Synthesis and optoelectronic properties of thermally cross-linkable fluorene derivative containing hole-transporting triphenylamine terminals

Article abstract of DOI:10.1016/j.polymer.2011.05.036

This paper describes the synthesis of a solution-processable and thermally cross-linkable 2,7-bis-[4-bis(4-vinylphenyl)aminophenyl]-9,9-dihexylfluorene (VTF) and its application as hole-transporting layer in multilayer polymer light-emitting diodes (PLEDs). The thermal, photophysical, and electrochemical properties of VTF were investigated by differential scanning calorimetry, thermogravimetric analysis, optical spectroscopy, and cyclic voltammetry. The VTF is readily cross-linked via vinyl groups by heating at 180 °C for 30 min to obtain homogeneous film with excellent solvent resistance. Multilayer PLEDs (ITO/PEDOT:PSS/cured-VTF/MEH-PPV/Ca/Al) were readily fabricated by spin-coating process using cross-linked VTF as hole-transporting layer (HTL). The maximum brightness (13,640 cd/m²) and current efficiency (0.69 cd/A) were superior to those without HTL (ITO/PEDOT:PSS/MEH-PPV/Ca/Al: 7810 cd/m ², 0.28 cd/A). In addition, the cured-VTF could replace conventional hole-injection layer (PEDOT:PSS) to reveal comparable performance (8240 cd/m², 0.44 cd/A). Current results indicate that the VTF with four thermally cross-linkable terminal vinyl groups is a promising optoelectronic material, which is readily processed by wet processes.

Full text of DOI:10.1016/j.polymer.2011.05.036

Polymer 52 (2011) 3311e3317
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and optoelectronic properties of thermally cross-linkable fluorene
derivative containing hole-transporting triphenylamine terminals
*
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 solution-processable and thermally cross-linkable 2,7-bis-[4-
bis(4-vinylphenyl)aminophenyl]-9,9-dihexylfluorene (VTF) and its application as hole-transporting
layer in multilayer polymer light-emitting diodes (PLEDs). The thermal, photophysical, and electro-
chemical properties of VTF were investigated by differential scanning calorimetry, thermogravimetric
analysis, optical spectroscopy, and cyclic voltammetry. The VTF is readily cross-linked via vinyl groups by
heating at 180 ꢀC for 30 min to obtain homogeneous film with excellent solvent resistance. Multilayer
PLEDs (ITO/PEDOT:PSS/cured-VTF/MEH-PPV/Ca/Al) were readily fabricated by spin-coating process using
cross-linked VTF as hole-transporting layer (HTL). The maximum brightness (13,640 cd/m2) and current
efficiency (0.69 cd/A) were superior to those without HTL (ITO/PEDOT:PSS/MEH-PPV/Ca/Al: 7810 cd/m2,
0.28 cd/A). In addition, the cured-VTF could replace conventional hole-injection layer (PEDOT:PSS) to
reveal comparable performance (8240 cd/m2, 0.44 cd/A). Current results indicate that the VTF with four
thermally cross-linkable terminal vinyl groups is a promising optoelectronic material, which is readily
processed by wet processes.
Received 23 March 2011
Received in revised form
16 May 2011
Accepted 21 May 2011
Available online 27 May 2011
Keywords:
Thermally cross-linkable
Hole-transporting
Fluorene derivative
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
vacuum evaporation, which usually requires strict control of
thermal deposition conditions. On the contrary, film formation by
Organic and polymer light-emitting devices (OLEDs and PLEDs)
have attracted considerable attention due to their potential appli-
cations in solid-state lighting and full-color flat-panel displays
[1e6]. The major issues in such applications lie in the feasibility of
fabricating large-area devices and the cost of manufacturing tech-
nology [7]. Solution processes, such as spin-coating, are believed to
be more cost-effective way of fabricating large-area devices. Device
performance is another critical factor in determining their potential
for applications. To achieve optimum device performance, charges
injection and transport from cathode and anode must be balanced
to raise the recombination ratio of charges [8]. Multilayer devices
are usually fabricated to attain this balance by inserting hole-
transporting (or injecting) layer (HTL) or electron transporting (or
injecting) layer (ETL) in addition to emitting layer [9e14]. For
example, Tang and VanSlyke demonstrated efficient multilayer
OLEDs using small molecular triphenylamine as HTL and Alq3 as
emitting and electron-transporting layers to balance carriers
transport and injection [15]. However, the low molecular weight
hole-transporting and emitting materials must be deposited by
solution processes, such as spin-coating and ink-jet printing
methods, are relatively easy and cost-effective. Some low molecular
weight hole-transporting materials whose films can be obtained by
wet processes have been synthesized and applied in OLEDs. For
example, Promarak et al. [16] fabricated a multilayer OLED device
by wet processes using amorphous fluorene derivative [2,7-bis(4-
diphenylaminophenyl)-9,9-bis-n-hexylfluorene] as HTL and
thermal deposited Alq3 as emitting layer. The device exhibited
bright green emission of 6500 cd/m2 and low turn-on voltage of
3.8 V. However, the thermal deposition step eliminated advantages
gained by the inherent simplicity of wet processing [17]. Therefore,
fabrication of multilayer device by subsequent spin-coating is
highly desirable. A major challenge for solution processing is the
erosion or dissolution of previously deposited layer by subsequent
coating solution.
To avoid this undesirable erosion or dissolution, the utilization
of orthogonal solvents for processing has been reported [7,18]. For
example, the water soluble poly(styrenesulphonate)-doped
poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) has been widely
used in OLEDs and PLEDs as a hole-injection layer to enhance the
device performance [19]. However, the PEDOT has serious draw-
backs such as corrosion of the ITO anode [20,21], poor surface energy
match with aromatic emissive layer and exciton quenching at the
* 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 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.05.036

3312
W.-F. Su, Y. Chen / Polymer 52 (2011) 3311e3317
interface between PEDOTand emissive materials [10]. Furthermore,
whether PEDOT possesses adequate electron-blocking capacity is
another issue needs to be considered [9,22]. To avoid this undesir-
able erosion or dissolution, cross-linkable precursors with specific
functional groups become a necessary for the fabrication of multi-
layer devices. Several thermally curable triarylamine derivatives
containing trifluorovinyl ethers (TFVE) or styryl groups were
successfully used as HTLs and demonstrated excellent solvent
resistance [8,9,23e25]. Moreover, the optimal HTL should possess
notonlyappropriate HOMO energy level (lying between those of ITO
and emitting layer) tofacilitatehole-injectionbut alsohighelectron-
blocking capacity [22]. However, the cross-linkable HTLs containing
trifluorovinyl ether groups (TFVE) required both high curing
temperatures (>225 ꢀC) and long curing times (1e2 h). This pro-
longed high-temperature curing tends to degrade the previously
deposited layers. Accordingly, it is highly desirable to develop cross-
linkable HTLs which are readily curable at lower temperature
(<200 ꢀC). In addition to thermal curing conditions, solvent resis-
tance of the cured film is also an essential requirement for practical
applications. For this reason, we attempted to design solution-
processable and thermally cross-linkable HTLs which can be ther-
mally cured below 200 ꢀC to obtain highly solvent-resistant films.
Previously, we successfully developed thermally cross-linkable
poly(fluorene-co-triphenylamine) (PFTV) and solution-processable
4,40,400-{[9,9-bis(hexyl)-9H-fluorene-2,4,7-triyl]tri-2,1-ethenediyl}
tris(N,N-diphenyl) benzeneamine (TF) as hole-transporting mate-
rials and obtained good device performances [26,27]. However,
some issues left to be solved to enhance device performance further,
such as increasing solvent resistance and thermal cross-linking
reactivity. Therefore, in this study, we synthesized a new solution-
processable, thermally cross-linkable 2,7-bis-[4-bis(4-vinylphenyl)
aminophenyl]-9,9-dihexylfluorene (VTF) to be applied as hole-
transporting layer or hole-injection layer. The VTF comprises rigid
fluorene as core and hole-transporting triphenylamine as terminals,
aiming to retain its thermal and morphological stability and to
enhance device performance, respectively [28]. The VTF revealed
high thermal cross-linking reactivity (curable below 200 ꢀC within
30 min); its cured film was homogeneous and highly solvent resis-
tant. We find that the VTF integrates thermal cross-linking and
hole-transporting ability with high reproducibility in fabricating
multilayer PLEDs by successive spin-coating processes. The perfor-
mance of MEH-PPV device is greatly enhanced by inserting the
thermally cured VTF as hole-transporting layer, with the maximum
luminance and current efficiency being 13,640 cd/m2 and 0.69 cd/A,
respectively. In addition, the cured-VTF can also be applied as hole-
injection layer to replace conventional PEDOT:PSS. The new VTF is
a promising hole-transporting material for optoelectronic devices;
and moreimportantly, it can be deposited byspin-coating processes.
internal standard. The FT-IR spectra were measured as KBr disk using
a Fourier transform infrared spectrometer, model 7850 from Jasco.
Mass and elemental analysis were carried out on a JEOL JMS-700
spectrometer and Heraus CHN-Rapid elemental analyzer, respec-
tively. Thermogravimetric analysis (TGA) was performed under
nitrogen atmosphere at a heating rate of 20 ꢀC/min, using a Perki-
nElmer TGA-7 thermal analyzer. Differential scanning calorimetry
(DSC) was performed on a Mettler Toledo DSC 1 Star System under
nitrogen atmosphere at a heating rate of 10 ꢀC/min.Absorptionspectra
were measured with a Jasco V-550 spectrophotometer and photo-
luminescence (PL) spectra were obtained using a Hitachi F-4500
fluorescence spectrophotometer. The cyclic voltammograms were
recorded using a voltammetric apparatus (model CV-50 W from BAS)
at room temperature under nitrogen atmosphere. The measuring cell
was composed of an 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) [31,32]. An atomic force
microscope (AFM), equipped with a Veeco/Digital Instrument Scan-
ning 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-transporting and emitting layers were
measured by a surface profiler, a-step 500. The static contact angles
were measured with water by using a contact angle meter (GBX,
PX610, France).
2.2. Synthesis of compound 5 and tetra-functional VTF (Scheme 1)
2.2.1. Synthesis of 2,7-bis-[4,40-(phenylazanediyl)dibenzaldehyde]-
9,9-dihexylfluorene (5)
A mixture of 4,40-(4-bromophenylazanediyl)dibenzaldehyde (3:
0.8 g, 2.1 mmol), 9,9-dihexylfluorene-2,7-bis(trimethyleneborate)
(4, 0.5 g, 1 mmol), (PPh3)4Pd(0) (50 mg, 0.04 mmol) and several
drops of Aliquat 336 were added to a mixture consisting of toluene
and aqueous solution of K3PO4 (2 M). The mixture was first purged
with nitrogen and then stirred rigorously at 90 ꢀC for 24 h. The
mixture was poured into distilled water after cooling to room
temperature and extracted with chloroform. The organic layer was
dried over anhydrous magnesium sulfate and vacuum concen-
trated. The crude product was purified by column chromatography
using CHCl3 as an eluent. The product was re-dissolved in chloro-
form and poured into a large amount of ethanol; the appearing
precipitates were collected by filtration to give yellow solids of 5.
The yield was 61% (0.57 g). FT-IR (KBr pellet, cmꢁ1):
2931, 2848, 2734, 1691 (eCHO), 1585, 1504, 1467, 1315, 1284, 1213,
1162, 821, 732, 690. 1H NMR(CD2Cl2, ppm):
9.93 (s, 4H, eCHO),
n 3062, 3033,
d
2. Materials and methods
7.81e7.83 (d, 10H, AreH, J ¼ 8 Hz), 7.72e7.74 (d, 4H, AreH, J ¼ 8 Hz),
7.62e7.65 (m, 4H, AreH), 7.27e7.31 (m, 12H, AreH), 2.07 (s, 4H,
eCH2-), 1.08w1.13 (m, 12H, eCH2-), 0.75w0.79 (m, 10H, eCH2e &
2.1. Materials and characterization
eCH3). 13C NMR (CD2Cl2, 400 MHz, ppm):
d
180.54, 142.09, 134.97,
130.44, 129.39, 129.23, 121.71, 121.30, 118.76, 117.37, 116.01, 113.11,
111.47, 110.34, 30.56, 21.68, 19.84, 14.04, 12.73, 3.92. ELEM. ANAL
4-Bromotriphenylamine (2) and 4,40,-(4-bromophenylazanediyl)
dibenzaldehyde (3) were prepared according to previously reported
procedures [29,30]. 9,9-Dihexylfluorene-2,7-bis(trimethyleneborate),
Aliquat 336 and tetrakis(triphenylphosphine)palladium [Pd(PPh3)4]
were purchased from Aldrich Co., Alfa Aesar Co. and Strem Chemicals
Inc., respectively. Methanol (ECHO Co.), toluene (Tedia Co.), chloro-
form (CHCl3, Tedia Co.) and other solvents were HPLC grade reagents.
All reagents and solvents were used without further purification. All
newly synthesized compounds were identified by 1H NMR, 13C NMR,
and elemental analysis (EA). 1H NMR and 13C NMR spectra were
recorded with a Bruker AVANCE-400 NMR spectrometer, with the
chemical shifts reported in ppm using tetramethylsilane (TMS) as an
.
Calcd. for C65H60N2O4 (%): C, 83.66; H, 6.48; N, 3.00. Found: C,
83.65; H, 6.59; N, 2.90. FAB-MS: m/z: 933 [Mþ] (calcd: 932.5).
2.2.2. Synthesis of 2,7-bis-[4-bis(4-vinylphenyl)aminophenyl]-9,9-
dihexylfluorene (VTF)
To a mixture of methyltriphenylphosphonium bromide (1.92 g,
5.36 mmol) in 15 mL of THF was added t-BuOK (5 mL, 5.00 mmol)
under cooling and then 5 (0.50 g, 0.535 mmol). The mixture was
stirred for 24 h at room temperature, poured into distilled water
and extracted with chloroform. The organic layer was dried with

W.-F. Su, Y. Chen / Polymer 52 (2011) 3311e3317
3313
Br
Br
O
O
B
B
O
O
POCl3
NBS
4
N
N
N
DMF
OHC
CHO
1
3
2
OHC
CHO
CH3P+Ph3Br-
t-BuOK THF
N
N
N
N
OHC
CHO
VTF
5
Scheme 1. Synthesis of compound 5 and tetra-functional monomer VTF.
anhydrous magnesium sulfate and vacuum concentrated. The
crude product was purified by column chromatography using
n-hexane/dichloromethane (v/v ¼ 3/1). The product was re-
dissolved in chloroform and poured into a large amount of meth-
anol; the appearing precipitates were collected by filtration to give
light yellow solids of VTF. The yield was 58% (0.29 g). FT-IR (KBr
fabrication 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
pellet, cmꢁ1):
1180, 1112, 989, 900, 836, 821, 647. 1H NMR (CDCl3, ppm):
n 3083, 3029, 2927, 2856,1596,1504, 1461,1319,1280,
3.1. Synthesis and characterization
d
7.73w7.75 (d, 2H, AreH, J ¼ 8 Hz), 7.54w7.58 (m, 8H, AreH),
Tetra-aldehyde derivative of fluorene (5) was synthesized by the
palladium-catalyzed Suzuki coupling of 9,9-dihexylfluorene-2,7-
bis(trimethyleneborate) (4) and 4,40-(4-bromophenylazanediyl)
dibenzaldehyde (3) as shown in Scheme 1. The target thermally
cross-linkable hole-transporting VTF was synthesized by the well-
known Wittig reaction from 5 and methyltriphenylphosphonium
bromide. Chemical structure of 5 and target tetra-functional VTF
were satisfactorilyconfirmed by1H NMR,13C NMR spectra, elemental
analysis, and mass spectrometry. Fig. S1 (Supporting Information)
shows the 1H NMR spectra of 5, in which the characteristic chemical
shift at 9.93 ppm is assigned to protons of the aldehyde substituents
(eCHO). And as shown in Fig. 1, the characteristic peak at 9.93 ppm
disappears completely in 1H NMR spectra of tetra-functional VTF,
with new proton peaks appearing obviously at 6.65w6.72 and
5.17w5.69 ppm attributable to vinyl substituents (¼CH- and ¼ CH2).
The proton’s absorption peaks was assigned according to the HMBC
7.32w7.34 (d, 8H, AreH, J ¼ 8 Hz), 7.18w7.20 (d, 4 H, AreH,,
J ¼ 8 Hz), 7.09w7.11 (d, 8 H, AreH,, J ¼ 8 Hz), 6.65w6.72 (dd, 4H,
AreCH ¼ CH2, J1 ¼ 17.6 Hz, J2 ¼ 10.8 Hz), 5.65w5.69 (d, 4H,
AreCH ¼ CH2, J ¼ 17.6 Hz), 5.17w5.20 (d, 4H, AreCH ¼ CH2,
J ¼ 10.8 Hz), 2.00w2.05 (m, 4H, eCH2-), 1.05w1.31 (m, 12H, eCH2-),
0.73w0.77 ( m, 10H, eCH2-&-CH3). 13C NMR (CDCl3, 400 MHz,
ppm): d 151.63, 147.04, 146.50, 139.79, 139.25, 136.16, 132.34, 127.84,
125.55, 124.46, 124.02, 120.90, 119.90, 112.39, 55.21, 40.47, 31.45,
29.69, 23.77, 22.55, 13.97. ELEM. ANAL. Calcd. for C69H68N2 (%): C,
89.57; H, 7.41; N, 3.03. Found: C, 89.32; H, 7.50; N, 2.96. FAB-MS: m/
z: 925 [Mþ] (calcd: 924.54).
2.3. Fabrication and characterization of electroluminescent devices
Multilayer light-emitting diodes [ITO/PEDOT:PSS/HTL/MEH-
PPV/Ca/Al] or [ITO/HIL/MEH-PPV/Ca/Al] were fabricated to inves-
tigate their optoelectronic characteristics. The ITO-coated glass
substrate was washed successively in ultrasonic baths of neutral
cleaner/de-ionized water (v/v ¼ 1/3) mixture, de-ionized water,
acetone and 2-propanol, followed with treatment in a UV-Ozone
chamber. A thick hole-injection 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 hole-transporting layer (HTL)
was formed by spin-coating a solution of VTF in toluene (5 mg/mL,
2000 rpm) on top of the PEDOT:PSS layer and thermally treated at
180 ꢀC for 30 min under nitrogen atmosphere. Then the emitting
MEH-PPV layer (EML) was spin-coated onto the HTL. The VTF and
MEH-PPV solutions were filtered through a syringe filter (0.2 mm)
before the spin-coating. The actual thickness of HTL and EML are
about 30 and 100 nm, respectively. Finally, a thick layer of cathode
was deposited by successive thermal evaporation of Ca (50 nm) and
Al (100 nm) under 110ꢁ6 Torr. The luminance versus bias, current
density versus bias, and emission spectra of the PLEDs were recor-
ded using a combination of Keithley power source (model 2400) and
Ocean Optics usb2000 fluorescence spectrophotometer. The
Fig. 1. 1H NMR spectrum of VTF in CDCl3.

3314
W.-F. Su, Y. Chen / Polymer 52 (2011) 3311e3317
(Heteronuclear Multiple Bond Coherence) spectrum of VTF
[Fig. S1(b)]. Successful synthesis of 2,7-bis-[4-bis(4-vinylphenyl)
aminophenyl]-9,9-bis-n-hexylfluorene (VTF) was ascertained from
the 1:1 area ratio of peak a (¼CH2 in vinyl) over peak i (eCH2 in alkyl
chain of fluorene). The characteristic chemical peaks at 6.65w6.72
and 5.17w5.69 ppm confirms the existence of vinyl groups, from
which thermally cross-linkable character can be expected. In
combination with the data of mass and elemental analysis, the
successful synthesis of VTF has been confirmed. The VTF is soluble in
common organic solvents such as chloroform, THF, toluene, and
chlorobenzene.
1st heating scan
2nd heating scan
2nd heating scan after
isothermal at 180 oC
o
Tg = 198
C
Temperature ( C)
H = 86.1 J/g
3.2. Thermal cross-linking properties and surface morphology of
VTF
40
60
80
100
120
140
160
180
200
220
Since VTF contains reactive vinyl groups which will give rise to
networks structure (cross-linking) under proper thermal treat-
ment. Thermal cross-linking and thermal resistant characteristics
of VTF were studied by differential scanning calorimetry (DSC) and
thermogravimetric analysis (TGA), respectively (Table 1). The
thermal decomposition temperature (Td, 5% weight loss) of VTF is
512 ꢀC under nitrogen atmosphere, indicating that the VTF is highly
thermal stable (Fig. S2). As shown in Fig. 2, during the first heating
scan the DSC trace of VTF shows an exothermic peak at 184 ꢀC, in
which the exotherm starts at about 175 ꢀC. The heat released
(86.1 J/g) is attributed to cross-linking reaction of the periphery
vinyl groups. However, no obvious exotherm is observed in the
second heating scan, indicating the cross-linking has been
completed during the first scan. Furthermore, the glass-transition
temperature (Tg) of the cured VTF is about 198 ꢀC with no detect-
able crystallization and melting transition, suggesting that it is
basically an amorphous material (as shown in inset). The high Tg is
highly desirable to prevent the crystallization process, during
device operation or thermal annealing, which deteriorates long-
term morphological stability. The reaction time required to cure
VTF was estimated using isothermal DSC technique. The
exothermic thermal cross-linking of VTF under 180 ꢀC is completed
within 30 min (Fig. S3). This can be confirmed by the absence of
exothermic peak in the DSC trace of the cross-linked sample. To
ensure complete cross-linking in the following study, the cross-
linking of all VTF films was conducted at 180 ꢀC for 30 min under
nitrogen atmosphere. Moreover, the cross-linking behaviors of the
VTF film were investigated by FT-IR spectra (Fig. S4). The peaks
around 900 and 987 cmꢁ1 corresponding to the vinyl CeH out-of-
plane deformation vibration and 1625 cmꢁ1 corresponding to the
vinyl C]C stretching vibration decreased significantly after the
thermal curing. The cured films became insoluble in solvents. These
results indicate that the cross-linking of vinyl groups has been
completed under these conditions.
Temperature (oC)
Fig. 2. DSC traces of VTF at a heating rate of 10 ꢀC/min. The first scan was used to
observe the reaction heat of vinyl groups.
However, NPB layer must be deposited by thermal evaporation
under vacuum due to its poor film-forming property obtainable in
solution processes such as spin-coating. Homogeneous film is
a prerequisite to fabricate OLEDs and PLEDs with high device
performance and long lifetime. Furthermore, to achieve efficient
emission in PLEDs, it is imperative to obtain highly homogeneous
film for each layer. The morphology of VTF film obtained by spin-
coating on ITO substrate, before and after thermal curing, was
investigated using atomic force microscope (AFM: tapping mode).
As shown in Fig. 3, the surface morphology of VTF film before
thermal curing is quite homogeneous, with the root-mean-square
(rms) roughness being 1.63 nm. However, the film homogeneity
is further improved after the thermal curing at 180 ꢀC, 30 min. The
cured film is pinhole- and aggregate-free, with the rms roughness
being reduced to 1.24 nm. This homogeneous surface morphology
of VTF after thermal cross-linking is beneficial for obtaining high
performance PLED devices.
3.3. Optical and electrochemical properties
Fabrication of multilayer PLEDs by solution processes absolutely
requires that previously deposited layer should be resistant to the
subsequent coating solutions. To examine solvent resistance of the
VTF films before and after thermal curing (180 ꢀC, 30 min), they
were spin-coated with chlorobenzene (good solvent of VTF) to
evaluate their dissolution behaviors. As shown in Fig. 4, the
absorption spectrum of pristine VTF film disappears almost
completely after the spin-coating, meaning that the VTF film is
dissolved by chlorobenzene during the coating process. However,
the absorption spectrum of thermal cured-VTF film remains almost
unchanged after the spin-coating. This indicates that cured-VTF is
highly solvent-resistant, which is attributed to its highly cross-
linked structure after the thermal curing. This characteristic
makes VTF suitable for the fabrication of multilayer PLEDs by
solution processes.
Low molecular weight hole-transporting materials, such as
N,N0-di-[(1-
naphthalenyl)-N,N0-diphenyl]-(1,10-biphenyl)-4,40-
diamine (NPB), are widely used in the fabrication of OLEDs.
Table 1
Thermal and Electrochemical Properties of VTF.
Cyclic voltammetry (CV) is an effective method in investigating
reduction-oxidation behaviors of thin films insoluble in electrolyte
solution [33,34]. Because the VTF is readily formed as homogeneous
film that is insoluble in acetonitrile, it was deposited on an ITO glass
to be applied as the working electrode. The electrode was supported
in anhydrous acetonitrile containing 0.1 M tetra-n-butylammonium
perchlorate [(n-Bu)4NClO4] for CV measurements. The highest
occupied molecular orbital (HOMO) which corresponds to ioniza-
tion potential (IP) can be estimated from the onset oxidation
opt
Tg
Td
Eox vs. FOC
EHOMO
(eV)d
ELUMO
(eV)e
Eg
(ꢀC)a
(ꢀC)b
(V)c
(eV)f
VTF
198
512
0.49
ꢁ5.29
ꢁ2.39
2.90
a
Determined by differential scanning calorimetry (DSC) at a heating rate of 10 ꢀC/min.
The temperature at 5 wt% loss in nitrogen atmosphere, measured by TGA.
EFOC ¼ 0.48 V vs. Ag/AgCl.
b
c
d
e
f
EHOMO ¼ -(Eox, FOC þ 4.8) eV.
ELUMO ¼ Eg þ EHOMO
.
Band gaps obtained from onset absorption (lonset): Eg ¼ 1240/lonset
.

W.-F. Su, Y. Chen / Polymer 52 (2011) 3311e3317
3315
Fig. 3. The AFM images of VTF films coated on ITO glass: (a) pristine film, rms roughness ¼ 1.63 nm; (b) film thermally treated at 180 ꢀC for 30 min, rms roughness ¼ 1.24 nm.
potential (Eox) revealed in CV by the equation EHOMO ¼ -(Eox þ 4.8)
eV. Fig. S4 shows the cyclic voltammograms of the cured-VTF film
(180 ꢀC for 30 min), the onset oxidation potential is observed at
0.49 V (versus ferrocene) and the HOMO level estimated to be
ꢁ5.29 eV. The lowest unoccupied molecular orbital (LUMO) level is
estimated tobe ꢁ2.39 eV, using the HOMO level and the optical band
gap (Eg) calculated from onset absorption (Table 1). Fig. 5 illustrates
the energy levels of cured-VTF, PEDOT:PSS and work functions of
electrodes. Clearly, the HOMO level of cured-VTF (ꢁ5.29 eV) is close
3.4. Optoelectronic properties of multilayer PLEDs using VTF as
hole-transport layer
To evaluate the applicability of the cured-VTF as hole-
transporting layer, multilayer polymer light-emitting diode, with
a device configuration of [ITO/PEDOT:PSS/cured-VTF/MEH-PPV/
Ca(50 nm)/Al(100 nm)], was investigated by successive spin-
coating process to investigate their optoelectronic properties. The
device without cured-VTF as hole-transporting layer (ITO/
PEDOT:PSS/MEH-PPV/Ca/Al) was also fabricated simultaneously for
comparative study. The transparent and insoluble cured-VTF film
was fabricated by spin-coating VTF solution on top of the
PEDOT:PSS layer and then curing at 180 ꢀC for 30 min under
nitrogen atmosphere. The current density versus bias and bright-
ness versus bias curves (JeBeV) of the electroluminescent (EL)
devices are shown in Fig. 6, with the corresponding characteristic
data summarized in Table 2. Fig. 7 shows the electroluminescent
spectra of PLED using cured-VTF as hole-transporting layer (HTL),
the main emission peaks are situated around 584 and 622 nm. Its EL
spectrum is analogous to that of device without VTL layer, sug-
gesting that the emission is originated from emitting MEH-PPV
layer. Furthermore, inserting a cured-VTF as HTL significantly
improves device performance, i.e., the maximum brightness and
maximum current efficiency are enhanced from 7810 cd/m2 and
0.28 cd/A (without HTL) to 13,640 cd/m2 and 0.69 cd/A, respec-
tively. The turn-on voltage is increased from 3.1 V to 4.1 V when the
cured-VTF is inserted as HTL (Fig. 6 & Table 2). Inserting a cured-
VTF layer reduces the current density at lower voltage (<6.5 V),
which is probably caused by the retarded hole-injection due to its
to that of ITO/PEDOT:PSS (ꢁ5.2 eV) and the barrier height (
DEh) for
hole injection between PEDOT:PSS and VTF is 0.09 eV, facilitating
the injection and transport of holes. Accordingly, the cured-VTF is
a potential hole-injection and transporting material.
0.20
a
VTF
spin-coating with
chlorobenzene
0.15
0.10
0.05
0.00
b
cured-VTF
0.15
spin-coating with
chlorobnzene
LUMO
-2.39
-2.70
-2.9
Ca
0.10
0.05
0.00
-4.3
Al
-5.02
MEH-PPV
-5.2
PEDOT
300
400
500
600
700
-5.29
cured-VTF
Wavelength (nm)
HOMO
Fig. 4. Absorption spectra of pristine and cured-VTF films: (ꢁ) before and (—) after
spin-coated with pure chlorobenzene. The cross-linking was conducted at 180 ꢀC for
30 min under nitrogen atmosphere.
Fig. 5. The energy level diagrams of cured-VTF, PEDOT:PSS and electrodes.

3316
W.-F. Su, Y. Chen / Polymer 52 (2011) 3311e3317
2500
2000
1500
1000
500
1.2
PEDOT(HIL)
PEDOT(HIL)/cured-VTF(HTL)
cured-VTF(HIL)
PEDOT(HIL)
PEDOT(HIL)/cured-VTF(HTL)
cured-VTF(HIL)
1.0
0.8
0.6
0.4
0.2
0
0.0
105
500
550
600
650
700
750
800
Wavelength (nm)
104
103
102
101
Fig. 7. Electroluminescent spectra of polymer light-emitting diodes. Device structure:
(ꢁ) ITO/PEDOT:PSS/MEH-PPV/Ca/Al; (ꢁꢁ) ITO/PEDOT:PSS/cured-VTF/MEH-PPV/Ca/Al;
(ꢁꢂꢂꢁ)ITO/cured-VTF/MEH-PPV/Ca/Al; HIL: hole-injection layer, HTL: hole-
transporting layer.
Moreover, a PLED with cured-VTF as hole-injection layer (HIL)
[ITO/cured-VTF/MEH-PPV/Ca/Al] was also fabricated to evaluate
the applicability of VTF as HIL. The turn-on voltage (at 10 cd/m2) is
increased from 3.1 V to 4.6 V when the PEDO:PSS is replaced with
cured-VTF as HIL (Table 2). This indicates that cured-VTF layer
raises the barrier height for hole injection when compared to
PEDOT:PSS layer. The aqueous contact angle of ITO substrate, ITO/
PEDOT:PSS and cured-VTF are <5ꢀ, 10e20ꢀ and 78ꢀ, respectively.
The surface energy mismatch between hydrophilic ITO and
hydrophobic cured-VTF is likely to cause poor physical-electronic
contact at the ITO/cured-VTF interface [22]. However, its
maximum brightness (8240 cd/m2) and maximum current effi-
ciency (0.44 cd/A) are still superior to those with PEDOT:PSS as HIL
(7810 cd/m2, 0.28 cd/A). This suggests that the cured-VTF acts as
buffer layer with electron-blocking feature that results in more
balanced hole and electron fluxes [22,23]. To conclude, the cured-
VTF is a promising hole-transporting and hole-injection material
for the fabrication of efficient multilayer PLEDs by successive spin-
coating processes.
0
2
4
6
8
10
Bias (V)
Fig. 6. Current density versus bias and brightness versus bias characteristics of the
polymer light-emitting diodes. Device structure: (ꢁ) ITO/PEDOT:PSS/MEH-PPV/Ca/Al;
(ꢁꢁ) ITO/PEDOT:PSS/cured-VTF/MEH-PPV/Ca/Al; (ꢁꢂꢂꢁ) ITO/cured-VTF/MEH-PPV/
Ca/Al; HIL: hole-injection layer, HTL: hole-transporting layer.
lower HOMO level (ꢁ5.29 eV) than that of MEH-PPV (ꢁ5.02 eV).
Higher voltages raise the current density quickly owing to the fact
that many holes gain enough energy to conquer the barrier height.
The responses of the current efficiency to the operating voltage are
depicted in Fig. S6. Inserting cured-VTF as hole-transporting layer
raises the device current efficiency for about two times. The
performance enhancement is attributable to more balanced
charges injection and transport in the MEH-PPV layer due to the
presence of the cured-VTF as hole-transporting layer. As mentioned
above, the HOMO level of cured-VTF (ꢁ5.29 eV) is close to that of
PEDOT:PSS (ꢁ5.2 eV) to facilitate hole injection and transport.
Additionally, the cured-VTF also acts as electron-blocking layer
because of its high LUMO level (ꢁ2.39 eV, Fig. 5). Therefore, the
cured-VTF layer not only effectively confines the excitons in MEH-
PPV layer but also prevents the PEDOT:PSS chain from penetrating
into the MEH-PPV layer to quench the emission [9,25,35].
4. Conclusions
We have successfully prepared a hole-transporting 2,7-bis-[4-
bis(4-vinylphenyl)aminophenyl]-9,9-dihexylfluorene (VTF) con-
taining thermally reactive vinyl groups by the Wittig reaction. The
VTF demonstrated high thermal stability with thermal decompo-
sition (Td) being 512 ꢀC and exothermic peaks at 184 ꢀC due to
thermal cross-linking of the vinyl groups. The glass-transition
temperature was 198 ꢀC after thermal curing at 180 ꢀC for
30 min. The cured-VTF was highly solvent-resistant, attributed to
its highly cross-linked structure after the thermal curing. Moreover,
the thermal curing resulted in homogeneous surface morphology,
with the root-mean-square roughness being 1.24 nm. Multilayer
devices using cured-VTF as hole-transporting layer [ITO/
PEDOT:PSS/cured-VTF/MEH-PPV/Ca/Al] were readily fabricated by
successive spin-coating processes. The device performance was
significantly enhanced, by using the cured-VTF as hole-
transporting layer. The optimal maximum luminance and
maximum current efficiency were 13,640 cd/m2 and 0.69 cd/A,
respectively, which were superior to those without cured-VTF as
hole-transporting layer (7810 cd/m2, 0.28 cd/A). Moreover, the
cured-VTF was also used as hole-injection layer, instead of
conventional PEDOT:PSS, to obtain comparable device performance
Table 2
Optoelectronic Properties of the Multilayer PLEDs.a
HIL/HTL
Von (V)b Lmax (cd/m2)c LEmax(cd/A)d CIE 1931 (x, y)e
PEDOT(HIL)
PEDOT(HIL)/
cured-VTF(HTL)
cured-VTF(HIL)
3.1
4.1
7810
13,640
0.28
0.69
(0.58, 0.42)
(0.58, 0.42)
4.6
8240
0.44
(0.58, 0.42)
a
Device structure: ITO/HIL/HTL/MEH-PPV/Ca/Al; HIL: hole-injection layer, HTL:
hole-transporting layer.
Turn-on voltage defined as the bias at a luminance of 10 cd/m2.
b
c
Maximum luminance.
d
Maximum luminance efficiency.
The 1931 CIE coordinates at ca. 1000 cd/m2.
e

W.-F. Su, Y. Chen / Polymer 52 (2011) 3311e3317
3317
(8240 cd/m2, 0.44 cd/A). Current results reveal that the thermally
cross-linkable VTF is a promising hole-transporting and hole-
injection material, applicable for the fabrication of multilayer
PLEDs by successive wet processes.
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Acknowledgment
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The authors thank the National Science Council of Taiwan for
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Appendix. Supplementary material
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