On the road to non-toxic and stable perovskite solar cells
The illustration shows the changes in the structure of FASnI3:PEACl films during treatment at different temperatures. © Meng Li/HZB
The promising halide perovskite materials for solar energy conversion show high efficiencies, but this comes at a cost: The best perovskite materials incorporate toxic lead which poses a hazard to the environment. To replace lead by less toxic elements is not easy since lead-free perovskites show lower stability and poor efficiencies. Now, an international collaboration has engineered a new hybrid perovskite material with promising efficiency and stability.
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.
- Long-term stability for perovskite solar cells: a big step forwardPerovskite solar cells are inexpensive to produce and generate a high amount of electric power per surface area. However, they are not yet stable enough, losing efficiency more rapidly than the silicon market standard. Now, an international team led by Prof. Dr. Antonio Abate has dramatically increased their stability by applying a novel coating to the interface between the surface of the perovskite and the top contact layer. This has even boosted efficiency to almost 27%, which represents the state-of-the-art. After 1,200 hours of continuous operation under standard illumination, no decrease in efficiency was observed. The study involved research teams from China, Italy, Switzerland and Germany and has been published in Nature Photonics.
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- Electrocatalysis with dual functionality – an overviewHybrid electrocatalysts can produce green hydrogen, for example, and valuable organic compounds simultaneously. This promises economically viable applications. However, the complex catalytic reactions involved in producing organic compounds are not yet fully understood. Modern X-ray methods at synchrotron sources such as BESSY II, enable catalyst materials and the reactions occurring on their surfaces to be analysed in real time, in situ and under real operating conditions. This provides insights that can be used for targeted optimisation. A team has now published an overview of the current state of knowledge in Nature Reviews Chemistry.