Don't be Afraid of OLED!

Fachbeitrag MEDInternational 2019

OLEDs are weak in light, short-lived and unreliable – these are common prejudices against this technology. This may be true of about displays made 15 years ago, but the technology has evolved since then.

OLED Whiteboard

Since large-area OLED displays can be manufactured efficiently and are commercially available, this technology has also moved into the consciousness of electronics market customers.

What does OLED mean? The abbreviation stands for “Organic Light Emitting Device”. “Organic” refers to the materials that contribute to the function of the display - they are materials of organic chemistry. In contrast, TFTs are based on semiconductor materials of inorganic chemistry. In the following, the generic term “LCD” will stand for all liquid crystal technologies. In addition, only passive displays are considered, i.e. those where the picture element switches by applying a voltage alone and where a transistor is not involved as an active component, as is the case with TFTs.

Basic differences to LCD

LCDs act as a valve for existing light, which usually comes from a light source behind the display. The transparency of all layers is well below 10%, which means that 90% of the light is lost. Figure 1 shows a comparison of the typical parameters of two equivalent modules in OLED and TFT technology.

Figure 1: Comparison of OLED and TFT specifications
Figure 1: Comparison of OLED and TFT specifications

OLED and LCD compared

OLEDs do not require a backlight and are therefore thinner than LCDs. While the power consumption for LCD is mainly determined by the backlight and is constant, in OLED only the actively illuminating picture elements consume power. OLEDs have a wide viewing angle of almost 180° with no colour deviation or loss of contrast. OLEDs with their luminous materials achieve a large colour gamut (see Figure 2). Their contrast is very high, since in dark areas no background illuminated by the backlight shines through. The technology allows a wide temperature range.

 

Figure 2: Comparison of OLED and TFT colour spaces
(Click image for full size)

Implementations

Only “low end” applications require segmented displays. The most versatile applications are offered by OLED dot matrix display. They are available in various (monochrome) colours such as yellow, green, blue-green, orange, white, red and blue. If two dyes are applied next to each other on the same substrate, two screen areas with different colours can be displayed using the “Area Colour" effect.

Figure 3 shows monochrome displays in different colour designs, clockwise from top left green, yellow, orange, blue-green and white in the centre.

If you arrange three basic colours in strip form as with TFT, you get a colour display that can display thousands of different colour tones by combining them.

Figure 3: Monochrome Displays
Figure 3: Monochrome Displays

Are OLED dark?

When studying a data sheet, it is noticeable that the indicated brightness is rather low compared to TFT: depending on the colour, it lies between 80 and 150 cd/m2. To conclude from this that OLEDs are difficult to read is not correct: the readability depends on the contrast, i.e. the ratio between light (pixel switched on) and dark (background). The background of OLEDs is very dark, because no backlight shines through from behind. Unlike TFT, OLEDs do not need a polarizing filter for their function, but they can increase contrast by eliminating reflections from the incident light. Absolutely high brightness is not necessary for reading. Examples include sports watches with heart rate monitors that can also be read outdoors at high brightness levels.

Life expectancy

The service life of OLEDs is defined in the same way as that of TFTs: it describes the time that elapses before the initial brightness drops to 50%. With TFT the brightness of the LED backlight decreases, with OLED the display itself. During operation, the service life depends on various factors, especially temperature and brightness. Depending on the colour emitted, luminous materials have different lifetimes, from blue with 30,000 hours to yellow with 150,000hours. Environmental influences are independent of the operation and limit the storage life span. Humidity and oxygen react chemically with the organic materials. They are well controllable by the design of the encapsulation of the cell and have only a minor role.

Figure 4: Readability of an OLED in incident light (top left)
(Click image for full size)

Undesired Display Effects

The effect of (differential) aging of individual pixels by operation is known under various terms: Burn in, image sticking, persistence or ghosting. The human eye recognizes differences in brightness quite well. Therefore, the GUI designer should ensure that all pixels of a display are switched on for approximately the same time and avoid static image contents. In some applications this is not possible, so other strategies have to be followed. “Screen savers" are suitable if the display content does not have to be read permanently. The protection is effective if the user does not actively work with the display, e.g. with status displays, energy meters or measuring devices. The original image content is reactivated by pressing a key, touching a button or values are changed.

OLEDs are used wherever a small display with not too much information content is needed. Small medical devices, especially portable ones, are predestined for this. Seniors who monitor glucose or blood pressure themselves prefer clear, high-contrast displays. The low-power technology enables a long service life without charging or battery replacement. Oxygen therapy devices, defibrillators and electro-medical devices also benefit from OLEDs, as do devices for medical or biochemical laboratories, as they can be read from all sides thanks to an all-round viewing angle.

Technology outlook

The manufacturers of OLEDs are working on the further development of their technology. Driven by their use in consumer devices, future display generations will be refined: in the first step, substrate glasses will become thinner, contours do not necessarily have to be rectangular, and through optimized production, the edges of the display can be slimmer. In the next generation, flexible substrates are available that enable 2D curved surfaces or bendable displays. Applications include “wearable" displays that can be integrated on the body or clothing.

A further step is the optimization of optical properties. So far, OLED layers offer only a limited transparency of a few 10%. Future materials will enable a significantly increased light transmission, which will pave the way for new applications. Head-up displays or spectacles for augmented reality, but also optical measuring instruments such as magnifying glasses with crosshairs or rulers, are in the pipeline.

In the age of the Internet of Things, where every device collects, bundles and sends data to the cloud, interaction with sensors does not necessarily focus on local visualization, because this takes place where the data is aggregated and filtered according to certain criteria. This reduces the complexity of the local display. Despite of this, the demand for small-format displays is growing, because each sensor must be setup and parameterized - e.g. its IP address and measuring range, wants to transmit local messages - e.g. to request maintenance or to display the battery status, or wants to transmit a trend.

OLEDs are particularly suitable for this because they have low power consumption, high contrast and are easy to control even by low-power CPUs. With their bright colours, they can be easily integrated into many devices, from design-driven coffee machines to portable blood pressure monitors. The potential of the technology has not yet been exhausted, and further developments make the displays even more attractive.

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