Researchers at ETH Zürich, with international colleagues, have developed a new ultra-slim, bendable light-emitting diode (LED) that produces the purest green light ever achieved. The LED could help to introduce the next generation of ultra-high resolution displays used for smartphones and televisions, electronic devices that depend on ultra-pure red, blue and green light to produce images that are clearer and richer in detail, and offer a more refined range of colors.

Ultra-pure green light is much more difficult to produce than red and blue as the human eye can differentiate between more intermediary green hues. The purity of the colors also has to be within the international Rec.2020 standard, which defines the technical requirements for ultra-high resolution displays. The purer the base colors, the better the range of hues that can be displayed on a screen.

Achieving ultra-pure green light has been a challenge as it cannot be realized through conventional technologies such as inorganic LEDs, organic LEDs or quantum dots LEDs in the same was as ultra-pure blue and red light emissions. However, in this study, as reported in Nano Letters [Kumar et al. Nano Lett. (2017) DOI: 10.1021/acs.nanolett.7b01544], the new LED was found to produce up to 99% ultra-pure green of the Rec. 2020 standard based on straightforward room temperature processes, comparing well with the purest color television displays, which cover up to 77.72%.

“We believe our results pave the way for development of ultra-high resolution vivid displays, such as mobile phones and televisions”Sudhir Kumar

In helping to bridge the problematic “green gap” for the next generation of displays, as the roll-to-roll fabrication process used was carried out at room temperature it could lead to simple and relatively inexpensive industrial production of the flexible diodes in the future. As co-lead author Sudhir Kumar said “We believe our results pave the way for development of ultra-high resolution vivid displays, such as mobile phones and televisions”.

The team used nanomaterials to develop the LED technology. Although LEDs normally contain a semiconductor crystal, indium gallium nitride, which converts the electrical current passed through it into radiant light, it does not offer the ideal properties for producing ultra-pure green light. Therefore, the team used emitters made from perovskite, a cheap material used in fabricating solar cells and which is able to convert electricity into light relatively efficiently. The quality of the color is dependent on the thickness and form of the nanocrystal used, so the perovskite at only 4.8 nanometres was ideal.

There is still much to do before such ultrapure-green LEDs can find industrial applications, as operational stability, shelf life and energy efficiency need to be improved before final integration in new displays. The next objective is for the next version of he LED to be two or three times more efficient than anything currently available.