Compositional Engineering To Improve the Stability of Lead Halide Perovskites: A Comparative Study of Cationic and Anionic Dopants

The instability of perovskite solar cells is the single greatest barrier to their commercialization. While a number of studies have now looked at the effect of perovskite composition on device stability, many of these have examined only a single compositional variable. With many of these studies having been carried out under different environmental conditions, and still others lacking environmental controls entirely, it is often difficult to compare the relative effect of various cationic or anionic dopants. To address this knowledge gap, we fabricated CH3NH3PbI3-based solar cells where either the methylammonium or iodide ions were replaced with 20 mol % of a dopant ion (ethylammonium, formamidinium, bromide, or chloride). We then assessed their stability either in a controlled 85% relative humidity environment or under 1 sun illumination in air; both conditions have been previously shown to rapidly decompose CH3NH3PbI3. Of the dopants studied, the formamidinium cations imparted the best moisture resistance, and the resulting perovskite displayed the lowest photochemical reactivity. We attribute the improved stability of the formamidinium-doped perovskite to the more delocalized positive charge of the formamidinium cation.