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220 R.F. Cossiello et al. / Synthetic Metals 158 (2008) 219–225
Fig. 1. Chemical structures of (a) poly[2-methoxy-5(2 -ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) and (b) poly(styrene-co-acrylic acid-co-1-pyrenylmethyl
methacrylate) (SAA), x = 96.93, y = 3.01, z = 0.06; characterization obtained from 13 NMR and UV–vis measurements.
In this work we studied the role of the morphology on as internal standard. Absorption spectra were measured on a
device performance in blends of MEH-PPV with the copolymer Hewlett-Packard-8452A UV–vis spectrometer.
poly(styrene-co-acrylic acid) (SAA) containing with 3 mol% Molar percentages of styryl–styryl and styryl–acrylic
of acrylic acid units (Fig. 1) where the acrylic groups behave sequences were determined by integrating the areas of
ionomerically. The electrical properties of the blends with differ- the peaks at 146.6–144.8 ppm (i.e. adjacent styryl units)
ent concentrations of MEH-PPV were correlated with the blend and of 144.7–142.4 ppm (styryl units in an electron-
morphology analyzed by epiﬂuorescence microscopy (EFM). withdrawing microenvironment) [31,32]. The molar content of
Because this ionomer is not intrinsically ﬂuorescent, it was 1-pyrenylmethyl methacrylate (SAA-py) was determined to be
labeled with a very small amount of pyrenyl groups, strongly 0.06% using a calibration curve obtained from absorbance in the
emitting blue ﬂuorescent molecules emitting in resonance with UV–vis spectral range in chloroform solution.
MEH-PPV absorption. Two types of ﬁlms were prepared. For morphological anal-
ysis using epiﬂuorescence microscopy and scanning electron
2. Experimental microscopy (SEM), ﬁlms of MEH-PPV/SAA 25/75, 50/50,
75/25, 90/10 and 100/00 (wt.%) blends (thickness ∼80 m)
2.1. Materials were prepared by casting from 5 mg/mL chloroform solutions.
After slow evaporation under a saturated solvent atmosphere at
Acrylic acid (AA) (Sigma–Aldrich 99.0%) and styrene (S) room temperature for 30 h, they were annealed at 100 ◦ C under
(Sigma–Aldrich, 99.5%) were washed with 5% sodium hydrox- dynamic vacuum in an oven for 12 h to minimize the thermal
ide and distilled water. After, they were dried over anhydrous stress and erase thermal histories. Film thicknesses were ca.
sodium sulfate, vacuum distillated and stored under refrigera- 30–40 m. In addition, ﬁlms were also prepared by spin coating
tion. The ﬂuorescent monomer 1-pyrenylmethyl methacrylate of the same solutions, with spin rate of 1500 rpm. After deposi-
(MMA-py, 97%, polysciences), potassium persulfate (KPS, tion, these ﬁlms were also annealed under the same conditions.
99%, Sigma–Aldrich), sodium dodecyl sulfate (SDS, 98%, Film thickness was around 200 nm.
Merck) and sodium bicarbonate (99%, Synth) were used as
supplied. Chloroform (Merck), dichloromethane (Merck), and 2.2. Methods
methanol (Merck) were of analytical grade. Poly[2-methoxy-
5-(2 -ethylhexyloxy)-p-phenylene-vinylene] (Mn = 86 kg/mol,
¯ Epiﬂuorescence microscopy was performed using an inverted
Sigma–Aldrich) was used as received. Conventional emulsion microscope (Leica DM IRB) employing a mercury arc lamp
copolymerization of the monomers S, AA and MMA- (HBO-100 W) for UV–vis excitation in the wavelength range
py were carried out using previously reported protocols of 330–380 nm selected by optical ﬁlters. The emission image
[29,30]. was selected from the excitation beam by a dichroic mir-
The copolymer was characterized using FTIR, UV–vis and ror (λexc > 410 nm) [31–34]. Objective magniﬁcations of 50×,
NMR (1 H and 13 C) spectroscopies. The FTIR spectra of the 100×, 200× and 500× were used and the images were taken
copolymers were acquired using a Bomem MB-series model with a digital camera (Samsung SDC-311) processed by Linksys
B-100 infrared spectrophotometer by casting thin ﬁlms of the v. 2.38 software. Images were obtained using the epiﬂuorescence
copolymers from chloroform solutions over NaCl windows. All conﬁguration and were observed in terms of a blue-to-green
spectra were recorded at room temperature. Sixty-four scans region due to the ﬂuorescence from the pyrenyl-labeled copoly-
were signal-averaged at a resolution of 2 cm−1 over the spec- mers, and a red-to-yellow region related to the emission from
tral range from 4000 to 600 cm−1 . The high-resolution 1 H and MEH-PPV. Images of thinner and thicker ﬁlms were recorded.
13 C NMR spectra of the copolymers were recorded using a Scanning electron microscopy of the thinner and thicker
Bruker AC300/P, 300-MHz FT-NMR spectrometer operating at (fracture) ﬁlms was obtained using a JSM-6340F microscope
resonance frequencies of 300.15 and 75.048 MHz for 1 H and operating with an accelerating voltage of 20 kV. Samples were
13 C, respectively, using CDCl as solvent and tetramethylsilane coated with a platinum/gold alloy with thicknesses around 4 nm.
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R.F. Cossiello et al. / Synthetic Metals 158 (2008) 219–225 221
Original magniﬁcation was 10,000× and 5000×. Thicker ﬁlms FTIR spectra (see Supporting Information—Fig. 1) exhibit
were cryogenically fractured. several IR-band characteristic of the acrylate segments, such
Steady-state photoluminescence (PL) spectroscopy was per- as: antisymmetric stretching vibrations of the CH3 groups
formed on ﬁlms using an ISS-PC1 spectroﬂuorimeter with a (2985–2994 cm−1 ); symmetric stretching vibrations of the CH3
photon counting detection system and back-face illumination. groups (2952–2862 cm−1 ) overlapped with the stretching vibra-
The samples were excited at 348 and 460 nm and the ﬂuo- tions of the CH2 groups (2845–2852 cm−1 ); C O stretching
rescence emission was collected in the range of 360–690 and vibrations (1730–1720 cm−1 ), bending vibrations of CH3 (1452
480–720 nm, respectively. Excitation spectra were collected and 1371 cm−1 ) and of CH2 groups 1492 cm−1 , rocking vibra-
in the range of 230–360 nm, using λem = 370 nm. Slits were tions of CH2 (757 cm−1 ) and COC stretching (1068 cm−1 ). The
selected for a spectral resolution of ±0.5 nm. IR spectra also show the characteristic absorption bands of the
Because the molar content of the 1-methylpyrenyl methacry- styrene block, such as the phenyl C C stretching vibra-
late moieties was very small, techniques like RMN and FTIR tions (1601 cm−1 ); C–H bending (698 cm−1 ); CH stretching
cannot detect the presence of such groups. Therefore, the molar of the aromatic ring (3026 cm−1 ) and a strong band from the
amount with the pyrenyl groups was determined spectropho- ( CO OH) (3432 cm−1 ).
tometricaly using a calibration curve of standard solutions of The proton 1 H NMR spectra of the copolymer show chemi-
MMA-py in dichloromethane (with molar concentrations rang- cal shifts for: the phenyl groups of the styrene blocks (6.4–7.8
ing from 5 × 10−7 to 5 × 10−5 mol L−1 ) and then measuring and 3.06–3.10 ppm), for methine ( CH ) and methylene groups
the absorbance of polymer samples in dichloromethane solu- ( CH2 ) (1.23–2.8 ppm), for methyl from 0.7 to 1.1 ppm,
tion using the value of molar absorptivity of 0.59 × 105 cm−1 at with deutered chloroform at 7.27 ppm. The 13 C NMR spec-
490 nm. tra show peaks of carbonyl in the range of 175.5–176.6 ppm
(see Supporting Information—Fig. 2), the carbon of the phenyl
2.3. Fabrication of the electroluminescent device ring bonded to the main chain occurs at 144.3–146.5 ppm,
carbons of the styrene groups are in the range from 124.2
The EL properties of the copolymers were investigated to 128.3 ppm, carbon of the methyleneoxy ( OCH2 ) are at
using a double-layer device fabricated in the conﬁgurations 66.1–67.8 ppm, carbon of the methyne of the styryl moieties
ITO/PEDOT-PSS/MEH-PPV/SAA/Ca/Al and ITO/PEDOT- ( –C –H) are in the range of 58.0–59.1 ppm, carbon of the
PSS/MEH-PPV/Ca/Al. For blends MEH-PPV/SAA blends the methylene are at 39.9–47.5 ppm, and carbon of the methyl
compositions studied were 25, 50, 75 and 90 wt.% of MEH-PPV. ( CH3 ) at 11.3–14.7 ppm. The ratio between the styryl–styryl
The ITO substrates were ﬁrst cleaned by sonication in deter- and styryl–acrylic acid can be estimated using the integrated
gent followed by repeated rinsing in deionized water, acetone area observed at 146.6–144.8 (chemical shift of the styryl–styryl
and isopropanol and ﬁnally treated with dilute ozone gas. A hole units) and 144.7–142.4 ppm from the styryl groups in a microen-
injection layer of PEDOT-PSS (Bayer Co.) was spin-coated on vironment with high electronegativity such as the acrylic acid
the ITO/glass substrate (spinning speed 1500 rpm) and baked . Peak of the solvent (deutered chloroform) is at 76.5 and
at 120 ◦ C for 2 h. The ﬁlm thickness was 100 nm. MEH-PPV 77.9 ppm. Peaks from the pyrenyl moieties were not observed
and MEH-PPV blends in solutions (20 mg/mL in chloroform) due to the lower concentrations.
were ﬁltered through 0.2 m Millex-FGS ﬁlters (Millipore) and The molar content of 1-pyrenylmethyl methacrylate (MMA-
than spin-coated onto the PEDOT:PSS layer in an inert nitrogen py) was z = 0.05 wt.% for the SAA copolymer, determined
gas environment. A 5 mg/mL MEH-PPV and MEH-PPV blends spectrophotometrically (see Supporting Information—Fig. 3)
solution in chloroform was used. The polymer ﬁlms were typi- [33,34].
cally 75 nm thick, measured by ellipsometry, using a Si wafer.
Calcium electrodes 400 nm thick were evaporated onto the poly- 3.2. Morphology of MEH-PPV and its blends
mer ﬁlms at about 0.1 Pa pressure through a mask, followed by
deposition of a protective coating of aluminum. The diode area Morphology was analyzed using thinner (200 nm) and thicker
was 6 mm2 . The devices were characterized using a spectral (30–40 m) ﬁlms. For thicker ﬁlms, the EFM micrographs of
measurement system constructed as described elsewhere [4,35]. MEH-PPV/SAA blends show bright spots depending on the
The devices were operated using pulsed voltage for spectral mea- composition (Fig. 2). For the 25% MEH-PPV blend discrete red
surements and dc voltage for current density determinations. The domains of MEH-PPV dispersed in a blue emitting matrix of
devices that have the emission spot occurring closer to the cen- the styrene–acrylic copolymers containing pyrenyl moieties are
ter of the device typically are less bright and less uniform than observed. For blends with higher MEH-PPV compositions the
those with emission near the cathode. morphology is more complex, in which domains of the copoly-
mer are dispersed in a continuous red matrix. However, these
3. Results and discussion domains seem to be interconnected or at least surrounded by
an interconnected phase. Images by SEM of the cryogenically
3.1. Characterization of the copolymer fractured surfaces added evidence of phase interconnection for
blends with compositions of 25% and 50% of MEH-PPV. How-
The copolymer was characterized by three techniques: ever, for compositions of 75% and 90% of MEH-PPV discrete
1H and 13 C NMR, FTIR and UV–vis spectroscopies. The domains with low interface adhesion were observed. There-
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222 R.F. Cossiello et al. / Synthetic Metals 158 (2008) 219–225
Fig. 2. Epiﬂuorescence optical (scale 30 m) and SEM fracture micrographs (magniﬁcation 200×, scale 10 m) of thicker MEH-PPV:SAA ﬁlms (30–40 m) with
compositions of 25, 50, 75 and 90 wt.% of MEH-PPV. Films were deposited on glass by casting of chloroform solutions.
fore, although phase separation is evident, indicating that the from pure SAA, indicating that some color mixture is occurring
mutual solubility of the components is small, the phase separa- in the matrix, probably due to interpenetration of domains of
tion mechanism is complex and some spinodal decomposition is both polymers. Interpenetration is more clearly observed in both
involved. This result is in agreement with a previous report show- SEM and epiﬂuorescence images for blends with 50% of MEH-
ing that MEH-PPV displays some miscibility when blended PPV, where the matrix seems to be richer in the SAA component
with styrene–acrylic copolymers [33,34], although it seems to while diffuse brown–red domains are distributed in the matrix.
be immiscible with poly(methyl methacrylate) and polystyrene Interconnected domains become more visible for thicker ﬁlms,
. It is important to point out that the color of the blue emit- as shown in Fig. 2. For the 75 wt.% of MEH-PPV there is an
ting copolymer is quenched because of the strong absorption of inversion of the phases, the matrix becomes red as shown by
the MEH-PPV in the same region. epiﬂuorescent microscopy, although the color is not equal to that
The morphology of the thinner ﬁlms was analyzed using observed for the pure MEH-PPV. Similar behavior is observed
both epiﬂuorescence and scanning electron microscopy of the for blends with 90% of MEH-PPV where the matrix is more
ﬁlm surfaces. According to the SEM micrographs of MEH- clearly formed by the conjugated polymer while the copolymer
PPV/SAA the ﬁlm of blends revealed thicknesses of about composes the dispersed phase.
200 nm. The colors of the emission observed in the epiﬂuorescence
Images by SEM and by epiﬂuorescence of the thinner ﬁlms micrographs are in agreement with the photoluminescence spec-
of polymer blends with MEH-PPV content from 25% to 90% are tra of the same blends (Fig. 4). The blue emission between 360
in Fig. 3. For the blend with 25 wt.% of MEH-PPV a red colored and 460 nm is characteristic of the domains containing SAA with
disperse phase formed by small domains is distributed in a blue attached emissive pyrenyl moieties and the red emission between
emissive matrix of SAA. However, the blue emission color of 550 and 650 nm is characteristic of the MEH-PPV domains. As
the matrix observed by epiﬂuorescence is distinct of the color we can see, pyrenyl emission is completely quenched above
Fig. 3. Epiﬂuorescence optical (scale 30 m) and SEM micrographs (magniﬁcation 200×, scale 1 m) of thinner ﬁlms (200 nm) with compositions of 25, 50, 75
and 90 wt.% of MEH-PPV. Films were deposited on glass by spin coating of chloroform solutions.
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R.F. Cossiello et al. / Synthetic Metals 158 (2008) 219–225 223
Fig. 4. Normalized photoluminescence spectra (SAA (from 350 to 450 nm),
MEH-PPV (from 500 to 650 nm)) of the same samples of MEH-PPV/SAA
blends as Fig. 2. λexc = 350 nm.
Fig. 5. Normalized EL spectra of ITO/PEDOT-PSS/MEH-PPV/SAA/Ca/Al
devices. Voltage = 5 V.
25% of the MEH-PPV due to overlap with MEH-PPV absorp-
tion. The photoluminescence spectrum of the 25% blend seems a larger distribution of excitons with different energies and
to be sharper and slightly blue-shifted (Table 1). Although we in different microenvironments that recombine excited states
did not perform systematic studies using ﬁlms with different that undergo relaxation, leading to inhomogeneous spectral
thicknesses, this result suggests that inner ﬁlter effects may play broadening.
a role for samples more concentrated in MEH-PPV due to its
high molar absorptivity.
3.3. Electroluminescent properties of MEH-PPV/SAA
The performance of the electroluminescent devices was eval-
uated by several parameters. Firstly we considered the EL
spectral (Fig. 5) changes with blend composition. Although the
spectral proﬁle is almost independent of the composition, there
is a small red-shift of the band position with the increase of the
MEH-PPV concentration (Table 1), which could be attributed
to the inner ﬁlter effect. Nevertheless, since these samples are
thinner than those used in the PL studies of cast ﬁlms we may
also consider the possibility that there is a dilution effect  in
blends with lower MEH-PPV concentration with a consequent
decrease of the more aggregated species.
Fig. 6 compares the EL emission of the devices and PL
emissions of a 50 wt.% blend. There is no signiﬁcant change
of the peak position although a greater spectral broaden-
ing is observed for EL emission. A possible explanation for
this broadening is that while the PL arises principally from
a lower energy interchain exciton, the EL could arise from
Spectral characteristics of the PL in blends of MEH-PPV/SAA and of EL of
MEH-PPV Electroluminescence Photoluminescence
Peak (nm) FWHM (cm−1 ) Peak (nm) FWHM (cm−1 )
25 578 1863 581 1060
50 580 1642 583 1538
75 582 1792 583 1538 Fig. 6. Normalized EL (- - -) (using electric ﬁeld on forward bias excitation)
90 582 1676 584 1538 spectra of (a) ITO/PEDOT-PSS/MEH-PPV/Ca/Al device and (b) ITO/PEDOT-
100 584 1844 585 1096 PSS/MEH-PPV:SAA/Ca/Al devices with a 50/50 wt.% blend. For comparison
we also show the PL spectra (—) (λexc = 350 nm) of the spin-coated ﬁlms.
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224 R.F. Cossiello et al. / Synthetic Metals 158 (2008) 219–225
to the onset of signiﬁcant hole injection, as mentioned above
Comparing the EL behavior with the blend morphology
deﬁned by epiﬂuorescence microscopy, we observe that blends
containing interconnected domains (50 and 75 wt.%) corrobo-
rate the assumption that the interfaces among domains induce
concentration of charge at the interfaces that facilitate charge
recombination [20,37]. This phenomenon arises from the self-
organizing properties of the blends, in which entropy driven
phase separation of the constituent polymers gives rise to
submicron-sized domains having characteristic compositions of
emission. Emission from domains of different composition is
controlled by the ease with which charge is injected, which in
turn depends on the applied voltage .
It is worth noting that the compositions that provided
improvement of the performance are similar to those observed by
others  using MEH-PPV/PSS-ionomer for which the PL and
EL intensities for a 50/50 blend ratio were roughly doubled com-
pared with pure MEH-PPV ﬁlm. These results were explained
by a dilution effect that reduces quenching processes. In addition
to the increase of PL and EL intensities it was observed that the
turn-on voltage and the working voltage decreased signiﬁcantly.
We also compared the EL versus emission intensity
for a ITO/PEDOT-PSS/MEH-PPV/Ca/Al device and several
ITO/PEDOT-PSS/MEH-PPV/SAA/Ca/Al devices (Fig. 8). As
can be seen the best device performance was obtained with
the ITO/PEDOT-PSS/MEH-PPV/SAA/Ca/Al device using a
50/50 wt.%. For comparison, the electroluminescence quantum
Fig. 7. (a) Luminance vs. voltage and (b) current density vs. voltage of the EL
yield when blended with 50 wt.% of SAA increased more than
devices ITO/PEDOT-PSS/MEH-PPV/SAA/Ca/Al at various MEH-PPV concen- four times (from 508 to 2264 cd m−2 ). The current density
trations (wt.%). The turn-on current density is ∼50 mA/cm2 at 3 V. increased almost two times, 541 mA cm2 compared with the
MEH-PPV system of 338 mA cm2 .
Fig. 7 shows the luminance versus voltage curve and the The EL spectrum in Fig. 9 is blue-shifted when the bias
current density versus voltage for the ITO/PEDOT-PSS/MEH- voltage increases. As an example, for a device using 75% of
PPV/SAA/Ca/Al devices. In general, these devices show that MEH-PPV and 25% of SAA the emission peak is observed at
both the current density and luminance increase sharply with 573 nm for 4 V, 575 nm for 8 V and 582 nm for 11 V. One expla-
voltage and have a diode character. The behavior dependents on nation for this behavior is that there is hole accumulation at the
blend composition and the brightest devices were those with 50 anode/polymer interface due to lower mobility, compared with
and 75 wt.% blends, at least for voltages up to 5 V. When the hole mobility located at the cathode interface. In other words,
imposed voltage exceeds 6 V the device with the 75 wt.% blend although the dilution effect may be important for performance
has its brightness reduced due to the lower efﬁciency of charge
injection. At a voltage of 5 V, the luminance of the 50 wt.% blend
is around seven times brighter than pure MEH-PPV. In addition,
the voltage corresponding to the brightest luminance decreased
from 10 V for the MEH-PPV device to 6 V for the 50 wt.% blend
device. Similar results using MEH-PPV blended with sulfonated
polystyrene have been reported and the best performance was
also observed for a device with 50/50 MEH-PPV/PS .
In observing the electrical response of the I–V curve (Fig. 7),
three ranges of voltage can be identiﬁed. At low voltage, space
charge limited current and ninj dominate the injected charge
contribution. Current in this regime is determined by the bulk
properties of the solid rather than contact effects. Increasing
the forward bias ﬁlls the limited number of traps occasioning a
rapid increase in the effective hole mobilities, and, therefore, a
rapid power-law increase in current (J ∝ Vn ). Above −7 V, the Fig. 8. EL vs. wavelength plots for a ITO/PEDOT-PSS/MEH-PPV/Ca/Al device
data deviate from the linear Fowler–Nordheim prediction due and ITO/PEDOT-PSS/MEH-PPV/SAA/Ca/Al devices. Voltage = 5 V.
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