Ultra-pure green light leads to ultra-high definition displays

Chih-Jen Shih is very satisfied with his breakthrough: “To date, no one has succeeded in producing green light as pure as we have,” says the Professor of Chemical Engineering in his laboratory at the Hönggerberg campus. He points at an ultra-slim, bendable light-emitting diode (LED), which displays the three letters “ETH” in a fine hue of bright green.

Shih’s progress is significant, particularly in terms of the next generation of ultra-high resolution displays used for TVs and smartphones. Electronic devices must first be able to produce ultra-pure red, blue and green light in order to enable the next generation of displays to show images that are clearer, sharper, richer in detail and with a more refined range of colours. For the most part, this is already possible for red and blue light; green light, however, has hitherto reached the limits of technology.

This is due mainly to human perception, since the eye is able to distinguish between more intermediary green hues than red or blue ones. “This makes the technical production of ultra-pure green very complex, which creates challenges for us when it comes to developing technology and materials,” says Sudhir Kumar, co-lead author of the report.

Up to 99 percent ultra-pure green

It becomes clear from reference to the Rec.2020 standard just how much progress Shih’s ultra-green light has made in the development of the next generation of displays. The international standard defines the technical requirements for ultra-high resolution (known as “Ultra HD”) displays and provides a framework for further research and development. The requirements also include an improvement in colour quality visible to the naked eye. The standard provides the colour scale that a display can reproduce and therefore a broader range of colour hues.

Ultra-pure green plays a key role in extending the colour range, or gamut. Ultimately, new hues are created through the technical mixture of three base colours: red, blue and green. The purer the base colours, the broader the range of hues a screen can display. Shih’s new LED is in line with 97 to 99 percent of the Rec. 2020 standard. By comparison, the purest colour TV displays currently available on the market cover on average only 73.11 to 77.72 percent; none exceeds 80 percent.

Inexpensive, producible LED technology

Wendelin Stark, ETH Professor of Functional Materials Engineering, along with researchers from South Korea and Taiwan, also contributed to the project results, which have been published in the scientific journal Nano Letters. Shih not only made a breakthrough in terms of the results, but also in the material and method. He and his colleagues have effectively developed an ultra-thin, bendable light-emitting diode able to emit pure green light using simple room-temperature processes. Shih says that this is the second aspect of his breakthrough and is at least equally important, as until now high-temperature processes were required to produce pure light with LED technology. “Because we were able to realise the entire process at room temperature, we’ve opened up opportunities for the simple, low-cost industrial production of ultra-green light-emitting diodes in the future,” says Jakub Jagielski, co-lead author of the report.

Learn more: Green light for ultra-fine display colours

 

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