Researchers at the University of Michigan and Arizona State University have examined bulky "defect pacifying" molecules as a way to increase the stability and overall lifespan of perovskite materials.
The team expects this novel way of preventing perovskite materials from degrading quickly could help enable solar cells estimated to be two to four times cheaper than today's thin-film solar panels.
Perovskite crystals contain lead atoms that aren't fully bound to the other components within the perovskite. Such "undercoordinated sites" are defects often found on the crystal surfaces and at grain boundaries where there's a break in the crystal lattice. These defects hinder the movement of electrons and speed up the decay of the perovskite material.
Scientists have already found that mixing defect pacifying molecules into the perovskites can help lock up the undercoordinated lead, in turn preventing other imperfections from forming at high temperatures. But until now, it wasn't clear exactly how a given molecule affected the hardiness of perovskite cells.
"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.
To investigate the problem, the team created three additives with a range of shapes and sizes and added them into thin films of perovskite crystals, which can absorb light and convert it to electricity. Each additive contained the same or similar chemical building blocks, which made size, weight and arrangement the main properties differentiating them.
Then, the team measured how strongly the different additives interacted with perovskites and consequently influenced the formation of defects in the films. Larger molecules by mass were better at sticking to the perovskite because they had more binding sites that interact with perovskite crystals. As a result, they tended to be better at preventing defects from forming.
The best additives also needed to take up a lot of space. Large but skinny molecules resulted in smaller perovskite grains during the manufacturing process. Smaller grains aren't ideal because they also create perovskite cells with more grain boundaries, or more areas for defects to form. In contrast, bulky molecules forced larger perovskite grains to form, which in turn reduced the density of grain boundaries in the film.
Heating the perovskite films to over 200 degrees Celsius confirmed that bulky additives helped the films retain more of their characteristic slate black color and develop fewer structural defects.
"Both the size and configuration are important when designing additives, and we believe this design philosophy could be implemented across various perovskite formulations to further improve 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.
