‘Tandem solar cell’ could slash industry costs

Source: By Parker M. Shea, E&E News reporter • Posted: Monday, March 9, 2020

Researchers have created a “tandem solar cell” that they say could produce more power than existing solar technology and slash costs for industry.

The technology — a blend of an emerging solar technology called perovskites and traditional silicon solar cells — has a 25.7% efficiency rate and can withstand high temperatures without affecting performance, according to a paper published in Science last week. For comparison, solar company SunPower Corp. claims to have the most efficient traditional photovoltaic cell on the market today at a little over 22% efficiency. The new cell is groundbreaking, researchers said, because it is both cheaper to produce and longer-lasting than others of its kind.

“Adding a layer of perovskite crystals on top of textured silicon to create a tandem solar cell is a great way to enhance its performance,” said Yi Hou, a University of Toronto postdoctoral researcher and first author on the paper.

Although researchers said the stability of the cell has a ways to go before it’s commercially viable, this is the best stability of a perovskite-solar tandem cell reported so far, according to Hou.

“It’s ramping up so fast. This never happened before,” added Chuanxiao Xiao, a researcher at the Department of Energy’s National Renewable Energy Laboratory and study co-author. NREL has been a leader in experimental solar cell technology and has tracked the rate of improvement of different solar technologies over time.

According to the International Renewable Energy Agency, perovskite cells “have the potential to change the dynamics and economics of solar power because they are cheaper to produce than solar cells and can be produced at relatively low temperatures, unlike silicon.” There’s also a limit on how much more efficient silicon can be on its own, researchers said. Yet perovskites have faced stability and manufacturing challenges, raising interest in the idea of a blend with existing silicon cells.

Researchers have known for years that adding a thin layer of solution-processed perovskite on top of the textured silicon wafer in a traditional cell can greatly improve the cell’s efficiency. But that approach came with two problems.

First, today’s solar cell industry doesn’t manufacture smooth silicon wafers, researchers said. The surfaces of most commercial wafers instead are textured, and polishing the silicon just to add perovskite would be prohibitively expensive for producers. Second, perovskite-silicon tandem cells tend to be unstable when using the materials that most researchers have been working with.

The researchers say their new method comes close to solving both problems by applying a thick enough layer of perovskite to cover all the peaks and valleys on a silicon wafer. The result is a PV cell that operates with 25.7% efficiency and is stable, confirmed by third-party experts at the Fraunhofer Institute for Solar Energy in Freiburg, Germany, according to a press release from the University of Toronto. The technology allows a “drop-in solution” for existing silicon cells so their manufacturing process doesn’t have to be revamped, according to Hou.

Although the cell was also able to withstand 85 degrees Celsius (185 degrees Fahrenheit) for up to 400 hours without affecting performance, the researchers are aiming to get the cell to last at that temperature for at least 1,000 hours, a benchmark for today’s commercial cells.

“The perovskite may degrade faster than the silicon,” Xiao said, adding that despite the new cell being a huge advance, “it may still have a long way to go on stability.”

Thomas White, a professor at the Australian National University who works on cutting-edge perovskite-silicon technology, similarly told E&E News earlier this year that the last big challenge is making the cells last (Climatewire, Jan. 13).

NREL notes that perovskite-silicon tandem cells have reached up to 29.1% conversion efficiency just since 2018, making them the most rapidly improving experimental PV cell technology.

According to researchers, the theoretical limit for a silicon-only PV cell — as efficient as any could ever be — is about 29%, although none has approached that. For perovskite cells, efficiency could be as high as 35%, and they could also be much cheaper.

Chris Case is chief technology officer of Oxford PV, a company developing a commercial perovskite-silicon tandem solar cell. Oxford PV ran a successful pilot for its products in Germany and is now building a 125-megawatt factory to commercially manufacture its perovskite-silicon cells.

“What [Thursday’s paper] confirms is something the academics and the research community has understood … that this material wants to sort of crystallize and be a solar cell. It almost doesn’t care how you mix and produce it,” Case said.

“All the companies that have come along in the past 10 years have failed, because they’ve gone head to head against silicon, which is 95% of the market,” Case said. By contrast, perovskite-silicon tandem cells don’t directly compete but complement the silicon technology already out there, he said.

Last month, NREL announced an answer to what it called “one of the most vexing, last-mile challenges” to bringing perovskite-silicon cells to the commercial market. With that breakthrough, NREL researchers discovered a cheap way to use “off-the-shelf” materials to stop toxic lead from leaking out of the cells, patching up what would have been a major environmental problem posed by the technology (Energywire, Feb. 21).

Reporter Christa Marshall contributed.