Improved Pb-Sn Perovskite Solar Cells through Additive Engineering with Phenethylammonium Halides

Lead-tin (Pb-Sn) metal halide perovskites (MHPs) with a ∼1.24 eV bandgap are ideal bottom cells in all-perovskite photovoltaic tandems, but Sn 2+ instability limits their performance. Traditional additives such as SnF 2, 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 cm 2 and a device architecture of glass/ITO/PEDOT:PSS/Cs 0.05 FA 0.70 MA 0.25 Pb 0.50 Sn 0.50 I 3 /C 60 /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/cm 2, 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.