Roles of MACl in Sequentially Deposited Bromine‐Free Perovskite Absorbers for Efficient Solar Cells

So far, the combination of methylammonium bromide/methylammonium chloride (MABr/MACl) or methylammonium iodide (MAI)/MACl is the most frequently used additives to stabilize formamidinium lead iodide (FAPbI3) fabricated by the sequential deposition method. However, the enlarged bandgap due to the addition of bromide and the ambiguous functions of these additives in lead iodide (PbI2) transformation are still worth considering. Herein, the roles of MACl in sequentially deposited Br‐free FA‐based perovskites are systematically investigated. It is found that MACl can finely regulate the PbI2/FAI reaction, tune the phase transition at room temperature, and adjust intermediate‐related perovskite crystallization and decomposition during thermal annealing. Compared to FAPbI3, the perovskite with MACl exhibits larger grain, longer carrier lifetime, and reduced trap density. The resultant solar cell therefore achieves a champion power conversion efficiency (PCE) of 23.1% under reverse scan with a stabilized power output of 23.0%. In addition, it shows much improved photostability under 100 mW cm−2 white illumination (xenon lamp) in nitrogen atmosphere without encapsulation. The role of methylammonium chloride (MACl) in sequentially deposited bromine (Br)‐free formamidinium lead iodide (FAPbI3)‐based perovskite is systematically demonstrated to regulate the PbI2/FAI reaction, tune the phase transition at room temperature, and adjust the PbI2 residual through an intermediate‐related perovskite decomposition during thermal annealing. The resulting optimized solar cells achieve a remarkable efficiency of 23.1% with considerably improved photostability.