Solar power still needs to get much cheaper. Are perovskites the answer?

In the future, solar power won't just come from bulky blue panels on rooftops. The solar panels of tomorrow will be transparent, lightweight, flexible, and ultra-efficient. We'll be able to coat shingles or skylights or windows with them — and it'll all be as cheap as putting up wallpaper.

And these solar cells will be made from a new material called perovskite.

At least, that's the vision sketched out in this fascinating Scientific American essay by three solar researchers: Varun Sivaram, Samuel Stranks, and Henry Snaith. They note that scientists are doing incredible things with perovskite solar cells in the lab, achieving huge leaps in performance very rapidly. In theory, perovskites should be able to vastly surpass silicon, the material we currently use for solar panels — and, potentially, make solar power much, much cheaper.

"Many of us believe this is the field's biggest breakthrough since the invention of the solar cell"

"Time will tell," Sivaram has written elsewhere of perovskites, "but many of us believe this is the field’s biggest breakthrough since the original invention of the solar cell sixty years ago."

That line piqued my interest, so I called up Sivaram, now the Douglas Dillon Fellow at the Council on Foreign Relations, to chat about perovskites. What he stressed, however, was that there are still huge, daunting hurdles to overcome. Perovskites perform spectacularly well in laboratory settings, but they still degrade when they come in contact with moisture and aren't yet durable enough to be deployed in the real world.

And here's the hitch: Most academics aren't terribly interested in mundane questions like improving durability. Meanwhile, the US solar industry is still focused on the boom in old-fashioned silicon solar panels — and ignoring perovskites for now. Perovskites could be a potentially revolutionary technology, but they're having real trouble making the leap from lab to private sector.

In our talk, Sivaram elaborated on why existing solar technology isn't good enough, why we need major advances, and what could prevent perovskites from actually panning out. The end result was a fascinating portrait of how technological innovation happens — and how potentially sweeping energy breakthroughs could fail to catch on.

Brad Plumer: Right now, silicon solar panels are the technology of choice for solar power, and they're proliferating rapidly. So why isn't that enough? Why do we need a new breakthrough?

Varun Sivaram: I think solar today is getting tantalizingly close to what people think of as "grid parity" — it’s getting down to the costs that everyone once thought would allow it to compete with natural gas and coal and nuclear.

But it turns out that's not enough. Once solar really starts displacing fossil fuels, that cost target ends up moving downward. That's what MIT's big "Future of Solar" report concluded. When solar starts to be deployed en masse, it starts competing with itself, particularly during sunny times, and drives its marginal value down. So the cost target for unsubsidized solar to be competitive moves downward, from 8 cents per kilowatt hour down to 6 to 4 cents per kilowatt-hour... [Note: For more on this, read "The economic limitations of wind and solar power."]

So if you really want to deploy solar power at scale, and account for the cost of storage technologies that will help dispatch power when it's most needed [and, crucially, store electricity for when the sun isn't shining], then solar panels are going to have to become as cheap as laying carpet or painting your wall.

Silicon solar panels are booming, but they may not be enough. (Shutterstock)

BP: Is there reason to think existing silicon solar technology won't get us down to that cost target?

VS: There is, despite the impressive declines in cost to date through greater manufacturing experience. Silicon solar panels look the way they do — heavy, rigid, often kind of ugly — because of how they're made. Silicon goes through some pretty intensive processing, at temperatures over 1,500°F, in order to make a pretty brittle wafer. And then you take that wafer, and you package it up into a solar cell, and you need to protect it with glass. So you can’t do much with the aesthetics of what that panel looks like or how much it weighs because of all this protection.

But in order to have this wallpaper-like cheapness, you'd need a solar technology that’s much lighter, that you can produce on a roll-to-roll process, rather than one solar panel at a time. You'd need solar that could be placed on roof shingles, or on windows, or coating literally any surface you can think of. That’s what’s going to make solar truly cheap enough to be deployed at scale and displace fossil fuels.

BP: In this Scientific American essay, you discuss solar perovskites as a technology that might bring those costs down dramatically. Now, I see press releases all the time for over-hyped clean tech research that never pans out. What makes perovskites any different?

VS: From a scientific point of view, this is a bigger deal than many, if not all, of the breakthroughs in solar we’ve seen in the past. Full stop.

There's a reason this can be confusing for folks outside the scientific community. The bar you need to clear to get a press release is, say, a single Nature or Science article. But in the past three years, there have been over 50 articles on perovskites in Science and the Nature family of journals. It has taken our community by storm.

The reason perovskites are such a big deal is that they shatter this long-standing tradeoff between performance and efficiency on the one hand and all these other things like cost, flexibility, weight, transparency, on the other. In the past, we've always had a fixed budget: You couldn't get any of these cool add-ons without compromising on efficiency. But perovskites are the first technology where you don't have this trade-off. There’s no fundamental reason why you couldn’t make super cheap, flexible, lightweight, and highly efficient perovskite solar cells.

That's shocking to scientists. We believe the challenges, at this point, lie in product development rather than fundamental science — because, theoretically, this is an almost optimal technology on several dimensions.

Dennis Schroeder/NREL

The band gap of perovskites can be adjusted by changing their compositions to access different parts of the sun's spectrum. (Dennis Schroeder/NREL)

BP: Optimal how? You could get higher efficiency solar cells than with silicon?

VS: It’s definitely the case that you could get a higher efficiency than with silicon. The silicon solar panel is governed by an efficiency ceiling, called a thermodynamic limit, and it sits right around 30 percent.

Perovskite solar is cool because you can stack multiple perovskite layers on top of each other, each one slightly different and harnessing a different part of the solar spectrum, different colors of light. You can even stack a perovskite right on top of a silicon solar cell. By doing that, you can more efficiently turn sunlight into electricity. Theoretically, we should be able to achieve efficiencies above 30 percent for the silicon/perovskite combination or 40 percent even for a stack of several perovskite layers. Whereas the maximum efficiency achieved by silicon solar cells in the lab has topped out at about 25 percent for the last couple decades.

In the lab, we've seen efficiencies for perovskites rise to 20 percent faster than any other technology. It’s shocking.

Certified solar cell record efficiencies for silicon and perovskite technologies (date axis truncated to better show perovskite efficiency trajectory—silicon solar cells were invented in 1954; data from National Renewable Energy Laboratory) (

That said, I want to be careful not to come across as absolutely confident that perovskites will succeed. There are still serious challenges that need to be surmounted.

BP: Right. You mentioned there are still big hurdles in taking perovskites from the lab into the real world. Can you talk about those?

VS: There are two major challenges that we’ve identified. Perhaps the biggest challenge is stability. [Perovskite cells can degrade rapidly, particularly if exposed to moisture.] Silicon solar panels may be an old technology, but they're famously reliable. They have warranties for 25 years and many last much longer than that. But if a perovskite cell is made in the lab and degrades in a couple hours, no investor is going to take it seriously.

Some scientists, including those in my lab in Oxford, are looking at how to improve this stability. Initial studies suggest that if you seal the perovskite from moisture, it won’t degrade. But a whole lot of testing needs to be done on that. And, unfortunately, that’s not the focus of perovskite research in academia.

The second barrier that seems a little more surmountable is toxicity. One of the constituent atoms of perovskites is lead. So any perovskite solar cells would have to undergo safety testing to prove that the lead can’t get out.

A big problem I've identified is that academics are mostly focused on the physics of the technology, but not mundane things like real-world usage and effective lifetime. The currency of academia are the really prestigious journal articles. But you get those by proving the highest efficiency for a cell under a perfectly simulated solar noon for about two seconds. You don't have to pay attention to cost-effectiveness or life-cycle use — the things that are important for commercialization.

On the other hand, there isn’t strong industrial research going on into how to make perovskites usable for real-world applications. And that's really dangerous when you've got academia focused on intellectually interesting but practically limited questions.

A perovskite solar cell made of tin. (University of Oxford Press/Flickr)

BP: Why isn’t there more industry research into perovskites? What accounts for this disconnect?

VS: First of all, the large solar companies are pretty much focused on silicon. Only a couple are branching out into non-silicon technologies.

Second, you’d expect this gap to be filled by startups and earlier-stage companies with a different risk profile. But in solar, unfortunately, startups and venture capitalists were burned in the early 2010s, when a slew of solar companies went belly-up because of market forces outside of their control — including, arguably, Chinese government support for silicon solar-panel manufacturers. So there’s no appetite right now for Silicon Valley VCs to go invest in a new solar technology.

Third, public support for these advanced, innovative technologies is unfortunately being replaced by support for mature technologies. The Department of Energy spends more on existing silicon technology and its accessories than on emerging materials like perovskites, both in basic lab research and in loan guarantees for demonstration support. All of these reasons bias the field against new technologies like perovskites.

BP: We often hear that fossil-fuel companies are standing in the way of clean energy. But it seems like there's a risk that first-generation clean tech can become so established that it stands in the way of next-generation technologies, no?

VS: Yes, first-generation clean energy technologies are standing in the way of the second-generation. They’re acting as barriers not bridges.

We’ve seen this in the past with nuclear power. By historical accident, we became locked into a light-water reactor design that's arguably more expensive, performs less well, and is less safe than advanced reactor designs known as Generation IV designs. And there's a danger of those being locked out because of the dominance of light-water reactors and the way our regulatory structure preferences those.

Solar is an example in the present. As silicon-focused companies scale up their manufacturing operations and create economies of scale, it becomes even more difficult for smaller competitors without scale to enter the market — even if they have a technology that, down the road, could have far superior cost and performance.

An example in the future might be batteries. Lithium-ion batteries, just by virtue of being scaled up in the near future, could end up locking out other battery technologies with higher energy densities that could provide a more compelling alternative to the internal combustion engine or work in tandem with renewable energy to reliably supply the electricity grid.

This doesn’t have to be the case. First-generation technologies could be a bridge, not a barrier. For instance, there’s been so much financial innovation in the solar sector, around things like solar leasing, that could help prime the market for new technologies down the road. So it’s a fine line to cross between being a bridge and a barrier, and I think we’re on the wrong side of that line.

BP: So how do we change that? How do we make sure we're not locking out more advanced clean-energy technologies?

VS: I think there are things that will need to happen in both the private and the public sectors.

In the private sector, I think the current utility business model where we have given these monopolies to regulated entities has been particularly stifling for innovation and product development. I draw a comparison to LEDs. With lighting, we originally had a mediocre technology called the compact fluorescent lamp that got usurped by superior LEDs. So why weren't LEDs locked out by CFLs? The answer I usually come to is that innovation and the diffusion of innovation is easier if you have a robust consumer market—LEDs had several "stepping-stone" markets, leading them from consumer electronics and displays to mass market lighting.

Solar doesn't have that yet—the ultimate consumer of solar, the utility, is a monopsonist (a monopoly buyer in a market). However, I believe that utility reform — particularly what they're doing up in New York, with Reforming the Energy Vision, making utilities respond to market incentives for distributed energy resources — could spur innovation in clean tech. [Full disclosure: Sivaram is working in an advisory capacity with the governor's office on New York's reforms.]

There are other things, too. I ask professionals in the semiconductor industry, "How do we emulate what you guys did?" (After all, efficiency improvements in silicon solar panels have lagged Moore’s Law improvements in computing power by a factor of over a billion.) Those guys will say, just wait, if you wait a little while, there will be conferences that bring industry and the labs together at every stage of development from making a basic solar cell to integrating it into a product. Just wait.

I’m not convinced we have the time to wait, though. So I also imagine that a more obvious public role is necessary.

BP: So what sorts of public policies would help bridge the gap between early clean tech and more advanced clean tech?

VS: You'd want to do two things: avoid reinforcing the barrier and enable the bridge.

So on not reinforcing the barrier: one thing is to be careful about thinking certain policies are technology-neutral when they’re not. Policies like renewable portfolio standards or carbon taxes might sound technology-neutral, but they tend to preference mature technologies that are already on the market.

Then there's enabling the bridge, which can be done by addressing the two valleys of death for new technologies. The first valley of death is basic research and protoyping. Public support for R&D is just a slam dunk. The other is demonstration support, which is crucial. The government fills the gap where private investors are too risk-averse — from scaling the technology up from a prototype to pilot scale to factory scale. And that involves making some pretty large and risky investments.

We did have a loan-guarantee model at the Department of Energy that did some of this, but that program was widely criticized. Ironically, it was criticized for two different reasons. Republicans criticized it because a $500 million loan to Solyndra went bad. But those of us who want innovative technologies to succeed criticized it because 80 percent of the loan guarantee money actually went to mature technologies, or tech that weren’t fundamentally disruptive.

The "valley of death" for energy technology comes right around the commercialization stage. (Sandia National Laboratory)

But I think there’s a lot of room to innovate on this model of public support for demonstration technologies at scale. Perhaps it won’t look like loans. Perhaps it would involve an equity investment made by the government. Or perhaps it wouldn't give out $500 million chunks of money but would give out smaller, $50-100 million chunks to diversify your risk. Still, one way or another, there has to be a way to fill the gap where private investors won’t take on that risk. That’s crucial.

BP: Going back to something you said earlier, it seems like we'd also want to find ways to bridge the gap between industry and academia — so that interesting research in the lab is getting commercialized. Because the links between the two won't necessarily emerge automatically.

VS: Totally. There has to be an intricate set of conferences and organizations that link academia and industry every step of the way. Or that enable different companies to collaborate with each other on shared interests, the same way that SEMATECH [a partnership between the US government and various semiconductor manufacturers] was used for the semiconductor industry in the 1990s.

So these are arguments in favor of a hands-on public approach that carefully guides industry development. But I do want to caution by saying that oftentimes these processes take unexpected turns. And I bet if you asked many of the folks in successful industries, they’d be wary of excessive government involvement. So there’s a balance to be struck. Again, you have to be careful that public policy is reinforcing that bridge, not the barriers.

Further reading

-- Here's some of Sivaram's work on why there's no Moore's law for solar, why the world needs post-silicon solar technologies, and what perovskites need to do to escape the confines of academia.

-- A solar future isn't just likely — it's inevitable

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