Plastics: Semiconductors of New Millennium

Science, and particularly physics, is continually expanding to conquer realms that until recently were considered science fiction. Only recently, a group of scientists have begun to make their mark in the sector of semiconductors (encountered in all areas of modern life) known as conjugated polymers. Their goal is to create useful products from low-cost materials, some of which are also environmentally friendly. For industry, it means that the manufacture of these products will not be nearly as time-consuming as it is today. In Greece, organic materials have started to gain attention and there are already research groups working in this area (for example Prof. P. Lianos’s group at the University of Patras.) Low-cost products Semiconductors can be found at work in almost every aspect of modern life. Such diverse fields as telecommunications, medical technology and electronics (e.g. computers, TVs, CD/DVD players) have been revolutionized by their impact. At the same time, plastics are also abundant in our world – it seems that just about everything is made of plastic these days. Car bumpers, bottles and toys are just a few examples. Semiconductors enabled us to do things that were never imagined before their advent or that were only found in the domain of science-fiction literature. On the other hand, plastics have caused a major impact due to their low cost, high performance and ease of processing. Is there a way to combine these two pillars of the modern world? That is, to create electronic devices that are much cheaper and more easily fabricated than at present. Not long ago, the answer would have been no. But that is no longer the case. The story goes like this. Electroluminescence (generation of light by electrical excitation) from an organic material was first reported by M. Pope (NY University) in 1963. Following this, the organic thin film EL was stimulated in the 1980s by the work of C.W. Tang & S.A.Van Slyke (Kodak) who used molecular materials which are now referred to as «small molecules.» As far as polymers (plastics) are concerned, we think of them as being somehow the opposite of metals. That is, they do not conduct electricity. In 1977 A. Heeger (UCSB, USA), A. MacDiarmud (Pennsylvania, USA) and H. Shirakawa (Tsukuba, Japan) changed this view by making a polymer that could conduct like a metal. For their discovery, they were awarded last year’s Nobel Prize in chemistry. Since then, the science of electrically conductive polymers has advanced very rapidly. Emission of light But the real revolution did not come until 1990 when D.D.C. Bradley, J.H. Burroughs, R.H. Friend and co-workers at Cambridge University (UK) demonstrated the first polymer light-emitting device. This opened up a whole new field of research with tremendous potential. There is a large variety of possible applications, and their commercial realization is being addressed by interdisciplinary teams of chemists, physicists and engineers in both industrial and university laboratories. So how can certain polymers conduct when most commercial plastics are insulators? We are talking about a special class of materials, which are known as conjugated polymers. In metals, such as aluminum, atoms are packed so tightly together that the electrons (current carriers) on the outside of each atom get lost in a crowd, which can be easily made to move in one direction. That is why metals are good conductors. In most plastics, the electrons are connected to atoms more firmly and cannot move freely. So current cannot flow and they are insulators. In conjugated polymers, the electrons attached to one molecule can interact with those in neighboring molecules. That limited movement allows some current to flow. A useful analogy for the above is the spread of the flu. If a person who has the flu walks around freely, the virus will spread quickly. If, on the other hand, he stays at home, the virus does not spread at all. In the case where he only goes to his neighbors, the virus will still spread but less dramatically. Polymers fall into the last category, which is why they are classed as semiconductors. Light emission is observed when electrical current passes through them. The beauty of these materials is that their properties (for example, the color of the emitted light) can be chemically adjusted to suit specific requirements. Currently, there is very strong interest in developing devices based on semiconducting polymers for electronic and optical applications. Work is being carried out both at academic institutions and in major industrial companies. For example, the major universities involved are Imperial College of the University of London, Cambridge, Sheffield, Hull, St Andrews (UK), Princeton, UCSB, UCLA, Pennsylvania (USA), Potsdam (Germany) and Yamagata (Japan). Some of the companies active in this area are The Dow Chemical Company, Sharp, CDT, Dupont, Siemens, Philips, Kodak, TDK, Pioneer, Seiko-Epson and NEC. The Imperial College group, headed by Professor Donal D.C. Bradley, is widely considered to be one of the leaders in the field. His 25-strong team is conducting cutting-edge research in various aspects of polymer-based optoelectronics, by making use of state-of-the-art facilities and various academic and industrial collaborations. The group is interested in studying low-power-consumption light-emitting elements for flat panel displays or as backlights in liquid crystal screens (like the screens usually found in mobile phones) and also fabricating the electronics (TFTs) to drive them. Furthermore, there is strong interest in developing polymer photodiodes and solar cells as means of producing wavelength-sensitive detectors and solar-power harvesting cells offering environmentally friendly energy sources. The group is also developing polymers for use as the gain medium in devices such as amplifiers and lasers, or as an integral part of ultrafast optical data communication networks. Polymer LEDs are now being used as backlights in LCDs. Monochrome polymer displays have shown satisfactory performance and are very close to commercialization. Furthermore, polymer transistors have been successfully demonstrated. Other applications are still under development with very promising results so far. Of course, problems do exist (such as stability and efficiency) but the great potential advantages that polymers have to offer provide a strong driving force to overcome them. The possibilities are endless and it is no surprise that organics have captured the attention of those looking for the next big thing. Imagine an ultrathin, lightweight screen that can be rolled up and carried away. Additional advantages of such screens would be wide-viewing angles and that they won’t crack if dropped. Furthermore, electronics consisting of all-plastic, low-cost circuitry might replace conventional chips in mass-produced applications. Organic white light sources will be cheaper and environmentally friendlier than conventional fluorescent lamps that use toxic materials. In the marketing sector, they could be used in identification and tagging and large-area signs or even light-emitting goods packaging! An imaginative person might even suggest an illuminating wall acting as a giant video screen, where you can view whatever you feel like (maybe see the sunset from your favorite island!). With a global market of electronic displays alone worth $50 billion, it’s little wonder that the companies are investing in new types of organic materials in an effort to capture a share of this market. The universities are receiving grants from those companies but also from their own governments in order to advance the field and explore the new physics involved. Organic materials are making an impact on both fundamental physics and technological applications. Future students may study physics in classrooms with organic display boards lit by organic lights. – The authors contributed this article to Kathimerini.

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