Yu-Hsuan Ho

Sensing performance of precisely ordered TiO2 nanowire gas sensors fabricated by electron-beam lithography.

In this study, electron beam lithography, rather than the most popular method, chemical synthesis, is used to construct periodical TiO2 nanowires for a gas sensor with both robust and rapid performance. The effects of temperature on the sensing response and reaction time are analyzed at various operation temperatures ranging from 200 to 350 °C. At the optimized temperature of 300 °C, the proposed sensor repeatedly obtained a rise/recovery time (ΔR: 0.9 R0 to 0.1 R0) of 3.2/17.5 s and a corresponding sensor response (ΔR/R0) of 21.7% at an ethanol injection mass quantity of 0.2 μg.

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.

High-Efficiency Fluorescent Blue Organic Light-Emitting Device with Balanced Carrier Transport

We demonstrate a blue fluorescent organic light-emitting device having a current efficiency of 19.2 cd/A and an external quantum efficiency of 8.32% at 100 cd/m 2 . While driving at 20 mA/cm 2 , the device showed a current efficiency of 16.5 cd/A with a CIE coordinate at (0.155, 0.239) and an operation voltage of 5.14 V. The estimated half-decay lifetime is 15,611 h at an initial luminance of 1000 cd/m 2 . The improved performance is the result of better carrier balance which is achieved by using bis(10-hydroxybenzo[h]quinolinato)beryllium (Bebq 2 ) as the electron-transport layer.

Efficiency improvement and image quality of organic light-emitting display by attaching cylindrical microlens arrays

In this paper, cylindrical microlens arrays with two different alignments were proposed to be applied in a commercial mobile phone having an organic light-emitting diode (OLED) panel. It was found that the parallel-aligned cylindrical array had better performance than the vertical-aligned one for the OLED panel. The parallel-aligned cylindrical microlens array can increase the luminous current efficiency at surface normal and the luminous power efficiency of the OLED panel by 45% and 38%, respectively. Besides, it can also make the spectrum of the OLED panel more insensitive to the viewing angle. Though it can slightly blur the image on the OLED panel, the universal image quality index can be maintained at a level of 0.8630.

Enhanced light out-coupling of organic light-emitting diode using metallic nanomesh electrodes and microlens array

A precisely controlled metallic nanomesh was fabricated by using nanosphere lithography to pattern the silver thin film to form hexagonal nanohole arrays with excellent uniformity, high conductivity and good transparency. An Alq(3) based OLED, with the silver nanomesh electrode of high ðll factor of 70.2% demonstrated a considerable luminous efðciency of 4.8 cd/A, which is 60.9% higher than the referenced device with ITO anode. The periodical nanohole array not only increased the transparency but also helped extracting surface plasmonic wave in organic layers. By attaching the microlens array to further extract the trapped light in substrate, the extraction efficiency enhancement of device with nanomesh anode was 73.8% higher than 50.2% of the referenced device with ITO anode. And the overall current efficiency of device with nanomesh anode was 87.7% higher than traditional ITO based device.

Efficiency enhancement of flexible organic light-emitting devices by using antireflection nanopillars.

We present an antireflection structure consisted of irregular nanopillars to increase light extraction efficiency of flexible organic light-emitting devices. The nanopillars were made by imprinting the anodized aluminum oxide on polycarbonate substrates. The thermal viscosity effect formed the nanopillars with tapered shapes. Such nanopillars show excellent antireflection properties for a wide range of incident angles and wavelengths. The normal transmittance was improved from 85.5% to 95.9% for 150-nm-height nanopillars. The transmittance was greatly improved from 52.8% to 89.1% at 60° incident angle. With this antireflection structure, the device efficiency was improved 69% as compared to devices with flat substrates. Due to wide-angle antireflection, the image contrast ratio was also significantly improved.

Emitter apodization dependent angular luminance enhancement of microlens-array film attached organic light-emitting devices

Taking organic emitter apodization calculated from electromagnetic theory as input, the angular luminance enhancement of a microlens-array-film (MAF) attached OLED (organic light-emitting device) can be further evaluated by ray-tracing approach. First, we assumed artificial emitters and revealed that not every OLED with MAF has luminance enhancement. Then, the OLEDs of different Alq(3) thickness were fabricated and their angular luminance measurement validated simulation results. Mode analyses for different layers were performed to estimate the enhancement potential of the MAF attached devices. In conclusion, the organic emitters with higher off-axis-angle luminous intensity cause lower out-coupling efficiency but gain higher enhancement after the MAF attached.

Ultraviolet-enhanced room-temperature gas sensing by using floccule-like zinc oxide nanostructures

The self-aggregation of floccule-like ZnO nanostructures that were shaped by an anodic aluminum oxidation (AAO) template to improve photoactivation and sensing performance was demonstrated. Because of differences in the surface energy between the densely distributed nanopores of AAO templates, sputtered ZnO materials were located in constricted regions and aggregated into roughened nanostructures with a high surface-to-volume ratio. Because of the generation of oxygen ions by ultraviolet illumination, the room-temperature-sensing responses showed a high degree of linearity with a resistance variation of 1.758% per 100 ppm of octane gas. The optimized sensing performance of the self-organized ZnO nanostructures was increased and was 15.4 times higher than that of an unpatterned ZnO thin film.

Improve efficiency of white organic light-emitting diodes by using nanosphere arrays in color conversion layers

The authors demonstrated an efficient color conversion layer (CCL) by using nanosphere arrays in down-converted white organic light-emitting diodes (WOLEDs). The introduced periodical nanospheres not only helped extract the confined light in devices, but also increased the effective light path to achieve high-efficiency color conversion. By applying a CCL with red phosphor on a 400-nm-period nanosphere array, we achieved 137% color conversion ratio for blue OLEDs, which was 2.68 times higher than conventional flat CCL. The resulting luminous efficiency of WOLEDs with patterned CCLs (20.97 cd/A, 1000 cd/m2) was two times higher than the efficiency of the flat device (10.26 cd/A, 1000 cd/m2).