Potassium iodide reduces the stability of triple-cation perovskite solar cells

electron transport layers (ETL), we propose that defect passivation using KI is highly sensitive to the composition of the perovskite-ETL interface. We reconfirm findings from previous reports that KI preferentially interacts with bromide ions in mixed-halide perovskites, and - at concentrations >5 mol% in the precursor solution - modifies the primary absorber composition as well as leading to the phase segregation of an undesirable secondary non-perovskite phase (KBr) at high KI concentration. Importantly, by studying both material and device stability under continuous illumination and bias under ambient/high-humidity conditions, we show that this secondary phase becomes a favourable degradation product, and that devices incorporating KI have reduced stability.