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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.
© 2013 Optical Society of America
1. Introduction
The antireflection (AR) techniques characterized by a gradient refractive index has inspired the development of new structures and artificial materials with antireflection surface properties, which have wide applications of optical and optoelectronic equipment in areas as diverse as military purposes to consumer electronics [1–7]. Compared with conventional single-layer or multilayer AR coatings for a limited spectral range and optimized to a specific viewing angle, the graded-index AR method was theoretically predicted to have broadband AR behavior of up to one order of magnitude and omnidirectional AR effect. They help to maximize the transmission of light through optical surfaces and reduce the reflection glare, which is not only over a broad spectral range but also over wide angles of light incidence. There are two major ways to achieve a gradient refractive index AR surfaces. The first is the multilayer structures of successively reducing refractive index. Its AR effect strongly depends on the smoothness of the index profile. Mathematically, the final reflectance is the integration of the differential reflectance at each interface of the AR layered structures. So a smooth transition of refractive index can help optical impedance matching and improve the AR performance [8, 9]. The second major approach to produce continuously varying refractive-index structures is to use tapered morphology [1, 10–16]. Generally, the nanostructures must be homogeneously distributed, and its feature size is maintained to be smaller than the wavelength of incident light to avoid light scattering. The effective refractive index of the proposed nanostructures was changed by introducing the ambient materials. Compared to the multilayered AR nanostructures, the AR nanostructures of tapered morphology are often monolithic and comprised of one single material, which have more robust properties [16]. These AR nanostructures could be summarized in two types as negative (concave) and positive (convex) modes [5, 10, 15–21]. Negative AR nanostructures are generally comprised of etched nano-voids with subwavelength feature size on the substrate surface. A great deal of research has been undertaken especially relating to porous silicon and glass, which were relied on wet or dry etching with or without mask in fabrication process [13, 17–26]. The refractive index gradually varies from the top of the structure to the bulk substrate, so great AR effect has been achieved. But the surface damages from etching process somtimes leads to electricity issues for optoelectronic devices [27]. The positive AR nanostructures are usually formed from physical or chemical growth through assembly of polymer or metal-oxide materials [10, 23–26]. Recently, due to the progress of nanoimprint techniques, more researches have been proposed to fabricate the positive AR nanostructures by imprinting from concave templates [6, 7, 15, 16, 28–33]. The AR effects of the resulting positive nanostructures can be well controlled by tuning the feature size, period and tapered shape of the porous achietecture. So a hard template with tapered concave morphology is the most important for the nanoimprint methods of AR techniques. AAO membrane as the imprint template is a widely-used approach for antireflection purposes, which has periodically close-packed nanostructures and flexibility to tailor the shape, aperture ratio and periodicity [15, 34]. But the complicated fabrication process and fragility of aluminum oxide limit the application of massive production. Especially for large-area fabrication, it’s difficult to obtain a homogeneously distributed nanoholes on the AAO membrane. In this work, we present a uncomplicated and low-cost method to fabricate a robust and graded-density nanoporous silicon as the nanoimprint template for polycarbonate AR thin film. Also we applied it on the commercial OLED display panel to show the improvement of image quality under bright condition by reducing the ambient reflection.
2. AR film nanoimprinted by density-graded nanoporous Si template
Figure 1 shows the schematic for the formation of positive AR nanostructure on the surface of polycarbonate substrate by thermal nanoimprinting using silicon nanoporous template. The silicon template with graded density morphology was fabricated by using nanoparticle-catalyzed etching. The subwavelength nanoholes were formed by the etchants mixture HF: H2O2: H2O in the ratio of 1: 5: 2 with an equal volume of aqueous gold-containing solution [17]. In this process, the patterned Si wafer was soaked in a sonication bath for times from 5 to 9 min. The pictures under bright ambient and SEM images were shown in Fig. 2. The ambient reflection was obviously reduced after etching process.
Fig. 1 Diagrams of fabrication process of graded-index antireflection nanopillars and its application in commercial OLED display panel.
The etched nanoholes are very dense near the surface and result in concave nanostructures on silicon substrate. When the etching time increases, the nanoholes cluster into wider pores and extend more deeply. The colloidal Au creates high-aspect-ratio nanoholes with diameters of 20 to 200 nm. Larger etched nanoholes can help the molten polycarbonate material inject more easily during nanoimprint process. The graded-index antireflection structures were fabricated by the thermal nanoimprint technique with the silicon template. In the imprinting process, the proposed silicon template is covered with polycarbonate film and set in a hermetic chamber under controlled temperature and pressure. To have a uniform air pressure on the PC film, a thin polyethylene terephthalate (PET) film was placed on the PC film and sealed by the steel chamber. After the chamber temperature is increased higher than glass transition temperature of polycarbonate, high chamber pressure is applied to help molten polycarbonate inject into the concave nanostructures of silicon template. The pressure in the chamber was increased to about 1 kgw m−2 so that the nanoholes on the template surface were filled with the molten polycarbonate. After ten minutes of imprinting, the whole system was cooled down to room temperature and the air pressure was released. Then the silicon template was removed immediately, and nanopillars with graded-density surface were formed on the polycarbonate substrate surface. The remaining silicon particles were cleaned in the IPA solution by using ultrasonic cleaner for hours. At last, the nanoimprinted AR thin film was attached on the commercial OLED display panel with index matching oil to extract the confined light in the substrate and reduce the ambient reflection to improve the display image quality under strong light illumination.
In order to clearly evaluate the surface morphology variation under different etching time, we convert the SEM graphs into binary images and filter the tiny black spots to calculate the aperture ratio of the etched nanoholes, which is defined as the area of nanoholes divided by total area. The binary SEM images and corresponding aperture ratios were shown in Fig. 3(a). Increasing the etch time_of nanoporous silicon template results in the aggregation of the nanoholes and increase the aperture ratio. The relation between aperture ratio and etching time is shown in Fig. 3(b). After 9-minute-etching, the average diameter of nanoholes is about 200 nm, which is small enough to avoid the scattering effect and applicable for thermal nanoimprint. The applying pressure is kept at 1 kgw m−2 which is relatively low compared to our previous works to prevent damages to the silicon template [6]. So the feature size of the etched nanoholes is the dominating parameter of the AR nanostructures fabrication, which directly influences the average height of the imprinted nanostructures. The pictures of the designed template and polycarbonate AR film are shown in Fig. 4. The hazy region of Fig. 4 (right) is the polycarbonate film without imprinted nanostructures. Because the picture shown in Fig. 4 (right) was taken under bright light illumination, the haze mainly came from the strong ambient reflection. It’s also evidence that the imprinted nanostructures can effectively reduce the light reflection and improve the image clarity.
Fig. 3 (a) Binary SEM images of the proposed silicon template after nanoparticle-catalyzed etching (b) aperture ratio under different etching time (dashed line: exponential fitting curve).
Fig. 4 Silicon template with concave nanostructures (left) and nanoimprinted polycarbonate AR film (right).
Figure 5 show the AFM images of the positive imprinted nanostructures by the silicon template under different etching conditions. It can be seen that the positive nanostructures formed graded-density surface with irregular distribution, which make these polycarbonate film a good antireflection surface. The average heights of the imprinted nanopillars were positively related to the aperture ratio of the nanoporous silicon template. Generally, the higher nanostructures make the effective refractive index changing more smoothly and lead to better AR effects. Figure 6(a) shows the reflectance spectra. The average height of imprinted nanostructures was ranged from 20.4 to 200.7 nm. The referenced light was incident on the sample at the normal direction, and the reflected light was measured by an integral sphere. As average height increased, the reflectance spectrawas reduced and became flat. Figure 6(b) shows the relation between average height and resulting AR effects in visible wavelength region (400-800 nm) for silicon concave molds with different nanoparticle-catalyzed etching time When we used the silicon molds with the longer etching time, the lower average reflectance was found due to the more graded-density surface morphology. If we keep on etching to expand the nanoholes on template, the reflectivity of proposed AR film would increase rapidly due to scattering effects from large aggregated nanostructures. Compared with the flat polycarbonate substrate, the average reflectivity of the patterned substrate was reduced from 10.2% to 4.8% in our optimized case.
Fig. 5 AFM images of the imprinted polycarbonate film by Si concave molds with the nanoparticle-catalyzed etching time of (a) 7 and (b) 9 minutes.
Fig. 6 (a) Reflectance spectra of patterned thin film with different average height (b) average height and resulting AR effects in visible wavelength region by using silicon concave molds with different nanoparticle-catalyzed etching time.
3. AR film applying on the commercial OLED display panel
Figure 7(a) shows the emitting images of the OLED panel displaying red, green and blue frames with and without graded-index AR thin film attachment. The film is imprinted by the 9-min-etched template. The luminous enhancement ratio was defined as the luminance of display panel at normal direction with AR structures patterning divided by the one without AR structures. Obvious luminous enhancement is observed after the AR film is attached on the display panel. Generally, the luminous enhancement is positively related to the AR effects of the applying subwavelength nanostructures [6]. Due to the difference of OLED materials, layered structures and emission pixel areas, different luminous improvement was observed for different colors [35–37]. In this work, the emission area was thought to be the main factor for the luminous enhancement. As shown in Fig. 7(a), the blue pixel has the largest emission area compared to the green and red pixels. For the large blue pixel, the applied nanostructures extract the light from the area away from the measured region, which contributes to a significant luminous enhancement. On the other hand, for a small red pixel, such as red pixel, only little waveguiding light from the area away from the measured region can be extracted. So the luminous enhancement of blue pixels at normal direction was 54.8%, which was higher than green pixels (48.0%) and red pixels (41.6%). And for specific OLED, the luminous enhancement is positively related to the AR effects of the applied nanostructures because the enhancement mainly comes from the reduction of total internal reflection on substrate surface. The applied nanostructures with best AR effects also lead to best light coupling improvement, so we chose the AR film imprinted by 9-min-etched Si template for the image test.
Fig. 7 Images of single color frame (a) and Lena standard (b) under dark ambient and luminous enhancement (c) of commercial OLED panel with and without polycarbonate AR film attachment.
The angle-depend luminous enhancement was shown in Fig. 8. In order to further understand the image improvement after applying the graded-index AR nanostructures under strong ambient illumination, Lena standard test image was displayed on the commercial OLED panel, which is shown in Fig. 8. And universal image quality index (Q) was introduced as [38]:
Here, and are the gray scale values of each pixel of the OLED panel in dark condition and bright ambient. N is the total pixel number. and are the averages, σx and σy are the standard deviations, and σxy is the covariance of x and y, respectively, Hence, the first term of Eq. (1) illustrates the correlation between the OLED panel pictures under different ambient condition. The second one shows the luminance difference between the two images. And the final term is the contrast distortion. The calculated results of image quality index are shown in Table 1. The panel attached by the AR nanopillars exhibited a much higher image quality index (0.92) than the regular one (0.854). The main reason came from the smaller contrast distortion term (0.99) after attaching AR nanostructures, compared to the case with flat surface (0.94). Generally, the contrast ratio in display application can be expressed as [39]:where Lon and Loff are the luminous intensity of turn-on and turn-off pixel, respectively. And Lambient is the luminous intensity of the reflected ambient light. By depressing the ambient reflection, the contrast ratio was greatly improved.Fig. 8 Lena standard test images with and without graded-index AR thin film attachment under bright ambient condition.
Table 1. Image Quality Index of OLED Panel in Bright Ambient Condition
4. Conclusions
In summary, we demonstrated a relatively convenient and low-cost method to fabricate large-area antireflection nanostructures by using nanoimprint techniques with graded-density Si template. The average reflectivity of the patterned polycarbonate substrate in visible wavelength region was depressed from 10.2% to 4.8% for the Si template with nanoparticle-catalyzed etching for 9 minutes. After attaching on the commercial OLED panel, the enhancement of light extraction and reduction of reflected ambient light were observed. The luminous enhancement ratio at normal direction for RGB single frame is 41.6%, 48.0%, and 54.8% respectively. Due to the improvement of image contrast, the quality index of the Lena image test on the OLED panel was improved from 0.85 to 0.92 under strong ambient illumination.
Acknowledgments
This work was supported by National Science Council, Taipei, Taiwan, under Contract No. NSC-101-2627-E-002-005, NSC-100-2120-M-007-006, NSC-100-2221-E-001-010-MY3 and SC 102-2221-E-131-030-MY2.
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