Partial Substitution of Methylammonium Halides (Cl, Br, and I) in Double-Cation Tin Perovskite: Relative Impact on Material Properties and Solar Cell

Tin-based perovskites (TBPS) are promising lead-free alternatives for photovoltaic applications; however, they suffer from high defect densities and rapid oxidation of Sn2+ to Sn4+, leading to poor film quality and instability. This study investigates the impact of partially substituting formamidinium iodide (FAI) with methylammonium halides (MAX, X = Cl, Br, or I) on the properties, stability, and device performance of the PEAFASnI3 film. MAX incorporation reduces the band gap, with MAI exhibiting the largest reduction. MACl induces highly oriented crystallization with reduced defects, while MABr leads to the largest crystallites, lowest defect density, and most effective suppression of Sn2+ oxidation. These improvements enhance energy level alignment, enabling MABr-based devices to achieve significantly enhanced VOC (0.53 V) and fill factor (70%), as well as superior reproducibility compared to the control. Despite these improvements, MAX substitution accelerates the degradation of the perovskite film under ambient conditions, which is attributed to modified surface chemistry. X-ray photoelectron spectroscopy reveals that MAX-modified films exhibit a reduced surface SnF2 content, which prevents the formation of protective SnF2/SnO2 layers and renders them more vulnerable to oxidation. These results provide valuable insights into the influence of small cation and halide substitutions in tuning the material properties, surface chemistry, phase stability, and photovoltaic performance of TBPS.