Researchers crack secret to long

“Silicon solar cells are great because they are very efficient and can last for a very long time, but the high efficiency comes with a high cost,” said Xiwen Gong, U-M assistant professor of chemical engineering. “To make high-purity silicon, temperatures over 1,000 degrees Celsius are needed. Otherwise, the efficiency won’t be as good," he added.

Perovskites are a promising alternative for solar panels as they can be produced at lower temperatures. However, they have a short lifespan because they tend to degrade when exposed to heat, moisture, and air. This limitation restricts their commercial competitiveness in the solar panel industry.

Gong’s latest study, published in Matter, suggests that bulky "defect pacifying" molecules are best for improving the stability and lifespan of perovskite solar cells. Perovskite crystals contain lead atoms that are not completely bonded to the other components within the perovskite. These "undercoordinated sites" are defects often found on the surfaces of the crystal and at the grain boundaries where there is a break in the crystal lattice. These defects impede the movement of electrons and accelerate the deterioration of the perovskite material.

However, this can be mitigated by using additives to the perovskites to prevent defects from forming at high temperatures by stabilizing the undercoordinated lead. However, the precise way each molecule affects perovskite cells' durability has remained unclear until now.

“We wanted to figure out what features on the molecules specifically improve the perovskite’s stability,” said Hongki Kim, a former postdoctoral researcher in chemical engineering and one of the study’s first authors. So, Gong's team tested three differently sized and shaped additives added to perovskite crystal films that convert light to electricity. The additives had similar chemical building blocks but differed in size, weight, and arrangement.

The team studied how the different additives interact with perovskites and affect the formation of defects in films. They found that larger molecules are better at preventing defects due to their ability to adhere to the perovskite crystals, as they had more binding sites by mass. To verify this, the team found that heating the perovskite films to over 392 degrees Fahrenheit (200 degrees Celsius) showed that bulky additives helped the films retain more of their characteristic black color and develop fewer structural defects.

Size does matter

“Both the size and configuration are important when designing additives, and we believe this design philosophy could be implemented across various perovskite formulations to improve further the lifetime of perovskite solar cells, light emitting devices, and photodetectors,” said Carlos Alejandro Figueroa Morales, a doctoral student in macromolecular science and engineering and one of the study’s first authors.

You can review the study for yourself in the journal Matter.