Examples of electroluminescent polymers
Techniques used to investigate devices
Advantages of Polymer Light Emitting Diodes
Highlights of research within ESPRIT and elsewhere
The ESPRIT Basic Research project 8013 "LEDFOS" - "Light Emitting Devices from Organic Semiconductors" is funded by the European Union, through the ESPRIT project. Under this project we investigate an unusual class of polymer which emits light when current passes through it. This phenomenon is called "electroluminescence" (sometimes shortened to "EL"), and these novel materials look very promising for making displays, of the sort that one might find in digital watches, clocks and possibly, eventually, television screens. The materials are also strongly fluorescent.
Polymer light emitting diodes were first discovered in the University of Cambridge in 1990 (references).
Polymers are long chains of carbon atoms. How they are arranged, and with what other atoms, determines the mechanical, optical and electrical properties of the material. Chemists can make a huge variety of polymers - often referred to as "plastics". Most of them are good electrical insulators.
Electroluminescent polymers have a large number of "double bonds" between the carbon atoms. Electrons can move more easily along the chain and this gives the materials their distinct colours and unusual electrical properties. We can think of them as "organic semiconductors". Like silicon or gallium arsenide, it is possible to make transistors and diodes from them.
The LEDFOS ESPRIT project is a collaboration between chemists and physicists in five European countries. In it, we want to develop the know-how in the technology of the organic light emitting diode and to use this to improve our understanding of the underlying physics and chemistry which controls the properties of these devices. In Cambridge there are also projects to standardise assessment and manufacturing techniques (BRITE EURAM "PolyLED") and to study the use of these materials in photovoltaic ("solar") cells.
It takes a lot of effort to make new materials, so at present, polymer quantities are typically less than one gramme and active areas of devices are a few millimetres square. Ultimately, devices could be much larger, and mass-production techniques will be required for volume applications, but such research is not covered in the ESPRIT LEDFOS project, where the aim is to "break new ground".
These include:
Polymer light emitting diodes ("PLED"s) are not yet available in the shops!
In order to develop them commercially, it would have to be shown that they complement or have advantages over existing technologies such as the Cathode Ray Tube (TV screen), Liquid Crystal Displays (LCDs) and semiconductor-based Light Emitting Diodes (LEDs).
Polymer EL displays are lightweight, thin, potentially cheap, using standard semiconductor processing steps, can be made in large areas, have efficiencies comparable to or better than existing displays, come in a wide range of colours, can be flexible or curved, and have properties easy to tailor by altering the chemistry and processing.
Within the ESPRIT collaboration, Philips are best placed to exploit this research, because of their direct interest in consumer electronics. The University of Cambridge formed a separate company - Cambridge Display Technology (CDT) - to which it transferred its patent rights, and with whom, because of its origins, the Cambridge group has a close association. CDT is not directly part of the ESPRIT project and has its own programme of research and development, better geared to the exploitation of these materials.
Picture of Polymer Light Emitting Diode provided courtesy of Cambridge Display Technology (CDT).
Outside ESPRIT, there is growing interest in the field. Companies active in this research include:
Marks denote European collaborative affiliations: * =ESPRIT # =BRITE EURAM
There are also many universities across the world interested in this field.
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Comments, please, to
Richard Gymer (OE), Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE rwg11 @ .cam.ac.uk