Caffeinated solar panels

30 April 2019

Interview with 

Paul Coxon, University of Cambridge

COFFEE-BEANS

A cup on its side, with coffee beans spilling out of it

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Most of us have experienced the “pick me up” that comes from a cup of coffee or tea. But this week, scientists in the US and China made the surprising announcement that adding caffeine to their solar panels made them work better! To find out why, Katie Haylor stuck the kettle on and took a look at the results with Cambridge University solar panel expert Paul Coxon...

Paul - Solar cells take light from the sun. They're usually made of a semiconductor material which is halfway between a metal and an insulator. They're a special sandwich of different materials together and they absorb the light inside the crystal of this material, you excite the atoms inside making these charges, these positive and negative particles, this is forming your current.

Katie - Currently, if I look at some solar panels on my roof or if I travel past a solar farm, how much of the light going in is actually been converted to electricity at the end?

Paul - Well, most of the solar panels which you see out and about today will be made from silicon. About 93% of the world's solar electricity is made with silicon solar cells and so they have a limit of about 20 to 23% at the moment.

Katie - So what did this group do then, because they weren't looking at silicon were they?

Paul  - No. They were working on a new classification of material called perovskites, and these have attracted a lot of attention in recent years. These are quite different to silicon solar cells in that you can make the material at very low temperature. They're just wet chemicals that you can mix together and they form a crystal structure. This crystal structure is very very good at turning light into electricity. What they did was they added some caffeine into the mix and this affected how the crystals In these layers grew.

Katie - So hang on. I mean I'm sitting here with a cup of tea, you've got a cup of coffee, what made them decide to put caffeine in a solar cell?

Paul -  Well, caffeine has got special chemical groups called carboxyl groups. This is a carbon atom joined to an oxygen atom with two bonds. By adding this into the chemical mix of your perovskite solar cell, this passivated or stabilised the material in a special way which influenced how the crystals grew within the solar cell layer, and this made the layer slightly more crystalline. It gave you better crystals, and better quality crystals which are bigger crystals, and this improved how the solar cell behaved.

Katie - Why is that then? What's the caffeine actually doing to benefit this process?

Paul - It actually made it slightly harder to grow lots and lots of little crystals. So as you're coating these wet chemicals together and forming a crystalline layer, imagine you've got lots and lots of little crystals growing in all different directions, and when these little crystals come together at the boundaries, these are where defects can occur and these can impinge or slow down the charges moving through the film. By adding the caffeine, you made it slightly harder for these random crystals to grow and it made the crystals grow in a preferential direction, in one way more likely. This gave you bigger crystals so it gave you a more uniform layer. So by having bigger crystals in the layer, it changed how the electrons moved through the layer, and it led to a higher voltage of your solar cell and it gave you a higher power conversion efficiency. So they managed to increase it using caffeine to about 20%, which is a great achievement for them.

Katie - So is it fair to say that because of the effect the caffeine had on the growth of the crystals, these perovskite crystals, this could potentially increase the efficiency of solar panels made from perovskite, which are currently not mainstream, right?

Paul - No. They're still quite new so there's a lot of research in this area. So yes, by adding the caffeine in this way you can influence how these crystals grow and give higher conversion efficiencies. But also, it made these crystals more stable against heat. One of the problems which perovskite can have is that under heat the layer can degrade, but they found that the caffeine influenced how the ions, the charged particles in these layers moved, and this meant that they were more stable at higher temperatures. This means that out and about in a real-world situation they're more stable and they could last longer.

Katie -  I can see why having a material that degrades with heat is a bit of a problem when it comes to trying to make a solar panel. What is the point of doing this with perovskite if most solar cells are made out of silicon? Can we put caffeine in silicon and make that more efficient?

Paul  -   Well, you can't put the caffeine in the silicon, but we can put the perovskite, maybe with the caffeine, on top of silicon. And this means that we can tune the top layer of your perovskite to harvest the blue light from the sun and the red light passes through and get absorbed by the silicon underneath. So imagine having the perovskite  piggybacking on top of the silicon. This allows you to capture more of that solar light and turn it into electricity much more efficiently.

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