This article appeared in a journal published by Elsevier. The attached
copy is furnished to the author for internal non-co...
Author's personal copy

                                                  Available online at

Author's personal copy

220                                              R.F. Cossiello et al. / Synthetic Metals 158 (20...
Author's personal copy

                                          R.F. Cossiello et al. / Synthetic Metals 158 (2008) 219...
Author's personal copy

222                                             R.F. Cossiello et al. / Synthetic Metals 158 (200...
Author's personal copy

                                               R.F. Cossiello et al. / Synthetic Metals 158 (2008...
Author's personal copy

224                                                 R.F. Cossiello et al. / Synthetic Metals 158 ...
Author's personal copy

                                                  R.F. Cossiello et al. / Synthetic Metals 158 (2...
Upcoming SlideShare
Loading in...5

Cossiello Synthetic Metals Capa


Published on

Cover of Magazine "Synthetic Metals" was published on 4th April 2008

Published in: Technology, Education
  • Be the first to comment

  • Be the first to like this

No Downloads
Total Views
On Slideshare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide

Cossiello Synthetic Metals Capa

  1. 1. This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit:
  2. 2. Author's personal copy Available online at Synthetic Metals 158 (2008) 219–225 Electroluminescence of (styrene-co-acrylic acid) ionomer/conjugated MEH-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 Qu´mica, Caixa Postal 6154, Campinas 13084-971, SP, Brazil ı b Department of Polymer Science and Engineering, Conte Research Center, University of Massachusetts, Amherst, MA 010003, USA c Departamento de Qu´mica, Universidade Federal do Paran´ , Centro Polit´ cnico, Caixa Postal 19081, Curitiba 81531-990, PR, Brazil ı a e Received 10 July 2007; received in revised form 17 December 2007; accepted 11 January 2008 Available 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 copolymer was synthesized with 3 mol% of acrylic acid units to simultaneously enhance blend miscibility and charge transport in MEH-PPV. The morphology was studied using epifluorescence microscopy and scanning electron microscopy in which the pyrenyl-labeled copolymer is used to enhance microscopic contrast. Device performances were compared: those using MEH-PPV have a turn-on voltage of 3.5 V, luminance of 500 cd/m2 and current 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, a luminance 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 ful selection of the blend components and their concentration are important factors in overcoming the limitations of charge Since the discovery of organic electroluminescent devices transport in MEH-PPV [8,15]. (OLED) and in particular with the development of poly- Electroluminescent devices using MEH-PPV blended with meric electroluminescent devices (PLED) efforts have expanded inert matrix polymers such as poly(methylmethacrylate) or toward developing new materials and systems that include new polystyrene have shown enhanced performance compared to polymers, copolymers or mixture of components [1–6]. The devices using pure MEH-PPV [8,16,17]. Possible reasons for use of polymer blends represents a less expensive approach to the better performance include a dilution effect [8–10], con- prepare materials for new polymeric electroluminescent (EL) centration of excitons at the domain interface [18,19] and the devices with improved performance [7–9]. improvement of the charge transport and recombination pro- A polymer often used in electroluminescent devices cesses [20,21] all of which have been proposed to explain the is poly[2-methoxy-5(2 -ethylhexyloxy)-p-phenylenevinylene] enhanced performance. To distinguish between the effect of dilu- (MEH-PPV), a conjugated polymer emitting in the red whose tion and ionic influences on charge injecting, Kim et al. [22] conductivity is dominated by hole transport [8,10–13]. To compared MEH-PPV blended with polystyrene (PS) and with improve device performance, an electron transport material must ionic sulfonated polystyrene (PSS). Polystyrene decreases the be introduced in the device to balance the charge concentration interchain interaction of the conjugated polymer and diminishes [14]. This additional component can be used as an additional exciton quenching while the ionomer may also facilitate elec- layer or blended with the electroluminescent polymer. Care- tron injection [22–24]. In addition to the improvement of the charge mobility of holes or electrons ionomers are also more adherent to the cathode and to the conjugated polymer [25–29]. ∗ Corresponding author. Tel.: +55 19 35214729; fax: +55 19 35214885. However, questions related to the role of the blend morphology E-mail address: (T.D.Z. Atvars). are 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
  3. 3. Author's personal copy 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. 3
  4. 4. Author's personal copy 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 [33]. 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-
  5. 5. Author's personal copy 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, [29]. 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.
  6. 6. Author's personal copy 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 blends 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 [22] 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 Table 1 Spectral characteristics of the PL in blends of MEH-PPV/SAA and of EL of ITO/PEDOT-PSS/MEH-PPV/SAA/Ca/Al devices MEH-PPV Electroluminescence Photoluminescence (wt.%) 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.
  7. 7. Author's personal copy 224 R.F. Cossiello et al. / Synthetic Metals 158 (2008) 219–225 to the onset of significant hole injection, as mentioned above [36]. 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 [38]. It is worth noting that the compositions that provided improvement of the performance are similar to those observed by others [22] 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 [24]. 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.
  8. 8. Author's personal copy R.F. Cossiello et al. / Synthetic Metals 158 (2008) 219–225 225 [2] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, A.B. Holmes, Nature 347 (1990) 539–541. [3] L. Akcelrud, Progr. Polym. Sci. 28 (2003) 875–962. [4] A.M. Machado, J.D.D. Neto, R.F. Cossiello, T.D.Z. Atvars, L. Ding, F.E. Karasz, L. Akcelrud, Polymer 46 (2005) 2452–2460. [5] N.R. Evans, L.S. Devi, C.S.K. Mak, S.E. Watkins, S.I. Pascu, A. Kohler, R.H. Friend, C.K. Williams, A.B. Holmes, J. Am. Chem. Soc. 128 (2006) 6647–6656. [6] C. Huang, C.G. Zhen, S.P. Su, C. Vijila, B. Balakrisnan, M.D.J. Auch, K.P. Loh, Z.K. Chen, Polymer 47 (2006) 1820–1829. [7] G. Yu, H. Nishino, A.J. Heeger, T.A. Chen, R.D. Rieke, Synth. Met. 72 (1995) 249–252. [8] A. Babel, S.A. Jenekhe, Macromolecules 37 (2004) 9835–9840. [9] G.C. Oh, J.J. Yun, S.M. Park, S.H. Son, E.M. Han, H.B. Gu, S.H. Jin, Y.S. Yoon, Mol. Cryst. Liq. Cryst. 405 (2003) 43–51. [10] Q.M. Shi, Y.B. Hou, J. Lu, H. Jin, Y.B. Li, Chin. Phys. Lett. 23 (2006) 950–952. Fig. 9. Normalized intensity of the EL vs. wavelength for the ITO/PEDOT- [11] Y. Zhang, J.B. Peng, Y.Q. Mo, Y. Cao, Appl. Phys. Lett. 85 (2004) PSS/MEH-PPV/SAA/Ca/Al device with 75 wt.% under forward bias at 4, 8 and 5170–5172. 11 V. [12] Q.M. Shi, Y.B. Hou, J. Lu, H. Jin, Y.B. Li, Y. Li, X. Sun, J. Liu, Chem. Phys. Lett. 425 (2006) 353–355. when blends are employed, the resultanting interfaces may also [13] J. Chappell, D.G. Lidzey, P.C. Jukes, A.M. Higgins, R.L. Thompson, S. play an important role, since charge concentration at the domain O’Connor, I. Grizzi, R. Fletcher, J. O’Brien, M. Geoghegan, R.A.L. Jones, Nat. Matter 2 (2003) 616–621. interfaces increases the probability of charge recombination and [14] T.Q. Nguyen, R.C. Kwong, M.E. Thompson, B.J. Schwartz, Appl. Phys. exciton formation is probably also increased [8,35,39]. Lett. 76 (2000) 2454–2456. [15] A. Petrella, M. Tamborra, M.L. Curri, P. Cosma, M. Striccoli, P.D. Cozzoli, 4. Conclusions A. Agostiano, J. Phys. Chem. B 109 (2005) 1554–1562. [16] T.W. Lee, O.O. Park, L.M. Do, T.Y. Zyung, T. Ahn, H.K. Shim, J. Appl. Phys. 90 (2001) 2128–2134. EL devices using blends of MEH-PPV/SAA of several [17] G. He, Y. Li, Appl. Phys. Lett. 80 (2002) 4247–4249. composition ratios were characterized and showed better perfor- [18] S. Sinha, A.P. Monkman, J. Appl. Phys. 97 (2005) 114505–114512. mances for luminescence intensity (500 cd m−2 for MEH-PPV [19] C. Silva, D.M. Russell, A.S. Dhoot, L.M. Herz, C. Daniel, N.C. Greenham, and 2300 cd m−2 for the 50/50 wt.% blend), current den- A.C. Arias, S. Setayesh, K. Mullen, R.H. Friend, J. Phys. Condens. Matter 14 (2002) 9803–9824. sity (350 mA cm−2 for MEH-PPV and 550 mA cm−2 for the [20] M.M. Alam, C.J. Tonzola, S.A. Jenekhe, Macromolecules 36 (2003) 50/50 wt.% blend) as well as and lower turn-on voltage. Epifluo- 6577–6587. rescence microscopy demonstrated phase separation by spinodal [21] N. Rappaport, O. Solomesch, N. Tessler, J. Appl. Phys. 98 (2005) decomposition at the 50/50 wt.% composition ratio with the 33714–33718. presence of interconnected domains. The enhancement of device [22] T.H. Kim, J.H. Park, T.W. Lee, O.O. Park, Polymer 45 (2004) 8567–8571. [23] B.N. Yoon, I.H. Chang, S.H. Choi, D.H. Choi, M.J. Cho, J.H. Kim, Synth. performance can be associated either with the dilution effect Met. 150 (2005) 213–216. that decreases the quenching processes or with the presence of [24] T.W. Lee, O. Park, H.M. Lee, L.M. Do, T. Zyung, Synth. Met. 111/112 interfaces among the domains which could concentrate holes (2000) 225–227. and electrons to facilitate charge recombination or even both [25] M. Esteghamatian, G. Xu, Synth. Met. 75 (1995) 149–152. phenomena. [26] T.P. Nguyen, P. Le Rendu, O. Gaudin, A.J.T. Lee, R.B. Jackman, C.H. Huang, Thin Solid Films 511 (2006) 338–341. [27] D. Poplavskyy, J. Nelson, D.D.C. Bradley, Appl. Phys. Lett. 83 (2003) Acknowledgments 707–709. [28] J.C. Scott, S. Karg, S.A. Carter, J. Appl. Phys. 82 (1997) 1454–1460. TDZA, LA and RFC thank FAPESP, MCT/PADCT/IMMP, [29] N.A. Iyengar, B. Harrison, R.S. Duran, K.S. Schanze, J.R. Reynolds, Macromolecules 36 (2003) 8978–8985. ¸˜ CNPq, CAPES, Fundacao Arauc´ ria and FAEPEX/Unicamp for a [30] G. Yang, Y. Li, J.O. White, H.G. Drickamer, J. Phys. Chem. B 103 (1999) financial support and fellowships. FEK and AC acknowledge 7853–7859. support from AFOSR. The authors acknowledge Prof. Carol [31] M.L. de Andrade, T.D.Z. Atvars, J. Br. Chem. Soc. 17 (2006) Collins for useful discussions. 1000–1003. [32] M.L. de Andrade, T.D.Z. Atvars, J. Phys. Chem. B 108 (2004) 3975– 3984. Appendix A. Supplementary data [33] M.L. de Andrade, T.D.Z. Atvars, Macromolecules 37 (2004) 9096–9108. [34] M.L. de Andrade, T.D.Z. Atvars, Macromolecules 37 (2004) 9626–9630. Supplementary data associated with this article can be found, [35] B. Hu, F.E. Karasz, Chem. Phys. 227 (1998) 263–270. in the online version, at doi:10.1016/j.synthmet.2008.01.011. [36] J.H. Park, H.Y. Yu, J.G. Park, B. Kim, S.H. Lee, L. Olofsson, S.H.M. Persson, Y.W. Park, Thin Solid Films 393 (2001) 129–131. [37] J.S. Kim, P.K.H. Ho, C.E. Murphy, R.H. Friend, Macromolecules 37 (2004) References 2861–2871. [38] C.C. Huang, H.F. Meng, G.K. Ho, C.H. Chen, C.S. Hsu, J.H. Huang, S.F. [1] S. Miyate, H.S. Nalva, Organic Electroluminescent Materials and Devices, Horng, B.X. Chen, L.C. Chen, Appl. Phys. Lett. 84 (2004) 1195–1197. Gordon and Breach, Tokyo, 1998. [39] M. Zhu, T.H. Cui, K. Varahramyan, Microelectr. Eng. 75 (2004) 269–274.