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-1 Pbn I3n+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)3 Pb4 I13 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 Pb4 I13 , the MA-containing analogue and a renowned stable 2D perovskite, both the (PDA)(FA)3 Pb4 I13 films and their derived devices exhibit exceedingly higher thermal stability.
Keywords: X-ray diffraction; formamidinium; perovskite; solar cells; thermostability.
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