Unraveling Bulk versus Surface Passivation Effects in Highly Efficient p–i–n Perovskite Solar Cells Using Thiophene‐Based Cations

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. Herein, two different thiophene salts that are used as passivating agents are compared, namely thiophene methylammonium chloride and thiophene ethylammonium chloride (TEACl), which are used for the passivation of bulk and surface defects in triple‐cation‐based metal halide perovskites. First, it is observed 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 with the reference. Second, it is demonstrated that the chemical structure of the cation dictates its capability either in passivating bulk defects effectively or to form a superficial two‐dimensional/three‐dimensional 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.