Non-Toxic and Stable Perovskite Solar Cells

Illustration of the changes in the structure of FASnI3:PEACl films during treatment at different temperatures. (Source: M. Li, HZB)

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, Helmholtz Zentrum Berlin 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 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. (Source: HZB)

Reference: M. Li et al.: Tin halide perovskite films made of highly oriented 2D crystals enable more efficient and stable lead-free perovskite solar cells, ACS Energy Lett., online 5 May 2020; DOI: 10.1021/acsenergylett.0c00782

Link: Active Materials and Interfaces for Stable Perovskite Solar Cells (A. Abate), Helmholtz Zentrum Berlin HZB, Berlin, Germany

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