Displays from Transparent Films of Natural Nanofibers
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Transparent Films of Natural Nanofibers
Antonio Norio Nakagaito, Masaya Nogi, and Hiroyuki Yano Abstract Organic light-emitting diodes bring a whole new level of image quality, power consumption, and very thin profiles to displays. In addition, with the appropriate choice of a flexible substrate, paper-like flexible displays that are lightweight, robust, and conformable can be produced. This will make it possible to roll or fold the displays for portability or incorporate them in clothing as wearable displays. Plastic substrates are considered prospective materials due to their inherent flexibility and optical qualities. However, one of the major drawbacks of plastics is the large thermal expansion. The thermal expansion of the substrate has to be compatible with those of the layers deposited on it, otherwise these layers will become strained and crack during the thermal cycling involved in the display manufacture. One of the proposed solutions to reduce the thermal expansion of plastics without appreciable loss in transparency is to reinforce them with nanofibers. These nanofibers are already available in enormous quantities in nature, in the form of cellulose, with the caveat that they have to be extracted properly. Here we present the methodologies required to obtain the cellulose nanofibers and to produce optically transparent composites for use in flexible displays.
Introduction Organic light-emitting diodes (OLEDs) represent a display technology that brings bright, vivid color, high-contrast-ratio images with fast response time and wide viewing angles to the next generation of televisions. Because the luminescent materials emit light, the backlights and color filters of traditional liquid crystal displays (LCDs) are not required, so the screens can be made extremely thin with low power consumption. A LED is comprised of a light-emitting polymer (LEP) film approximately 100 nm in thickness sandwiched between a metallic cathode and an optically transparent anode, all deposited on a transparent substrate. The cathode is typically a metal, like calcium (Ca) or magnesium (Mg), whereas the anode is usually indium tin oxide. When an electrical signal is applied to the electrodes, charge carriers in the form of holes from the anode and electrons from the cathode are injected into the LEP layer. There, by electron-hole recombination, excitons are generated by
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radiative decay in a phenomenon called electroluminescence (EL), which is in essence the emission of light.1 OLEDs are considered the preferred technology to make flexible displays. The all solid-state structure with active layers less than 1 μm thick is highly appropriate to attain flexibility. The current commercial display technology relies on rigid glass substrates where the active layers are deposited, as in the case of LCDs. However, flexible displays offer many benefits, such as thinness, lightness, robustness, conformability, and the convenience of rolling them away when not in use. The natural candidate material to substitute glass in flexible di
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