The results could allow engineers to systematically identify the most effective molecules to extend the life of perovskite solar cells and move away from relying on time-consuming trial and error methods.

A discovery at the University of Michigan provides important insight into preventing rapid degradation of perovskite semiconductors. The technology could potentially lead to the production of solar cells that are two to four times cheaper than existing thin-film solar panels.

Perovskites can also be combined with the silicon-based semiconductors that dominate today’s solar cells to create “tandem” solar cells that can exceed the maximum theoretical efficiency of silicon solar cells.

“Silicon solar cells are great because they are very efficient and can run for a very long time, but high efficiency comes at a high cost,” said Xiwen Gong, an associate professor in the university’s chemical engineering department. “To make high-purity silicon requires temperatures over 1,000 degrees Celsius. Otherwise, the efficiency won’t be that good.”

Problems with perovskite solar cells

Higher temperature is accompanied by higher economic and environmental costs. However, although perovskites can be produced at lower temperatures, they break down when exposed to heat, moisture and air. As a result, the lifetime of perovskite is currently too short to be commercially competitive in solar panels.

Gong’s research aims to create more powerful perovskite solar cells, and his latest work, published in the journal Matter, shows that bulk molecules with “silent defects” best increase perovskite stability and overall lifetime.

Understanding Perovskite Defects

Perovskite crystals contain lead atoms that are not fully bonded to other perovskite components. Such “uncoordinated domains” are defects frequently found on crystal surfaces and at grain boundaries where there is a break in the crystal lattice. These defects hinder the movement of electrons and accelerate the breakdown of the perovskite material.

Engineers already know that mixing defect-corrupting molecules into perovskites can help lock poorly coordinated lead, thus preventing other defects from forming at high temperatures. But until now, engineers didn’t know exactly how this molecule affects the stability of perovskite cells.

“We wanted to find out which molecular features specifically improved the stability of the perovskite,” said Hongki Kim, a former doctoral student in chemical engineering and one of the study’s first authors.

Research on perovskite additives

To investigate the problem, Gong’s team created three dopants of different shapes and sizes and added them to thin films of perovskite crystals that can absorb light and convert it into electricity. Each supplement contained the same or similar chemical building blocks; this made size, weight and location the main distinguishing features.

The team then measured how strongly different additives interacted with the perovskites, which in turn affected the formation of defects in the films. Larger molecules adhere better to perovskite because they have more binding sites that interact with perovskite crystals. As a result, they tended to be better at preventing defects from occurring.

But the best supplements also have to cover a lot of space. Large but thin molecules resulted in smaller perovskite grains during the fabrication process. Smaller grains are not ideal because they also create perovskite cells with more grain boundaries or more space for defects to form. On the contrary, bulky molecules forced the formation of larger perovskite grains, which reduced the density of grain boundaries in the film.

Effect of additional size and shape

Heating perovskite films to temperatures exceeding 200 degrees Celsius confirmed that bulk additives helped the films retain more of their characteristic slate black color and create fewer structural defects.

“Both size and configuration are important in additive design, and we believe that this design philosophy can be applied to a variety of perovskite compositions to further extend the lifetime of perovskite solar cells, light-emitting devices, and photodetectors,” said Carlos Alejandro Figueroa Morales. PhD student. student