Elucidation of Chemical Species and Reactivity at Methylammonium Lead Iodide and Cesium Tin Bromide Perovskite Surfaces via Orthogonal Reaction Chemistry

We quantified the chemical species present at and reactivity of the (100) face of tetrahedral single-crystal methylammonium lead iodide, MAPbI₃(100), and polycrystalline cesium tin bromide, CsSnBr₃. For these ABX₃ perovskites, experiments utilized the orthogonal reactivity of the A⁺-site cation, the B²⁺-site cation, and the X–-site halide anion. Ambient pressure exposure to BF₃ solutions probed the reactivity of interfacial halides. Reactions with p-trifluoromethylanilinium chloride probed the exchange reactivity of the A⁺-site cation. A complex-forming ligand, 4,4′-bis(trifluoromethyl)-2,2′-bipyridine, probed for interfacial B²⁺-site cations. Fluorine features in X-ray photoelectron spectroscopy (XPS) quantified reaction outcomes for each solution-phase species. XPS revealed adsorption of BF₃, indicating surface-available halide anions on both MAPbI₃(100) and on CsSnBr₃. Temperature-programmed desorption quantified a ∼200 kJ mol–¹ desorption activation energy from MAPbI₃(100) and a ∼215 kJ mol–¹ desorption energy from CsSnBr₃. Adsorption of the fluorinated anilinium cation included no concomitant adsorption of chlorine as revealed by the absence of Cl 2p features within the limits of XPS detection. We interpret the observation of the anilinium species as exchanging for interfacial methylammonium species on MAPbI₃(100) surfaces and interfacial cesium on the polycrystalline CsSnBr₃ surface. Within detection limits, the bipyridine ligand demonstrated no adsorption to MAPbI₃(100), suggestive of a Pb²⁺ deficient surface, but adsorption to the polycrystalline CsSnBr₃ that suggests surface-accessible Sn²⁺. The combination of results implies that methylammonium cations and iodide anions dominate tetragonal MAPbI₃(100) surface that, respectively, enables cation exchange and Lewis adduct formation for surface derivatization. We discuss the present results in the context of interfacial stability, passivation, and reactivity for perovskite-based energy conversion.