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Keeping track of chlorine in perovskite stability

Scientists in Japan delved deep into the crystalline structure of a perovskite solar cell in order to understand how chlorine helps to improve the cells’ stability. By varying the concentration of chlorine in the material they were able to find an optimal level for increased stability, and open new doors on the way to understanding the specific role of chlorine and the mechanism behind the improvement.

As our understanding of perovskite solar cells improves, scientists are looking deeper and deeper into the material to observe the structure, the way electrons flow through it, and any obstacles limiting the performance.

Defects in the material, particularly at the boundaries between crystal grains, have been shown to cause problems, causing the structure to quickly degrade when exposed to UV light or moisture. Among the many approaches suggested to solve this, doping the cells with chlorine has shown significant improvements.

However, scientists have struggled to explain why chlorine had these effects, or whether it even remained present in the material after deposition. This was the starting point for a group of scientists led by the Okinawa Institute of Science and Technology (OIST). “The research community had no idea why they were seeing these improvements,” explained OIST scientist Afshan Jamshaid. “Once added, the researchers couldn’t track the chlorine – they couldn’t tell whether the chlorine was incorporated deep into the perovskite material, stayed at the surface or even left the material during the manufacturing process.”

Confirming chlorine

Aiming to provide a definitive answer, Jamshaid and the group set out to fabricate chlorine doped perovskite solar films and pry into their structure right down to the atomic level, to confirm the presence or absence of chlorine. Using an imaging technique called scanning tunnelling microscopy, they observed dark depressions on the surface of the films, which were not present in samples without the chlorine doping.

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Dark spots on the film’s surface were confirmed to be chlorine atoms.
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Darker spots on the film’s surface were confirmed to be chlorine atoms.Image: OIST

They were then able to calculate (with the help of collaborators at China’s Soochow University) that these dark patches were spots where chlorine had replaced iodine in the perovskite structure. And since more chlorine appeared to be present at the grain boundaries, they now suspect that chlorine serves to reduce the number of surface defects in the material, increasing its stability. Their work is described in full in the paper Atomic-scale insight into the enhanced surface stability of methylammonium lead iodide perovskite by controlled deposition of lead chloride, published in Energy & Environmental Science.

Concentration

The group also trialled different deposition times, varying the final concentration of chlorine in the material. Through this, they are able to claim that 14.8% represents the ideal surface concentration of chlorine to boost stability. Any lower, and the improvements would be less pronounced, any higher and the chlorine will start to drastically alter other properties of the perovskite.

They plan to continue investigating how chlorine behaves as a dopant for perovskites, with the next step to create a fully operational solar cell based on a chlorine doped perovskite layer. “…fundamental studies like these are so important – they help device engineers pin down the most optimal manufacturing process without as much trial and error,” said Jamshaid. “By understanding how the dopants improve the material, it can also guide us towards new chemical mixes that might work even better.”

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Source: pv magazine