Kuan-Yu Chen

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.

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.

Fabrication of surface metal nanoparticles and their induced surface plasmon coupling with subsurface InGaN/GaN quantum wells.

Based on the fabrication of Ag nanoparticles (NPs) with controlled geometry and surface density on an InGaN/GaN quantum well (QW) epitaxial structure, which contains indium-rich nano-clusters for producing localized states and free-carrier (delocalized) states in the QWs, and the characterization of their localized surface plasmon (LSP) coupling behavior with the carriers in the QWs, the interplay behavior of LSP coupling with carrier delocalization in the QWs is demonstrated. By using the polystyrene nanosphere lithography technique with an appropriate nanosphere size and adjusting the post-fabrication thermal annealing condition, the induced LSP resonance wavelength of the fabricated Ag NPs on the QW sample can match the QW emission wavelength for generating the coherent coupling between the carriers in the QWs and the induced LSP. The coupling leads to the enhancement of radiative recombination rate in the QWs and results in increased photoluminescence (PL) intensity, red-shifted PL spectrum, reduced PL decay time, and enhanced internal quantum efficiency. It is found that the observed effects are mainly due to the LSP coupling with the delocalized carriers in the QWs.

Radiation simulations of top-emitting organic light-emitting devices with two- and three-microcavity structures

We demonstrate the simulation results of the radiation properties from top-emitting organic light-emitting devices (top-emitting OLEDs) with two- and three-microcavity structures based on the general electromagnetic theory. The parameters of the layer thickness and complex refractive index of each layer, the locations and density of the oscillating dipoles, and the emission photoluminescence spectrum are varied to optimize the device performance. In evaluating the deice performances, the output spectrum, the intensity distribution, and the viewing-angle characteristics of a top-emitting OLED are concerned. The simulation results are consistent with the Fabry-Perot cavity equation, which can be used as a guideline for designing a two-cavity top-emitting OLED. In such a design process, the dipole position is chosen first. Then the thicknesses of the whole organic layer, the semitransparent cathode, and the dielectric layer are adjusted for optimizing the device performance. In a three-cavity top-emitting OLED, not only the emission intensity and the viewing angle can be optimized at the same time, but also the emission wavelength can be independently tuned. Besides, the use of a three-cavity structure helps to narrow the spectral width and increase the color purity

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.

P-179: Low Blur Effect and High Light Extraction Efficiency Enhancement of Organic Light Emitting Displays with Novel Microstructure Attachment

Microstructure film attachment and surface roughing techniques had long been utilized to improve the light extraction efficiency from a light source with a high refractive index. Instead of merely concerning the efficiency for lighting purposes, we must take both the light extraction efficiency and image quality into account for display applications. The image blur was observed to decrease the contrast ratio and thus lower image quality. In our previous work, we have studied the image blur effect quantitatively and correlated it to the coverage ratio and light extraction efficiency. In this paper, we apply an innovative microstructure array arrangement to planar light emitting device, i.e., for organic light emitting display (OLED) to reduce the blur effect and keep almost the same efficiency as that obtained by applying a traditional microstructure array.

Omnidirectional antireflection polymer films nanoimprinted by density-graded nanoporous silicon and image improvement in display panel

We present a low-cost method to fabricate large-area polycarbonate AR nanostructures to improve the luminous intensity and image clarity of a commercial 2.0-inch display panel in bright condition. The polycarbonate AR nanostructures were nanoimprinted by the graded-density nanoporous silicon template with nanoparticle-catalyzed etching. The average reflectivity of the AR film in visible wavelength region was reduced from 10.2% to 4.8% in the optimized case. After attaching on the display panel to reduce the light reflection on the substrate, the brightness enhancement and decrease of ambient light reflection were observed. Due to the enhancement of contrast ratio, the quality index of the Lena image test was improved from 0.85 to 0.92 under strong ambient illumination.

Device‐dependent angular luminance enhancement and optical responses of organic light‐emitting devices with a microlens‐array film

— By taking the organic emitter apodization calculated from electromagnetic theory as input, the angular luminance enhancement of organic light-emitting devices (OLEDs) with a microlens-array film (MAF) can be further evaluated by the ray-tracing approach. First, the OLEDs of different Alq3 thickness are fabricated and their angular luminance measurements are compared to simulation results. Second, mode analyses for different layers are performed to estimate the enhancement potential of the MAF-attached devices. Finally, by decreasing the Alq3 thickness, increasing the viewing angle, and attaching the MAF, the EL spectral peak shifts of the OLEDs seem irregular, but the spectral blue shifts induced by the optical structures are all explained by the optical responses (EL spectra divided by the intrinsic PL spectrum). In conclusion, the organic emitters with higher off-axis-angle luminous intensity cause lower out-coupling efficiency but gain higher enhancement after the MAF is attached. With the choices of apodizations and microstructures, the tailored or customized angular radiation patterns can be also made possible.