Volatile 2D Ruddlesden‐Popper Perovskite: A Gift for α‐Formamidinium Lead Triiodide Solar Cells

Abstract Perovskite solar cells with increasingly pure composition of α‐formamidinium lead triiodide (α‐FAPbI 3 ) perovskite are utilized to set more and more record‐breaking efficiencies. However, pure α‐FAPbI 3 perovskite is unstable and difficult to prepare. Here, a series of bulky alkylammoniums known as the spacer cations (RP cations) of 2D Ruddlesden–Popper perovskites (2D perovskites) are used to prepare α‐FAPbI 3 perovskite films. The deprotonation process of RP cations during annealing removes the in situ generated 2D perovskites from the film, which determines the phase and compositional purity, crystallinity, and stability of α‐FAPbI 3 perovskite films and depends on the design of RP cations. Only a small number of residual RP cations (0.3–2.3%) are found anchoring at grain boundaries. As a result, α‐FAPbI 3 perovskite solar cells prepared from RP cations, especially 2‐thiophenemethylammonium, show higher efficiency and stability than control devices prepared from the most commonly used methylammonium. It is believed that in situ generated 2D perovskites are ideal additives for α‐FAPbI 3 perovskite, because a large addition (20%) of 2D perovskites ensures the preparation of high‐quality phase‐pure α‐FAPbI 3 perovskite films, while a small number of residual RP cations anchored at grain boundaries guarantee the performance and stability of α‐FAPbI 3 perovskite solar cells.