NanoLEDs double as light detectors

Article By : Julien Happich

A research group has developed nanoLEDs that can be switched between light-emitting and light-detecting modes by simply changing voltage bias.

A research group from the United States and South Korea has presented a type of nanoLEDs with brightness levels exceeding 80,000cd/m², and capable of operating as light emitters and detectors.

In their study entitled “Double-heterojunction nanorod light-responsive LEDs for display applications,” the dual-mode LEDs enable types of interactive displays. About 50nm long and 6nm in diameter, the all-solution-processed DHNR light-responsive LEDs include QD directly in contact with two different semiconductor materials within the nanorod. In this configuration and depending on the voltage bias, the quantum dots can improve radiative recombinations or lead to efficient separation of photo-generated carriers.

[eetimes europe low and high magnification fig1 (cr)]
Figure 1: Low- and high-magnification scanned transmission electron microscopy images of DHNRs (right) and magnified image of the region within the white dotted box (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, thus 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 of around 1.7V and a maximum brightness in excess of 80,000cd/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 1000cd/m² under 2.5V bias. But in one experiment, the researchers operated a 10px x 10px 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.

[eetimes europe Energy band diagram of DHNR-LED (cr)]
Figure 2: Illustration of the energy band of DHNR-LED along with the 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 features could easily be implemented by integrating a control circuit to translate any detected signal into brightness adjustments.

Brightness could be automatically adjusted in response to external light–intensity change. But since light detection could perform at pixel-level, shadows across a screen could be compensated for, or approaching fingers could be detected and interpreted as 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 parallel data communication and processing devices performing direct display-to-display data communication.

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 super capacitor.

First published by EE Times Europe.

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