Acid-Catalyzed Reactions Activate DMSO as a Reagent in Perovskite Precursor Inks

Proton transfer from methylammonium (CH₃NH₃⁺) to dimethylsulfoxide (DMSO), a common Lewis-base solvent, initiates the production of ammonium (NH₄⁺) and dimethylammonium ([CH₃]₂NH₂⁺). We propose two parallel reaction pathways initiated by this proton transfer. Using DMSO-d₆ to elucidate reaction schemes, we demonstrate that protonation is followed either by methyl group transfer between the resulting CH₃NH₂ and residual CH₃NH₃⁺, or by transmethylation to CH₃NH₂ from DMSOH⁺. The former reaction yields NH₄⁺ and (CH₃)₂NH₂⁺ and is the dominant pathway at processing relevant temperatures; the latter yields (CH₃)₂NH₂⁺ in addition to methylsulfonic acid and dimethylsulfide. In the preparation of hybrid organic-inorganic perovskite (HOIP) thin films for photovoltaic applications, the substitution of CH₃NH₃⁺ with NH₄⁺ and (CH₃)₂NH₂⁺ in the HOIP crystal results in deviations from the tetragonal structure expected of phase-pure CH₃NH₃PbI₃, with a deleterious effect on the absorptivity of the resulting films. These results emphasize the importance of elucidating the under-appreciated precursor/solvent reactivity, the products of which, when incorporated into the solid state, can have profound effects on HOIP composition and structure, with a commensurate impact on macroscopic properties and device performance.