Defect-Driven Local Structures in 3D Hollow Hybrid Perovskites Resolved by Solid-State NMR Spectroscopy

Metal halide perovskites (MHPs) are versatile semiconductors with high defect tolerance. Among emerging MHP architectures, the 3D “hollow” perovskites incorporating ethylenediammonium ( en ) dications represent a unique defect stabilization paradigm by retaining structural order, stability, and performance. However, the local chemical environments of defects that underpin bulk properties have remained unresolved, due to the inherently averaging nature of diffraction techniques. Here, we address this information gap by probing both crystallinity and defect-specific local structures in three representative hollow systems, en MAPbI 3, en FAPbI 3, and en FAPbBr 3 (MA: methylammonium; FA: formamidinium), using powder X-ray diffraction (PXRD), periodic density-functional theory (DFT), and solid-state nuclear magnetic resonance (ssNMR) spectroscopy. Specifically, ssNMR allows to identify two distinct vacancy regimes: a low- en (below 30%) regime with trans conformers that stabilize the framework by simultaneously generating Pb deficiency, and a new high- en (above 30%) regime with conformationally disordered en dications that lead to distortions in PbX 6 octahedra and crystallinity features. Insights into defect sites and their local chemical environments are obtained by analyzing 1D 1 H, 207 Pb, and 2D 1 H– 1 H and 1 H– 14 N correlation NMR spectra. These findings help explain defect compensation mechanisms, reconciling correlated trends between bandgap evolution and stability across iodine- and bromine-based hollow perovskites, and are expected to provide a framework for rationalizing defect engineering strategies in hybrid perovskites and related metal halides.