Dimensional Regulation in Metal‐Free Perovskites by Compositional Engineering to Achieve Record Low X‐Ray Detection Limits

Abstract Utilizing the manipulation of perovskite dimensions has been proven as an effective approach in regulating perovskite properties. Nevertheless, achieving precise control over the dimensions of perovskites within the same system poses a significant challenge. In this study, we introduce a sophisticated method to attain precise dimensional control in metal‐free perovskites (MFPs), specifically through the process of octahedron tailoring by compositional engineering. Accordingly, we successfully instigated a transition from HPIP−NH 4 I 3 ⋅ H 2 O (3D), HPIP 2 −NH 4 I 5 (2D) and HPIP 3 −NH 4 I 7 (1D) structures. Notably, HPIP 2 −NH 4 I 5 is the first 2D MFP. As anticipated, these perovskites exhibited completely distinct fluorescence and X‐ray detection capabilities due to their differing dimensions. Remarkably, the 2D HPIP 2 −NH 4 I 5 device effectively hindered ion migration perpendicular to the 2D layers, achieving the lowest detection limit of 12.2 nGy air s −1 among metal‐free single crystals‐based detectors. This study expands the dimensionality control strategies for MFPs and introduces, for the first time, the potential of 2D MFPs as high‐performance X‐ray detectors, thereby enriching the diversity of the MFPs family.