After two decades of intensive study and testing, flexible displays are just about ready to take off. Researchers have strived to find the right combination of flexible glass, polymer, and metal-foil substrates along with thin-film-transistor (TFT) backplates—a combination that will turn the flexible display into a commercial reality. Ultimately, they're looking to produce a thin, flexible, clear substrate with the barrier properties of glass.
Anticipated mass-market applications include newspapers, books, and magazines as alternatives to paper; point-of-sale (POS) terminals; outdoor and indoor signage; smart cards; and labeling for retail shelves. The technology's potential in the automotive market looms particularly large, with windshields and dashboards as well as bumper stickers, upholstery, GPS, and other infotainment functions.
Market research company iSuppli Corp. expects the flexible-display market to ramp up from nearly nothing today to about $338 million by 2013. Market analysis firm NanoMarkets has forecast a $668 million "paperlike" displays market by 2008, though most electronic-paper displays are rigid, not flexible. Often called electronic ink (e-ink), electronic paper involves the deposition of electronic functions embedded in ink films that are deposited on a flexible substrate material.
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Manufacturers of flexible displays are looking at existing processes used to make rigid displays, like LCDs, to create low-cost flexible displays. They're also investigating flexible substrate materials like plastic, flexible glass, metal foils, and polymers, as well as display materials like electronic ink (electrophoretics), LCDs, organic LEDs (OLEDs), and even LEDs.
Yet they're finding that moving from traditional rigid substrates used in the manufacture of ICs and display materials to a flexible substrate isn't easy. Many flexible materials can't handle the high processing temperatures encountered when making rigid displays.
No single material can satisfy the requirements of both substrate and deposited electronics during manufacturing. Such bendable materials can't reliably operate at high temperatures without being affected by stresses. There's also a need for laminate adhesives that can perform reliably at high temperatures without being affected by stresses.
The technology often is confused with electrochromic displays, another form of electronic ink. According to Dave Jackson, director of marketing and planning at E Ink Inc., electrophoretic displays function by moving clusters of charged, colored particles in an electric field. In contrast, electrochromic displays contain chemical compounds that change color when an electric current is applied.
Both are reflective and can hold their image with no power. Yet electrochromic displays typically require a large amount of power to drive the color-changing reaction, making them less energy-efficient than electrophoretic displays.
So far, electrophoretic inks printed on rigid plastic substrates have had the most commercial success in devices that are generally known as electronic-paper displays (EPDs). E Ink has pioneered and holds many patents for e-ink technology. EPD substrates comprise tiny pockets containing charged particles suspended in an opaque liquid ink.
An electrophoretic display is an information display that forms visible images by rearranging charged pigment particles via an applied electric field (Fig. 1). Electrophoretic displays are considered prime examples of the electronic-paper category because of their paper-like appearance and low power consumption.
In the simplest implementation of an electrophoretic display, titanium-dioxide particles approximately 1 mm in diameter are dispersed in a hydrocarbon oil. A dark-colored dye is also added to the oil, along with surfactants and charging agents that cause the particles to take on an electric charge. This mixture is placed between two parallel, conductive plates separated by a gap of 10 to 100 mm.
When a voltage is applied across the two plates, the particles migrate electrophoretically to the plate bearing the opposite charge from that on the particles. When the particles are located at the front (viewing) side of the display, the display appears white, because light is scattered back to the viewer by the high-index titania (titanium-dioxide) particles.
When the particles are located at the rear of the display, it appears dark because the colored dye absorbs the incident light. If the rear electrode is divided into a number of small picture elements (pixels), an image can be formed by applying the appropriate voltage to each region of the display to create a pattern of reflecting and absorbing regions.
Examples of commercial electrophoretic displays include the high-resolution active-matrix displays used in the Sony Librie and Sony Reader, as well as the iRex iLiad e-readers. These displays are constructed from an electrophoretic imaging film manufactured by E Ink. Also, the Motorola Motofone uses the technology to achieve its remarkable slimness.
The Sony Librie is the first commercial product using an EPD. Launched in April 2004 in Japan, this electronic reader utilizes E Ink's Imaging Film EPD. VIT is manufacturing transportation signage in Europe using E Ink's EPD technology. Neolux and Midori Mark take advantage of it in their retail signage applications located across Japan. Various watch and clock companies have developed new product concepts with E Ink's technology as well.
More recently, Netherlands-based Polymer Vision came up with an e-book reader based on E Ink's technology. Known as the librofonino, the 5-in. (13-cm) rollable display includes a cellular connection. Telecom Italia will introduce it later this year in Italy.
Plastic Logic Corp. is continuing work on flexible all-plastic displays using E Ink's imaging film and a novel printed backplane manufacturing process. The company recently partnered with Innos Ltd. to establish the world's first production facility for organic semiconductor-based rollable displays.
Plastics themselves aren't very friendly for flexible displays. They require relatively low manufacturing-process temperatures, which are typically much lower than those used in the processing of display materials. Finding the right balance between the characteristics of the plastic material and the display material's manufacturing processing requirements has been very difficult. Alternatives like flexible glass and steel metal foils are more attractive.
Nevertheless, because plastics are so pervasive, many companies are still searching for the right plastic materials in the creation of flexible displays. Plastic Logic's attempt at flexible all-plastic displays uses E Ink's imaging film as well as a unique printed backplane manufacturing process. The firm refined a process for organic TFT backplane deposition that's fully compatible with very low-glass-transition temperatures and inexpensive plastic substrates (Fig. 2).
Other efforts target printed inks. Bridgestone's Electronic Liquid Powder bi-stable image technology promises to challenge LCDs in flexible displays. The company's Quick Response Liquid Powder Display (QR-LPD) prototype is 0.29 mm thick and has a rapid response time of 200 ms.
But EPD switching speeds are lacking. On the order of 100 to 500 ms, they're too slow for video, which typically requires a refresh rate of 15 ms. Active-matrix LCDs can achieve switching speeds from 10 to 50 ms, while OLEDs can switch even faster at approximately 100 ms. Nevertheless, researchers feel they're on track to improving EPD switching speeds, which can reach 100 ms.
In a radically different approach, Imaging Systems Technology implements a plasma gas. The company's new flat-panel-display technology uses gas to encapsulate microspheres (Plasmaspheres) as the pixel element. The Plasmaspheres consist of encapsulated glass shells that contain ionized gas and can be deposited on a variety of rigid as well as flexible substrates for practically any display size (Fig. 3 and Table 1). Because the Plasmaspheres act essentially as capacitors, they don't need any power for refreshing.
A CLOSER LOOK AT LARGE LCDS
Many display experts believe LCDs are most likely to succeed in large-scale, cost-effective flexible displays (Fig. 4). Many developments strengthen this view. For example, Samsung has crafted a prototype 5-in. diagonal LCD panel that uses amorphous silicon TFTs. Also, Fujitsu has shown two versions (monochrome and color) of a cholesteric LCD, flexible, 3.8-in. diagonal panel.
On another front, HP Laboratories has designed an electrophoretically controlled nematic (EPCN) flexible LCD prototype. The device uses bistable, passive-matrix color LCDs on a plastic substrate. Unlike active-matrix displays, which have a TFT embedded in each liquid-crystal pixel, this prototype needn't be refreshed. Thus, it remembers its on and off states for as long as needed.
Researchers at South Korea's Hangyang University are proposing pixel-isolated LCDs (PILCDs) for enhancing a flexible LCD's mechanical stability (Fig. 5 and Table 2). PILCDs with 3-in. diagonals are possible, say the researchers. Since the liquid crystal molecules are isolated in pixels by patterned or phase-separated microstructures, the LC alignment is stable, better suiting their design for continuous-roll processing (see "Roll-To-Roll Processing Becomes A Priority,").
However, LCDs don't operate well on a bent substrate. In a conventional LCD, two glass plates sandwich a TFT layer embedded in amorphous silicon. Using a flexible polymer film instead of glass affects the LCD's image quality. This quality depends on the cell gap between the polymers and distorts the image, making it very difficult to view at different angles.
OLEDs are another competitive technology (Fig. 6). Based on the electroluminescence of organic compounds, OLEDs are brighter than LCDs. They also offer wider viewing angles and faster response times. Unlike LCDs, OLEDs don't need a backlighting source, so they're thinner and lighter than LCDs, too. These are crucial attributes for many military applications, where soldiers are otherwise burdened with loads of electronic gear.
One of the biggest attributes of OLEDs is their low power dissipation. Bi-stable OLEDs draw power only when they're on, which means lower power dissipation and longer battery life—obviously very attractive attributes for portable electronics. OLEDs are also more durable than LCDs. They're about 10 times more impact-resistant than plastic-substrate LCDs and 100 times more than glass LCDs.
OLEDs have some important drawbacks, though. They require a strong barrier against moisture. Using them on plastic substrates lets moisture move easily through the substrate. Though their lifetimes have improved more recently, they're limited, particularly for the color blue. And, compared with LCDs, OLEDs have limited lifetimes of about 20,000 hours, which translates into over two years of continuous use.
Nevertheless, hundreds of companies and academic laboratories are working on developing the right combination of substrate materials and electronics to enable the large-scale manufacture of flexible displays. For instance, DuPont is investigating the use of polyester films. The firm's Teijin facility in the United Kingdom is putting together a family of engineered substrates for flexible displays and their attendant electronics.