What are OLED monitors?
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.
The advantages of OLED monitors
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 predminant 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 whilst the rather rare ‘full array’ variety can be accurately and independently controlled so that specific areas of monitor can become lighter or dimmer depending on the image to be displayed. This has allowed high-end monitors and TV screens to become thinner, lighter and more efficient superior dynamic contrast ratios contrast.
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.
The disadvantages of OLED monitors
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. 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. Samsung are confident with their current progress and have invested over $2.2bn in new production plants. 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 not too distant future.
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. At various roadshows, including the Gadget Show Live event in the UK, LG showed off their new 15-inch OLED TV; the LG 15EL9500. This is now available in limited capacity at UK retail for around 1500-2000 GBP. At CES 2011 a followup and potentially very exciting follow-up OLED TV was announced to be launched in 2011; this time a 31 inch model. This never reached the market, however. LG have also demonstrated a technology known as WOLED (white organic light emitting diode) that uses individually controlled organic white pixels as a kind of backlight to shine light through a colour filter. This helps to overcome differential degredation of traditional coloured substrates; WOLED displays may offer a bridge between existing technologies and large OLED monitors using coloured pixels. LG Display have further evolved this concept with an implementation known as RGBW which features 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 a fully filtered design and as with other implementations benefits from luminance control on a per-pixel basis. The most recent release using the WOLED implementation is a 20.7 medical display with a staggering QSXGA resolution of 2560×2048, an amazing static contrast ratio of >100,000:1 and literally brilliant peak luminance of 900cd/m2.
LG WOLED monitor
As far as future models for consumer use go LG plans to release the EM9600 (EM960V UK designation) 55″ Full HD 3D OLED TV in limited retail capacity some time during 2012, with an expected retail price of around $8000. The EM9600 uses LG’s colour-filtered RGBW design and was demonstrated as a real crowd-pleaser at CES 2012, shown briefly in the video below. A similar 55″ display (the KN55ES9000) was demonstrated by Samsung during CES 2012 with the release schedule likely to be competitive with the LG model but a slightly higher expected price tag of around $9000. The Samsung is a ‘conventional’ OLED design (dubbed ‘Super-OLED’) with direct colour light emission from its organic subpixels. It isn’t likely that the Samsung model uses the solution coating process described in the previous paragraph so its suitability as a large PC monitor remains to be seen. In other exciting news, LG plans to have their 5.5 gen OLED production facility producing their ‘IT line’ up and running during 2012-2013 which, alongside DuPont and Samsung’s new processing techniques, could finally give us a taste of the technology on our desktops.
It is also known that Sony and Samsung are rolling out new mobile AMOLED displays and, in Sony’s case, rather expensive professional screens. They will continue to do so in the future – really, it is only a matter of time (at most a few years if we’re optimistic) before we see the technology rolling out for the high-end computer market and later hitting the mainstream market. An equally exciting technology that should bring similar advantages to OLED is also being developed, by a cooperative partnership between LG Display and QD Vision; QLED (Quantum dot Light Emitting Diode). 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.