Light-emitting polymers: Increasing promise

05/01/1997

TECHNOLOGY TOPIC

Light-emitting polymers: Increasing promise

W. Conard Holton

Light-emitting polymers (LEPs) are showing increased potential for display technologies as lifetime and manufacturing issues are addressed. Two small companies, several major electronics corporations, and academics around the world are developing the technology. The next few years will show whether LEPs can compete with more established technologies in applications ranging from backlighting products and simple dot-matrix displays to full-color flat panel displays.

Electroluminescence from ultrathin layers of organic materials has been of interest for several decades. Much of the initial development in organics was not of polymers but of small molecules made up of a series of rings, such as tris(8-quinolinolato) aluminum (III) (Alq) (Fig. 1a). These thin, sublimed molecular films remain a more developed technology, but polymers are beginning to close the gap (see "Small molecule, big competitor"on p. 164).

LEPs belong to a class of materials known as conjugated polymers. The semiconductor characteristics of these materials result from a molecular framework of alternating carbon-carbon single and multiple bonds (Fig. 1b). A delocalized p-electron system runs along the polymer chain and gives it the ability to support positive and negative charge carriers with high mobilities. To make a light-emitting diode (LED), one or more thin layers of polymer are sandwiched between two electrodes, one of which is transparent. The cathode injects electrons into the structure and the anode draws them out, in effect injecting holes behind. At the emissive layer between the poles, the high-energy electrons drop into low-energy holes, giving up their excess energy as photons (Fig. 2). Performance can be improved by techniques such as building heterostructures for carrier confinement. The transparent metal indium tin oxide (ITO) is commonly used to inject holes and transmit light. Calcium or aluminum are standards for electrodes.

In 1976, the first conjugated polymer was made by accident while researchers were synthesizing polyacetylene in the laboratory of chemist Hideki Shirakawa at the Tokyo Institute of Technology. Shirakawa then joined Alan Heeger and Alan MacDiarmid at the University of Pennsylvania. Here they vastly increased conductivity by doping the polymer with iodine [1]. In 1989, Richard Friend, Andrew Holmes, and colleagues at Cavendish Laboratory, Cambridge University, generated yellow-green light from a poly(p-phenylene vinylene) (PPV) LED [2]. Interest in commercialization grew quickly. To this end, Uniax Corp., Goleta, CA, was founded in 1990 by Alan Heeger, now at the University of California, Santa Barbara, and fellow professor Paul Smith. Similarly, the Cambridge researchers spun off Cambridge Display Technology (CDT), Cambridge, UK, in 1992.

Figure 1. Basic structures of a) Alq and b) poly (p-phenylene vinylene) (PPV).

Advantages and challenges

"The commercial viability of the technology [including Alq] will turn on small differences," says James Sheats, a scientist at Hewlett-Packard Laboratories in Palo Alto, CA. "Manufacturers look at the bottom line - what will do it at the least expense. Time to market is also important, and one technology may always be used if it`s used first." He adds, "At Hewlett-Packard, we`re trying to understand everything we can about the electroluminescence process in polymers. Once we understand it in one, we will understand it in others." He identifies temperature stability and mechanical robustness as two other important issues for commercializing LEPs.

The rewards of meeting these challenges remain high, since success will produce a technology with low-DC voltage demands (under 5 V), fast switching, patternability, flexibility, high resolution, and simple, low-cost manufacturing. To date, researchers have been able to demonstrate polymer LEDs with lifetimes (to 50% of output emission) of about 10,000 hr at 100-150 cd/m2. Other LEDs have lasted 2000 hr at 400 cd/m2 [3, 4]. A lifetime of 10,000 hr is often cited as the standard commercial goal, although it is far more than needed for short-lived consumer products and considerably less than the 100,000 hr that an inorganic LED may last. Approximately 3000 hr at 30 cd/m2 is considered necessary for low-illumination backlight applications.

A full range of colors from polymer LEDs has been achieved by researchers in the US, Europe, and Japan, and red-green-blue LEDs from Link?ping University (Sweden) and the Technical University of Graz (Austria) were reported at the Fall `96 Materials Research Society meeting. LEPs in the infrared and ultraviolet range have also been reported. Common polymers being researched include PPV, soluble PPV derivatives like poly (2-methoxy-5-(2`-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV), poly(p-phenylene), soluble polythiophene derivatives, and polyanilene.

Figure 2. Schematic polymer light-emitting diode.

Packaging a polymer LED to prevent degradation from moisture and oxygen in the atmosphere remains a significant problem, although progress is being made in developing plastic-based encapsulation, according to Mark Gostick, business development manager at Cambridge Display Technology. The current method, glass encapsulation, negates the flexibility that polymers would otherwise provide, but is suitable for many applications and provides manufacturing benefits. "This problem can be solved in time," he says.

"CDT is doing 90% of its work on green," he continues. "We`re still working with PPV to understand it from a synthesis point of view, and green is a good commercial color. We`re looking at blues seriously now, but lifetime is generally lacking so far." CDT is actively selling its precursor PPV material to encourage others to experiment with the material and discover how easy it is to work with. CDT unveiled its prototype LEP dot-matrix display at last year`s Society for Information Display meeting in San Diego, CA. The company is not planning product manufacturing, but aims to license and transfer the technology. It has recently licensed its technology in a nonexclusive agreement with Philips Components BV that will allow Philips to scale existing laboratory processes and to develop its own manufacturing processes for small displays.

Make it manufacturable

Ease of manufacturing is one of the biggest advantages of LEPs over small organic molecules and, for that matter, over traditional inorganic LEDs, both of which have crystalline structures that must be grown under vacuum conditions using costly equipment. Polymers can be worked easily in solvent form. To produce the polymer thin film, Gostick thinks that doctor-blade coating is best for simple products (Fig. 3), and says it is analogous with reel-to-reel coating. In high-value-added products on small substrates, either traditional spin or doctor-blade coating is appropriate. At Uniax, spin coating is the preferred technique, although marketing development manager Nick Colaneri says processes such as doctor-blade and extrusion coating are possible.

Colaneri says, "We work mostly in orange using MEH-PPV. We`ve made other colors, but each color requires lots of development work and there is commercial opportunity with monochrome." The company is currently building a prototype assembly line with a cleanroom to determine what one would look like. Perhaps another company will make the product and Uniax will market and sell it, or perhaps the company will enter into joint ventures. Uniax also researches the use of conjugated plastics in applications such as photodetectors. Last year, Uniax founder Alan Heeger at UCSB and CDT founder Richard Friend at Cavendish, announced that each had achieved lasing from LEPs.

A novel process

For production manufacturing, Hewlett-Packard`s James Sheats says, "I think you want something other than spin coating, which is wasteful. You`ll want large substrates for scale, and as substrates get larger, spin coating is less efficient." He notes that scale cost is not driving the manufacturing yet. However, "the integrated circuit business is very conservative," he continues. "If it`s a cost issue, the industry doesn`t want to do it." He would like to see substrates on the order of 1 m2, which isn`t feasible with spin coating. Sheats thinks that a new molecular self-assembly technique holds very interesting promise for LEP manufacturing.

The self-assembly technique is being developed by Michael Rubner and colleagues at the Massachusetts Institute of Technology. "We`re trying to develop molecular-level manipulation tools to control film thicknesses at tens of angstroms," says Rubner. "We use dipping machines and water-based solutions in a simple adsorption process to make large-area, uniform thin films." He adds, "It`s new and exploratory, and premature to suggest it will take off."

Nonetheless, his group has succeeded in fabricating light-emitting devices from polymethacrylic acid PPV that exhibit luminance levels in the range of 20-60 cd/m2. Ideally, the technique could be used to create multilayer heterostructures in which functionally different layers within the device are used to control the key elements of operation, such as charge injection or carrier transport. The process is based on the alternate spontaneous adsorption of molecular layers of oppositely charged polymers [5].

Future prospects

The time scale for commercializing light-emitting polymers is beginning to clear, at least for simple products such as alphanumeric displays and backlights that might work on cellular phones or pagers. CDT and Uniax both expect to have sample products out within a year. Their named and un-named research partners may have similar products in the same time frame. As for flat panel displays, they are still 5-10 years off, assuming that the manufacturing issues can be resolved and competing display technologies do not become too well established.

Figure 3. Light-emitting polymer display fabrication process.

"Things are happening remarkably fast in the whole field of organic electroluminescence," Rubner concludes. "The basic understanding of the science is falling into place. The bottom line is manufacturing high-yield devices that are acceptable to the marketplace." Like most working in this field, he reports lots of inquiries from companies who want to catch up. "If someone pulls it off and makes money, it`ll be great. If not, then it doesn`t bode well for the science. Most of us are very optimistic."

References

1. R.B. Kaner, A.G. MacDiarmid, "Plastics that Conduct Electricity," Scientific American, pp. 106-111, 1988.

2. J.H. Burroughes et al., "Light-emitting Diodes Based on Conjugated Polymers," Nature, Vol. 347, pp. 539-541, 1990.

3. A.J. Heeger, J.L. Long, Jr., "Optoelectronic Devices Fabricated from Semiconducting Polymers," Optics and Photonics News (Optical Society of America), pp. 24-30, August 1996.

4. J.R. Sheats et al., "Organic Electroluminescent Devices," Science, Vol. 273, pp. 884-888, 1996.

5. O. Onitsuka et al., "Enhancement of Light-emitting Diodes Based on Self-assembled Heterostructures of poly(p-phenylene vinylene)," J. Appl. Phys., Vol. 80, No. 7, pp. 4067-4071, 1996.

W. CONARD HOLTON is a freelance science and technology writer based in Troy, NY; ph 518/279-1569, e-mail [email protected].