QLED tech aims light one way to up TV energy efficiency

UV light shines on a pane of glass, coated with several layers of two-​dimensional semiconductor nanoplatelets, which emits blue light. (Credit: Jakub Jagielski/ETH Zurich)

A new technology increases the energy efficiency of quantum dot light emitting diodes for screens by emitting high-intensity light in one direction, researchers report.

By minimizing the scattering losses of light inside the diodes, they emit a larger proportion of the light generated towards the viewer.

Quantum dot light emitting diode (QLED) screens have been on the market for a few years now. They are known for their bright, intense colors, which they produce using quantum dot technology.

Conventional QLEDs consist of a multitude of spherical semiconductor nanocrystals, known as quantum dots. In a screen, when UV light excites the nanocrystals from behind, they convert it into colored light in the visible range. The color of light each nanocrystal produces depends on its material composition.

“…our technology requires only half as much energy to generate light of a given intensity.”

However, the light these spherical nanocrystals emit scatters in all directions inside the screen; only about one-fifth of it makes its way to the outside world and is visible to the observer.

To increase the energy efficiency of the technology, scientists have been trying for years to develop nanocrystals that emit light in only one direction (forward, towards the observer)—and a few such light sources already exist. But instead of spherical crystals, these sources are composed of ultra-thin nanoplatelets that emit light only in one direction: perpendicular to the plane of the platelet.

If the scientists arrange these nanoplatelets next to each other in a layer, they produce a relatively weak light that is not sufficient for screens. To increase the light intensity, scientists are attempting to superimpose several layers of these platelets. The trouble with this approach is that the platelets begin to interact with each other, with the result that the light is again emitted not only in one direction but in all directions.

Chih-Jen Shih, professor of technical chemistry at ETH Zurich, and his team of researchers have now stacked extremely thin (2.4 nanometers) semiconductor platelets in such a way that they are separated from each other by an even thinner (0.65 nanometer) insulating layer of organic molecules. This layer prevents quantum-physical interactions, which means that the platelets emit light predominantly in only one direction, even when stacked.

“The more platelets we pile on top of each other, the more intense the light becomes. This lets us influence the light intensity without losing the preferred direction of emission,” says first author Jakub Jagielski, a doctoral student in Shih’s group. That’s how the scientists managed to produce a material that, for the first time, emits high-intensity light in only one direction.

Using this process, the researchers have produced light sources for blue, green, yellow, and orange light. They say that they can’t realize the red color component yet, which is also required for screens, with the new technology.

In the case of the newly created blue light, around two-fifths of the light generated reaches the eye of the observer, compared to only one-fifth with conventional QLED technology.

“This means that our technology requires only half as much energy to generate light of a given intensity,” Shih says. For other colors, however, the efficiency gain achieved so far is smaller, so the scientists are conducting further research with a view to increasing this.

Compared to conventional LEDs, the new technology has another advantage: the novel stacked QLEDs are easy to produce in a single step. It is also possible to increase the intensity of conventional LEDs by arranging several light-emitting layers on top of each other; however, this needs to be done layer-by-layer, which makes production more complex.

The research appears in Nature Communications.

Source: ETH Zurich