2D Ruddlesden‐Popper versus 2D Dion‐Jacobson Perovskites: Of the Importance of Determining the “True” Average n‐Value of Annealed Layers.

2D Ruddlesden‐Popper (RP) and Dion‐Jacobson (DJ) halide perovskite structures are characterized by the <italic>n</italic> parameter that represents the number of PbI6 layers in the spacer‐separated perovskite slabs. In most of the literature, <italic>n</italic> is referred to the initial composition of the precursor solution and, then, to the initial spacer content. In this approach, the fact that the spacer can be partially lost upon annealing is not considered. Here, the chemical processes occurring upon the preparation of reduced‐dimensional halide perovskite thin films of both families are investigated with a special focus on the organic cations composition and, especially, the evolution of <italic>n</italic> values. For each family, several spacers are investigated. It is shown that a large amount of monoammonium spacer cations is progressively eliminated upon annealing for the 2D‐RP perovskites, while, on the contrary, the final composition is close to the initial one in the case of the diammonium spacer cations of the 2D‐DJ perovskites. The elimination is related to the volatilization of the species after their deprotonation and the phenomenon can be regulated by halide additives. This work provides an important insight into the chemistry of the 2D perovskite layers and shows the importance of determining their “true” <italic>n</italic> value. [ABSTRACT FROM AUTHOR]