Among the new materials for solar cells, the halide perovskites are considered particularly promising. Within a few years, the efficiency of such perovskite solar cells raised from a few percents to over 25%. Unfortunately, the best perovskite solar cells contain toxic lead, which poses a hazard to the environment. However, it is surprisingly challenging to replace the lead with less toxic elements. One of the best alternatives is tin. Halogenide perovskites with tin instead of lead should show excellent optical properties, but in practice, their efficiencies are mediocre and decrease rapidly. And this rapid "aging" is their main disadvantage: the tin cations in the perovskite structure react very quickly with oxygen from the environment so that their efficiency drops.
Now, an international cooperation led by Antonio Abate, HZB, and Zhao-Kui Wang, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, China, has achieved a breakthrough that opens up a path to non-toxic perovskite-based solar cells that provides stable performance over a long period. They also use tin instead of lead but have created a two-dimensional structure by inserting organic groups within the material, which leads to so-called 2D Ruddlesden-Popper phases. "We use phenylethylammonium chloride (PEACl) as an additive to the perovskite layers. 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" explains first author Dr Meng Li. Li is a postdoc in Abate's group and has organised the close cooperation with the Chinese partners. At the Shanghai Synchrotron Radiation Facility (SSRF), they were able to precisely analyse the morphology and crystal characteristics of the perovskite films after different annealing treatments.
The best of these lead-free perovskite solar cells achieved an efficiency of 9.1% and high stability values, both under daytime conditions and in the dark. 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 oxidising. "This work paves the way for more efficient and stable lead-free perovskite solar cells," Abate is convinced.