How do you choose the most suitable display technology for your application?

Jan Jongman, Senior Manager Technology Development

May 18, 2020

In our everyday lives we are surrounded by devices with displays and spend a lot of time looking at screens - be it the screens of our smartphones, laptops or TVs. Apart from these high resolution active matrix displays, we are also accustomed to looking at low resolution displays such as those of our alarm clocks, smart thermostats and kitchen appliances. We even rely on displays when driving – from the car dashboard to the infotainment system, the surface areas devoted to displays in cars is growing rapidly.

HDR TVAll these display applications have specific requirements, and display system designers have to choose from a plethora of display technologies. In order to select the most suitable technology, they first need to decide which display parameters are most important for the application in mind.

In this blog we list the most important display parameters that system designers need to take into consideration when looking for the most suitable technology for their application. We also compare some of the display technologies available using these parameters.

Display parameters

  • Display resolution: Display resolution can be specified as the number of pixels per inch (PPI) or the total number of pixels in the display. Top of the range large displays have 7680 x 4320 pixels.
  • Color gamut: The color gamut defines how well colors can be represented. A commonly used standard is the percentage of NTSC (National Television Standards Committee) a display can reproduce. For example monitors for video editing must be able to reproduce 100% NTSC.
  • Update speed: This is the speed with which a new image can be written or updated. For reading applications, a page update in a fraction of a second is sufficient, while for gaming monitors an update frequency of 240Hz may be required.
  • Lifetime: Different applications are expected to have different lifetimes. For example, for mobile phones this is around three years, while for white goods and cars the required lifetime is more than 10 years.
  • Contrast ratio: The contrast ratio refers to the ratio of the luminance of a white image divided by the luminance of a black image. The contrast ratios for LCD screens are measured in a dark room and can vary from 1:1000 up to 1:1500. In practice, reflections from external illumination are present and this is the reason why all emissive displays are difficult to read on a sunny day.
  • Power consumption: This is the electrical power required to display an image or video. This is a very important consideration for portable devices as it will impact on the time the battery lasts between charges.
  • Conformal and bendable: This attribute refers to what extent the display can be bent or folded repeatedly. The emergence of conformable and foldable displays enables novel designs for a wide range of applications.
  • Cost: When selecting a display for a particular application, it is important to consider all costs related to the display, including the cost of the display component itself, housing, backlight, electronic interface and power supply.

Display technologies

Below we summarise some of the key display technologies system designers can choose from and compare them using the display parameters listed above.

Key Display parameter

LCD

OLED

QDOT OLED

Flex OLED

µLED

OLCD

Bend radius

1 1 1 5 3 4

Lifetime

5 3 2 1 5 5

Resolution

4 5 5 5 4 4

Update speed

4 5 5 5 5 4

Color gamut

4 (5+) 4 4 4 5 4 (5+)

Contrast ratio

4 (5*) 5 5 5 5 4 (5*)

Cost

5 2 2 1 1 5

We use a scale from 0 to 5, with 5 meaning: very flexible; long life time; high resolution; fast update speed; large color gamut; high contrast ratio; low cost. + refers to QDOT while * refers to dual cell technology.

LCD

Historically LCD displays were slow and and did not have a good contrast ratio and color gamut at all viewing angles. These problems have been resolved, and today LCD accounts for more than 90% of the displays sold. The color gamut of LCD depends on the color filter used and is normally not as wide as that of an OLED display. With the introduction of QDOT and dual-cell technology similar performance as OLED can be achieved in both color and contrast.

Quantum Dot (QDOT) films placed between the backlight and the LCD panel, enable very saturated colours to be displayed. The quantum dots convert the broad LED spectra into narrow red, green and blue spectra, significantly improving the colour gamutwith out the need for a narrow transmission band color filter.

For the dual cell technology a monochrome LCD display is placed between the RGB LCD display and the backlight. This enables a much higher contrast ratio (1,000,000 : 1) giving it a similar performance to OLED, but at a lower cost.

OLED displays

OLED displays have been under development for a very long time. Lifetime has been their weakest point, especially for the blue emitters. Lifetimes are even further reduced at high brightness. OLED displays can be divided into two types RGB OLED and WOLED.

RGB OLED displays have the red, green and blue OLED material patterned using a Fine Metal Mask (FMM) and emit light only when required. The spectrum of the red, green and blue emitter is narrow enough to produce a wide color gamut. However patterning these OLED materials still represents many technical challenges in mass production especially for large displays and therefore at high volume only small RGB OLED displays are available.

Large OLED displays mainly use WOLED technology. A white emitting OLED material is deposited on top of the active matrix backplane and a red, green, blue and white colour filter array (CFA) is placed on top of white light emitting OLED, compromising the color gamut and power efficiency.

QDOT OLED displays

To overcome the weaknesses of WOLED Samsung is investing heavily in QDOT OLED display technology for large displays. Photoluminescent red and green quantum dots are printed on top of a blue OLED back plane. This solution addresses two of the weaknesses of WOLED. Firstly a uniform blue emitting OLED material can be deposited, avoiding the need for Fine Metal Masks. Secondly the red and green quantum dots convert the blue light into red or green light instead of absorbing the complementary colours from white light. This has a major power efficiency improvement and the high contrast ratios typical for OLED displays can still be achieved.

Flex OLED displays

The majority of OLED dispays are made on glass substrate since glass is a perfect cheap oxygen and water barrier material, crucial for OLED. Flexible OLED displays are mainly made on polyImide substrate, which will require and additional barrier layer, increasing the cost. However flexible barrier materials are not as good as glass, compromising the lifetime.

µLED displays

In the last few years several companies have been betting on mini and micro LED (µLED) displays. Inorganic LEDs have high efficiency, long life time and a good color performance, and are better than most other illumination technologies, making them suitable for emissive displays. In a micro LED display, each pixel contains a red, green and blue LED. These LEDs are made on silicon wafers in a wafer fab, similar to LEDs for lighting applications. After slicing and dicing the LEDs, the yielded ones need to be transferred onto a display backplane. However, these displays contain millions of pixels and transferring all these mini or micro LEDs from wafers onto display backplanes with high yield is an enormous engineering challenge.

OLCD

OLCD is a glass-free display technology that combines the benefits of LCD technology with the inherent flexibility of a high-performance organic-thin-film transistor (OTFT) backplane (which replaces the amorphous-silicon backplane used in glass displays). It enables conformable and shapeable displays that can be cost-effectively scaled to large area sizes in the same way glass LCD is scaled. OLCD displays can also be combined with a QDOT film to enhance the color performance and with a dual-cell OLCD to obtain contrast ratio of around 1,000,000 : 1.

Dual-cell OLCD brings additonal advantages over glass dual-cell LCD including thinnes, lightness and flexibility. For example the use of 40μm TAC film instead of 400μm glass substrates allows for extremely thin modules even with two cells, and can be manufactured in a simpler way (than dual cell glass LCD and OLED) at lower cost with a higher optical performance.

With so many and versatile display technologies there isn’t a universal one that fits all applications – each one of them fulfills different requirements and is therefore suitable for different applications. What makes the display industry so exciting is that display technologies keep evolving and getting better and at FlexEnable we are proud to play a role in these innovations.

If you are looking for a scalable, low cost and long lifetime flexible display technology, then please get in touch with FlexEnable at info@flexenable.com.

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