HSE Scientists Develop Method to Stabilise Iodine in Solar Cells
Scientists at HSE MIEM, in collaboration with colleagues from China, have developed a method to improve the durability of perovskite solar cells by addressing iodine loss from the material. The researchers introduced quaternary ammonium molecules into the perovskite structure; these molecules form strong electrostatic pairs with iodine ions, effectively anchoring them within the crystal lattice. As a result, the solar cells retain more than 92% of their power after a thousand hours of operation at 85°C. The study has been published in Advanced Energy Materials.
Perovskites are materials with a specific crystal structure in which lead and halogen atoms (such as iodine) are combined with small organic or inorganic ions. This crystal lattice absorbs light efficiently and converts it into electricity. Over the past decade, perovskite solar cells have evolved from laboratory experiments into a major area of solar energy research. They can potentially be manufactured at a lower cost than silicon-based cells and have nearly matched them in terms of efficiency.
Iodine-based perovskites achieve efficiencies of over 26%, delivering the best performance among perovskite materials. This is because their energy structure is well suited to absorbing sunlight and capturing a large fraction of the incoming radiation. Charge carriers in these crystals have longer lifetimes, can travel greater distances, and suffer fewer losses from structural defects, which minimises undesirable charge recombination. However, iodide perovskites also have a drawback: under prolonged exposure to light and heat, their crystal lattice loses iodine, leading to erosion of the metal electrodes. As a result, the solar cell gradually deteriorates and its efficiency declines.
Previous attempts to address this issue have focused on strengthening the crystal structure or introducing molecules that bind iodine through hydrogen bonds. However, these bonds are not strong enough to retain iodine effectively during prolonged device operation. A more logical approach would be to use stronger electrostatic interactions, in which positively charged molecules firmly bind negatively charged iodine ions. Until now, however, it has been unclear how to incorporate such molecules into the perovskite crystal lattice without disrupting its structure.
In a new study, scientists from HSE MIEM, East China Normal University, Hengyang Normal University, and Ningbo University have developed a method to electrostatically retain iodine within the perovskite structure, significantly enhancing the resistance of solar cells to prolonged photothermal stress.
First, the researchers carried out theoretical calculations to identify molecules that could most effectively bind triiodide anions (I₃⁻). Quaternary ammonium compounds—molecules in which the nitrogen atom is fully surrounded by hydrocarbon groups—proved to be the most effective. This configuration enables strong confinement of triiodide ions, and subsequent experiments therefore employed tetrabutylammonium iodide (TBAI).
In the experimental phase, the researchers added TBAI to the solution used to form perovskite films and compared the resulting films with control samples prepared without additives. Films containing TBAI preserved their structural integrity and chemical stability under photothermal conditions, whereas the controls gradually degraded. This difference was evident from measurements of metallic lead, a marker of perovskite degradation: after 250 hours of photothermal exposure, its concentration in the modified films remained nearly unchanged, while in the control films it increased by approximately 1.5 times. In addition, the presence of TBAI almost completely suppressed the migration of iodine and copper between layers.
The researchers then tested full-scale solar cells. The addition of TBAI not only prevented degradation but also improved the properties of the material: the perovskite grains became larger and more ordered, the density of defects decreased, and the power conversion efficiency increased from 24.14% to 26.23%. For state-of-the-art perovskite devices operating close to their physical limits, such an improvement is substantial. TBAI also notably enhanced device stability: after 1,000 hours of operation at 85 °C, cells containing TBAI retained 92.5% of their initial efficiency, whereas the control device degraded to 43.8% after just 288 hours.
The authors hope that this approach to regulating electrostatic interactions in perovskites will contribute to the development of more durable solar cells.
Andrey Vasenko
'In theory, this approach could be applied to other types of halide perovskites, including materials in which iodine is combined with bromine. This would enable the development of solar panels that combine high efficiency with thermal stability,' comments Prof. Andrey Vasenko of HSE MIEM.
The study was carried out as part of the research project 'Engineering of Highly Efficient and Stable Perovskite Solar Cells.'
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