Two-dimensional perovskite templates for durable, efficient formamidinium perovskite solar cells.

We present a design strategy for fabricating ultrastable phase-pure films of formamidinium lead iodide (FAPbI) by lattice templating using specific two-dimensional (2D) perovskites with FA as the cage cation. When a pure FAPbI precursor solution is brought in contact with the 2D perovskite, the black phase forms preferentially at 100°C, much lower than the standard FAPbI annealing temperature of 150°C. X-ray diffraction and optical spectroscopy suggest that the resulting FAPbI film compresses slightly to acquire the (011) interplanar distances of the 2D perovskite seed. The 2D-templated bulk FAPbI films exhibited an efficiency of 24.1% in a p-i-n architecture with 0.5-square centimeter active area and an exceptional durability, retaining 97% of their initial efficiency after 1000 hours under 85°C and maximum power point tracking.