Image source, Linköping University
- Author, Chris Baraniuk
- Role, Technology reporter
At first glance it seems like a relic from the 1980s. A small computer screen with flickering low-resolution text scrolling across it. But this could be the future.
The screen is made using perovskite light emitting diode (PeLED) technology. It’s radically different from the LED technology used in your smartphone display today, and it could lead to devices that are thinner, cheaper, and have longer battery life.
Not only that, PeLEDs are very unusual in that they can both absorb and emit light, meaning you can use the same material to integrate touch, fingerprint and ambient light sensors, says Feng Gao of Linköping University in Sweden.
“This is difficult, but we think it is possible.”
In today’s smartphones, such functions are performed by electronic components separate from the phone’s screen itself.
“It’s a very nice demonstration… it’s very new,” said Daniele Braga, head of sales and marketing at Fluxim, a technology research company in Switzerland. Although he notes that optimizing all the different features promised here could make it difficult to bring these types of displays to market quickly.
Prof. Gao shows the latest version of the technology via a video call. It’s a small screen again, but this time the pixels per inch (ppi), a measure of the screen’s sharpness, have almost doubled: 90 ppi.
A simple animation plays on the screen, showing two stick figures fighting. An article has just been published with further details about this prototype.
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Perovskite the mineral contains calcium, titanium and oxygen arranged in a crystal structure. It was discovered in the 19th century, but people later realized that they could make other types of perovskites that shared the same structure while having different elements or molecules as components.
Depending on the materials chosen, perovskites can, for example, conduct electricity very well or radiate light.
“By slightly tuning the chemical composition, you can cover the entire visible spectrum,” says Dr. Braga, explaining that making perovskites is a relatively simple and inexpensive process. “If you think about mass production, this is huge.”
However, there are some problems.
PeLEDs are notoriously unstable; they break down when exposed to moisture or oxygen, for example. Loreta Muscarella from the Vrije Universiteit Amsterdam is working on the development of new types of PeLEDs.
She says that if you leave a PeLED for a few hours or days, the color of the light it emits will gradually deteriorate or shift to a less pure version of, say, green than the green you want.
And this undermines the whole point of perovskites. They are desirable in part because they can be tuned to emit a very specific, very pure form of red, green or blue – the main hues required for digital color displays.
To keep them stable, PeLEDs can be encapsulated in glue or resin, says Prof. Gao. But researchers are still working to ensure the technology doesn’t falter over a long period of time.
Dr. Muscarella says traditional LEDs have a lifespan of 50,000 hours or more, while PeLED’s lifespan is still in the hundreds to thousands of hours.
It could be years before you buy a commercial product that contains a PeLED, she adds.
But there is another type of luminous perovskite that you may encounter first on the market.
It depends on photoluminescence. This is not an LED as such, but rather a filter or film-like material that absorbs and re-emits light of a certain color.
In some TVs on the market today, a colored filter provides the crucial red, green and blue colors used in every pixel on the screen.
By mixing these colors at different levels you can obtain the range of shades necessary to display a complete image.
The red, green and blue filters are illuminated by an LED backlight. But today’s filters actually block much of that light.
Photoluminescent perovskites, on the other hand, transmit almost all light, which would mean a big increase in brightness and efficiency.
Helio, a British company, is working on this. A video on their website shows how a red or green colored perovskite film can emit blue light almost perfectly as red or green.
The technology that Prof. Gao and his colleagues are developing is very different. They experiment with screens that emit light using LEDs that are made with perovskites.
These are known as electroluminescent perovskites. Working with them is difficult because they are sensitive to electric fields and, as mentioned, are not very stable. But ultimately, they could be even more efficient options for illuminating the red, green and blue pixels in a smartphone, tablet or TV screen without the need for color filters at all.
The main benefits of switching to this technology could be in reducing the cost of these devices and reducing their energy consumption.
No one knows exactly how much less energy a future PeLED screen could consume compared to, say, an OLED screen, but laboratory experiments suggest that PeLEDs are already competitive with OLEDs and could one day significantly surpass them in terms of efficiency, says Dr. Muscarella. .
Prof. Sir Richard Friend, from the University of Cambridge, is one of the co-founders of Helio, along with Prof. Henry Snaith from the University of Oxford. He points out that one of the challenges with PeLEDs is getting them to emit light in the right direction. This is really important for displays.
“You need to make sure the light is emitted in a forward direction, rather than getting stuck going sideways,” he explains.
Researchers are experimenting with many different techniques to tackle this problem. Dr. For example, Muscarella and colleagues attempted to print a nanoscale bumpy pattern on the surface of PeLEDs, which appears to improve light emission.
However, for Prof. Gao, who has published with Prof. Sir Friend and received his PhD from the University of Cambridge in 2011, the promise of PeLED screens that do so much more than just emit light beckons.
From fingerprint authentication to heart rate monitoring and light detection, it could all one day be done using a single sheet of layered materials with the all-important light-absorbing perovskite at the center.
“It is really very unique,” he says enthusiastically. “This is not possible with other LED technologies.”