One innovation the world could really use right now is a better way to harness the energy of the sun.
Solar panels have been rapidly growing in use. But they still make up only a small part of the global energy mix.
If you could design a solar cell that is cheaper, easier to install, and generates more electricity than the current technology, that could go a long way toward helping us phase out fossil fuels and avoid the worst effects of climate change.
The work of University of Toronto physicist Sajeev John might make such an innovation possible. The recipient of numerous honours — most recently the $1 million Gerhard Herzberg Medal — he has figured out a way to trap light. This discovery has already been applied to lasers used by surgeons; in the future, it could be used to design supercomputers superior even to the quantum computers researchers are trying to build today.
TVO.org speaks with John about his quest to develop a high-efficiency material to convert sunlight into electricity that would be both relatively inexpensive and environmentally friendly.
TVO.org: In general terms, how would this material work?
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Sajeev John: We are dealing with silicon materials, which are the most abundant and most environmentally friendly material you could use for solar cells. And silicon solar cells have been around for a long time — they’re 95 per cent of the market, approximately. And they’re very good materials in certain respects. But one of the biggest problems with silicon is that it’s not really a good absorber of the longer wavelengths of sunlight, say, starting from 800 nanometers up to 1200 nanometers: the red to near-infrared. And so that limits their efficiency or their ability to capture and absorb the sunlight in the first place.
What people have been doing over the past 30 to 40 years is to make silicon rather thick, so that by having a thick chunk of silicone — maybe 300 microns in thickness — they could eventually absorb enough of the sunlight. That’s what you see in standard solar panels now. The problem with doing that is if the silicon is very thick, then when a photon is absorbed, it creates certain charge carriers and electron hole pairs that have to migrate from where they were created to the contacts where they can deliver power to an external load. And if they have to travel 300 microns, there’s a very good likelihood that they will recombine or simply lose their energy and produce heat. And that’s what is limited the efficiency of silicon solar cells.
I’ve developed a design and done numerical simulations to show that there’s a particular architecture that you can place on a silicon solar cell that allows it to trap light very efficiently, even when the material is made very thin. And by thin, I mean 10 microns, 15 microns — a factor of 20 less than standard silicon solar cells. And if you’re able to trap all the light or almost all the light in a thin structure like that, that means that the charge carriers you produce only have to travel a short distance, and they’re very likely to reach their destination. So this actually can increase the efficiency of a solar cell.
John: The material that we’re looking at is high-quality silicon — that’s very different from what people talk about in terms of paints. Paints are very disordered materials, and when you create charge carriers in those, they cannot travel very far. And paints also are not very good absorbers of sunlight. So you’re bad on two points. The only advantage that it has is that it’s cheap.
Ours is good-quality crystalline or polycrystalline silicon. It might be encapsulated in a polymer. But it’s something that is so thin, the silicon now becomes flexible. It’s not like your standard, thick solar panel, which is just a heavy, inflexible chunk of silicon. This thin silicon would be something that you could coat on a variety of surfaces: on automobiles, building surfaces — maybe even clothing.
It is the opportunity to make solar-light capture really ubiquitous and not just limited to a few solar farms and a few rooftops, to have a very high-efficiency material that is pretty inexpensive and very environmentally friendly that you could coat on all these different surfaces while maintaining the efficiency or even improving upon the efficiency of standard solar panels. Our predictions are that the efficiencies of these thin photonic crystal solar cells could approach 30 per cent. The best solar panels out there made of thick silicon are around 20 per cent.
TVO.org: I know you don’t have a crystal ball. But what’s your best guess as to how close you are to making this material efficient enough and cheap enough to compete with conventional solar panels?
John: There are multiple steps in that process. So, right now, even as we speak, I’m working with one of the leading groups in Germany that has set world records for silicon solar-cell efficiency. They have fabricated sort of solar-cell-ready materials, and those samples have already been shipped to a group that I’ve been working with for a long time that has a lot of experience and expertise in fabricating the photonic crystal architecture. So those are going to be placed on the top side of the solar cells. And then they’ll be sent back to the original group in Germany for testing of solar efficiency, the device efficiency. And we’re going to start with relatively thick solar cells, maybe 100 to 300 microns, just to see how that works out, because the ability to handle these thin films is something that has to be fully developed. So once we get that proof of principle, and we’re hopefully able to see a high efficiency — maybe higher than the current world record — then we’ll start to work on thinning it down and seeing if the efficiency can go up even further, ultimately making a thin, flexible film.
TVO.org: I was just kind of wondering when I might be able to expect to be able to put some of your film on my house, for example.
John: I think it’s probably going to be a few years before you’re going to be able to put it on your house. There are various considerations that have to go into it: The durability — is it going to last 10 or 20 years? What is the aesthetic appeal of it? And, finally, of course, how efficient it is.
TVO.org: One reason I wanted to talk to you is that this Monday, TVO’s current-affairs program, The Agenda, had a discussion examining Canada’s often mediocre record on innovation. It seems as if, while Canadian scientists and researchers are great at making discoveries, it is often companies in other countries that commercialize and reap the benefits. I’m wondering how well-positioned Canadian companies are to help you commercialize your research.
John: As you know, it’s a very competitive world out there, and there are capabilities outside of Canada for making things very inexpensively. And I like to work with some of the best people that I can find around the world — for example, in Germany, in the United States, in Australia. Right now, I’m focused on getting the job done rather than expecting that everything is going to be done just in Canada.
In Canada, there are definitely smaller solar-cell companies that are looking for opportunities. I have joint funding with another faculty member, and we occasionally organize little conferences where we invite some other solar-cell companies within Ontario to participate in that. So that’s also a conversation that’s going on.
This interview has been condensed and edited for length and clarity.