Effects of water on the forward and backward conversions of lead(II) iodide to methylammonium lead perovskite

This work has chemically addressed the enigmatic effects of water on solar cell devices based on an organic-inorganic hybrid perovskite, i.e., water can exert either beneficial or detrimental effects on device fabrication and device stability. We have examined in detail a known conversion of lead(II) iodide crystals to a polymer strip of a hydrated plumbate dimer in water and analyzed the time course of the conversion of the fiber crystals of the hydrate to perovskite crystals using fiber X-ray diffraction analyses. We found that the first step, the conversion of lead(II) iodide to a plumbate intermediate, is a fast solid-to-solid conversion reaction, and that the second step, the conversion of this intermediate to the perovskite, is also very fast because of easy loss of water. Overall, we propose that the water present as a minute constituent in DMF or DMSO actively participates as a catalyst in the whole reaction scheme, acting in the first step as a hydrated methylammonium to assist the cleavage of Pb-I bridges, and in the second step through quicker loss from a plumbate intermediate because of a lower hydrogen-bond accepting ability than DMF or DMSO.