Two-way LEDs could turn screens into touchless chargers
Could it one day be possible to top up your phone's battery from ambient light? Companies like Japan's Kyocera, who have solar-powered displays in the works, certainly think so, as do material scientists at the University of Illinois at Urbana-Champaign who have developed multi-purpose LED arrays that absorb light and turn it into electricity, (and pack a number of other neat tricks as well).
The LED arrays consist of tiny nanorods arranged on a thin film that are made from three types of semiconductor material and measure less than five nanometers in diameter. One of these materials both emits and absorbs visible light, while the other two materials facilitate how electrons flow through the first. This combination gives the LEDs the ability to emit, sense and respond to visible light.
They do this simultaneously by switching really quickly between emitting mode and detecting mode. This happens so quickly, three orders of magnitude faster than a standard display refreshes, that it is imperceptible to the human eye, meaning that the display appears as if it is constantly alight.
When it does detect light, it behaves in a similar way to a solar cell, absorbing it through the photovoltaic effect. At the moment, this is only on a very small scale, but the researchers are buoyed by their early results and believe that they can work toward a self-powered LED display that doesn't compromise on performance.
"The key improvement would be in the device being able to absorb much more of the ambient light," Moonsub Shim, lead author of the study, explains to New Atlas. "However, displays also need to emit light and that imposes a limitation. I think there are ways around this problem but further research is needed."
Whether this means that all of – or just the majority – of the display's power would come from the array itself is unknown at this stage, nevertheless Shim tells us he is "optimistic about the prospect of powering by harvesting ambient light."
Using this light-detection capability to generate power might be the long game, but the researchers say it could offer some highly useful functionality in the shorter term, too. Because the arrays can be programmed to react to light signals, it could be used to create interactive displays that recognize objects or respond to touch-less gestures, such as a wave, an approaching finger or a laser stylus in a electronic whiteboard-type setup.
What's more, they could automatically adjust their brightness depending on ambient light. On one hand, this is similar to how some laptops and mobile devices will automatically dim when the room is brighter, on the other hand, it could offer something more.
"Most tablets and cell phones have a separate photodetector somewhere on the device to monitor ambient lighting to adjust screen brightness," explains Shim. "But because there is only one, or a limited number, the brightness of the entire screen is adjusted to the same level. Furthermore, you need to build in these photodetectors separately in addition to the components of the display. With our LEDs, if they were integrated into displays as emissive pixels, you wouldn't need separate photodetectors."
Where this could come in especially handy is when using a device in an unevenly lit environment, because it means that each pixel's brightness could be adjusted independently.
"So if your screen was partly under the shade, it would adjust each part differently so that the viewing is much more uniform," Shim continues. "This ability to detect light at the individual pixel level while having each pixel appear to continuously emit light provides a whole new platform to develop tablets, cell phones, TVs and other displays with new capabilities, not just adjusting brightness."
It is clear that a lot more work is needed, and the researchers say they are only touching the surface of what is possible, but Shim tells us the early results have them very excited. Their demonstrations so far have involved only red LEDs and they are now working on methods that would incorporate green and blue pixels too, as well as tweaking the composition of the nanorods with a view to boosting their light-harvesting capacity.
The research was published in the journal Science.