The Promise Of Perovskite Solar Panels

IRA FLATOW: This is Science Friday. I’m Ira Flatow, live from the Chautauqua auditorium in Boulder, Colorado.

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I’ve got to tell you, this week I’ve been driving all through Colorado, and I’ve noticed something really interesting that I was very happy to see, and that was a lot of solar panels sprouting up. Yeah, I mean, if you listen to this show, you know I’m a huge fan of solar energy. I have solar panels on my roof.

And if you’ve had your eyes on the solar industry for as long as I have been watching it, you know that solar technology has come a long way, and it is rapidly changing. They’ve become more efficient. They’re less expensive. And that’s what we’re going to talk about now, and lucky for us, we have a lot of groundbreaking work on solar energy done right here in Colorado.

So joining me to talk about the new research on solar panels are my guests, Dr. Joseph Berry, Senior Research Scientist at the National Renewable Energy Laboratory in Golden, Colorado. Welcome to Science Friday. And Dr. Laura Schelhas, Senior Scientist also at NREL. Isn’t how we say it? Yeah, National Renewable Energy Laboratory. Welcome to Science Friday also.

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I remember when that was established, your center, going back into the ’70s That’s another thing to talk about. But for now, let’s talk about over the years, Joe, you’ve been in this industry just a little bit, maybe like 25 years, something like that.

JOSEPH BERRY: I’ve been at NREL for now a little better than what– I’m coming up on 18, I think, years.

IRA FLATOW: Yeah, so you’ve seen some changes coming. Tell me about the changes that you have seen over the years.

JOSEPH BERRY: I mean, there are some technologies that we deploy today, like what you have on your house, that have been around for a long, long time. There are other technologies that have come up to try to do it cheaper, better, faster that have had a little bit of challenges to make it to market and maybe have receded from prominence. And it’s been kind of a cycle and churn of new technologies trying to basically break through and enter into the market.

So I’ve seen, I don’t know, half a dozen different technologies kind of come, and two of them have really stuck around, I would say.

IRA FLATOW: Two of them?

JOSEPH BERRY: Two of Them.

IRA FLATOW: Do I have to pry it out of you?

JOSEPH BERRY: Silicon and CadTel are the two big ones that are deployed at multiple-gigawatt scale. So that’s what’s on most people’s roofs, and that’s what’s in most utility-scale power plants.

IRA FLATOW: Let’s talk about solar panels, even the ones that I can talk from personal experience on my roof. They don’t last forever, do they? What’s the life expectancy.

LAURA SCHELHAS: It can vary. Warranties are increasing now, so you’re seeing warranties pushing over 30 years.

IRA FLATOW: 30 years?

LAURA SCHELHAS: Yeah, which is insane.

IRA FLATOW: That is insane.

LAURA SCHELHAS: If you think about the lifetime of a car, which sees a lot of the same stress as a solar panel, how long–

IRA FLATOW: My roof is not going to last that long.

LAURA SCHELHAS: Exactly. So it’s astonishing to me that you can create something this cheap and it sees the sun, hail, rain, hot, cold.

IRA FLATOW: And it keeps it all off of my roof too.

LAURA SCHELHAS: And it just keeps trucking. It’s kind of amazing.

IRA FLATOW: So we should expect that you should change them now, the ones on the market, every 20 years or?

LAURA SCHELHAS: It depends. So I think rooftop, probably 20, 25 years. And then with utility, it can be even longer. But with climate change, sometimes you don’t get a chance because maybe a hurricane comes through or hail. That’s kind of hard to prevent breaking the panel.

IRA FLATOW: Yeah, and I was surprised when I was doing research on this. I always thought, let’s have a south-facing roof, but they said the west-facing roof was more efficient because that’s the time you use it and need it most.

JOSEPH BERRY: I mean, you’re raising another really interesting question. One of our other colleagues at NREL developed what they call the duck curve. And if you look at energy and electricity usage throughout the day, you get these spikes at the beginning and end of the day, which is why siting something that you would have thought would be suboptimal is actually the most impactful to your utility bill.

IRA FLATOW: I know that you two work on some cutting-edge materials for solar panels. Joe, let’s talk about this thing called perovskite. What is that, and why should we be excited about that?

JOSEPH BERRY: Well, perovskite, strictly speaking, is a crystallographic structure, but if you think about any PV device or photovoltaic solar cell, it’s made from a semiconductor, and the different atoms that are in there form some structure.

And so perovskite is a particular structure, and we use it as a shorthand for what are technically metal-halide perovskites. So these metal-halide perovskites, when I started at the lab, they were not a thing.

IRA FLATOW: Not a thing.

JOSEPH BERRY: They were not a thing. They weren’t even an idea, for the most part. And they’ve come about kind of in the last, let’s say 12 to 15 years, have really become– we can make these particular devices, at least at the lab scale, at higher efficiency than any other kind of comparable system.

IRA FLATOW: Really? How efficient are we talking?

JOSEPH BERRY: So at lab scale, these can now do better than 26%.

IRA FLATOW: Wow. And we started out years ago just a few percent, right?

JOSEPH BERRY: There’s actually a story about one of our colleagues that we could share.

IRA FLATOW: Tell me. Tell me. Go ahead.

JOSEPH BERRY: Yeah, one of our colleagues was approached with this perovskite technology at one point, and the efficiency was only like 2% or 3%. And they’re like, yeah, no, we aren’t interested in that until it’s more efficient. And so it was a little bit later before some of the rest of us decided that, oh well, we will look at these.

IRA FLATOW: And then a really interesting thing about this– besides how efficient it is– is, Laura, you can actually paint this on a side of a building. It’s paintable.

LAURA SCHELHAS: Yeah. So the panels that you have on your roof are likely silicon. So that’s a wafer. You have to pull this big boule and cut it and put it together. But what you’ll see is our colleague just painting this on a conductive substrate and then the numbers changing on the voltmeter. And so that’s really the power being produced by the light we’re just shining on it during that process.

IRA FLATOW: So the crystals align themselves in the right way as you’re painting them?

LAURA SCHELHAS: They don’t have to be aligned. They just need to be crystallized. So there’s some benefit, maybe, to them aligning, but they just have to form into that crystalline matrix. But it doesn’t have to be a single crystal. It can just be kind of a hodgepodge of stuff, and you’re off to the races.

IRA FLATOW: Wow. But you’re in the silicon camp, aren’t you, still?

LAURA SCHELHAS: I am a all hands on deck. I think–

IRA FLATOW: [LAUGHS] That was no cop out here. Let’s not–

LAURA SCHELHAS: A little bit. Honestly, I think for the energy transition, we need everything. And so I think that perovskites represent a really new cool technology that’s high efficiency, and we certainly can make a lot of it, but there’s work to be done still. So yeah.

IRA FLATOW: And so you’re still in the camp that we can make silicon more efficient and work with that?

LAURA SCHELHAS: Well, the cool thing is actually when you put these two together. And so you can actually make your perovskite semi-transparent, and then you can put it right on top of your silicon and make a tandem. And so then you have the benefit of the silicon and the benefit of the perovskite, and now you’ve just made a happy solar sandwich.

JOSEPH BERRY: This is one of the things that– I mentioned that there’s technologies that have come and some that have gone. One of the reasons why the perovskites are especially interesting is this thing that Laura has just highlighted is the ability for perovskites not only to be really efficient, like we just talked about, but to really add value and overall efficiency to incumbent technologies. So we can take a silicon cell, and thermodynamics kind of says that a silicon cell can only kind be on the order of 30% efficient. We can basically exceed that by pairing a perovskite with the silicon, right?

IRA FLATOW: Whoa. So you’ve reached the limit, almost.

LAURA SCHELHAS: You’ve almost beat the limit.

JOSEPH BERRY: Well, we can go beyond what we would call the conventional detailed balance limit for a single-junction device. That’s a lot of words, but what that basically means is that we have to make compromises. If the sun was only one color, we could make an 80% efficient device really easily, but it’s not one color. There’s a full spectrum, and so that means that we have to make some compromises if we’re just going to have a single device harvest that energy.

And that’s what we do for silicon, and it can still be very efficient. But if you really want to get very, very high efficiencies, you’ve got to have multiple cells that are optimized for each of the different sections of the spectrum.

IRA FLATOW: I’ve got a question for you about that, but first, I want to tell our audience, the mics are open. If you want to get to the microphones and ask questions about solar cells, please do that.

One question I have for you is that question itself about the broad spectrum. One of the parts of the broad spectrum that we don’t see is infrared, the heat section, right? Couldn’t solar cells be working at night when the heat is still out there if you can find a way to get the solar cells to do that?

JOSEPH BERRY: So it’s a little bit of a question about power density. So we do have some colleagues who are working on what we consider to be kind of thermal PV devices, but they’re going to basically aim those devices not at just general thermal radiation that you might capture in, say, a night-vision camera. They’re going to aim it at a really hot source and essentially use that like we would use the sun, just that the sun is a little bit more of a ubiquitous source that has high-quality energy coming from it at high density.

LAURA SCHELHAS: Yeah, and if you actually take the other end of the spectrum too, UV, you can use that during the day. If you can collect the UV, it ends up being transparent, and now you can introduce things like solar windows that you can still see through.

IRA FLATOW: Wow. UV, ultraviolet, during the day, I mean, can the perovskite do that or you have to find another crystallized structure that…

JOSEPH BERRY: We’re working on systems that will, in fact, target those parts of the spectra as well. But again, in the context of solar, we’re kind of stuck with the sun we’ve got, and so we want–

IRA FLATOW: Oh, that’s awful. But there’s a whole lot of solar radiation coming on the Earth isn’t there?

JOSEPH BERRY: But when we think about different sections, if we think about how much energy is really available in some of these other parts of the spectrum, there is energy available, and we can take advantage of it, but it’s not where you get the biggest bang for your buck.

IRA FLATOW: All right, let’s see how much of bang for our buck we can get in the audience. Let’s go over here. Yes?

AUDIENCE: Thank you. I love this stuff about the solar sandwich. I have really two questions. One is price. I know silicon is very inexpensive now, but I think maybe perovskites are as well. And then I’ve heard perovskites are very unstable, and I don’t know if that’s been solved.

JOSEPH BERRY: As far as price goes, if we think about the fundamental thing, what really causes things to be expensive? It ends up being the amount of energy we put into something. And silicon, it takes a lot of energy both to purify the material and then to get it into the right structure. We have a lot more of an entitlement to essentially lower that energy expenditure to make the material to begin with.

Having said that, it’s one thing to do it or have that entitlement. It’s another thing to realize that entitlement. And yeah, we’re always trying to make everything cheaper, better, faster, but so are the people who make silicon. It’s just a question of how to get there.

LAURA SCHELHAS: Yeah, and then the stability question, I think, is a really important one. When we think of silicon, it’s silicon, and it’s a rock, and it’s really stable. Perovskites, like Joe said, is this crystal structure that can be a nonstable material.

When we first started working on it, it was. One of our colleagues tells this joke. The first time they made a little device, they had to run over to the tester to even get a measurement. Now we have these sitting outside for months and even going on a year. So in terms of the material stability, will it just stay in that structure? We’ve made a lot of progress as a field.

Field performance, reliability, and durability kind of factors in the whole module and everything else that’s going on. We’re not there yet, but it is certainly promising what we’re seeing.

JOSEPH BERRY: I mean, the one other thing I would say is that at a basic level, your photovoltaic panel is not dissimilar from your cell phone, and if you put your cell phone out on your roof for 30 years, you wouldn’t expect it to do terribly well, and that’s the challenge we’re trying to address.

At the same time, when we walk into the lab, on our research team, I’m involved in the stuff that’s easy, which is, you can walk into the lab. You can measure it. You can know its efficiency. Laura is involved in the stuff that’s really, really hard, which is, what measurement can you do in five minutes that will tell you where you’re going to be in 30 years? And that’s just kind of really demanding technically.

IRA FLATOW: Right. Let’s see what’s demanding in the audience here. Yes, go ahead.

AUDIENCE: Hi. So I learned how to drive in the ’70s, and ever since then, I figured out– I imagined electric vehicles back in those days and solar powered, specifically solar-powered electric vehicles. So this paint-on technology seems very promising to literally paint a car with this product, and it literally powers itself, either on the fly or charges the battery. So tell me what your prediction is for that.

IRA FLATOW: Yeah, could you charge the battery? I mean, getting the car to run, you’d need a lot of solar cells, but maybe trickle charging or something.

LAURA SCHELHAS: And there are certainly people looking at it. We see some folks looking at putting it on the front of the car. I think there’s even been commercial product out where they’ve used it to power just your seat heaters and stuff like that.

I think one of the challenges with getting a lot of power out of something on a car is one of the issues with solar is shade. There’s ways that you can get around it, but thin film technologies in particular do struggle with shading. And so if you have these really high-efficient things that you can paint all over the place but you’re going under shady things all the time, it may short out your device. It’s not impossible. It’s just an engineering challenge that we’d have to overcome.

IRA FLATOW: But you could paint the top of a carport in a parking lot or something.

LAURA SCHELHAS: And there are definitely carports that do this and employ silicon and things like that. I used to live in California, and they put in a new beautiful carport at the mall with these glass-glass modules. You could kind of see through them, and they were beautiful, maybe because I’m a nerd, but I thought they looked really cool.

IRA FLATOW: I’m with you on that one. Let’s go over here. Yes?

AUDIENCE: Is it possible that solar panels could use moonlight to power?

LAURA SCHELHAS: Oh, I love that question.

IRA FLATOW: See, they always ask the best questions, the kids.

LAURA SCHELHAS: And it does. It works.

IRA FLATOW: Say that again?

LAURA SCHELHAS: You can see some energy generation from moonlight in certain circumstances. So I used to work on some projects where we were doing a lot of long-term fitting of time-series data of solar panels, and we kept seeing this weird stuff at night. And my colleague, Bennett Myers at SLAC in California, figured out it was actually the moonlight that was causing a little bit of power generation. So yeah.

JOSEPH BERRY: One way to think about it is that the moon is like a big reflector. The catch is that it’s not reflecting kind of directionally at a particular target. It’s diffuse reflection, but that’s still light. And if it’s light, you can totally get it.

IRA FLATOW: My brother, who was a commercial photographer for many years, and he would actually meter a full moon. He says it has the same light as daylight on pavement on the street. That’s how intense– you don’t want to look through it with a telescope even, you know? And that’s what science is all about, finding out these answers to these questions.

Well, I’m sorry we don’t have any more time for questions, but thank you all for taking time to be with us today. Thank you, both of you. Dr. Joseph Berry, senior research scientist at NREL in Golden, and Dr. Laura Schelhas, senior scientist also at the National Renewable Energy Laboratory, thank you for dropping by and telling us all about these new advances.

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