This illustration shows the changes in the structure of FASnI3:PEACl films during treatment at different temperatures. Image: HZB/Meng Li.
This illustration shows the changes in the structure of FASnI3:PEACl films during treatment at different temperatures. Image: HZB/Meng Li.

Among new materials for solar cells, halide perovskites are considered particularly promising. Within a few years, the efficiency of perovskite solar cells has risen from just a few percent to over 25%. Unfortunately, the best perovskite solar cells contain toxic lead, which poses a hazard to the environment, but replacing the lead with less toxic elements has proved surprisingly challenging.

One of the best alternatives is tin. Halogenide perovskites with tin instead of lead should show excellent optical properties, but in practice their efficiencies decrease rapidly. This rapid ‘aging’ is the main disadvantage of tin-based perovskites and is due to tin cations in the perovskite structure reacting very quickly with oxygen in the environment, causing their efficiency to drop.

Now, an international team of scientists, led by Antonio Abate at Helmholtz-Zentrum Berlin in Germany and Zhao-Kui Wang at the Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, in China, has achieved a breakthrough that opens up a path to non-toxic perovskite-based solar cells that provide stable performance over a long period. The team reports its breakthrough in a paper in ACS Energy Letters.

Also utilizing tin rather than lead, the scientists have created a two-dimensional (2D) structure by inserting organic groups within the perovskite material FASnI3, which leads to so-called 2D Ruddlesden-Popper phases.

"We use phenylethylammonium chloride (PEACl) as an additive to the perovskite layers," explains Meng Li, a postdoc in Abate's group and first author of the paper. "Then we carry out a heat treatment while the PEACl molecules migrate into the perovskite layer. This results in vertically ordered stacks of two-dimensional perovskite crystals."

At the Shanghai Synchrotron Radiation Facility (SSRF), the scientists were able to precisely analyze the morphology and crystal characteristics of the perovskite films after different annealing treatments. This revealed that the PEACl molecules accumulate between the crystalline perovskite layers as a result of the heat treatment and form a barrier that prevents the tin cations from oxidizing.

In tests, the best of these lead-free perovskite solar cells were able to convert light to electricity with an efficiency of 9.1 % and achieve high stability values, both under daytime conditions and in the dark. "This work paves the way for more efficient and stable lead-free perovskite solar cells," says Abate.

This story is adapted from material from Helmholtz-Zentrum Berlin, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.