OLED monitors
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 computer monitors that use LCD (liquid crystal display) technology, OLED monitors do not require backlighting to function. The principle of OLED 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:
The OLED 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 OLED monitor screen. The layering of an OLED cell can be seen below:
How an OLED cell is layered
The colour of light emitted from the emissive layer of an OLED monitor depends on the exact organic makeup of the molecules within. As with LCD (liquid crystal display) monitors, 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 monitor 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 screen
Passive-matrix OLED (PMOLED) screens consist of OLED 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 diagram
Active-matrix OLED monitors
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 an AMOLED cell is shown below, again with only two pixel colours for diagrammatic purposes. Note that the AMOLED 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 diagram
Because TFT matrices are more efficient than the external circuits of PMOLED displays, AMOLED monitors are extremely energy efficient when compared to PMOLED displays. The TFT array controls current very rapidly and accurately, and is not held back by liquid crystals; AMOLED screens therefore have exceptional response times and colour reproduction.
Other OLED technologies
Although AMOLED screens are where the money is (literally) in terms of PC monitors and TV screens, there are several additional OLED 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 TVs and PC Monitors that you can literally see through, TOLED has particularly interesting applications in the military in aircraft, vehicle and soldier-mounted HUDs (heads-up displays).
Perhaps not the most useful application for TOLED, but an impressive display of the technology nonetheless
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 OLED technology, which has actually featured in the news recently, is 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 (TOLED) so that they could act as windows during the day and lights during the evening – perhaps even allowing them to black out when you want to sleep.
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-carbon fluorescent) backlights of LCD monitors with LED backlights using either white or coloured LED arrays. An LED backlight is lighter, thinner and more efficient than a CCFL-backlight and 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 monitors and TV screens to become thinner, lighter, more efficient and with superior contrast, brightness and colour reproduction enhancing the overall image quality
Not only is an OLED monitor exceptionally thin and light, by doing away with the backlight you also save a tremendous amount of power; when OLED PC monitors hit the mass-market, they could be over 10 times as efficient as the best LED-backlit LCD monitor of today. Because light is emitted directly from the emissive layers of OLED displays, the viewing angles are vastly superior to any LCD display. The most common technology used in LCD PC monitors; TN (twisted nematic), is widely criticised for distortion of the picture from significantly off-centre viewing angles.
Although not necessarily widely applicable to PC monitors, as we explored in the previous section; OLED displays can be flexible and/or transparent, allowing them to be used for certain specialist applications.
The disadvantages of OLED monitors
Unlike the advantages of OLED monitors, the disadvantages are not so numerous and are, for the most part, currently being rectified. The largest problem facing OLED 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 OLED 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).
Although OLED is a constantly evolving technology, and we’ve come a long way over the past few years; the video below highlights some important advantages and disadvantages of OLED technology, at the level it was in 2007-08:
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 and by using slightly different molecules, it is thought that the lifetime of ‘blue’ OLED 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 OLED 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 [source]. Samsung are confident with their current progress and have invested over $2.2bn in new production plants – so 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 OLED 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. LG have also demonstrated a technology known as WOLED (white organic light emitting diode) that uses white OLED pixels as a kind of backlight to shine light through a colour filter. The manufacturing process is cheaper and less time consuming than implementing seperate red, green and blue pixels; WOLED displays may offer a bridge between existing technologies and large OLED monitors using coloured pixels. It is probable that the contrast and colour performance of a WOLED display is inferior to a normal OLED display and the idea of using a white backlight and a colour filter isn’t exactly a radical departure from existing LCD technologies – but the self illuminated nature of the pixels is certainly attractive. The most recent release using this technology 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. You can read more about this here.
LG WOLED monitor (source: OLED-Display.net)
It is also known that Sony and Samsung are rolling out new mobile AMOLED displays and, in Sony’s case, small professional OLED 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 OLED PC Monitors rolling out for the high-end market and later hitting the mainstream market. PC Monitors will continue to bring you the latest OLED monitor news as soon as we can and we will hopefully be able to test some of the first OLED computer monitors when they become available – for the latest OLED monitor developments and news click here .
This article is brilliant – you’ve explained the technology so well! I can’t wait for OLED monitors and I have read this article several times just to take in all the greatness
So how long do you think until OLED monitors become available?
Although guessing is the best we can do, I would say that you might see semi-affordable OLED monitors in the UK no sooner than 2013. Being optimistic, it may be only beyond 2015 they really start to become affordable and replace LCD screens in our homes. Given this, I wouldn’t hold your breath. I could be wrong and the pace could pick up, but I’d say you could buy a new monitor today and not worry about it becoming obsolete for at least a few years.
How good are OLED monitors going to be compared to existing LCD monitors with LED backlights?
Answer posted here.
What is a PHOLED monitor?
‘PHOLED’ stands for phophorescent organic light emitting diode. Special materials are used that emit light in this highly efficient form of OLED (roughly 4 times as efficient as standard fluorescent OLEDs). Samsung is one of the main companies interest in OLED (and indeed PHOLED) materials for use in displays – they currently have a contract with UDC (Universal Display Corporation) to produce red (and soon green) PHOLED materials. Wikipedia has some decent information on PHOLED.