Compositional modifications and passivating additives have been key enablers to approach operationally stable halide perovskite devices with excellent optoelectronic properties. Thermal and structural instability of the most desirable single-cation metal...

Perovskite solar cells (PSCs) have reached a formidable power conversion efficiency of 25.7% over the years of development. One of the strategies that has been responsible for the development of stable and highly efficient PSCs is modifications of the monovalent A‐site cations (methylammonium, MA; formamidinium, FA; cesium, Cs, etc.) in lead halide perovskites. Herein, the impact of modifying the monovalent cation (MA, FAMA, CsFAMA, potassium‐passivated CsFAMA, rubidium‐passivated CsFAMA) in lead halide perovskite on their optoelectronic, charge transport, and photovoltaic behavior is systematically studied. Reduced trap density and improved charge carrier mobility after introduction of FA and Cs in the MAPb(I0.85Br0.15)3 system are confirmed. Further passivation of the triple‐cation perovskite with K and Rb enhances the optoelectronic characteristics, charge transport, and charge extraction efficiency in halide perovskite solar cells.