Propylammonium Chloride Additive for Efficient and Stable FAPbI₃ Perovskite Solar Cells

To achieve high efficiency perovskite solar cells (PSCs) based on α-phase formamidinium lead iodide (FAPbI₃), addition of methylammonium chloride (MACl) in the precursor solution is commonly used, mainly because of phase stability and improvement of grain size and crystallinity. However, the instability of MA in the perovskite limits the device long-term stability. In this report, n-propylammonium chloride (PACl) is proposed as an alternative to MACl for more stable and efficient FAPbI₃-based PSCs. Perovskite grain size is increased after addition of PACl. Unlike the MA cation, the propylammonium cation passivates the grain boundary rather than being incorporated into the perovskite lattice due to larger ionic size, which minimizes the change in bandgap. Carrier lifetime is significantly increased by more than five times from 405 to 2110 ns with the PACl additive with negligible trap-mediated recombination, while only four times longer carrier lifetime is observed by MACl additive. As a result, a power conversion efficiency over 22.2% is achieved by 20 mol% PACl additive, which is one of the best efficiencies among the MA-free and Br-free PSCs. In addition, stability against moisture is much better for PACl than for MACl due to an in situ formed barrier at the bulk perovskite.