Developing Next-Gen Solar Cells

Andrew Corselli

The solar energy world is ready for a revolution. Scientists are racing to develop a new type of solar cell using materials that can convert electricity more efficiently than today’s panels.

In a paper published in the journal Nature Energy, a CU Boulder researcher and his international collaborators  unveiled an innovative method to manufacture the new solar cells, known as perovskite cells, an achievement critical for the commercialization of what many consider the next generation of solar technology.

Today, nearly all solar panels are made from silicon, which boast an efficiency of 22 percent. This means silicon panels can only convert about one-fifth of the sun’s energy into electricity, because the material absorbs only a limited proportion of sunlight’s wavelengths. Producing silicon is also expensive and energy intensive.

Enter perovskite. The synthetic semiconducting material has the potential to convert substantially more solar power than silicon at a lower production cost.

“Perovskites might be a game changer,” said Michael McGehee, Professor in the Department of Chemical and Biological Engineering and Fellow with CU Boulder’s Renewable & Sustainable Energy Institute.

Scientists have been testing perovskite solar cells by stacking them on top of traditional silicon cells to make tandem cells. Layering the two materials, each absorbing a different part of the sun’s spectrum, can potentially increase the panels’ efficiency by over 50 percent.

“We're still seeing rapid electrification, with more cars running off electricity. We’re hoping to retire more coal plants and eventually get rid of natural gas plants,” said McGehee. “If you believe that we're going to have a fully renewable future, then you're planning for the wind and solar markets to expand by at least five to ten- fold from where it is today.”

To get there, he said, the industry must improve the efficiency of solar cells.

But a major challenge in making them from perovskite at a commercial scale is the process of coating the semiconductor onto the glass plates which are the building blocks of panels. Currently, the coating process has to take place in a small box filled with non-reactive gas, such as nitrogen, to prevent the perovskites from reacting with oxygen, which decreases their performance.

McGehee and his collaborators set off to find a way to prevent that damaging reaction with the air. They found that adding dimethylammonium formate, or DMAFo, to the perovskite solution before coating could prevent the materials from oxidizing. This discovery enables coating to take place outside the small box, in ambient air. Experiments showed that perovskite cells made with the DMAFo additive can achieve an efficiency of nearly 25 percent on their own, comparable to the current efficiency record for perovskite cells of 26 percent. The additive also improved the cells’ stability.

Here is an exclusive Tech Briefs interview, edited for length and clarity, with McGehee.

Tech Briefs: What was the biggest technical challenge you faced while adding DMAFos?

McGehee: The challenge was making these solar cells in air. That's the challenge that has existed for years, and the dimethylammonium formate is the solution to that. Formate is a reducing agent, and it can keep the iodide from oxidizing. Then what also had to be figured out is that formate is an anion, so there has to be a cation that goes with it to balance out the charge, and you don't want to modify the perovskite itself; you don't want the cation to go into the crystalline structure. Dimethylammonium was a good choice because it's fairly large and it doesn't go into the perovskite crystal structure. That was a bit of the thought process. First, we need a reducing agent, formate is a good choice, and then we need a large cation to pair with it.

Tech Briefs: Can you explain in simple terms how the team built the solar cells?

McGehee: You take the salt solutions that are the precursor for the perovskite, and when you spincast it, you get a thin film and the solvent evaporates. When there's nothing left but the salt, the perovskite crystal structure forms. That's generally how perovskite solar cells are made. There are other layers as well, but that layer gets all the attention.

And part of the story here is that the overwhelming majority of researchers would do that in a glove box filled with nitrogen. So, picture this box that's almost two-meters wide and two- to three-feet deep, and it's got these big rubber gloves, and you're pumping nitrogen through it, and you're filtering out any trace amounts of oxygen and water, and you stick your hands into these gloves, and you do all your work in there. It's not too bad at a research stage, but when you go to make products in a factory, it's just that it's costly if you have to do all these steps in that atmosphere. So, if you can have it not be sensitive to oxygen and water, and you can just do it in air, it makes the research more convenient, and it certainly makes manufacturing less expensive.

Maybe another aspect of this is if you make a solution and it only lasts for hours before some chemistry in the bile starts changing the precursor, that's just a real inconvenience to have to constantly make fresh solution. And you're going to waste a lot. It’s better to have a good shelf life. In some cases, chemical companies would love to sell you bottles of solution with the salt already dissolved. And for them to be able to do that, it must have a great shelf life. The have to be able to ship it to you, and it has to last for weeks. This additive really improves shelf life, which is great from a manufacturing point of view.

Tech Briefs: The article I read says, “The team is actively developing tandem cells with a real-world efficiency of over 30 percent that have the same operational lifetime as silicon panels.” Do you have any updates you can share about this?

McGehee: That's something that we started about a decade ago, and it's progressed tremendously. Right now, the world record for a perovskite silicon tandem is 33.9 percent efficiency. For reference, silicon by itself is 26.9 percent, so that's adding seven efficiency points to the record. That's a product that's well over a $30 billion-per-year product — just the silicon by itself. So, one way to think of this is putting perovskite cells on top is a major boost in performance for the cells. That part is going great; it’s not a product yet.

The leading company is probably Oxford PV; their R&D is in Oxford, England, while the factory is in Brandenburg, Germany. They can cover full-size silicon wafers; they have made full-size panels. Swift Solar in the U.S. does this as well — full disclosure, I’m an advisor to them. They’re not quite as far along.

If you’re wondering why there are no products yet, they have the efficiency and have passed the stability tests, but they’re getting their pilot lines working well. However, there’s still bugs to work out, but this technology is pretty far along and looking exciting.

Tech Briefs: The article also says that the study brings solar cells one step closer to commercialization. How close would you estimate we are?

McGehee: I think Oxford PV could put something out on a limited scale in a year or two. Of course, things don't go from nothing to a gigawatt in one year. They already are testing some panels outside.

It wouldn't be surprising if they did some 20-, 30-, 40-kilowatt installations in the next year. I think we'll start to see products in a small scale in one to three years.

Tech Briefs: Do you have any advice for engineers or researchers aiming to bring their ideas to fruition, broadly speaking?

McGehee: Understand what you're good at and what you can do and where you need help, and don't hesitate to get that help. Be sure to collaborate with good people. Things aren't done these days by individuals — they're done by teams.

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