05-06-2017 | | By Mark Patrick
Organic Light-Emitting Diode (OLED) displays are front-page news again following the industry rumour that Apple is considering the thin, lightweight screen technology for the forthcoming iPhone 8. It has also distinguished its MacBook Pro with an innovative OLED touch bar. In addition to this, Samsung, a major OLED maker, has featured curved OLED displays in several of its flagship phones and LG’s large OLED TVs are winning plaudits for their vibrancy and sharpness, despite high prices.
But OLED’s success has in some respects been a long time coming. Those with long memories might recall the first wave of OLED display fever 20 years ago when news reports extolled the benefits of the new technology: cheaper, brighter and lighter than LCD screens. What’s more, thin OLED displays could be flexible – soon we could roll up our mobile phones and TVs just like a newspaper. As usual, the reports were far ahead of reality. As every engineer knows, it’s one thing to make a research breakthrough in the lab, and quite another to scale up that discovery into mass production – but we are finally seeing OLED displays as a component that can be part of any project.
OLED vs LED vs LCD
OLEDs are based on electroluminescence in organic compounds. They rely on the same basic physical principle as the older, non-organic LEDs – efficiently generating light from electricity by continuously pumping electrons up to a higher energy state and inducing them to emit photons to release that energy. But OLEDs produce relatively diffuse light with a wide viewing angle and good heat dissipation; they are particularly suitable for production in large sheets of closely spaced elements; and the small organic molecules are almost transparent, making them easy to layer without light loss. Despite the misnamed “LED TV” (actually an LCD panel with a few white LED backlights), LEDs are not suitable for the high-pixel-density displays required by consumer devices.
These characteristics, and others, of OLEDs promise many advantages over LCD displays, such as naturally wide viewing angles, low driving voltage, and high contrast and colour range. Most importantly, unlike LCDs, the individual elements of an OLED are luminescent and so need no backlight. This makes the display thinner and lighter, offers potential for power saving, and simplifies design and manufacturing – and it also produces a vibrant image with darker blacks and higher contrast compared to the subtractive filtering approach inherent in LCD design.
Early OLED display makers suffered from low production yields, particularly for larger displays. Yields below 30% were common. And what’s worse, those costly, bright and clear OLED displays weren’t lasting long enough. Customers complained of screen burn-in (a permanent ghost image of static user interface elements), fading brightness, and colour shifts. Sometimes, the deterioration was visible within a year or two of manufacture.
These problems caused a slower-than-predicted roll-out for OLED technology. Small displays led the way, as they naturally have higher yields. Moreover, owners typically replace mobile phones more frequently than TVs and notebook PCs, so may never notice a phone’s display quality is degrading. Display panel yields are particularly vulnerable to flaws because every fault is visible to end users. Other complex semiconductor devices, like CPUs and memory chips, can be sold with defective areas of the chip disabled, perhaps as cheaper, lower-spec components.
Yields have improved, but for the toughest challenge, large screens, there is room for improvement. LG, which dominates the still-nascent OLED TV market, with over 90% of the approximately one million TVs sold in 2016, reported yields of 65% for UHD screens. The company trumpeted a threefold increase in mean lifetime, up to 100,000 hours – eleven years of continuous viewing – but it’s notable that LG attributed this to better post-production testing, not manufacturing yield. LG’s success is partly thanks to its patented manufacturing process, relying on white OLEDs – other manufacturers use different technology, typically based on RGB OLEDs.
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