Flexible Displays Power
New Applications
Flexible materials will replace glass in ubiquitous LCD displays.
On the train home from work, a businesswoman reads the newspaper, then rolls it up and carries it home. She then unrolls the paper and slaps it against the wall of her kitchen, where it begins showing her favorite television program as she cooks her dinner. It is science fiction today, but with advances in flexible display technology, it is perhaps not many years from becoming reality.
Most displays today, on computers, cellphones, and televisions, are built on glass substrates. Aside from the obvious issue of fragility, there are secondary concerns of weight, scalability, and portability that all drive the desire to make flexible displays. In addition, flexible substrates offer the hope of substantially lower costs if they can be moved from the batch processing required for fixed displays to continuous roll-to-roll processing. There is a substantial obstacle, however, in the form of high processing temperatures.
The traditional liquid crystal display (LCD) is a sandwich construction, starting from a bottom layer for the backlight and its associated optics, then a pair of glass layers on either side of a layer of nematic liquid crystals, a polarizing layer, a color filter, and perhaps an independent environmentally protective layer. The problem lies in the glass/liquid crystal sandwich layer. A strength of LCD display designs is that they take advantage of materials and methods from semiconductor manufacturing, but with the disadvantage of high processing temperatures. So when designers look to build a system around a flexible substrate, the first issue they must address is that of temperature compatibility, and that is determined by which type of display elements will be used.
Light reading
By combining
an array of organic transistors on a flexible substrate with a sheet of electrophoretic ink microspheres, Polymer Vision is able to create “electronic paper” that can be rolled into diameters as small as 7.5 mm. The rollable display can be hidden within a small device, meaning the display can actually be larger than the basic device itself. Image: Polymer Vision |
The traditional LCD architecture is a transmissive design, where light comes from the back and propagates through the layers to the viewer. Reflective designs use ambient light to provide contrast when reflected from dark and light display pixels. In emissive displays, the pixels are each individual light sources. One method of producing a reflective display is using electronic ink, a medium that changes its visible reflectance with an applied voltage. E Ink Corp., Cambridge, Mass., uses an electrophoretic electronic ink. Electrophoresis is the process by which charged colloidal particles move through a liquid under the influence of an electric field. If the voltage is applied in a spatial pattern, that pattern will be duplicated by the charged particles and appear as a visible display.
The E Ink technical approach is to put both negatively-charged black-pigmented and positively-charged white-pigmented particles in microcapsules, each just a few tens of microns in diameter. The filled microspheres are manufactured as a thin, flexible plastic imaging film. E Ink provides the frontplanes to their development partners, who then attach the frontplane to an electrode array. Polymer Vision, Eindhoven, the Netherlands, laminates the E Ink frontplane to a flexible backplane that contains an active matrix thin film transistor array.
According to Edzer Huitema, chief technical officer of Polymer Vision, the key to introducing flexibility into the backplane was to fabricate many of the layers of the active matrix from organic materials rather than inorganic semiconductors. The organic materials can be applied at room temperature and retain a minimum feature size of 5 µm. This approach, he says, “enables the processing of a complete active matrix display on 25-µm-thick plastic substrates.” The assembled front- and back-planes form a display only about 100 µm thick, important because the minimum roll radius of the finished display is about 50 times the thickness.
The approach has paid off, as evidenced by the recent announcement of Polymer Vision’s new mobile device, smaller than a typical cell phone, that incorporates a 12.7 cm display which can be rolled out from and returned into the handset. The device allows users to download many forms of information available through Telecom Italia’s network. “Our vision is that over the coming years we will see a rollable display in every mobile device,” says Huitema.
Heavy lifting
Imaging Systems Technology (IST), Toledo, Ohio, is looking to move into the opposite end of the display market: large displays. Flexible displays offer advantages in this market due to the lower weight and decreased fragility of the display panels.
IST uses an emissive plasma display approach. Traditional plasma displays encapsulate an ionizable gas between two sheets of glass. IST’s approach is to replace the continuous plasma enclosure with a large number of discrete “plasma spheres”, which can be produced in a range of diameters from 0.25 to 3 mm. Where a traditional plasma display needs to maintain a uniform air gap and dielectric layer thickness across a large display area, the plasma sphere approach levies those requirements at a sphere-to-sphere level, easing the constraints on the substrate. The bottom substrate can be flexible, and the top substrate is not even necessary. The reduction in weight provides for cascading reductions in shipping and installation costs, but even more significant is the potential savings with roll-to-roll manufacturing.
According to Carol Wedding, IST’s president, the advantages of their approach extend well beyond cost of manufacturing. Output of the plasma spheres is voltage-controlled, which reduces the complexity of the electrode structure and makes it easy to scale to very large sizes. The plasma sphere technology is also an emissive—and very bright—technique, which lends itself to high-update rates suitable for video images. They have produced laboratory prototypes of monochrome displays with a brightness of 14,000 cd/m2.
Everything in between
There are dozens of other companies working on one form or another of flexible display technology. Another emissive display technology, using organic light emitting diodes (OLEDs), is compatible with low temperature processing, and companies are working on integrating OLEDs with flexible substrates, transparent electrodes, and low temperature active matrix arrays. Coming at the problem from the opposite side, Schott North America, Elmsford, N.Y., is offering a high-temperature flexible substrate: glass; but a glass so thin that it can be conformally wrapped for unique display applications.
The U.S. Display Consortium (USDC), San Jose, Calif., is working to support the development of flexible display technology by sponsoring work across the spectrum of technologies. Brett Bryars, director of technology programs at the USDC, notes that there are still major challenges in going to flexible displays on plastic. In addition to the low temperature processing requirement, plastics are typically permeable to oxygen and moisture, which can wreak havoc on the internal display structures. But, Bryars notes, “the technology for barrier layers is just being commercialized by several companies.”
Looking at the global issues that new approaches must address in order to be successful, Barry Young, senior VP at Display-Search, Austin, Texas, sees opportunities in the short term as replacements for rigid displays, but with the real opportunity lying in new applications, where glass does not fit. When will it happen? According to Young, “flexible displays will thrive when the front of the screen image is acceptable to the viewer and the production is based on a roll to roll process to keep the prices down.”
—Richard Gaughan
Founder and chief engineer at Mountain Optical Systems Technology, www.mountainoptical.com.
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