A Flexible Approach To New Computer Displays

Tag: Computer Display

But the glass substrate makes LCD displays rigid and fragile,limiting their use. Now display manufacturers are working todevelop a new generation of robust, flexible displays that can becurved to fit the shape of a product or even rolled up like amagazine. The question is, which of the technologies underdevelopment is the best? Big industrial names such as Nokia, Thales and Philips, as well asuniversities, research centres and many small and medium-sizedbusinesses have pooled their skills and expertise to thoroughlytest a large number of materials and techniques.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 depositedthe grid of thin-film transistors (TFTs), which control the stateof each pixel in the display. To create a flexible display theFlexiDis researchers needed to find an alternative to glass.

One possibility was thin metal, which is particularly attractivefor a promising new kind of light-emitting element called an OLED(organic light-emitting diode). Unlike an LCD an OLED emits its ownlight rather than filtering light from a background source and sohas the potential to create full-colour displays using much lesspower than LCDs.

OLEDs can also switch on and off much faster than an LCD makingthem suitable for video displays such as TV sets."The initial guess was to work with metal substrates becausemetal is a very good barrier to water and oxygen both of which areknown to degrade the lifetime of OLEDs," says Dr Haskal.Metal also has the advantage of being stiff enough to be handled infactories designed to manufacture displays based on glass, a veryimportant economic consideration.

It turned out that constructing a metal-based backplane suitablefor OLEDs was very difficult, so the partners also decided toinvestigate the possibility of constructing OLED displays on aplastic backplane."We had to introduce a method of making thin-film transistorson plastic in a process which can be run in a productionfacility,” says Dr Haskal. “Ultimately that was thebiggest problem."

Conventional transistors are typically made at temperatures around280°C, which is too hot for most plastics. Rather than try toreduce the temperature of a standard process, the researchersdecided to develop two alternatives.One method used a heat resistant plastic called polyimide at280°C. The other alternative was to use organic TFTs, which canbe deposited at much lower temperatures.

Electronics on plastic. The FlexiDis partners have now developed three new technologies forproducing flexible plastic backplanes. The first, called EPLaR(electronics on plastic by laser release), uses polyimidespin-coated on to a glass plate. The TFTs are formed on the plasticin the usual way and the whole backplane assembly is then releasedfrom the glass by a laser process.The other two technologies use organic TFTs deposited at120-150°C, a temperature at which many more plastics can beused. In one process the TFTs are built up by ink-jet and in theother a spin-coating process is used.These three technologies have found their first commercialapplications with a monochromatic display that can showhigh-resolution images.So-called ‘electrophoretic’ displays are the basis ofwhat has been called ‘e-paper’, which reflects lightjust like normal paper and can hold an image without consuming anypower. The glass-based version of this technology has beencommercialised in the Sony Librié, the Amazon Kindle and theIRex Iliad.

Two European companies are launching e-readers based on theFlexiDis technologies. A factory in Taiwan has been licensed tomass produce flexible displays for the consumer market. ThalesAvionics LCD are planning to industrialise flexible displays forthe avionics sector.

Although the OLED technology is further from the market –FlexiDis partners demonstrated the first flexible OLED display in2007 – it offers the best prospects for creating flexibledisplays that can support full colour and video.In the longer term, the development of full colour displays couldmake possible the kind of moving newspaper pictures seen in moviessuch as the Harry Potter series and Minority Report."Everyone in this industry has watched Minority Reportbecause of the ideas about working with newspapers which showconstantly updating information in full colour and fullvideo," says Dr Haskal.FlexiDis received funding from the EU's Sixth Framework Programmefor research.This feature is part 1 of a two-part series on the FlexiDisproject.