Hung-Yi Lin

Patterned microlens array for efficiency improvement of small-pixelated organic light-emitting devices

In this paper, we experimentally and theoretically investigated the optical characteristics of organic light-emitting devices (OLEDs), having different pixel sizes and attached with patterned microlens array films. For a regular microlens array, though it can extract the waveguiding light and increase luminous current efficiency for a large-pixelated OLED, we observed that it decreases the luminance to an even lower level than that of the planar OLED as its pixel size is close to the microlens dimension. Although a microlens can effectively outcouple the light rays originally at incident angles larger than the critical angle, it also can impede the outcoupling for the light rays originally at incident angles smaller than the critical angle. Enhancement or reduction of the light extraction depends on the relative positions of the light emitting point and the microlens. Therefore, we proposed a center-hollowed microlens array, of which the microlenses directly upon the pixel are removed, and proved that it can increase the luminous current efficiency and luminous power efficiency of a small-pixelated OLED. By attaching this patterned microlens array, 87% of luminance enhancement in the normal direction was observed for a 0.1x0.1 mm2 OLED pixel. On the other hand, a regular microlens array resulted in 4% decrease under the same condition.

Efficiency improvement and spectral shift of an organic light-emitting device by attaching a hexagon-based microlens array

In this paper, we present and analyze the influences of the fill factor and the sag of hexagon-based microlenses on the optical characteristics of an organic light-emitting device (OLED), such as spectral shift, CIE (abbreviation of the French ‘Commission internationale de l’´ eclairage’) coordinates, viewing angle dependence, luminous current efficiency and luminous power efficiency. Both the luminous current efficiency and luminous power efficiency of the OLED were found to increase linearly on increasing the fill factor of the microlenses. It is also found that the full width at half maximum (FWHM) of the OLED spectra and CIE coordinates decreased linearly on increasing the fill factor of the microlenses. Besides, the efficiency improvement of the OLED increased with the height ratio of attached microlenses. Compared to the OLED, the luminous current efficiency and luminous power efficiency of the device can be enhanced by 35% and 40%, respectively, by attaching a microlens array having a fill factor of 0.90 and a height ratio of 0.56. We also observed blue shifts at different viewing angles when microlens arrays were attached to the OLED, which is evidence that the waveguiding modes are being extracted. In our planar OLED, the peak wavelength blue shifted and the FWHM decreased on increasing the viewing angles, due to the microcavity effect.

Enhancing light extraction efficiency of polymer light-emitting diodes with a 12-fold photonic quasi crystal.

This work demonstrates the enhancement of light extraction of polymer light-emitting diodes (PLEDs) by incorporating a 12-fold photonic quasi crystal (PQC) in the device structure. Multi-exposure two-beam interference technique combined with inductively coupled plasma etching was employed to pattern a 12-fold PQC structure on the ITO film on a glass substrate of the diode. The air-hole coverage (AHC) and etching depth dependences of the light emitting performance of the 12-fold PQC patterned PLEDs were investigated. For AHC within the range between 6.4% and 32.3%, a nearly constant enhancement of the luminance efficiency of the PQC PLEDs was observed. On the other hand, the light emitting performance of the PQC PLEDs is very sensitive to the etching depth. The photoluminescence intensity of the PQC PLEDs increases monotonically with the etching depth. In contrast, the electro luminance efficiency shows a non-monotonic dependence on etching depth with a maximum occurring at 55 nm etching depth. The maximum improvement of luminance efficiency of the 12-fold PQC PLEDs reaches nearly 95% compared with an un-patterned PLED at an injection current of 110 mA.

Enhancement and Saturation Phenomena on Luminous Current and Power Efficiencies of Organic Light-Emitting Devices by Attaching Microlens Array Films

Compared to the OLED with planar substrate, the luminous current efficiency and luminous power efficiency of the device are shown 25% and 36% enhancement, respectively, by attaching the microlens array film (MAF) having a fill factor of 0.788 and a height ratio of 0.46. Both the luminous current efficiency and luminous power efficiency of the organic light-emitting device (OLED) increased monotonically with increasing the fill factor of MAFs, regardless of their arrangements. However, the curve-fitting equation of the luminous power efficiency has a negative second-order term, which shows a saturation phenomenon. Based on the experimentally verified configuration, simulation showed that the luminous current and power efficiencies also increase along with increasing height ratio and they were found to have more pronounced saturation phenomena. As for the spectral characteristics, the peak wavelength of the planar OLED spectra got blue-shift and the full-width-at-half-maximum (FWHM) of its spectra decreased with increasing the viewing angles due to the microcavity theories. After MAFs are attached to the OLED, the FWHM of the OLED spectra decreased linearly by increasing the fill factor of the MAFs. We also observe blue shifts at different viewing angles which is the evidence that the waveguiding modes are being extracted. The saturation phenomena of efficiency enhancement imply that high fill factor and large height ratio of MAFs will certainly benefit for efficiency enhancement, however, full factor cases are not always the most desirable. Optimal fill factor and height ratio, which may be less than unity, will gain the best efficiency enhancement, suffer from less color deviation, and make fabrication easier.

Optical characteristics of silver film on the moth-eye structure

The development of coating optics to lower the reflected light and thereby to increase the optical efficiency of an optical system has been a very important issue for many years. Conventional solutions to this, such as multilayered alternation of high and low refractive index layers, often lead to an expensive coating process. Recently, the use of antireflection structured (ARS) surface, which is called “moth-eye structure”, has been proposed as an applicable option based on both the theoretical and experimental studies. In the current study, the experimental results of the reflectance and transmittance of two different thicknesses of silver films deposited on the moth-eye structure were carried out. The moth-eye structure arrays were fabricated by holographic exposure and photolithographic processes on the polymer film. The structure arrays were consisted of periodic length of about 300 nm, with the diameter of about 250 nm and the height of 150 nm. Compared with the silver coating film on the flat PET substrate, the optical property of the silver coating film on the moth-eye structure showed a better result for the anti-reflection application. The 25 nm-thick silver film on the moth-eye structure is suggested to be applied for the car window glass of antireflective films to obtain the high performance of heat insulation with acceptable transparency in the visible range.

Roll to Roll Processing for Flexible Nanophotonics

This article discusses the current status and achievements of R2R technology for large area nano-scaled optical devices developed at MSL/ITRI. Firstly, a single layer of nanostructure on polymer film is designed for anti-reflection purpose by finite difference time domain (FDTD) method in the visible light spectrum. The conical array with around 1 aspect ratio, like moth-eye shape and showing superior performance in the optical simulation, has been adapted for the R2R experiments. The development of R2R process includes roller machine design and fabrication, roller mold design and making, development of rolling imprint process, characterization of rolled devices. In this study, large area (200mm *200mm) Ni template was fabricated with DUV exposure, followed by dry etching and electroforming process, respectively. Then, the template was bonded on the roller mold with magnetic film to make nanostructure roller mold. With the delicate nanostructure roller mold, systematic experiments have been conducted on the home-made roller machine with various parameters, such as linear speed, dose rate, and material modifications. The duplicated nanostructure films show very good optical quality of anti-reflection (AR < 1%) and are in good agreement with the theoretical predictions. Besides, the duration of the roller mold has been highly promoted to hundreds of imprint in the UV embossing process.Copyright

The Effect of Sub-Wavelength Structure Morphology on Anti-Reflection Properties

In this paper, we present a new thermal reflow method to transform photresist pillars into the shapes of spherical caps and month-eyes. We also investigated the effect of this sub-wavelength structure morphology on anti-reflection properties of these photoresist patterns. The diameter, the height, and the pitch of this regularly arranged pillar array were set at 200, 250, and 340 nm, respectively. After the thermal reflow, experimental results showed that the reflectivity of the heat-treated photoresist patterns can be reduced to approximately one fourth of that of the polished surface of the silicon wafer.

Surface Finishing Using Carbon Nanotube Forest

The use of carbon nanotube forest as a brush tool to refine the surface roughness of brass in nanometer scale was reported. The carbon nanotube with a height of 700∼800 μm was first synthesized on silicon wafer by chemical vapor deposition. The carbon nanotube forest on silicon wafer was then transferred and bonded onto a stainless steel wheel. The wheel with carbon nanotube forest was installed on a precision milling machine and used to brush the surface of a brass specimen. The influence of various feed amount, brushing speed and brushing time on the surface roughness of the brass specimen were experimental evaluated. The results show that the increasing feed amount effectively reduces surface roughness of the brass specimen from 20 nanometers to 1 nanometer. Scanning electron microscopy images reveal the evidence of the bushing process that brass chips were melted on the tip of carbon nanotube forest.Copyright

Outcoupling Efficiency Improvement of Planar OLED Devices with Square-based Microlenses

In this paper, analysis of outcoupling efficiency improvement and optical characteristics of organic light-emitting devices (OLEDs) attached with square-based microlens arrays are presented. A square-based microlens array, having a base length of 35.4 μm and a fill factor of 89.6%, increases the luminous current efficiency and outcoupling efficiency of the OLED device by 42% and 47%, respectively. Optical properties of the OLED device, such as optical emitting spectrum and CIE coordinate, have also been investigated when attached with microlens arrays having different fill factors.

Effects of mold pattern with anti-adhesive treatment in nanoimprint lithography

A study of a top-down technique of nanoimprint lithography (NIL) for developing a novel bottom-up approach of chemical vapor deposition to generate anti-adhesive coatings between Ni mold stamps and polymethyl methacrylate (PMMA ) substrate is presented in this work. A polybenzoxazine ((6,6’-bis(2,3-dihydro-3-methyl-4H-1,3-benzoxazinyl))) molecule self-assembled monolayer (PBO-SAM) regarded as an anti-adhesive coating agent demonstrates that non-fluorine-containing compounds can improve the nanoimprint process in Ni/PMMA substrates. In this experiment, the nanostructure-based Ni stamps and imprinted PMMA molds are fabricated using an electron-beam lithograph (EBL) and a homemade nanoimprint equipment, respectively. The diameter of pillar patterns obtained is 200 nm with 400 nm pitch on Ni stamp surface. Based on the hydrophobic PBO-SAM surface in this forming condition, the results of Ni mold stamps indicate over 90% improvement in controlling quality and quantity.