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We have demonstrated a simple and efficient method to fabricate OLEDs with enhanced out-coupling efficiencies and with low pixel blurring by inserting nano-pillar arrays prepared through the lateral phase separation of two immiscible polymers in a blend film. By selecting a proper solvent for the polymer and controlling the composition of the polymer blend, the nano-pillar arrays were formed directly after spin-coating of the polymer blend and selective removal of one phase, needing no complicated processes such as nano-imprint lithography. Pattern size and distribution were easily controlled by changing the composition and thickness of the polymer blend film. Phosphorescent OLEDs using the internal light extraction layer containing the nano-pillar arrays showed a 30% enhancement of the power efficiency, no spectral variation with the viewing angle, and only a small increment in pixel blurring. With these advantages, this newly developed method can be adopted for the commercial fabrication process of OLEDs for lighting and display applications.
© 2016 Optical Society of America
1. Introduction
Organic light emitting diodes (OLEDs) have drawn increased attention following the success of mobile phones using OLED displays and large area displays with curved screens. State-of-the-art phosphorescent OLEDs achieved high external quantum efficiencies (EQEs) of over 30% as a result of optimized device structures using exciplex forming co-hosts and phosphorescent dyes with horizontally oriented emission dipoles [1]. However, since more than 60% of the generated light is still confined inside OLEDs in surface plasmon polariton (SPP) modes, waveguide modes, substrate confined modes, etc; increasing the out-coupling efficiency has been the main issue in OLEDs not only for solid state lighting but also for display applications.
Micro-lens arrays [2] or textured surfaces [3,4] have been used to extract the light confined in the glass mode. Internal light extraction structures have been reported to extract waveguide modes and SPP modes. Examples include the insertion of low index materials [5,6], adoption of high index substrates [7,8], the use of patterned grid [9,10], photonic crystal [11–13], randomly dispersed nano-structure [14–16], corrugated structure [17–20], nano-particle [21–23], plasmonic nano-cavity [24] and moth-eye structures [25]. Large enhancements in light extraction have been achieved using these methods. Unfortunately, most of these methods also increase the haze of light emitting area, resulting in pixel blurring in displays [26]. Hence, as of now, commercial displays do not use light extraction methods. It is only recently that attention is being paid to increase light extraction without increasing the pixel blurring.
We recently reported the use of nano-pores, formed by the phase separation of a polymer blend, as a light extraction layer for obtaining OLEDs with low pixel blurring, where pore patterns were converted to pillar patterns through nano-imprint lithography. Although nano-imprint lithography is a convenient and cost effective process for fabricating nano-structures, a large area mold is inconvenient to handle and detaching large area substrates is cumbersome. Therefore, it is desirable to fabricate nano-patterns directly on a substrate without the need for further processing.
In this paper, we demonstrate a simple method for the direct fabrication of nano-pillar arrays (NPA) by the phase separation of a polymer blend. Phase-separated patterns formed by spin coating gave low haze values. The method allowed us not only to fabricate pores and pillars selectively, but also to control the shape, size and distribution of the patterns by controlling the composition and solvent of the blend. We could thus increase the light extraction efficiency by 30% with a low pixel blurring by using the directly formed nano-pillar arrays as a light extraction layer in a phosphorescent OLED. This technique has the potential to be applied in an actual commercial fabrication process for OLEDs used in lighting and display applications.
2. Experimental methods
2.1 Polymer Solutions
Tetrahydrofuran (THF), methyl ethyl ketone (MEK), methylene chloride, chloroform and dichloroethane (DCE) were purchased from Sigma-Aldrich and used as solvents. Polymeric acrylic resin, Optmer SS series (Mw = 10,000 g/mol, 15.1 wt% solution with PGMEA, 25 ml) was obtained from JSR Micro and was diluted with solvents (75 ml) for viscosity control. Polystylene (PS) (Mw = 10,000 g/mol, 0.04 g, purchased from Sigma-Aldrich) was dissolved in the same solvents as poly(methyl methacrylate) (PMMA) (1.96 g) to make 2 wt% solutions. After complete dissolution, the PS and PMMA solutions were blended.
2.2 Patterning and characterization
Polymer blend films were spin coated on glass substrates using the blend solutions (100 μl) while controlling the speed at 2000, 3000 and 4000 rpm. After the spinning and evaporation of the solvent, the coated glass was heat-treated for 10 minutes on a hot plate at 230°C, after which it was immersed in cyclohexane for 60 s to remove the PS phase. The haze of the resulting nano-pattern was measured using a NDH-5000 haze meter from Nippon Denshoku and the surface was examined with an XE-100 atomic force microscope from Park Systems.
2.3 OLED fabrication and characterization
Polymeric titanium alkoxide, purchased from Brewer Science [15, 27], was diluted with ethanol [polymeric titanium alkoxide:solven = 20:80 wt%] and spin coated for 40 s at 3000 rpm on a patterned glass as a planarizing layer. After annealing in an oven at 190°C for 10 minutes, an interconnected TiO2 network with a refractive index of 1.8 at 632.8 nm was formed through a drying and densification process. A 150 nm thick indium zinc oxide (IZO, 40 ohm/sq) was deposited on the planarized substrate as a transparent anode using a facing target sputtering system. The organic layers and cathode were deposited by thermal evaporation in the following sequence without breaking vacuum: ReO3 (1 nm)/TAPC (40 nm)/HATCN (5 nm)/ TAPC (40 nm)/HATCN (5 nm)/TAPC (40 nm)/HATCN (5 nm)/TCTA (10 nm)/TCTA:B3PYMPM:Ir(ppy)2acac [54:40:8 in wt%] (30 nm) /B3PYMPM (40 nm)/ LiF (1 nm)/Al (100 nm), where TAPC represents 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane, HATCN 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile, TCTA 4,4',4”-tris(carbazol-9-yl)-triphenylamine, Ir(ppy)2acac bis(2-phenylpyridyl) iridium(III) acetyl-acetonate, and B3PYMPM bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine, respectively. The emissive area of the OLED was 2 × 2 mm2 and thickness of the substrate was 0.7 mm. Three stacks of TAPC (40 nm)/HATCN (5 nm) layers and TCTA layer were used as hole transporting layers (HTL) to prevent leakage currents at the edge of the transparent electrode and B3PYMPM as the electron transporting layer. The devices were encapsulated in glass canisters. A Keithley 2400 semiconductor parameter analyzer and a Photo Research PR-650 spectrophotometer were used to measure the electrical and luminescence characteristics of the devices, respectively. The image quality of the OLEDs with a phase separation layer was compared with the OLED with a micro-lens array (MLA) where a micro-lens array film consisting of half spheres with a diameter of 70 μm was attached to glass substrates using the index matching optical adhesive NOA65 (Norland optical adhesive, n = 1.52) after completion of the OLEDs.
3. Results and discussion
Randomly dispersed nano-pores or pillars are formed by the phase separation of two immiscible polymers in a blend. PS and PMMA were dissolved in solvents and blended to spin-coat on substrates. After the evaporation of the solvents and treatment with a solvent to selectively dissolve one phase, nano-patterns were formed as illustrated schematically in Fig. 1.
Fig. 1 Schematic illustration of fabrication process of nano-pillar array. (a) Spin coating of blend solution on glass substrate, (b) vertical phase separation occurs during spin-coating, (c-e) spinodal dewetting occurs following vertical phase separation, and (f) formation of nano-pillar array after treatment of selective solvent.
The morphologies of the nano-patterns strongly depend on the solvent, the composition and the film thickness. For instance, the polymer blend dissolved in THF forms nano-pores very well as shown Fig. 2 and as described before [15]. The patterns with nano-pores were formed using thermally curable PMMA mixed with PS in the weight ratio of 8:2 and the concentration of the mixed polymer in THF was 3.07wt%. The mixed solution was coated on glass substrates at spinning speeds of 1000, 3000, and 5000 rpm. The fast Fourier transforms (FFTs) of the nano-pores formed in each case show broad halos, indicating that the nano-pore arrays have a range of diameters with random orientations. The diameters of the pores and the distances between the holes are in the 200-400 nm range so as to produce scattering in the visible light region. The surface profiles of the patterns are shown in Fig. 2(b). The depth of the pores decreases from 240 to 160 to 120 nm as the spinning speed increases progressively from 1000 to 1500 and to 2000 rpm, respectively. The diameters of the pores also decrease with reduced film thickness, which is expected due to the restricted growth of domains for films of low thickness [28]. The phase separation of an immiscible polymer blend such as PMMA and PS was reported to be proceeded by the formation of a bilayer and followed by dewetting in that the bilayer [29]. However, dewetting could occur either by the nucleation of holes or by a fluctuation of the interface. In a spinodal dewetting case, the resulting pattern has a certain preferred distance between pits and hills, which is called the spinodal wavelength λS and this is related to the effective interface potential (the free energy per unit area), Φ of the system [30,31].
Fig. 2 (a) AFM images of nanopatterns on glass substrates formed by phase separation followed by selective removal of PS in the blend films composed of PMMA:PS (8:2) which were spin coated at 1000 rpm (left), 3000 rpm (center), and 5000 rpm (right) (dimension: 10 μm x 10 μm) (Insets: 2D-FFT images of pore patterns), respectively. THF was used as the solvent. (b) Surface profiles of phase separation layer along the dashed lines in Fig. 2(a). (c) Wavelength dependency on the thickness of film.
σ denotes the liquid-air surface tension and Φ(h) is equal to -A/(12πh2) where A is the Hamaker constant. In Fig. 2(c), the wavelength shows a strong dependence on the square of film thickness and the pores are believed to be created as a result of spinodal dewetting.
Based on the observations and with the aim to obtain nano-pillars instead of nano-pores, the concentration of PS was further increased since it has been shown that the domain structure and the surface morphology strongly depend on the concentration [32–35]. Figure 3 shows the AFM images of the films with increasing concentration of PS namely, 30, 40, 50, 60 and 80 wt%, spin coated at 1000 rpm. Pores are coagulated to form a large pore with increasing PS weight ratios and the PS phase becomes the major phase. Pillar-like shapes are formed for the composition where PS is above 80 wt%. However, the pattern is composed of craters rather than pillars. The difference in solubility between PS and PMMA plays a role in forming the patterns. THF is a better solvent for PS than PMMA, and PMMA has a preference for polar substrate surfaces such as glass. During spin coating, a transient bilayer is formed with a PMMA rich bottom layer. Therefore, PMMA is depleted first from the solvent to form a bottom layer with hillocks after which PS fills the pit formed by PMMA. After the removal of PS by cyclohexane, only the hillocks of PMMA are observed. Hence, the bottom layer of PMMA with a certain thickness is always observed. As the weight ratio of PS increases, a thin bottom layer of PMMA and a lateral phase separated structure are formed. Therefore, it is difficult to fabricate pillar arrays with appropriate diameters and distribution using THF as the solvent.
Fig. 3 AFM images of nanopatterns on glass substrates formed by phase separation followed by selective removal of PS in the blend films with different compositions of PMMA:PS, (a) 30:70, (b) 40:60, (c) 50:50, (d) 60:40 and (e) 80:20. Dimensions: 10 μm × 10 μm. THF was used as the solvent and the speen speed was 1000 rpm.
On the other hand, solvents such as MEK, chloroform and DCE are better solvents for PMMA than for PS. PS is expected to be depleted of these solvents first and form isolated island structures over the PMMA solution layer. PMMA is solidified to fill the remaining area and pillar arrays are formed when the weight ratio of PS is high. Figure 4 shows pillar arrays formed by the phase separation of PMMA and PS when better solvents for PMMA such as MEK, chloroform, MC and DCE are used and where the PMMA to PS weight ratiois 2:8.
Fig. 4 AFM images of nanopatterns on glass substrates formed by phase separation followed by selective removal of PS in the blend films (PMMA:PS = 2:8 in weight) spin coated at 1000 rpm using the solutions dissolved in different solvents; (a) MEK, (b) chloroform, (c) methylene chloride, and (d) DCE. Dimensions: 10 μm × 10 μm.
The solution was further coated with different spinning speeds of 2000, 3000 and 4000 rpm. Figure 5(a) shows the AFM images of nano-patterns formed by the spin coating of the PMMA: PS (2:8) blend solution in chloroform at different spin speeds of 2000, 3000 and 4000 rpm denoted as C20, C30 and C40, respectively. The thicknesses of the coated films are almost thesame as the heights of the pillars, indicating that the phases are separated laterally. By increasing the spinning speed, the average pillar height of the nano-pillar arrays is decreased from 300 nm to 230 nm and the diameter of the pillars is also decreased (Fig. 5(b)).
Fig. 5 Patterns of nanopillars on glass substrates formed by phase separation followed by selective removal of PS in the blend films (PMMA:PS = 2:8 in weight) with different coating thicknesses using chloroform as the solvent. AFM images of the patterns (dimensions: 5 μm × 5 μm) when spin-coated at 2000 rpm (left, C20), 3000 rpm (center, C30), and 4000 rpm (right, C40). (b) Surface profiles of C20, C30, and C40 films along the dashed lines in (a).
To evaluate the pattern with nano-pillars as light extraction layers, the transmittance haze was measured for the samples. The transmittance haze is defined as the ratio of the diffused light to the total transmitted light. Scattering media usually increase the haze because the light is scattered in all directions. In display applications, the increased haze blurs the pixel and drastically reduces the resolution of the device. In the case of pillars formed by lateral phase separation, the pillars have cylindrical shapes and hence do not scatter incident light with normal incidence, resulting in low haze values. The haze values of C20, C30 and C40 measured with normal incident light were 4.2, 2.6 and 1.5%, respectively. The haze value for glass without any pattern is 0.2% and the haze of glass with 70 μm diameter micro lens arrays (MLAs) is 84.8%. The haze values of the glass substrates with the nano-pillars are much lower than those for the same substrates with MLAs. As a result of the low haze values, no significant difference in clarity is observed between samples with and without the nano-pattern when letters are seen through the C30 patterned glass, as shown in Fig. 6(a). The surface roughness of the patterned layer (C30) was reduced to the peak-to-valley roughneess of 28.6 nm by spin coating the planarization layer as shown in Fig. 6(b).
Fig. 6 (a) Photograph of letters with and without the glass with the nanopillar arrays, C30 (Haze: 2.6%). C30 shows only a small increment in haze. Dimensions of glass substrates: 25 mm × 25 mm. (b) AFM images of the C30 patterns when planarized by polymeric titanium alkoxide (dimensions: 5 μm × 5 μm).
Highly efficient green phosphorescent organic light emitting diodes with Ir(ppy)2acac doped TCTA and B3PYMPM co-host EML were fabricated to evaluate the performance of organic light emitting diodes using the nano-patterned layers with nano-pillar arrays formed by the phase separation of the polymer blend as a light extraction layer [1]. The PhOLED structure is shown as the inset in Fig. 7(a). The patterned layer was planarized by the spin coated TiO2 layer before the deposition of the IZO transparent electrode as described in the experimental section. Thick HTL multilayers were used to reduce the leakage currents at the edge of thetransparent electrode [36]. Current density-voltage-luminous characteristics of OLEDs with and without nano-patterns are shown in Fig. 7(a). All devices turned on at the same voltage of 2.4 V, and showed low leakage currents. Current and power efficiencies with and without the nano-pillars are compared in Fig. 7(b). The OLED with C30 showed the largest enhancement of 24% in current efficiency (from 88.9 cd/m2 without any extraction layer to 110 cd/m2) and21% in power efficiency (from 91 lm/W to 110 lm/W at 1 mA/cm2). The enhancement is even larger at 100 mA/cm2 (luminance of about 10000 cd/m2) with an enhancement of 30% from 57.6 lm/W to 74.7 lm/W.
Fig. 7 Device performance of OLEDs without nanopillar arrays as a reference (black) and with nanopillar arrays of a C20 layer (red), a C30 layer (green), and a C40 layer (blue) as the light extraction layer. (a) Current density-voltage-luminance characteristics with the inset showing the device structure. (b) Current and power efficiencies as a function of current density (mA/cm2) for the OLEDs. (c) Angular dependence of the EL intensities for OLEDs. All data were normalized with the intensity of the reference device in the normal direction. Orange line represents a guide to the Lambertian pattern. (d) EL spectra of OLEDs acquired from an integrating sphere at 1 mA/cm2.
Apart from the efficiency enhancement factor, obtaining an even enhancement over all wavelengths and emission patterns close to Lambertian emission are important for lighting applications. Comparisons of the spectra from the integrating sphere showed that light beams from 380 to 780 nm were enhanced by 26% without spectral variation (Fig. 7(d)). In other words, the external quantum efficiency was enhanced by 26% without spectral distortion. The OLEDs with nano-pillar arrays showed little angle dependent spectral change (Fig. 8(a)), in contrast to the reference device. More importantly, the enhancement of the efficiencies and improved emission characteristics were obtained with little image blurring as demonstrated by the sharp edges of the active area as shown in Fig. 8(b). In contrast, the OLEDs with a micro lens array showed a slightly higher enhancement of light extraction but with a large image distortion and hence cannot be used for practical applications in displays.
Fig. 8 Angle dependent emission spectra of the OLED with the C30 layer as the light extraction layer at 1mA/cm2 (Inset: angle dependent emission spectra of the reference OLED without the light extraction layer). (b) Emission images at the normal direction of the reference device (left) and the OLEDs with C30 (center) and MLA (right), respectively.
4. Conclusion
A simple method has been developed for the direct fabrication of nano-pillar arrays by the phase separation of a polymer blend followed by selective dissolution of a single phase to use as a light extraction layer in OLEDs without a pattern transfer process such as nano-imprint lithography. Nanopatterns formed by the process were significantly influenced by solvent as well as composition and thickness of the blend films. The method allowed us not only to fabricate nano-pores and nano-pillars selectively, but also to control the shape, size and distribution of the patterns by controlling the composition and solvent of the blend. Using chloroform as solvent for PMMA and PS blend, we were able to fabricate random nano-pillar arrays of PMMA. We could thus increase the light extraction efficiency by 30% with a low pixel blurring using the directly formed nano-pillar arrays as a light extraction layer in a phosphorescent OLED. This technique has the potential to be applied in an actual commercial fabrication process for OLED lighting and display.
Acknowledgments
This work was supported by the IT R&D program of MKE/KEIT (10041062, Development of fundamental technology for light extraction of OLED).
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