Nanoplates make QLED screens more energy efficient
QLED screens have been available for a few years now. They are known for their bright, intense colors, which are produced with the so-called quantum dot technology. QLED stands for Quantum Dot Light Emitting Diode. Researchers at ETH Zurich now have developed a technology that increases the energy efficiency of QLEDs. The researchers achieved this by minimizing the scattering losses of light inside the diodes. As a result, a larger proportion of the light generated is emitted to the outside.
Conventional QLEDs consist of a high number of spherical semiconductor nanocrystals, also known as quantum dots. In a screen, these nanocrystals are excited from behind with UV light. The crystals convert this into coloured light in the visible range. Depending on the material composition of the nanocrystal, a different colour is generated.
However, these spherical nanocrystals scatter the generated light inside the screen to all sides. As a result, only around one-fifth of the light generated escapes the screen and becomes visible to the user. To increase the energy efficiency of the technology, scientists have been trying for years to develop nanocrystals that emit light only in the direction of the viewer. The first such light sources already exist. They do not consist of spherical crystals, but of ultra-thin nanoplates. These emit light in only one direction – perpendicular to the plane of the platelets.
If these nanoplates are arranged next to each other in a layer, they generate a relatively weak light that is not sufficient for screens. In order to increase the light intensity, scientists are pursuing the approach of superimposing several layers of such platelets. However, the plates begin to interact with each other, and the light is emitted not only in one direction but on all sides.
The researchers, led by Chih-Jen Shih, Professor of Technical Chemistry at ETH Zurich, have now stacked extremely thin (2.4 nm) semiconductor wafers so that they are separated from each other by an even thinner (0.65 nm) insulating layer of organic molecules. This layer prevents quantum-physical interactions, so that the wafers emit light predominantly in only one direction, even when stacked.
“The more platelets we stack on top of each other, the more intense the light becomes. We can thus influence the light intensity without losing the preferred direction of emission,” says Jakub Jagielski, PhD student in Sith’s group and first author of the report. This is the first time scientists have produced a material that emits light at high intensity in only one direction.
The researchers were able to create sources of blue, green, yellow and orange light. However, the red colour component, which is also required for screens, cannot yet be realised with the new technology, according to the scientists.
For the newly created blue light, the following applies: instead of one-fifth of the light generated as with conventional QLED technology, around two-fifths of it now reaches the eye of the observer. To generate light of a certain intensity, this technology requires only half as much energy as conventional QLED technology. For other colours, however, the gain in efficiency is currently even smaller. The scientists are therefore attempting to increase this gain there as well in further research work.
Compared to conventional LEDs, the new technology has another advantage, as the scientists emphasize: The novel stacked QLEDs are very easy to manufacture in a single step. With conventional LEDs, it is also possible to increase the intensity by arranging several light-emitting layers on top of each other. However, their production is layer by layer and is therefore more complex.
Original Publication: Nature Communications https://dx.doi.org/10.1038/s41467-019-14084-3