Light-detecting LEDs promise interactive, energy harvesting displays

February 13, 2017 // By Julien Happich
An international team of researchers from the US and South Korea have reported a novel type of nanoLEDs with unprecedented brightness levels (in excess of 80,000 cd/m2) capable of operating both as light emitters and light detectors.

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 of two different types, one of which can enhance radiative recombinations (useful for LEDs) while the other type leads to efficient separation of photo-generated carriers.

Low- and high magnification scanning transmission electron microscopy images of DHNRs (right) magnified image of the region within the white dotted box on the left.

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.

Energy band diagram of DHNR-LED along with directions of charge flow for light emission (orange arrows) and detection (blue arrows) and a schematic of a DHNR.

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.