Defect passivation is nowadays considered a must-have route for high-efficiency perovskite solar cells. However, a general rule that correlates the choice of passivating agents with performance enhancements is still missing. This work compares two different thiophene salts used as passivating agents, namely thiophene methylammonium chloride (TMACl) and thiophene ethylammonium chloride (TEACl), used for the passivation of bulk and surface defects in triple-cation (CsFAMA) based metal halide perovskites. First, we observe that the surface passivation method leads to better device performances reaching a power conversion efficiency of 23.56%, with reduced voltage losses and increased fill factor when compared to the reference. Second, we demonstrate that the chemical structure of the cation dictates its capability either in passivating bulk defects effectively or to form a superficial 2D/3D heterostructure, which happens only for the TEACl case. The chemical composition and the cation dimension are responsible for device performance enhancement as observed by a joint spectroscopic and density functional theory simulations study, providing rational guidelines for further smart device design.This article is protected by copyright. All rights reserved.