Highly thermostable and efficient formamidinium‐based low‐dimensional perovskite solar cells

Currently, most two-dimensional (2D) metal halide perovskites are of the Ruddlesden-Popper type and contain the thermally unstable methylammonium (MA) molecules, which leads to inferior photovoltaic performance and mild stability. Here we report a new type of MA-free formamidinium (FA) based 2D Dion-Jacobson phase perovskites, featuring a general formula of (PDA)(FA) n-1 Pb n I 3n+1 with propane-1,3-diammonium (PDA) as the organic spacer cation. The 2D perovskite films with well-oriented crystal grains are attained under the assistance of the FACl additive, where the role of Cl is investigated through the grazing-incidence X-ray diffraction technique. The photovoltaic device based on the optimized (PDA)(FA) 3 Pb 4 I 13 film demonstrates a remarkable power conversion efficiency of 13.8%, the highest record for the FA-based 2D perovskite solar cells. In addition, compared to (PDA)(MA) 3 Pb 4 I 13 , the MA-containing analogue and a renowned stable 2D perovskite, both the (PDA)(FA) 3 Pb 4 I 13 films and their derived devices exhibit exceedingly higher thermal stability.