Growth prospects for low power microdisplays continue to gain momentum — especially in mobile applications such as music/video players; viewfinders in cameras and video recorders; and in consumer video glasses.

There are a number of miniature display technologies competing for the space:

• transmissive displays – in particular miniaturised liquid-crystal technologies — have matured through the growing popularity of large-screen TVs and monitors
  
• reflective technologies — including digital light processing (DLP) and liquid-crystal on silicon (LCOS) — have advantages particularly for projection systems. These have also been implemented in popular applications for a number of years and undergone significant development
  
• emissive technologies such as OLED (organic LED) are relatively new but can already compete with LCD and LCOS technology on price and performance. Furthermore, being relatively early stage technologies, they have greater headroom for future improvement.
  

OLED displays can use either small organic molecules or polymers. Light-emitting polymers have the major advantage that they are soluble and therefore can readily be deposited in solution onto a display substrate — for example, by spin-coating or ink-jet printing — without the need for a temperature-controlled vacuum-environment. P-OLED (polymer OLED) displays can have larger screen sizes than can be made than with small-molecule OLEDs  as there is no need for the shadow masks required by the vacuum deposition processing of the latter. P-OLED displays arguably also operate at lower voltage and are more power-efficient than small-molecule ones.

Displays of all sizes
P-OLED technology development really got going during the early 1990s, when UK start-up Cambridge Display Technology (CDT) spun out of Cambridge University to develop light emitting polymers – the fluorescent materials at the heart of P-OLED displays.

Colour coding LED info
	Driving into the blue
	Organic - grow your own
	Green chandeliers
	Into the black?
	Display Trilogy
	Headlight glare
	Phosphorous
	Sensing light
	Grey area
An alternative look at the hues and shades of LED technology

As if these advantages were not enough, the most tangible difference between LCD and P-OLED is picture quality.

Although quality is largely subjective, there are a number of definable objective factors that make one display look subjectively better than another. Among these are brightness, contrast ratio, the darkness of black areas, pixel sharpness, natural colours and the ability to handle fast-moving images. P-OLEDs excel in all of these respects.

As one would expect, the transmissive nature of LCDs means that brightness depends on the power consumption of the backlight and the efficiency of liquid-crystaltransmission. P-OLEDs are inherently more efficient and the image remains visible over a wider viewing angle.

High contrast enhances the visual experience by producing an illusion of depth. Contrast is affected by ambient light but in EVF or enclosed head-sets, where ambient light is excluded, the contrast ratio is not significantly reduced. Contrast is also limited with LCDs, and significantly enhanced with P-OLEDs by light leakage.

Backlight can leak through a dark pixel in an LCD so they are never truly black – some light is transmitted through even the darkest pixels. With P-OLEDs, a black pixel is truly black — it really is an absence of light. The result is a display that appears to be almost 3D, thanks to its brightness and contrast.

The sharpness of a displayed image enhances perceived quality because the eye sees it as being more realistic. In electronic displays, sharpness is degraded significantly by the appearance of pixilation, which will happen if the pixel dots of the display that light up are far apart or the dark gaps between them are visible. Parts of the overall image then appear jagged rather than smooth or continuous. Worse, the viewer sees individual dots instead of a continuous picture.

From a technical viewpoint, the crucial factors are the number of pixels and the percentage of a pixel’s area that is lit. The more of it that is lit, and the more uniformly it is lit, the better the image quality. If too small an area of the pixel is lit, the screen appears like a wire mesh instead of contiguous patches of colour. The fill factor of P-OLEDs is around 80 per cent, leading to much less pronounced pixelation than alternative LCDs, where the fill factor is often less than 25 per cent.

Whilst these considerations apply equally to static and moving images, fast moving video really separates the display sheep from the goats. It is one of the best tests of perceived real-life image quality.

Blurring and flickering
Blurring and flickering result when display pixels do not switch on and off quickly enough. The effect is that moving objects lag or leave a trail of slower dots. Here again, P-OLEDs exhibit superior characteristics compared with LCDs. Typical refresh times for LCDs are around 15ms: P-OLEDs switch in a fraction of a millisecond, producing good motion video without slow-switching artefacts.

Image quality followed by battery life (power consumption) are the main considerations for consumers. But device makers have also to consider how displays are integrated into an overall system, in particular to a device’s electronics.

Miniature LCD microdisplays typically feature a liquid crystal array on a glass substrate, making it impossible to fully integrate with the control circuitry. MED’s P-OLED microdisplays have a CMOS silicon substrate, so electronics are very readily integrated meaning design and development times are shorter, and manufacturing is relatively straightforward.

Despite all these advantages, designers may be tempted to ignore the potential of OLED technology because of its historical reputation for short product lifetimes and lower reliability.

Perceptions of lower longevity have some historical basis in fact. Under certain operating conditions OLED displays will get dimmer over time. Nevertheless, such displays have more than adequate lifetime for consumer oriented near-to-eye applications such as video cameras and video glasses, and will amply outlast the device through normal usage. Today, we could expect a P-OLED microdisplay to survive being used for several hours a day, every day for several years with acceptable performance.

There is the issue of differential aging — blue-wavelength OLED materials do not maintain their light output for as long as red or green. The result is that RGB OLED displays can exhibit a yellow-coloured cast after a period of time. MED’s answer is to use white P-OLEDs with red, green and blue filters, which ensures that any dimming is consistent across the spectrum. There is no shift in the colour balance.

With advantages like these, microdisplay products based on P-OLED technology will continue to penetrate the mainstream over the next five years. There will be an evolution from smaller to larger displays and the emergence of a whole host of non-display applications including P-OLED backlights for LCDs, P-OLED panels for lighting, and the emergence of revolutionary products like P-OLED sticking plasters for the treatment of skin cancers. The future is brighter with P-OLEDs.

Professor Ian Underwood is chief technology officer and co-founder of MicroEmissive Displays