Author: Adam Simmons
Last updated: May 23rd 2014
What are they?
OLED monitors are flat computer displays which consist of pixels made from OLEDs (Organic Light Emitting Diodes) rather than liquid crystal filled units. Unlike LCD (Liquid Crystal Display) technology, OLED does not require backlighting to function. The principle of this technology is that when current flows between a cathode and an anode, an emissive layer of organic molecules (e.g. polyaniline, green in diagram) sandwiched between these electrodes can become illuminated (electroluminescence). For this to happen efficiently, a layer known as the conductive layer (orange in diagram), made up of organic plastic molecules such as polyfluorene, lies between the emissive layer and the anode. The anode is positively charged and therefore draws electrons from the conductive layer, leaving the conductive layer with a positive charge that draws electrons from the emissive layer. Light is emitted as a by-product, in a process known as electrophosphorescence.
The OLED process is explained in the diagram below:
OLED process diagram (credit: HowStuffWorks)
The layers described above total a thickness of around 100-500 nanometres, which is around 100 times thinner than human hair. This makes them extremely fragile and hence they must be supported by an additional substrate layer. This substrate is usually clear plastic, foil or glass of varying thickness, and must be transparent, like the anode, so that the emitted light can be seen on the screen. The layering of an OLED cell can be seen below:
OLED cell diagram (credit: HowStuffWorks)
The colour of light emitted from the emissive layer depends on the exact organic makeup of the molecules within. As with LCD (liquid crystal display), OLED units are made up of ‘pixels’ of different colours; i.e. various organic molecules make up ‘pixels’ within the emissive layer which will emit light of different colours once illuminated. The brightness (light intensity) of an OLED is proportional to the current applied to the cell.
Types of OLED screen
There are several types of OLED cells which are being developed for possible incorporation into OLED monitors. The principles used in all are similar to those explained above, but the arrangement of the layers within the cells and the exact materials used differs slightly. Some technologies described below are not applicable to PC monitors and will be restricted to specialist applications such as heads-up-displays on aircraft or small bright clock screens; but we explore them anyway.
Passive-matrix OLED (PMOLED) screens consist of cells with opaque cathodes and transparent anodes laid perpendicular to one another in strips. Between these strips are the organic layers of alternate coloured light-emitting diodes and conductive molecules. Once power is switched on to external circuitry (voltage is applied), current flows through particular cathode and anode strips, so that light of selected colours and brightness are emitted through the electrode intersections according to the molecules illuminated and current applied (respectively). The PMOLED process is shown diagrammatically below, with only two pixel colours shown for simplicity:
PMOLED cell (credit: HowStuffWorks)
Active-matrix OLED (AMOLED) screens are currently receiving massive research and development funds from the likes of Samsung, LG and Sony for incorporation into HDTVs and PC monitors. AMOLED cells contain organic molecule layers and anodes arranged in small sheets (pixels), sandwiched between a larger cathode sheet and integrated into a TFT (thin film transistor) matrix. The TFT matrix not only acts as the supporting substrate; it also controls which pixels become activated by switching on or off current flow to the appropriate pixels and hence drives them in a similar manner to TFT LCD monitors. The typical layout of such a cell is shown below, again with only two pixel colours for diagrammatic purposes. Note that the cell featured in the diagram is bottom-emitting (i.e. has a transparent TFT backplane that light passes through). AMOLED cells may also be top-emitting, meaning that light passes through a transparent cathode rather than the substrate (TFT backplane), which is reflective or transparent.
AMOLED cell (credit: HowStuffWorks)
Because TFT matrices are more efficient than the external circuits of PMOLED displays, AMOLED is extremely energy efficient in comparison. The TFT array controls current very rapidly and accurately, and is not held back by liquid crystals; Active Matrix OLED screens therefore have exceptional response times and colour reproduction.
Other OLED technologies
Although AMOLED is where the money is (literally) for monitors and TVs, there are several additional technologies which have rather particular specialist applications. Transparent OLEDs (TOLEDs) make use of a transparent cathode in addition to the already transparent anode and substrate to produce a screen that is over 80% as transparent as the substrate used, when the pixels are in the off state. Although this could potentially be used in high-end displays (that you can literally see through), this application is limited by the inability of the TOLED matrix to display ‘black’. Nonetheness; this has particularly interesting applications in the military in aircraft, vehicle and soldier-mounted HUDs (heads-up displays).
By using a highly flexible substrate, such as thin foils or plastics, it is also possible to make a durable, lightweight and even foldable OLED screen (FOLED). These have interesting applications for both civilians and military personnel, as they have be integrated into clothing. Another emerging technology involves the use of ‘pure’ white OLEDs as an efficient lighting alternative. The light emitted is more energy-efficient, brighter and whiter than fluorescent or incandescent light bulbs. By producing OLEDs in large sheets, which is an advantage of the current ‘printing’ manufacturing process, it is possible to make large thin sheets of light for use on walls and ceilings. It is even possible to make them transparent so that they could act as windows during the day and lights during the evening – perhaps even allowing them to black out.
In 2009 and into 2010, a great drive has been made by PC monitor and TV manufacturers (in particular LG and Samsung) to replace the usual CCFL (Cold Cathode Fluorescent Lamp) backlights of LCD monitors with LED backlights using either white or coloured arrays. The predominant modern form of this backlight uses strips or clusters of white LEDs behind the edges of a monitor – a backlight typed dubbed WLED. A WLED backlight is lighter, thinner and more efficient than a CCFL-backlight but have until recently been very limited in their colour gamut output.
OLED technology is the next step in the evolution of the display, as it does away with the backlight entirely. With only a thin transparent film in the way of the light emitted by the pixels, you get an image with previously impossible contrast, greater apparent brightness and vivid, lifelike colours with an exceptionally wide gamut. Response times and refresh rates are also significantly enhanced over even the best LCDs – an OLED monitor could theoretically have a response time of around 0.01ms and a refresh rate exceeding 1 KHz (1000Hz). Manufacturers are also experimenting with multiple emissive layers to enhance the brightness, which is possible due to the exceptionally thin nature of the cells. The end result of all this is images that are much more vivid and lifelike than anything produced by an LCD. No picture or video could ever do these changes justice but this one gets the point across quite nicely.
OLED image quality
Not only is the hypothetical OLED monitor exceptionally thin and light, by doing away with the backlight you also save a tremendous amount of power; when these hit the mass-market they could be over 10 times as efficient as the best LED-backlit LCD monitor of today. As the technology stands at the moment they are considerably more efficient than LCD screens of comparable size when displaying mainly blacks and dark colours – but a lot of white on screen drives current power consumption up significantly. One undeniable advantage of is viewing angles that are vastly superior to any LCD display; light is emitted directly from the emissive layers of OLED displays. The most common technology used in LCD, TN (Twisted Nematic), is widely criticised for distortion of the picture from significantly off-centre viewing angles.
Although not necessarily widely applicable to larger screens, OLEDs can be flexible and/or transparent. This allows them to be used for certain specialist applications as explored in the previous section.
Unlike the advantages the disadvantages are not so numerous and are, for the most part, currently being rectified. The largest problem facing manufacturers is that organic materials used in OLED displays degrade over time, like any organic matter. The most troublesome element of this degradation is that blue-emissive pixels degrade more rapidly than their red and green counterparts. This could potentially lead to colour balance issues over time and is of great concern for PC monitors due to how frequently they would be used (unlike a small smartphone screen, for example, which spends most of its time on standby).
Where are we now?
Fortunately, great strides are being made by Samsung and partners to increase the lifetime of OLED pixels of all colours. By using improved technology to ‘spray’ organic materials onto the substrate surface and by using slightly different molecules, it is thought that the lifetime of ‘blue’ pixels could be extended from 14,000 hours to 60,000 hours (nearly 7 years). This would mean that all pixel colours would degrade at similar rates and would give the monitor a useful life of several years. This same spraying process should reduce manufacturing costs (a large problem for OLED screens today) by reducing wasted materials and the completion of important and expensive research. A recent ‘spraying’ process referred to as ‘solution coating technology’ is being developed by DuPont and is showing great improvements in key areas including manufacturing efficiency and material longevity. In November 2011 DuPont signed an agreement to allow panel manufacturers such as Samsung to adopt their solution coating process commercially which is a very important step indeed. We should expect some great things from them in the future.
Scientists in Michigan have also been looking at increasing the lifetime of the blue OLED substrate my taking a different approach. According to Kieffer (the lead researcher), by reconfiguring the molecular structure itself it should be possible to significantly extend the useful lifetime of blue substrates – effectively ‘doubling’ their efficiency. These are just examples of important research which will one day overcome the hurdles placed before the commercially viable monitor.
The other good news for the consumer is that Samsung are not the only manufacturer investing heavily in OLED technologies at the moment. LG have recently announced that they are tripling their investment in such technology and really ramping up their production capacity.
As far as models for consumer use go LG had planned to release the 55EM9600 (55EM960V UK designation) 55″ Full HD 3D OLED TV in limited retail capacity some time during 2012 (above). Its expected retail price sits at around $10000 (£7000) or more. This product was available to consumers from early 2013 instead as the updated 55EM9700 (55EM970V for UK consumers). The EM9700 uses a colour-filtered WRGB design and was demonstrated as a real crowd-pleaser at CES 2013. WRGB is essentially an evolution of WOLED, featuring colour filters over three organic white subpixels with a fourth subpixel that emits ‘naked’ unfiltered white light. This is designed to enhance the luminance efficiency compared to an RGB-OLED or WOLED design with the white pixel able to assist in brightening the image or displaying pure white on its own. As with other implementations this system benefits from luminance control on a per-pixel basis, allowing a pretty much infinite contrast ratio to be achieved.
A similar 55″ display (the ES9500) was demonstrated by Samsung during CES 2012 with a similar expected price tag. This was released on the Korean market during summer 2012. It has been re-branded the F9500 for the international market and is due for release in 2013. During CES 2013 Sony and Panasonic also unveiled some prototype large OLED displays with 4K (4096 x 2160) resolutions. These use a technology called ‘Super Top Emission’ which incorporates an RGB pixel design and colour filters, which the company claims enhances colour purity and overall efficiency. Sony is also pushing out a number of professional monitors using this technology, including a 30″ 4K monitor which they are hoping to launch in April 2014. At CES 2013 LG and Samsung also demonstrated another innovation; 55″ curved OLED TVs. This model was released in the UK and Europe in September priced at £6999, following an unveiling at IFA as the KE55S9C. This TV takes advantage of the technology’s thin and flexible nature and are intended to provide a more engrossing viewing experience. Since then Samsung have released a number of other large displays using similar technology.
Samsung curved OLED TV
For their large displays Samsung use a ‘conventional’ RGB-OLED design (dubbed ‘Super-OLED’) with direct colour light emission from its organic subpixels. There are rumours that Samsung are now also starting to develop WRGB-OLED technology for large displays similar to that used by LG. This is certainly a key technology which could prove popular for PC monitors in the future due to its luminance efficiency and lack of differential colour degradation problems.
QLED - an alternative
Samsung are suffering from a number of issues such as yield and production prices with their ‘Super Top Emission’ technology and have scrapped plans to build a new manufacturing plant for large OLED displays. They are still focusing efforts on flexible technologies and ‘small to medium’ sized displays used on phones and tablets and hope to return to larger displays if technological advances improve the economic viability of their preferred Super Top Emission designs. LG on the other hand continues to invest heavily in new OLED fabrication facilities, including those used for large displays. It is generally hoped that WRGB-OLED, LG’s current preferred method, could finally give us a taste of the technology on our desktops. An equally exciting technology that should bring similar advantages to OLED is also being developed. QLED (Quantum dot Light Emitting Diode), above, is the product of a cooperative partnership between LG Display and QD Vision. We will continue to bring you the latest QLED and OLED monitor news as it rolls out and will hopefully be able to test some of the first consumer displays as they become available.