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.