Perovskite solar cells are cheaper to make than traditional silicon cells and the efficiency with which they can convert sunlight into electricity has been increasing rapidly in recent years. But to be commercially viable, they need to scale up from lab size.
Using a newly developed fabrication method, scientists have attained better than a 15-percent energy conversion efficiency from perovskite solar cells larger than one square centimeter in area.
Researchers had reported efficiency in perovskite cells of higher than 20 percent, which rivals traditional silicon cells. But those high efficiency ratings are only possible using cells that are a tenth of a square centimeter—fine for lab testing, but too small to be used in a solar panel.
“The use of tiny cells for efficiency testing has prompted some to question comparison of perovskite solar cells with other established photovoltaic technologies,” says Nitin Padture, professor of engineering at Brown University. “But here we have shown that it is feasible to obtain 15-percent efficiency on cells larger than a square centimeter through improved processing. This is real progress.”
Maintaining high efficiency on larger perovskite cells has proved to be a challenge, Padture says. “The problem with perovskite has been that when you try to make larger films using traditional methods, you get defects in the film that decrease efficiency.”
The fabrication process builds on a previously reported method developed by Yuanyuan Zhou, a graduate student in Padture’s lab. Perovskite precursors are dissolved in a solvent and coated onto a substrate. Then the substrate is bathed in a second solvent (called anti-solvent) that selectively grabs the precursor-solvent and whisks it away. What’s left is an ultra-smooth film of perovskite crystals.
In this new study, published in the journal Advanced Materials, Zhou and Mengjin Yang, a postdoctoral researcher at National Renewable Energy Laboratory, developed a trick to grow the perovskite crystals to a larger size. The trick is to add excess organic precursor that initially “glues” the small perovskite crystals and helps them merge into larger ones during a heat-treatment, which then bakes away the excess precursor.
“The full coverage and uniformity over a large area come from the solvent method,” Padture says. “Once we have that coverage, then we increase the size of the crystals. That gives us a film with fewer defects and higher efficiency.”
The 15-percent efficiency reached in this latest work is a good start, Padture says, but there’s still room to improve. Ultimately, researchers would like to reach 20 to 25 percent in large-area cells, and think that mark could be within reach using this method or a similar one.
The US Department of Energy and the National Science Foundation funded the work.
Source: Brown University