Joong Yeon Cho

The fabrication of a patterned ZnO nanorod array for high brightness LEDs

In this study, a patterned ZnO nanorod array was formed on the ITO layer of GaN-based light-emitting diodes (LEDs), to increase the light extraction efficiency of the LED. The bi-layer imprinted resin pattern was used for selective growth of the ZnO nanorod array on the ITO layer. Compared to conventional LEDs grown on patterned sapphire substrate (PSS), the deposition of the blanket ZnO layer on the ITO layer increased the light extraction efficiency of the LED by about 10%. Further growth of the ZnO nanorod layer on the blanket ZnO layer increased the light extraction efficiency of the LED by about 23%. In the case that a patterned ZnO nanorod layer was formed on a blanket ZnO layer, the light extraction efficiency increased by about 34%. These enhancements of the device were caused by modulation of the refractive-index in ZnO layers and the surface roughening effects because of the unique design of the pattern, which was nanostructure-in-nanopattern, resulting in the formation of many escape cones on the LED surface.

Fabrication of SiNx-based photonic crystals on GaN-based LED devices with patterned sapphire substrate by nanoimprint lithography

SiNx-based photonic crystal (PhC) patterns were fabricated on the ITO electrode layer of a GaN-based light-emitting diode (LED) device on a patterned sapphire substrate (PSS) by a UV nanoimprint lithography process in order to improve the light extraction of the device. A three-dimensional finite-difference time-domain simulation confirmed that the light extraction of a GaN LED structure on a PSS is enhanced when SiNx PhC patterns are formed on the ITO top layer. From the I-V characteristics, the electrical properties of patterned LED devices with SiNx-based PhC were not degraded compared to the unpatterned LED device, since plasma etching of the p-GaN or the ITO layers was not involved in the patterning process. Additionally, the patterned LED devices with SiNx-based PhCs showed 19%-increased electroluminescence intensity compared with the unpatterned LED device at 445 nm wavelength when a 20 mA current is driven.

Fabrication of 3D nano-structures using reverse imprint lithography

In spite of the fact that the fabrication process of three-dimensional nano-structures is complicated and expensive, it can be applied to a range of devices to increase their efficiency and sensitivity. Simple and inexpensive fabrication of three-dimensional nano-structures is necessary. In this study, reverse imprint lithography (RIL) with UV-curable benzylmethacrylate, methacryloxypropyl terminated poly-dimethylsiloxane (M-PDMS) resin and ZnO-nano-particle-dispersed resin was used to fabricate three-dimensional nano-structures.UV-curable resins were placed between a silicon stamp and a PVA transfer template, followed by a UV curing process. Then, the silicon stamp was detached and a 2D pattern layer was transferred to the substrate using diluted UV-curable glue. Consequently, three-dimensional nano-structures were formed by stacking the two-dimensional nano-patterned layers. RIL was applied to a light-emitting diode (LED) to evaluate the optical effects of a nano-patterned layer. As a result, the light extraction of the patterned LED was increased by about 12% compared to an unpatterned LED.

Forming the graded-refractive-index antireflection layers on light-emitting diodes to enhance the light extraction

Distributed antireflection (AR) layers with different composition ratios of ITO and SiO(2) formed on an ITO electrode of GaN-based LEDs provide substantial enhancement in light-extraction efficiency. By using the coradio frequency magnetron sputtering deposition, four 50 nm thick AR layers with graduated refractive indices were fabricated. The effect of the AR layers on enhancing the efficiency of the LED device was analyzed by electroluminescence (EL) and I-V measurements. As a result, the EL intensity of the LED device grown on the patterned sapphire substrate with AR layers was increased by up to 13% compared to the conventional patterned sapphire substrate-applied LED device without AR layers at a drive current of 20 mA. The AR layers on top of the LED device gradually changed the refractive indices between ITO (n=2.1) and air (n=1.0), which minimized the total internal reflection of generated light. And no degradation in the electrical characteristic of the LEDs was observed according to the I-V measurements.

Fabrication of functional nanosized patterns with UV-curable polysilsesquioxane on photovoltaic protective glass substrates using hybrid nano-imprint lithography

UV-curable polysilsesquioxane materials were used to incorporate moth-eye structures on photovoltaic (PV) protective glass. These patterns were formed using a hybrid nanoimprint lithography technique and annealed at 100 °C to evaporate the solvent (xylene). Compared to the bare, un-patterned PV protective glass, the PV protective glass patterned on both sides had superior optical properties. Transmittance of the PV protective glass patterned on both sides increased by up to 3.13% and reflectance decreased by up to 3.42%, and the transmittance was increased for all angles of incidence. Furthermore, the JSC of devices with the PV protective glass patterned on both sides increased by up to 3.15%. Finally, a monitoring system was set up to measure electricity generated by PV modules. The efficiency of the PV module with PV protective glass patterned on both sides was enhanced by up to 12.16% compared with that of the PV module with un-patterned PV protective glass.

Enhancement of light extraction efficiency of OLEDs using Si 3 N 4 -based optical scattering layer

An optical scattering layer, consisting of a Si3N4 nano-pillar array and a spin-coated hydrogen silsesquioxane (HSQ) planarization layer, was introduced to an organic light-emitting diode (OLED) substrate to increase the out-coupling efficiency. After plasma enhanced chemical vapor deposition (PECVD) of the Si3N4 layer, the nano-pillar array was created using nanoimprint lithography and reactive ion etching. As the Si3N4 pillar array has a refractive index of 2.0, photons generated in the organic layer are scattered by the Si3N4 structures and thus have a higher chance of being emitted from the device. The spin-coated HSQ planarization layer produces a flat substrate, which is essential for depositing a uniform organic material layer and assuring the electric conductivity of the transparent conducting oxide (TCO) layer. In this study, Si3N4 nano-structures with a height of 100 or 300 nm were used to enhance the out-coupling efficiency of the OLED devices. Although the electrical conductivity of the TCO layer deposited on the light scattering layer was slightly degraded, the OLED devices formed with the light scattering layer exhibited a higher luminous power at given electrical power. Consequently, the use of a planarized 300-nm-thick Si3N4 layer increased the external quantum efficiency of the OLED device by 50% at 10,000 cd/m2 compared to the reference OLED device fabricated on a flat glass substrate.

Improvement of photon extraction efficiency of GaN-based LED using micro and nano complex polymer structures

A micro- and nanoscale complex structure made of a high refractive index polymer (n = 2.08) was formed on the ITO electrode layer of an edge-emitting type GaN blue light-emitting diode (LED), in order to improve the photon extraction efficiency by suppressing total internal reflection of photons. The nanoimprint lithography process was used to form the micro- and nanoscale complex structures, using a polymer resin with dispersed TiO2 nano-particles as an imprint resin. Plasma processing, such as reactive ion etching, was used to form the micro- and nano-scale complex structure; thus, plasma-induced damage to the LED device can be avoided. Due to the high refractive index polymeric micro- and nanostructure on the ITO layer, the electroluminescence emission was increased up to 20%, compared to an identical LED that was grown on a patterned sapphire substrate to improve photon extraction efficiency.

Direct Indium Tin Oxide Nanoparticle Printing Technique for Improvement of Light Extraction Efficiency of GaN-Based LEDs

To enhance the light extraction efficiency of the GaN-based light emitting diode (LED), indium tin oxide (ITO) nanoparticle photonic crystal patterns are fabricated on the surface of the GaN-based blue LED device using the direct printing technique of the ITO nanoparticles. According to electroluminescence (EL) measurements, the EL intensity of the GaN-based blue LED with photonic crystal patterns is 28% higher than an identical LED without photonic crystal patterns. Printing the ITO nanoparticles eliminates the need for a plasma etching process of the ITO layer so that the current-voltage characteristics do not degrade.

In Situ Nanolithography with Sub-10 nm Resolution Realized by Thermally Assisted Spin-Casting of a Self-Assembling Polymer.

In situ nanolithography is realized based on warm spin-casting of block copolymer solutions. This advancement is based on Si-containing block copolymers with an appropriate thermodynamic driving force for spontaneous phase-separation combined with the thermal assistance provided by slight temperature elevations during the spin-casting. Sub-10 nm half-pitch nanoscale patterns are produced within 30 s without a separate annealing process.

Improvement of light out-coupling efficiency in organic light-emitting diodes with variable nanopatterns

We present a viable method to enhance the current efficiency of organic light-emitting diodes (OLEDs) by imprinting cylindrical nanopatterns (NPs) onto a glass substrate. Herein, OLEDs equipped with an optimum NP array having a diameter of 300 nm and a periodic distance of 500 nm show a 33.4% improvement in current efficiency compared with a device without NPs. The light extraction efficiency of OLEDs was enhanced using a nanopatterned glass substrate, without any electrical degradation owing to the formation of NPs on opposite sides of the organic layers. In addition, top-emitting OLEDs can be fabricated easily by patterning the encapsulation glass.