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.

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?

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.

“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.”

Alternatives to glass

Displays have two principal assemblies: a ‘backplane’ with the electronics that drive the display, and a ‘frontplane’ containing the actual display elements.

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.

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.

OLEDs can also switch on and off much faster than an LCD making them suitable for video displays such as TV sets.

“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.

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.

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.

“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.”

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.

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.

Electronics on plastic

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.

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.

These three technologies have found their first commercial applications with a monochromatic display that can show high-resolution images.

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.

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.

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.

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.

“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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