Making solar energy more accessible
A new form of solar cell that uses plastic instead of silicone may make solar panels a cheap and effective way of producing electricity. David Lidzey from the University of Sheffield explains more...
David Lidzey - The materials that we are interested in are very special plastics so they have been synthesized for us by synthetic chemists here at Sheffield. They've got two really important properties; firstly they absorb light over a broad range of wavelengths, so where you might see a film of polythene which is largely transparent, the materials that we have can absorb light all the way from green wavelengths right the way through to the near infrared, and of course if you want to harvest sunlight energy you want to pick up as much of the sunlight as possible. So that's one property which is important. The second property is that they are electrical semi-conductors; so basically once you put a charge carrier, like an electron for example, in these materials, the charge will flow through the material if you apply a field to it. So, the photovoltaics are made from these thin films of semiconductors and essentially we have our plastic which is mixed with another material which is called a Fullerene. Fullerene is basically carbon-60, so this is a molecule which was discovered 20 years ago or so. It's one of the pure forms of carbon.
Meera - So how would this actually work? I assume it's reasonably new, you're always looking to use plastic and people haven't up to now, so what are the challenges really faced with using this as a material?
David - Well there are a number of challenges. One is extending the red absorption wave length, so we want to go further and further into the infrared, and the second challenge is actually getting these charges out of the thin films. So basically you have this Fullerene molecule and when the polymer is excited by the absorption of a photon, an electron jumps from the polymer molecule to the Fullerene molecule and this act of what's called 'charge transfer' essentially creates a tiny current inside the device. When that happens over millions and millions of molecules, we can extract enough of a current out of the device because of this charge transfer process.
The problem is though, when we take our polymer and we mix it with the fullerene molecule, the polymer and the fullerene phase separate. You've seen a salad dressing made by the mixture of oil and vinegar and of course the oil and vinegar separate; when we mix the polymer and the fullerene together, they'll also phase separate in a similar way. Now this is beneficial for us because we want to create parts of the film which are polymer-rich and other parts of the film which are fullerene-rich, and these almost create little charge-transfer wire inside the material. If they actually phase separate on too large a length scale, then this makes the device not work very efficiently. If it phase separates on a very fine length scale then they also don't work very efficiently because you can't form these little charge-transfer pathways. So essentially one of the tricks is to actually know how to get the film to phase separate on exactly the right length scale and this is one of the things that we've been very interested in. And we've been using the Diamond Light Source and other techniques to actually study the structure of the film.
Meera - What has this enabled you to see then in detail about the process?
David - Now what we find is that for most of the time when this solution is drying, actually nothing much happens at all. I guess this is a sort of 'watching the paint dry' phase. But when the solvent evaporates to leave only about 50% of the film containing solvent, things suddenly get very exciting. What happens is we see a very rapid crystallization of the polymer. This process happens in about 5-10 seconds and suddenly we see that as the polymer molecules start to crystallize, it's very difficult for the solvent to leave the film. The kinetics of the crystallization of the polymer tells us that the crystals first form around impurities, or little aggregates, that exist within the film and we see a crystallization that happens essentially in one dimension. So we see a one-dimensional crystallization of the polymer.
And thirdly, we actually find that as the polymer crystallizes, we can see that the crystallization and the arrangement of the molecules in the crystallites improves, basically their packing gets tighter and tighter, there's a general reduction in kinks and twists in the molecules. So it's really the whole picture, the way that the solvent evaporates, the dynamics of the crystallization, and the improved packing of the molecules. It gives us an overall picture of the processes that occur as the film dries.
Meera - So having been able to see this; what stage would you say your research is at the moment? Have you got a final design that you would want to go into production to have plastic incorporated into our solar cells?
David - Well at the moment the sort of solar cells that we are producing here at Sheffield, and that other groups are producing around the world, have efficiency between 4 and 8%. So this means that between 4 and 8% of the sun's energy that is absorbed by the solar cell is actually converted into electrical power. Silicone, our big rival as it were, has an efficiency of between 15 and 20%, so you can see that we're actually quite far behind silicone really. So one of the big challenges now is to improve the efficiency of the organic based system up towards the efficiencies of the inorganic based systems. It is possible that we will never actually get to the efficiencies that you get with the inorganic, but the big advantage of using polymers is that they are very cheap to produce and make thin films from.
Meera - So although it may not match the efficiency of silicone, the fact that it's cheaper will hopefully mean that more places and more people will be using it?
David - Well exactly. The idea is that if you could produce these things on a plastic substrate then essentially you could go down to you local DIY Supermarket and buy a big roll of this stuff and just pin it to your side of your house and produce your electricity very, very cheaply. And really there's a huge amount of land area which is redundant in parts of the country, like the roofs of out-of-town supermarkets or the sides of railways or motorways. You don't want to do very much with these places and if you could actually cover them at very low cost with a thin, flexible photovoltaic film, you could produce a huge amount of electricity.
Meera - David Lidzey from the University of Sheffield.