Copper solar cell creates "all in one" reaction factory

Researchers at Cambridge University have developed a way to use low-cost copper-based semiconductors to produce photovoltaic - solar - panels that capture the Sun’s energy but then use it to drive chemical reactions on their surfaces. Effectively it’s an all-in-one chemical factory that could produce simple fuels like hydrogen, or even more complex carbon-based molecules. It’s the brainchild of physicist Sam Stranks…

Sam - So generally what we're trying to do is harvest sunlight and convert that sunlight to useful energy to do chemistry. And so here we're splitting water to produce hydrogen, which can be used as a fuel.

Chris - So what is the actual problem you're trying to solve?

Sam - One of them is how efficient we can harvest the sunlight and convert it to energise electrons. Typically this is done with silicon. It's a very good material that can do this very, very efficiently, but silicon's quite expensive to produce. So we've been using copper based materials, which are at the moment very inefficient. We'd be making them more efficient. These are cheap to make and cheap to use.

Chris - If you can pull this off, what sort of improvement do you think you're going to get?

Sam - It's really a step change in what we can do with sunlight. So rather than just thinking about producing electricity, we can start making very interesting materials and making green fuels. So this is for example, carbon based fuels that could be used either in energy applications or in materials developments as well. So this is all done by green solar power.

Chris - And why have you picked on copper?

Sam - Copper is earth abundant, that's a very important thing we want with these sorts of applications. We want to be able to scale them up in a very large way. So we need materials that are readily available and can be easily resourced. Copper is one of those. It's also a very good material that does harvest light quite well and can energise those electrons to do the chemistry that we want to do.

Chris - So talk us through actually how you make it, what you end up with. If you had a physical lump of this in front of us that we could see, what would I be looking at? What does it look like and how does it actually work?

Sam - Yeah, so we make these through a process. It's called electrodeposition. So we use chemicals dissolved in solution that can then be converted to a solid crystal film that will look relatively dark and absorbing because it's absorbing the sunlight quite well. And then we have electrodes on them as well. So we can wire them up and measure the current, for example, that we're getting from these devices

Chris - To all intents and purposes. Then does it work just like a solar panel put on my roof?

Sam - Very much so. It does. So in the same way as a solar cell has an absorbent material that absorbs the light and then the energised electrons are collected as current. We have that in the same way in this material, but the difference is that we have catalysts on the surface that when the electrons reach that catalyst, then they can do some chemistry to be able to reduce water to hydrogen.

Chris - And what was stopping people doing this before then?

Sam - So the concept's been around for a while on copper based materials. They've been limiting because there's defects in these materials where electrons lose their energy, to heat for example. So what we've really done is being able to grow very, very high quality crystals with fewer defects. That means those energised electrons can travel a lot further.

Chris - And is it literally just electricity coming out of wires and then you do something with the electricity or is the step here that you're doing something on the surface of the material? So you're doing sort of two jobs in one, capturing the light, converting it into useful energy, electricity or whatever, and then you're doing chemistry and other exciting things in situ there and then, so fewer losses.

Sam - Exactly. It's integrated on the chip, all in one. So here that's the advantage that we don't have to wire it up and then have a separate electrolyser where the water splitting can happen separately. This is all integrated into one. So it's a very elegant solution and in principle could be very efficient because you've got the absorber itself energising the electrons and then the chemistry is done on the surface of that absorber.

Chris - I'm just trying to visualise that. So would you have it sitting underwater then, or do you have a thin film of water, or have you got little channels so the water or the solution or whatever it is, runs across the surface? How do you do that?

Sam - So it is immersed in water and typically in our cells that we make in the lab, we have essentially a little beaker that it's soaking in. That will be a challenge when we think about scaling it up and there's different solutions for that, but that's for future work.

Chris - So the ultimate goal is it will produce electricity which will split water into hydrogen and oxygen. What, you then tap off the hydrogen and use that? How do you fit this into a sort of production line?

Sam - That's right. So it's extracting out the hydrogen in particular, in this case. And that's something that then that hydrogen could be stored, and it could be used in applications either on site or offsite. So thinking about things like green steel for example, you could use hydrogen to produce that. But there's also some exciting applications beyond just hydrogen. So this is these materials particularly that go beyond silicon in terms of the voltage they can produce. You can start to do interesting chemistry in higher order chemistries. So thinking about making carbon molecules, multi-car molecules as well. And particularly when we can start getting to these higher voltages that materials like copper oxide or other related materials can enable that silicon can't at the moment. So we can start to think about, for example, reducing carbon dioxide to produce carbon molecules. It's really that the holy grail of this solar fuel world is to get to the C2 and above carbon chemistries.

Chris - C2 as in two carbons linked together.

Sam - Exactly right. Yep.

Chris - Is anyone else doing this the way you're trying to do this or have you got something unique here?

Sam - There aren't many others in the world working on the broad field of solar fuels, so it's very much an emerging field. What's exciting about this result is that the photo currents we're producing are going well beyond those results that have been out there so far. In fact, we're moving from something around four milliamps per centimetre squared to something about seven milliamps per centimetre squared under operational current. So we're almost doubling the currents which is really exciting.

Chris - And how does that compare with what silicon can do?

Sam - For silicon, it's more like 20 milliamps per centimetre squared. The problem is there's a compromise in the voltage, so silicon's voltage is much, much lower, whereas we can get much higher. And that's where the exciting new chemistry comes in.