Organic Light Emitting Diode (OLED) displays are now available not only in smartphones and televisions, but also laptops and PC monitors. We explore how OLED screen technology compares and competes with LCD products.
OLED displays have been used in smartphones for a while now, so it was only a matter of time before scaling and manufacturing yields improved to the point where large displays suitable for televisions became economically viable. LG launched production OLED TVs in January 2013, albeit at the eye-watering price of $14,999 (about £10,300). A year later that price had halved, and the current price for an LG OLED Ultra HD 4K TV is below £3,000. To put that in perspective, LG will sell you a 55-inch TV based on (much more mature) LCD technology for under £800.
According to analytics company IHS Technology, sales of AMOLED TVs (Active Matrix OLED) are due to hit the two-million mark in 2016 and 10 million by 2018. And, as usually happens once a display technology is rolled out into the TV market, we're beginning to see OLEDs filter down into the computer display market. At the recent CES show, for example, we saw OLEDs in laptops such as Lenovo's ThinkPad X1 Yoga, HP's Spectre x360 and Samsung's Galaxy Tab Pro S, and also in Dell's 30-inch monitor.
So how do these new OLED screens compare to the established LCD technology? Do they offer markedly better performance? What about yields and cost, and how about longevity? These are still early days and OLED technology will improve, but let's see how it stacks up now.
Manufacturing yield and its effect on cost
In any manufacturing process the number of fully functional units coming off the production line is known as the yield, usually expressed as a percentage. Lower yields result in a higher price to the end user. Display screens require micron-perfect precision over a large area, with extreme levels of quality control. Getting manufacturing yields up to an economically sustainable level and keeping them there required gradual improvements to the underlying technology, improvements to the manufacturing methods and constant fine tuning.
The various companies involved in OLED development and production are exploring variations on the basic technology in pursuit of high-performance, long-lasting display panels with high yields.
When others in the industry doubted OLED's potential, Korean company LG Display purchased the technology from Kodak in 2009. LG has invested millions in developing WOLED (White OLED) technology and on building plant to manufacture large-format panels. According to Ken Hong, LG's Global Communications Director, said in 2014 "The fact that nobody is even chasing us on that [WOLED] is an amazing benefit, an advantage that we'll probably feel for ten years. No-one will catch us for 2-3 years, that's a pretty big lead."
How OLED displays work
Organic LEDs are called 'organic' because they use a (less toxic) carbon/hydrogen-based chemistry rather than a metallic element based chemistry. OLEDs typically consist of two layers of these organic molecules: the electron transporting and emitting layer (ETL) and the hole transporting layer (HTL). These layers are sandwiched between a cathode and a transparent anode. When a suitable voltage is applied between the cathode and the anode, the layers emit light. Different colours can be produced by varying the chemistry of the layers.
An OLED screen has no backlight, because each sub-pixel is an OLED that directly emits light, with the video signal controlling its brightness. The depth of black, where the OLEDs are off, is only limited by any ambient light reflecting off the display. Active Matrix OLED (AMOLED) displays share technology with LCD panels, since each OLED is driven by a Thin Film Transistor (TFT) addressed through a row-and-column matrix. Without the various inefficient light guides, diffusers, polarising films and colour filters found in LCD panels, the OLED pixel LEDs can deliver high brightness for high peak whites. OLED technology has the potential to deliver high peak brightness and a wide range of contrast and colour gamut. OLEDs also have the advantage in response time, with claims of 0.1 milliseconds (ms).
Early research into OLED panels focussed on the obvious idea of having individual red, green and blue OLEDs in each pixel, and the majority of companies developing OLED screens are still working with this idea. Depending on the frequency and purity of the light from each OLED in a pixel, this offers the widest possible colour gamut and contrast. However at present, blue OLEDs suffer from efficiency and longevity problems.
Another approach -- known as White Organic LED - Colour Filtered (WOLED-CF) or White RGB, which simplifies the active elements and results in better yields -- is to manufacture all the LEDs on the display as White OLEDs. Like the LCD panels, the 'white' light -- in this case from the pixel LEDs rather than from a liquid-crystal-modulated backlight -- is then filtered through red, green and blue filters. This produces a display with good contrast (a wide dynamic range), but any colour impurity in the combination of the light from the WOLEDs and the filtration will compromise colour gamut.
LG calls its configuration of WOLED M+ technology, which as well as using colour filters in front of WOLED sub-pixels also adds an unfiltered white sub-pixel. Red, Green, Blue and White (RGBW) sub-pixels are combined in four combinations; RGB, WRG, BWR and GBW. The unfiltered white sub-pixels make high peak brightness possible and M+ panels are therefore capable of HDR images. LG also uses the M+ configuration on high-definition LCD panels.
A Liquid Crystal Display (LCD) screen works by modulating the light from a separate backlight through polarising electronic shutters and colour filters. Limits on the dynamic range of the polarising cells, from passing to blocking the light from the backlight, and on the colour purity of the filters and backlight, place limits on contrast and on colour gamut. The unpolarized light from the backlight must be passed through an initial polarising filter for the system to work and this immediately reduces brightness of the backlight by 50 percent or more, reducing efficiency. Overall light efficiency for an LCD can be as low as 5 percent, from the backlight LEDs to the front of screen!
But what if the existing LCD technology could be massively improved, without having to throw out the existing manufacturing plant and methods? This is exactly what can be done by replacing the older (inorganic) White LED edge-illuminated backlights with backlights based on quantum dot technology. The backlight in an LCD panel is a mechanically separate unit and it's relatively straightforward to introduce a different type of backlight into a current LCD panel manufacturing plant.
Quantum dots are an application of nanotechnology that makes it possible to manufacture crystals of a semiconductor material to a precise size. When illuminated with high-frequency (ultraviolet or blue) light, these crystals fluoresce, converting the high-frequency light to light of a lower frequency. What's more, the frequency or colour of the light they emit is directly related to the size of the crystals. The conversion is almost 100 percent efficient, it's narrow band and the output wavelength is determined by the crystal size.
This means that a mixture of crystals of two specific sizes can be produced, embedded in a sheet of plastic. When that sheet of plastic is illuminated with high-frequency light from a conventional blue LED, the quantum dots convert some of that light into down-shifted, narrow-band, red and green. The exact frequencies of the quantum dots can even be tuned to match the response of the dye-based RGB filters at the front of each LCD pixel. A quantum-dot-doped plastic sheet can be used with distributed LED illumination, or a bar of doped plastic can be used with edge-lit displays.
Nanosys, the leading company in this field, calls such sheets QDEF (Quantum-Dot Enhancement Film).
Current LCD backlights are mostly based on edge illumination from inorganic 'White' LEDs -- blue LEDs with an added yellow phosphor (most commonly this phosphor is YAG, ytrium-aluminium-garnet) that down-shifts some of the blue light. The light from the yellow phosphor contains both green and red frequencies, but isn't efficiently matched with the green and red filters of the LCD pixels. This limits the maximum brightness and the colour gamut. By contrast, a quantum dot backlight, driven by inorganic blue LEDs distributed across the back of the display area, produces higher peak brightness, a greatly expanded colour gamut, higher efficiency and allows for 'local dimming' contrast enhancement.
Quantum dots can also be used by integrating them directly into the structure of a LED, but the QDEF backlight technology is the simplest way of using quantum dots to considerably improve LCD design.
Colour display standards
Compared to the capabilities of the new OLED and quantum dot LCD displays, the range of colours and brightness that most people see on their televisions and computer displays today is surprisingly small. Colour gamut standards for today's HDTV broadcast are defined by ITU-R Recommendation BT.709 and the colour co-ordinates specified in that are little different from those of the phosphors of colour CRTs from 1953. The ITU-R Rec. BT.2020 specifies the colour gamut that can be expected from UHDTV in future and, as the included gamut plots on the CIE XYZ colour diagram show, this represents a huge step forward.
However, in order for this wider gamut to be visible it has to be present in the source material. Current professional digital movie cameras capture wide-gamut, high-dynamic-range images, but as soon as this source material is processed for broadcast or optical disc distribution the gamut and dynamic range are reduced to meet current rec.709 standards.
Also known as HDRTV (High Dynamic Range TeleVision), Dolby Vision requires a display capable of high contrast and high brightness. The second piece of the puzzle is Dolby's proprietary signal processing. This cleverly uses the capabilities of existing video codecs to encode high dynamic range video. An HDR display equipped with a decoding chip running Dolby's proprietary algorithms can decode these signals to drive the display to the full extent of its available gamut, while at the same time the signal remains compatible with older displays.
Seeing the bigger picture
This article started life as a look at OLED technology, but the real story is that the colour displays now starting to reach the mass market offer huge improvements in colour gamut. Whether this is achieved via OLEDs or through improvements to existing LCD technology, is almost of secondary importance.
The latest professional digital movie/video cameras are capable of producing content with wide gamuts and high dynamic range, most of which is being lost thanks to broadcast standards that are still stuck in the 1950s. It's now time for content creators and delivery systems to step up, implement the latest HDR technology and take full advantage of these new displays.
With a spread of technologies becoming available, and possibly large differences in performance, buyers will need to be careful when selecting a new display product. The future for displays really does look bright -- very bright indeed, in fact.
Acknowledgements Jeff Yurek, Corporate Communications Manager at Nanosys for providing information on QDEF for this article; and LG Display for information on its OLED displays.