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We examined the light diffusing effects of nano and micro-structures on microcavity designed OLEDs. The results of FDTD simulations and experiments showed that the pillar shaped nano-structure was more effective than the concave micro-structure for light diffusing of microcavity OLEDs. The sharp luminance distribution of the microcavity OLED was changed to near Lambertian luminance distribution by the nano-structure, and light diffusing effects increased with the height of the nano-structure. Furthermore, the nano-structure has advantages including light extraction of the substrate mode, reproducibility of manufacturing process, and minimizing pixel blur problems in an OLED display panel. The nano-structure is a promising candidate for a light diffuser, resolving the viewing angle problems in microcavity OLEDs.
© 2014 Optical Society of America
1. Introduction
Organic light emitting diode (OLED) display panels are employed in various devices such as smartphone displays and televisions. Continuous technological development has led to an expansion of applications of OLED display panels. An OLED display has several advantages over a liquid crystal display (LCD) due to the absence of a backlight unit. OLED display panels are thinner, lighter, and can be more flexible than LCD panels. They also have a high contrast ratio and relatively wide viewing angle. However, the electric power consumption of an OLED television is currently somewhat higher than that of an LCD television of the same size, due to the low out-coupling efficiency of the OLED display panel [1]. Highly efficient OLEDs are being actively developed, and they are becoming more important for their potential application to flat panel displays and lighting [2–4]. Among approaches for realizing OLED display panels with high power efficiency, methods of OLED light extraction have been attracting great attention. Several trials have been conducted introducing new structures for enhancing light extraction efficiency by out-coupling of the waveguide mode in an OLED-stack and substrate.
For example, microstructuring [5–7], microlenses [8–10], microcavities [11–14], nano-structures [15–17], a high index substrate [18], and photonic crystals [19] have been investigated. Those methods increase the light extraction efficiency significantly, but many of them have serious drawbacks, such as the complexity and difficulty of the fabrication process.
The micro-cavity effect can be used to extract light efficiently in an OLED display panel by using a relatively simple process. The microcavity enhances color saturation through spectrum narrowing, as well as luminous efficacy. However, typical microcavity structures will result in a large angular dependence on the color and luminance of the emitting light [20], which causes serious problems for display applications. In the case of top-emitting OLEDs (TOLEDs), a microcavity effect also exists, but it also leads to the strong viewing-angle dependence mentioned above [21–23].
In the microcavity OLED, organic layers are sandwiched between the reflective electrode and semitransparent electrode. This structure is effectively a Fabry-Perot resonator, which induces the microcavity effect in OLEDs. The cavity length is fixed by the distance between the two parallel electrodes and defines the resonant wavelength. Only a certain wavelength corresponding to the fixed cavity length is emitted in a given angle, which results in the narrowed and angular-dependent emission. The spectrum narrowing is basically good for the image quality of the display panel because color saturation increases. However, the viewing-angle characteristic is a key issue for high quality flat panel displays, and thus angular-dependent emission intensity and peak wavelength need to be resolved in order to use the microcavity effect.
Several methods have been proposed for resolving the viewing angle problems in microcavity OLEDs, including an irregular micro lens array (MLA) [24], dispersive gratings [25], sand-blasted substrate [26], microstructured cavity [27], and optimization of the microcavity [28]. However, their suppression of the angular dependency is insufficient, or the size of the light diffusing structure is as large as several tens of micrometers. The OLED lighting panels based on white OLEDs cannot use the strong microcavity effect, thus the microcavity effect is useful only for OLED display panels composed of red, green and blue sub-pixels. Several tens of micrometers is comparable to the size of the sub-pixel of an OLED display. Large-sized structures, such as MLA, used light diffusing may generate a pixel blur problem in an OLED display panel unless the structure is precisely aligned with a pixel. Therefore the light diffusing structure should have a much smaller size than that of the sub-pixel of an OLED display.
We developed a random nano-structure (RNS) and micro concave structure (MCS) for a light diffuser to reduce the viewing angle problems. The sizes of the RNS and MCS were several hundred nanometers and smaller than 5 um, respectively.
2. Experimental
Firstly, the RNS was manufactured by the following process. On a glass substrate, a 500 nm-thick SiO2 layer and a 60 nm-thick Ag film were deposited sequentially. SiO2 and Ag were deposited by plasma-enhanced chemical vapor deposition and thermal evaporation methods, respectively. To form the Ag dewetting mask, the Ag-coated sample was heated to 400 °C. The Ag droplets resulting from the dewetting of the Ag thin films were used as a hard mask to dry etch the SiO2 film on glass. Ag thin films were agglomerated and formed nano-sized particles with a random size distribution on the SiO2 layer/glass substrate through solid or liquid state dewetting. The exposed region between the Ag droplets was dry etched using an inductively coupled plasma (ICP) method to form random nano-scale ellipse pillars. After the dry etching process, the Ag mask was removed using diluted HNO3. The height and diameter of the resultant nano-structures were about 360 nm and 100 - 500 nm, respectively. This method is cost effective and scalable.
Secondly, the MCS was fabricated by a two-step dipping process. The first etching process (frosting process) used a solution that was mainly composed of NH4F, and the second etching (smoothing process) was carried out in a HF mixed solution. The nonuniform etching rate on the glass surface was considered to be the principal driving force, producing an irregular concave structure [29]. The MCS manufacturing processes were carried out by U plus Vision Co., Ltd.
The luminance angular distributions of microcavity OLED devices combined with RNS and MCS were measured to evaluate their light diffusing power. The RNS and MCS were formed on the outer surface of the glass substrate of the OLEDs. The schematic diagram of the microcavity OLED equipped with RNS or MCS is shown in Fig. 1.
The bottom emission type OLEDs in this study were fabricated using the following configuration: ITO (40 nm)/Ag (20 nm)/ITO (40 nm)/ NPB (40 nm)/HAT-CN (10 nm)/NPB (30 nm)/HAT-CN (10 nm)/NPB (30 nm)/HAT-CN(10 nm)/NPB (35 nm)/TCTA (5 nm)/DCzPPy:Irppy3(3%) (20 nm)/BmPyPB(55 nm)/LiF (1 nm)/Al (120 nm), where NPB is N,No-diphenyl-N,No-bis(1-naphthyl)-1,1°-biphenyl-4,4-diamine, HAT-CN 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile, TCTA tris(4-carbazoyl-9-ylphenyl)amine, DCzPPy 2,6-bis-[3-(carbazol-9-yl)phenyl]pyridine, and BmPyPB 1,3-bis(3,5-dipyrid-3-yl-phenyl)benzene. The OLED-stack and cathode were deposited by a thermal evaporation method in a high vacuum chamber below 5X10−7 torr on an ITO/Ag/ITO coated glass substrate. The OLED grade materials were purchased and used without further purification.The microcavity was formed between cathode (the reflective electrode; Al film) and anode (the semitransparent electrode; ITO/Ag/ITO film). The OLED devices were transferred directly from a vacuum into an inert environment of a glove box, where they were encapsulated using a UV-curable epoxy, and a glass cap with a moisture getter. The luminance angular distribution and electroluminescence (EL) spectra were measured with a goniometer equipped spectroradiometer (Minolta CS-2000), at a constant current density level of 1.5 mA/cm2. Hazes of the RNS and MCS on glass substrates were measured with a hazemeter (Nippon Denshoku Industries NDH-5000) with JIS K-7361 method.
3. Results and discussion
3.1 Light diffusing by random nano-structure
The light diffusing effects were examined on microcavity OLEDs by comparing the luminance distribution of microcavity OLED devices without RNS, and with RNS. Figure 2 shows the morphology of the RNS surface which was like an irregular ellipse pillar array. A rectangular RNS cross-section was fabricated due to the dry etching process. The ICP etching process with a Ag dewetting mask made an irregular ellipse pillar array, thus the cross-section of each pillar was determined by the Ag dewetting mask, and the height of the pillar could be controlled by etching time. The diameter of the RNS was 100 - 500 nm. RNS with 200 nm height (RNS 200), 350 nm height (RNS 350) and 500 nm height (RNS 500) showed 11.7%, 21.2% and 27.4% of haze, respectively. The haze of the RNS increased with the height of the RNS.
Fig. 2 SEM images of the surface of the RNS (a), cross-section of RNS 200 (b), RNS 350 (c), and RNS 500 (d).
Haze represents the fraction of light which is diffused away from the normal incident direction, thus optical structures with high haze may disperse the incident light effectively. Figure 3 shows luminance distributions of the microcavity OLEDs with and without RNS. The luminance in the direction perpendicular to the substrate (0°) of the microcavity OLED was significantly larger than that of high (inclined) angle.
Fig. 3 Luminance distribution curves of the reference microcavity OLED, OLED with RNS 200, RNS 350, and RNS 500.
The difference in luminance between 0° and high angles decreased in the OLEDs with RNS, and it decreased with the increasing height of the RNS. The luminance distribution of the microcavity OLED was changed near to Lambertian distribution with the increasing height of the RNS. A notable point regarding the luminance distribution curves of the RNS is that the luminance of 0° didn’t decrease. This is an important factor for a light diffusing layer on display panels because high luminance of the front view can enhance the quality of image.
Figure 4 shows spectrum changes with viewing angle of the microcavity OLEDs with and without RNS. Peak wavelengths of 0° spectrum and 70° spectrum for the microcavity OLED were 508 nm and 511 nm, respectively; it shifted by 3 nm. Those for the microcavity OLED with RNS were 509 nm and 510 nm, respectively, irrespective of RNS height. The shifts of peak wavelength for RNS’s were 1 nm which was smaller than that of the reference microcavity OLED. The color differences depending on viewing angle are significantly reduced by the RNS and decreased with increasing the RNS height as shown in Table 1.. The difference of peak intensity between 0° and high angle for the RNS were remarkably decreased compared to that of the microcavity OLED. Peak intensity ratios of 0° to 70° for the microcavity OLED, RNS 200, RNS 350 and RNS 500 were 0.52, 0.62, 0.68 and 0.81, respectively.
Fig. 4 Normalized emission spectra of the reference microcavity OLED (a), OLED with RNS 200 (b), RNS 350 (c), and RNS 500 (d).
Table 1. Optical Properties of Microcavity OLEDs with RNS and MCS
It is considered that the scattering effect produced by the irregular ellipse pillars of the RNS not only expand luminance distribution but also improve spectral change with viewing angle. The RNS scattering mechanism was considered to be Rayleigh scattering because the scattering particle of RNS was smaller than the wavelength of incident light. Rayleigh scattering power is represented as,
where σs is the scattering cross-section, d the diameter of particle, λ wavelength of incident light, n refractive index of particle [30]. Therefore, the scattering power of RNS increased with RNS height, as shown in the results of the experiments.External quantum efficiencies (EQEs) were significantly enhanced by the RNS. EQEs of the reference microcavity OLED, and the OLED with RNS 200, RNS 350, and RNS 500 were 9.67%, 10.91%, 11.45% and 12.13%, respectively. The enhancements of EQE for RNS 200, RNS 350 and RNS 500 were 12.8%, 18.4% and 25.4%, respectively. The RNS equipped OLEDs showed an improvement of luminous efficacy as well as luminance distribution of the microcavity OLED. The RNS is considered to have the function of light extraction from the substrate mode in addition to scattering power, to expand luminance distribution. The enhancement of light extraction increased with the height of the RNS. Several optical characteristics are shown in Table 1.
3.2 Light diffusing by micro concave structure
We examined the light diffusing effects of the MCS, as had been done with the RNS. Microstructures on the substrate surface should affect the luminance distribution of bottom emission type OLEDs. Figure 5 shows the morphology of the microstructure of the MCS surface. It looks like irregular concave pits. The irregular shape of the MCS is thought to have originated from the nonuniform wet etching process. The MCS with 42.0% haze (MCS 42) was manufactured by the frosting process, and that with 18.3% haze (MCS 18) was manufactured by the frosting and smoothing process. MCS 42 showed a very rough and irregular surface morphology, and it was composed of multiple craters, with large and shallow (larger than one micrometer) craters containing small and sharp craters. The concave structure of MCS 18 was relatively simple. It was composed of shallow craters with about 2 μm diameters. The smoothing process appeared to etch out the small craters resulting in a relatively smooth surface.
The luminance distribution of the microcavity OLED was changed near to Lambertian distribution by applying the MCS 42, while it was relatively far from Lambertian distribution for the MCS 18. The scattering mechanisms of MCS 18 and MCS 42 are considered to be different from each other, because the surface of MCS 18 consisted of relatively smooth concave pits with a size larger than 1 μm, while the surface of MCS 42 was nearly an irregularly rough surface. Light incident to the surface will be scattered by a combination of refraction, reflection, and diffuse scattering for the MCS 18, and almost by diffuse scattering for the MCS 42.
Figure 6 shows the luminance distributions of the microcavity OLEDs with and without MCS. The luminance difference between 0° and the high angles decreased in the OLEDs with MCS, and it decreased with the increasing haze of the MCS, as was the case with the RNS.
Fig. 6 Luminance distribution curves of the reference microcavity OLED, OLED with MCS 18 and MCS 42.
Figure 7 shows the spectrum change with viewing angle of the microcavity OLEDs with and without MCS. Peak wavelengths shift between 0° and 70° spectrum was 3 nm for the reference microcavity OLED. The shifts of peak wavelength for MCS 18 and MCS 42 were also 3 nm, the same as the reference OLED. However, differences of peak intensity between the 0° and 70° spectrum for MCS 18 and MCS 42 were remarkably decreased, compared to that of the reference microcavity OLED. Peak intensity ratios of 0° to 70° for the reference microcavity OLED, MCS 18 and MCS 42 were 0.52, 0.64 and 0.73, respectively. Optical characteristics depending on viewing angle were improved by the MCS. It is thought that the scattering effect produced by the irregular shape of the MCS can also resolve the viewing angle problems of the OLED with microcavity structure.
Luminance distribution was improved by the MCS, whereas external quantum efficiency (EQE) was not significantly enhanced by the MCS. EQEs of the reference microcavity OLED, and the OLED with MCS 18 and the OLED with MCS 42 were 9.67%, 9.77% and 10.44%, respectively. The enhancement of EQE was just 8%, even in the case of MCS 42, whose haze was 42.0%. And the luminance at 0° decreased compared to the reference OLED, which is a disadvantage for a display panel application.
To compare the scattering power of the MCS and the RNS, the ratio of luminance at 40° to the luminance at 0° was calculated. The ratio for the reference microcavity OLED was 0.74. Those for RNS 200, RNS 350 and RNS 500 were 0.85, 0.90 and 0.98, respectively. Those for MCS 18 and MCS 42 were 0.85 and 0.89, respectively. The scattering power and the light extraction of the RNS were higher than those of the MCS considering their haze. Furthermore, the RNS has advantages for manufacturing display panels because its size is much smaller than the pixel size and it can be fabricated by a highly controllable and reproducible process. The small size of the scattering center is advantageous to reduce pixel blur problems.
3.3 FDTD and ray tracing simulations
Optical modelling using the finite difference time domain (FDTD) simulation method has been widely used for the prediction and analysis of optical effects of submicron structures in OLEDs [31, 32]. The light diffusing effects of MCS and RNS on a microcavity OLED were analyzed by two dimensional (2-D) FDTD simulation method. The FDTD simulation program provided by Lumerical Solutions, Inc. was used in this study.
A typical microcavity OLED consisting of ITO/Ag/ITO, OLED-stack and Al cathode on the glass substrate was designed for the simulation with the FDTD program, as shown in Fig. 8.A hole transporting layer, a light emitting layer and an electron transporting layer in the OLED-stack have similar refractive indices and optical absorption coefficients, thus they were regarded as one optical layer. Considering the total calculation time and required memory, the size of the FDTD simulation was limited. The size of simulation was 30.0 μm (width) x 13.3 μm (height), and the thickness of the glass substrate was 10 μm.
Fig. 8 Schematic diagrams of the model of FDTD simulation for the microcavity OLED; (a) is for OLED with flat glass, (b) for the MCS 10 μm, and (c) for the RNS 500 nm.
The RNS and the MCS were set between glass and air and were represented by cross-section in the FDTD models. The RNS was represented by rectangles with 500 - 600 nm width, and the MCS by partial circles with 250 - 500 nm height. For the FDTD simulation of an OLED, a planar light source is preferable to a single point light source because the single point source gives significantly different results depending on the location of the source [31]. But a planar light source cannot be expressed perfectly in an FDTD simulation.
We carried out twenty FDTD simulations with 20 sets of dipole sources in which the dipole sources in one set had an x, y and z direction, respectively. Each set of dipole sources was set at a different location, determined randomly, to avoid interference among the light sources. The center wavelength and full width at half maximum (FWHM) of the source were 515 nm and 80 nm, respectively. After the simulations, all the resulting electric field power values depending on viewing angle, through far field calculation for each set of FDTD simulations, were summed arithmetically.
We compared the electric field power distributions calculated from the FDTD simulations depending on the height of the RNS and radius of the MCS, as shown in Fig. 9.The angle widths at half maximum of calculated power distributions (half width) were used to compare the light diffusing effects of RNS and MCS. The half width of the microcavity OLED was 85.3°. The half widths for the OLED with MCS decreased a little compared to the reference microcavity OLED. The half widths for 1 μm, 5 μm, 10 μm and 50 μm radius MCS were 80.5°, 80.7°, 78.1° and 83.0°, respectively.
Fig. 9 Normalized power distribution curves depending on viewing angle calculated by FDTD simulations for the OLED with MCS (a) and RNS (b).
It was determined that the MCS was not effective in expanding the viewing angle regardless of MCS radius. The half widths of the OLED with RNS increased compared to the reference microcavity OLED and increased with the height of the RNS. The half widths for 100 nm, 200 nm, 350 nm and 500 nm height RNS were 88.8°, 95.7°, 96.6° and 102.6°, respectively. The FDTD simulation results showed that the RNS was effective for light diffusing of the microcavity OLED.
The simulation results for the RNS corresponded well to the experimental results. Because of the RNS’ relatively simple structure and cross-section, the 2-D FDTD simulation was considered to exactly describe a real system of OLED with RNS. The principal origin of scattering power of the RNS is considered to be Rayleigh scattering, due to its small size, as mentioned above. Rayleigh scattering power increases with increasing particle size and refractive index difference between particle and environment. Therefore, the increase of RNS height can enhance the scattering power and then increase the light diffusing effect in the microcavity OLEDs with RNS.
However, for MCS the simulation results were not in accordance with the experimental results. Two reasons can be considered for the discrepancy between the simulation and experimental results. One is discordance between the real surface morphology of the MCS and modelling of a 2-D FDTD. Incident light is scattered by a combination of several origins such as refraction, reflection, backscattering and diffuse scattering [33]. The morphology of MCS was irregular, especially for the MCS 42. Small bumps within the concave pits and flat areas on the edges, as seen in Fig. 2, should affect the light diffusing in the case of MCS 18. The surface morphology of MCS 42 is considered to be totally different from a concave pit array.
Describing the exact MCS morphology in the FDTD modelling within such small dimensions was very difficult. We described the surface of the MCS as a smooth line because of difficulties of modelling, which was believed to lead to the discrepancy. The other reason is the limitation of the FDTD simulation size. The dimensions of the FDTD simulation were limited to reduce calculation time, so that the optical phenomena in the real system were somewhat different from the FDTD simulation.
To verify the results of the FDTD simulations for MCS, optical simulations by ray tracing method with LightTools (Synopsis OSG) were carried out. Figures 10 11and 12show the results of ray tracing for a flat glass, a deep pit array of 50 μm radius and 50 μm height, and a shallow pit array of 50 μm radius and 10 μm height, respectively. Near parallel rays (radiation angle: 1°) were well diffused by the deep pit array compared to flat glass. However out-coupled ray distributions of flat glass and the deep pit array were scarcely different from each other for distributed incident rays (radiation angle: 45°). In the case of a shallow pit array, the diffusing angle of out-coupled rays significantly decreased, even for the near parallel rays, and the out-coupled distribution for the distributed incident rays was somewhat reduced compared to the flat glass. The RNS was concluded to be more effective than the MCS for light diffusing of the microcavity OLEDs, based on the results of simulations and experiments. That is, Rayleigh scattering by nano-sized RNS is more powerful for light diffusing compared to scattering by micro-sized MCS.
Fig. 10 Ray tracing results of the flat glass for 1° (a) and 45° (b) of radiation angle of incident rays.
Fig. 11 Ray tracing results of the deep pit array for 1° (a) and 45° (b) of radiation angle of incident rays.
Fig. 12 Ray tracing results of the shallow pit array for 1° (a) and 45° (b) of radiation angle of incident rays.
The primary reason why the MCS was not so effective for light diffusing is that the concave pit surface feature was micrometer size. When rays are irradiated to the smooth surface of a micro-scale concave pit of glass, the rays are refracted and reflected according to Snell’s law and Fresnel’s principle, rather than scattered. A concave lens is known to expand a beam of light by refraction at the concave surface. But this is true only for near parallel rays perpendicular to the lens. However, the expanding effect will be almost invalid when the incident angles are distributed. The secondary reason is that the pits were shallow. The distribution of refracted rays from a shallow concave pit will be narrow as shown in Fig. 12. Light from an OLED light source is widely distributed, even in the microcavity OLEDs, unless it is a laser-like light source. Thus, a shallow and micro-size concave pit structure is not considered to be effective for light diffusing of the microcavity OLEDs.
4. Conclusion
In this work, we have investigated the light diffusing effects of RNS and MCS on microcavity designed OLEDs, using both experiments and simulations. In both structures, the high haze values contributed not only to the light diffusing but also to spectral stability. In addition, the RNS showed significant light extraction effects, which increased with the height of the RNS. The results of FDTD simulations and experiments showed that the RNS was more effective than the MCS for light diffusing of the microcavity OLEDs as well as external light extraction. Rayleigh scattering by nano-sized RNS is considered to be the more powerful origin of the light diffusing, compared to scattering by micro-sized MCS. In addition to these results, the RNS has advantages from the viewpoint of display panel manufacturing because its size is much smaller than pixel size. Such a small sized structure can minimize pixel blur problems, and it can be fabricated by a highly controllable and reproducible process. The RNS emerges as a promising candidate for a light diffuser, resolving the viewing angle problems in microcavity OLEDs.
Acknowledgments
This work was supported by the IT R&D program of MOTIE/KEIT under Grant 10041062 (Development of Fundamental Technology for Light Extraction of OLED). We greatly appreciate U plus Vision Co., Ltd for the wet etching process.
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