Oxidation in Tin Halide Perovskites: Influence of Acidic and Basic Additives

Tin-containing halide perovskites (Sn-HPs) are promising alternatives to their lead-containing counterparts; however, Sn-HPs suffer from higher defect concentrations due to the oxidation of Sn2+ to Sn4+. To minimize oxidation, improve film morphology, and reduce defect concentrations, additives that interact with SnI2 are typically incorporated into Sn-HP precursor solutions. These additives are often acidic, yet acidic conditions tend to be more oxidizing. In this work, we determine how oxidation of Sn-HP precursors is influenced by the chemical environment in solution, including the presence of acidic and basic additives, solvent system, oxygen exposure, and water exposure. We find that strong Br & oslash;nsted-Lowry acids increase oxidation of SnI2 and I-, whereas addition of basic salts decreases oxidation. When the solvent system contains dimethyl sulfoxide (DMSO), a conventional solvent used for the fabrication of Sn-HPs, strong acids lead to the fast oxidation of SnI2 at room temperature. Through the use of Sn-119 and H-1 NMR, UV-vis absorbance, and controlled atmosphere exposure, we propose oxidation reactions that depend on the chemical environment, including the presence of DMSO, oxygen, water, and acidic additives. Because of the less oxidizing conditions, Sn-HP films processed with basic salts display increased performance in perovskite solar cells relative to when the conjugate acid is used or no additive is included.