A Molecular Glue That Boosts the Efficiency of Perovskite Solar Cells

In a step toward improving the long-term reliability of perovskite solar cells, a research team at Brown University has devised a molecular glue that keeps a key interface inside cells from degrading.
In a study published in the journal Science, the researchers explained that the treatment dramatically increases cells’ stability and reliability over time while also improving the efficiency with which they convert sunlight into electricity.
Commenting on the emerging clean energy technology, Nitin Padture, a professor of engineering at Brown University and senior author of the new research, said, “There have been great strides in increasing the power-conversion efficiency of perovskite solar cells. But the final hurdle to be cleared before the technology can be widely available is reliability – making cells that maintain their performance over time. That’s one of the things my research group has been working on, and we’re happy to report some important progress.”
Padture’s research group won a $1.5 million grant from the U.S. Department of Energy to expand its work.
Perovskites are a class of materials with a specific crystalline atomic structure. When a little over a decade ago, researchers showed that perovskites are very good at absorbing light, the findings led to a flurry of research into perovskite solar cells. The efficiency of those cells has rapidly increased and is now comparable to traditional silicon cells, the researchers said. The difference is that perovskite light absorbers can be made at near-room temperature, whereas silicon needs to be grown from a melt at a temperature approaching 2,700 degrees Fahrenheit. Perovskite films are also about 400 times thinner than silicon wafers. The comparatively easier manufacturing processes and the use of less material make perovskite cells cheaper. These cells can be potentially made at a fraction of the cost of silicon cells, the experts said.
While the efficiency improvements in perovskites have been remarkable, Padture said that making the cells more stable and reliable has remained challenging. Part of the problem has to do with the layering required to make a functioning cell. Each cell contains five or more distinct layers, each performing a different function in the electricity-generation process. Since these layers are made from different materials, they respond differently to external forces, he added. Also, temperature changes that occur during the manufacturing process and service can cause some layers to expand or contract more than others. That creates mechanical stresses at the layer interfaces that can cause the layers to dissociate. If the interfaces are compromised, the performance of the cell drops.
The weakest of those interfaces is between the perovskite film used to absorb light and the electron transport layer, which continues to carry current through the cell.
“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 to take place,” said Padture, adding, “If we can strengthen that, then we can start making real improvements in reliability.”
To achieve that, Padture and his colleagues began experimenting with compounds known as self-assembled monolayers (SAMs).
“This is a large class of compounds,” Padture said, elaborating further that when these are deposited on a surface, the molecules assemble themselves in a single layer and stand up like short hairs. By using the right formulation, strong bonds between these compounds and all kinds of different surfaces could be formed, the researcher explained.
Padture and his team found that a formulation of SAM with silicon atom on one side, and iodine atom on the other, could form strong bonds with both the electron transport layer (which is usually made of tin oxide) and the perovskite light-absorbing layer. The team hoped that the bonds formed by these molecules would strengthen the layer interface. The researchers seemed to be heading in the right direction.
“When we introduced the SAMs to the interface, we found that it increases the fracture toughness of the interface by about 50%, indicating that any cracks that form at the interface tend not to propagate very far,” Padture said. “Therefore, in effect, the SAMs become a kind of molecular glue that holds the two layers together,” he elaborated.
Testing solar cell function showed that the SAMs increased the functional life of the perovskite cells to a large extent. Non-SAM cells prepared for the study retained 80% of their peak efficiency for around 700 hours of lab testing. Meanwhile, the SAM cells were still going strong after 1,300 hours of testing. Based on those experiments, the researchers project the 80% efficiency life of the SAM cells to be about 4,000 hours.
“One of the other things we did, which people don’t normally do, is breaking open the cells after testing,” said Zhenghong Dai, a Brown doctoral student and lead author of the research.
In the control cells without the SAMs, the researchers saw damages such as voids and cracks. But with the SAMs, the toughened interfaces looked unaffected, they revealed. The improvement reportedly surprised the researchers.
Padture said the improvement in toughness did not come at the cost of power-conversion efficiency. The SAMs improved the cell’s efficiency by a small amount, he added. That seemingly 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. “The fact that we could improve reliability without losing efficiency – and even improving efficiency – was a nice surprise,” he said.
The SAMs themselves are made from readily available compounds and easily applied with a dip-coating process at room temperature. So the addition of SAMs would potentially add little to the production cost, Padture said.
The researchers plan to build on this success. Having strengthened the weakest link in the perovskite solar cell stack, they’d like to move onto the next weakest link until they’ve fortified the entire stack, they said. That work will involve strengthening not only the interfaces but also the material layers.
“This is the kind of research that’s required in order to make cells that are inexpensive, efficient, and perform well for decades,” Padture said.
In a long list of research announced on perovskite solar cells, research teams from Helmholtz-Zentrum Berlin, Oxford PV, and Gwangju Institute of Science and Technology, among others, have worked to increase the efficiency of these cells.
Experts from Helmholtz-Zentrum had attained a conversion efficiency record of 29.15% in a tandem solar cell made of perovskite and silicon.
Oxford PV researchers, on their part, coated ordinary silicon solar cells with a thin film of perovskite material to better use photons across the solar spectrum. The method resulted in 29.52% efficiency.
Scientists from South Korea-based Gwangju Institute used L-alanine as an additive to perovskite materials to passivate defects and increase grains in perovskite solar cells to overcome the problem. They confirmed that solar cell efficiency increased to 20.3% from 18.3%.