A protective layer of fluoride ions could increase the efficiency and stability of perovskite solar cells, which are a promising alternative to their silicon peers.
Perovskite is low cost, easy to produce and almost as efficient as silicon-based solar cells. This has helped stimulate research into the material that has seen its efficiency rise from under four per cent in 2009 to over 24 per cent, which is close to traditional silicon cells. Tandem cells, which combine silicon and perovskite cells, achieve an efficiency of over 28 per cent.
Perovskite does, however, contain a number of defects due to the nature of the material and the way it is manufactured. Over time, vacancies in the atomic structure of the metal halide trigger the degradation of the perovskite under the influence of moisture, light and heat.
Now, researchers from Eindhoven University of Technology, DIFFER (Dutch Institute for Fundamental Energy Research), Peking University and University of Twente have discovered that a small amount of fluoride increases the stability of the material and the solar cells significantly. The solar cells are claimed to retain 90 per cent of their efficiency after 1,000 hours of operation at extreme testing conditions. The findings are published in Nature Energy.
The researchers have experimented with a new type of perovskite by adding a small amount of fluoride in the production process. The fluoride ions form a protective layer around the crystal, preventing the diffusion of the harmful defects.
“Our work has improved the stability of perovskite solar cells considerably”, said Shuxia Tao, assistant professor at the Centre for Computational Energy Research, a joint centre of the Department of Applied Physics of TU/e and DIFFER, and co-author of the paper. “Our cells maintain 90 per cent of their efficiency after 1,000 hours under extreme light and heat conditions. This is many times as long as traditional perovskite compounds. We achieve an efficiency of 21.3 per cent, which is a very good starting point for further efficiency gains”.
Much of the work of the team from Eindhoven has gone into explaining why fluoride is such an effective ingredient compared to other halogens. Using computer simulations they conclude that part of its success is due to the small size and high electronegativity of fluoride ions. The higher the electronegativity of an element, the easier it attracts electrons of neighbouring elements. This helps fluoride ions to form strong bonds with the other elements in the perovskite compound, forming a stable protective layer.
“We expect it will take another five to ten years for these cells to become a commercially viable product. Not only do we need to further improve their efficiency and stability, we also need to gain a better theoretical understanding of the relevant mechanisms at the atomic scale. We still don’t have all the answers to why some materials are more effective than others in increasing the long-term stability of these cells”, said Tao.