Surface passivation with 2D perovskites is a powerful strategy to achieve improved stability and performance in perovskite solar cells (PSCs). Various large organic cations have been successfully implemented, led by phenylethylammonium (PEA+) and its derivatives. However, systematic studies on large sets of cations to understand the effect of substituent position on 2D perovskite passivation and device performance are lacking. Herein, a collection of halogenated PEA+ iodide salts (x-XPEAI where x: ortho (o), meta (m), para (p), X: F, Cl, Br) are synthesized by a facile method and deposited on top of 3D perovskite. The 2D perovskite layer formation is confirmed by X-ray diffraction (XRD) and grazing-incidence wide-angle X-ray scattering analyses for all cations, regardless of the nature and position of the halogen. Density functional theory analysis reveals that lower formation energies and higher interfacial dipoles achieved by m-substituted cations are responsible for enhanced performance compared to their o- and p- counterparts. While the m-BrPEAI-treated device shows a champion efficiency of 23.42%, (VOC=1.13 V, FF=81.2%), considering average efficiencies, stability, and reproducibility, the treatment with m-ClPEAI salt yields the best overall performance. This comprehensive study provides guidelines for understanding the influence of large cation modification on performance and stability of 3D/2D PSCs.