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Synthetic Metals 158 (2008) 219–225

Electroluminescence of (styrene-co-acrylic acid) ionomer/conjugatedMEH-PPV blends

Rafael F. Cossiello a, Ali Cirpan b, Frank E. Karasz b, Leni Akcelrud c, Teresa D.Z. Atvars a,∗a Universidade Estadual de Campinas, Instituto de Quımica, Caixa Postal 6154, Campinas 13084-971, SP, Brazil

b Department of Polymer Science and Engineering, Conte Research Center, University of Massachusetts, Amherst, MA 010003, USAc Departamento de Quımica, Universidade Federal do Parana, Centro Politecnico, Caixa Postal 19081, Curitiba 81531-990, PR, Brazil

Received 10 July 2007; received in revised form 17 December 2007; accepted 11 January 2008Available online 7 March 2008

Abstract

This work reports the electroluminescence of poly[2-methoxy-5(2′-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) and poly(styrene-co-acrylic acid-co-1-pyrenylmethyl methacrylate) (SAA) blends in ratios from 0 to 100 wt.% in mass of MEH-PPV. The styrene-co-acrylic copolymerwas synthesized with 3 mol% of acrylic acid units to simultaneously enhance blend miscibility and charge transport in MEH-PPV. The morphologywas studied using epifluorescence microscopy and scanning electron microscopy in which the pyrenyl-labeled copolymer is used to enhancemicroscopic contrast. Device performances were compared: those using MEH-PPV have a turn-on voltage of 3.5 V, luminance of 500 cd/m2 andcurrent density of 430 mA/cm2 at 5 V, while MEH-PPV blended with 50 wt.% styrene–acrylic copolymer showed a turn-on voltage of 2.5 V, aluminance of 2300 cd/A and a current density of 640 mA/cm2 at 5 V.© 2008 Elsevier B.V. All rights reserved.

Keywords: MEH-PPV; Electroluminescence; Polymer blend

1. Introduction

Since the discovery of organic electroluminescent devices(OLED) and in particular with the development of poly-meric electroluminescent devices (PLED) efforts have expandedtoward developing new materials and systems that include newpolymers, copolymers or mixture of components [1–6]. Theuse of polymer blends represents a less expensive approach toprepare materials for new polymeric electroluminescent (EL)devices with improved performance [7–9].

A polymer often used in electroluminescent devicesis poly[2-methoxy-5(2′-ethylhexyloxy)-p-phenylenevinylene](MEH-PPV), a conjugated polymer emitting in the red whoseconductivity is dominated by hole transport [8,10–13]. Toimprove device performance, an electron transport material mustbe introduced in the device to balance the charge concentration[14]. This additional component can be used as an additionallayer or blended with the electroluminescent polymer. Care-

∗ Corresponding author. Tel.: +55 19 35214729; fax: +55 19 35214885.E-mail address: [email protected] (T.D.Z. Atvars).

ful selection of the blend components and their concentrationare important factors in overcoming the limitations of chargetransport in MEH-PPV [8,15].

Electroluminescent devices using MEH-PPV blended withinert matrix polymers such as poly(methylmethacrylate) orpolystyrene have shown enhanced performance compared todevices using pure MEH-PPV [8,16,17]. Possible reasons forthe better performance include a dilution effect [8–10], con-centration of excitons at the domain interface [18,19] and theimprovement of the charge transport and recombination pro-cesses [20,21] all of which have been proposed to explain theenhanced performance. To distinguish between the effect of dilu-tion and ionic influences on charge injecting, Kim et al. [22]compared MEH-PPV blended with polystyrene (PS) and withionic sulfonated polystyrene (PSS). Polystyrene decreases theinterchain interaction of the conjugated polymer and diminishesexciton quenching while the ionomer may also facilitate elec-tron injection [22–24]. In addition to the improvement of thecharge mobility of holes or electrons ionomers are also moreadherent to the cathode and to the conjugated polymer [25–29].However, questions related to the role of the blend morphologyare far from reaching a consensus.

0379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.synthmet.2008.01.011


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-pyrenylmethylmethacrylate) (SAA), x = 96.93, y = 3.01, z = 0.06; characterization obtained from 13NMR and UV–vis measurements.

In this work we studied the role of the morphology ondevice performance in blends of MEH-PPV with the copolymerpoly(styrene-co-acrylic acid) (SAA) containing with 3 mol%of acrylic acid units (Fig. 1) where the acrylic groups behaveionomerically. The electrical properties of the blends with differ-ent concentrations of MEH-PPV were correlated with the blendmorphology analyzed by epifluorescence microscopy (EFM).Because this ionomer is not intrinsically fluorescent, it waslabeled with a very small amount of pyrenyl groups, stronglyemitting blue fluorescent molecules emitting in resonance withMEH-PPV absorption.

2. Experimental

2.1. Materials

Acrylic acid (AA) (Sigma–Aldrich 99.0%) and styrene (S)(Sigma–Aldrich, 99.5%) were washed with 5% sodium hydrox-ide and distilled water. After, they were dried over anhydroussodium sulfate, vacuum distillated and stored under refrigera-tion. The fluorescent monomer 1-pyrenylmethyl methacrylate(MMA-py, 97%, polysciences), potassium persulfate (KPS,99%, Sigma–Aldrich), sodium dodecyl sulfate (SDS, 98%,Merck) and sodium bicarbonate (99%, Synth) were used assupplied. Chloroform (Merck), dichloromethane (Merck), andmethanol (Merck) were of analytical grade. Poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene-vinylene] (Mn = 86 kg/mol,Sigma–Aldrich) was used as received. Conventional emulsioncopolymerization of the monomers S, AA and MMA-py were carried out using previously reported protocols[29,30].

The copolymer was characterized using FTIR, UV–vis andNMR (1H and 13C) spectroscopies. The FTIR spectra of thecopolymers were acquired using a Bomem MB-series modelB-100 infrared spectrophotometer by casting thin films of thecopolymers from chloroform solutions over NaCl windows. Allspectra were recorded at room temperature. Sixty-four scanswere signal-averaged at a resolution of 2 cm−1 over the spec-tral range from 4000 to 600 cm−1. The high-resolution 1H and13C NMR spectra of the copolymers were recorded using aBruker AC300/P, 300-MHz FT-NMR spectrometer operating atresonance frequencies of 300.15 and 75.048 MHz for 1H and13C, respectively, using CDCl3 as solvent and tetramethylsilane

as internal standard. Absorption spectra were measured on aHewlett-Packard-8452A UV–vis spectrometer.

Molar percentages of styryl–styryl and styryl–acrylicsequences were determined by integrating the areas ofthe peaks at 146.6–144.8 ppm (i.e. adjacent styryl units)and of 144.7–142.4 ppm (styryl units in an electron-withdrawing microenvironment) [31,32]. The molar content of1-pyrenylmethyl methacrylate (SAA-py) was determined to be0.06% using a calibration curve obtained from absorbance in theUV–vis spectral range in chloroform solution.

Two types of films were prepared. For morphological anal-ysis using epifluorescence microscopy and scanning electronmicroscopy (SEM), films of MEH-PPV/SAA 25/75, 50/50,75/25, 90/10 and 100/00 (wt.%) blends (thickness ∼80 �m)were prepared by casting from 5 mg/mL chloroform solutions.After slow evaporation under a saturated solvent atmosphere atroom temperature for 30 h, they were annealed at 100 ◦C underdynamic vacuum in an oven for 12 h to minimize the thermalstress and erase thermal histories. Film thicknesses were ca.30–40 �m. In addition, films were also prepared by spin coatingof the same solutions, with spin rate of 1500 rpm. After deposi-tion, these films were also annealed under the same conditions.Film thickness was around 200 nm.

2.2. Methods

Epifluorescence microscopy was performed using an invertedmicroscope (Leica DM IRB) employing a mercury arc lamp(HBO-100 W) for UV–vis excitation in the wavelength rangeof 330–380 nm selected by optical filters. The emission imagewas selected from the excitation beam by a dichroic mir-ror (λexc > 410 nm) [31–34]. Objective magnifications of 50×,100×, 200× and 500× were used and the images were takenwith a digital camera (Samsung SDC-311) processed by Linksysv. 2.38 software. Images were obtained using the epifluorescenceconfiguration and were observed in terms of a blue-to-greenregion due to the fluorescence from the pyrenyl-labeled copoly-mers, and a red-to-yellow region related to the emission fromMEH-PPV. Images of thinner and thicker films were recorded.

Scanning electron microscopy of the thinner and thicker(fracture) films was obtained using a JSM-6340F microscopeoperating with an accelerating voltage of 20 kV. Samples werecoated with a platinum/gold alloy with thicknesses around 4 nm.


R.F. Cossiello et al. / Synthetic Metals 158 (2008) 219–225 221

Original magnification was 10,000× and 5000×. Thicker filmswere cryogenically fractured.

Steady-state photoluminescence (PL) spectroscopy was per-formed on films using an ISS-PC1 spectrofluorimeter with aphoton counting detection system and back-face illumination.The samples were excited at 348 and 460 nm and the fluo-rescence emission was collected in the range of 360–690 and480–720 nm, respectively. Excitation spectra were collectedin the range of 230–360 nm, using λem = 370 nm. Slits wereselected for a spectral resolution of ±0.5 nm.

Because the molar content of the 1-methylpyrenyl methacry-late moieties was very small, techniques like RMN and FTIRcannot detect the presence of such groups. Therefore, the molaramount with the pyrenyl groups was determined spectropho-tometricaly using a calibration curve of standard solutions ofMMA-py in dichloromethane (with molar concentrations rang-ing from 5 × 10−7 to 5 × 10−5 mol L−1) and then measuringthe absorbance of polymer samples in dichloromethane solu-tion using the value of molar absorptivity of 0.59 × 105 cm−1 at490 nm.

2.3. Fabrication of the electroluminescent device

The EL properties of the copolymers were investigatedusing a double-layer device fabricated in the configurationsITO/PEDOT-PSS/MEH-PPV/SAA/Ca/Al and ITO/PEDOT-PSS/MEH-PPV/Ca/Al. For blends MEH-PPV/SAA blends thecompositions studied were 25, 50, 75 and 90 wt.% of MEH-PPV.

The ITO substrates were first cleaned by sonication in deter-gent followed by repeated rinsing in deionized water, acetoneand isopropanol and finally treated with dilute ozone gas. A holeinjection layer of PEDOT-PSS (Bayer Co.) was spin-coated onthe ITO/glass substrate (spinning speed 1500 rpm) and bakedat 120 ◦C for 2 h. The film thickness was 100 nm. MEH-PPVand MEH-PPV blends in solutions (20 mg/mL in chloroform)were filtered through 0.2 �m Millex-FGS filters (Millipore) andthan spin-coated onto the PEDOT:PSS layer in an inert nitrogengas environment. A 5 mg/mL MEH-PPV and MEH-PPV blendssolution in chloroform was used. The polymer films were typi-cally 75 nm thick, measured by ellipsometry, using a Si wafer.Calcium electrodes 400 nm thick were evaporated onto the poly-mer films at about 0.1 Pa pressure through a mask, followed bydeposition of a protective coating of aluminum. The diode areawas 6 mm2. The devices were characterized using a spectralmeasurement system constructed as described elsewhere [4,35].The devices were operated using pulsed voltage for spectral mea-surements and dc voltage for current density determinations. Thedevices that have the emission spot occurring closer to the cen-ter of the device typically are less bright and less uniform thanthose with emission near the cathode.

3. Results and discussion

3.1. Characterization of the copolymer

The copolymer was characterized by three techniques:1H and 13C NMR, FTIR and UV–vis spectroscopies. The

FTIR spectra (see Supporting Information—Fig. 1) exhibitseveral IR-band characteristic of the acrylate segments, suchas: antisymmetric stretching vibrations of the CH3 groups(2985–2994 cm−1); symmetric stretching vibrations of the CH3groups (2952–2862 cm−1) overlapped with the stretching vibra-tions of the CH2 groups (2845–2852 cm−1); C O stretchingvibrations (1730–1720 cm−1), bending vibrations of CH3 (1452and 1371 cm−1) and of CH2 groups 1492 cm−1, rocking vibra-tions of CH2 (757 cm−1) and COC stretching (1068 cm−1). TheIR spectra also show the characteristic absorption bands of thestyrene block, such as the phenyl C C stretching vibra-tions (1601 cm−1); C–H bending (698 cm−1); CH stretchingof the aromatic ring (3026 cm−1) and a strong band from the( CO OH) (3432 cm−1).

The proton 1H NMR spectra of the copolymer show chemi-cal shifts for: the phenyl groups of the styrene blocks (6.4–7.8and 3.06–3.10 ppm), for methine ( CH ) and methylene groups( CH2 ) (1.23–2.8 ppm), for methyl from 0.7 to 1.1 ppm,with deutered chloroform at 7.27 ppm. The 13C NMR spec-tra show peaks of carbonyl in the range of 175.5–176.6 ppm(see Supporting Information—Fig. 2), the carbon of the phenylring bonded to the main chain occurs at 144.3–146.5 ppm,carbons of the styrene groups are in the range from 124.2to 128.3 ppm, carbon of the methyleneoxy ( OCH2 ) are at66.1–67.8 ppm, carbon of the methyne of the styryl moieties(�–C�–H) are in the range of 58.0–59.1 ppm, carbon of themethylene are at 39.9–47.5 ppm, and carbon of the methyl( CH3 ) at 11.3–14.7 ppm. The ratio between the styryl–styryland styryl–acrylic acid can be estimated using the integratedarea observed at 146.6–144.8 (chemical shift of the styryl–styrylunits) and 144.7–142.4 ppm from the styryl groups in a microen-vironment with high electronegativity such as the acrylic acid[33]. Peak of the solvent (deutered chloroform) is at 76.5 and77.9 ppm. Peaks from the pyrenyl moieties were not observeddue to the lower concentrations.

The molar content of 1-pyrenylmethyl methacrylate (MMA-py) was z = 0.05 wt.% for the SAA copolymer, determinedspectrophotometrically (see Supporting Information—Fig. 3)[33,34].

3.2. Morphology of MEH-PPV and its blends

Morphology was analyzed using thinner (200 nm) and thicker(30–40 �m) films. For thicker films, the EFM micrographs ofMEH-PPV/SAA blends show bright spots depending on thecomposition (Fig. 2). For the 25% MEH-PPV blend discrete reddomains of MEH-PPV dispersed in a blue emitting matrix ofthe styrene–acrylic copolymers containing pyrenyl moieties areobserved. For blends with higher MEH-PPV compositions themorphology is more complex, in which domains of the copoly-mer are dispersed in a continuous red matrix. However, thesedomains seem to be interconnected or at least surrounded byan interconnected phase. Images by SEM of the cryogenicallyfractured surfaces added evidence of phase interconnection forblends with compositions of 25% and 50% of MEH-PPV. How-ever, for compositions of 75% and 90% of MEH-PPV discretedomains with low interface adhesion were observed. There-



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) withcompositions 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 themutual solubility of the components is small, the phase separa-tion mechanism is complex and some spinodal decomposition isinvolved. This result is in agreement with a previous report show-ing that MEH-PPV displays some miscibility when blendedwith styrene–acrylic copolymers [33,34], although it seems tobe immiscible with poly(methyl methacrylate) and polystyrene[29]. It is important to point out that the color of the blue emit-ting copolymer is quenched because of the strong absorption ofthe MEH-PPV in the same region.

The morphology of the thinner films was analyzed usingboth epifluorescence and scanning electron microscopy of thefilm surfaces. According to the SEM micrographs of MEH-PPV/SAA the film of blends revealed thicknesses of about200 nm.

Images by SEM and by epifluorescence of the thinner filmsof polymer blends with MEH-PPV content from 25% to 90% arein Fig. 3. For the blend with 25 wt.% of MEH-PPV a red coloreddisperse phase formed by small domains is distributed in a blueemissive matrix of SAA. However, the blue emission color ofthe matrix observed by epifluorescence is distinct of the color

from pure SAA, indicating that some color mixture is occurringin the matrix, probably due to interpenetration of domains ofboth polymers. Interpenetration is more clearly observed in bothSEM and epifluorescence images for blends with 50% of MEH-PPV, where the matrix seems to be richer in the SAA componentwhile diffuse brown–red domains are distributed in the matrix.Interconnected domains become more visible for thicker films,as shown in Fig. 2. For the 75 wt.% of MEH-PPV there is aninversion of the phases, the matrix becomes red as shown byepifluorescent microscopy, although the color is not equal to thatobserved for the pure MEH-PPV. Similar behavior is observedfor blends with 90% of MEH-PPV where the matrix is moreclearly formed by the conjugated polymer while the copolymercomposes the dispersed phase.

The colors of the emission observed in the epifluorescencemicrographs are in agreement with the photoluminescence spec-tra of the same blends (Fig. 4). The blue emission between 360and 460 nm is characteristic of the domains containing SAA withattached emissive pyrenyl moieties and the red emission between550 and 650 nm is characteristic of the MEH-PPV domains. Aswe 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, 75and 90 wt.% of MEH-PPV. Films were deposited on glass by spin coating of chloroform solutions.



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/SAAblends as Fig. 2. λexc = 350 nm.

25% of the MEH-PPV due to overlap with MEH-PPV absorp-tion. The photoluminescence spectrum of the 25% blend seemsto be sharper and slightly blue-shifted (Table 1). Although wedid not perform systematic studies using films with differentthicknesses, this result suggests that inner filter effects may playa role for samples more concentrated in MEH-PPV due to itshigh molar absorptivity.

3.3. Electroluminescent properties of MEH-PPV/SAAblends

The performance of the electroluminescent devices was eval-uated by several parameters. Firstly we considered the ELspectral (Fig. 5) changes with blend composition. Although thespectral profile is almost independent of the composition, thereis a small red-shift of the band position with the increase of theMEH-PPV concentration (Table 1), which could be attributedto the inner filter effect. Nevertheless, since these samples arethinner than those used in the PL studies of cast films we mayalso consider the possibility that there is a dilution effect [22] inblends with lower MEH-PPV concentration with a consequentdecrease of the more aggregated species.

Fig. 6 compares the EL emission of the devices and PLemissions of a 50 wt.% blend. There is no significant changeof the peak position although a greater spectral broaden-ing is observed for EL emission. A possible explanation forthis broadening is that while the PL arises principally froma lower energy interchain exciton, the EL could arise from

Table 1Spectral characteristics of the PL in blends of MEH-PPV/SAA and of EL ofITO/PEDOT-PSS/MEH-PPV/SAA/Ca/Al devices

MEH-PPV(wt.%)

Electroluminescence Photoluminescence

Peak (nm) FWHM (cm−1) Peak (nm) FWHM (cm−1)

25 578 1863 581 106050 580 1642 583 153875 582 1792 583 153890 582 1676 584 1538

100 584 1844 585 1096

Fig. 5. Normalized EL spectra of ITO/PEDOT-PSS/MEH-PPV/SAA/Ca/Aldevices. Voltage = 5 V.

a larger distribution of excitons with different energies andin different microenvironments that recombine excited statesthat undergo relaxation, leading to inhomogeneous spectralbroadening.

Fig. 6. Normalized EL (- - -) (using electric field on forward bias excitation)spectra of (a) ITO/PEDOT-PSS/MEH-PPV/Ca/Al device and (b) ITO/PEDOT-PSS/MEH-PPV:SAA/Ca/Al devices with a 50/50 wt.% blend. For comparisonwe also show the PL spectra (—) (λexc = 350 nm) of the spin-coated films.



Fig. 7. (a) Luminance vs. voltage and (b) current density vs. voltage of the ELdevices ITO/PEDOT-PSS/MEH-PPV/SAA/Ca/Al at various MEH-PPV concen-trations (wt.%). The turn-on current density is ∼50 mA/cm2 at 3 V.

Fig. 7 shows the luminance versus voltage curve and thecurrent density versus voltage for the ITO/PEDOT-PSS/MEH-PPV/SAA/Ca/Al devices. In general, these devices show thatboth the current density and luminance increase sharply withvoltage and have a diode character. The behavior dependents onblend composition and the brightest devices were those with 50and 75 wt.% blends, at least for voltages up to 5 V. When theimposed voltage exceeds 6 V the device with the 75 wt.% blendhas its brightness reduced due to the lower efficiency of chargeinjection. At a voltage of 5 V, the luminance of the 50 wt.% blendis around seven times brighter than pure MEH-PPV. In addition,the voltage corresponding to the brightest luminance decreasedfrom 10 V for the MEH-PPV device to 6 V for the 50 wt.% blenddevice. Similar results using MEH-PPV blended with sulfonatedpolystyrene have been reported and the best performance wasalso observed for a device with 50/50 MEH-PPV/PS [24].

In observing the electrical response of the I–V curve (Fig. 7),three ranges of voltage can be identified. At low voltage, spacecharge limited current and ninj dominate the injected chargecontribution. Current in this regime is determined by the bulkproperties of the solid rather than contact effects. Increasingthe forward bias fills the limited number of traps occasioning arapid increase in the effective hole mobilities, and, therefore, arapid power-law increase in current (J ∝ Vn). Above −7 V, thedata deviate from the linear Fowler–Nordheim prediction due

to the onset of significant hole injection, as mentioned above[36].

Comparing the EL behavior with the blend morphologydefined by epifluorescence microscopy, we observe that blendscontaining interconnected domains (50 and 75 wt.%) corrobo-rate the assumption that the interfaces among domains induceconcentration of charge at the interfaces that facilitate chargerecombination [20,37]. This phenomenon arises from the self-organizing properties of the blends, in which entropy drivenphase separation of the constituent polymers gives rise tosubmicron-sized domains having characteristic compositions ofemission. Emission from domains of different composition iscontrolled by the ease with which charge is injected, which inturn depends on the applied voltage [38].

It is worth noting that the compositions that providedimprovement of the performance are similar to those observed byothers [22] using MEH-PPV/PSS-ionomer for which the PL andEL intensities for a 50/50 blend ratio were roughly doubled com-pared with pure MEH-PPV film. These results were explainedby a dilution effect that reduces quenching processes. In additionto the increase of PL and EL intensities it was observed that theturn-on voltage and the working voltage decreased significantly.

We also compared the EL versus emission intensityfor a ITO/PEDOT-PSS/MEH-PPV/Ca/Al device and severalITO/PEDOT-PSS/MEH-PPV/SAA/Ca/Al devices (Fig. 8). Ascan be seen the best device performance was obtained withthe ITO/PEDOT-PSS/MEH-PPV/SAA/Ca/Al device using a50/50 wt.%. For comparison, the electroluminescence quantumyield when blended with 50 wt.% of SAA increased more thanfour times (from 508 to 2264 cd m−2). The current densityincreased almost two times, 541 mA cm2 compared with theMEH-PPV system of 338 mA cm2.

The EL spectrum in Fig. 9 is blue-shifted when the biasvoltage increases. As an example, for a device using 75% ofMEH-PPV and 25% of SAA the emission peak is observed at573 nm for 4 V, 575 nm for 8 V and 582 nm for 11 V. One expla-nation for this behavior is that there is hole accumulation at theanode/polymer interface due to lower mobility, compared withhole mobility located at the cathode interface. In other words,although the dilution effect may be important for performance

Fig. 8. EL vs. wavelength plots for a ITO/PEDOT-PSS/MEH-PPV/Ca/Al deviceand ITO/PEDOT-PSS/MEH-PPV/SAA/Ca/Al devices. Voltage = 5 V.



Fig. 9. Normalized intensity of the EL vs. wavelength for the ITO/PEDOT-PSS/MEH-PPV/SAA/Ca/Al device with 75 wt.% under forward bias at 4, 8 and11 V.

when blends are employed, the resultanting interfaces may alsoplay an important role, since charge concentration at the domaininterfaces increases the probability of charge recombination andexciton formation is probably also increased [8,35,39].

4. Conclusions

EL devices using blends of MEH-PPV/SAA of severalcomposition ratios were characterized and showed better perfor-mances for luminescence intensity (500 cd m−2 for MEH-PPVand 2300 cd m−2 for the 50/50 wt.% blend), current den-sity (350 mA cm−2 for MEH-PPV and 550 mA cm−2 for the50/50 wt.% blend) as well as and lower turn-on voltage. Epifluo-rescence microscopy demonstrated phase separation by spinodaldecomposition at the 50/50 wt.% composition ratio with thepresence of interconnected domains. The enhancement of deviceperformance can be associated either with the dilution effectthat decreases the quenching processes or with the presence ofinterfaces among the domains which could concentrate holesand electrons to facilitate charge recombination or even bothphenomena.

Acknowledgments

TDZA, LA and RFC thank FAPESP, MCT/PADCT/IMMP,CNPq, CAPES, Fundacao Araucaria and FAEPEX/Unicamp forfinancial support and fellowships. FEK and AC acknowledgesupport from AFOSR. The authors acknowledge Prof. CarolCollins for useful discussions.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, at doi:10.1016/j.synthmet.2008.01.011.

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Technology

Cossiello Synthetic Metals Capa