MEH-PPV Immiscible Blends by

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Langmuir 2007, 23, 2184-2190

Thin Film Fabrication of PMMA/MEH-PPV Immiscible Blends by Corona Discharge Coating and Its Application to Polymer Light Emitting Diodes Hee Joon Jung,† Youn Jung Park,† Sang Hun Choi,† Jae-Min Hong,‡ June Huh,§ Jun Han Cho,| Jung Hyun Kim,⊥ and Cheolmin Park*,† Department of Materials Science and Engineering, Yonsei UniVersity, Seoul, Korea, Optoelectronic Materials Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea, Department of Materials Science and Engineering, Seoul National UniVersity, Seoul, Korea, Polymer Science and Engineering Department, Dankook UniVersity, Seoul, Korea, and Department of Chemical Engineering, Yonsei UniVersity, Seoul, Korea ReceiVed August 8, 2006. In Final Form: NoVember 3, 2006 We introduce a new and facile process, corona discharge coating (CDC), to fabricate thin polymer films of the immiscible poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) and poly(methyl methacrylate) (PMMA) blends. The method is based on utilizing directional electric flow, known as electric wind, of the charged unipolar particles generated by corona discharge between a metallic needle and a bottom plate under high electric field (5-10 kV/cm). The electric flow rapidly spreads out the polymer solution on the bottom plate and subsequently forms a smooth and flat thin film over a large area within a few seconds. The method is found to be effective for fabricating uniform thin polymer films with areas larger than approximately 30 mm2. The thin films obtained by CDC exhibit unique microstructures where well-defined spherical and cylindrical domains of approximately 50 nm in diameter coexist. These nanosized domains are found to be much smaller than those in films made by conventional spin coating, which suggests that CDC is beneficial for fabricating phase-separated thin film structures with significantly increased interfacial areas. The effects of the applied voltage, tip-to-plate distance, and substrates on the film formation as well as the resulting microstructure are investigated. Furthermore, the light emitting performance of a device prepared by CDC is compared with one made by spin coating.

Introduction The morphological control of immiscible polymer blend thin films has been of great importance because material properties and performance strongly depend on phase geometry and dimension influenced mainly by processing conditions.1-3 In most practical cases, the morphology of immiscible polymer blends is determined not solely by its thermodynamic conditions, but also from combination with the nonequilibrium nature of the processing employed for fabrication.4-7 In solvent-based processes, the most representative process being the spin coating used in fabricating thin polymer films for various organic electronic devices,9-12 a polymer blend starts out as a single * To whom correspondence should be addressed. Tel.: +82-2-21232833. Fax: +82-2-312-5375. E-mail: [email protected]. † Department of Materials Science and Engineering, Yonsei University. ‡ Korea Institute of Science and Technology. § Seoul National University. | Dankook University. ⊥ Department of Chemical Engineering, Yonsei University. (1) Morteani, A. C.; Choot, A. C.; Kim, J. S.; Silvia, C.; Greenham, N. C.; Friends, R. H.; Murphy, C.; Moons, E.; Cina, S.; Burroughes, J. AdV. Mater. 2003, 15, 1708-1712. (2) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789-1791. (3) Wang, H.; Composto, R. J.; Hobbie, E. K.; Han, C. C. Langmuir 2001, 17, 2857-2860. (4) Tanaka, K.; Takahara, A.; Kajiyama, T. Macromolecules 1996, 29, 32323239. (5) Kugler, T.; Lo¨gdlund, M.; Salaneck, W. R. IEEE J. Sel. Top. Quantum Electron. 1998, 4, 14-23. (6) Wang, P.; Koberstein, J. T. Macromolecules 2004, 37, 5671-5681. (7) Ton-that, C.; Shard, A. G.; Daley, R.; Bradley, R. H. Macromolecules 2000, 33, 8453-8459. (8) Hall, D. B.; Underhill, P.; Torkelson, J. M. Polym. Eng. Sci. 1998, 38, 2039-2045. (9) Lawrence, C. J. Phys. Fluids 1998, 31, 2786-2795.

phase solution, undergoes phase separation transitions during processing, and finally forms macroscopically phase separated structures when solvent removal leads to vitrification.13 A strong centrifugal field on a polymer solution induced by fast rotation of a substrate produces a highly uniform polymer thin film in which the microstructure caused by the phase separation of two components is evolved kinetically in nonequilibrium nature.13,14 Other well-known film preparation methods include dip coating, Langmuir-Blodgett, and slip casting. In principle, various physical fields such as mechanical, electrical, and magnetic can be applied for thin polymer blend film preparation for the purposes of certain industrial applications.15,16 In all cases, the film thickness can be orders of magnitude smaller than the typical size of phaseseparated domains observed in bulk phase separation, and thus the effects of two bounding surfaces must be taken into account: the surface between the polymer solution and air and one between the polymer solution and the substrate. The nonequilibrium morphologies frozen during the fast thin film formation of immiscible polymer blends, dependent upon a complex competition of a number of physical phenomena, are tunable for a specific application through proper control of the processing variables. For example, it has been of great interest to understand the factors affecting the development of phaseseparated structures from light emitting polymer blend solutions (10) Ton-That, C.; Shard, A. G.; Bradley, R. H. Polymer 2002, 43, 49734977. (11) Luo, S. C.; Craciun, V.; Douglas, E. P. Langmuir 2005, 21, 2881-2866. (12) Walheim, S.; Bo¨ltau, M.; Mlynek, J.; Krausch, G.; Steiner, U. Macromolecules 1997, 30, 4995-5003. (13) Wang, H.; Composto, R. J. Macromolecules 2002, 35, 2799-2809. (14) Li, X.; Han, Y.; An, L. Langmuir 2002, 18, 5293-5298. (15) Binder, K. J. Non-Equilib. Thermodyn. 1998, 23, 1-4. (16) Bates, F. S.; Wiltzius, P. J. Chem. Phys. 1989, 91, 3258-3274.

10.1021/la062341h CCC: $37.00 © 2007 American Chemical Society Published on Web 12/29/2006

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and to control the performance of a polymer diode relying on the phase separation.17-23 In particular, nano-size light emitting diodes with emitting domains on the order of 300-500 nm can be fabricated using light emitting polymer/poly(methyl methacrylate) (PMMA) blends.22,23 Such a small light emitting domain is used as a light source of scanning near-field optical microscopes.22 Efficient exciton formation in conjugate polymer and the following emission at the phase interface with inert PMMA are known to enhance the device performance with the microphase separated domains embedded in a light emitting polymer matrix as small as possible.17 Corona discharge, as a form of electrical breakdown in gases, occurs when the voltage difference applied between the sharp metal needle and the flat surface exceeds a threshold value such that the electric field strength in the vicinity of the needle becomes sufficiently large to ionize the gas molecules.24-30 Because the strength of the electric field decreases with distance from the coronating needle, the ionization zone usually remains localized around the needle surface. In the case of a positive corona, negative ions are drawn toward the needle, while positive ions move outside and drift along the electric field lines toward the flat bottom electrode. The outside of an ionization region is called a drift zone, where ions of one sign are dominant.26 A unipolar charge current is established as ions in the drift zone are set in motion in response to the electric field. Collisions between drifting ions and electrically neutral air molecules give rise to momentum transfer that leads to the electrohydrodynamic flow known as “corona wind”.26 The corona wind is physically driven by the Coulomb force from unipolar charge and the electric field, and its direction follows the direction of ion drift along the electric field lines radiating from the needle toward the bottom electrode. This physical force due to corona discharge has been applied for a variety of industrial applications such as contaminant removal, induced dipoles of liquid crystals and ferroelectric polymers, and surface treatment.29-31 We envision that the incompressible flow of electrically charged particles is strong and uniform enough to rapidly spread out the polymer solution and form thin films with fast evaporation of solvent, and thus it can be utilized as a driving force for fabricating polymer thin films. Furthermore, it is also interesting to investigate how the electric charge flow induced by corona discharge influences the phase-separated microstructure of immiscible polymer blends containing conducting or semiconducting polymers as one of the blend components which are active materials in many organic devices. (17) Iyengar, N. A.; Harrison, B.; Duran, R. S.; Schanze, K. S.; Reynolds, J. R. Macromolecules 2003, 36, 8978-8985. (18) Kim, J. S.; Ho, P. K. H.; Murphy, C. E.; Friend, R. H. Macromolecules 2004, 37, 2861-2871. (19) Ananthakrishnan, N.; Padmanaban, G.; Ramakrishnan, S.; Reynolds, J. R. Macromolecules 2005, 38, 7660-7669. (20) Moons, E. J. Phys.: Condens. Matter 2002, 14, 12235-12260. (21) Qin, D. S.; Li, D. C.; Wang, Y.; Zhang, J. D.; Xie, A. Y.; Wang, G.; Wang, L. X.; Yan, D. H. Appl. Phys. Lett. 2001, 78, 437-439. (22) Granstro¨m, M.; Ingana¨s, O. AdV. Mater. 1995, 7, 1012-1015. (23) Granstro¨m, M.; Berggren, M.; Ingana¨s, O. Science 1995, 267, 14791481. (24) Cobine, J. D. Gaseous Conductors; Dover: New York, 1958. (25) Loeb, L. B. Electrical Corona; University of California Press: Berkeley, 1965. (26) Nygaard, K. J. ReV. Sci. Instrum. 1965, 36, 1320-1323. (27) Sugimoto, Y.; Higashiyama, Y.; Asano, K. Electr. Eng. Jpn. 1998, 122, 1-7. (28) Boeuf, J. P.; Pitchford, L. C. J. Appl. Phys. 2005, 97, 103307. (29) Giacometti, J. A.; Fedosov, S.; Costa, M. M. Braz. J. Phys. 1999, 29, 269-279. (30) Sano, N.; Yamamoto, D. Ind. Eng. Chem. Res. 2005, 44, 2982-2989. (31) Dao, P. T.; Williams, D.; McKenna, W. P.; Goppert-Berarducci, K. J. Appl. Phys. 1993, 73, 2043-2049.

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Here we present a new, robust, and fast method, corona discharge coating (CDC) in fabricating thin films of immiscible polymer blends of a light emitting conjugated polymer, poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylenevinylene] (MEHPPV), and poly(methyl methacrylate) (PMMA) using the incompressible flow of the electrically charged gas particles drifting from the sharp metallic needle to the bottom electrode under corona discharge. The polymer solution placed on the bottom electrode is instantly spread out, being transformed into a thin film within a few seconds at normal atmosphere. The uniform film formation depends on various factors including the voltage applied, the needle to bottom surface distance, and the substrates and solvents used. We also examine how the phaseseparated morphology of the immiscible MEH-PPV/PMMA blends is affected by the electric flow. CDC produces a unique phase-separated morphology in which the microdomains of the minor PMMA component are approximately 50 nm on average in diameter, much smaller than the ones observed in typical spin coating (∼300 nm). The origin of the smaller microdomains is hypothesized as being the strong interaction between the charged unipolar particles and the polar PMMA polymer. The light emitting performance of a device containing MEH-PPV/PMMA blends fabricated by CDC is briefly mentioned, compared with one by spin coating. Experimental Section Materials. Poly(methyl methacrylate) (PMMA) (MW ) 47 600 g/mol) and poly[2-methoxy-5-(2′-ethylhexyloxy)phenylenevinylene] (MEH-PPV) (MW ) 40 000 g/mol) were purchased from Aldrich and used without further purification. Polymer blend solutions in toluene at a concentration of 0.1 wt % were prepared by mixing each polymer with the weight ratios of 1:1, 1:3, and 3:1 of MEH-PPV and PMMA. Poly(styrene-block-4-vinylpyridine) (PS-b-P4VP), purchased from Polymer Source Inc. (Dorval, Canada), has 47 600 g mol-1 PS block and 20 900 g mol-1 P4VP block (polydispersity index ) 1.14). PS-b-P4VP was dissolved in toluene, a solvent in which PS block is well solvated and P4VP core and PS corona micelles are readily formed. The spin coating (spin coater SPIN 1200 Midas-system, Korea) of MEH-PPV/PMMA blends and PSb-P4VP was carried out at 2000 rpm for 1 min at room temperature. Corona Discharge Coating Process. Figure 1a illustrates CDC apparatus we manufactured. The apparatus involves two asymmetric electrodes, a needle-type anode made of tungsten and a flat copper plate cathode. The copper plate can also be a heater that can control the temperature with a maximum of 250 °C. The distance can be varied from 0.1 to 2 cm between these two electrodes using a screwtype manipulator. A direct current (dc) voltage (0.5-10 kV, MHP 10-10A, Wookyungtech, Korea) of positive polarity was applied on the tip of the needle anode and the cathode for corona discharge generation. Film formation by CDC starts with dropping the polymer solution using a micropipette onto the substrate positioned in the middle of the copper plate. The next step is to gradually ramp up the voltage to a target voltage to form a directional flow of discharged particles over the polymer solution. A heater can be switched on to evaporate the solvent during the film formation, but it is usually not used. A polymer thin film is formed within a few seconds with rapid evaporation of solvent. Microstructure Characterization. The microstructure of thin blend films by CDC was characterized by use of a tapping mode atomic force microscope (AFM; Nanoscope IIIa Dimension 3100) with height and phase contrast and a transmission electron microscope (TEM; Phillips X60, 50 kV). TEM samples were prepared by depositing thin films onto carbon-coated TEM grids by taking floating films out of the water. Films were floated on the water from small sections of carbon and polymer films detached from the substrate by employing a poly(acrylic acid) (PAA; Aldrich) solution. Device Fabrication. Polymer light emitting devices (PLEDs) of MEH-PPV/PMMA blends were fabricated as follows: Indium tin

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Figure 1. (a) Corona discharge coating (CDC) apparatus. Two different kinds of corona discharge generated in the apparatus: (b) glow and (c) spark. oxide (ITO) glass (Rs ) 15 Ω/square) was patterned using a gas mixture of hydrochloric acid (HCl) and nitric acid (HNO3) with a volume ratio of 1:1, and ITO stripes of 3 mm size were obtained. The patterned ITO substrate was cleaned in an ultrasonic bath with a detergent, acetone, and isopropyl alcohol and rinsed in deionized water. This was followed by spin coating with poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) and drying at 130 °C for 5 min. After the film formation of MEH-PPV/PMMA blend by CDC, thermal annealing was done at 60 °C for 5 min. Aluminum (thickness 100 nm) was deposited by a thermal evaporator with a deposition rate of 0.5 Å/s under a pressure of 5 × 10-6 Torr. The current-voltage and the luminescence-voltage characteristics were measured using a Keithley 2400 digital voltage meter and a photodiode and a Keithley 487 picoampere meter, respectively.

Results and Discussion A positive corona discharge was successfully generated in our apparatus when high voltage between a sharp needle anode and plate-type bottom cathode was applied at the minimum value of approximately 4 kV. Positively ionized gas molecules, repulsively interacting with the anode, drifted toward the bottom plate electrode with the direction of the electric field. The subsequent momentum transfer of the ionized gases to neutral surrounding gases causes the so-called electric wind, the directional flow of the ionized gas molecules which can be utilized as a driving force for fabricating thin polymer films. There are two different types of electric wind, glow and spark, which depend on the voltage applied as clearly visualized in the photographic images of Figure 1b and Figure 1c, respectively. The spark-type corona discharge frequently occurs under high voltage, resulting in

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Figure 2. Proposed mechanism of thin film formation by CDC. A positive corona is initiated by an exogenous ionization event in a region of high potential gradient. The electrons resulting from the ionization are attracted toward the needle, and the positive ions are repelled from it. A directional flow of positively charged particles was formed over the substrate as shown in (b). (c) The flow of particles above the droplet induces the deformation of solution. (d) After the deformation of solution, the droplets spread onto the substrate and solvent evaporation accelerates the film formation.

continuous electrical breakdown across the two electrodes. In contrast, the glow-type discharge displays a smooth drifting of the ionized gases toward the bottom surface. Thin polymer films were rapidly formed within a few seconds when a polymer solution was placed on a substrate underneath the electric wind. The polymer solution droplet, mechanically bombarded by the positively charged gas molecules, spread out in a radial direction, and subsequently a thin film was fabricated as the solvent was evaporated with a certain rate controllable via the temperature of the substrate. The effect of the substrate temperature on film preparation was marginal, and for most cases the film preparation was performed at room temperature. Thin polymer film formation is schematically illustrated in Figure 2. We systematically examined the formation of thin blend films of PMMA/MEH-PPV (1/3) as a function of both the magnitude of applied voltage and the tip-to-substrate distance. The film formation strongly depends on both parameters, as shown in Figure 3. No thin films were observed at the voltage of approximately 4 kV, below which proper corona discharge was not formed. A nonuniform film of approximately 500 nm in thickness was prepared mainly by quiescent solvent evaporation. The spark-type discharge observed at high voltage regions failed to obtain thin uniform film because highly localized electric gas flow at the center of a film significantly deteriorated and sometimes destroyed the film. The regions of the spark formation varied with the tip-to-substrate distance as shown in Figure 3. Below a certain voltage at a fixed distance, corona discharge

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Figure 3. Thin film formation capability of CDC as functions of the applied voltage and the tip-to-substrate distance. The criteria for a good film formation are uniform thickness, large coverage on a plate, and flatness of film.

turned into a glow-type mode in which highly uniform thin blend films were obtained over large areas (30 mm2). The force per unit volume (f) generated by the electric wind of glow-type corona discharge is simply expressed by f (N/m3) ) J/µ, where J and µ denote the corona current density (A/m2) and ion mobility (m2/V‚s), respectively.26 One can assume that µ is independent of the electric field and that J is proportional to V(V - Vs), where V is the voltage across the gap and Vs is the voltage that is required to start the discharge. In addition, the wind velocity (V) is characterized by V (m/s) ) E‚µ, where E is the electric field that decays in a radial direction from the center. The force per unit volume exerted on a polymer solution in our system is on the order of 104 N/m3.28 Compared to spin coating, our method has more serious film edge thickening due to the unique film formation force, i.e., the radial decaying driving electric force from the center. The film becomes abruptly much thicker in the regions outside the uniform area. Wafer-scale, large area film formation requires very a large voltage with sufficient gap distance in our method which is rarely accessible in a normal power supply and consequently makes our method seem less efficient for industrial applications. The method, however, is found to be very adequate for thin films having an area larger than approximately 30 mm2 with relatively low cost. In particular, the film preparation on a selective area with needle arrays seems to be plausible since arrays of individually addressable metallic needles are fabricated with ease. We are currently designing a multiple-needle system for a feasibility test. We investigated the microstructure and film roughness of the PMMA/MEH-PPV blend (1/3) prepared by CDC with TEM and tapping mode AFM. The bright field TEM image in Figure 4a clearly shows a microphase separated blend structure with both spherical and short cylindrical PMMA microdomains of approximately 50 nm in diameter. The size distribution of the microdomains is very narrow, as shown in the inset of Figure 4a. The root-mean-square film roughness measured from an AFM image in height contrast (Figure 4b) is approximately 6 nm. The selective dissolution of PMMA domains with acetone allowed us to identify the PMMA phase more clearly. Figure 4c displays an AFM image of the PMMA/MEH-PPV blend (1/3) film prepared by spin coating. The spin coating of the 1 wt % blend solution with a speed of 2000 rpm allowed us to prepare a thin film with a thickness of approximately 50 nm, which is close to that prepared by CDC. The averaged diameter of phase-separated PMMA domains in the spin-coated blend film is approximately 300 nm, 5 times larger than the ones prepared by CDC. The

Figure 4. Bright field TEM (a) and tapping mode AFM (b) images of the microstructure of thin blend films of PMMA/MEH-PPV (1/3) by CDC. (c) AFM image of thin blend film of PMMA/MEH-PPV (1/3) by spin coating. The size distributions of microdomains in thin film by CDC and spin coating are shown in the insets of (a) and (c), respectively.

microdomains have much a broader size distribution, ranging from 50 to 500 nm as shown in the inset of Figure 4c. Corona discharge is highly effective to fabricate nanometer-scale polymer domains with a narrow size distribution in the PMMA/MEHPPV blend thin film. Interestingly, the cylindrical PMMA microdomains evidenced in CDC (Figure 4a,b) were not observed in the thin film prepared

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Figure 5. Bright field TEM images of thin film of PS-b-P4VP micelle solution prepared by (a) spin coating and (b) CDC.

by spin coating. The PMMA cylindrical microdomains of several hundred nanometers in length are frequently found in CDC film, which is possibly due to a directional perturbing field by corona discharge that makes PMMA microdomains interact anisotropically with other domains. To gain further understanding of the origin of the cylindrical PMMA microdomains, we employed a PS-b-P4VP block copolymer micelle solution in which nearly monodisperse micelles had been already self-assembled and randomly dispersed with a polar P4VP core and nonpolar PS corona.32 The subsequent thin film formation of these colloidal micelles using either corona discharge or spin coating allows us to investigate the effect of either pure mechanical centrifugal force from spin coating or electrohydrodynamic force from CDC on nanostructure formation. The micelle solution of PS-b-P4VP copolymer with a concentration of 0.5 wt % was spin-coated on a silicon substrate. Closely packed PS-b-P4VP micelles of approximately 40 nm in diameter are clearly visible in a thin film prepared by spin coating (Figure 5a). We always observed only spherical micelle structure when varying the spin speed ranging from 500 to 5000 rpm to examine the effect of the centrifugal force on nanostructure formation. In contrast, CDC gives rise to the cylindrical micelles (32) Shin, H.-I.; Min B. G.; Jeong, W.; Park, C. Macromol. Rapid Commun. 2005, 26, 1451-1457.

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similar to the ones found in the PMMA/MEH-PPV blend as shown in Figure 5b, which implies that the initially spherical micelles were fused with each other to form cylindrical micelles under corona discharge. Considering that the cylindrical micelle formation is independent of mechanical centrifugal force under our experimental conditions, we attribute the cylinder formation to a directional-specific interaction induced by massive charged particles falling on the micelles during CDC. PMMA is one of the well-known electret polymers that can be easily polarized under an electric field due to its unique molecular structure with a large dipole moment, which is capable of accumulating high surface charges.33 The positively charged gas molecules traveling through the PMMA/MEH-PPV solution droplet during the corona discharge interact with PMMA chains preferentially when the solution reaches a critical concentration of phase separation. The charges selectively trapped on PMMA molecules can play the role of nucleating agent from which PMMA domains grow. A large number of the charged PMMA nuclei created instantly in the blend solution become mature PMMA domains of approximately 50 nm in diameter, which are much smaller than the ones prepared by spin coating. Additionally, the rapid evaporation of the solvent during the process does not provide sufficient time for domain growth, resulting in PMMA microdomains with relatively a narrow size distribution, as shown in Figure 4a. It should be noted that the nanometer-scale PMMA domain by CDC is kinetically driven and may still contain some amount of MEH-PPV due to the fast phase separation. The individual PMMA microdomains formed with charged character are easily produced under the corona discharge due to the strongly polar nature of PMMA and therefore connect with each other due to electrostatic or permanent dipole interaction, leading to the formation of the short cylindrical microdomains observed in Figure 4b. In the case of PS-b-P4VP block copolymer, the nucleation of P4VP microdomains with the charged particles did not occur because the P4VP core in the preformed micelles is shielded by outer corona PS blocks from intrusion of charged particles into the core. However, the selective interaction of polar P4VP blocks, similar to the case of PMMA, with positively charged molecules staying initially outside the P4VP core enabled the spherical micelles to interact with each other as the charged molecules gradually intrude into the P4VP core of micelle, ultimately leading to cylindrical micelles, as shown in Figure 5b. We have also observed similar spherical and cylindrical nanostructures in other immiscible blend films, including PS/ PMMA by CDC. We investigated the microstructures of the PMMA/MEH-PPV blends with different compositions prepared by CDC. Three different thin films of PMMA/MEH-PPV blends with the compositions of 1:3, 1:1, and 3:1 were prepared at the applied voltage of 5 kV with the tip-to-substrate distance of 2 cm. Similar to the case shown in Figure 4, closely packed short cylindrical PMMA microdomains were observed in the blend film with the composition of 1:3 (Figure 6a). Interestingly, a bicontinuous microstructure that has been generally observed in PMMA/MEHPPV blends with the blend ratio of 1:117 was not formed in our system (Figure 6b). The microstructures of the blends with different ratios prepared by spin coating were confirmed to be similar to those obtained by Iyengar et al.17 The phase inversion in the spin-coated film also occurred near 50% PMMA. Instead of phase inversion, spherical PMMA domains of approximately 200 nm in diameter appear dominantly over the entire film surface, indirectly supporting our argument that the PMMA phase is selectively nucleated by the charged particles. When we increased (33) Mazur, K. J. Phys. D: Appl. Phys. 1997, 30, 1383-1398.

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Figure 7. AFM images of thin film of PMMA/MEH-PPV (1/3) blend by CDC on various substrates including (a) SiO2 and (b) ITO as shown in height images, and (c) mica and (d) Au in phase images.

Figure 6. Bright field TEM images of thin films by CDC with different blend compositions between MEH-PPV and PMMA corresponding to (a) 3:1, (b) 1:1, and (c) 1:3.

the amount of PMMA to 75%, bicontinuous morphology was observed as shown in Figure 6c, which indicates that the threshold composition of the phase inversion increases in the CDC film far more than that in the spin-coated film. We also prepared CDC films on various substrates in order to investigate the effect of bounding surface on the phase-separated structure. Figure 7 shows the microstructures of thin PMMA/ MEH-PPV 1/3 blend films prepared by CDC on various substrates

including Au, ITO, SiO2, and mica. The films formed on SiO2 and mica surfaces (Figure 7a and Figure 7c, respectively) exhibit well-defined spherical or short cylindrical PMMA microdomains with the diameters of approximately 50 nm, while those on ITO and Au surfaces (Figure 7b and Figure 7d, respectively) display completely different microstructures in which the phase boundary is unclear with large surface roughness. We speculate that the poor film formation with the different microstructures on the conductive ITO and Au substrates is ascribed to the relatively small amount of charged particles possessed by PMMA domains. The positively charged particles drifted by the electric field may pass directly through the blend solution and go into the conductive substrate rapidly without sufficient time to accumulate in the solution. Consequently, the lack of the charged particles in the solution hinders the formation of stable PMMA nuclei from which well-defined PMMA domains grow. On the other hand, the blend solutions on an insulating substrate such as SiO2 and mica may have relatively larger amounts of charged particles during the film formation, leading to the nanodomains we observed. We also measured the light emitting properties of a device prepared by CDC of a PMMA/MEH-PPV (1/3) thin film of approximately 1 µm in thickness on a spin-cast PEDOT/PSS layer. Figure 8a shows typical diode behavior with current and power outlet observed only in the forward bias. The current begins to increase at the voltage of approximately 14 V, which corresponds to an onset voltage of exciton recombination at the corresponding voltage as confirmed by the beginning of luminescence at approximately 14 V in Figure 8b. We also prepared a device where a blend active layer was spin cast with a thickness of approximately 1 µm. The performance of the device fabricated by CDC is comparable to that fabricated by spin coating. It should be noted that the performance of the devices we fabricated is much worse than that of the device made by state-of-the-art methods because the Al cathode is used in our measurement. Utilization of a Ca cathode and/or an interlayer such as LiF can, we believe, significantly improve the performance. In fact, we expected better light emitting performance of our device arising from the significantly increased

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components may lead to enhanced exciton dissociation and reduced charge recombination. In addition, the use of multiple arrays of sharp metallic anode needles for micropatterned blend thin film fabrication is being pursued.

Conclusions

Figure 8. (a) Current density-applied voltage and (b) luminescenceapplied voltage of the device with PMMA/MEH-PPV (1/3) blend thin film as a light emitting layer fabricated by CDC (open circles) and spin coating (solid circles).

interfacial areas between PMMA and MEH-PPV during corona discharge. However, the CDC on the semiconducting PEDOT/ PSS layer was not successful for creating the well-defined nanosized PMMA domains that were previously observed on the insulating surfaces. We are currently trying to optimize the device performance by developing a way to create nanoscale phase-separated structures even on conducting surfaces. One plausible way is to coat the conducting surface with self-assembled monolayers (SAMs). As another application, the nanoscale phase separation controlled by CDC may be beneficial for the design and construction of efficient polymeric photovoltaic devices because such nanoscale structures incorporated with conducting

We have developed a new, simple, and efficient method, corona discharge coating (CDC), which can easily produce thin films of PMMA/MEH-PPV blends of nanometer-scale domain size. The electric wind of the ionized gas molecules created under needle-plate type corona discharge was highly effective to spread out a polymer blend solution and to form a thin film within a few seconds. CDC generated a unique microstructure of PMMA/ MEH-PPV (1/3) blend that exhibits well-defined PMMA micodomains with a much narrower size distribution and smaller domain size (∼50 nm in diameter) than the ones (∼300 nm) prepared by spin coating. This result implies that our method is beneficial for fabricating phase-separated thin film structures with significantly increased interfacial areas between PMMA domains and active MEH-PPV. The formation of short cylindrical PMMA microdomains during CDC was explained by specific electrostatic or dipole interaction between the charged PMMA microdomains. The light emitting properties of PMMA/MEHPPV (1/3) blend film fabricated by CDC were comparable to those fabricated by spin coating. Acknowledgment. We thank the Ministry of Commerce, Industry and Energy (MOCEI) through research and development projects for Opto-electromagnetic Advanced Materials and NGNT (No. 10024135-2005-11). This research was also supported by a grant (f0004091) from the Information Display R&D Center, one of the 21st Century Frontier R&D Programs funded by the Ministry of Commerce, Industry and Energy of the Korean Government. This work was supported by the Second Stage of Brain Korea 21 Project in 2006. LA062341H