Abstract
The viewing angle characteristics and light extraction efficiency of organic light-emitting diodes (OLEDs) with a micro-cavity structure were enhanced. This was accomplished by inserting a diffusion layer composed of nano-sized structures of a transparent polymer poly(methyl methacrylate) (PMMA) combined with a zinc oxide (ZnO) semi-planarization layer with a high refractive index (n = 2.1) into the devices. The PMMA nanostructures were fabricated by employing a reactive ion etching (RIE) process. The height and density of the PMMA nanostructures were controlled by varying the speed at which the PMMA was spin-coated onto the substrate. The insertion of the diffusion layer into the micro-cavity OLEDs (MC-OLEDs) improved the external quantum efficiency (EQE) by as much as 17% when compared to that of a MC-OLED without a diffusion layer. Furthermore, adjustment of the viewing angle from 0° to 60° halved the peak shift distance of the electroluminescence (EL) spectra from 42 to 20 nm. Additionally, changing the viewing angle from 0° to 60° changed the color coordinate movement distance of the MC-OLED with the diffusion layer to 0.078, less than half of the distance of the MC-OLED without the diffusion layer (0.165).
Highlights
Organic light-emitting diodes (OLEDs) are being extensively examined as next-generation technologies in the field of display and solid-state lighting owing to their low power consumption, high color purity and gamut, fast response, and applicability to flexible display d evices[1,2,3,4,5]
It is necessary to deposit aplanarization layer on the nano-sized structure to reduce its surface roughness such that the structure is appropriate for incorporation into an organic light-emitting diodes (OLEDs) device
In contrast to the tendency exhibited by the current efficiency, the micro-cavity OLED (MC-OLED) (Devices A and B), which include the diffusion layer, have external quantum efficiency (EQE) values that are 17.2% and 14.5% higher, respectively, compared with those of the Micro-Cavity device (Non-Cavity: 1.21%, Micro-Cavity: 1.45%, Device A: 1.7%, Device B: 1.66% at 50 mA/cm[2]). These results demonstrate that the light emitted by the MC-OLEDs in the direction normal to the surface is effectively diffused owing to the inserted diffusion layer and that the light extraction efficiency improved such that light is observed at all viewing angles
Summary
Organic light-emitting diodes (OLEDs) are being extensively examined as next-generation technologies in the field of display and solid-state lighting owing to their low power consumption, high color purity and gamut, fast response, and applicability to flexible display d evices[1,2,3,4,5]. The light trapped in the waveguide mode, which results from the TIR at the interface between the ITO (n = 1.9) and glass substrate (n = 1.5), was extracted by utilizing a randomly distributed nano-sized structure, an internal scattering layer, a periodic photonic crystal, or a layer with a low refractive index[17,18,19,20,21,22,23]. We used a simple reactive ion etching (RIE) process to fabricate a transparent layer consisting of nano-sized structures of a polymeric material We subsequently inserted this layer between the glass substrate and the indium zinc oxide (IZO) anode of the micro-cavity OLED (MC-OLED) device in conjunction with a semi-planarization layer to form a diffusion layer with a high refractive index to lessen the viewing angle dependency of the MC-OLED. The semi-planarization layer serves to enhance the optical efficiency of the MC-OLED compared with that of a conventional MC-OLED device
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