A group of chemists at Russia’s RUDN University has developed a series of compounds based on methylammonium iodide and iodine, which it says could be used to produce perovskite solar devices without the need for toxic solvent materials. This would also allow perovskite cells to be manufactured without chemical products to be disposed of.
The increasing presence of perovskites on the PV manufacturing roadmap poses a whole series of questions, not least of which is the environmental impact of their production.
Perovskites potentially offer high-efficiency solar generation at a much lower cost than today’s technologies, and their production would likely have lower material and energy requirements, and therefore a lower carbon footprint than crystalline silicon. But the presence of lead, and the use of solvents in the production of perovskite solar cells still represent significant concerns.
A group at the Polish Academy of Sciences developed a potential solution to this back in 2017, with a steel ball mill grinding process that produced a lead halide perovskite that achieved 16.8% efficiency when incorporated into a solar cell.
A new discovery by a chemistry group at RUDN University could open up a new approach to solvent-free perovskite production. The group synthesized a series of four stable compounds from the reaction between methylammonium iodide and iodine, which it says could be used as precursors for perovskite solar cell production.
Creating a ‘melt’ to be applied to a thin film of metallic lead could be used as an alternative to solvent-based perovskite thin-film production. With this in mind, the group began to study various compounds of methylammonium iodide and iodine in search of a suitable candidate for perovskite production.
Several of the compounds studied, described in the paper Methylammonium Polyiodides: Remarkable Phase Diversity of the Simplest and Low-Melting Alkylammonium Polyiodide System, published in the Journal of Physical Chemistry Letters, were found to have the right properties. They melted at room temperature and formed ionic liquids, these could then be applied evenly to large surfaces, such as a metallic lead thin film, to produce solar cells.
The investigation found that these polyiodide liquids would melt at room temperature only in the presence of large organic cations, such as those present in methylammonium. “The methylammonium cation has a large dipole moment and is capable of forming a large number of hydrogen bonds,” explained RUDN University chemist Victor Khrustalev. “At small cation sizes, this leads to increased entropy during melting, which lowers the melting temperature.”
Under certain conditions, the group found that four compounds MAI2, MAI2.67, MAI4, and MAI5.5 evolved from the melt. By conducting a thermodynamic analysis of these they were able to define general conditions under which the melt can exist, and found that similar effects in certain other perovskite formulations – including several based on formamidine and polybromides.
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