Enhanced light extraction efficiency and viewing angle characteristics of microcavity OLEDs by using a diffusion layer.

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).

Extraction of Light Using Random Nanocone on Poly(vinyl-butyral) for Flexible OLEDs.

In this study, we designed a smooth, highly flexible, mechanically robust poly(vinyl-butyral) (PVB)/silver nanowire (AgNW) composite transparent conducting electrode (TCE) integrated with a random nanocone (RNC) to enhance the light extraction of flexible organic light-emitting diodes (OLEDs). The RNC was fabricated by reactive-ion etching (RIE) on AgNW embedded in PVB. As the etching time increased, the size of the RNC became larger. The sheet resistance and transmittance of PVB/AgNW with the RNC was 21.7 Ω/sq and ~87%, respectively. For the PVB/AgNW, the change in sheet resistance was only 2.6% when a 2,000-bend test was performed. The maximum external quantum efficiency was 28.3% when RNC 700 s was used as a green phosphorescent OLED. In addition, for current efficiency and power efficiency, RNC 700 s increased 1.4 times over RNC 0 s. RNC is free of viewing-angle-dependent color and brightness distortion. PVB/AgNW and RNC are practical ways to overcome the brittleness of conventional indium tin oxide and improve the efficiency of flexible OLEDs. Finally, this product is expected to be applied to various flexible optical devices.

Optical characteristics of refractive-index-matching diffusion layer in organic light-emitting diodes.

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.

Solution Processable, Flexible, and Transparent Hybrid Electrodes Using Tungsten Oxide Buffer Layer on Silver Nanowires.

In this study, we fabricate solution processable, flexible, and transparent hybrid electrodes. The hybrid electrodes are designed by depositing a tungsten trioxide (WO3) buffer layer on silver nanowires. The fabrication method is solution based, and the electrodes can be fabricated directly on a flexible substrate at low temperatures. This fabrication method is cost-effective and scalable. The hybrid electrodes show high flexibility, transparency, and conductivity. In addition, since the thickness of the WO3 buffer layer can be controlled, the transparency, conductivity, and surface roughness of the hybrid electrodes can be tailored. These hybrid electrodes are potential alternatives to conventional indium tin oxide electrodes for flexible electronic devices.

Ag fiber/IZO Composite Electrodes: Improved Chemical and Thermal Stability and Uniform Light Emission in Flexible Organic Light-Emitting Diodes.

Electrospun metal fiber is a promising flexible transparent electrode owing to its extremely long length and facile fabrication process. However, metal-fiber electrodes have problems with chemical and thermal stability and nonuniform emission in organic light-emitting diode (OLED) at low luminance. In this study, we proposed a Ag fiber/IZO composite electrode with high stability. Ag fiber/IZO composite electrodes exhibited chemical and thermal stability. In addition, it was demonstrated that the OLED with the Ag fiber/IZO composite electrode operated stably, and the uniform emission of the OLED with metal-fiber electrodes improved by using highly conductive IZO film.

Enhanced light extraction from organic light-emitting diodes using a quasi-periodic nano-structure.

Organic light-emitting diodes with a quasi-periodic nano-structure (QPS) were fabricated via a combination of laser interference lithography (LIL) and reactive ion etching (RIE). The LIL process was used to generate a periodic pattern, whereas the RIE process was used as a supplement to add randomness to the periodic pattern. The period of the fabricated periodic pattern was determined by finite difference time domain solutions. The height and density of the QPS were controlled by the RIE etching time and were optimized. The resulting quasi-periodic nanostructure comprised silicon dioxide (SiO2) with a low refractive index (n = 1.4-1.5), and an external quantum efficiency enhancement of 18% was achieved using the QPS device, without any viewing angle problems or spectral distortion, which are serious drawbacks of periodic patterns.

Light Extraction Enhancement in Flexible Organic Light-Emitting Diodes by a Light-Scattering Layer of Dewetted Ag Nanoparticles at Low Temperatures.

We demonstrated light extraction improvement by applying a scattering layer of Ag nanoparticles physically synthesized through a low-temperature annealing process to flexible organic light-emitting diodes (OLEDs). In general, increasing the size of Ag nanoparticles is preferred to increase light scattering, but a high-temperature annealing process (∼400 °C) is required to produce them. However, flexible substrates generally cannot withstand high-temperature processes. In this study, we formed Ag nanoparticles at a low temperature of ∼200 °C by inserting a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate buffer layer, thus promoting Ag dewetting. As a result, the scattering layer of enlarged Ag nanoparticles formed at low temperatures increased the external quantum efficiency by 24% in a flexible OLED compared to a reference device.

Simple method for fabricating scattering layer using random nanoscale rods for improving optical properties of organic light-emitting diodes.

We investigated a low-temperature mask-free process for preparing random nanoscale rods (RNRs) as a scattering layer. The process involves spin coating and dry etching, which are already widely applied in industry. Our film exhibited 17-33% optical haze at 520 nm wavelength and 95% total transmittance in the visible range. Therefore, this film can be used as a scattering layer for improving viewing angle characteristics and decreasing substrate mode loss in organic light-emitting diodes (OLEDs). Specifically, we focussed on varying the height and density of the RNRs to control the optical characteristics. As a result, the OLEDs with RNRs revealed a variation in colour coordinates of Δ(x, y) = (0.007, 0.014) for a change in the viewing angle, which was superior to those without the RNRs that displayed a variation of Δ(x, y) = (0.020, 0.034) in CIE 1931. Moreover, the OLEDs with RNRs exhibited 31% enhanced external quantum efficiency compared to those of the OLEDs with the bare substrate. The flexibility of the polymer used for the RNRs and the plasma treatment suggests that the RNRs can be applied to flexible OLED displays and lighting systems.

Junction-Free Electrospun Ag Fiber Electrodes for Flexible Organic Light-Emitting Diodes.

Fabrication of junction-free Ag fiber electrodes for flexible organic light-emitting diodes (OLEDs) is demonstrated. The junction-free Ag fiber electrodes are fabricated by electrospun polymer fibers used as an etch mask and wet etching of Ag thin film. This process facilitates surface roughness control, which is important in transparent electrodes based on metal wires to prevent electrical instability of the OLEDs. The transmittance and resistance of Ag fiber electrodes can be independently adjusted by controlling spinning time and Ag deposition thickness. The Ag fiber electrode shows a transmittance of 91.8% (at 550 nm) at a sheet resistance of 22.3 Ω â–¡-1 , leading to the highest OLED efficiency. In addition, Ag fiber electrodes exhibit excellent mechanical durability, as shown by measuring the change in resistance under repeatable mechanical bending and various bending radii. The OLEDs with Ag fiber electrodes on a flexible substrate are successfully fabricated, and the OLEDs show an enhancement of EQE (≈19%) compared to commercial indium tin oxide electrodes.

Spectral-distortion-free light extraction from organic light-emitting diodes using nanoscale photonic crystal.

Despite their generally good performance, photonic crystal (PC)-based organic light-emitting diodes (OLEDs) encounter a serious spectral distortion problem. In this study, we obtained spectral-distortion-free PC-based OLEDs by lowering the pitch (period of the PC) to less than a half the emission wavelength, using a simple and scalable nanoscale process of laser interference lithography. The demonstrated OLEDs with 200 nm pitch-size nanoscale periodic hole arrays exhibited negligible changes in the Internal Commission on Illumination 1931 color coordinate of Δ (0.0104, 0.0078) and a peak wavelength of Δ0 nm (relative to the reference), while maintaining the function of the internal light extraction layer, manifested as a 23% enhancement of the external quantum efficiency (EQE). The enhancement of the EQE reached 85% after incorporating a micro-lens array. The improved light extraction, spectral-distortion-free characteristic, and excellent color stability over a broad range of viewing angles were successfully derived by performing finite difference time domain simulations.

An extremely low-index photonic crystal layer for enhanced light extraction from organic light-emitting diodes.

This paper reports organic light-emitting diodes (OLEDs) with improved light extraction fabricated by embedding an extremely low-index photonic crystal (LIPC) layer. The LIPC layer increases the optical efficiency through the reduced wave-guide mode between the substrate and anode both by increased light resonance and by a strengthened diffraction effect from an extremely low-refractive-index medium, specifically a line structure composed of a vacuum gap. As a result, the current efficiency and power efficiency of the LIPC-OLEDs are 1.51 and 1.93 times higher, respectively, than the reference device at 1000 cd m(-2). Because most of the light extraction is significant, especially in the forward direction, at the specific wavelengths satisfying the Bragg's diffraction equation, it is possible to calculate the anomalous spectrum of the LIPC-OLED through the finite-difference time domain (FDTD) method.

High-performance hybrid buffer layer using 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile/molybdenum oxide in inverted top-emitting organic light-emitting diodes.

A high-performance 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HATCN)/molybdenum oxide (MoO3) hybrid buffer layer with high hole-injection efficiency and superior plasma resistance under the sputtering process was developed. The HATCN enhances the hole-injection efficiency, and the MoO3 effectively protects the underlying organic layers from plasma damage during deposition by sputtering. This improves the characteristics of inverted top-emitting organic light-emitting diodes using a top transparent conductive oxide electrode. The device using the hybrid buffer layer showed the highest electroluminescence characteristics among devices with other buffer layers. The high hole-injection efficiency of HATCN was shown by the J-F curve of hole-only devices, and the plasma protection performance of MoO3 was shown by atomic force microscope surface morphology images of the buffer layer film after O2 plasma treatment.

Enhanced light out-coupling efficiency of organic light-emitting diodes with an extremely low haze by plasma treated nanoscale corrugation.

Extremely low-haze light extraction from organic light-emitting diodes (OLEDs) was achieved by utilizing nanoscale corrugation, which was simply fabricated with plasma treatment and sonication. The haze of the nanoscale corrugation for light extraction (NCLE) corresponds to 0.21% for visible wavelengths, which is comparable to that of bare glass. The OLEDs with NCLE showed enhancements of 34.19% in current efficiency and 35.75% in power efficiency. Furthermore, the OLEDs with NCLE exhibited angle-stable electroluminescence (EL) spectra for different viewing angles, with no change in the full width at half maximum (FWHM) and peak wavelength. The flexibility of the polymer used for the NCLE and plasma treatment process indicates that the NCLE can be applied to large and flexible OLED displays.

Novel composite layer based on electrospun polymer nanofibers for efficient light scattering.

We fabricated a PAN (polyacrylonitrile) NF (nanofiber)-embedded composite layer to adjust the light-control layer in light-emitting-diode (LED) and organic-light-emitting-diode (OLED) lighting systems with unique optical characteristics, for effective light scattering. The newly designed light-control composite layers with a composition of PAN NF/SU-8 exhibited a change in the optical properties, which was identified by the diameter control of the NF using a simple process. The change in the optical properties was largely dependent on the embedded NF's features. Therefore, the NF can be applied in different types of lighting systems, depending on each lighting device's purpose.

Effective indium-doped zinc oxide buffer layer on silver nanowires for electrically highly stable, flexible, transparent, and conductive composite electrodes.

We demonstrate a flexible, transparent, and conductive composite electrode comprising silver nanowires (Ag NWs), and indium-doped zinc oxide (IZO) layers. IZO is sputtered onto an Ag NW layer, with the unique structural features of the resulting composite suitable as a flexible, transparent, conductive electrode. The IZO buffer layer prohibits surface oxidation of the Ag NW, and is thereby effective in preventing undesirable changes in electrical properties. The newly designed composite electrode is a promising alternative to conventional ITO films for the production of flexible and transparent electrodes to be applied in next-generation flexible electronic devices.