Nanosinusoidal Surface Zinc Oxide for Optical Out-coupling of

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Nano-sinusoidal Surface Zinc Oxide for Optical Outcoupling of Inverted Organic Light-Emitting Diodes Dohong Kim, Kie Young Woo, Jun Hee Han, Tae-Woo Lee, Ho Seung Lee, Yong-Hoon Cho, and Kyung Cheol Choi ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00710 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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Nano-sinusoidal Surface Zinc Oxide for Optical Out-coupling of Inverted Organic Light-Emitting Diodes Dohong Kim,† Kie Young Woo,‡ Jun Hee Han,† Tae-Woo Lee,† Ho Seung Lee,† Yong-Hoon Cho,‡ and Kyung Cheol Choi*,† †

School of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea

‡

Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea

ABSTRACT: A zinc oxide (ZnO) layer having a random nano-sinusoidal surface on one side is reported. Our new ZnO nanostructure is fabricated in a few minutes at room temperature by simplified one-step nanoparticle lithography utilizing spin-coated SiO2 nanoparticles as a temporal mask. Inverted organic light-emitting diodes adopting the nano-sinusoidal surface ZnO in the bottom cathode exhibit better angular characteristics and 35% enhancement of efficiency in both phosphorescent and fluorescent emission through the liberation of waveguide and surface plasmon modes while maintaining good electrical characteristics and lifetime. The fabrication

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recipe can be freely modified and expanded to create various or more complicated nano-shapes of ZnO films, which can inspire other possible applications of the ZnO nanostructures.

KEYWORDS: nanostructure, nanoparticle, reactive ion etching, lithography, cathode, lifetime

For high-performance inverted organic light-emitting diodes (OLEDs), various efficient cathode structures have been studied and applied using n-type metal oxides, especially zinc oxide (ZnO) or aluminum doped zinc oxide (AZO), and an electron injection interlayer, such as caesium carbonate (Cs2CO3) or polyethylenimine (PEI), for the purpose of higher electron injection rates to balance charges and achieve a high internal quantum efficiency.1–5 However, even though high internal quantum efficiency is attained, much of the generated light in the phosphorescent emitting layer is trapped inside OLEDs as waveguide modes from the total internal reflection by using high refractive index layers or as surface plasmon modes from the existence of dielectric and metal layers.6 Thus far, several approaches, such as the use of substrate modification,7 microlenses,8 scattering layers,9 microcavity,10 or nanostructures,11–13 have been proposed to enhance the out-coupling efficiency.14 In particular, quasi-periodic nanostructures inside OLEDs are efficient to extract both waveguide and surface plasmon modes without significant change of the spectra in viewing angles.15–17 Fabricating nanostructures occasionally requires high temperature, high-cost or complicated processes and techniques such as electron beam lithography, laser interference lithography, or nanoimprinting with molding and replicating.18–21 To fabricate nanostructures more simply, nanoparticles can be employed with a solution process at low cost.15,22,23 However, additional


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nanoparticles or extra layers with residues could alter the electrical characteristics, charge balance, and lifetime of an optimized OLED. Furthermore, in the case of inserting insulating nanostructures between the substrate and the bottom electrode, contact between a TFT and an OLED pixel would be unfeasible without an additional nanostructure patterning process.24–26 Additionally, only a few papers have reported on the lifetime of OLEDs having out-coupling nanostructures,26,27 presumably due to the usual adverse effect of locally high current densities or defects on the lifetime of OLEDs depending on the nanostructure’s shape and fabrication methods.28–30 Therefore, an alternative way of easily fabricating nanostructure by utilizing a simple and low-cost nanoparticle solution process to improve the out-coupling efficiency of an inverted OLED without changing the electrical characteristics or deteriorating the lifetime of the device should be developed. In this paper, a fabrication method to prepare a ZnO film where one side of the film has randomly nano-sinusoidal grooves is presented. The nano-sinusoidal surface ZnO film (sin. ZnO film) was easily fabricated by simple nanoparticle lithography using spin-coated SiO2 nanoparticles with one step etching in a few minutes. An inverted phosphorescent OLED with a bottom cathode containing the sin. ZnO film is also described. Compared with an OLED with a flat ZnO film, the OLED with the sin. ZnO film showed higher efficiency without change of the electrical characteristics, angular spectrum dependence or lifetime deterioration due to the broad periodicities and smooth shape of the sin. ZnO film.

RESULTS AND DISCUSSION Nano-sinusoidal surface of ZnO nanostructure from nanoparticle lithography. Figures 1a-d show schematic illustrations of the fabrication steps for the sin. ZnO film, and Figures 1e-g show actual SEM images. A flat ZnO film is prepared by atomic layer deposition for easy


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control over the thickness and to produce a neat film (Figure 1a). SiO2 nanoparticles of 350 nm size are simply spin-coated and are randomly distributed as a monolayer on the ZnO film, as shown in Figures 1b,e.15 The SiO2 nanoparticles and ZnO film are simultaneously etched by the reactive ions of CF4 and Ar gases in an ICP-RIE system (Figure 1c). After the one-step etching process, the SiO2 nanoparticles are fully etched in a few minutes without need for a further stripping process, and the surface of ZnO naturally has a randomly sinusoidal shape with tens of nanometers of groove depth (Figures 1d,f,g), following the difference in the etching rates of the SiO2 nanoparticle and the ZnO film.31,32 Details of the fabrication strategy are provided in the methods section later.

Figure 1. Schematic illustrations and SEM images of fabrication steps for a ZnO film with a randomly nano-sinusoidal surface on one side (sin. ZnO): (a) flat ZnO film by atomic layer deposition, (b) spin-coated SiO2 nanoparticles on flat ZnO, (c) reactive ion etching using CF4 and Ar gases on SiO2 nanoparticles and the ZnO film, and (d) sin. ZnO. SEM images of (e) spincoated SiO2 nanoparticles on the flat ZnO in a top view, (f) sin. ZnO in a top view, and (g) sin. ZnO in a tilted view.


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The characteristic factors of the ZnO nanostructure could be easily controlled in this nanoparticle lithography method. The surface periodicity of the sin. ZnO nanostructure follows the size and density of the SiO2 nanoparticles. The groove depth of the sin. ZnO nanostructure is changed according to the flow rate of CF4 gas in the RIE step (Figures 2a-d). As the CF4 flow rate is increased, the etch rate of SiO2 nanoparticles rises and the groove depth of the ZnO films subsequently decreases since the mask of SiO2 nanoparticles for the sin. ZnO nanostructure is early etched out while the etch rate of the ZnO films, based on the physical etching of Ar gas, is not severely changed. Since the groove depth of the ZnO nanostructure is adjustable in a broad range, as shown in Figure 2e, the groove depth can be almost independently controlled even though the size of the SiO2 nanoparticles is changed.

Figure 2. SEM images and groove depth of sin. ZnO films from the reactive ion etching process of CF4 and Ar gases: (a) 0 sccm of CF4, (b) 4 sccm of CF4, (c) 8 sccm of CF4, and (d) 12 sccm of CF4. (e) Groove depth of the sin. ZnO films fabricated by varying the flow of CF4 gas.


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Inverted phosphorescent OLED with ZnO nanostructure of randomly nano-sinusoidal surface. The sin. ZnO films prepared by the suggested simple fabrication method combining SiO2 nanoparticles and a RIE process using CF4 and Ar gases have potential to be useful in various fields related to nanostructures. For example, performance enhancement of organic photovoltaics or OLEDs is possible by manipulating light paths. Here, a sin. ZnO film with broad quasi-periodicities of a few hundred nanometers from randomly distributed 350 nm SiO2 nanoparticles and a groove depth of approximately 60 nm is fabricated to effectively enhance the out-coupling efficiency of OLEDs.30,33 The structures of the devices used in this paper are illustrated in Figure 3. The flat or sin. ZnO film on ITO used as a cathode is shown in Figures 3a,b. The successive layers above the ZnO films were thermally evaporated and deposited for efficient and reliable red inverted phosphorescent OLEDs (Figures 3c,d). The Cs2CO3 layer was used as an electron injection layer. The Ir(piq)3 dopant in the Bebq2 host allowed phosphorescent emission. The NPB and MoO3 layers were employed to transport holes from the Al anode.


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Figure 3. SEM images of (a) flat ZnO film and (b) sin. ZnO film on ITO coated glasses in a tilted view. Schematic illustrations of red inverted phosphorescent OLEDs with (a) flat ZnO film or (b) sin. ZnO film.

Figure 4 shows the performance of the red inverted OLEDs with the flat or sin. ZnO film. The graphs of current density versus voltage of the OLEDs are similar (Figure 4a), which shows the electrical characteristics were not altered. The generally occurring voltage drop phenomenon29,34 from the effectively reduced organic layer thickness around the inclined side of nanostructures did not occur because the length from the bottom side of the sin. ZnO nanostructure to the top of the nano-sinusoidal ZnO peak is greater than the thickness of the flat ZnO and hence the increased ZnO path appears to compensate the effectively reduced organic layers. In this aspect, layering the out-coupling ZnO nanostructure above a flat electrode would have strengths in


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designing an OLED structure. The current efficiency of the red OLED with the sin. ZnO film is 11.5 cd/A at 1,000 cd/m2, which is an improvement of 35% from the value of 8.5 cd/A of the OLED with the flat ZnO film, as shown in Figure 4b. The external quantum efficiency (EQE) and power efficiency were calculated from measured data with variation of the viewing angles from 0o to 70o by 10o degrees. The EQE and power efficiency of the OLED with the sin. ZnO are also enhanced at all luminance levels compared with the OLED with the flat ZnO, as shown in Figures 4c,d. The overall optical efficiency was improved due to the enhanced out-coupling efficiency from the structural change that extracts waveguide and surface plasmon modes. Other sin. ZnO films of lower depths, or smaller structural change, showed insufficient out-coupling enhancement (Supporting Information Figure S1).


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Figure 4. Characteristics of the red inverted phosphorescent OLEDs with the flat ZnO (black) or the sin. ZnO (red) having a depth of 60 nm and a broad quasi-periodicity from 400 nm to 800 nm: (a) current density-voltage (b) current efficiency-luminance, (c) external quantum efficiency (EQE)-luminance, and (d) power efficiency-luminance.

The angular characteristics of the devices with the flat ZnO or the sin. ZnO were compared through luminous intensity and electroluminescence (EL) spectra according to the viewing angle at constant same current driving (Figures 5,6). As shown in Figure 5, the luminous intensity of the OLED with the sin. ZnO is greater than that of the OLED with flat ZnO at all measured


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angles, and the angular distributions of the intensities of the OLEDs are similar for both with the case of a Lambertian source without distinct angular dependence even though the ZnO nanostructure exists. Figure 6 shows that there is less change in the shape of the EL spectra of the OLED with the sin. ZnO depending on the viewing angle.

Figure 5. Normalized luminous intensity of the OLEDs with the flat ZnO (black) or the sin. ZnO (red) having a depth of 60 nm and a broad quasi-periodicity from 400 nm to 800 nm according to the viewing angle.


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Figure 6. Normalized electroluminescence (EL) spectra of the OLEDs with (a) the flat ZnO and (b) the sin. ZnO having a depth of 60 nm and a broad quasi-periodicity from 400 nm to 800 nm according to the viewing angle.

In addition to phosphorescent OLEDs, fluorescent OLEDs were also fabricated and measured. The structures of Alq3-based green inverted fluorescent OLEDs are shown in Supporting Information Figure S2. The enhancement in the performance and optical characteristics of the fluorescent OLED with the sin. ZnO was respectively the same as that of the phosphorescent OLED (Figures S3,S4). The constantly enhanced luminous intensity without any spectrum change depending on the viewing angle of the sin. ZnO-based OLED results from the quasi-periodicities of the nanostructure. A periodic structure extracts a specific matched wavelength light of trapped modes in an OLED to a specific angle by Bragg diffraction condition, which results in improved out-coupling efficiency but also angular dependence.35 By using nanostructures with broad periodicities, the overall wavelengths of guided modes at varying angles are matched and light escapes from the OLED devices.15 Figure 7a shows the distribution of electric field intensities


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from the waveguide mode, surface plasmon mode, and hybrid mode of TE0, TM0, and TM1 inside the OLED calculated by a transfer matrix; the Cs2CO3 layer of 1 nm was ignored because of the ineffective thickness in this optical calculation. The wavelengths of light, matching the periods of the nanostructure, to be out-coupled from each mode in a direction perpendicular to the substrate are calculated (Figure 7b). Due to the broad region of periods from 400 nm to 800 nm in the sin. ZnO, as shown in Figure 7c, the guided modes are diffracted and extracted over the entire emission wavelengths, which results in an enhanced spectrum of the OLED with the sin. ZnO whereas the spectrum shape of the flat OLED is unchanged (Figure 7d).


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Figure 7. Optical out-coupling of the OLED with the sin. ZnO by diffraction of trapped modes: (a) calculated normalized electric field intensity of the trapped modes in the OLED structure of glass/ITO(150 nm)/ZnO(30 nm)/Bebq2:Ir(piq)3 8%(70 nm)/NPB(70 nm)/MoO3(5 nm)/Al(100 nm) at 626 nm wavelength of the peak emission. TE or TM respectively denotes a transverse electric or transverse magnetic mode. (b) Calculated relation between a grating period and a matched wavelength of light that would be out-coupled by Bragg diffraction of the trapped modes (TE0 (blue), TM0 (red), and TM1 (orange) modes). The greenish shaded region represents the periods on the sin. ZnO film. (c) Averaged and normalized power spectrum density (PSD) of the morphology of the sin. ZnO film. Four 2.5 um x 2.5 um AFM images of the sin. ZnO fabricated from randomly distributed 350 nm SiO2 nanoparticles as inset. (d) Relative


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EL spectra of the OLEDs with the flat ZnO (black) or the sin. ZnO (red) having quasi-periodicity of a few hundred nanometers and a depth of 60 nm.

Figure 8 shows the operating lifetime of the OLEDs at constant current driving for the initial luminance of 1,000 cd/m2, which is a high level of luminance considering red emission devices. The luminance changed slightly within a few hours (Figure S5), which is thought to result from the stabilized interface and breaking the leakage path while aging the devices electrically at the early stage. The lightly higher luminance of the OLED with the sin. ZnO is considered under disadvantageous conditions in the lifetime test. The lifetime of the OLED with sin. ZnO does not show a sharp decline but rather is similar to that of the OLED with the flat ZnO. The lifetime LT80, the time of the luminance to reduce to 80% of the initial luminance, of the OLED with the sin. ZnO was 507 h, which is much higher than that (165 h) of an inverted OLED having the same emission layer reported in other paper, and is similar to the value (482 h) of a stable noninverted OLED.36,37 The stable operation of the sin. ZnO-based OLED results from not only lower current driving from efficiency enhancement but also the smooth change of the surface slope on the nano-sinusoidal structure, in contrast with the abrupt slop change of stepwise or protruding structures causing locally much strengthened current paths that degrade organic layers.


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Figure 8. Operating lifetime of the OLEDs with the flat ZnO or the sin. ZnO at each constant current for the initial luminance of 1,000 cd/m2.

CONCLUSIONS A randomly sinusoidal surface of ZnO on one side with periods of hundreds of nanometers and groove heights of tens of nanometers was fabricated by a simplified nanoparticle lithography strategy using a one-step RIE process of CF4 and Ar gases in a few minutes and utilizing spincoated SiO2 nanoparticles as a temporal mask. The replacement of the flat ZnO by the sin. ZnO constituting a cathode enhanced the efficiency of the inverted OLEDs by out-coupling guided modes. Angular dependence was diminished due to the broad periods of the sin. ZnO film. The relatively stable operating and unaltered electric characteristics of the OLED are caused by the smooth shape of the sin. ZnO even though the OLED was nanostructured. Additionally, the possible various shapes of ZnO nanostructures by free modification of the suggested fabrication method may expand their area of application.

METHODS


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Synthesis of SiO2 nanoparticles solution. SiO2 nanoparticles of approximately 350 nm in size were synthesized based on the Stöber method at room temperature.15,38 In detail, tetraethyl orthosilicate (TEOS, 99.999%, Sigma-Aldrich) 7.5 mL and ethanol 26.5 mL were mixed and stirred by a stir bar at 700 rpm for 10 min. After adding DI water 10.8 mL for hydrolysis, the solution was further stirred for another 10 min. Ammonia 0.7 mL (NH3 28-30%, Sigma-Aldrich) was lastly added and the solution was stirred for 3 hours. The reaction of synthesizing SiO2 nanoparticles was finished and the SiO2 nanoparticles were about 350 nm in size. Fabrication of nano-sinusoidal surface ZnO nanostructure. The ZnO film was deposited by an ALD system (LucidaTM D100, NCD) using diethylzinc (DEZ) precursor and H2O reactant at a chamber temperature of 140 oC.39 The synthesized SiO2 nanoparticle solution diluted with ethanol by 50% v/v in this paper was spin-coated on a cleaned ZnO film at 3,000 rpm for 40 sec. The 350 nm SiO2 nanoparticles were properly distributed as a monolayer at random positions on the ZnO film. The sample of SiO2 nanoparticles on the ZnO film was transferred to an Oxford Plasmalab 100 ICP system, and then etched under parameters of 200W ICP power, 150W bias power, 4 sccm CF4 gas, 20 sccm Ar gas, and 10 mTorr pressure. As a result, a randomly nanosinusoidal surface of the ZnO film was fabricated with a peak-to-peak depth of approximately 60 nm while SiO2 nanoparticles were fully etched. The etch rate of the ZnO film was approximately 30 nm/min in this setup. Fabrication of OLEDs. The flat or sin. ZnO film on ITO coated glass was patterned for a bottom cathode by typical photolithography using CR-7 chromium etchant (CYANTEK Corporation). The successive layers for OLEDs were deposited in a thermal evaporator with shadow masks for an active area of 9 mm2 at a base pressure of 3×10-6 Torr. The deposition rate and thickness of the materials were monitored by a quartz crystal oscillator. The detailed


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structure of phosphorescent red inverted OLEDs is as follows: Glass/ITO(150 nm)/flat or sin. ZnO(base 30 nm)/Cs2CO3(1 nm)/Bebq2:Ir(piq)3 8%(70 nm)/NPB(70 nm)/MoO3(5 nm)/Al(100 nm). The OLEDs were encapsulated with a glass-lid and a UV-curable sealant (XNR5570-Ba, Nagase Chemtex). Measurement of OLEDs. Voltage-current-luminance characteristics and EL spectra of the OLEDs were measured using a Keithley 2400 sourcemeter and a Konica Minolta CS-2000 spectroradiometer controlled by a computer. The luminous intensity and EL spectra at different viewing angles were measured by rotating a stage with OLEDs. Based on the data at different viewing angles with constant current driving, the EQE and power efficiency of the OLEDs were calculated. The lifetime of the OLEDs was measured by driving OLEDs at a constant current for initial luminance of 1,000 cd/m2.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Author Contributions D.K. conceived of the presented idea. D.K. and K.Y.W. planned and carried out the experiments. D.K. performed the measurements. D.K., J.H.H., T.-W.L., and H.S.L. contributed to the interpretation of the results. D.K. took the lead in writing the manuscript. All authors provided critical feedback and helped shape the research, analysis, and manuscript.

ACKNOWLEDGMENT


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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) and was also supported by the Technology Innovation Program (20000489, Interactive fiber based wearable display platforms for clothing displays) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).

ASSOCIATED CONTENT Supporting Information Available: Relative EL spectra of the OLEDs with the sin. ZnO films of different depths; characteristics of a green inverted fluorescent OLED with the flat ZnO or sin. ZnO; luminance change at the early stage in the lifetime test (PDF). This material is available free of charge via the Internet at http://pubs.acs.org

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Hauss, J.; Riedel, B.; Bocksrocker, T.; Gleiss, S.; Huska, K.; Geyer, U.; Lemmer, U.; Gerken, M. Periodic Nanostructures Fabricated by Laser Interference Lithography for


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For Table of Contents Use Only


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Figure 1. Schematic illustrations and SEM images of fabrication steps for a ZnO film with a randomly nanosinusoidal surface on one side (sin. ZnO): (a) flat ZnO film by atomic layer deposition, (b) spin-coated SiO2 nanoparticles on flat ZnO, (c) reactive ion etching using CF4 and Ar gases on SiO2 nanoparticles and the ZnO film, and (d) sin. ZnO. SEM images of (e) spin-coated SiO2 nanoparticles on the flat ZnO in a top view, (f) sin. ZnO in a top view, and (g) sin. ZnO in a tilted view. 177x79mm (300 x 300 DPI)


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Figure 2. SEM images and groove depth of sin. ZnO films from the reactive ion etching process of CF4 and Ar gases: (a) 0 sccm of CF4, (b) 4 sccm of CF4, (c) 8 sccm of CF4, and (d) 12 sccm of CF4. (e) Groove depth of the sin. ZnO films fabricated by varying the flow of CF4 gas. 177x64mm (300 x 300 DPI)


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Figure 3. SEM images of (a) flat ZnO film and (b) sin. ZnO film on ITO coated glasses in a tilted view. Schematic illustrations of red inverted phosphorescent OLEDs with (a) flat ZnO film or (b) sin. ZnO film. 177x110mm (300 x 300 DPI)


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Figure 4. Characteristics of the red inverted phosphorescent OLEDs with the flat ZnO (black) or the sin. ZnO (red) having a depth of 60 nm and a broad quasi-periodicity from 400 nm to 800 nm: (a) current densityvoltage (b) current efficiency-luminance, (c) external quantum efficiency (EQE)-luminance, and (d) power efficiency-luminance. 177x140mm (300 x 300 DPI)


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Figure 5. Normalized luminous intensity of the OLEDs with the flat ZnO (black) or the sin. ZnO (red) having a depth of 60 nm and a broad quasi-periodicity from 400 nm to 800 nm according to the viewing angle. 84x63mm (300 x 300 DPI)


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Figure 6. Normalized electroluminescence (EL) spectra of the OLEDs with (a) the flat ZnO and (b) the sin. ZnO having a depth of 60 nm and a broad quasi-periodicity from 400 nm to 800 nm according to the viewing angle. 177x73mm (300 x 300 DPI)


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Figure 7. Optical out-coupling of the OLED with the sin. ZnO by diffraction of trapped modes: (a) calculated normalized electric field intensity of the trapped modes in the OLED structure of glass/ITO(150 nm)/ZnO(30 nm)/Bebq2:Ir(piq)3 8%(70 nm)/NPB(70 nm)/MoO3(5 nm)/Al(100 nm) at 626 nm wavelength of the peak emission. TE or TM respectively denotes a transverse electric or transverse magnetic mode. (b) Calculated relation between a grating period and a matched wavelength of light that would be out-coupled by Bragg diffraction of the trapped modes (TE0 (blue), TM0 (red), and TM1 (orange) modes). The greenish shaded region represents the periods on the sin. ZnO film. (c) Averaged and normalized power spectrum density (PSD) of the morphology of the sin. ZnO film. Four 2.5 um x 2.5 um AFM images of the sin. ZnO fabricated from randomly distributed 350 nm SiO2 nanoparticles as inset. (d) Relative EL spectra of the OLEDs with the flat ZnO (black) or the sin. ZnO (red) having quasi-periodicity of a few hundred nanometers and a depth of 60 nm. 177x140mm (300 x 300 DPI)


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Figure 8. Operating lifetime of the OLEDs with the flat ZnO or the sin. ZnO at each constant current for the initial luminance of 1,000 cd/m2. 84x63mm (300 x 300 DPI)


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Nanosinusoidal Surface Zinc Oxide for Optical Out-coupling of

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