When physics students are taught about condensed matter and materials they mostly learn about metals, alloys, semiconductors and other inorganic materials. Organic and molecular materials are confined to chemistry and biology. But that may change in the future with the development of polymers and other molecular materials that can emit light and conduct electricity.

These organic materials offer many advantages over their inorganic counterparts: they are durable, flexible and cheap to mass-produce. It is little wonder that many of the leading electronics companies are embracing this expanding field of research. Product designers are dreaming up giant video screens that can be rolled up and carried from room to room, mobile phones with all-plastic circuitry that will not break when dropped and lightweight luminous panels that could replace the bulky light fittings in aeroplanes and cars.

The advantages of plastic

So how can plastics conduct when most commercial polymers are insulators? A special class of materials known as conjugated polymers – such as polyaniline and polyacetelyne – have the right physical and chemical properties to conduct electricity. In these so-called ¼-conjugated polymers, the electrons are delocalized along the backbone of the polymer molecule. By injecting charge carriers into the material and applying a high enough voltage, the charge carriers are able to jump from molecule to molecule. Electrons and holes can also “recombine” to generate light over a range of wavelengths. The beauty of these polymer materials is that their properties can be adjusted to suit specific requirements. For example, depending on the level of doping, the material can be a semiconductor or a conductor.

In Organic displays Junji Kido describes how the light-emitting properties of organic materials can be exploited in industry. While it has been known since the early 1960s that single organic crystals can emit light, the electric fields needed were prohibitively high. But the development of thin-film devices in the last decade has meant that organic materials are now emerging as realistic candidates for display applications. With the global market for electronic displays estimated at $50bn per year and rising, the giants of the display industry are eagerly investigating the new materials.

The colour of the output can be tuned by adding fluorescent laser dyes, and white light can be produced by having several layers, each doped with a different dye. Multilayer devices are already being used in real applications. The big advantage that polymers offer is that they are easy to fabricate. Indeed, standard inkjet printing technology is being adapted to make the different coloured pixels needed for full-colour displays.

Polymers have already been used as the active material in transistors, and are now being used for the conducting parts of devices as well. Philips Research in the Netherlands has gone a stage further and combined many components to make all-polymer integrated circuits, as Dago de Leeuw reports on plastic electronics. These devices are likely to replace silicon chips in mass-produced applications that use simple circuits, such as bar codes that can be read remotely, and other applications that demand lightweight and flexible devices.

While prototype devices have been demonstrated, the technology needs to be improved in several respects before plastic electronics can make an impact on the market. The switching speed of the plastic transistors is rather slow compared with silicon due to the relatively poor mobility of the charge carriers in the polymer material. In terms of mobility, the state-of-the-art polymers are 300 times better but are not easy to process using the current technology, although it is likely that these problems will be resolved in the future. But as improvements are made, it is important to maintain the flexibility of the circuits and their cost advantage over inorganic circuits.

Back to basics

Organic molecules are not confined to technological applications. Bernard Barbara and Leon Gunther describe in another article how certain magnetic nanomolecules are being used to investigate the boundary between quantum and classical mechanics. These molecules consist of a cluster of metal ions – manganese or iron – surrounded by water and acetate molecules. Applications are still a long way off and the unusual magnetic properties of nanomolecules look set to keep physicists busy for years to come.