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 low‐dimensional perovskites, featuring a general formula of (PDA)(FA)n−1PbnI3n+1 with propane‐1,3‐diammonium (PDA) as the organic spacer cation. The 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)3Pb4I13 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)3Pb4I13, the MA‐containing analogue and a renowned stable 2D perovskite, both the (PDA)(FA)3Pb4I13 films and their derived devices exhibit exceedingly higher thermal stability. A new type of methylammonium‐free formamidinium (FA) based perovskites is reported. The low‐dimensional perovskite films are obtained in the presence of the FACl additive, and the role of Cl is investigated through grazing‐incidence X‐ray diffraction. Solar cell devices based on (PDA)(FA)3Pb4I13 films show extremely high thermal stability and a remarkable PCE of 13.8 %.