged with Argon and stirred at 100 ꢁC for 48 h
under vigorous stirring. It was poured into water (50 mL) and
extracted twice with dichloromethane (250 mL). The combined
organic extracts were dried (MgSO4) and concentrated by rotary
evaporation. Further purification by column chromatography on
silica gel (ethyl acetate/n-hexane) afforded product 3 as white
solids (yield: 65%, melting point: 160e161 ꢁC). FT-IR (KBr pellet,
2. Materials and methods
2.1. Materials and characterization
Tri(4-bromophenyl)amine (2) [37] and 4-(3-methylpropyl)-N,N-
bis(4-bromophenyl)aniline (5) [38,39] were synthesized according
to the procedures reported previously. 9,9-diarylfluorene-2,7-
diboronic acid bispinacol ester (4) and poly(9,9-dioctylfluorene)
(PFO) were prepared according to our previously reported proce-
dures [40]. All reagents and solvents were purchased from Acros,
TCI, Aldrich, Lancaster Chemicals Co. and used without further
purification. All the solvents such THF and acetonitrile were dried
with appropriate drying agents (Na or CaCl2), then distilled under
reduced pressure and stored over 4 Å molecular sieves before use.
The polymerization catalyst was tetrakis(triphenylphosphine)
palladium [Pd(PPh3)4] procured from Strem. Newly synthesized
compounds were identified by 1H NMR, 13C NMR spectroscopy,
mass spectrometry, and elemental analysis (EA). 1H NMR and 13C
NMR spectra were recorded with Bruker AVANCE-400 and 500
NMR spectrometers respectively, and the chemical shifts are
reported in ppm using tetramethylsilane (TMS) as an internal
standard. Mass and elemental analysis were carried out on a JEOL
JMS-700 spectrometer and Heraus CHN-Rapid elemental analyzer,
respectively. The FT-IR spectra were measured as KBr disk using
a Fourier transform infrared spectrometer, model 7850 from Jasco.
Molecular weight and molecular weight distribution of the poly-
mer were determined by a gel permeation chromatograph (GPC)
using THF as an eluent at a flow rate of 1 mL/min at 40 ꢁC. Mono-
disperse polystyrene standards were used for molecular weight
calibration. Thermogravimetric analysis (TGA) was performed
under nitrogen atmosphere at a heating rate of 20 ꢁC/min, using
a PerkinElmer TGA-7 thermal analyzer. Thermal curing behaviors
and thermal transitional properties of the polymer were investi-
gated using a differential scanning calorimeter (DSC), Mettler DSC
1, at a heating rate of 10 ꢁC/min. Absorption spectra and photo-
luminescence (PL) spectra were measured with a Jasco V-550
spectrophotometer and a Hitachi F-4500 fluorescence spectro-
photometer, respectively. Cyclic voltammograms were recorded
using a voltammetric analyzer (model CV-50W from Bioanalytical
Systems, Inc.) under nitrogen atmosphere. The measuring cell was
consisted of a polymer-coated ITO glass 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
acetonitrile containing 0.1 M (n-Bu)4NClO4 as electrolyte. The
energy levels were calculated using ferrocene (FOC) as standard
(ꢀ4.8 eV with respect to vacuum level which is defined as zero)
cmꢀ1):
3033, 3054, 3081. 1H NMR (400 MHz, CDCl3, ppm):
n
719, 815, 898,1068,1286,1317,1484,1521,1579,1602,1621,
7.54e7.45 (m,
d
6H, Ar-H), 7.37e7.35 (d, 4H, Ar-H, J ¼ 8 Hz), 7.11e7.09 (d, 2H, Ar-H,
J ¼ 8 Hz), 6.99e6.97 (d, 4H, Ar-H, J ¼ 8 Hz), 6.78e6.71 (dd,1H, ¼ CH-
, J1 ¼ 16 Hz, J2 ¼ 16 Hz), 5.80e5.76 (d, 1H, ¼ CH2, J ¼ 16 Hz),
5.30e5.26 (d, 1H, ¼ CH2, J ¼ 16 Hz). 13C NMR (500 MHz, CDCl3,
ppm):
d 146.38, 146.24, 139.68, 136.45, 136.39, 135.82, 132.43,
127.88, 126.74, 126.69, 126.02, 125.63, 124.45, 115.74, 113.8. ELEM
ANAL. Calcd. for C26H19Br2N (%): C, 61.81; H, 3.79; N, 2.77. Found: C,
61.70; H, 3.85; N, 2.71. EI-MS (m/z): calcd: 504.99; found: 505.00.
2.2.2. Synthesis of poly(fluorene-co-triphenylamine) (PFTV)
The synthesis of poly(9,9-dioctylfluorene) (PFO) and copoly-
mer PFTV was carried out using a palladium-catalyzed Suzuki
coupling reaction. For instance, purified N,N-bis(4-bromophenyl)-
p-(4-vinylphenyl)aniline (3: 0.304 g, 0.6 mmol), 9,9-diarylfl-
uorene-2,7-diboronic acid bispinacol ester (4: 3.843 g, 5 mmol),
4-(3-methylpropyl)-N,N-bis(4-bromophenyl)aniline (5: 2.02 g,
4.4 mmol) and (PPh3)4Pd(0) (0.104 g, 0.09 mmol) were dissolved
in a mixture consisting of THF (30 mL) and aqueous solution of
2 M K3PO4 (16 mL). The mixture was first purged with Argon and
then stirred at 100 ꢁC for 72 h under vigorous stirring. Finally,
monomer 4 and 1-bromo-4-tert-butylbenzene were added to the
mixture to end-cap the polymer chain. The mixture was poured
into a large amount of methanol; the appeared solid was collected
by filtration and washed successively with methanol, 2-propanol
and hexane, followed by Soxhlet extraction with acetone to
remove trace oligomers. The residual palladium catalyst was
removed by stirring together with a silica gel (Silicycle, Si-Thiol) in
toluene. Then the solution was further extracted with de-ionized
water three times to reduce the concentration of metal ions. It was
then poured into a large amount of methanol to afford light-
yellow fiber of PFTV (yield: 75%). The PFTV was soluble in
conventional organic solvents such as toluene, xylene, THF and
chloroform. 1H NMR (400 MHz, CDCl3, ppm):
d 7.78e7.76 (d, Ar-H,
J ¼ 8 Hz), 7.58e7.56 (d, Ar-H, J ¼ 8 Hz), 7.49e7.44 (m, Ar-H),
7.26e7.04 (m, Ar-H), 6.94e6.87 (m, Ar-H), 6.64e6.62 (m,]CHe),
5.80e5.76 (d,]CH2, J ¼ 16 Hz), 5.27e5.24 (m,]CH2, J ¼ 12 Hz),
3.94e3.91 (t, eOCH2e, J ¼ 12 Hz), 2.58e2.56 (m, 1H, eCHe), 2.20
(s, 3H, eCH3), 2.10 (s, 3H, eCH3), 1.80e0.83 (m, eCH2e and eCH3).
13C NMR (500 MHz, CDCl3, TMS, 25 ꢁC):
d
155.86, 152.83, 147.04,
142.79, 139.60, 138.83, 136.63, 134.63, 134.42, 132.36, 131.31,

W.-F. Su, Y. Chen / Polymer 52 (2011) 77e85
79
130.58, 127.47, 126.62, 126.25, 125.93, 125.08, 124.20, 120.34,
110.32, 66.17, 65.28, 40.98, 39.19, 37.26, 36.32, 31.34, 29.91, 27.93,
24.65, 22.67, 22.58, 21.81, 21.24, 19.73, 16.58, 12.27. ELEM. ANAL.
Calcd. for PFTV (%): C, 88.43; H, 7.67; N, 1.71. Found (%): C, 87.64;
H, 7.71; N, 1.28.
of the devices was done in ambient conditions, with the following
performance tests conducted in a glove-box filled with nitrogen.
3. Results and discussion
3.1. Synthesis and characterization
2.3. Fabrication and characterization of light-emitting devices
The hole-transporting poly(fluorene-co-triphenylamine) (PFTV)
was synthesized by the palladium-catalyzed Suzuki coupling of 9,9-
diarylfluorene-2,7-diboronic acid bispinacol ester (4) and dibro-
motriphenylamine derivatives (3, 5) as shown in Scheme 1. The
emitting poly(9,9-dioctylfluorene) (PFO) was also synthesized by
the Suzuki coupling reaction. Tri(4-bromophenyl)amine (2) and
4-(3-methylpropyl)-N,N-bis(4-bromophenyl)aniline (5) were syn-
thesized according to the procedures reported previously [37e39].
9,9-diarylfluorene-2,7-diboronic acid bispinacol ester (4) and poly
(9,9-dioctylfluorene) (PFO) were prepared according to our previ-
ously reported procedures [40]. Monomer 3 was synthesized by the
Suzuki coupling of p-vinylphenylboronic acid with stoichiometric
amount of tri(4-bromophenyl)amine (2), which was obtained by
complete bromination of triphenylamine with N-bromosuccini-
mide (NBS). Chemical structure of monomer 3 was satisfactorily
confirmed by 1H NMR, 13C NMR spectra, elemental analysis, and
mass spectrometry. 1H NMR spectrum of 3 verifies the character-
istic chemical shifts at 6.78e6.71 ppm and 5.26e5.80 ppm assigned
to protons of the vinyl substituents (]CH2 and ]CHe). The
structure of PFTV was satisfactorily characterized by its 1H NMR,
13C NMR spectra (Fig. S1 in Supporting Information) and elemental
analysis. The actual composition of pendent styryl group is ca.
5 mol%, as estimated from the areas of the peaks at 5.24e5.80 ppm
Multilayer light-emitting diodes [ITO/PEDOT:PSS/HTL/PFO/LiF/
Ca/Al] or phosphorescent light-emitting diodes [ITO/PEDOT:PSS/
PFTV:Ir(piq)2acac/Ca/Al] were fabricated to investigate their
optoelectronic characteristics. The ITO-coated glass substrate was
washed successively in ultrasonic baths of neutraler reiniger/de-
ionized water (1:3 v/v) mixture, de-ionized water, acetone and
2-propanol, followed with treatment in a UVeOzone chamber. A
thick PEDOT:PSS layer was spin-coated on top of the freshly
cleaned ITO glass and annealed at 150 ꢁC for 15 min in a dust-free
atmosphere. The PFTV solutions were filtered through a syringe
filter (0.2 mm) before the spin-coating. The hole-transporting layer
(HTL) was formed by spin-coating a solution of PFTV in toluene
(10 mg/mL, 2000 rpm) on top of the PEDOT:PSS layer and ther-
mally treated at 230 ꢁC for 30 min under nitrogen atmosphere.
Then the emitting layer (EML) was spin-coated onto the HTL. The
actual thickness of HTL and EML is 40 and 70 nm, respective.
Finally, a thick layer of cathode was deposited by successive
thermal evaporation of LiF (1 nm), Ca (50 nm) and Al (100 nm)
under 110ꢀ6 Torr. The luminance versus bias, current density
versus bias, and emission spectra of the PLEDs were recorded using
a combination of Keithley power source (model 2400) and Ocean
Optics usb2000 fluorescence spectrophotometer. The fabrication
O
B
Br
N
Br
NBS
N
Br
N
Br
O
Br
1
2
3
Br
N
Br
O
O
Br
N
Br
B
B
O
O
OC10H21
4
5
3
Pd(PPh3)4, K3PO4
THF
N
N
0.88
0.12
OC10H21
OC10H21
PFTV
C8H17
C8H17
n
PFO
Scheme 1. Synthesis of monomer 3 and copolymer PFTV, and chemical structure of poly(9,9-dioctylfluorene (PFO).

80
W.-F. Su, Y. Chen / Polymer 52 (2011) 77e85
Table 1
Thermal and electrochemical properties of PFTV and PFO.
a
pristine film
treated with toluene
1.0
Tg (ꢁC)a Td (ꢁC)b Eox vs. FOC (V)c EHOMO (eV)d ELUMO (eV)e Egopt (eV)f
PFTV 225
419
432
0.47
0.90
ꢀ5.27
ꢀ5.70
ꢀ2.41
ꢀ2.77
2.86
2.93
0.8
0.6
0.4
0.2
PFO
68
a
Determined by differential scanning calorimetry (DSC) at a heating rate of
10 ꢁC/min.
b
c
d
e
f
The temperature at 5 wt% loss in nitrogen atmosphere, measured by TGA.
EFOC ¼ 0.47 V vs. Ag/AgCl.
EHOMO ¼ ꢀ(Eox, FOC þ 4.8) eV.
ELUMO ¼ Eg þ EHOMO
.
Band gaps obtained from onset absorption (lonset): Eg ¼ 1240/lonset
.
(]CH2) and 3.91 ppm (eOCH2e). The characteristic chemical shifts
at 5.24e5.80 ppm confirm the existence of pendent styryl groups,
from which thermally cross-linkable character can be expected. The
PFTV and PFO are soluble in common organic solvents such as
toluene, THF, and chloroform. The weight-average molecular
weight (Mw) of PFTV and PFO was 6.9 ꢂ 104 and 14.4 ꢂ 104,
respectively, with the polydispersity indexes (PDIs) being 1.8, as
determined by gel permeation chromatography using mono-
disperse polystyrenes as calibration standards.
0.0
1.0
o
cross-linked at 230
treated with toluene
C
b
0.8
0.6
0.4
0.2
1.0
0.8
0.6
0.4
0.2
0.0
cross-linked PFTV/DCP
at 230
treated with toluene
C
3.2. Thermal cross-linking properties and surface morphology of
PFTV
Since the copolyfluorene (PFTV) contains reactive pendent styryl
groups which will give rise to networks structure (cross-linking)
under proper thermal treatment. Thermal cross-linking and thermal
stability characteristics of PFTV were studied by differential scanning
calorimetry (DSC) and thermogravimetric analysis (TGA), respec-
tively. The thermal decomposition temperature (Td, 5% weight loss) of
PFTVis419 ꢁC undernitrogenatmosphere, indicatingthatthe PFTV is
highly thermal stable (Table 1). As shown in Fig. 1 (inset), during the
firstDSC heatingscanofmonomer3aclear-cutmeltingpointatabout
161 ꢁC is observed, followed closely by an exothermic peak around
163 ꢁC. The exothermic peak is attributed to the reaction of styryl
groups which is induced by the melt of 3. Thermal cross-linking
conditions of PFTV were first determined by a similar DSC heating
scan to 300 ꢁC (Fig. 1). During the first heating scan, PFTV showed
broad an exothermic peak around 230 ꢁC, in which the exotherm
starts at about 210 ꢁC. This exothermic peak (230 ꢁC) of PFTV is much
higher than that of monomer 3 (163 ꢁC), which is ascribed to high
300
400
500
600
700
Wavelength (nm)
0.0
1.0
c
o
C
cross-linked at 240
treated with toluene
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
6
4
2
0
Monomer 3-1st
Monomer 3-2nd
1st heating scan
2nd heating scan
Wavelength (nm)
3
2
Fig. 2. Absorption spectra of pristine and cross-linked PFTV films: (ꢀ) before and (---)
after spin-coated with pure toluene. The cross-linking was conducted at 230 ꢁC or
240 ꢁC for 30 min under nitrogen atmosphere. The inset in (b) shows the absorption
spectra of PFTV film containing 1 wt% of dicumyl peroxide (DCP).
-2
-4
-6
o
T
= 161 C
m
-8
-10
-12
-14
1
glass transition temperature of PFTV comprising rigid aromatic main
chain structure and bulky aromatic substituents at the C-9 position.
The exothermic heat (35.25 J/g) is attributable to the thermal cross-
linking reaction of the pendent styryl groups. No obvious exotherm is
observed in the second heating scan, indicating the cross-linking has
been completed during the first scan. Furthermore, the glass transi-
tion temperature (Tg) of the cured PFTV is about 225 ꢁC with no
detectable crystallization and melting transitions, suggesting that it is
basically an amorphous material. The high Tg is a highly desirable
propertyforpolymersusedinPLEDs, sinceitpreventsthedetrimental
crystallization process during device operation or thermal annealing
which deteriorates long-term morphological stability.
40 60 80 100 120 140 160 180 200 220
o
Temperature ( C)
0
ΔH = 35.25J/g
-1
-2
Tg = 225 oC
200
50
100
150
250
300
o
Temperature ( C)
Fig. 1. DSC traces of PFTV at a heating rate of 10 ꢁC/min. The first scan was used to
observe the reaction heat of styryl groups. The inset shows the DSC trace of monomer 3
at a heating rate of 10 ꢁC/min.
Solvent resistance of the cured PFTV film was investigated by
the absorption variations (300e700 nm) before and after washing

W.-F. Su, Y. Chen / Polymer 52 (2011) 77e85
81
with a good solvent (toluene). As shown in Fig. 2, the absorption
spectrum of the pristine PFTV film disappears almost completely
after rinsing with toluene, meaning that the PFTV film is dissolved
out during spin-coating process. However, the solvent resistance of
PFTV was enhanced by the thermal curing processes. For instance,
the remaining absorption intensity of the thermally cross-linked
PFTV film (at 230 ꢁC for 30 min) was about 27% after rinsing with
toluene. Moreover, its solvent resistance is further promoted when
cured at higher temperature (240 ꢁC for 30 min), i.e., more than 48%
of absorption intensity was remained after rinsing with toluene. In
addition, the degree of cross-linking could be increased by adding
dicumyl peroxide (DCP) as cross-linking agents. The remaining
absorption intensity of thermally cross-linked PFTV film, added
with DCP (1 wt%) and cured at 230 ꢁC for 30 min, was about 37%
after rinsing with toluene. These results indicate that both higher
curing temperature and cross-linking agent are effective in
increasing cross-linking density, which can be evidenced by
elevated glass transition temperatures [32]. Therefore, the solution-
processable and thermally cross-linkable characteristics of PFTV
are useful to fabricate multilayer PLEDs by spin-coating processes.
To achieve high performance in multilayer PLEDs, it is impera-
tive to obtain highly homogeneous film for each layer. Homoge-
neous film is a prerequisite to obtain PLEDs with high device
performance and long lifetime. The morphology of PFTV spin-
coated onto ITO substrate, before and after thermal curing, was
investigated using an atomic force microscope (AFM). As shown in
Fig. 3, the PFTV film exhibits a uniform surface morphology with no
observable pinhole or aggregate after thermal cross-linking at
230 ꢁC for 30 min. Moreover, the average root-mean-square (rms)
roughness of the cross-linked PFTV film surface is 0.85 nm
(0.88 nm, 0.82 nm, 0.85 nm), which is slightly lower than 0.94 nm
of pristine film (0.92 nm, 0.99 nm, 0.90 nm). According, the thermal
treatment not only promotes solvent resistance but also smoothes
out the film surface of PFTV. Most of the reported thermally curable
HTLs are low molecular weight compounds [28,31e33], although
polymers with trifluorovinyl ether pendent groups were also
investigated [29,33]. The film-forming property and thermal
stability of low molecular weight HTLs are usually inferior to
polymeric counterparts. Moreover, thermal cross-linking of the
polymers containing trifluorovinyl ether pendent groups required
both high temperatures (>225 ꢁC) and long times (1e2 h). There-
fore, the thermally cross-linkable PFTV should be a potential
candidate for hole-transporting materials from the viewpoint of
thermal property and processing feasibility.
-2.41
-2.77
-2.9
Ca
-5.0~-5.2
PEDOT
-5.27
PFTV
-5.70
PF
LiF
Fig. 4. The energy level diagrams of PFTV, PFO, PEDOT:PSS and Calcium electrode.
3.3. Electrochemical properties
Cyclic voltammetry (CV) is a commonly used technique to
investigate electrochemical properties of conjugated polymers
[43,44]. An ITO glass coated with PFTV was used as the working
electrode, supporting in anhydrous acetonitrile containing 0.1 M
tetra-n-butylammonium perchlorate [(n-Bu)4NClO4], to measure its
CV. The highest occupied molecular orbital (HOMO) which corre-
sponds to ionization potential (IP) can be estimated from the onset
oxidation potential (Eox
) revealed in CV by the equation
EHOMO ¼ ꢀ(Eox þ 4.8) eV. The lowest unoccupied molecular orbital
(LUMO) level is calculated by ELUMO ¼ ꢀe(EHOMO þ Eogpt), where the
optical band gap (Eogpt) is estimated from onset absorption. The
HOMO and LUMO levels of PFTV are estimated to be ꢀ5.27 eV and
ꢀ2.41 eV, respectively (Table 1). Fig. 4 depicts the energy levels of
PFTV, PFO, and PEDOT:PSS. Clearly the HOMO level of PFTV
(ꢀ5.27 eV) situates between those of PEDOT:PSS (ꢀ5.0 ∼ ꢀ5.2 eV)
[45] and PFO (ꢀ5.70 eV) [40], forming a stepwise energy ladder of
HOMO levels to facilitate hole-injection. This means that the hole-
injection from PEDOT:PSS to PFTV then transport to PFO is much
easier than direct injection from PEDOT:PSS to PFO. Therefore, the
improvement in hole-injection and transport are expected to
enhance device efficiency.
Fig. 3. The AFM images of PFTV films coated on ITO glass: (a) pristine film, average rms roughness ¼ 0.94 nm; (b) film thermally treated at 230 ꢁC for 30 min, average rms
roughness ¼ 0.85 nm.

82
W.-F. Su, Y. Chen / Polymer 52 (2011) 77e85
1600
1400
1200
1000
800
600
400
200
0
105
PFO
1.0
cross-linked PFTV/PFO
cross-linked PFTV
ITO/PEDOT:PSS/PFO/LiF/Ca/Al
ITO/PEDOT:PSS/cross-linked PFTV/PFO/LiF/Ca/Al
104
0.8
103
102
101
0.6
0.4
0.2
0.0
0
2
4
6
8
10
Bias (V)
400
450
500
550
600
650
700
Fig. 5. Brightness versus bias and current density versus bias characteristics of
multilayer PLEDs. Device structure: ITO/PEDOT:PSS/(PFTV)/PFO/LiF/Ca/Al. The PFTV
film was cross-linked at 230 ꢁC for 30 min.
Wavelength (nm)
Fig. 6. Emission spectra of light-emitting diodes using PFO or cross-linked PFTV as
emitting layer. Device structures: (ꢀ) ITO/PEDOT:PSS/PFO/LiF/Ca/Al; (ꢀꢀ) ITO/
PEDOT:PSS/cross-linked PFTV(HTL)/PFO/LiF/Ca/Al; (ꢀ,ꢀ,) ITO/PEDOT:PSS/cross-
linked PFTV/LiF/Ca/Al.
3.4. Optoelectronic properties of multilayer PLEDs using PFTV as
hole-transport layer
cross-linked PFTV, hole-only devices [ITO/PEDOT:PSS/with or
without PFTV/PFO/Au(50 nm)/Al(100 nm)] were fabricated to
investigate their current density versus electric field characteristics.
Inserting a cross-linked PFTV layer shifts the curve to the left greatly,
indicating that the cross-linked PFTV layer effectively increases the
device’s current density under the same external field (Fig. S2).
The EL spectra of the multilayer devices reveal similar emission
peaks at 437 nm corresponding to S10 / S00 vibronic transition of
fluorene segments (Fig. 6). To verify the EL emission is mainly
contributed from emitting PFO layer, the device [ITO/PEDOT:PSS/
PFTV/LiF/Ca/Al] using cross-linked PFTV as emitting layer was
fabricated to investigate its EL spectrum. The EL spectrum of using
PFTV as emitting layer device shows a main peak at 479 nm and
a shoulder at 444 nm, which are very different from that of
multilayer devices. This indicates that the electrons and holes
recombine mainly in the emitting PFO layer.
To evaluate the applicability of PFTV as hole-transporting layer
for multilayer devices,
a polymer light-emitting diode [ITO/
PEDOT:PSS/PFTV/PFO/LiF(1 nm)/Ca(50 nm)/Al(100 nm)] was
fabricated by successive spin-coating process. The PFTV was spin-
coatedon topof the PEDOT:PSS layerand treated at 230 ꢁC for 30 min
under nitrogen atmosphere, followed by the spin-coating of PFO
solution. The PFO and LiF were used as emitting layer and electron-
injection layer, respectively. The device without hole-transporting
layer (ITO/PEDOT:PSS/PFO/LiF/Ca/Al) was also fabricated simulta-
neously for comparative study. Fig. 5 shows the current density
versus bias and brightness versus bias curves (JeVeB) of the devices,
with the characteristic data summarized in Table 2. The maximum
brightness and maximum current efficiency of the device with cured
PFTV as HTL are 4870 cd/m2 and 1.15 cd/A, respectively. These
performances are much better than those of the devices using
pristine PFTV as HTL (2560 cd/m2, 0.49 cd/A) or without PFTV layer
(2410 cd/m2, 0.50 cd/A). The result indicates that uncross-linked
PFTV (pristine) contributes little in promoting the performance
when compared with the device without PFTV as HTL. This is due to
substantial dissolution of PFTV film during spin-coating of the
emitting PFO layer. However, when thermally cross-linked PFTV is
used as hole-transporting layer, both current efficiency and bright-
ness are greatly enhanced (about 2 times). The enhancement is
attributable to improved hole-injection from PEDOT:PSS to PFTV
layers, which is much easier than direct injection from PEDOT:PSS to
PFO layers. As mentioned above, the HOMO level of PFTV forms
a stepwise energy ladder with those of PEDOT:PSS and PFO to
facilitate hole-injection. To confirm hole-injection property of the
As mentioned previously, the degree of cross-linking can be
increased by adding suitable cross-linking agents. The hole-trans-
porting PFTV was thermally cured in the presence of dicumyl
peroxide (DCP: 1 wt%) during the fabrication of a multilayer device
[ITO/PDEOT:PSS/PFTVþDCP/PFO/LiF(1
nm)/Ca(50
nm)/Al
(100 nm)]. The maximum brightness and maximum current effi-
ciency are 5560 cd/m2 and 2.27 cd/A (Table 2), respectively, which
are higher than those without adding DCP (4870 cd/m2, 1.15 cd/A).
The results demonstrate that increasing cross-linking density in
PFTV layer gives rise to improved device performance. To conclude,
the performance enhancement of the EL devices using PFTV as
hole-transporting layer is attributed to promoted hole-injection
ability. However, the PFTV layer should be thermally cross-linked
during the fabrication processes to fully display its hole-trans-
porting function. The PFTV is a promising hole-transporting
material which can be employed in the fabrication of multilayer
devices by successive spin-coating processes.
Table 2
Optoelectronic properties of the multilayer PLEDs.a
b
HTL
Von (V) Lmaxc (cd/m2) LEmax (cd/A)d CIE 1931 (x, y)e
None
5.1
5.6
5.7
2410
4870
2560
5560
0.50
1.15
0.49
2.27
(0.16, 0.08)
(0.17, 0.08)
(0.17, 0.10)
(0.17, 0.10)
PFTV (cured)f
PFTV (uncured)
3.5. Optoelectronic properties of electrophosphorescent devices
using PFTV as host
PFTV þ DCP (cured)f 6.1
a
Device structure: ITO/PEDOT:PSS/HTL/PFO/LiF/Ca/Al; HTL: hole-transporting
layer.
The efficacy of PFTV as an efficient hole-transporting material
has been verified in multilayer PLEDs mentioned above. The PFTV
seems applicable as host for electrophosphorescent devices
because it contains triphenylamine moieties in main chain.
However, the triplet energy (ET) of the host should be larger than
b
c
d
e
f
Turn-on voltage defined as the bias at a luminance of 10 cd/m2.
Maximum luminance.
Maximum luminance efficiency.
The 1931 CIE coordinates at ca. 1000 cd/m2.
Treated at 230 ꢁC for 30 min.

W.-F. Su, Y. Chen / Polymer 52 (2011) 77e85
83
1.0
0.8
0.6
0.4
0.2
0.0
1.0
UV Ir(piq)2acac
PL PFTV
PFTV2
PFTV4
PFTV8
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 7. Absorption spectrum of Ir(piq)2acac and PL spectrum of PFTV film coated on
quartz plate (with lex ¼ 385 nm).
Fig. 9. EL spectra of the PFTV films doped with different amounts of Ir(piq)2acac. The
number represents weight percent of the Ir(piq)2acac in the film.
and Ir(piq)2acac (2e8 wt%) as emitting layer were fabricated to
investigate their optoelectronic characteristics. The main EL emis-
sion peak is around 620 nm with degenerated PFTV emission
(400e530 nm). The EL emission of PFTV is almost completely
that of phosphorescent emitter to improve the efficiency of tri-
pletetriplet energy transfer [46e48]. Therefore, phosphorescent
emission spectrum of PFTV at 77 K in a frozen solution of toluene
was measured to estimate its triplet energy. The phosphorescent
emission maximum of PFTV is 559 nm, which corresponds to
a triplet energy gap of 2.21 eV. This ET of PFTV is higher than that of
red-emitting dopant Ir(piq)2acac (2.0 eV) [49], ensuring that the
back energy transfer from Ir(piq)2acac to PFTV can be effectively
suppressed. Moreover, the PL spectrum of PFTV overlaps with that
of Ir(piq)2acac (Fig. 7), suggesting that efficient energy transfer
from PFTV to Ir(piq)2acac can be expected. As shown in Fig. 8, the
PL intensity of PFTV doped with Ir(piq)2acac decreases gradually
with increasing weight percent of the dopant (from 2 wt% to 8 wt%).
The emission spectra show two peaks at 442 and 620 nm, which
originate respectively from PFTV host (PL) and Ir(piq)2acac dopant
(radiative decay from triplet state to ground state) [14]. The effi-
cient energy transfer from host to dopant and suppressed back
energy transfer are prerequisites for attaining high performance in
host-guest electrophosphorescent devices.
1400
PFTV2
PFTV4
1200
PFTV8
1000
800
600
400
200
0
Therefore,
a series of EL devices, ITO/PEDOT:PSS/PFTV:Ir
0
2
4
6
8
10
12
14
(piq)2acac(110 nm)/Ca(50 nm)/Al (100 nm), using blends of PFTV
Bias (V)
104
103
102
101
PFTV2
PFTV4
PFTV8
1.6
PFTV2
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
PFTV4
PFTV8
0
2
4
6
8
10
12
14
Bias (V)
400
450
500
550
600
650
700
Wavelength (nm)
Fig. 10. Current density versus bias and brightness versus bias characteristics of PLEDs
using Ir(piq)2acac-doped PFTV as emitting layer. The number represents weight
percent of the Ir(piq)2acac in the film; devices structure: ITO/PEDOT:PSS/PFTV:
Ir(piq)2acac/Ca(50 nm)/Al(100 nm).
Fig. 8. PL spectra of the PFTV films doped with different amounts of Ir(piq)2acac (with
lex ¼ 385 nm). The number represents weight percent of the Ir(piq)2acac in the film.

84
W.-F. Su, Y. Chen / Polymer 52 (2011) 77e85
above 419 and 225 ꢁC, respectively. Thermal curing of PFTV at
230 ꢁC for 30 min led to homogeneous surface morphology with
the root-mean-square (rms) roughness being 0.88 nm. The HOMO
level of PFTV (ꢀ5.27 eV) lies between those of PEDOT:PSS
(ꢀ5.0 ∼ ꢀ5.2 eV) and PFO (ꢀ5.8 eV), forming a stepwise energy
ladder to facilitate hole-injection. The multilayer devices using
cross-linked PFTV as hole-transporting layer [ITO/PEDOT:PSS/
PFTV/PFO/Ca/Al] were readily fabricated by successive spin-
coating processes. The device performances were significantly
enhanced, by using the cured PFTV as hole-transporting layer. The
best maximum current efficiency and brightness were 5560 cd/m2
and 2.27 cd/A, respectively. Moreover, the PFTV was confirmed as
effective host for electrophosphorescent devices with Ir(piq)2acac
as red emitter. The maximum brightness and maximum current
efficiency reached 5300 cd/m2 and 2.64 cd/A, respectively. Current
results reveal that the PFTV is a promising optoelectronic material
which is applicable not only as hole-transporting polymer in
multilayer PLEDs but also as host polymer for electro-
phosphorescent LEDs.
Table 3
Optoelectronic properties of the electrophodphorescent devices using PFTV as
a host polymer doped with Ir(piq)2acac.
Hosta
Vonc (V)
Lmaxd (cd/m2)
LEmax (cd/A)e
CIE 1931 (x, y)f
PFTV2b
PFTV4
PFTV8
PFTV4g
6.1
6.7
7.1
6.7
4880
5580
4890
5300
0.83
2.15
0.65
2.64
(0.58, 0.30)
(0.64, 0.32)
(0.66, 0.32)
(0.65, 0.32)
a
Device structure: ITO/PEDOT:PSS/PFTV:Ir(piq)2acac/Ca/Al.
b
c
d
e
f
The number presents weight percent of Ir(piq)2acac in the film.
Turn-on voltage defined as the bias at a luminance of 10 cd/m2.
Maximum luminance.
Maximum luminance efficiency.
The 1931 CIE coordinates at ca. 1000 cd/m2.
g
Device with additional LiF as electron-injection layer: ITO/PEDOT:PSS/PFTV:
Ir(piq)2acac/LiF/Ca/Al.
quenched at 4 wt% and 8 wt% Ir(piq)2acac. The EL spectra peaked at
622 nm are similar to the PL spectrum of Ir(piq)2acac (Fig. 8),
indicating that the same emitting species are involved in both
conditions. However, the EL emission spectra (Fig. 9) are very
different from corresponding PL spectra (Fig. 8). This is presumably
attributed to different underlying energy transfer mechanisms
occurred under photo- and electro-excitation. The energy transfer
from PFTV to Ir(piq)2acac is more complete under electro-excita-
tion which usually results in about 75% triplet excitons. Moreover,
the Ir(piq)2acac may act as carriers trapping sites due to its low
band gap. Fig. 10 shows the current density versus bias and brig-
htness versus bias curves (JeVeB), with the characteristic data
summarized in Table 3. The maximum brightness and maximum
current efficiency of the devices are 4880e5580 cd/m2 and
0.65e2.15 cd/A, respectively. The turn-on voltage is increases from
6.1 V to 7.1 V with the increase of Ir(piq)2acac concentration (2 wt%
to 8 wt%), confirming that the Ir(piq)2acac acts as charges trapping
sites under electro-excitation. Because the HOMO (ꢀ5.17 eV) and
LUMO (ꢀ3.23 eV) levels of Ir(piq)2acac [33,49] lie between those of
PFTV (ꢀ5.27 eV, ꢀ2.41 eV), both holes and electrons are readily
trapped and recombined in Ir(piq)2acac (Fig. 10).
The PFTV4-based device exhibits the best performance, with
the maximum brightness and maximum current efficiency being
5580 cd/m2 and 2.15 cd/A, respectively. With increasing Ir(piq)2a-
cac concentration the maximum current efficiency is first enhanced
from 0.83 cd/A (2 wt%) to 2.15 cd/A (4 wt%) and then dropped
abruptly to 0.65 cd/A (8 wt%). The abrupt reduction in efficiency at
8 wt% Ir(piq)2acac is probably due to concentration quenching. In
order to improve the EL performance, a device with extra lithium
fluoride (LiF) layer was fabricated to balance charges injection and
transport; the device structure was ITO/PEDOT:PSS/PFTV4
(110 nm)/LiF(1 nm)/Ca(50 nm)/Al(100 nm). Its maximum bright-
ness and maximum current efficiency were 5300 cd/m2 and
2.64 cd/A, respectively (Table 3). This efficiency enhancement (from
2.15 to 2.64 cd/A) is attributed to improved electron-injection by LiF
layer that results in more balanced charges injection and transport.
Current results demonstrate that PFTV is not only an efficient hole-
transporting polymer for electroluminescent devices but also
applicable as host for electrophosphorescent devices.
Acknowledgment
The authors thank the National Science Council of Taiwan for
financial aid through project NSC 98-2221-E-006-002-MY3.
Appendix. Supporting information
Supporting information associated with this article can be
found, in the online version, at doi:10.1016/j.polymer.2010.10.065.
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