Perovskite cells are an alternative to the traditional silicon cells widely used to harvest the Sun’s energy. They are easier and cheaper to make and research has dramatically improved their efficiency in recent years, but until now they have lacked the same longevity as silicon cells. The new research, carried out at Rhode Island’s Brown University, demonstrates a method to significantly improve perovskite durability, open up the possibility of commercial adoption in the future.
"There have been great strides in increasing the power-conversion efficiency of perovskite solar cells," said Nitin Padture, a professor of engineering at Brown University and senior author of the study, which appears in Science. "But the final hurdle to be cleared before the technology can be widely available is reliability - making cells that maintain their performance over time.”
Perovskite cells are constructed from multiple layers that each perform a different role in electricity generation. Padture and his team identified that weakest of the interfaces between those layers is the one between the perovskite film used to absorb light and the electron transport layer, which keeps current flowing through the cell. Drawing on his materials experience developing ceramic coatings for aircraft engines, he began experimenting with compounds known as self-assembled monolayers (SAMs) to help strengthen this layer.
"A chain is only as strong as its weakest link, and we identified this interface as the weakest part of the whole stack, where failure is most likely," said Padture. "If we can strengthen that, then we can start making real improvements in reliability.
"When we introduced the SAMs to the interface, we found that it increases the fracture toughness of the interface by about 50 per cent, meaning that any cracks that form at the interface tend not to propagate very far. So in effect, the SAMs become a kind of molecular glue that holds the two layers together."
Testing of solar cell function showed that the SAMs dramatically increased the functional life of the perovskite cells. Non-SAM cells prepared for the study retained 80 per cent of initial efficiency for around 700 hours of lab testing. Meanwhile the SAM cells were still going strong after 1,330 hours of testing. Based on these experiments, the researchers project the 80 per cent-retained-efficiency life to be about 4,000 hours.
Notably, the improvement in durability did not come at the cost of efficiency. In fact, the SAMs actually improved the cell's power conversion by a small amount. This occurred because the SAMs eliminated tiny molecular defects that form when the two layers bond in the absence of SAMs.
"The first rule in improving the mechanical integrity of functional devices is 'do no harm,” Padture said. "So that we could improve reliability without losing efficiency - and even improving efficiency - was a nice surprise."