Talking science in sunny California

IMG_0024I started this post on the Boeing 777 whose left wing is in the foreground of the picture above. The flight was from San Francisco to London. It took off in the evening, so we got to see a stunning sunset with a beautiful blood red sky. In the plane, as we were so high up, the view of the sun and backlight clouds was amazing.

I’ve spent a the last couple of days at the University of California Santa Barbara, talking to a whole bunch of smart enthusiastic scientists, about a lot of things, but in particular about solar panels made not from silicon but from molecules, referred to in the trade is organic photovoltaics or OPVs. These are films of polymers or organic molecules that turn light into electricity. We need to generate more power for our societies, while reducing the amount of carbon dioxide we produce, and OPVs are one way of doing this.

But making OPVs is hard. I still know very little about them but this week have learnt a lot from the experts. First a photon of light hits the OPV and is absorbed. Its energy produces what is called an exciton. So first, the OPV needs to be able to absorb light and generate these excitons. But this exciton is neutral, it is uncharged and so can’t create a current – electric currents are flows of charged particles.

So this exciton needs to wander off to a boundary between two types of material where it splits into an electron (negatively charged) and a hole (positively charged). These two charges then go into the two types of material, and need to be pulled out of the opposite sides of the film, to generate the current. But if at any point the electron and hole bump into each other, they can recombine, which then releases the energy that went into creating the exciton. This energy may be in the form of heat – which we don’t want, as converting photon energy into heat is not what we want. We don’t want heat, we want electricity.

Like many devices, there are trade offs in the design of OPVs. For example, we need two types of material, one to conduct electrons and one to conduct holes, and there has to be a lot of surface area between the two, as this surface is where the excitons split into to electrons and holes. Too little surface area means this splitting is difficult. But this surface is also where electrons and holes can recombine, so too much surface means too many pairs of electrons and holes recombining, which wastes energy as heat.

Also, the materials should allow the electrons and holes to move swiftly through them, which typically means they must be crystalline, because crystals typically conduct the best. But if the crystals are too large they have little surface area per unit volume and so we are back in trouble with too little surface for the excitons to split on.

So making an OPV work means reconciling a bunch of contradictory requirements as best you can. This makes it hard, it also makes it fun, as you have to work out what is going on, and which of the many possibilities is limiting your efficiency in a given OPV.

As someone who works on trying to understand crystallisation, this looks like heaven. There are a whole bunch of smart people who need to understand crystallisation better to make their devices better, and understanding crystallisation is what I do. There could be some fun collaborations ahead.

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