Miniature OLED displays explained
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.
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Today, P-OLED technology can be used to create displays of all sizes and performances, ranging from simple monochrome versions to video-capable, full-colour graphic displays.
Moreover, according to industry analyst firm NanoMarkets LC , the wider technology of organic electronics is rapidly making its way out of the lab and into real world applications. It predicts the market for such products as OLEDs, organic thin-film transistors, etc, will grow from $1.4bn in 2007 to $19.7bn by 2012, rising to $34.4bn in revenues by 2014. By 2012, the OLED industry alone is expected to reach $10.8bn.
Microdisplays, where the display is integrated with its driver and control electronics onto a single silicon substrate, show major promise, especially in near-to-eye and projection applications.
The near-to-eye space, for which P-OLED microdisplays offer the greatest advantages, splits into two main sub-spaces. The first is where the microdisplay module is built into a product which is held up to the eye — for example, electronic viewfinders for consumer use, night-vision scopes, electronic binoculars and telescopes.
The other is where the microdisplay module is placed close to the eye in a hands-free configurasuch as in head-mounted displays for personal media players, watching TV via mobile phone and playing games on the move.
An example of a P-OLED microdisplay for such applications is represented by the eyescreen ME3204. Developed by Edinburgh-based MicroEmissive Displays (MED), this provides a complete digital microdisplay system with a high level of electronic and optical integration and excellent image quality coupled with ultra-low power consumption. It provides QVGA resolution (320×240, 230k dots) with 0.24in. diagonal pixel array.
The emissive P-OLED technology, which requires no backlight components, integrated display driver electronics and digital video interface allow glue-less integration into a wide range of systems and enables product designers to develop smaller and lighter weight products.
Key factors for microdisplays are power consumption, image quality and longevity. Power consumption is an issue that has more impact on video glasses than in viewfinders. Viewfinders are just one of the powered components in a handheld device, whereas in video glasses the display is effectively the major powered component.
Power hungry screen
In a digital camera, the LCD screen is probably the component with the single highest power consumption. For instance, an LCD screen with a typical 320×240 pixel display may use 300 or 400mW, whereas a typical LCD microdisplay uses less than 200mW. The equivalent P-OLED microdisplay consumes just 50mW, which would equate to a significant improvement in battery life. LCDs need a very bright backlight because they are transmissive and inefficient. P-OLEDs, emit the light by themselves – very efficiently.
In video glasses, where the microdisplay is the single highest power component, consumption of 50mW equates to a theoretical battery life of 30 hours from a single alkaline AA cell. An LCD microdisplay would last less than nine hours.
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.
Wi th 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