Quantum Dots for Solar Cells

PMMA (clear plastic) with the new quantum dots embedded.

PMMA (clear plastic) with the new quantum dots embedded.

 

Researchers at the Los Alamos National Laboratories,  in collaboration with scientists from University of Milano-Bicocca,  have created a new type of quantum dot which could allow the windows of your house to be converted into a new type of solar cell. Previous types of quantum dots have lacked a particular feature called a Stokes Shift which barred them from this use.

Quantum Whats?

A quantum dot is a very small lump of matter, usually made of between 100 and 100,000 atoms arranged in a repeating, ordered pattern. They are so small that you can’t see them. Many can’t even be seen through a microscope. Their extremely small size causes them to absorb and emit light highly effectively from very specific sets of colours. For example, one quantum dot might absorb sky blue, acid green, and neon purple, but not any other color. The size of the quantum dot determines which colors it will absorb.

Atoms and molecules also do this, but the big difference is that quantum dots can be engineered to pick out certain colours fairly easily, whereas it is difficult to do this with molecules, and impossible with atoms.

After a quantum dot, molecule, or atom has absorbed some light, it can emit light. This can be the same colour or a different colour. Molecules behave differently to atoms and normal quantum dots in doing this: atoms and normal quantum dots only emit colours that they can also absorb, but molecules often emit colours different to what they can absorb. The difference between the absorbed colour and the emitted color is called the Stokes Shift, named in honour of Sir George G. Stokes who first described the phenomenon.

 

Stokes Shifts Actually Matter

These “solar panel windows” would actually be more like antennas or collector dishes. They would collect sunlight then re-direct it onto a solar panel on the edge of the glass, letting you deliver a lot of light to a small panel to get a lot more energy out than otherwise.

The quantum dots would absorb some of the light hitting the window, then emit it inside the glass. Quantum dots emit light in all directions, no matter what direction the original absorbed light came from. Some of the emitted light would shine straight onto the solar pannels. Some of the light would bounce around inside the glass and reach the edges slightly less directly, a lot like light travelling through a fiber optic cable. Some of the light would still leave the glass, but a large amount would have reached the solar panels.

The problem if you try to do this with regular quantum dots is that any light emitted can then be absorbed again. You tend to lose more of the light energy this way, making the solar collector window much less efficient. If the quantum dots had a Stokes Shift, they would not absorb the light their neighbors had emitted.

 

So How Did They Do It?

The Los Alamos researchers have made their quantum dots like tootise pops: instead of being made of the same stuff the whole way through, these quantum dots have a thick outer shell wrapped around a small inner core.

Most of the absorption happens in the outer shell, then the energy of the light passes into the inner core and gets emitted from there. The quantum dot can’t absorb the light it is emitting, so the light escapes from the quantum dot. The neigbours can’t absorb the color that has been emitted, so it gets past them, and (probably) travels through the glass to the solar panel in the edge.

 

Where can I buy it?

These solar collector windows, properly called “luminescent solar concentrators” have been demonstrated using other techniques, but they tend to be difficult to make or use expensive components. Quantum dots are relatively easy to make, easy to customise, and can potentially make this technology cheap and accessible. This is early days, though, and there are no release dates for buying solar collector windows for your house just yet.

 

Original paper published in Nature Photonics.

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