One of the best-known properties of polymers – materials that include plastics in general – is their electrical insulation capacity, which makes them widely used for covering wires, thus avoiding shocks and short circuits. In the mid-1970″s, a discovery made by Japanese and American researchers showed that this is not true for all types of these materials. Some have the ability to conduct electricity and are being quoted as substitutes for LCD or plasma TV and computer screens, with advantages, in addition to their possible use in solar cells, transistors and other electronic devices. These new electricity-conducting polymers are part of a line of research of various groups in the world, including in Brazil, with researchers from the University of São Paulo (Poli-USP), who are led by Professor Adnei Melges de Andrade, from the Institute of Electrotechnics and Energy (IEE) at USP, where they belong to the Molecular Electronics Group (GEM).
The researchers work mainly with the polymer light-emitting diodes (PLED) and organic light-emitting diodes (OLED), two types of light emitting diode (LED) that are different from those sold today – produced from inorganic semiconductor material – because they are organic and basically composed of carbon molecules. The group is also working to develop other devices, such as thin-film transistors, organic solar cells and sensors.
Supported by regular help projects from FAPESP, the group is moving towards developing new electronic devices, using polymeric material. “Now we”re not only interested in whether the devices work,” explains Professor Fernando Josepetti Fonseca, who is also part of GEM. “This is the minimum and we did that that in the previous project that started in March 2004 and ended in February 2006. Now, we want to know how long they work for and how they can be made more efficient, in addition to being made more simply and cheaply.”
Nobel Prize in Chemistry
The work of the group from Poli is only possible due to two major scientific discoveries made at the end of the 20th century. As sometimes happens in science, the first of them occurred by accident or mistake, which occurred in 1976, in the laboratory of Japanese researcher Hideki Shirakawa from the Tokyo Institute of Technology. In an attempt to synthesize polyacetylene – a simple polymer formed from carbon and hydrogen, which appears as a black powder – one of Shirakawa”s Chinese students got the “recipe” wrong. As a result, instead of the desired polymer, he produced a glossy silver film, as shiny as a sheet of aluminum. Seeking to understand where he had gone wrong, the student saw that he had used an amount of catalyst (a substance used to speed up chemical reactions) a thousand times greater than necessary. Shirakawa kept the film and later showed it to American chemist, Alan MacDiarmid, from the University of Pennsylvania, who was visiting Japan. Shirakawa was then invited to form a partnership with MacDiarmid and the American physicist, Alan Heeger. Working together in 1977, the three found that after dosing the polyacetylene with iodine the flexible silver film became a golden metallic foil, whose electrical conductivity increased significantly. They had discovered semiconductor polymers, which earned the three researchers the 2000 Nobel Prize in Chemistry.
The second discovery occurred in 1990, when Jeremy Burroughes, Richard Friend and Donald Bradley, from Cambridge University in England, created the first device with an electroluminescent semiconductor polymer that emits light when it receives an electrical charge. More specifically, they noted that certain structures of semiconducting polymers could be assembled in order to make light emission possible. They had created, therefore, organic light emitting diodes, OLEDs, which are being incorporated into TV screens and computers, as well as displays in portable devices, like cell phones.
In the laboratory at Poli, the researchers are not producing polymers; they are developing devices. “We get them from academic institutions that collaborate with us,” explains Fonseca. Among the contributors is the Federal University of Parana (UFPR), through the group of professor, Leni Akcelrud, the Institute of Chemistry of USP, through its professor, Neyde Yukie Murakami Iha, and the Department of Metallurgical and Materials Engineering at Poli-USP, with its professor, Wang Hui Shui. “In the environment of the physics and the manufacture of devices we are working with professor Luiz Pereira, from the Department of Physics of the University of Aveiro, in Portugal.” With the electroluminescent polymers they receive they make PLEDs and OLEDs. The group has been working on developing transistors for two years. The great difference in relation to silicon transistors is the lower manufacturing costs. As for organic solar cells the research is in its initial stages.
The biggest efforts of the group of researchers from USP are currently directed towards the development of two organic light-emitting diodes. The difference between the two types is in the manufacturing process and in the components from which they are made. “OLEDs is the name generally given to organic LEDs made from small molecules,” explains researcher, Gerson dos Santos, a member of the group. “PLEDs, in turn, are made from long polymeric chains.” Despite these differences the term “OLED” has become dominant in the world to indicate the two types of organic LED.
There are many possibilities for assembling these devices and the researchers are looking to produce them in an efficient, reproducible and long-lasting way. The simplest structure for an OLED comprises a transparent substrate (glass or an inert polymer), onto which an electrode is deposited, made from a metallic oxide that has high ionization potential – indium tin oxides are normally used for this. Over this layer goes what may be called the heart of the OLED: the light-emitting polymer. This, in turn, is covered by an electrode, a fine metallic layer, generally made from a metal with low ionization potential (loss of electrons), such as aluminum, calcium or magnesium.
The exciton decays
The functioning principle of the OLED is also relatively simple. “Electrons are injected from one electrode, while gaps (also called holes, a term very common in physics and electricity, which means the absence of electrons in certain positions) are introduced by another when an electric voltage is applied between the two,” explains researcher John Paul Hempel Lima. “These charges move through the polymer chains and can recombine to form an electronically stimulated species, the exciton. This exciton decays radioactively, emitting light. In other words, the radioactive decay of the exciton is the electroluminescence of the OLED.”
The assembly of such devices begins with the choice of materials, which will define the color it will emit. Furthermore, to produce efficient devices it is essential that the morphology and thickness of each layer are controlled. There are several techniques for depositing them from the substrate, one on top of the other. Much of the assembly of OLEDs is done inside a glove box, a machine whose model was designed by researchers in conjunction with a national manufacturer. It allows an atmosphere to be created inside it, consisting of nitrogen with reduced concentrations of oxygen and water to avoid degradation of the devices. The OLEDS are also encapsulated within the chamber, a stage that consists of placing a capsule of glass over the material. “Because an imported machine was very expensive we had to order a similar, national model from a Brazilian company,” says Andrade. Most of the funding received from FAPESP in the first project, about R$ 300,000 out of the total of just over R$450 000, was used to make the glove box. “Besides this equipment, we developed a robot for applying the layers, in a technique known as self-assembly, and we adapted a commercial printer for applying the polymers (a technique known as ink-jet deposition).”
GEM”s researchers have already produced various types of OLED in different colors, using more than 20 types of polymer. The group”s work, according to Fonseca, is half way between basic research, scientific discovery and industry. “Our objective is not to produce devices that are ready for market, but to go beyond basic research in order to develop technologies that industries can themselves further develop and manufacture on a large scale.” So, the great challenge to be overcome is to increase the life of OLEDs, which is still just a few hours, not only aiming at their immediate applications, but also to provide the material with greater stability and the OLEDS with good luminance. Another of the group”s challenges is to produce an OLED that emits white light, a feat that has been attempted by companies and research institutions all over the world. “Producing this type of device is no easy task. A balanced combination of the emission of the basic colors – red, green and blue – needs to be achieved, which is difficult,” says Andrade. The group is testing two ways of achieving this. In one, the components are mixed to produce an emission that is close to white light. In the other, an OLED is produced that has more than one polymer emitter, each one emitting a color, which implies more process stages.
According to Andrade, the advent of this white light emitter is going to revolutionize our way of life, because it profoundly alters the way environments will be lit. Today, everyone is used to using conventional lights, such as incandescent bulbs and, more recently, compact fluorescent bulbs, but it will be possible to substitute them in the future with very thin, flat devices, which are no thicker than a sheet of writing paper and consume less energy, an important feature, considering that natural resources will become increasingly limited.
1. Research and development of electroluminescent devices, using semiconductor polymers (nº 03/07454-5); Type Regular Research Awards; Coordinator Adnei Melges de Andrade – USP; Investment R$ 352,347.45 and US$ 51,268.94 (FAPESP)
2. Study and development of organic LEDs, solar cells, thin film transistors and sensors based on semiconductor polymers (nº 09/05589-7); Type Regular Research Awards; Coordinator Adnei Melges de Andrade – USP; Investment R$ 135,293.81 and US$ 121,643.95 (FAPESP)