Swiss researchers set new flexible solar cell efficiency record

Solar energy technology may just have taken a leap forward

Swiss laboratory Empa has taken a major step in the development of solar power cells. Scientists have improved low-cost flexible cells to almost match the efficiency rates of more expensive silicon or glass cells.

 

It's being billed as a small, but significant breakthrough in harnessing renewable energy sources. On Thursday, Empa, the Swiss Federal Laboratories for Materials Science and Technology, announced that it had boosted the efficiency of flexible solar cells to a new world record of 18.7 percent – a significant improvement over the previous record of 17.6 percent (also set by Empa).

That might not sound like a huge improvement, but it potentially puts flexible solar cells on a par with silicon or glass-based cells - which are more efficient, but also heavier, bulkier and more expensive.

Efficiency is everything

Large flexible solar cells currently on the market have an efficiency of only three or four percent, according to Ayodhya Tiwari, who led the team at Empa's Laboratory for Thin Film and Photovoltaics. Silicon solar panels, meanwhile, have an efficiency of 18 to 20 percent.

Silicon requires purification before it can be used for solar panels

Flexible solar cells are made of a semi-conductor compound material called copper indium gallium (di)selenide (known as CIGS for short), which has a more complex cell structure than silicon.

Tiwari's team made the breakthrough by improving the structural properties of the CIGS layer, which reduced recombination losses, and using a polymer film rather than a metal foil as a carrier substrate for the cells.

"In the field of solar energy, the bottom line is to have a low cost of electricity that competes with conventional sources of power," Tiwari told Deutsche Welle. "It's not yet there, but flexible solar cells have the potential to give a lower cost – in manufacture, but also in transportation and installation."

Friedrich Kessler, who leads a research team at the Center for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW), said Empa's achievements were significant.

"I rate this very highly," he told Deutsche Welle. "It's a great result that really gives me hope. I've made the measurements here, and I can confirm these figures are real."

"I would say this group around Tiwari and Empa leads the field in the area of plastic-based flexible solar cells," he added. "There's no-one else in the world likely to take away their world record, which they've held for years."

More complex, more flexible

The more conventional silicon solar cells have a high manufacturing cost because they require special purification steps, while CIGS cells can be mixed into an ink and printed onto a substrate material. This makes CIGS cells thinner and more flexible.

CIGS cells have been known for over a decade, and were first manufactured commercially in 2007, but up until now it has always been accepted that, despite the cost advantages, silicon is more efficient.

But Tiwari says it's just a matter of catching up.

"Silicon technology is simply more mature than flexible solar cell technology," he said. "But the significance of the new development is to show that flexible solar cell technology can give you devices that are as efficient as the ones you get with competing technologies."

CIGS cells are lighter, so they have more uses

Like potato chip packets

Another reason why flexible cells are easier to make than silicon panels is because the technology can be adopted from other areas of mass-manufacturing.

"Food packaging has been revolutionized by roll-to-roll manufacturing," noted Tiwari. "Take a potato chips packet - if you peel it off, it's nothing but plastic coated with a metal layer produced on a roll-to-roll machine. That type of machine can be adapted to produce flexible solar cells."

The key advantage of CIGS is not just the lower manufacturing costs, but also the lighter weight.

"It looks like a carpet. You can fold it, you can roll it," said Tiwari. "And it has a few more applications - for instance if you want to install on buildings that cannot support a heavy structure."

The perils of scaling up

The next challenge for Empa is to scale up their new CIGS cells to manufacturing sizes without sacrificing too much efficiency. Generally, the bigger the surface area of solar panels, the less efficient they become.

Tiwari cannot yet say what efficiency levels they will achieve when they scale up their new CIGS cells, but previous results suggest 16 percent efficiency is achievable. That would certainly give CIGS the potential to compete with silicon solar panels.

"You have to make clear that these results have been achieved in a laboratory, not in a production plant." warned Kessler. "They have to show that this is possible in roll-to-roll production. But if they do that, it is potentially technology that could be used both in energy production and day-to-day application."

Author: Ben Knight
Editor: Cyrus Farivar