Building (001) oriented FAPbI 3 films for high-performing perovskite solar cells.

The solution processed FAPbI 3 perovskite usually suffers from chaotic orientations. Herein, a template structure of oriented 2D perovskite is used to obtain a high-quality FAPbI 3 film with (001) preferred orientation by cation exchange. The highly oriented BA 2 PbI 4 serves as a growth template and promotes the (001) orientation of the 3D perovskite. The dominantly (001) orientated FAPbI 3 perovskite exhibits uniform surface morphology and suppressed film defects.