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Metal halide perovskites of the general formula ABX₃—where A is a monovalent cation such as caesium, methylammonium or formamidinium; B is divalent lead, tin or germanium; and X is a halide anion—have shown great potential as light harvesters for thin-film photovoltaics^{, , , –}. Among a large number of compositions investigated, the cubic α-phase of formamidinium lead triiodide (FAPbI₃) has emerged as the most promising semiconductor for highly efficient and stable perovskite solar cells^{, , –}, and maximizing the performance of this material in such devices is of vital importance for the perovskite research community. Here we introduce an anion engineering concept that uses the pseudo-halide anion formate (HCOO⁻) to suppress anion-vacancy defects that are present at grain boundaries and at the surface of the perovskite films and to augment the crystallinity of the films. The resulting solar cell devices attain …