Ju Hyun Hwang

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.

Silver Nanowire-IZO-Conducting Polymer Hybrids for Flexible and Transparent Conductive Electrodes for Organic Light-Emitting Diodes.

Solution-processed silver nanowire (AgNW) has been considered as a promising material for next-generation flexible transparent conductive electrodes. However, despite the advantages of AgNWs, some of their intrinsic drawbacks, such as large surface roughness and poor interconnection between wires, limit their practical application in organic light-emitting diodes (OLEDs). Herein, we report a high-performance AgNW-based hybrid electrode composed of indium-doped zinc oxide (IZO) and poly (3,4-ethylenediowythiophene):poly(styrenesulfonate) [PEDOT:PSS]. The IZO layer protects the underlying AgNWs from oxidation and corrosion and tightly fuses the wires together and to the substrate. The PEDOT:PSS effectively reduces surface roughness and increases the hybrid films’ transmittance. The fabricated electrodes exhibited a low sheet resistance of 5.9 Ωsq−1 with high transmittance of 86% at 550 nm. The optical, electrical, and mechanical properties of the AgNW-based hybrid films were investigated in detail to determine the structure-property relations, and whether optical or electrical properties could be controlled with variation in each layer’s thickness to satisfy different requirements for different applications. Flexible OLEDs (f-OLEDs) were successfully fabricated on the hybrid electrodes to prove their applicability; their performance was even better than those on commercial indium doped tin oxide (ITO) electrodes.

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 NFs features. Therefore, the NF can be applied in different types of lighting systems, depending on each lighting devices purpose.

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 Braggs 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.

Low-haze light extraction from organic light-emitting diode lighting with auxiliary electrode by selective microlens arrays.

Improved out-coupling efficiency and low haze of organic light-emitting diode (OLED) lighting with an auxiliary electrode are demonstrated by selective microlens arrays (SMLAs). The microlens arrays, aligned with the auxiliary electrode, were selectively fabricated, since the fully packed microlens arrays lead to OLED lighting with high haze. The external quantum efficiency and power efficiency of the devices with the SMLAs increased by 32% when compared with the devices without these arrays. Using the SMLAs, dark grid lines in the emission region became brighter, with a low haze, and the spectra of the emitted light had no shift.

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.