Why is there a push towards phosphor free led technology?

Phosphors are a class of materials that absorb shorter wavelengths of light and emit longer wavelengths. They do this very very fast. Phosphors have allowed for a more compact led package. In phosphor based leds The active layer emits high energy wavelength. Some of this energy is absorbed by phosphor coating that converts it into low energy wavelengths. Thus result is that the overall energy radiating from the led package appears to be white. Mixture of blue, green, yellow, red. Before phosphors it was a common practice to use single colored LEDs. To produce white light then it was necessary to use atleast three LEDs. This was expensive,inefficient and a major reason why it took so long for leds to succeed even though they were discovered in early 1900s way before the invention of modern semiconductors. Phosphors have been revolutionary and yet there is a great deal of research being done to replace them why?

One reason is that they loose energy. The overall efficiency of the led package depends not only on the efficiency of the active emissive layer but also on the ability of the phosphor to convert the energy into different colors. The more layers you add to a system the less efficient it is going to get. Phosphors can be difficult to synthesize. Most of them are complex compounds of multiple elements. And because each and every led is coated with phosphors they need to be produced at scale. This complicates manufacturing process as not only do you have to synthesize active emissive layer but also the converting phosphor in equivalent quantity.

Many phosphor materials are made of rare earth materials. For something that is as important as an led you want to make it using materials that are abundant, easy to acquire recyclable. Long life span does not matter if the product is discarded and components are not recycled. Moreover you now have 2 points of failure in the system. The light source is going to stop working as intended if either the phosphor or the active emissive layer fails. It is for this reason that alternatives are being looked into.

Thats all well and good but what will phosphors be replaced with? So conceptually phosphors wont really be replaced. Pure materials are not capable of emitting white light. Band gap limits not just the amount of energy that can be absorbed but also the amount of energy that can be emitted. Since band gap is an intrinsic material property it follows that all materials would have a signature emission spectrum. As unique as a finger print. And sure enough spectral lines are proof that materials emit at well defined energies.

So while the concept of phosphors won't be replaced new kinds of phosphor technologies are being created. The most important of these are those that integrate with active emissive layer instead of being coated on the outer package of the led. So that in a single package we can have as close to white light emission as possible. Quantum dots, organic LEDs and donor/acceptors dopants in wide band gap semiconductor are some of the most promising ones.

We will be looking at these technologies and the research being done on them in future posts.

#light #energy #matter

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Notes by Akshat Jiwan Sharma

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