Lead-tin (Pb-Sn) metal halide perovskites (MHPs) with a ∼1.24 eV bandgap are ideal bottom cells in all-perovskite photovoltaic tandems, but Sn²⁺ instability limits their performance. Traditional additives such as SnF₂, metallic Sn, or organic reductants mitigate oxidation but fail to control rapid crystallization, leading to poor film morphology in thick absorbers. Here, phenethylammonium iodide (PEAI), 4-fluorophenethylammonium iodide (4F-PEAI), and 4-fluorophenethylammonium bromide (4F-PEABr) are explored to regulate Pb-Sn perovskite crystallization and enhance stability. These additives improve crystallization, morphology, and device performance. Among them, 4F-PEABr yields the best operational stability under maximum power-point tracking due to fluorine and bromide functionalities that promote slower crystallization and a (111) orientation. The optimized p-i-n perovskite solar cell with an active area of 0.128 cm² and a device architecture of glass/ITO/PEDOT:PSS/Cs_0.05FA_0.70MA_0.25Pb_0.50Sn_0.50I₃/C₆₀/BCP/Ag achieved a power conversion efficiency of 21.5%, with an open-circuit voltage of 0.83 V, a short-circuit current density of 32.5 mA/cm², and a fill factor of 80%. This study demonstrates halogen and molecular engineering as effective strategies to achieve efficient and stable Pb-Sn perovskite solar cells for tandem applications.