Capturing Sunlight on a Nano Scale

Diana -   Solar cells aren't a new concept, but they are an area of massive growth.  In fact, photovoltaics as they're known are thought to be the fastest growing energy technology.  But great as solar sounds, it's long been held back by the relatively poor efficiency of the cells themselves which is what researchers are trying to improve.  Niraj Lal is a researcher from the NanoPhotonics group at Cambridge University's Cavendish Laboratory.  Hi, Niraj.

Niraj -   Good day.

Diana -   So, can you tell us a little bit about why current cells are poor performers?

Niraj -   I think the reason why you don't see solar cells on roofs everywhere you go is because they're expensive to make.  The materials that they are made out of are pretty expensive and also, they're not as efficient as we'd want them to be.  So, when sunlight hits a solar cell, some of it gets reflected off the top of the solar cell and some of it goes straight through and then out and back again without even being absorbed.

Diana -   So how can we use nanotechnology to try and improve that?

Niraj -   Scientists all around the world are finding that when we structure a material on the nanoscale, a really small scale, it interacts with light in different ways - ways that we haven't seen before.  And one of those ways is to concentrate light in regions that we haven't been able to concentrate light in before - on the top of a metal surface.  That's what's known as plasmons and we can use these concentrated bits of light to increase absorption in the solar cell and increase efficiency.

Diana -   But how does that actually work?

Niraj -   For certain metals - typically gold and silver or sometimes with copper and aluminium you get the same thing - when you shine light on them, if you shine light with just the right colour, and you have the structure just right, you can set up a resonance inside the metal, inside the charges of the metal.  So if you shine light on a metal surface and you excite these charges going backwards and forwards, you can setup a concentrated bit of light along the top surface of the metal, and that's what's known as a plasmon.  That's what we're trying to do.  We're trying use those plasmons to increase the efficiency of solar cells.

Diana -   And you've got an audible example of this setting up of resonance with you.  Could you give us a demonstration?

Niraj -   Yeah.  So what I've got here is a Buddist singing bowl; monks use it to help them meditate.  It's a brass bowl, about the same size as a cereal bowl.  What I've got here is a wooden rod and I'm kind of rotating it against the edge of the bowl.  If I drive it with just the right frequency, I can set up a resonance response inside the bowl.  So that's a mechanical response, a mechanical resonance, but the same thing happens with light for structures about 100,000 times smaller.  That's what my PhD is about - using structures like this to increase the absorption, increase the efficiency of solar cells.

Diana -   So how do you make these nanostructures then?

Niraj -   It's pretty cool.  What they do is they get the beans from a bean bag, so polystyrene spheres, but a lot smaller and they put them on a clean surface of a metal in a solution and they dry them.  As they dry, they pack into a hexagonal lattice.  When they're all dry and all set, we grow gold or silver or another metal from behind them using electro-chemistry.  As we grow them we get to about half height or a little bit higher than half height, we stop and then we dissolve away the spheres - dissolve away the bean bags, and we're left with a field of little bubbles everywhere and that's what we call nanovoids.  We use their plasmonic resonance to increase or try to increase efficiency of solar cells.

Diana -   And just how much more efficient can they become with this technology?

Niraj -   So last week we made a batch of solar cells that showed about twice as much electricity at certain wavelengths.  It's a preliminary result and we're still trying to figure out exactly what's going on but it's exciting.

Diana -   So presumably, these haven't been deployed anywhere yet and no one's using them across the world?

Niraj -   Not yet.  I think it'll be a while before anything like this actually gets on top of a roof but efficiencies are increasing every year and I think, if this works, in 5 to 10 years, we'll see them on roofs.

Diana -   Fantastic!  Well thanks Niraj.  That's Niraj Lal, he's based in the NanoPhotonics group at Cambridge University.