Thin, flat, and low power: The ideal display is (still) just around the corner

This year will mark a golden anniversary; in December 1953, the Federal Communications Commission approved RCA's proposal for a color-television service in the United States. It wasn't the first color-TV service, but it was the first to be based on a single CRT, and RCA developed the shadow-mask tube for it in the remarkably short time of only months. That RCA team would have no difficulty recognizing the CRT in almost every TV set produced worldwide today. The basic format of the shadowmask tube has remained essentially unchanged for 50 years, and it remains one of the most pleasing, versatile, and efficient large-display vehicles at our disposal today.

Domestic television is one of the few areas in which CRTs still hold sway, however, and the race is on to replace these CRT TVs with the long-sought, large-screen, flat-panel product. PDPs (plasma-display panels) and large TFT (thin-film-transistor) liquid-crystal panels have so far penetrated only the premium domestic market and look unlikely to achieve the manufacturing cost levels they will need to penetrate volume consumer markets. Meanwhile, the large-format, rear-projection domestic TV is almost entirely a US phenomenon and makes up a very small part of the European market. Technologies such as OLEDs (organic light-emitting diodes), which some hail as the ultimate victors in this race, are still a long way from fulfilling that promise—but they are starting to appear in smaller display formats and offer a new and useful option for human-interface components.

Smaller displays are differentiated more by application but have not for some time seen many fundamental innovations. However, the situation is now changing, and a wide range of consumer-, industrial-, instrumentation-, and portable-product options are coming to the market now or within months. Much depends on the appropriate level of complexity for the display function you need to implement. To begin at the top end of the spectrum, if your application demands full graphics (with or without color), the major option remains the LCD panel in passive-matrix (for smaller units) or active-matrix form.

One display trend has been to make LCD panels more versatile and readable in a wider range of ambient-lighting conditions; backlighting a standard LCD panel with enough intensity to provide full daylight readability requires considerable power. If you succeed, the power that the light source absorbs will likely consume a large part of your overall power budget.

Most of the major LCD-panel suppliers have been adding transflective full-color panels to their ranges. As their name implies, these panels work in either transmissive mode (with backlighting) or in reflective mode (using incident ambient light). However, their designers face an overall "photon-budget" problem. For a display to operate in reflective mode, each pixel site must contain a path for ambient light to reach a reflective element lying behind the active layers of the display; the light reflects and returns, through the liquid-crystal shutter and filters of the active pixel itself, to your eye. LCD designers must allocate part of the area of each pixel site to provide that window for incoming light, reducing the percentage of the pixel site that's available for the display itself. Because the display loses efficiency in light transmitted in backlit mode, you must step up the backlighting level to achieve equivalent display brightness. As a consequence, little incident light ever makes it back out to the outside world in reflective mode. (Recall that, for an average TFT LCD panel, the fraction of the light incident on its rear surface that makes it to the outside world is a single-digit percentage. The LCD makes a better shade than a window!)

Transflective-display readability can look good in reflective mode, but, to achieve this readability, the display requires incident-light levels approaching full sunlight or at least bright daylight. You cannot run displays in reflective mode in most indoor environments and expect the readability to be comparable with backlit mode in low ambient light. However, backlit and incident-light modes are additive (allowing for reflection off the front of the panel), and if your power budget allows the use of continuous backlighting, you can have a pleasing display that is readable in a wide range of ambient-light levels.

Other factors to take into account include the viewing angle. Although designs vary, the reflection of incident light back to the pixel uses some degree of scattering or diffusion rather than simple specular reflection, which would produce a pronounced viewing-angle-pattern shape. Nevertheless, designers will have made some assumptions about the likely positioning of the ambient-light source relative to the viewer's eye, and you should expect some limitation or variation of the acceptable viewing angle as displays change from predominantly backlit to reflective mode. Published light-level figures and viewing-angle polar diagrams are a guide, at best; you will need to power up a sample and view it from all angles while altering the ambient lighting to get a real grasp of its appearance in your application.

But what if your application has a low power budget and cannot afford the backlight or even the few milliamps necessary to continuously drive the LCD panel itself, but you would like the panel to display information all the time? About to come to market is a technology that, in effect, gives an LCD cell a "memory." With this construction, you will be able to write an image to an LCD and remove the power, and the display will retain the image virtually indefinitely. ZBD (Zenithal Bistable Devices) created this technique, which adds a finely ridged grating to the inner glass surface of an LCD cell of STN (super-twist-nematic) construction. (Recall that a passive LCD is multiplexed; the drivers scan the pixels row by row, and persistence effects give a continuous display—hence, the need to move to an active device, or TFT, with per-pixel transistor switches, when pixel count rises.) The presence of the grating "latches" the polarization state of the liquid crystals and retains it when you remove the power. This ability allows passive STN devices to compete with TFTs, because they retain the state of each pixel from one scan to the next, and they retain the overall image with no power. Despite the fact that the latching is in some sense a mechanical effect—at the molecular level—ZBD says that mechanical shock does not disturb the stored image; it remains until you apply power and rewrite it. Further, variations in the grating structure allow parts of a pixel cell to latch at different applied voltages, so a display can store a gray-scale image of at least seven and possibly as many as 64 levels, the company says.

And what of those small color panels that the latest generation of cell phones features? Those, too, are becoming available to a more general market. At Sony, LCD product manager Matthew Tapping notes that only two factors really matter for the cell-phone market: how good the display readability looks and, especially, appearance and its power consumption. Sony is building active panels using low-temperature polysilicon with all the drivers on the glass itself, including dc/dc converter, row and column drivers, chip selects, and a timing generator. For handheld products, this setup yields a 1.94-in. (49.3-mm) display of 128×160 pixels (portrait format) using a vertical-RGB-stripe mode that measures 37.1×51.5×3.2 mm. Cell phones will first use the unit, which will become available for sampling in June and begin production in August. Driving this display with moving pictures, at 25 frames/sec and using a full palette of 65,000 colors will require about 3 mW, Tapping notes. This requirement is dwarfed by the 150 to 200 mW necessary for the backlight. If a battery drives your handheld application, you may need to reduce the inactivity time-out until the backlight switches off to only a few seconds.

Work is also continuing on improving the characteristics of the deposited silicon (on the glass panel of the LCD cell) in which the TFTs are fabricated. Polysilicon is better than amorphous silicon and better still is so-called C-G (continuous-grain) silicon. C-G silicon exhibits electron mobilities several times those of polysilicon and several hundred times those of amorphous silicon. C-G silicon's properties are sufficiently close to those of bulk silicon to allow you to integrate more complex circuitry, such as drivers, on the same substrate. Sharp, for example, calls this concept its System LCD. As the driver devices diffuse into the deposited silicon, the space that packaged or surface-mounted chips occupy becomes smaller, and the assembly can be significantly thinner.

Nevertheless, some industry watchers expect some form of OLED to eventually supersede LCD, and manufacturers have spent years developing these devices. So, you can now buy and use displays based on OLEDs. OLED technology is emissive; layered deposition of organic semiconductor materials, rather than the more familiar metallic semiconductors, form light-emitting structures. Two fundamental technologies, generically known as "small molecule" and "polymer," are under development. Their key proponents are Kodak and CDT (Cambridge Display Technology), respectively. Both enterprises license the IP in their technologies to display manufacturers. After years of development, OLEDs have enough absolute brightness to offer good contrast in bright ambient lighting. They are thin and require no backlighting, and—as the light is emitted from (close to) the surface of the structure—they are efficient, with none of the losses of, for example, an LCD panel in which the light is attenuated as it passes through polarizers and filters. Further efficiency comes from the fact that only the active pixels generate light and consume power; the subtractive structure of an LCD requires full backlighting all the time. OLEDs have a wide viewing angle, and they are fast, active-electronic devices. Full video data rates and operation at low temperatures are well within their switching rates. You can design OLEDs to emit all three primary colors with good purity, and these colors combine to give a good color gamut.

Beyond those features, however, OLEDs offer the possibility of large, low-cost displays on flexible substrates. Manufacturers can deposit the organic materials by printing processes; CDT and other companies have demonstrated the feasibility of ink-jet printing of full-color displays. As with other matrix-based displays, you can structure OLEDs as passive-matrix devices, which are adequate for lower resolution and display size, or active-matrix devices for higher resolution and panel size. And, as with other active-matrix devices, you can build the switching transistors in any of the available deposited-silicon types. The ultimate combination of the possibilities that OLEDs hold might be a 1m or larger, full-color graphics display that you produce in volume by ink-jet printing the active materials onto a flexible substrate, perhaps thin enough to call a film or foil, on a continuous, reel-to-reel production facility.

Needless to say, that dream is some ways off. The multifaceted problems of handling the organic chemistry that OLEDs involve hamper their development. Although researchers have produced all three primary colors, blue has been more difficult to create than red or green and still lags behind those colors in ultimate lifetime, constraining progress toward full-color displays. As the emitters age, a display built in early materials would suffer a color shift due to differential aging effects. Absolute lifetimes (defined as the time to the –3-dB light-output level) now extend to acceptable values for commercial devices in all three colors. CDT quotes 11,000 hours for blue emitters versus 40,000 hours for green and 60,000 for red (Figure 1). Therefore, many of today's first product offerings are monochrome, although with a choice of color. However, Kodak has announced the AM550L full-color OLED module, which it uses as a digital-camera-back display on its Easyshare LS633 camera. The QVGA (521×218-pixel) module measures 5.5 cm diagonally, and you can evaluate it with the AMEV1-100 kit, which includes a controller, software, and interfaces to standard video-driver boards. Kodak produces the display through a joint venture with Sanyo (SK Display Corp) and, also with Sanyo, has demonstrated 5.5- and 15-in.-diagonal, active-matrix, full-color panels.

Contact with the atmosphere and with water vapor in particular rapidly degrades OLED chemistry, so sealing the deposited chemistry is critical. For this and other reasons, first offerings have been on glass substrates; plastic and flexibles will follow.

One of the first of CDT-technology-based OLED displays to appear on a product is on the Norelco Spectra electric razor.

Typical of the first-generation displays beginning to emerge is the product currently shipping from DuPont Displays, a passive-matrix monochrome device of 128×64 pixels with 2.1-in. diagonal measurement and a yellow/green emission (Figure 2). DuPont has announced a 160×160-pixel version of the same technology and a QVGA color active-matrix display that measures 4 in. diagonally. DuPont, developing polymer OLED technology from CDT, exemplifies the network of intercompany cooperation that is attempting to drive OLED forward; it has agreements, for instance, with UDC (Universal Display Corp), covering aspects of small-molecule and polymer chemistry.

Other companies that have announced or are shipping OLED products include Philips and Densitron, which offers a dot-matrix part of 64×128 pixels, plus a 132×176-pixel, 2-in.-diagonal, full-color display that offers 40 to 50-cd/m² brightness. Densitron has also built a 250-cd/m², single-digit, 47-mm-high display with OLED technology for use in public-information displays.

OLEDs are current-driven devices that require dedicated driver ICs to achieve the correct performance in lifetime, brightness, and gray scale. One of the first sources of such drivers is Clare Micronix (Figure 3); offerings are also available from TDK, STMicroelectronics, and Philips Semiconductors.

Operating on a somewhat different structure is a small color panel from TDK; TDK can supply multicolor but not full-color OLED devices, using a composite OLED structure to produce white light, which it then filters to yield the colors (Figure 4). You can have 16 colors (two base colors with four-level gray-scale drive) or 64 colors (three base colors). TDK has for some time been supplying monochrome OLED displays in small dot-matrix panels to the automotive market and will develop the technology toward full color. The current product offers an active area of 23.5×79 mm with 240×64 dots.

Meanwhile, a consortium of companies cooperating with the objective of establishing flat-panel OLED displays in Germany has formed DFF, the Flat-Panel Display Forum (www.displayforum.de). The forum has 79 members, 13 of which are moving toward a pilot manufacturing facility.

At a lower level of complexity, a vast market exists for products that present a limited amount of data. Chris Barnardo, marketing director at Pelikon, quotes estimates that the segmented, nonmultiplexed-display market is worth some $4 billion worldwide. Pelikon is exploiting electroluminescent-emission technology to produce a custom product called InterfaceDisplay. Using this technology, you can produce a display with fixed segments or areas of illumination as small as 1×1 mm or as large as required in a flexible substrate less than 0.5 mm thick. The display is not usable outdoors, because its absolute brightness is too low, but it is readable in all indoor ambient-light conditions, and you can dim it to provide a low brightness level for use in darkened conditions. The technology sandwiches the light-emitting material between two planar electrodes; the front-face one is transparent and excited by an ac drive voltage of 100 to 200V at 200 Hz to 1 kHz. Pelikon has developed a 32-channel ASIC to accompany the printed-pattern technology to generate the driving voltage from supplies as low as one battery cell. The ASIC includes a microcontroller core, and Pelikon can package the entire offering as a customized option with a tailored display pattern and software.

Author Information

You can reach Editor Graham Prophet at +44 118 935 1650, fax +44 118 935 1670, e-mail gprophet@reedbusiness.com.