Protective layer of fluoride can prevent degradation of perovskite solar cells

By adding a small amount of fluoride to the production process of perovskite, solar cells based on that material are better protected. The discovery is a step in the right direction, but perovskite cells are far from being practically applicable.

Researchers from TU/e, research institute DIFFER, the University of Twente and Peking University have conducted experiments with adding fluoride to perovskite. They conclude that solar cells made on the basis of that material still retain 90 percent of their efficiency after a thousand hours under extreme light and heat conditions. That is, according to the researchers, ‘many times longer’ than traditional perovskite compounds.

Perovskite is a promising material for solar cells because it is cheap and easy to make. However, it degrades quickly, causing the efficiency of the solar cells to deteriorate rapidly. Although the addition of fluoride provides protection, it is not enough to make perovskite solar cells applicable in practice. In the solar energy industry, an efficiency retention of 85 percent is assumed after 10 to 15 years and the perovskite cells are nowhere near that.

Perovskite cells degrade under the influence of moisture, light and heat through gaps in the atomic structure of the metal halide. By adding fluoride during the production process, the fluoride ions form a protective layer around the crystal, preventing harmful defects from spreading in the material.

The team of researchers involved in the project from Eindhoven University of Technology concludes on the basis of simulations that the success is partly due to the small dimensions and high electronegativity of fluoride ions. At a high electronegativity electrons from neighboring elements are more easily attracted. The fluoride ions can therefore form a strong bond with other elements in the perovskite compound and thus form a protective layer.

Research has been going on for years into making perovskite solar cells. Despite the new invention, the researchers expect that it will take another ten to fifteen years before it becomes commercially interesting. The research is published in Nature Energy.

The atomic structure of fluoride (NaF) containing metal halide perovskite (FAPbI3). Due to the high electronegativity, the fluoride stabilizes the lattice of perovskite by means of strong hydrogen and ion bonds at the surface of the material. Illustration | TU/e