Flexible perosvkite solar cell sets efficiency record

A perovskite solar cell has set a new efficiency record for flexible thin-film photovoltaics, with an independently-verified power conversion efficiency of 24.4%, (Nat. Energy 2022, DOI: 10.1038/s41560-022-01045-2). The device is a tandem cell, and beats the previous flexible thin-film champ by about 3 percentage points.

“That’s an impressive jump in efficiency,” says Axel Palmstrom at the US National Renewable Energy Laboratory, who helped develop the flexible perovskite cell that formerly occupied the top slot (Joule 2019, DOI: 10.1016/j.joule.2019.05.009), and was not involved in the new research. “Being able to increase the efficiency and put these devices on a flexible substrate will enable lightweight photovoltaic applications.”

Today’s solar power market is dominated by rigid silicon cells. Although flexible solar cells can be shaped to fit onto wearable devices or electric vehicles, and are potentially easier to manufacture and install on buildings than conventional rigid cells, they remain a niche product because they are typically less efficient and less robust. Some researchers hope that perovskites could give flexible cells a much-needed boost.

When light strikes a cell’s perovskite layer, it excites electrons and their positively-charged counterparts, holes. These charges flow into adjacent layers to generate a current. High-performing cells efficiently separate the holes from the electrons before they can recombine, and this partly relies on having good interfaces between the layers.

Researchers increasingly use nickel oxide (NiO) nanocrystals to form the hole-transporting layer because the material is cheap, robust, and can be processed at low temperature. However, a mismatch between the perovskite and NiO energy bands has limited these cells’ performance, says Hairen Tan at Nanjing University. Meanwhile, defects around the interface can trap holes and allow them to recombine with electrons, wasting the solar energy they carry. “Managing the interfaces is really the key to get very efficient flexible devices,” Tan says.

Tan’s team overcame these problems by adding a monolayer of carbazole phosphonic acid molecules that bridge the perovskite and NiO layers. The molecules help holes to zip across the interface, and they also lower the energy band of the NiO nanocrystals closest to the perosvkite, smoothing out the troublesome energy difference.

The researchers united this souped-up cell with a second perovskite cell, producing a tandem device that comes close to the efficiency of the best rigid all-perovskite tandem cell (Nature 2022, DOI: 10.1038/s41586-021-04372-8). The bridging molecules also seem to make the cell more durable: it can withstand 10,000 bends with no loss in performance. Tan hopes that such performance improvements could help flexible solar cells find wider commercial use, and the researchers recently launched a start-up company called Renshine Solar to develop the devices further.