Progress in Plastic Electronics Research

Progress is being made in the long quest to develop plastics with electronic properties.

Researchers at Cambridge's Cavendish Laboratory have already pioneered polymer light-emitting diodes (LEDs), which are currently being developed into a commercial display technology by Cambridge Display Technology (CDT) and other companies. Now they are working on using polymers as transistors.

If the research pays off, it could lead to the development of a whole range of "smart" electronic devices - including identification tags, flat-panel displays and programmable credit cards - and all made from plastic.

Philips Research Laboratories in Eindhoven, Netherlands, have already made prototypes of complex integrated logic circuits on flexible, plastic substrates based on all-polymer transistors.

One of the problems in achieving adequate operating speed has always been the mobility of the charge carriers in the polymer semiconducting layer. The mobility, which is a measure of the velocity of the charge carriers in the device, determines the magnitude of the transistor current and the switching speed of the device, and so is crucial to the performance of polymer transistors. So far most mobilities found have been far lower than required - a major obstacle to developing workable applications.

However, most of the work has so far been done on "disordered polymers" - where the polymer chains are tangled, rather like a bowl of spaghetti, resulting in highly localized charge carriers. Now, in work published in Nature, Cavendish Professor Richard Friend and a European team of colleagues have found experimental evidence that in some recently discovered polymers which tend to self-organize, the charge carriers are more extended than usual. This makes the mobility much higher - indeed close to the target mobilities needed for some of the circuit applications.

Dr. Henning Sirringhaus, who is leading the research on polymer transistors at the Cavendish, commented, "These new results contribute to our understanding of the relationship between structure and charge transport in these non-conventional polymer semiconductors. The electrical and optical properties of self-assembled organic structures, that is, ordered structures which, by some fundamental interaction between the molecules, form on their own, without requiring too much complicated (and usually expensive) help from us physicists, is an exciting field which is just starting to develop."

Two-dimensional charge transport in self-organised, high-mobility conjugated polymers by H. Sirringhaus, P.J. Brown, R.H. Friend (Cavendish Lab, University of Cambridge), M. M. Nielsen, K. Bechgaard (Cond. Matter Phys. and Chemistry Dept., Riso National Lab, Roskilde), B M W Langeveld-Voss, A.J.H. Spiering, R.A.J. Janssen, E.W. Meijer (Lab. of Macromolecular and Organic Chemistry, Eindhoven University of Technology), P. Herwig, D. M. de Leeuw (Philips Research Labs, AA Eindhoven) was published in Nature on 14 October.