Plastic field emitter could simplify display development

Plastic field emitter could simplify display development
LIVERPOOL, England — Electronic devices made from polymers could be used as broad-area emitters for a field-emission display and as an active matrix for driving it, said Liverpool University researchers. Their work is based on a polymer class called polyalkylthiophenes.

The result could be lighter, thinner displays aimed at mobile terminals, said Bill Eccleston, a professor at the university's electronic engineering department, who has led the research.

"We've shown that conjugated polymers can be made to emit large numbers of electrons at low-threshold fields. Polymers can also be used to make transistors for driving a display. Both could be laid down as one process," Eccleston said. "We believe this could be the basis of low-cost displays that could also reduce power consumption."

Akhter Computers Ltd. (Harlow, England), whose chief executive, Humayun Mughal, is a former student of Eccleston's, has struck a deal to be the primary licensee of Liverpool's patented technology. Akhter, an assembler of PCs, servers and notebook computers, is setting up a subsidiary, UK Light Thin Displays, to develop the technology with a pilot manufacturing operation at an Akhter factory in Burnley, England.

"We're going to create new products based on the technology and then look for partners," said Mughal.

Mughal said the facility could be prepared with an adequate clean room in months' time, and he expected UK Light Thin Display to have a prototype display within two years.

Liverpool University's Eccleston said the ability to use polymer transistors to drive a display depended on layout configuration to achieve ultrashort channel lengths. The technology will therefore depend on photolithographic patterning rather than the less-precise printing techniques used by other display firms.

"The anode voltage scheme could be as low as 20 V, although a lot depends on the phosphors and we may have to go to higher voltages," Eccleston said. "At the moment the polymer transistor needs 30 V, but we think we've got a scheme to bring it down to 10 V."

Ordered structures
Though not as thoroughly studied as other conducting polymers that are finding their way into flat-panel displays, polyalkylthiophenes are getting more interest from researchers because of their unusual ability to produce ordered structures from polymer chemistry. In general, polymers form in long, tangled molecular chains. Despite the random ordering of the chains, these materials can conduct electrons and act like semiconductors.

Conduction does not result from a drift of free electrons through the material. Instead, the disordered chains overlap, creating the ability for electrons to hop from one segment to the next along the backbone of the molecules. While conferring processing advantages, the amorphous structure results in lower performance compared with inorganic semiconductors. In a highly structured material such as silicon, semiconductor properties result from the interaction of the electron's field with the regular lattice. In conducting polymers, the overlapping of nearby electron orbitals creates conduction and valence bands. Thus, while the origins of semiconductor effects is different in polymers and inorganic semiconductors, both materials systems can lead to the same types of devices.

But crystalline inorganic materials, such as silicon, demand highly specific environmental conditions to form a perfectly regular structure. Polymers offer the same effects in a much less rigid structure that is easier to work with. Polymer researchers start with polythiophene and then add alkyl side chains. The side-chain formation can take place in three possible monomer-to-monomer alkylthiophene couplings, termed head-to-tail, head-to-head and tail-to-tail.

Thin films cast from these polymers have been found to have highly specific regularities with regard to a given type of side-chain bonding. Essentially the same structures can have different proportions of each type of alkylthiophene couplings, resulting in different electronic properties. The added regular structure has produced electron mobilities that are much higher than amorphous conducting polymers. — Additional reporting by Chappell Brown.