Spray-on solar cells

Scientists promise us a future where solar cells can be sprayed onto surfaces, so cars, buildings and streets could be covered in them.
01 October 2014

Interview with 

Michael Price, Cambridge University


Traditional solar cells are made using silicon crystals. While these have their the sunbenefits, such as efficiency, they are relatively bulky, expensive to make, and can't be bent. So now scientists are looking into new materials for solar cells which may have better properties. Michael Price works on organic solar cells which can be made cheaply and are thin and light. Unfortunately, at the moment, they aren't as efficient as conventional cell. Michael showed Chris Smith how he's looking for ways to improve the efficiency of these materials. 

Michael -  I'm looking mostly at the physics of new types of material for solar panels.  We call them solar cells and a solar panel is just made up of lots of solar cells.

Chris -  Now, when you look on the roof and you see these solar panels, what are they actually doing?  How do they work?

Michael -  Yeah, so to convert the energy of the sun which is obviously our most abundant resource of energy, there are a number of steps that you need to undergo.  So, we think the first step that you need to achieve in good solar cells, you need to absorb the light.  So, if you can think of light as particles - we call them photons - you need to absorb that particle of light in your solar material.  In the second step, in the process of absorption, you create an excited electron.  So, your photon comes into your material, hits that material and it's going to excite an electron, got a bunch of these electrons sitting dormant in your material, in your semiconductor and once you've excited one of these, the electron needs to travel through the material and then out into a conductor like just a normal copper wire. And that flow of electrons is what we call electricity.  There are a bunch of problems that could happen.  The electrons can crash into each other.  They can crash into other atoms and you lose energy as heat.

Chris -  So, the light coming in from space is hitting the material and is dislodging some electrons or putting them into a state where they can actually flow around a circuit.  How does that get into the grid because obviously, people want to use their cells on their roof to reduce their energy bill don't they and they do that by selling the electricity back to the grids.  So, how does that happen?

Michael -  As we all know, grid power works on an AC (alternating current) and so, you need to convert your direct current because the sun is a constant source. It doesn't flick on and off at frequency of 50 hertz.  You need to have just an electronic device to convert your direct current from the sun into an alternating current and then you've got power for your microwave.

Chris -  Sounds like a done deal.  I mean, how good are these panels, the ones you see on people's roofs?  How much energy are they converting into electricity they can use and sell?

Michael -  So, 90% roughly of the solar panels that you see on people's roofs are made from silicon.  These are pretty good.  People have been working on them for 20 years and they're getting up towards 20%.  Maybe most...

Chris -  So when you say they're getting up towards 20%, you mean, of the energy that hits them, about 20% turns into useful electricity.

Michael -  Yeah, that's right.

Chris -  What happens to the other 80% then?

Michael -  A lot of what I study, the goal of improving efficiency of solar materials is to get that number up.  But there's a fundamental limit.  The most efficient single layer of solar panel you can get based on thermodynamics, that's just looking at the principles of energy conservation - the energy you put in has to equal the energy you get out and 30% is the number that we talk about.  That's the upper limit.  Of course, you can make solar cells more efficient by putting them on top of each other or concentrating the light that goes in on them.

Chris -  So, tell us about this that you've got in front of you - a little demo there.

Michael -  This solar panel is made of plastic essentially.  That's the active material.  If I hold it really close to the light up here, it's going to make a really annoying buzzing noise which is perfect for radio.  So, I'm just holding it up, really close to a light on the roof simulating the sun and it's attached to this frustrating little buzzer here.  And you can see at the same time, I can flex the solar panel.  It's really thin.  It's really light because it's made of electrically active polymers - plastics rather than...

Chris -  So, this is different than what you would find on top of roofs.

Michael -  And this is what we study in my lab, different types of materials.

Chris -  So, you're trying to re-invent what's on people's roofs at the moment and make thinner, cheaper, lighter materials.

Michael -  Yeah.  We're not necessarily going to replace what's on people's roofs.  We might be able to put a very cheap coating onto silicon and improve their efficiency.  In fact, that's happened in the last couple of years, but there are a whole bunch of uses for cheaper, perhaps not quite as efficient, but cheaper, lighter solar.  It's really useful on the developing world.  For instance, there's still a billion people who are off-grid who don't have mains power.  If we can supply them with something that's cheap, that's not going to break if you drop it then that could be a really good thing.

Chris -  Who's got some questions about solar panels, how solar cells work?

George -  My name is George from Ely.  I'm just wondering how you can get the solar cells into a material.

Michael -  Basically, what consists of a solar cell, you need two electrical contacts, so they're going to be made of metal.  The way we do it, we start with a piece of glass and then we have a transparent kind of metallic conductor and then we - just because the materials we work with don't require extreme processing, we just squirt them on.  We can just squirt these materials and then we evaporate on another metallic contact.

Chris -  What's the recipe for the stuff you can just squirt on?

Michael -  Well, so there's a whole range of things you can do.  I was reading today, there's a guy that's been harvesting salmon sperm for use in LEDs which are just like solar cells in that a solar cell will convert photons to electrons, electricity.  An LED will convert electricity to photons.  But in our lab, we use our polymers, plastics.  We also have been researching this new type of material called perovskite solar cells.

Chris -  Yeah, that's getting a lot of attention, isn't it?  What's special about that?

Michael -  Basically, they have all of the good things of polymer solar cells there.  They're cheap, flexible, you can just squirt them on.  You don't need to grow them painstakingly like you need to grow a crystal of silicon.  And they're also really efficient.

Chris -  So, could you have a paint that you could slap the stuff up the wall and turn your garden fence into solar cells?

Michael -  People always ask about painting solar cells, but you need the electrodes as well.  So, it's not quite as simple as just...

Chris -  Maybe wallpaper or some things like that.

Michael -  There's already Oxford photovoltaics who pioneered another work on these perovskites.  They're working on building integrated photovoltaics.  So, that means that all of the glass that goes on your skyscrapers could have a thin layer of semi-transparent solar panel on that glass and so, they could generate electricity while you work.

Chris -  While you're zapping the buildings across the street which happened with the building in London last summer, wasn't it?

Michael -  Yeah.

Holly -  This is Holly.  I've seen lots of solar panels everywhere in England but it's always cloudy.  Do they still work?

Michael -  Yes, they do.  The new types of materials that we're working on actually work better in low light than the crystal and silicon ones.  But it's obviously true that there are places that are better for solar than England.  Yeah, the solar panels will work in low light levels but just not particularly well.

Will -  Hi.  I'm Will from Swavesey and I was wondering, does the colour of the cells change the amount of light it absorbs?

Michael -  Yes, that's a really good question.  The ideal solar cell is a black one because it absorbs all of the light in a visible spectrum but you can make solar cells of all different colours.  But a transparent solar cell that lets all of the light through that our eyes can see is not going to be as efficient as something that harvests that light.  You can make transparent solar cells that absorb the infrared.  So, those are the really long wavelengths of light that our eyes can't see, but they're not going to be very efficient.

Chris -  So, just let them sort of model-T Ford of the solar panel world.  You're going to have any colour you like as long as it's black.

Michael -  Exactly.

Dwight -  I'm Dwight.  I'm also from Ely.  Why do the solar cells wear out and why do you have to replace them after a while?

Michael -  That's also a very good question.  In an area of active research there are an awful lot of reasons.  Most solar cells, we try and encapsulate to protect them from the elements.  Usually, if the encapsulation fails, then the solar cell is going to wear out.  In the case of silicon, these solar cells lasts upwards over 20 years.  The reasons for that can be varied.  It can be due to oxygen degradation or water or probably, before the actual material fails, the electronics associated with it are going to fail as well.

Anthony -  Hi.  I'm Anthony from Cambridge.  I was wondering, how long does it take a solar panel to generate as much energy as was used to manufacture it?

Michael -  Again, similar to the wind question, the energy payback time for our solar panels obviously varies greatly with where it is.  If it's in a very sunny place, then the energy payback time actually can be very small and the energy payback time is going down on time, but I think I've heard, less than a year, it can be.  But if you put it in a cloudy place, the energy that you've used to create the thing has been - if you haven't made it very efficiently, then the energy payback time is going to be a bit more.

Chris -  What is that thing that looks like a giant till roll in front of you?

Michael -  This is a giant roll of polymer solar panel.  It's rolled up here because it's a really good demonstration of the fact that you could print these solar cells off like newspaper and that's the goal.  That's what can make them so much cheaper.  I also got - it says it on the front here - you're looking at the world's first coloured production really plastic display that is flexible.  This is from Plastic Logic which is a spinoff company of one of the professors in my group and basically, it's a flexible kind of eReader like if you can imagine your Kindle was in colour and you could bend it.

Chris -  I mean, handy because the people in customs did that to my Kindle when I was travelling recently.  It didn't take too kindly to it.  What's your name?

Malcolm -  My name is Malcolm and I'm from Long Stanton.  So, the dark of the colour of the solar panel means, the more sunlight it absorbs and the more electricity is made.

Michael -  Yeah, that's it in a nutshell.


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