One of the leading approaches to enhancing the performance and stability of perovskite solar cells (PSCs) involves passivating the perovskite surface and grain boundaries with large ammonium salts. Here, we report the synthesis of furan-, thiophene-, and selenophene-functionalized phenyl methanaminium iodide salts (FPMAI, TPMAI, and SPMAI) and their application as passivating agents on 3D [(Cs0.04FA0.85MA0.11)Pb(I0.96Br0.01Cl0.03)3] perovskite. The TPMAI-passivated PSCs performed the best and achieved a power conversion efficiency (PCE) of 23.15% compared to the reference (without a passivating agent) at 20.91%. Efficiencies reduced to 98 and 54% of the initial value after 1250 h of continuous illumination for TPMAI-treated PSCs and the reference, respectively. DFT calculations revealed that TPMAI offers superior passivation, disfavoring iodine vacancy formation. Our findings highlight the potential of functionalized PMAI salts as passivation agents for improved efficiency and stability in PSCs.