Perovskite production without the side-effects

The increasing prominence of perovskites in the PV manufacturing industry poses a 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 current 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 represent significant concerns.

Researchers at the Polish Academy of Sciences developed a potential solution in 2017, with a steel ball, mill grinding process that produced a lead halide perovskite which achieved 16.8% efficiency in a solar cell.

Now, a discovery by a chemistry group at the Peoples’ Friendship University of Russia (RUDN University) could open up a new approach to solvent-free perovskites. The group synthesized four stable compounds from the reaction between methylammonium iodide and iodine in a process the researchers say could be used as a precursor for perovskite solar cell production.

Ionic liquids

Creating a ‘melt’ to be applied to a thin film of metallic lead could be an alternative to solvent-based perovskite thin-film production. With that in mind, the RUDN group began to study 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 which could be applied evenly to large surfaces such as a metallic lead thin film to produce solar cells.

The scientists observed the polyiodide liquids melted 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,” said 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 four compounds – MAI₂, MAI_2.67, MAI₄ and MAI_5.5 – evolved from the melt. By conducting thermodynamic analysis of them, the researchers were able to define general conditions under which the melt could exist and found similar effects in certain other perovskite formulations – including several based on formamidine and polybromides.