USING Polymer Technology FOR HIGH-INFORMATION CONTENT FPDs.

Polymers, long valued for their low cost, ease of manufacture, strength, and ability to fit a wide range of applications, may prove to be the best road to inexpensive, high performance Flat Panel Displays (FPDs). A number of organizations are developing flat panel display technologies using easily processed organic or polymer compounds in designs based on electroluminescent devices. The first polymer flat panels will be available commercially later this year.

Of the various technologies vying to displace Liquid Crystal Displays (LCDs) as the dominant flat panel technology, Electroluminescent Displays (ELDs) are the only completely solid state display design and are unmatched for ruggedness. Inorganic electroluminescent displays have established a solid niche in several applications, including medical, scientific and industrial instrumentation, transportation, and specialized business applications.

The highest volume application for ELDs today is switchable illumination for wristwatches and clocks. In their quest to come up with computer monitors and other high information content displays based on ELDs, industry developers are struggling with durability issues, and concerns such as inadequate gray scales and incomplete color ranges. Another issue for ELDs is their cost, still relatively high compared to competing technologies.

Organic LEPs

A number of organizations are avoiding the problems of conventional ELDs by developing designs that use organic materials to fabricate organic Light Emitting Polymers (LEPs) as the basic light-emitting element in the display. Organic materials offer display engineers the ability to fine-tune a wide range of properties, including the conduction of electricity and the emission of light. LEPs are layered structures, similar to conventional ELDs, wherein an organic compound is sandwiched between two electrode layers. The choice of materials and deposition processes distinguishes the various LEP technologies.

Designing LEP Displays

The design of displays with LEPs is simple: a conducting transparent anode layer such as indium tin oxide is deposited on a glass substrate, followed by a conducting polymer layer (See Fig). After the conducting polymer layer is dried, the emissive polymer is deposited in a pattern and can include any of a wide range of suitable polymers, including Polyphenylene Vinylene (known as PPV), polyfluorenes, or others. For color displays, three emissive polymers will be deposited in an appropriate pattern to provide the three primary colors. The last layer is a cathode metal, such as calcium. Connections are then formed and the panel is encapsulated.

The LEP process can be adapted from an established LCD production line, dramatically simplifying the manufacturing process. For both processes, making the backplane with TFT devices and wiring is the same. The LEP process from this point requires only printing polymers, application of the cathode layer, forming of connections, and encapsulation. The LCD manufacturing process by contrast involves placing an alignment layer, inserting spacer beads, placing the top glass layer, filling the cavity with liquid crystal and sealing, placement of color filters and polarizers, backlights, etc.

In comparing the manufacturing costs of LEP displays and LCDs, throughput is a major factor. Filling a 20-inch LCD panel with liquid crystal alone may take 12 hours. Industrial ink jet printers with resolution of 360 dpi (more than enough for passive displays) and three-inch print heads can deposit an LEP layer at 22 inches per second. Ten-inch print heads are now nearing commercial availability and will print at about the same speed.

The use of printing techniques to manufacture electronic devices is an important development in itself and may find use in a wide range of applications beyond displays, including electronic circuits. Printing of electronics is of great interest in reducing cost, pollution, and energy consumption. While printing is unlikely to make inroads in high-end microprocessors, there are a great many opportunities for low cost, low bandwidth electronics where printing could offer enormous advantages.

LEPs will enter the market primarily in low-information-content displays. One of the most promising is the use of LEP panels as backlights for LCDs. While initial implementations of LEP backlights will probably incorporate backlight and liquid crystal display in separate modules, manufacturers could ultimately deposit the LEP panel directly on the LCD glass plate for a completely integrated unit with sizeable savings in weight and better illumination than conventional backlights.

The ability to convert a LCD production line to LEP includes both passive and active matrix displays. Most early versions of LEP displays will probably be passive because of their relative ease of manufacture and resultant short learning curve. Passive displays do, however, present some liabilities. To preserve image brightness, passives need high voltage drive electronics and lead to a consequent 10 percent loss of luminous efficiency. Active matrix displays, on the other hand, are more expensive in fabrication because the electronics must be deposited with a polysilicon line (which, if already in place for LCDs, is easily adapted to LEPs). Active matrix panels are nearly quasi-dc systems, where pixels remain on during the entire refresh cycle. Therefore, equivalent brightness for the active matrix is achieved with lower supply voltage, near-peak efficiency, and superior resolution.

Active matrix technology also offers the option of integrating much of the display's electronics directly on the glass plate for additional weight reduction and simplified interconnection to other systems. The issue of passive versus active matrix LEP displays will likely be played out over several generations of product, each offering advantages in specific applications.

LEP display characteristics compare favorably with other flat panel displays. Pixels today are 30 microns wide for color displays--more than adequate for HDTV applications--but will need further reduction for high-resolution monitor standards such as QSVGA. The size of LEP displays is limited only by the available patterning technology. Unlike LCDs, viewing angle is unrestricted. Color range for LEPs is already complete and will easily offer the millions of colors needed for high-information-content displays such as computer monitors. Response times are less than 5nsec, so that smearing of moving images will not be a problem. Extrapolated lifetimes already exceed 20,000 hours.

Organic LED displays also offer all the ruggedness of electroluminescent displays. While LCDs distort their image when the display is under pressure, such as by a finger pressing on the glass, LEP displays suffer no such effects.

LCDs will probably retain limitations in performance, including a restricted viewing angle and 50,000 times greater response time compared to LEPs with a consequent smearing of video. Recently developed LCDs that improve on a characteristic such as wider viewing angle force tradeoffs in other parameters such as response time. Ruggedness, low temperature operation, and daylight viewing remain serious problems for LCDs.

Still, there are, of course, a few technological issues in the way of broad commercial acceptance of LEP displays. For one thing, the panels must be sealed to protect against moisture and oxygen, which can quickly destroy the functionality of the display. Ordinary polyester-type plastics are inadequate for such protection and LEP-based displays require more costly plastics with better properties. PPV materials are susceptible to electrochemical instability that can shorten display lifetimes, although improved synthesis and the array of new organic materials available appears to have solved the durability question. The use of industrial ink jet printers for patterning will require the development of polymer solutions specifically designed for compatibility with the ink jet patterning process. The ink jet process itself must be adjusted to accommodate the registration and repeatability required for LEP-based displays. For LEP displays, problems in lifetime, color range, and brightness have largely been solved, though other hurdles remain.

The LEP market is essentially the FPD market and there are few, if any, applications for which LEPs are unsuitable. Passive LEP panels are ready for commercial introduction for low-information-content displays such as those used for small appliances and backlights. Progress on high-information-content and active matrix displays is quite rapid and prototypes are available, but device efficiency needs to be better understood before commercial production can begin.

The first commercial LEP product will be Philips' monochrome passive display in mobile phones and pagers, expected to reach the market in late 1999. As production gears up for more sophisticated applications and includes active matrix displays, LEPs will find application in palmtop and handheld PCs, laptops, navigational and automotive instrumentation, handheld TVs, digital still cameras, camcorders, and later, as larger and higher performance displays are available, desktop displays, HDTVs, and others. By 2005, Display Search projects that non-PC applications of flat panel displays alone will form an $11 billion market. LEP developers are even anticipating the use of their semiconducting polymers in photovoltaic devices. In the display arena, LEPs will compete with other FPD designs such as field emission displays, plasma display panels, and other novel technologies, but given their low cost, ruggedness, and high performance, they will certainly establish a solid market in the near future.

Jeremy Burroughes is the technical director of Cambridge Display Technology (Cambridge, UK).