Passive Matrix OLEDs
A world of contrasts and colors
Vincenzo Santoro, Rutronik
The term OLED (organic light emitting diode) is becoming increasingly common in everyday life, for example with smartphones, which are often equipped with the currently popular AMOLED technology (active matrix OLED).
This displays a maximum resolution on the smallest active area. Retailers and specialist stores are also increasingly advertising OLED equipment – with even sharper images and more true-to-life colors. Smart watches glow in vibrant colors and deliver high contrasts, smart home displays offer aesthetic visualization and touch benefits. AMOLED technology has long since conquered the consumer sector and is now being used in more and more applications.
For some time now, passive matrix OLED (PMOLED) displays have also been hugely popular, for example in the industrial sector. Vibrant colors, unique contrasts, and high resolutions as well as a slender design make them interesting for product developers of industrial applications, especially when used as a replacement for a passive LCD display to enhance the application’s aesthetic appeal.
Structure and colors of PMOLEDs
A PMOLED consists of several layers. An anode represents the organic layer, the cathode is arranged in rows at right angles to the anode. Both of these layers have a strip-like structure. Actual control is achieved via both the anode and the cathode line, which activates the OLED at the respective intersection points and generates light.
PMOLED sizes range from less than 1 inch to 5.5 inches. In addition, they also enable an alphanumeric and graphic dot matrix resolution. The background of the displays is always black, while the pixels can be displayed in yellow, blue, white, amber, sky blue, and green. Unlike AMOLED technology, however, the developer must determine a color when selecting the PMOLED.
Advantages of Design-In-Process
Key points speak in favor of a PMOLED. One factor is the low installation height of the displays. Self-illuminating technology eliminates the need for backlighting of the display module. As a result, it consists only of the front and rear glass, thereby reducing the thickness of the LEDs by a few millimeters.
Further advantages are the “around-the-clock” angle of view and the high contrast. The PMOLEDdisplays the same high contrast (2000:1) from all sides, which is why there are no preferential angles of view. This is particularly useful if, for example, a status display is controlled or read out by one or more users from different angles.
Moreover, the PMOLED is extremely energy efficient and provides a basis for low-power applications. Only the pixel that is actually intended to light up consumes power, while the non-luminous pixels remain inactive.
PMOLEDs are sensitive
Before deciding whether a PMOLED is suitable for the respective application, it is also necessary to consider the weaknesses of the technology. These include, above all, the lifetime and burn-in effects. The service life of a PMOLED can be severely limited by various influencing factors, as the material reacts very sensitively to oxygen and moisture as well as constantly high temperatures. A PMOLED is therefore not really suitable for use in direct sunlight. Burn-in effects, i.e. the burning of phosphor into the screen, can occur if the display permanently controls the same pixels. A varied and homogeneous control is required to avoid this problem.
The cost factor also plays a significant role. Since the production of the display is very complex, a PMOLED still requires higher investment than, e.g., a passive LCD.
More aesthetics for many applications
PMOLED technology is suitable wherever the design should be aesthetically more appealing than that of a passive LCD. It is ideal for metering, as status displays for gas, electricity or water meters only require a selective display. PMOLED technology is also a suitable option for thermostats, white goods or applications with smaller, selective displays.
PMOLEDs are not (yet) suitable for 24/7 status displays due to the service life of the technology. Despite the still existing obstacles, PMOLED technology is gaining in popularity. The R&D departments of suppliers are continuously working on ways to extend the service life, minimize burn-in effects, and increase resolutions. The consumer market is definitely an incentive for the development of new OLED properties – be it through flexible OLEDs (e.g. smart watches) or transparent OLEDs (metering), which are also becoming increasingly popular in the industrial sector.
To cover the complete PMOLED portfolio, Rutronik works closely with the renowned OLED supplier Raystar. Its extensive standard portfolio includes alphanumeric displays in sizes from 8x2 to 20x4 rows and columns as well as in all color variations. Raystar and Rutronik also offer graphic COG (chip on glass) and COB (chip on board) PMOLEDs in sizes ranging from 0.49 inches to 5.5 inches.
All standard products can be customized and open up countless possibilities of the Design-In-Process that go far beyond the standard functions. In the next few years, the development in the field of PMOLEDs will show what is possible here – from trend technology to the replacement of passive LCDs. PMOLEDs already have a great influence when it comes to developing the design and are a real eye-catcher for all visualization topics.