A Brown University research team has taken an important step in improving the long-term reliability of perovskite solar cells, an emerging clean energy technology. In a study to be published on Friday, May 7 in the journal Science, the team demonstrates a “molecular glue” that prevents a key interface inside cells from breaking down. The treatment dramatically increases the stability and reliability of cells over time, while also improving the efficiency with which they convert sunlight into electricity.
“There have been great strides in increasing the energy conversion efficiency of solar cells to perovskite,” said Nitin Padture, professor of engineering at Brown University and lead author of the new research. “But the last hurdle before the technology becomes widely available is reliability – making cells that maintain their performance over time. This is one of the things my research group has been working on, and we’re happy to report some important things. progress.”
Perovskites are a class of materials with a particular crystal atomic structure. Just over ten years ago, researchers showed that perovskites absorb light very well, which sparked a flood of new research into perovskite solar cells. The efficiency of these cells has increased rapidly and now rivals that of traditional silicon cells. The difference is that perovskite light absorbers can be made at a temperature near room temperature, while silicon must be grown from a melt at a temperature approaching 2700 degrees Fahrenheit. Perovskite films are also about 400 times thinner than silicon wafers. The relative ease of manufacturing processes and the use of less material means that perovskite cells can potentially be manufactured at a fraction of the cost of silicon cells.
Although the improvements in the efficiency of perovskites have been remarkable, says Padture, making cells more stable and reliable has remained difficult. Part of the problem has to do with the layering required to make a functional cell. Each cell contains five or more distinct layers, each performing a different function in the process of generating electricity. Since these layers are made of different materials, they react differently to external forces. Also, temperature changes that occur during the manufacturing process and during service can cause some layers to expand or contract more than others. This creates mechanical stresses at the interfaces of the layers which can cause the decoupling of the layers. If interfaces are compromised, cell performance drops.
The weakest of these interfaces is the one between the perovskite film used to absorb light and the electron transport layer, which maintains current in the cell.
“A chain is only as strong as its weakest link, and we have identified this interface as the weakest part of the whole stack, where failure is most likely,” said Padture, who heads the Institute for Molecular and Nanometric Innovation at Brown. “If we can strengthen that, then we can start to make real improvements in reliability.”
To do this, Padture has drawn on his experience as a materials scientist, developing advanced ceramic coatings used in aircraft engines and other high performance applications. He and his colleagues began experimenting with compounds known as self-assembled monolayers, or SAMs.
“It’s a great class of compounds,” Padture said. “When you lay them on a surface, the molecules come together in a single layer and stand up like short hairs. Using the right formulation, you can form strong bonds between these compounds and all kinds of different surfaces.”
Padture and his team found that a formulation of SAM with a silicon atom on one side and an iodine atom on the other, could form strong bonds with both the elective transport layer (which is usually made of tin oxide) and the light absorbing layer of perovskite. . The team hoped that the bonds formed by these molecules could strengthen the interface of the layer. And they were right.
“When we introduced SAMs at the interface, we found that it increased the fracture resistance of the interface by about 50%, which means that any crack that forms at the interface tends to fail. not spread very far, ”Padture said. “So, in fact, SAMs become a kind of molecular glue that holds the two layers together.”
Solar cell function tests have shown that SAMs dramatically increase the functional lifespan of perovskite cells. The non-SAM cells prepared for the study retained 80% of their initial efficiency during approximately 700 hours of laboratory testing. Meanwhile, SAM cells were still performing well after 1330 hours of testing. Based on these experiments, the researchers predict that the 80% retained efficiency life is about 4,000 hours.
“One of the other things we did, which people don’t normally do, is we broke the cells after the tests,” said Zhenghong Dai, a doctoral student at Brown and lead author of the research. . “In the control cells without SAM, we saw all kinds of damage such as voids and cracks. But with SAMs the reinforced interfaces looked really good. It was a dramatic improvement that really shocked us.”
Importantly, said Padture, the improvement in toughness did not come at the expense of power conversion efficiency. In fact, SAMs actually slightly improved the efficiency of the cell. This happened because SAMs removed tiny molecular defects that form when the two layers bond in the absence of SAM.
“The first rule to improve the mechanical integrity of functional devices is to ‘do no harm’,” said Padture. “So that we could improve reliability without losing efficiency – and even improving efficiency – was a nice surprise.”
SAMs themselves are made from readily available compounds and are readily applied with a room temperature dip coating process. So adding SAM would potentially add little to the cost of production, Padture said.
The researchers plan to build on this success. Now that they have fortified the weakest link in the perovskite solar cell stack, they would like to move on to the next weakest, then the next and so on until they have fortified the entire stack. This work will involve strengthening not only the interfaces, but also the hardware layers themselves. Recently, Padture’s research group won a $ 1.5 million grant from the US Department of Energy to expand their research.
“This is the kind of research needed to make inexpensive, efficient, and performant cells for decades,” said Padture.