David Robson charts the progress of display technology
It sounds like the content of a sci-fi novel: high-quality displays printed on thin, flexible materials could allow moving pictures on the pages of a book, animated food packaging and home cinema screens papered to your walls. And in the realm of 3D display, researchers can now ‘immerse’ themselves in the Sun’s corona to study the shape and size of solar flares, and engineers can enter a virtual car before a prototype has even been built.
This new generation of displays has been slow to take off, but what started with an excited initial buzz may turn into an avalanche of new products. For the thin, flexible displays, we are only now seeing the fruits of decades of research. The first patent for the technology behind these displays, organic light emitting diodes (OLEDs), was filed in 1987 but it wasn’t until last year’s Consumer Electronics Show that we saw the technology reaching mainstream products, with Sony showcasing an 11-inch display just 3mm thick.
The rate of this development is escalating: this year, Sony upgraded its 11-inch OLED display to a 27-inch prototype. Previously, these displays had suffered from very short lifetimes and poor colour definition, but manufacturers are now ironing out these difficulties to produce bigger, brighter and higher definition displays than ever before.
OLEDs, on which these new displays rely, are made from very thin films (less than 500nm) of organic compounds, layered on a substrate, which emit light when a voltage is applied across the surface. Unlike conventional LCD displays, each pixel provides its own light source, so no backlight is required, allowing for a thinner and lighter display that uses less than 50 per cent of the electrical power. This means that the display has a very good contrast, and is viewable from any angle.
Developing organic substances that can withstand continued use has been one of the most challenging obstacles. ‘The lifetime has been the primary issue,’ admits Eric Mayes, senior commercial product manager at Cambridge Display Technologies, which supplies and develops OLEDs.
Different substances are used to produce the three primary colours – red, green and blue – from which all other colours in the display can be formed, and these typically decay at different rates.
Manufacturers think they have cracked this problem, with lifetimes of around 50,000 hours. The pixels, formed from many layers of the organic compounds, can be controlled via an ‘active’ or ‘passive’ technique. Using the passive technique, each pixel is controlled by passing a current through the corresponding row and column of the display.
This limits the size and the quality of the display, because the longer rows and columns needed to be supplied with a greater current which can be damaging to the OLED materials. However, this was still useful for smaller, simpler devices, such as those found in mobile phones, MP3 players and industrial devices.
One of the biggest advances in this technology has been to the development of active control for the displays, which uses a matrix of tiny transistors to control the current flowing through each pixel independently of the other pixels, in a similar way to current TFT monitors. Because much smaller currents are required to control each individual pixel, it is possible to have larger displays that can cope with a more complex output.
Possibly the most important development has been the way in which the thin films of organic compounds are layered onto the substrate. Previously the preferred process, developed by Kodak, was vapour deposition of small organic molecules through a mask that separated the boundaries for each pixel. In addition to being a very expensive process, this technique also limited the size of the possible displays, as the larger masks are very fragile and tended to sag at certain points, destroying the precise array of pixels.
However, a newer technique threatens to make vapour deposition redundant. Instead of using small organic molecules, Cambridge Display Technologies has been using organic polymers which can be dissolved in a solution and applied to the substrate with a printing process. In the past, the technique was similar to inkjet printing but, recently, in a collaboration with Toppan Printing, the company has developed a roll printing process, similar to newspaper production, which can print large displays with a higher resolution at a very low cost.
‘This has opened our eyes,’ says CDT’s Eric Mayes. ‘Solution processing is the best way to go – it’s scalable, and reduces the capital costs of production.’
Whereas the liquid substances in LCD displays freeze at cold temperatures, these polymers can function in much wider temperatures, making the displays suitable for rugged applications such as deep sea diving equipment and industrial production lines. The two processes make use of different substances in the display to emit the light: the vapour deposition technique spreads thin layers of short organic molecules, while the printing process deposits organic polymers. While both substances should achieve the same results eventually, the polymers have not quite reached the lifetimes and colour quality of the short molecules.
‘CDT is three to four years behind Kodak,’ says Martin Cobb, a member of the displays division. ‘But my guess is that CDT will triumph in the end.’ Once the printing process has been perfected, the display inks could feasibly be printed on any surface. Cobb envisions that, within the next five years, screens may be manufactured that can be rolled up after use like a pull-down projector screen: ‘You may even be papering your wall with it.’
Although manufacturers are already taking advantage of OLEDs to produce thin, lightweight displays, they need to overcome a major obstacle before flexible displays of this nature are possible. The screens need to be printed on a watertight substrate to protect the delicate electronics; currently most flexible plastics are too permeable, so most OLEDs are still mounted on glass. ‘Creating a new barrier material is key to creating flexible OLEDs,’ explains Mayes.
Texas Instruments has developed HDTV technology to enable 3D viewing in the home. Using its DLP technology, the company can display two separate image sources at the same time on a DLP HDTV screen.
The films of OLED materials are less than 1.5μm, so the displays are as thick as the substrate they are printed on. Finding a less brittle substance than glass would also allow even thinner devices.
With mobile phones and MP3 players now capable of playing high-quality video clips, many manufacturers are looking beyond LCD displays to meet the growing demands for display systems on the latest consumer gadgets. Until OLEDs have fully matured, we may need to rely on improving existing technologies to meet these requirements.
Projection technology seems to be a likely candidate for this. Devices such as Texas Instruments’ DLP chip, commonly found in many projectors, can now provide the colour definition and resolution for even the most advanced applications, meaning projectors may soon be used in everything from high-definition TVs to 3D displays. The chips are so small they can be integrated into mobile phones to allow users to project on any suitable surface.
The chips are made from thousands of tiny mirrors, each of which can be tilted to either reflect light (the ‘on’ position) or to not reflect light (the ‘off’ position) to form the image. It’s almost like spectators at a sports event holding white cards above their heads, and removing the cards at certain points to write a message.
The mirrors spin at very high speeds, and the proportion of ‘on’ time compared to ‘off’ time determines the shade of grey that is produced. When the mirrors are synchronised with a colour wheel or LED light source, the device can also produce a high definition colour output. The LEDs also provide a greater contrast between the different shades, because they can be dimmed to produce dark points in the image. The speed at which these mirrors can spin has grown so fast that they can now reproduce images at a rate of 120fps – which, in turn, has opened up the possibility of 3D projection systems.
When producing a 3D display, each eye needs to see a different image, from a different perspective, which the brain then combines to gain 3D information about the location of objects. Many 3D systems work by displaying images for the left and then the right eye in quick succession, while simultaneously blocking the alternate eye using special glasses synchronised with the projector.
When the images are produced fast enough, the brain does not notice the temporary loss of vision, and instead stitches the images together. The DLP mirrors can now spin so fast that they can reproduce the images at this rate, providing a highquality, high-resolution colour 3D display.
‘Historically, 3D vision systems have struggled with performance,’ says John Reder, EMEA business development manager for DLP front projection at Texas Instruments. ‘The projectors can now run at a full frame rate for each eye, which is very important for maintaining realism.’
Visitors to the See3D centre, unveiled in April 2007 at the University of Wales, Aberystwyth, UK, will be able to experience this reality for themselves in an immersive ‘cave’ installed by Mechdyne. The centre has recently been used by scientists to view 3D data from solar flares on the Sun’s surface, within 30 minutes of the event happening.
In the past, it had only been possible to view a 2D image of solar activity, making it impossible to fully comprehend the size and scale of solar storms. Now, the scientists at Aberystwyth can fully comprehend the size and magnitude of the storm.
They are even equipped with motion sensors that track their movement and change the display accordingly, so the scientists can peer around the side of one solar flare to view other activity that would otherwise have been hidden from view, giving the illusion that they are actually just feet away from the Sun.
The See3D centre demonstrates all the characteristics of the new trend for ‘immersive’ displays, which ‘fill the peripheral vision to give the sense of being present within a virtual environment’, according to Jeff Brum, the vice president of marketing and business development at Mechdyne.
It’s not just scientists that can take advantage of these systems: town planners, environmentalists and automotive engineers are now prepared to pay vast sums of money to see the effects of their decisions in a virtual world.
One of the problems has been that, although automotive companies are placing an increasing emphasis on developing virtual models of their vehicles, it has not always been possible to translate these designs to a 3D display.
‘I think the content is missing for these systems at the moment,’ says Brum. ‘But the pieces are coming together.’ To solve this problem, Mechdyne is now providing software that can achieve this translation for most CAD software. Not everyone agrees that 3D displays will take off, particularly for the consumer market. Pacer’s Martin Cobb considers them ‘gimmicky’, and believes that the requirement to wear glasses will be a discouragement for many people. He believes that, once they have been refined, holographic methods will prove much more successful.
What remains to be seen is whether the technology for 3D displays and thin, flexible OLEDs will ever converge. Will we ever see flat, flexible displays streaming 3D video from the walls of our homes?
One of the major advantages of OLEDs over LCDs is that they don’t require a backlight, allowing a thin and flexible display at a low cost.
A flexible, thin backlight from Design LED.
Design LED has found a way of overcoming this weakness by developing a method of printing a flexible backlight onto the back of the display. Normally, LED backlights are made via injection moulding, which is an expensive process, whereas the printing technique is much less expensive. However, Design LED has found a method of printing a light guide, which functions in a similar way to an optical fibre, to control the distribution of an LED light throughout the back of the display. By segmenting the light guide, it is also possible to direct different coloured light to different parts of the display.
‘Printing the light guide gives a bendable, flexible backlight,’ says James Gourlay, the technical director of Design LED. Because the light guide transfers light from an LED source held adjacent, rather than behind, the display, the devices can also be very thin.