The findings of a study conducted at Imperial College London (ICL) in the UK could help researchers to develop longer-lasting solar cells based on organic lead halide perovskites.
The work is a collaboration between the groups of Dr Saif Haque, a reader in materials chemistry in the Department of Chemistry at ICL, and Professor Saiful Islam at the University of Bath (also in the UK). The researchers report to have identified the underlying mechanism that causes dramatic drops in the efficiency of perovskite cells in natural conditions, in some cases within a mere few days. Previously, ICL chemists had already reported that so-called “superoxides” break down perovskite materials. “Superoxide degrades the methyl ammonium lead triiodide by deprotonating the methyl ammonium cation. So it is important to minimize presence of superoxide,” Haque notes. Now, the researcher and his collaborators have discovered how those compounds form and how they attack the perovskite material.
The importance of crystallite size
“The paper reports the influence of crystallite size on the stability of methyl ammonium lead triiodide films when exposed to oxygen — films with larger crystal sizes are found to be more stable,” says Haque, referencing the Nature Communications article “Fast oxygen diffusion and iodide defects mediate oxygen-induced degradation of perovskite solar cells.” The paper, which he co-authored, also reports on the role of iodide vacancies or defects in mediating the formation of superoxide from oxygen.
Haque’s team found that superoxides form in spaces in the perovskite that are normally taken up by molecules of iodide. Their press release states that although iodide is a component of the perovskite material itself, there are defects where iodide is missing. Superoxides then wrangle into these vacant spots.
Passivation with iodide salts could improve stability
Contributing to a deeper understanding of the inner workings of perovskite solar cells, the findings could help to design and manufacture more efficient, longer-lasting devices in the future. Haque and his team found that dosing the fabricated material with extra iodide, thus reducing the vacancies, can increase stability. “Stability can be improved by controlling crystal size and reducing iodide defects in the material,” the expert confirms. “In the paper, we show passivation of the film with iodide salts — post manufacture — improves the stability.”
“We plan to test the stability of such perovskite films and solar cells in real world conditions, where both oxygen and moisture are present,” Haque says, talking about what he and his team are up to next within the scope of this research endeavor.
Written by Sandra Henderson, Research Editor, Solar Novus Today