Rutronik News

OLEDS - Endless Possibilities


Displays are often a prominent feature in science-fiction and futuristic visions. We have now actually reached that future: OLEDs enable the creation of thin, flexible, transparent displays, and in new shapes too. They offer enormous opportunities to add new features to existing applications, or to create entirely new-style products.

Most displays are still based on LCD (liquid crystal display) or TFT (thin-film transistor) technology. But OLED (organic light emitting diode) displays are increasingly being used especially to create highly innovative displays, to meet very specific requirements, or to implement large diagonal formats. They use organic materials which emit light when connected to a power source. As opposed to TFTs, this means they are self-illuminating, and need no backlighting. The only thing standing in the way of a breakthrough for OLEDs at present is their relatively short lives. Single OLEDs have already achieved over 30,000 hours, and ongoing development will see that figure rise still higher. Manufacturers are also working to improve their lighting efficiency to above 30%.

OLEDs were initially based primarily on rigid glass substrates. Today curved displays, or even round ones such as those from DLC, are to be found in smart watches for example. Thin-film encapsulation is enabling even thinner and more flexible metal- and glass-based OLEDs to be created.

According to their control method, a distinction is made between Passive Matrix OLEDs (PMOLED) and Active Matrix OLEDs (AMOLED):

Passive Matrix OLED

PMOLEDs use a simple sequential control scheme. It activates one row (or line) of the display after another. As PMOLEDs contain no storage capacitor, most pixel rows are off most of the time. To nevertheless achieve a certain level of brightness, a higher voltage is required. This entails a number of issues: PMOLEDs are not particularly energy-efficient, and their relatively high voltage means their lives are short - to a maximum of [how long?]. This issue imposes limits in terms of resolution and size, too, because the required voltage increases with the number of rows. So PMOLED displays are typically not larger than 3".

Their major plus-point is their lower price, thanks to their simple, low-cost manufacture. Some manufacturers are already offering flexible PMOLED displays, such as in fitness bracelets. Transparent models are also available.

Active Matrix OLED

AMOLEDs use a thin-film transistor with a storage capacitor. The capacitor ensures that only one row of pixels changes at a time; all the others are lit the whole time. As a result, AMOLEDs need less power than PMOLEDs, deliver shorter refresh rates, and allow for larger displays with higher resolutions. They are more complex to manufacture, however, which makes the end-product more expensive.

Foil AMOLEDs are a special design form which permits the creation of curved displays. They are based on a flexible plastic substrate which provides the OLED with little protection against damage by damp and oxygen. Manufacturers enhance leak-tightness with optimized barrier layers.

Good prospects

At present OLEDs are used mainly as smartphone displays and in other consumer electronics products, in medical devices, as lighting solutions, and on-board cars, such as for dashboard and cabin lighting, as well as for entirely new applications such as head-up displays and digital interior rear-view mirrors. If manufacturers succeed in improving the energy and lighting efficiency of OLEDs, they will spread to increasingly wide-ranging applications in the coming years, and fuel developers' imaginations in creating new product ideas on many different markets.