Posted: June 1, 2008 |
A flexible approach to new displays |
(Nanowerk News) The moving pictures seen in books and newspapers in the Harry Potter films are a step closer to reality with new display technologies developed by European researchers.
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Flat screen displays currently used in computer monitors, television sets and numerous other electronic devices are all built on a glass base. Most use liquid crystal devices (LCDs), which filter light from behind to form an image.
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But the glass substrate makes LCD displays rigid and fragile, limiting their use. Now display manufacturers are working to develop a new generation of robust, flexible displays that can be curved to fit the shape of a product or even rolled up like a magazine. The question is, which of the technologies under development is the best?
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“Research to define the route forward for flexible displays is too great a job for one particular company, institute or university,” says Dr Eliav Haskal of Philips Research who is coordinating the EU-funded FlexiDis project. “When we started the project nobody knew exactly what to do, there were many different solutions.”
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Big industrial names such as Nokia, Thales and Philips, as well as universities, research centres and many small and medium-sized businesses have pooled their skills and expertise to thoroughly test a large number of materials and techniques.
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Alternatives to glass
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Displays have two principal assemblies: a ‘backplane’ with the electronics that drive the display, and a ‘frontplane’ containing the actual display elements.
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Backplanes are conventionally made of glass on which is deposited the grid of thin-film transistors (TFTs), which control the state of each pixel in the display. To create a flexible display the FlexiDis researchers needed to find an alternative to glass.
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One possibility was thin metal, which is particularly attractive for a promising new kind of light-emitting element called an OLED (organic light-emitting diode). Unlike an LCD an OLED emits its own light rather than filtering light from a background source and so has the potential to create full-colour displays using much less power than LCDs.
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OLEDs can also switch on and off much faster than an LCD making them suitable for video displays such as TV sets.
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“The initial guess was to work with metal substrates because metal is a very good barrier to water and oxygen both of which are known to degrade the lifetime of OLEDs,” says Dr Haskal.
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Metal also has the advantage of being stiff enough to be handled in factories designed to manufacture displays based on glass, a very important economic consideration.
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It turned out that constructing a metal-based backplane suitable for OLEDs was very difficult, so the partners also decided to investigate the possibility of constructing OLED displays on a plastic backplane.
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“We had to introduce a method of making thin-film transistors on plastic in a process which can be run in a production facility,” says Dr Haskal. “Ultimately that was the biggest problem.”
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Conventional transistors are typically made at temperatures around 280°C, which is too hot for most plastics. Rather than try to reduce the temperature of a standard process, the researchers decided to develop two alternatives.
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One method used a heat resistant plastic called polyimide at 280°C. The other alternative was to use organic TFTs, which can be deposited at much lower temperatures.
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Electronics on plastic
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The FlexiDis partners have now developed three new technologies for producing flexible plastic backplanes. The first, called EPLaR (electronics on plastic by laser release), uses polyimide spin-coated on to a glass plate. The TFTs are formed on the plastic in the usual way and the whole backplane assembly is then released from the glass by a laser process.
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The other two technologies use organic TFTs deposited at 120-150°C, a temperature at which many more plastics can be used. In one process the TFTs are built up by ink-jet and in the other a spin-coating process is used.
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These three technologies have found their first commercial applications with a monochromatic display that can show high-resolution images.
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So-called ‘electrophoretic’ displays are the basis of what has been called ‘e-paper’, which reflects light just like normal paper and can hold an image without consuming any power. The glass-based version of this technology has been commercialised in the Sony Librié, the Amazon Kindle and the IRex Iliad.
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Two European companies are launching e-readers based on the FlexiDis technologies. A factory in Taiwan has been licensed to mass produce flexible displays for the consumer market. Thales Avionics LCD are planning to industrialise flexible displays for the avionics sector.
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Although the OLED technology is further from the market – FlexiDis partners demonstrated the first flexible OLED display in 2007 – it offers the best prospects for creating flexible displays that can support full colour and video.
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In the longer term, the development of full colour displays could make possible the kind of moving newspaper pictures seen in movies such as the Harry Potter series and Minority Report.
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“Everyone in this industry has watched Minority Report because of the ideas about working with newspapers which show constantly updating information in full colour and full video,” says Dr Haskal.
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FlexiDis received funding from the EU's Sixth Framework Programme for research.
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