Yet some polymers can conduct and some can emit light, and what paper #2 reports is their bringing together to create the first all-polymer device. It is the work of Richard Friend, Henning Sirringhaus, and Nir Tessler of the Cavendish Laboratory at the University of Cambridge, England, and it brings the "organic" computer of the future one step closer.

The paper describes an organic semiconductor integrated optoelectronics device which consists of a high-mobility conjugated polymer transistor driving a polymer light-emitting diode (LED). (Polymer LEDs were discovered by Friend and fellow chemist Andy Holmes in 1989.) The other component, a so-called field-effect transistor (FET), was made from poly(hexylthiophene), which consists of linked five-membered sulfur-containing thiophene rings, each with a hexyl group attached. What is special about this polymer is the way in which it has been produced so that all these hexyl groups are orientated in the same direction.

The performance of the FET approaches that of an inorganic thin-film silicon FET with field-effect mobilities up to 0.1 cm² per volt and ON-OFF current ratios in excess of a million. Paper #2 explains how the LED and FET were integrated into a working device mounted on a silicon dioxide substrate, and it clearly demonstrated that the high-mobility FET has sufficient driving current to switch a polymer LED. Moreover, it was easy to make and robust in operation, albeit relatively crude, and while it does not address the complex issue of integrating a full pixel–which would require the use of a polymer insulator–it points the way forward.

Speaking to Science Watch, Sirringhaus suggests that paper #2 is being heavily cited for two reasons: 

1. It was the first demonstration of a solution-processed polymer transistor with a performance close to that of a silicon-based device.
2. It was the first example in which two microelectronic polymer devices had been combined, representing a major step forward in polymer optoelectronic technology.

In work following on from paper #2, the Cavendish group have been able to elucidate how the polymer is able to deliver high charge mobility. In a Nature paper last year, (see H. Sirringhaus, et al., Nature, 401[6754]:685-8, 1999), they explain how the polymer's self-organized structure is responsible for its ability to carry high electric current. They found that the structural order induced in the polymer film, and the close interaction between neighboring polymer chains, gives rise to a change in the nature of the charge carriers. These carriers spread our over neighboring chains and, as a consequence, become more extended and mobile than in disordered low-mobility polymers, in which carriers are strictly confined to individual chains, forming so-called polarons.

It now appears possible that, with further improvements in structural order, a completely delocalized "band"-like regime may be reached of the kind that exists in crystalline inorganic semiconductors like silicon. The Cambridge team are focusing on other self-organizing mechanisms to further enhance mobilities, and looking at ways in which an all-polymer transistor might be fabricated by direct printing techniques.