Transparent solar cell turns edge on and generates its own light

Glass plate converts blue light to red, guides it to edge of glass.

Solar panels on Google rooftop
Enlarge / Solar panels sit on the roof of Google headquarters in Mountain View.

I love solar energy. Thanks to solar energy, on average, I’ve not had to pay for electricity for the last two years. Just because solar is already pretty good, though, doesn’t mean that it can’t be even cooler. Ladies and gentlemen, I give you transparent solar panels.

This area of research is addressing a couple of places where solar panels could be improved. For instance, it would be quite nice if we could coat windows with solar collectors but still let the light through. And, as a side effect, it may allow us to use each photon of light a little more efficiently. But how should we do that?

Using each photon twice

It turns out that these goals are connected. One of the ways that solar panels lose energy is when the photons have more energy than the solar cell can cope with.

To generate an electron, the material (generally silicon) of the solar panel has to absorb a photon with an energy above a certain amount, known as the band gap. Photons with energy less than the band gap are unable to generate electrons. What about photons with energy much greater than the band gap?

A photon with much more energy than the band gap can still generate an electron, but that electron has a lot of excess energy, which it loses as heat. In the end, all the electrons exit the solar panel with about the same energy as the band gap of the solar cell material. Obviously, it would be nice to keep the excess energy from going to waste like this.

To solve this problem, a group of researchers used a nanoparticle that was doped with ytterbium (a rare-earth metal). Ytterbium happens to love emitting light at almost exactly the wavelength that silicon likes to absorb it (which, in a solar panel, generates electrical energy). Even better, ytterbium, under the right circumstances, will absorb one blue/violet photon and emit twophotons at silicon’s favored energy. Best yet, the ytterbium does not like to absorb the photons that it has emitted, keeping it from interfering with the panel’s operation.

This novel combination of properties makes for a rather unique solar cell. The idea is that a solar panel that has this material will absorb blue light, then emit two infrared photons for every blue photon. The infrared photons are ignored by everything except the silicon that is used as the solar cell material. The silicon absorbs the infrared photons, generating two electrons for every photon of blue light that hit a ytterbium atom.

The main, slightly cynical, effect of this is that researchers can make bold claims about achieving ~160 percent efficiency (you don’t quite get 200 percent because the process is not perfect). But there are some quite cool upsides as well.

Guiding light to a solar cell

This kind of two-photons-from-one process has some additional benefits. The researchers created a polymer-glass material with the nanoparticles embedded in it. The particles absorb ultraviolet/deep-blue light from the solar spectrum but allow the rest to pass. This creates an apparently clear-glass structure.

When the ytterbium emits infrared light, it will mostly do so in directions that trap the light within the glass. The glass guides the light to the edges, where it can be absorbed by a silicon solar cell. Think of a pane of glass with a photovoltaic frame.

The end result, at this stage, is a very inefficient solar collector. Blue light generates infrared light with an efficiency of about 180 percent. However, only three percent of the blue light gets absorbed. (Remember that it looks transparent? This is why.) Then there are transport losses, making the whole idea a bit of a stretch right now.

Invisible solar panels

Still, invisible solar panels might well be in our future. The absorption efficiency can be improved. The researchers claim that with their existing materials and methods, they should be able to see three-fold improvements.

Looking at the guiding system, I would say that they can probably reduce the transport losses and ensure that the vast majority of the infrared light makes it into the solar cell. Furthermore, if you don’t mind having a slightly yellow view, a change in materials can improve the efficiency even further by using photons from the lighter blue part of the spectrum.

Finally, silicon is not the only solar panel material. More efficient materials are available—they are just really expensive. But, in this configuration, you only need solar cell material at the edges of the glass sheet. This reduces the amount of photovoltaic material required per square meter of panel. At least that is what the researchers claim.

I’m not so confident of that argument. These high-efficiency materials are tuned so that their band gap matches the solar spectrum. Light absorption followed by re-emission at longer wavelengths will create a mismatch between the light spectrum and the material band gap. You can’t avoid that, because you need the absorption and re-emission to obtain light guiding, without which you don’t reduce the amount of material required for the solar panel.

I’m pretty sure that is the wrong thing to do. It would make more sense just to have a giant dish to track the Sun and focus the light directly on the expensive solar panel material (and watch the cell overheat). Still, I like the idea of transparent solar cells, and I hope the researchers make it work.

Nanoletters, 2018, DOI: 10.1021/acs.nanolett.8b03966 (About DOIs)