Methylammonium Chloride: A Key Additive for Highly Efficient, Stable, and Up-Scalable Perovskite Solar Cells

Lead halide perovskites of the type APbX(3) (where A = methylammonium MA, formamidinium FA, or cesium and X = iodide and bromide), in a single-crystal form or more often as polycrystalline films, have already shown unique optoelectronic properties, comparable with those of the best single-crystal semiconductors. To form a properly crystalline iodide or iodide/bromide, perovskite and achieve high performance in solar cells, sources containing only iodide and bromide salts (PbI2, PbBr2, MAI, FAI, CsI, MABr) are typically used as precursor materials. However, recently, most of the record perovskites contain MACl as additive to control their crystallization, revisiting the importance of methylammonium cation excess and chloride incorporation in perovskites, previously highlighted by Snaith's group back in 2012. Here, we review the background and recent progress in MACl-mediated crystallization of perovskites, as well as the impact of the additive in solar cells. In particular, we describe the current understanding of the mechanism of perovskite crystallization process and defect passivation at grain boundaries in the presence of MACl. We then discuss the spectacular results (in terms of record efficiencies, stability, and up-scaling) that have been delivered by solar cells employing MACl-incorporated perovskites, and give an outlook of future research avenues that might bring perovskite solar cells closer to commercialization.