The flexible display's substrate is critical to e-paper's and e-ink's development. Many different types of materials are under investigation, including glass, plastic, polymer films, and metallic foils.
Such a substrate must be readily bendable and rollable. Additionally, it must accommodate different types of display technologies, such as LCDs, organic LEDs (OLEDs), electrochromic inks, and electrophoretic inks at high levels of brightness, contrast ratios, and resolution. It must offer low power dissipation. And while most such flexible substrates display monochromatic images, they should be able to provide full-color capability to enhance their market acceptability.
Coming up with a cost-effective process to meet such a tall order of requirements hasn't been easy. Still, notable advances already show the feasibility of different flexible substrate approaches. Continuing research and development efforts are expected to yield even better substrates.
Fujitsu Laboratories Inc. demonstrateda film substrate-based bendable color display at the Fujitsu Forum, held last year at the Tokyo International Forum (Fig. A). The company claimed the display was the first of its kind. Its vivid color images retained their memory function without the need for any electric power.
Last year, Samsung Electronics announced a full-color 7-in. diagonal VGA (640 by 480 pixels) flexible display that uses a transparent plastic substrate that's thinner, lighter, and more durable than conventional LCD glass panels. The display uses transmissive thin-film transistor (TFT) LCDs. Samsung was able to overcome plastic's heat sensitivity to high-temperature process conditions. The company devised a low-temperature process method to manufacture the display's amorphous TFTs, color filters, and liquid crystals.
Researchers at General Electric Plastics created a plastic-substrate system that they claim will lead to more flexible, lighter, more durable OLEDbased displays. They also say that it will be less costly to build than those based on glass substrates. The system combines a high-temperature Lexan polycarbonate film with a transparent coating that protects the display from oxygen and moisture.
The high clarity and high temperature resistance of the Lexan film enable the construction of a 125-mm thick substrate. According to the company, the substrate can withstand the heat involved in OLED fabrication and still allow optimal light transmission.-In addition, GE says the system should be amenable to high-volume-manufacturing processes, driving down cost.
Working with Universal Display Corp., researchers at Princeton University used a plastic substrate to develop a flexible organic OLED display. The 9.5- by 9.5-cm plastic sheet can be bent and deformed into a dome with a radius of curvature of 8.5 cm and will continue to work, despite an average tensile strain of 2.2%. Conductive and resistive Bayton P polymer materials were both used.
One of the more notable flexible plastic-substrates offers a 10-in. diagonal display from Plastic Logic (Fig. B). The company claims that it's the largest display of its type in the world. The display has SVGA resolution (800 by 600 pixels) and a four-level gray scale. It uses E-Ink's ink and is less than 0.4 mm thick.