Ordered Crystallographic Orientation of Dion–Jacobson 2D Perovskites for Ultrastable Solar Cells

ABSTRACT Dion–Jacobson (DJ) 2D perovskites show great potential in stability but do not meet theoretical expectations for photovoltaics due to the random crystallographic orientation in films. We herein report ordered crystallographic orientation of the DJ 2D perovskite, achieved by modifying the intermediate‐phase film with a Ruddlesden‐Popper (RP) phase 2D perovskite based on the two‐step processing method. The introduction of the RP 2D perovskite lowers the energy barrier for the transition from the intermediate phase to the 2D perovskite phase with the ordered alignment, and this strategy is applicable to various organic diammoniums‐based DJ 2D perovskites. Solar cells with oriented DJ 2D perovskite films achieve an efficiency of 21.1% (19.05% for 1 cm 2 devices) and exhibit outstanding stability by retaining 98.1% of initial efficiency after 2000 h of maximum power point tracking at 65°C under continuous 1‐sun illumination (ISOS‐L‐2 protocol) and 98.7% after 2500 h testing at a relative humidity of 85%.