Abstract Sulfur‐based multifunctional additives are attractive for increasing not only the device power conversion efficiency but also the moisture stability of perovskite solar cells. The stability of the device against external and internal stress plays a pivotal role in the commercial endeavor of emerging technologies such as perovskite photovoltaics. However, the potential of sulfur‐based additives remains largely unexplored for perovskite solar cell fabrication. Here, a mechanism is deduced for the local nanoscopic humidity ingression into a multifunctional additiviated formamidinium‐loaded halide perovskites. By tuning the iodide and bromide tails of the additives, the influence of sulfur heteroatom containing ammonium‐amidinium salts on the photo‐physical and device properties of a formamidinium‐rich perovskite absorber is uncovered. In addition, the process of strong water adsorption is excluded through the proton‐migration mechanism, thereby significantly improving the moisture resistance of perovskite films. The high crystallinity and long lifetime decay allow a higher PCE of 25.14% to be achieved compared to the control at 22.49%, along with improved long‐term stability by retaining 99.6% of the initial PCE after 1000 h.

Uncovering the Nanoscopic Humidity Ingression in Multifunctional Addivated Halide Perovskites