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) Pb I 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) Pb I 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) Pb I , the MA-containing analogue and a renowned stable 2D perovskite, both the (PDA)(FA) Pb I films and their derived devices exhibit exceedingly higher thermal stability.