Ultra-High-Resolution Organic Light-Emitting Diodes with Color

Mar 28, 2018 - The ultra-high-resolution display with color conversion electrode could be a basic technology for the development of advanced high-reso...
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Letter Cite This: ACS Photonics 2018, 5, 1891−1897

Ultra-High-Resolution Organic Light-Emitting Diodes with Color Conversion Electrode Jun Hee Han, Dohong Kim, Tae-Woo Lee, Eun Gyo Jeong, Ho Seung Lee, and Kyung Cheol Choi* School of Electrical Engineering, Korea Advanced Institute of Science and Technology, Yuseong-gu, Daejeon 34141, Republic of Korea S Supporting Information *

ABSTRACT: The implementation of ultra-high-resolution displays is one of the important technologies for advanced displays. In this paper, an ultra-high-resolution organic light-emitting diode display is implemented without the fine metal mask method, but via a color conversion electrode. A red and green color ultra-high-resolution organic light-emitting diode display with a pixel size of 5 μm was experimentally realized without changing any aspects of the structure of the OLED display except for the precisely fine-patterned color conversion electrode. Furthermore, nanometer-scale pixel size can be expected through this method. The color conversion electrode is a multilayer structured nanometer thick conductive optical film, and its applicability was confirmed based on a theoretical analysis and optical simulation. The ultra-high-resolution display with color conversion electrode could be a basic technology for the development of advanced high-resolution displays. KEYWORDS: high resolution, subwavelength optics, color conversion, light resonance, optical film, organic light-emitting diodes

O

In this study, an ultra-high-resolution OLED display was implemented by introducing a color conversion electrode (CCE). The principle of changing the color using a CCE is the cavity effect. Unlike a typical cavity effect, however, the CCE can change the emissive color of the device while maintaining the thickness and type of organic materials and the electrical properties of the devices. Therefore, a red- and green-colored ultra-high-resolution OLED display with a pixel size of 5 μm was experimentally realized based on a yellow emission layer without changing any aspects of the structure of the display except the fine-patterned CCE. The principle of color tuning by the CCE was theoretically analyzed, and optimized conditions of the CCE were designed using an optical simulation. The CCE is advantageous in terms of efficiency, and this was confirmed by theoretical and experimental analyses. The experimental and simulation results demonstrate remarkable potential to realize variously shaped nanometer-scale pixels and utilization in the full-colored ultra-high-resolution OLED display composed of red, green, and blue subpixels. Figure 1(a) shows the structure of the OLED with a CCE for an ultra-high-resolution display. As shown in the structure, the device is a bottom emission display, in which a CCE, a hole transport layer (HTL), an emission layer (EML), and a cathode are stacked on a substrate. The CCE consists of a structure wherein silver (Ag) thin films and tungsten trioxide (WO3) thin films are alternately stacked. In this study, the key technology to realize the ultra-high-resolution OLED display is the CCE.

ne of the main streams of display development is ultrahigh-resolution displays. However, the development of such an ultra-high-resolution technology has not progressed, especially for the organic light-emitting diode (OLED) display. There are several problems that should be overcome for implementation of ultra-high-resolution OLED displays including the realization of a highly integrated driving circuit and enhancing the mobility of thin film transistors. Among the various problems, however, the most urgent issue to be solved is reducing the pixel size of the display.1,2 Especially for virtual reality displays, in which the feeling of immersion is important, a display resolution of at least 8K is required to reduce the screen door effect, reflecting a problem of pixel identification due to large pixel size, and this means that the pixel size should be reduced to sub-micrometer units. In the case of OLEDs, a fine metal mask is used for pixel discrimination. However, when this mask is used, there is a limitation in reducing the pixel size, which remains an obstacle to realizing ultra-high-resolution displays. In order to solve the high-resolution problem, a structure consisting of white emission and a color filter has been suggested. Therefore, various studies have been conducted to implement a color filter in a narrow region, because an important factor determining the pixel size is the color filter.3−5 However, the works in previous studies were difficult to commercialize due to technical reasons or a complicated fabrication process. The studies using an inkjet have been carried out to realize the fine pixel size, but the materials that can be used for the inkjet method have to be further developed and the process steps have to be simplified.6,7 © 2018 American Chemical Society

Received: February 20, 2018 Published: March 28, 2018 1891

DOI: 10.1021/acsphotonics.8b00230 ACS Photonics 2018, 5, 1891−1897

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beam interference factor (f TI) (eqs 1, 2, and 3)8,9 Figure 1(b) schematically shows two-beam interference and Fabry−Perot within the device. In eq 1, λ is wavelength and d1 is the distance between the dipole and reflector 1. A detailed formula for the cavity gain is summarized in the Supporting Information. Generally, in order to tune the emitting light color, a method of changing the distance between reflector 1 and reflector 2 (d2) is used.10−12 When using this method, the thickness of the organic layer such as HTL and EML must be changed for a specific color, and this change affects the electrical properties of the device such as charge balance. However, by using the CCE proposed in this study, it is possible to tune the emitted light color by changing the reflection coefficient and transmission coefficient of the CCE without changing the thickness of the organic layers. In other words, it is possible to control the electrical and optical properties of the device individually with the CCE. This is confirmed from the formula delineating that the Fabry−Perot factor can be changed by only changing the reflection coefficient and transmission coefficient of the CCE (eq 3). ⎛ i sin Δϕ1 ⎞⎛ i sin Δϕ2 ⎞ ⎛ E1 ⎞ ⎜ cos Δϕ1 ⎟⎜ cos Δϕ2 ⎟ Y1 ⎟⎜ Y2 ⎟... ⎜ ⎟=⎜ ⎝ H1⎠ ⎜ ⎟⎜ ⎟ ⎝ iY1sin Δϕ1 cos Δϕ1 ⎠⎝ iY2 sin Δϕ2 cos Δϕ2 ⎠ ⎛ i sin Δϕ(n − 1) ⎞ ⎜ cos Δϕ ⎟⎛ E ⎞ (n − 1) Y(n − 1) ⎟⎜⎜ n ⎟⎟ ⎜ ⎜ ⎟ ⎝ Hn ⎠ ⎜ iY ⎟ ⎝ (n − 1) sin Δϕ(n − 1) cos Δϕ(n − 1) ⎠ (where Δϕn = nk̃ 0dn)

(4)

⎛1⎞ ⎛ ⎛ B⎞ E H⎞ ⎜ ⎟ = [M ]⎜ ⎟ ⎜where B ≡ 1 , C ≡ 1 ⎟ Y ⎝C ⎠ En En ⎠ ⎝ n⎠ ⎝

(5)

YB − C R = |r | = 0 Y0B + C

2

2

2Y0 T = |t | = Y0B + C

(6) 2

2

Figure 1. Design overview of suggested color conversion electrode. (a) Schematic device structure of organic light-emitting diodes with color conversion electrode. (b) Graphical representation of the light interference within an organic light-emitting diode. (c) Graphical representation of the electric and magnetic field inside the multilayer structure.

Gcav (λ) = fFP (λ) fTI (λ ; d1)

The principle of changing the reflection coefficient of the CCE is also light interference. As shown in Figure 1(a), the CCE consists of a multistack of thin films. When light is incident on the multilayered film, transmitted light and reflected light are generated in each layer. Finally, reflection and transmission of the entire film are determined through interference of this light. A graphical representation of this phenomenon is shown in Figure 1(c). The reflection and transmission of the CCE can be calculated using a characteristic matrix, such as eq 4. The general form of eq 4 can be represented as eq 5, and reflectance and transmittance can be simply calculated using the variable of eq 5 (eq 6, eq 7).8,9,13 In eq 4 and eq 5, En and Hn are electric field and magnetic field at boundary n (interface between material n − 1 and material n), respectively, and Yn is the optical admittance of material n. In order to change the reflection coefficient and transmission coefficient of the CCE, it is necessary to change the refractive index of each layer or to change the thickness of each layer. In this study, optimized thicknesses of each layer were used to control the reflection coefficient and transmission coefficient of

(1)

fTI (λ ; d1) = 1 + (|r1| e−κk 02d1)2 + 2|r1| e−κk 02d1 cos ΔϕTI (2) 2

fFP (λ) =

|t 2|

(

(1 − R )2 + 4R sin

ΔϕFP 2 2

)

(7)

(3)

In the OLED device, which has a stacked thin film structure with a subwavelength-scale thickness, the light interference in the device affects the emitting color of the device. Light interference in the device can be expressed as cavity gain (GCAV), a product of the Fabry−Perot factor (f FP) and the two1892

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Figure 2. Simulation results of the optimization of the color conversion electrode. (a, b) Calculation results to obtain the optimized conditions of the color conversion electrode for green and red by a MATLAB simulation, respectively. (c, d) Calculated simulation data of the Fabry−Perot factor and emission spectrum with a green and red color conversion electrode, respectively. The solid lines indicate the Fabry−Perot factor. The colored area shows the emission spectrum. The yellow dashed line is the emission spectrum of the reference device.

the CCE, while fixing the material as Ag and WO3. A detailed solution of the characteristic matrix, the reflection coefficient, and the transmission coefficient is summarized in the Supporting Information. Previous works experimentally demonstrated that the multilayer structure of Ag and WO3 is a highly conductive optical film and that this film can be sufficiently applied as an electrode in OLED devices.5,13,14 Based on these results, a CCE consisting of Ag and WO3 was simultaneously used as an electrode and an optical film in the display device for this study. The optimized conditions of the CCE for green and red were obtained by calculating the optical functions with MATLAB. The CCE is defined as the optimized CCE when the color coordination of the emission light from the OLED with the CCE is located close to the green and red coordination of sRGB. Figure 2(a) and (b) show simulation results according to the thicknesses of the WO3 layers within the CCE. As the emission color coordination of the OLED device is closer to the color coordination of green and red of sRGB in the optimization process, the simulation results have a larger value because the reciprocal of the distance between the emission color coordination and sRGB coordination is set as the simulation results. A wider range of simulation results of Figure 2(a) and (b) can be seen in Figure S1. Table 1 summarizes the optimized layer thickness of the CCE to obtain the green and red light from the simulation results. Figure 2(c) and (d) show the simulation results of the emission spectrum of the OLED device with the optimized CCE for green and red color, respectively. The OLED structure

Table 1. Optimized Layer Thicknesses of the Color Conversion Electrode material color

Ag

2nd WO3 (y nm)

Ag

1st WO3 (x nm)

green red

10 nm 10 nm

76 nm 137 nm

20 nm 20 nm

109 nm 101 nm

used in the simulation is the same structure as in Figure 1(a). The reference OLED device uses a transparent electrode of a thin Ag film with a capping layer instead of the CCE to observe the optical properties, while the electrical characteristics are the same as those of the CCE (Figure S2) because electrical properties can affect the performance and hinder accurate analysis of the device. The refractive index for each material used in the simulation was obtained by measurement except for aluminum (Figure S3). Furthermore, measured data of the photoluminescence (PL) spectrum of the emission layer was used to calculate the emission spectrum (Figure S4). The colored region in each graph is the simulated OLED emission spectrum with a green and red CCE, respectively. The emission spectrum of a reference transparent electrode is shown as a yellow dashed line. The cavity gain value according to the CCE is also shown in the graph as a solid line. In the graph, it can be seen that the Fabry−Perot factors of green and red have maximum values in the region of green and red wavelengths, respectively. The emission spectra calculated by the product of the cavity gain and the PL intensity of the emission layer also have the shape of green and red. Simulation 1893

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Figure 3. Performance of organic light-emitting diodes with the color conversion electrode. (a) Simulated and measured emission spectrum of OLEDs with CCE. Green open and solid squares are simulated and measured data for the green emission device, respectively. Yellow open and solid triangles are simulated and measured data for the reference device, respectively. Red open and solid circles are simulated and measured data for the red emission device, respectively. (b) CIE 1931 diagram of green, red, and reference devices. Circles are experimental data and stars are simulated data. Inset images are the pixel images of the green, red, and reference devices. (c) Current density versus voltage characteristics. (d) Luminance versus current density characteristics. (e) Current efficiency versus luminance characteristics. For (c), (d), and (e), green solid squares, yellow solid triangles, and red solid circles are green emission device, reference device, and red emission device, respectively. (f, g, h) Scanning electron microscope images of reference device, green emission device, and red emission device, respectively. The scale bars are 100 nm.

makes efficient use of the cavity effect was compared with an OLED with a CCE. A Ag film of 30 nm thickness was used as the thick metal electrode, considering the color purity and efficiency (Figure S6). The thickness of 30 nm is also a value that agrees with the sum of the Ag thickness of the CCE. In the simulation, it is confirmed that there is almost no difference in the optical aspect of the CCE compared with the case of using the thick metal electrode (Figure S7). The simulation was also conducted to determine if the color can be changed in a device consisting of a thick metal electrode by varying the thickness of the capping layer while maintaining the distance between the cathode and the anode. However, the peak wavelength of the emission spectrum was barely changed (Figure S8). These results show that the use of the CCE can take advantage of not only the optical aspect of the thick metal electrode but also the same organic thickness and the same electrical property regardless of emission color. Tuning the color without changing the organic thickness is also an advantage in implementing an

results show that the reflection phase of each CCE and the transparent electrode is changed (Figure S5), and this confirms that the basic principle of color tuning through the CCE is to change the reflection coefficient and the transmission coefficient of the CCE while keeping the thickness of the organic layer constant (eq 3). The green and red emission spectra obtained from the OLED with the CCE show higher peak values than the reference device in the simulation. These results suggest that the CCE can achieve improved efficiency relative to the case using a color filter made with dye or pigment. Because the principle of the dye and pigment color filter is light absorption, the emission spectrum of the device with the color filter will always have a lower value than the reference device. The principle of efficiency improvement is based on the cavity effect using constructive light interference. In order to confirm the advantage of the CCE in terms of optical properties, an OLED with a thick metal electrode that 1894

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Figure 4. Ultra-high-resolution display. (a) Schematic of the fabrication process of the ultra-high-resolution display. (b) Atomic force microscope images of the patterned CCE. (c) Cross section profile of the patterned CCE obtained by atomic force microscope. (d, e) Optical microscope images of an ultra-high-resolution green and red display, respectively. The scale bars are 50 μm.

simulation values almost coincide. Figure 3(c) shows the current density (J) versus voltage (V) graph of the fabricated device. When fabricating the OLEDs with the CCE, there was no modification in the thickness of the organic layer, and hence the JV characteristics were the same irrespective of the emissive color. Even the charge balance was also the same, because the thicknesses of HTL and EML were equal. These results suggest that the optical and electrical properties can be controlled individually with the CCE. Previous reports and our experimental results confirmed that the thickness of the HTL affects the electrical properties and the performance of the device (Figure S10).10,15,16 Figure 3(d) shows the luminance versus current density results, and Figure 3(e) shows the current efficiency versus luminance results, averaged over five devices. In the simulation, the calculation is performed considering only the optical properties of the device, assuming that the electrical properties are the same. As Figure 3(a) shows, the simulation and measured data have a similar spectrum shape because the assumption that the devices have the same electrical properties including charge balance regardless of color was satisfied in the experiment. Another reason is that when the CCE is applied to an actual device, the light interference works as designed by theory and simulation. All

ultra-high-resolution display, because a high-resolution display made by adjusting the thickness of the organic materials cannot be realized with the existing fabrication method such as finepatterned thin metal mask technique. Since the yellow color emitting material was used as the emission layer, the colors of the OLEDs were experimentally tuned to green and red. If the emission layer can emit white light, it can be confirmed that the three basic primary colors of blue, green, and red can be implemented with the CCE by the simulation results (Figure S9). Figure 3(a) shows the emissive spectra of the fabricated devices and of the simulation. The solid mark is the measured data by the actual experiment, and the left y-axis corresponds to the data. Data obtained through simulation are open marks, and the right y-axis corresponds to the data. Figure 3(b) shows the measured color coordination from the fabricated device and the color coordination obtained from simulation results on CIE 1931. The color coordination marked on CIE 1931 shows that it shifts to green and red from the yellow when the CCE is applied to the OLED device, and the actual pixel image also shows that the color changed significantly. The color coordination values of green and red of the fabricated devices are (0.367, 0.618) and (0.615, 0.383), respectively. The expected color coordination from the simulation is (0.379, 0.607) and (0.626, 0.372), respectively. The experimental and 1895

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data. This shows that the CCE is patterned and strictly divided into about 5 μm width. When the AFM data were analyzed, the root-mean-square surface roughness value of the patterned CCE was about 3 nm. Flattening the electrode surface is an essential condition for stable OLED operation, and the finepatterned CCE satisfied this. Figure 4(d) and (e) show ultra-high-resolution green and red OLED pixel images fabricated through the process shown in Figure 4(a). Even with micrometer-scale pixel size, the colors can be represented through the CCE. The experimental data confirmed that the proposed CCE can realize a pixel size of micrometer scale without any problems. This pixel size is smaller than the pixel size of tens of micrometers used in conventional displays.20,21 The pixel was line shaped in a size of micrometer scale, because the PR was patterned in the laboratory unit. However, the implementation of variously shaped nanometer-scale pixels can be expected because nanometer-scale patterning of PR is possible in industry.22 Although a single-color ultra-high-resolution display was fabricated in this study, it would be easy to produce an ultrahigh-resolution display composed of basic colors such as blue, green, and red at an industrial level where the patterning process is well established. In summary, we demonstrated an ultra-high-resolution OLED display with a CCE. It was theoretically and experimentally confirmed that an ultra-high-resolution green and red display with a pixel size of a few micrometer scale can be successfully implemented with a fine-patterned CCE without changing the thickness and type of the organic materials and electrical properties of the devices. The results of this study demonstrate remarkable potential to realize variously shaped nanometer-scale pixels and also show the possibility of high utilization in the display industry. It is expected that this CCEbased technology can provide a foundation for the development of advanced displays requiring ultra-high resolution such as virtual reality displays, microdisplays, etc.

devices were measured with the same current density of 1 mA/ cm2. In Figure 3(a), the peak value of red is much higher than the peak value of green, but the efficiency of green is higher than that of red, as shown in Figure 3(e). This is because the current efficiency is measured considering the luminosity function, and the luminosity function has a higher value on the green region than on the red region. Figure 3(a) shows that the spectrum peak value of the device with the CCE is higher than that of the reference device, not only in the simulation but also in the experimental data. As described above, the results verify that higher device efficiency can be expected when fabricating OLEDs using the CCE rather than OLEDs using a color filter of dye and pigment. In Figure 3(f), (g), and (h), the cross sections of the reference device and that of green and red devices with the CCE were observed by a scanning electron microscope (SEM). The images confirm that the thicknesses are nearly equal to the optimized values by the simulation. Furthermore, it is shown that all devices have the same thickness of HTL, EML, and cathode regardless of the color. In the case of the reference, the thin Ag film is more clearly seen by the backscattered electrons (Figure S11). In addition, the angular characteristics of the emission spectrum were also analyzed. The spectrum tended to blue shift according to the angle in both the green and red devices. When the angle was changed to 60 degrees, the peak wavelengths were shifted by about 15 and 55 nm for the green and red spectrum, respectively (Figure S12). This angular dependency is due to the fact that the basic principle of controlling the color is based on the cavity resonance. The proposed method in this study would not be a problem when the viewer faces the screen in the normal direction, but further study on the robust angular property is needed to increase the field of view of the display. In this regard, various studies have been conducted. For instance, researchers have shown that the angular dependency can be reduced when a polarized transverse magnetic wave is induced into a multilayer-structured film like CCE because optical admittance of the film is negligibly changed when the angle of incidence of the transverse magnetic wave is different17−19 (Figure S13). Our previous reports have already confirmed that a conductive optical film consisting of WO3 and Ag can be fine patterned by photolithography.5,13 Since the CCE is also composed of WO3 and Ag, fine patterning is possible with photolithography. The use of a fine-patterned CCE enables easy implementation of ultra-high-resolution displays. Figure 4(a) shows a schematic illustration of the fabrication process of the ultra-high-resolution display with the CCE. First, the red or green CCE is formed on the patterned negative photoresist (PR) by thermal evaporation. The PR is then stripped, producing a fine-patterned CCE. An ultra-high-resolution display can be achieved simply by depositing an EML layer and a cathode on the fine-patterned CCE. As in the previous experimental confirmation, the thickness and material of all layers between the two electrodes are the same, and thus all fabrication processes are exactly the same regardless of color except for the use of the CCE. Furthermore, the OLED device in which red and green color pixels are alternately emitted can be easily implemented by repeating the patterning steps (Figure S14). This shows that the CCE is advantageous in terms of the fabrication process even for ultra-high-resolution displays. Figure 4(b) and (c) are atomic force microscope (AFM)

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METHOD The suggested ultra-high-resolution display was manufactured on a cleaned glass substrate. Negative photoresist (NR93000PY, Futurrex Inc.) was spin-coated on the cleaned substrate. Then the photoresist was soft-baked at 150 °C for 3 min in an oven. The photoresist-coated substrate was exposed to ultraviolet light (UV) with a 360 mJ/cm2 exposure dose with a patterned chrome mask. The photoresist-coated substrate was baked again at 110 °C in the oven for 2 min. After the baking process, the photoresist was developed for 17 s in an AZ300MIF developer. After the photoresist was patterned, color conversion electrodes and a hole transport layer were deposited on the patterned photoresist substrate by thermal evaporation with a vacuum pressure of less than 6 × 10−6 Torr: WO3 (x nm)/Ag (20 nm)/WO3 (y nm)/Ag (10 nm)/WO3 (70 nm). WO3 (1−4 mm pcs 4N, TASCO) and Ag (3−5 mm granule 4N, TASCO) were used for the fabrication of the CCE. After the CCE was deposited on the patterned photoresist, the photoresist was stripped by acetone sonication cleaning for 10 min. Super yellow (PDY-132) of 65 nm thickness was spincoated on the patterned CCE in a nitrogen atmosphere. The substrates were then loaded into a thermal evaporation chamber, where the cathode was deposited in a vacuum pressure of less than 6 × 10−6 Torr: lithium quinolate (Liq) (1 nm)/aluminum (Al) (100 nm). 1896

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by Using Laser-Patterned Polyimide Shadow Masks. Appl. Phys. Lett. 2014, 104 (5), 05330310.1063/1.4864269. (3) Do, Y. S.; Park, J. H.; Hwang, B. Y.; Lee, S. M.; Ju, B. K.; Choi, K. C. Color Filters: Plasmonic Color Filter and Its Fabrication for LargeArea Applications (Advanced Optical Materials 2/2013). Adv. Opt. Mater. 2013, 1 (2), 109−109. (4) Ellenbogen, T.; Seo, K.; Crozier, K. B. Chromatic Plasmonic Polarizers for Active Visible Color Filtering and Polarimetry. Nano Lett. 2012, 12 (2), 1026−1031. (5) Han, J. H.; Kim, D.-Y.; Kim, D.; Choi, K. C. Highly Conductive and Flexible Color Filter Electrode Using Multilayer Film Structure. Sci. Rep. 2016, 6, 29341. (6) Kim, K.; Kim, G.; Lee, B. R.; Ji, S.; Kim, S.-Y.; An, B. W.; Song, M. H.; Park, J.-U. High-Resolution Electrohydrodynamic Jet Printing of Small-Molecule Organic Light-Emitting Diodes. Nanoscale 2015, 7 (32), 13410−13415. (7) Li, J.; Xu, L.; Tang, C. W.; Shestopalov, A. A. High-Resolution Organic Light-Emitting Diodes Patterned via Contact Printing. ACS Appl. Mater. Interfaces 2016, 8 (26), 16809−16815. (8) Born, M. Principles of Optics: Electromagnitic Theory of Propagation, Interference and Diffraction of Light; Cambridge University Press, 2002. (9) Bahaa, E. A. S. Fundamentals of Photonics, 2nd ed.; Wiley, 2007. (10) Han, J. H.; Kim, D.-H.; Choi, K. C. Microcavity Effect Using Nanoparticles to Enhance the Efficiency of Organic Light-Emitting Diodes. Opt. Express 2015, 23 (15), 19863. (11) Kim, E.; Chung, J.; Lee, J.; Cho, H.; Cho, N. S.; Yoo, S. A Systematic Approach to Reducing Angular Color Shift in Cavity-Based Organic Light-Emitting Diodes. Org. Electron. 2017, 48, 348−356. (12) Preinfalk, J. B.; Schackmar, F. R.; Lampe, T.; Egel, A.; Schmidt, T. D.; Brütting, W.; Gomard, G.; Lemmer, U. Tuning the Microcavity of Organic Light Emitting Diodes by Solution Processable PolymerNanoparticle Composite Layers. ACS Appl. Mater. Interfaces 2016, 8 (4), 2666−2672. (13) Han, J. H.; Kim, D.; Jeong, E. G.; Lee, T.-W.; Lee, M. K.; Park, J. W.; Lee, H.; Choi, K. C. Highly Conductive Transparent and Flexible Electrodes Including Double-Stacked Thin Metal Films for Transparent Flexible Electronics. ACS Appl. Mater. Interfaces 2017, 9 (19), 16343−16350. (14) Han, J. H.; Choi, K. C. P-104: A Transparent, Flexible, Patternable Electrode Using a Multilayer Film Structure. Dig. Tech. Pap. - Soc. Inf. Disp. Int. Symp. 2016, 47 (1), 1519−1522. (15) Murawski, C.; Liehm, P.; Leo, K.; Gather, M. C. Influence of Cavity Thickness and Emitter Orientation on the Efficiency Roll-off of Phosphorescent Organic Light-Emitting Diodes. Adv. Funct. Mater. 2014, 24 (8), 1117−1124. (16) Kim, D.-H.; Kim, J. Y.; Kim, D.-Y.; Han, J. H.; Choi, K. C. Solution-Based Nanostructure to Reduce Waveguide and Surface Plasmon Losses in Organic Light-Emitting Diodes. Org. Electron. 2014, 15 (11), 3183−3190. (17) Han, J. H.; Kim, D.; Lee, T.; Lee, H.; Choi, K. C. Angle Insensitive Flexible Color Filter Electrodes. Dig. Tech. Pap. - Soc. Inf. Disp. Int. Symp. 2017, 48 (1), 1738−1741. (18) Yang, C.; Shen, W.; Zhang, Y.; Li, K.; Fang, X.; Zhang, X.; Liu, X. Compact Multilayer Film Structure for Angle Insensitive Color Filtering. Sci. Rep. 2015, 5.10.1038/srep09285 (19) Lee, J. Y.; Lee, K.-T.; Seo, S.; Guo, L. J. Decorative Power Generating Panels Creating Angle Insensitive Transmissive Colors. Sci. Rep. 2015, 4, 4192. (20) Lee, J.-H., Liu, D. N., W, S.-T. Introduction to Flat Panel Displays; John Wiley & Sons, 2008. (21) Takubo, Y.; Hisatake, Y.; Lizuka, T.; Kawamura, T. 64.1: Invited Paper. Dig. Tech. Pap. - Soc. Inf. Disp. Int. Symp. 2012, 43 (1), 869− 872. (22) Shao, D. B.; Chen, S. C. Surface-Plasmon-Assisted Nanoscale Photolithography by Polarized Light. Appl. Phys. Lett. 2005, 86 (25), 1−3.

In the case of the reference device, the transparent electrode and the hole transport layer were deposited on the cleaned substrate by thermal evaporation with a vacuum pressure of less than 6 × 10−6 Torr: WO3 (41 nm)/Ag (12 nm)/WO3 (70 nm). Super yellow (PDY-132) of 65 nm thickness was then spin-coated on the patterned CCE in a nitrogen atmosphere. The substrates were loaded into a thermal evaporation chamber, where the cathode was deposited in a vacuum pressure of less than 6 × 10−6 Torr: Liq (1 nm)/Al (100 nm). A sourcemeter (2400, Keithley) and a spectroradiameter (CS-2000, Konica Minolta) were used to measure the electrical and optical characteristics of the OLED. The refractive indices of Ag, WO3, Al, and PDY-132 were measured experimentally by a spectroscopic ellipsometer (M2000D). Calculations of the optical functions were performed using MATLAB (MathWorks).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.8b00230. Detailed formula for the cavity gain and characteristic matrix; optimization of reference device and results of optical and electrical phenomena of the OLED devices with and without a CCE; angular characteristis of the emission spectrum; fabrication process for the ultra-highresolution display (alternately emitted version) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(K.C. Choi) Phone: +82-42-350-3482. Fax: +82-42-350-8082. E-mail: [email protected]. ORCID

Jun Hee Han: 0000-0002-6397-2275 Kyung Cheol Choi: 0000-0001-6483-9516 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Engineering Research Center of Excellence (ERC) Program supported by National Research Foundation (NRF), Korean Ministry of Science & ICT (MSIT) (Grant No. NRF-2017R1A5A1014708). This research was supported by the Nano·Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2016M3A7B4910635). This work was also supported and funded by LG Display Co., LTD.

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DOI: 10.1021/acsphotonics.8b00230 ACS Photonics 2018, 5, 1891−1897