Solution-processable and thermally cross-linkable fluorene-cored triple-triphenylamines with terminal vinyl groups to enhance electroluminescence of MEH-PPV: Synthesis, curing, and optoelectronic properties

Article abstract of DOI:10.1002/pola.26184

This study reports the synthesis, curing, and optoelectronic properties of a solution-processable, thermally cross-linkable electron- and hole-blocking material containing fluorene-core and three periphery N-phenyl-N-(4-vinylphenyl) benzeneamine (FTV). Th

Full text of DOI:10.1002/pola.26184

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Solution-Processable and Thermally Cross-Linkable Fluorene-Cored
Triple-Triphenylamines with Terminal Vinyl Groups to Enhance
Electroluminescence of MEH-PPV: Synthesis, Curing, and
Optoelectronic Properties
Chia-Shing Wu, Ya-Ju Yang, Szu-Wen Fang, Yun Chen
Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan
Correspondence to: Y. Chen (E-mail: yunchen@mail.ncku.edu.tw)
Received 16 March 2012; accepted 12 May 2012; published online
DOI: 10.1002/pola.26184
ABSTRACT: This study reports the synthesis, curing, and optoe-
lectronic properties of a solution-processable, thermally cross-
linkable electron- and hole-blocking material containing fluo-
rene-core and three periphery N-phenyl-N-(4-vinylphenyl)ben-
zeneamine (FTV). The FTV exhibited good thermal stability
with Td above 478 ꢀC in nitrogen atmosphere. The FTV is read-
ily cross-linked via terminal vinyl groups by heating at 160 ꢀC
for 30 min to obtain homogeneous film with excellent solvent
resistance. Multilayer PLED device [ITO/PEDOT:PSS/cured-FTV/
MEH-PPV/Ca (50 nm)/Al (100 nm)] was successfully fabricated
using solution processed. Inserting cured-FTV is between
PEDOT:PSS and MEH-PPV results in simultaneous reduction in
hole injection from PEDOT:PSS to MEH-PPV and blocking in
electron transport from MEH-PPV to anode. The maximum
luminance and maximum current efficiency were enhanced
from 1810 and 0.27 to 4640 cd/m2 and 1.08 cd/A, respectively,
after inserting cured-FTV layer. Current results demonstrate
that the thermally cross-linkable FTV enhances not only device
efficiency but also film homogeneity after thermal curing. FTV
is a promising electron- and hole-blocking material applicable
for the fabrication of multilayer PLEDs based on PPV deriva-
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tives. 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym
Chem 000: 000–000, 2012
KEYWORDS: cross-linkable; flourene; luminescence; light-emit-
ting diodes (LED); triphenylamine; vinyl
INTRODUCTION Since the discovery of green-yellow electro-
luminescence (EL) of poly(p-phenylene vinylene) (PPV) by
Holmes et al. in 1990,1 polymeric light-emitting diodes
(PLEDs) continue to attract intensive interests because of
their potential applications in large-area display and solid-
state lighting.2 In general, polymeric materials possess many
advantages over inorganic ones, such as facile fabrication via
spin-coating, roll-to-roll processing and other printing meth-
ods,3,4 and susceptible to structural modification. However,
for conventional electroluminescent {poly[2-methoxy-5-(20-
ethylhexyloxy)-1,4-phenylenevinylene]} MEH-PPV, holes are
more readily injected and transported than electrons, leading
to unbalanced carriers currents in PLEDs.5–9 Maintaining a
balance between electron and hole currents in PLEDs is an
important factor for achieving high device efficiency. Various
methods have been attempted to improve the device effi-
ciency. One is to insert an additional electron injection/trans-
port layer between the emitter and the cathode or an addi-
tional hole injection/transport layer between the emitter and
the anode.10–12 Nevertheless, fabrication of the multilayer
polymer LEDs is usually a difficult task, such as one layer
might be redissolved during subsequent spin-coating of next
layer.
To prevent the drawback of redissolution during subsequent
spin-coating, another method is developed high solvent re-
sistance of polymers to increase their cross-linking density,
which can be attained by photoexcited13–15 or thermal16–19
reactions. Therefore, many research groups prepared ther-
mally cross-linkable polymers to be fabricated multilayer
polymer LEDs in an attempt to balance charge injection/
transport and then improve device performance. For
instance, Jen and coworkers synthesized a series of highly ef-
ficient hole-transporting polymers with triphenylamine
(TPA) or N,N0-bis(4-butylphenyl)-N,N0-diphenyl-1,10-diphenyl-
4,40-diamine (di-Bu-TPD) covalently attached, as side chain,
on the perfluorocyclobutane (PFCB) backbone to achieve an
efficient blue light-emitting PLED (external quantum effi-
ciency: 0.82%).16 Reynolds and coworkers reported a novel
AB2-type TPA to synthesize a hyperbranched polymer with
subsequently polymerizable vinyl groups at the periphery as
hole-transporting materials to fabricate multilayer polymer
LEDs.17(b) Heeger and coworkers designed
a thermally
Additional Supporting Information may be found in the online version of this article.
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2012 Wiley Periodicals, Inc.
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SCHEME 1 Synthesis of monomers 4 and 6.
curable arylamine containing a PFCB structure as a hole-
transporting polymer to enhance the current efficiency from
0.091 to 0.132 cd/A.19(a)
stability but also is readily cross-linked via vinyl groups by
heating to obtain homogeneous film with excellent solvent
resistance. The FTV is inserted via spin-coating and ther-
mally cured between PEDOT:PSS and MEH-PPV to enhance
both maximum luminance and maximum luminance effi-
ciency of MEH-PPV. The results reveal that thermally cross-
linkable fluorene-bridged triple-TPA with terminal vinyl
groups (FTV) is applicable to enhance emission efficiency of
MEH-PPV-based devices.
Among the typical fluorescent materials, fluorene-based
derivatives show good thermal stability,20–22 and the emis-
sion spectrum of fluorene derivatives overlaps partially with
the absorption spectrum of MEH-PPV to facilitate energy
transfer between them. TPA and its derivatives have
attracted considerable interest as hole transport materials
for use in organic EL devices.23 However, the highest occu-
pied molecular orbital (HOMO) level of TPA (ꢁ5.30 eV) is
lower than that of MEH-PPV (ꢁ5.02 eV) to form the hole-
blocking effect, whereas the lowest unoccupied molecular or-
bital (LUMO) level (ꢁ2.12 eV) is higher than that of MEH-
PPV (ꢁ2.70 eV) to induce the electron-blocking effect.24 TPA
derivatives are inserted between PEDOT:PSS and MEH-PPV,
which lead to more balanced charges recombination in the
MEH-PPV emitting layer because of holes are usually more
readily transported than electrons in MEH-PPV.
RESULTS AND DISCUSSION
Characterization of Thermally Cross-linkable
Monomer (FTV)
Scheme 2 illustrates the synthetic routes used in the prepa-
ration of novel thermally cross-linkable monomer FTV. 2,4,7-
Tris[methylene(triphenylphosphonium bromide)]-9,9-dihexyl
fluorene (4) and 4-(phenyl(4-vinylphenyl)amino)benzalde-
hyde (6) were prepared successfully according to the proce-
dures reported previously25 (Scheme 1). Figure S2 (Support-
ing Information) shows the 1H NMR, H-H COSY, 13C NMR,
and DEPT135 spectra of FTV. Assignments of each carbon
and proton were assisted by the NMR spectra, and the struc-
ture of FTV was also confirmed via FT-IR, elemental analysis
(EA), and mass spectrometry, which agree well with the pro-
posed molecular structure of FTV. The chemical shifts at
In this study, to more balanced charges recombination in the
MEH-PPV, we synthesize a thermally cross-linking electron-
and hole-blocking material containing triple TPAs with termi-
nal vinyl groups 4,40400-{[9,9-bis(hexyl)-9H-fluorene-2,4,7-
triyl]tri-2,1-ethenediyl}tris{(N-phenyl-N-(4-vinyl phenyl)}ben-
zeneamine (FTV). The FTV exhibits not only good thermal
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TABLE 1 Thermal Properties of Thermally Cross-linkable FTV
DHc (J/g)
ꢁ56.4
DH160 (J/g)
a
b
d
Compound
Td (ꢀC)
Tm (ꢀC)
FTV
478
91
ꢁ58.1
a
Td, The decomposition temperature at 5 wt % loss was measured by
TGA at a heating rate of 10 ꢀC/min under nitrogen atmosphere.
b
Tm, Melting point determined by DSC measurement.
DH, Enthalpy of cross-linking in the first heating scan a heating rate of
c
10 ꢀC/min.
d
DH160, Enthalpy of isothermal cross-linking at 160 ꢀC within 30 min.
7.87–7.86 ppm (a) and 7.82–7.79 ppm (b) have been
assigned to aromatic protons of fluorene and vinylene pro-
tons between fluorene and TPA moieties, respectively. The
characteristic chemical shifts at 6.73–6.66 ppm (h), 5.69–
5.65 ppm (i), and 5.19–5.16 ppm (j) indicate the existence of
terminal vinyl groups from which thermally cross-linkable
character can be expected (Fig. S2a). The FTV is soluble in
common organic solvents such as toluene, THF, chloroform,
and chlorobenzene.
FIGURE 2 Isothermal DSC traces of FTV at 160 ꢀC under nitro-
gen atmosphere.
nal vinyl groups have been reacted during the first heating
scan. Moreover, the cured FTV shows no obviously detecta-
ble glass transition and melting temperatures up to 220 ꢀC,
indicating that it would be basically an amorphous material.
Highly thermal stability is desirable of cured FTV to prevent
its crystallization during device operation or thermal treat-
ment. As shown in Figure 2, the exothermic thermal cross-
linking of FTV under 160 ꢀC is completed within 30 min.
The joule heat released (58.1 J/g) is very close to that
obtained for the first heat scanning (56.4 J/g). This can be
confirmed by the absence of exothermic peak in the DSC
trace of the thermally cross-linked sample. To ensure com-
plete cross-linking in the following study, the thermal treat-
Thermal Cross-linking and Thermal Resistant Properties
Thermal cross-linking and thermal resistant characteristics
of FTV were studied by differential scanning calorimeter
(DSC) and (thermogravimetric analysis) TGA, respectively.
The thermal decomposition temperature (Td) of FTV was
ꢀ
about 478 ꢀC and its residual weight (at 900 C) was above
40% under nitrogen atmosphere, indicating their highly ther-
mal stability (Fig. S3, Supporting Information; Table 1). Ther-
mal cross-linking reaction characteristics of FTV wereꢀfirst
investigated by two DSC heating scans from 30 to 250 C at
ꢀ
a rate of 10 C/min. In the first sꢀcan, the DSC trace showed
a melting temperature (Tm) at 91 C and an exothermic peak
at 160 ꢀC in which the exotherm started at about 130 ꢀC
(Fig. 1). The joule heat released (56.4 J/g) is due to thermal
cross-linking reaction of the terminal vinyl groups. Neverthe-
less, no apparently detectable exothermic peak is observed
upon the second heating, suggesting that most of the termi-
ꢀ
ment of FTV films was conducted at 160 C for 30 min.
Photophysical and Solvent Resistant Properties
Figure 3 illustrates the absorption and photoluminescence
(PL) spectra of monomer FTV in solution (CHCl3) and as
films spin-coated from CHCl3 solution with the characteristic
optical data summarized in Table 2. The absorption
FIGURE 1 Differential scanning calorimetric curves of FTV at a
heating rate of 10 ꢀC/min under nitrogen atmosphere. The first
run was used to observe the reaction heat of periphery or ter-
minal vinyl groups of FTV.
FIGURE 3 UV-vis absorption and photoluminescence spectra
of FTV in CHCl3 solution (kex ¼ 419 nm) and film state (kex
¼
429 nm).
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TABLE 2 Optical Properties of FTV in CHCl3 Solution and Film
State
Solutiona
PL
Film
UV-vis
UV–vis
PL
a
a
kmax (nm)
kmax (nm)
kmax (nm)
kmax (nm)
FTV
336, 419
477, 501
339, 429
485, 509
a
In chloroform (1 ꢂ 10ꢁ5 M).
maximum peaks of monomer FTV in CHCl3 and as films
appear at about 419 nm (with shoulder at 336 nm) and 429
nm (with a shoulder at 339 nm), respectively. The major
absorption of FTV (ca. 419 nm and 429 nm) can be attrib-
uted to the p–p* transitions of their conjugated flourene
core, whereas the shorter-wavelength absorption (ca. 336
nm and 339 nm) is definitely originated from terminal TPA
moieties. The major PL spectra of FTV in solution and solid
state locate at about 477 nm (with shoulder at 501 nm) and
about 485 nm (with large shoulder at 509 nm), respectively.
Solid state absorption and PL maxima of FTV are slightly
red-shifted (3–10 nm and 8 nm) relative to solution state,
probably due to aggregate formation via intra- or interchain
interactions.
FIGURE 5 FTIR spectra of FTV (–––) and cured-FTV (– – –). The
cured-FTV was obtained by treating at 160 ꢀC for 30 min under
nitrogen atmosphere.
cross-linked FTV film were also investigated. The intensity of
characteristic absorption of terminal vinyl groups (ca. 900
cmꢁ1) diminishes dramatically after thermal cross-linking
(Fig. 5). Therefore, the high solvent resistance of cured FTV
is attributed to its increased cross-linking density after the
thermal curing.
Highly resistant to solvents is necessary for the fabrication
of multilayer PLEDs by solution processes. Solvent resistance
ꢀ
of the thermally cross-linked FTV film at 160 C for 30 min
Electrochemical Investigations
was investigated by the absorption spectra before and after
spin-coating with chlorobenzene which is a good solvent for
the uncured film. As shown in Figure S4 (Supporting Infor-
mation), the absorption spectrum of the pristine FTV film
almost disappears after chlorobenzene spin-coating, implying
that the FTV film is readily dissolved out during the solu-
tion-coating process. However, the absorption spectral inten-
sity of the thermally cross-linked FTV film reduces only
slightly after the spin-coating (Fig. 4). In addition, the FTIR
spectral variations of the pristine FTV film and the thermally
Cyclic voltammetry was used to investigate the electrochemi-
cal properties of pristine FTV and thermally cross-linked
FTV. Pristine- and cured-FTV-coated ITO glasses were used
as the working electrode, immersed in anhydrous acetoni-
trile containing 0.1 M tetra-n-butylammonium perchlorate
(Bu4NClO4) as electrolyte. Their HOMO energy levels were
estimated by the equations: EHOMO (eV) ¼ - (Eox,FOC þ 4.8),
where Eox,FOC is the onset oxidation potentials, respectively,
relative to the ferrocene/ferrocenium couple whose energy
level is already known (ꢁ4.8 eV). Their LUMO energy levels
FIGURE 4 Absorption spectra of cured FTV film (at 160 ꢀC for
30 min under nitrogen atmosphere): before (–––) and after (– –
–) spin-coating with chlorobenzene.
FIGURE 6 Cyclic voltammograms of FTV and cured-FTV (inset)
coated on ITO glass as working electrode (scan rate:
100 mV/s).
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TABLE 3 Electrochemical Properties of FTV
opt
Monomer/
Polymer
Eonset(ox) vs.
FOC(V)a
EHOMO
(eV)b
ELUMO
(eV)c
Eg
(eV)d
FTV
0.40
0.39
ꢁ5.20
ꢁ5.19
ꢁ2.64
ꢁ2.64
2.56
2.55
Cured-FTV
a
EFOC¼ 0.50 V vs. Ag/AgCl.
b
c
EHOMO ¼ ꢁe(Eonset(ox), FOC þ 4.8 V).
ELUMO ¼ ꢁe(EHOMO þ Egopt).
d
opt
Band-gaps estimated from onset absorption (konset): Eg
¼
1240/
konset
.
were calculated by ELUMO ¼ ꢁe(EHOMO þ Egopt), where the
optical band-gap (Egopt) was estimated from onset absorp-
tion. The cyclic voltammograms are shown in Figure 6, with
the representative electrochemical data summarized in Table
3. The estimated HOMO energy levels of pristine and cured
FTV are similar to be ꢁ5.20 and ꢁ5.19 eV, respectively,
meaning that their electrochemical properties are almost not
affected by terminal vinyl groups. The estimated LUMO
energy levels of pristine and cured FTV are ꢁ2.64 eV using
the HOMO levels and the optical band gap (Eg) calculated
from onset absorption. Moreover, the HOMO energy levels of
PEDOT:PSS, cured-FTV, and MEH-PPV are ꢁ5.0 eV, ꢁ5.19 eV,
and ꢁ5.02 eV, respectively, whereas the LUMO energy levels
of cured-FTV and MEH-PPV are ꢁ2.64 eV and ꢁ2.70 eV,
respectively. The lower HOMO level of cured-FTV (ꢁ5.19 eV)
than MEH-PPV (ꢁ5.02 eV) result in blocking of holes,
whereas the higher LUMO level (ꢁ2.64 eV) than MEH-PPV
(ꢁ2.70 eV) induces electron-blocking effect.24 Therefore, the
cured-FTV can be inserted between PEDOT:PSS and MEH-
PPV to simultaneously reduce hole injection from PEDOT:PSS
to MEH-PPV and to block electron transport from MEH-PPV
to anode. Accordingly, this might be an effective strategy to
balance charges recombination in the MEH-PPV emitting
layer whose holes are usually more readily transported than
electrons.
FIGURE 7 Energy band diagrams of PEDOT:PSS, cured-FTV,
MEH-PPV, and cathode.
Figure 8 shows the current density and luminance versus
bias characteristics of the multilayer devices, with the corre-
sponding performance data summarized in Table 4. The
turn-on voltages are slightly increased to 3.6 V from 3.1 V of
the device without cured-FTV layer. This is probably attrib-
utable to hole-blocking effect of the cured-FTV layer that
extra bias is needed to turn on the multilayer device.26
Moreover, maximum luminance and maximum current effi-
ciency are enhanced from 1810 and 0.27 to 4640 cd/m2 and
1.08 cd/A, respectively, by adding cured-FTV layer (film
thickness: 55 6 5 nm) (Fig. 8; Table 4). The device perform-
ance enhancement results from more balanced charges injec-
tion and transport by inserting thermally cross-linked FTV.
However, further increase in cured-FTV layer thickness leads
to degradation in maximum luminance and maximum cur-
rent efficiency; i.e., diminish to 1880 cd/m2 and 0.58 cd/A,
respectively, with cured-FTV (90 6 5 nm). This performance
degradation is probably due to more hole-blocking effect to
result in imbalance of charge injection and transport with
higher film thicknesses of cured-FTV. Therefore, the
Performance Enhancement of MEH-PPV by Thermally
Cross-linkable FTV
According to the energy band diagrams depicted in Figure 7,
the energy barrier between Calcium (Ca) and MEH-PPV
(0.20 eV) is higher than that between PEDOT:PSS and MEH-
PPV (0.02 eV). Moreover, holes are usually more readily
transported than electrons in MEH-PPV. These two effects
will result in reduced recombination ratio of holes and elec-
trons. The imbalance of charges transport in MEH-PPV can
be mitigated by inserting cured-FTV between PEDOT:PSS
and MEH-PPV layers to reduce hole injection from
PEDOT:PSS to MEH-PPV and to block electron transport
from MEH-PPV to anode simultaneously. For this reason,
cured-FTV was inserted between PEDOT:PSS and MEH-PPV
to solve MEH-PPVs common defect of unbalanced charges
transport, and multilayer PLEDs [ITO/PEDOT:PSS/cured-FTV
(or not)/MEH-PPV/Ca (50 nm)/Al (100 nm)] were fabricated
via successive spin-coating processes to investigate their de-
vice performances.
FIGURE 8 Current density versus bias and luminance versus
bias characteristics of PLEDs without or with cured-FTV layers.
Device configuration: ITO/PEDOT:PSS/(cured-FTV)/MEH-PPV/Ca
(50 nm)/Al (100 nm).
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TABLE 4 Optoelectronic Properties of the Light-emitting Diodesa
CIE
Turn-on
voltage (V)b
Lmax
(cd/m2)c
LEmax
(cd/A)d
Emission
(kem, nm)e
Coordinates
(x, y)f
Cured-FTV layer
Non
3.1
3.1
3.2
3.6
1810
2060
4640
1880
0.27
0.34
1.08
0.58
580, 626
580, 626
575, 625
586, 625
(0.57, 0.43)
(0.57, 0.43)
(0.54, 0.46)
(0.59, 0.41)
cured-FTV (25 6 5 nm)
cured-FTV (55 6 5 nm)
cured-FTV (90 6 5 nm)
a
d
e
f
Device structure: ITO/PEDOT:PSS/cured-FTV or not/MEH-PPV/
Maximum luminance efficiency.
Electroluminescent spectra at maximum luminance.
The 1931 CIE coordinate at maximum luminance.
Ca(50nm) /Al(100 nm).
b
Turn-on voltage at about 10 cd/m2.
c
Maximum luminance.
appropriate film thickness of cured-FTV was benefit to
enhance the device performance of MEH-PPV.
about 10ꢁ5 cm2/Vs of conventional MEH-PPV.28 Therefore,
more balanced charges injection and transport in MEH-PPV
is attained by inserting thermally cross-linked FTV between
PEDOT:PSS and MEH-PPV layers.
Current density versus bias curve shifts horizontally to
higher bias after inserting cured-FTV layer (Fig. 8). The hori-
zontal shift suggests that the current density is reduced
under the same operating bias. For instance, at operating
bias of 7 V the current density decreases from 700 to 280
mA/cm2, which is probably due to increased hole blocking
by cured-FTV layer (Fig. 8). The hole-blocking effect reduces
hole injection from PEDOT:PSS to MEH-PPV solving the com-
mon defect of MEH-PPV in which holes are usually more
readily transported than electrons. At the same time, the
cured-FTV slightly blocks electrons to avoid quenching in an-
ode due to its higher LUMO energy levels (ꢁ2.64 eV) than
MEH-PPV (ꢁ2.70 eV) (Fig. 7). Moreover, a hole-only device
[ITO/PEDOT:PSS/cured-FTV (60 nm)/Au (20 nm)/Al (100
nm)] was fabricated to investigate the hole mobility (Fig. S5,
Supporting Information). Estimated hole mobility of the
cured-FTV film (film thickness: 60 nm) was about 4.42 ꢂ
10ꢁ6 cm2/Vs at 11.8 ꢂ 105 V/cm, using the space charge
limited current model.27 This holy mobility is smaller than
In addition, the morphology of pristine and cured-FTV were
also investigated by atomic force microscope (AFM) to inves-
tigate the effect of thermal treatment. As shown in Figure 9,
the surface homogeneity of cured-FTV (RMS roughness: 1.09
nm) is superior to than that of pristine FTV (RMS rough-
ness: 1.94 nm). Homogeneous surface morphology is advan-
tageous to enhance device performance. Figure 10 shows the
electroluminescent emissions of the devices are exclusively
originated from MEH-PPV, with full width at half-maxima
(fwhm) being about 70 nm (with cured-FTV) and 90 nm
(without cured-FTV). Furthermore, the 1931 CIE coordinates
(x, y) of the EL emission shift slightly from (0.57, 0.43) to
(0.54, 0.46) and (0.59, 0.41) after inserting the cured-FTV
layer. Consequently, through fabrication of multilayer device,
solution-processable and thermally cross-linkable FTV is
effective in enhancing device performance of the conven-
tional MEH-PPV.
FIGURE 9 Surface AFM images of FTV and cured-FTV film coated on ITO glasses on tapping modes.
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potassium tert-butoxide (t-BuOK, Lancaster), paraformalde-
hyde (Showa), hydrogen bromide solution (33 wt %) in gla-
cial acetic acid (Acros), triphenylphosphine (Alfa Aesar),
phosphous oxychloride (RDH), sodium hydroxide (J. T.
Baker), N,N-dimethylformamide (DMF, Tedia), n-hexane
(Tedia), toluene (Tedia), benzene (Fisher), chloroform
(Tedia), ethanol (J. T. Baker), dichloromethane (Seedchem),
and other solvents were of commercial sources and used
without further purification. 2,4,7-tris[methylene(triphenyl-
phosphonium bromide)]-9,9-dihexyl fluorene (4) was pre-
pared from 9,9-dihexylfluorene (2) by reacting consecutively
with paraformaldehyde, HBr aqueous solution (33 wt %),
and triphenylphosphine. 4-(Phenyl(4-vinylphenyl)amino)ben-
zaldehyde (6) was synthesized from 4,4’-(phenylazanediyl)di-
benzaldehyde (5) by reacting with (methyl)triphenyl phos-
phonium bromide (Scheme 1).
FIGURE 10 Emission spectra of PLEDs without or with cured-
FTV layers. Device configuration: ITO/PEDOT:PSS/(cured-FTV)/
MEH-PPV/Ca (50 nm)/Al (100 nm). Inset: current efficiency ver-
sus current density characteristics of PLEDs without or with
cured-FTV layers.
Synthesis of Thermally Cross-linkable Monomer (FTV)
A mixture of 2,4,7-tris[methylene(triphenylphosphonium bro-
mide)]-9,9-dihexyl fluorene (4: 1.33 g, 0.95 mmol) and 4-
(phenyl(4-vinylphenyl)amino)benzaldehyde (6: 1.00 g, 3.34
mmol) in 30 mL of N,N-dimethylformamide was stirred at
room temperature for 30 min (Scheme 2). It was added with
sodium ethoxide (3.2 mL, 8.55 mmol) and stirred for addi-
tional 24 h under nitrogen atmosphere. After cooling to
room temperature, the reaction mixture was precipitated
from distilled water. The precipitated solid was collected by
filtration, dried in vacuo, and purified by column chromatog-
raphy (eluent: n-hexane/CH2Cl2 ¼ 3/1). The concentrated
yellow solid was redissolved in CHCl3, precipitated from
ethanol, and dried in vacuo to afford yellow 4,40400-{[9,9-
bis(hexyl)-9H-fluorene-2,4,7-triyl]tri-2,1-ethenediyl}tris{(N-
phenyl-N-(4-vinylphenyl)}benzeneamine (FTV) (43%). FTIR
(KBr pellet, cmꢁ1): m 3028, 2925, 2854 (CAH stretching
vibrations of aliphatic and aromatic), 1593, 1504 (AC¼¼CA),
901 (AC¼¼CH2 out-of-plane). 1H NMR (CDCl3, 500 MHz): d
7.87–7.86 (d, 1H, J ¼ 6.6 Hz, Ar-H), 7.82–7.79 (d, 1H, J ¼
12.8 Hz), 7.64 (s, 1H), 7.56–7.55 (d, 2H, J ¼ 6.9 Hz), 7.53 (s,
1H), 7.49–7.45 (m, 5H), 7.43 (s, 1H), 7.36–7.28 (m, 13H),
7.24–7.05(m, 26H), 6.73–6.66 (m, 3H, vinyl, CH), 5.69–5.65
(d, 3H, J ¼ 17.6 Hz, vinyl, cis-CH), 5.19–5.16 (d, 3H, J ¼ 10.8
Hz, vinyl, trans-CH), 2.07–2.03 (t, 4H, J ¼ 8 Hz, ACH2A),
1.15–1.03 ( m, 12H, ACH2A), 0.78–0.75 ( t, 6H, J ¼ 5.6 Hz,
ACH3),0.71–0.64 (m, 4H, ACH2A) ppm. 13C NMR (CD2Cl2,
400 MHz): d 152.95, 152.70, 147.90, 147.77, 141.85, 138.24,
137.01, 136.75, 134.47, 132.87, 132.54, 131.41, 129.93,
129.91, 128.17, 127.87, 127.67, 127.65, 126.22, 126.00,
125.39, 125.35, 124.55, 124.52, 124.36, 124.32, 123.99,
123.93, 120.84, 119.72, 112.68, 41.43, 32.12, 30.30, 23.18,
14.35 ppm. FAB-MS (m/z): calcd: 1219.67; Found: 1222.00.
Anal. Calcd. (%) for C91H85N3: C, 89.54; H, 7.02; N, 3.44.
Found: C, 89.45; H, 7.09; N, 3.36.
EXPERIMENTAL
Measurements
Newly synthesized compounds were identified by NMR spec-
tra, FT-IR spectra, and EA. 1H NMR and 13C NMR spectra
were obtained on a Bruker AMX-400 MHz spectrometer, with
the chemical shifts reported in ppm using tetramethylsilane
as an internal standard. The FTIR spectra were measured as
KBr disk on a Fourier transform infrared spectrometer,
model 7850 from Jasco. Elemental analysis was carried out
on a Heraus CHN-Rapid elemental analyzer. TGA of polymers
was performed under nitrogen atmosphere at a heating rate
of 15 ꢀC/min using a PerkinElmer TGA-7 thermal analyzer.
Thermal transition properties of polymers were investigated
using a DSC, Mettler Toledo DSC 1 Stare System, under nitro-
gen atmosphere at a heating rate of 10 ꢀC/min. Absorption
and PL spectra were measured with a Jasco V-550 spectro-
photometer and a Hitachi F-4500 fluorescence spectropho-
tometer, respectively. Cyclic voltammograms were recorded
with a voltammetric analyzer (model CV-50W from Bioana-
lytical Systems, Inc.) at room temperature under nitrogen
atmosphere. The measuring cell was made up of a polymer-
coated ITO glass as the working electrode, an Ag/AgCl elec-
trode as the reference electrode, and a platinum wire as the
auxiliary electrode. The electrodes were immersed in aceto-
nitrile 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).29 An AFM,
equipped with a Veeco/Digital Instrument Scanning Probe
Microscope (tapping mode) and a Nanoscope IIIa controller,
was used to examine the morphology and to estimate the
thickness and root-mean-square roughness of deposited
films.
Fabrication of Multilayer Polymer Light-emitting
Diodes via Solution-Process
Multilayer polymer light-emitting diodes (ITO/PEDOT:PSS/
thermal cross-linkable FTV/polymer/Ca/Al) were fabricated
to investigate their optoelectronic characteristics. Transpar-
ent indium tin oxide (ITO) glass was successively cleaned
Materials
Fluorene (Acros), 1-bromohexane (Acros), triphenylamine
(Acros), (methyl)triphenyl phosphonium bromide (Acros),
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SCHEME 2 Synthesis of thermally cross-linkable monomer FTV and cured-FTV.
with neutraler reiniger/deionized water (1/3 ¼ v/v) mix-
ture, deionized water, acetone and 2-propanol, and then
dried in vacuo overnight. Aqueous dispersion of PEDOT:PSS
was spin-coated on top of the cleaned ITO glass as hole-
injection layer and dried at 150 ꢀC for 15 min. Thermal
cross-linkable FTV was then deposited by spin-coating (2000
rpm) onto the PEDOT:PSS layer from a polymer solution in
chlorobenzene (5 mg/mL) and then cross-linked at 160 ꢀC
for 30 min. Emitting layer was then deposited by spin-coat-
ing (2000 rpm) onto the cured FTV layer from a polymer
solution (MEH-PPV) in chlorobenzene (10 mg/mL). Finally,
calcium and aluminum were consecutively vacuum-deposited
as cathode using a vacuum coater at a pressure of 2 ꢂ 10ꢁ6
Torr. Luminance versus voltage and current density versus
voltage characteristics of the devices were measured using a
combination of a Keithley power source (model 2400) and
an Ocean Optics usb2000 fluorescence spectrophotometer.
The optoelectronic measurements were conducted in
glove-box filled with nitrogen.
a
CONCLUSIONS
We have successfully synthesized and characterized a ther-
mally cross-linkable electron- and hole-blocking FTV
8
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€
7 (a) Meyer, H.; Haarer, D.; Naarmann, H.; Horhold, H. H. Phys.
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ꢀ
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a promising electron- and hole-blocking layer for the per-
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