Abstract
We developed a diffusion layer with a refractive index-matching layer added to a transparent nanoscale polymer-based structure to obtain its effective scattering effects. The diffusion layer had higher haze when the refractive index-matching layer to a higher refractive index was used. This diffusion layer was applied to conventional organic light-emitting diodes (OLEDs) and micro-cavity OLEDs (MC-OLEDs) to evaluate the characteristics. When a diffusion layer was applied to conventional OLEDs, the external quantum efficiency (EQE) was 31.1% higher than that of the device without a diffusion layer due to the reduction of the substrate mode, and the viewing angle characteristic was also improved. Then, when the diffusion layer was applied to the MC-OLEDs, all devices showed similar EQE values regardless of the presence or absence of the diffusion layer, and the viewing-angle-dependent characteristics were greatly improved by the diffusion layer. Furthermore, when the diffusion layer was used with polarizer film, the black color implementation was not affected by the polarizer film, proving that it is applicable to actual OLED display products.
Highlights
Organic light-emitting diodes (OLEDs) are gaining popularity as next-generation displays and lighting taking advantage of their low thickness, self-luminous properties, high color purity and gamut, fast response speed, and various applicable characteristics like flexibility and foldability[1,2,3,4]
Representative methods include randomly distributed structures[8,9], photonic crystals[10,11], scattering layers[12,13], low-refractive-index grids[14], micro lens array (MLA)[15,16,17], patterned substrate surfaces[18], corrugated structures[19,20], high-refractive-index substrates[21], and micro-cavity structures[22,23,24]. These light-extraction techniques have been reported to improve the efficiency of organic light-emitting diodes (OLEDs), they have the disadvantages that the fabrication steps are complex and difficult and that they are difficult to apply to actual displays and lighting devices
O2-gas plasma etching was performed through reactive ion etching (RIE) to form poly(methyl methacrylate) (PMMA) nanoscale pillar structures[26,27]
Summary
Fabrication and optical properties of diffusion layer. Figure 1 shows the fabrication process of the diffusion layer and the field-emission scanning electron microscopy (FE-SEM) images of the fabricated diffusion layer. The EQE curves showed efficiencies of 1.06, 1.21, 1.29, and 1.39% at 10 mA/cm[2], respectively, and the efficiency of Device C using the ZnO refractive-index-matching layer was the highest These efficiency-enhancement results demonstrate that the diffusion layer applied to the OLED has a reduced substrate mode due to a superior scattering effect and a rough outer surface. The shift of the color coordinate according to the viewing angle decreases as the haze of the diffusion layer applied to the MC-OLEDs increases (Micro-Cavity: 0.038, Device 1: 0.034, Device 2: 0.029, and Device 3: 0.027). We fabricated a bilayer-type diffusion layer with different refractive indices to improve the light efficiency and viewing-angle characteristics of OLED devices. These results demonstrate that diffusion layers are effective in fabricating OLED displays with high efficiency and viewing-angle independence
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