LEDs: we haven’t seen the light yet

The expert: Géraldine Dantelle. A materials chemist in the Materials, Non-Linear Optics and Plasmonics team at Grenoble’s Institut Néel, she previously worked at the prestigious École Polytechnique’s Physics of Condensed Matter laboratory. An expert on LEDs and phosphors that regulate light, she won the French National Centre for Scientific Research (CRNS) bronze medal in 2014.

Leds are now everywhere and have multiple uses. What further progress can we expect?
Géraldine Dantelle: There is still progress to be made in several areas. On the quality of white light, because LED light is currently a little too blue and therefore “cold”. On brightness, because the current efficiency of LEDs drops at higher powers and the only solution to this is to use multiple LEDs, which can prove expensive. And also on materials. For example, some types of LEDs are made using a rare earth element called cerium. Deposits currently being mined are mostly found in China, which makes sourcing complicated and increases costs.
Further reading: Capsulight: the healing power of light

This kind of microscopic particle converts blue light from diodes into white light. Thanks to progress on a scale invisible to the naked eye, LEDs will emit more powerful, whiter light in years to come. © G. Dantelle
What are scientists like yourself working on to improve these issues?
What are scientists like yourself working on to improve these issues?
We are increasing the amount of light emitted for example. We can prevent the element that produces light from sending part of it back. As the light element (the diode) emits blue light, we need to apply a phosphor coating to convert it into white light. But this means part of the light is sent back due to diffusion. We are exploring an approach to reduce the size of the phosphor grains to nanoparticle scale, so that light can pass through the coating without too much diffusion. I started this work at the École Polytechnique Condensed Physics laboratory.
Why is LED light not completely white yet?Why is LED light not completely white yet?
Light-emitting diodes (LEDs) emit light when electric current passes through their semiconductor material – there is no filament or heat loss. This method for converting electricity directly into light was discovered in 1907. For a long time, however, we were only able to make LEDs that emitted red or green light. The inventors of blue LEDs completely changed the game and were even rewarded with the Nobel prize in 2014. You can obtain white from three colour diodes – or a white of sorts. The three colours do not actually produce the same output and we do not know how to mix them harmoniously. The most common method only uses blue diodes – the most efficient ones – which are coated with a substance that absorbs part of the blue light and re-emits it in another colour. This substance, phosphor, is the focus of every researcher’s attention. LED light is too “cold” (i.e. a little too blue) and not very powerful, so it is not well suited to lighting. But research is underway and there is still a lot to expect from fundamental studies being carried out in the lab. Reducing diffusion in phosphors, for example, increases light output using the same electricity consumption. Improving the efficiency of the electricity-to-light conversion enables us to produce powerful lights in the form of headlights, which are formed of just one element. So the multiple LEDs that are popular in cars at the moment will become redundant…

We are also trying to find cheaper and more efficient materials. For example, yttrium aluminium garnet (YAG) is very good but expensive. What makes it so good? If we find out why, we can look for another material. And we’ve worked it out! During a research project I took part in at the University of California, Santa Barbara, we discovered that YAG has a specific property (great interatomic rigidity) and we can now look for other materials that also have this property.
What improvements will be made in the lab?
The active research taking place is particularly focused on increasing the output of blue LEDs for higher levels of power. We will see cheap LED lighting replace today’s low-energy light bulbs. Even though the light might be a bit cold, it will be able to supply places that don’t have enough of it, such as developing countries. There is also research being carried out into red-orange phosphors, which will enable better control of the colour (or colours) of lighting. As a result, the use of LEDs will continue to expand in homes, public lighting systems and cars in years to come.

The Nobel prize was awarded to the inventors of blue LEDs, which have enabled any colour to be created – including white. © Akimbomidget, CC, by-sa 2.5On the same subject
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