Real‐Time Observation of Order‐Disorder Transformation of Organic Cations Induced Phase Transition and Anomalous Photoluminescence in Hybrid Perovskites

A fundamental understanding of the interplay between the microscopic structure and macroscopic optoelectronic properties of organic‐inorganic hybrid perovskite materials is essential to design new materials and improve device performance. However, how exactly the organic cations affect the structural phase transition and optoelectronic properties of the materials is not well understood. Here, real‐time, in situ temperature‐dependent neutron/X‐ray diffraction and photoluminescence (PL) measurements reveal a transformation of the organic cation CH 3 NH 3 + from order to disorder with increasing temperature in CH 3 NH 3 PbBr 3 perovskites. The molecular‐level order‐to‐disorder transformation of CH 3 NH 3 + not only leads to an anomalous increase in PL intensity, but also results in a multidomain to single‐domain structural transition. This discovery establishes the important role that organic cation ordering has in dictating structural order and anomalous optoelectronic phenomenon in hybrid perovskites.