Organic Materials Are Poised As Never Before To Transform the world of circuit and display technology. Major electronics firms such as Philips and Pioneer, and smaller companies such as Cambridge Display Technology, Universal Display, and Uniax, are betting that the future holds tremendous opportunity for the low cost and sometimes surprisingly high performance offered by organic electronic and optoelectronic devices. Using organic light-emitting devices (OLEDs), organic full-color displays may eventually replace liquid-crystal displays (LCDs) for use with laptop and even desktop computers. Such displays can be deposited on flexible plastic foils [Fig. 1], eliminating the fragile and heavy glass substrates used in LCDs, and can emit bright light without the pronounced directionality inherent in LCD viewing, all with efficiencies higher than can be obtained with incandescent light bulbs.
An organic passive-matrix display on a substrate of polyethylene terephthalate, a lightweight plastic, will bend around a diameter of less than a centimeter. The 18-mm-thick, 5-by-10-cm monochrome display consists of 128 by 64 pixels, each measuring 400 by 500 µm, and is being operated at conventional video brightness of 100 cd/m2. It was fabricated by Universal Display Corp., Ewing, N.J., with a moisture barrier built into the plastic that prevents degradation of the pixels.
Organic electronics are already entering the commercial world. Multicolor automobile stereo displays are now available from Pioneer Corp., of Tokyo, and Royal Philips Electronics NV, Amsterdam, is gearing up to produce both OLED backlights to be used in LCDs and organic integrated circuits. It is possible that soon, portable and lightweight roll-up OLED displays will cover our walls, replacing the bulky and power-hungry cathode ray tube that has been the television standard for 50 years.
Given the need for very low-cost (but not always high-performance) circuits for everything from smart cards carrying personal information, to building entry cards, to inventory control, it is reasonable to assume that within 10 years, the square footage of organic circuitry might exceed that of silicon electronics (though one expects that silicon transistors would still vastly outnumber and outperform those fabricated from organic materials.)
Organic semiconductors have been the subjects of intense scientific investigation for the past 50 years. During most of that time, these materials, primarily consisting of carbon, hydrogen and oxygen, were considered to be merely a scientific curiosity. Organic materials' weak intermolecular bonds in the solid state give them properties of both semiconductors and insulators; so their study has deepened our fundamental understanding of the electronic and optical properties of solids. But, organic semiconductors attracted industrial interest when it was recognized that many of them are photoconductive under visible light. This discovery led to their use in electrophotography (or xerography) and as light valves in LCDs. There were even hopes that very low-cost thin-film solar cells and superconductors could be made using such substances.
Unfortunately, the potential of active electronic devices such as solar cells, light emitters, and thin-film transistors remained unfulfilled for decades because organic materials have often proved to be unstable. Further, making reliable electrical contacts to organic thin films is difficult, and when exposed to air, water, or ultraviolet light, their electronic properties can degrade rapidly. Finally, the low carrier mobilities characteristic of organic materials obviates their use in high-frequency (greater than 10 MHz) applications. These shortcomings are compounded by the difficulty of both purifying and doping the materials.
But in 1987 Ching Tang and Steven Van Slyke of Eastman Kodak Co., Rochester, N.Y., successfully addressed many of these problems when they produced the first efficient light emission from a two-layer organic structure resembling a pn junction. The Kodak group used a class of synthetic dyes that is closely related to well-understood xerographic materials to develop a device called a small-molecule OLED that produced light with about 1 percent efficiency. The materials used consist of often no more than 30 or 40 atoms covalently bonded into stable, individual molecular units, called monomers.
While this first demonstration of reasonably efficient light emission at low voltage attracted interest from potential display manufacturers, particularly in Japan, the technology attracted public attention when, in 1990, researchers at Cambridge University in England under the direction of Richard Friend reported a similar effect in a semiconducting organic polymer film consisting of poly para-(phenylene vinylene), or PPV. Unlike small molecule compounds, polymers are long chain molecules whose monomer segments are attached in a continuous covalently bonded, high-molecular- weight chain. Polymers tend to be environmentally rugged and flexible although, like small molecules, their electronic properties can rapidly degrade when exposed to oxygen or water.