In Situ Grazing‐Incidence Wide‐Angle Scattering Reveals Mechanisms for Phase Distribution and Disorientation in 2D Halide Perovskite Films

2D hybrid halide perovskites with the formula (A′)2(A)n‐1PbnI3n+1 have remarkable stability and promising efficiency in photovoltaic and optoelectronic devices, yet fundamental understanding of film formation, key to optimizing these devices, is lacking. Here, in situ grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) is used to monitor film formation during spin‐coating. This elucidates the general film formation mechanism of 2D halide perovskites during one‐step spin‐coating. There are three stages of film formation: sol–gel, oriented 3D, and 2D. Three precursor phases form during the sol–gel stage and transform to perovskite, first giving a highly oriented 3D‐like phase at the air/liquid interface followed by subsequent nucleations forming slightly less oriented 2D perovskite. Furthermore, heating before crystallization leads to fewer nucleations and faster removal of the precursors, improving orientation. This outlines the primary causes of phase distribution and perpendicular orientation in 2D perovskite films and paves the way for rationally designed film fabrication techniques. The mechanism of 2D halide perovskite film formation is resolved using in situ grazing‐incidence wide‐angle scattering (GIWAXS). The film begins as a sol–gel precursor before first forming a 3D MAPbI3‐like phase at the air/liquid interface. This acts as a template for the highly textured 2D phase with the layers perpendicular to the substrate, which grows closer to the substrate.