Display Technology
Innovative printed OLED technology
By Justin Coleman, display division manager, Review Display Systems O
LED display technology provides exceptional optical performance. In terms of accurate colour reproduction, image quality and clarity it has few competitors.
Although successful in some markets, can OLED offer a viable, long-term replacement for more established display technology such as thin-film transistor (TFT) LCD?
OLED successes
OLED display technology has seen considerable success in high volume OEM applications such as smart watches, premium mobile phones, and large-size (>50-inch) smart TVs. OLED adoption in other markets and industrial applications has however been somewhat limited.
An evolving technology Achieving efficient and effective OLED manufacturing processes have until recently presented a barrier to cost-effective OLED production. One of the issues affecting OLED displays has been the limited lifetime of OLED materials - particularly the organic compounds used as light emitters.
OLED manufacturing
Existing manufacturing techniques for OLED display technology are complex and expensive, resulting in high production costs. This is one of the main reasons for the relative lack of success of OLEDs in the medium-size display sector. Conventional OLEDs are produced using a vapour deposition method. This production technique requires large vacuum chambers to accommodate a vapour deposition source and the panel substrate. For colour (RGB) OLEDs,
three individual mask layers are required, and as pixel resolutions increase aligning the pixel mask precisely becomes increasingly challenging. In addition, the vapour deposition method is relatively inefficient in its utilisation of EL materials.
Advantages of printed OLED Japan-based OLED manufacturer JOLED has developed a pioneering ink-jet printing technique which forms emissive RGB subpixels by printing directly onto the panel substrate. JOLED’s printing technology realises a more efficient and cost-effective production process - typically 30-50 per cent cheaper compared to vapour deposition methods.
The printed OLED substrate is processed under normal atmospheric conditions without the need for a vacuum chamber or any metal masking layers. Waste material is completely avoided as only the precise amount EL material is deposited on the substrate during the printing process.
The printing process is also scalable which enables a range of panel sizes to be mass produced using the same print technology. While the printing method still requires advanced technical capabilities, it represents a major innovation in OLED manufacturing.
OLED vs TFT LCD - performance characteristics
JOLED printed OLED panels, in comparison with a contemporary TFT LCD display module, provide significant performance advantages in terms of contrast ratio, colour gamut, response time and luminance characteristics. The leading features of printed OLED displays include:
colour space and defines the chromaticity coordinates of red, green, and blue (RGB) within the CIE 1931 Chromaticity diagram. Printed OLED displays with a 10-bit colour depth can now deliver 138% of the sRGB colour gamut which enables highly accurate and deeply saturated colour reproduction. In comparison, TFT LCD typically supports a sRGB colour gamut of 100%.
Response time
Response time represents how quickly a pixel switches from an active ‘on’ state to an inactive ‘off’ state. A faster response time will minimise ghosting and blurring effects. Printed OLED displays have a response time of just 0.1mS, which is significantly faster (>100 times) than a typical TFT LCD at around 15mS.
28 September 2021 Components in Electronics
Contrast ratio Contrast ratio is defined as a measure of the ratio of the luminance of the brightest shade of white to that of the darkest shade of black a display can produce. More so than colour, or any other aspects of image quality, contrast
has the biggest impact on image performance. Printed OLED displays can now achieve contrast ratios of 1,000,000:1. Contrast ratio of a TFT LCD is typically 1000:1.
Colour gamut
The colour gamut of a display is defined as the range of colour that a display can reproduce within the full spectrum of colours that are visible to the human eye. The sRGB gamut is a standard
ney eciency
OLED is a self-illuminating emissive display technology - each individual pixel emits light - and can be switched on/off independently. A TFT LCD operates as an electronic shutter mechanism and requires a power-hungry backlight.
Blue light emission
Monitors, laptops, and smartphones can emit significant amounts of high energy, short wavelength blue light. Prolonged exposure to blue light can lead to damage of the light sensitive cells in the retina. Printed OLED displays offer a more user-friendly solution with a 40% reduction in blue light emissions compared with a TFT LCD.
Structure and form factor The OLED printing process creates a simple, exceptionally thin display structure with a total display thickness of just 1.3mm. A 32-inch printed OLED display panel weighs just 1kg. An equivalent TFT LCD display panel weighs significantly more, typically 4-8kg.
A bright future for OLED JOLED was established in 2015, combining the OLED display development divisions of Sony and Panasonic, with the goal of developing and commercialising high performance medium-size printed OLED displays. An innovative printing process has been realised and now enables the efficient and more cost-effective production of medium-sized OLED display panels from 10 to 32-inch.
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