Light-responsive LEDs could re-invent displays
Publishing their findings in Science in a paper titled “Double-heterojunction nanorod light-responsive LEDs for display applications”, the researchers expect the dual-mode LEDs to enable new types of interactive displays.About 50nm long and 6nm in diameter, the all-solution-processed double-heterojunction nanorod (DHNR) light-responsive LEDs described in the paper include quantum dots directly in contact with two different semiconductor materials within the nanorod. In this configuration and depending on the voltage bias, the quantum dots can enhance radiative recombinations (useful for LEDs) or lead to efficient separation of photo-generated carriers.
The layered structures in these anisotropic nanorods can be tuned independently so as to fine-tune both recombination and charge separation in a single device, hence enabling a single nanorod to be electroluminescent and generate a photocurrent. Once appropriately stacked between electrodes, the nanorods can be arranged into pixels that can be switched between light-emitting and light-detecting modes by simply changing a voltage bias (forward or reverse).
Boasting a low turn-on voltage (around 1.7 V), and a maximum brightness in excess of 80,000 cd/m2, the devices also exhibit low bias and high efficiencies at display-relevant brightness. The authors report an external quantum efficiency of 8.0% at 1000 cd/m2 under 2.5 V bias. But in one experiment, the researchers operated a 10×10 pixel DHNR-LEDs array as a live photodetectors (under reverse bias), combined with a circuit board that supplied a forward bias to any pixel detecting incident light.
By alternating forward and reverse bias at a sub-millisecond time scale, they were able to continuously “read out” light-detecting pixels as they illuminated the array.
This experiment leads the researchers to think that several new display features could easily be implemented by integrating a simple control circuit to translate any detected signal into brightness adjustments.
For one, brightness could be automatically adjusted in response to external light–intensity change. But since light detection could performed at pixel-level, shadows across a screen could be compensated for, or approaching fingers could be detected and interpreted as new touchless commands.
In their paper, the researchers also envisage that the individual pixel sensing capability of such DHNR-LEDs could support direct imaging or scanning at screen level. Another possible use of the dual light-emitting and light-detecting operation modes of the DHNR-LEDs may be to turn closely coupled LED displays (in effect large arrays of nanoLEDs) into massively parallel data communication and processing devices performing direct display-to-display data communication.
Last but not least, the photo-detection in the DHNR light-responsive LEDs being akin to a photovoltaic effect, displays could be made to harvest or scavenge energy from ambient light sources without the need for integrating separate solar cells, making the display even more efficient. This last use-case was also proven in an experiment related in the paper, coupling the LEDs to a supercapacitor.
“We do have patent applications for this technology with our co-authors from Dow Chemical. So there is certainly commercial interest” briefly acknowledged Professor Moonsub Shim, leading this research at the University of Illinois and contacted by eeNews Europe.
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