Mechanochemical Origins of Hierarchical Defect Coupling in Zero‐Dimensional Cs ₄ PbBr ₆ Perovskites: Impact on White‐LED and CO ₂ Photoreduction Efficiencies

Zero‐dimensional cesium lead halide (Cs 4 PbBr 6 ) perovskites are distinguished by their structural robustness and defect‐mediated optical properties, yet the origin of their emissive behavior remains elusive. Here, we reveal a hierarchical defect architecture in Cs 4 PbBr 6 microcrystals, wherein embedded CsPbBr 3 inclusions hosting bromine vacancies (V Br ), form multilevel CsPbBr 3 ‐V Br coupling states that govern charge‐carrier dynamics. A micro‐solvent–assisted mechanochemical strategy employing trace DMSO solvent enabled precise defect modulation while achieving low process mass intensity (PMI ∼5, far below conventional solution routes, ∼180) and structural fidelity. Comparative analyses of dry‐synthesized (D‐Cs 4 PbBr 6 ), DMSO‐assisted (B‐Cs 4 PbBr 6 ), and ligand‐mediated (L‐Cs 4 PbBr 6 ) microcrystals reveals the presence of hierarchical defect states are confirmed by low‐temperature photoluminescence, cathodoluminescence, and transient absorption spectroscopy, with DFT calculations identifying mid‐gap states (∼1.93 eV) responsible for NIR‐range emission. While shallow states drive this emission, detrimental deep traps are effectively neutralized by sulfate‐ion passivation, extending carrier lifetimes from 35.48 to 41.43 ns and enabling suppression in photoluminescence quenching. The optimized bromine‐vacancy densities facilitate enhanced CO 2 photoreduction (70.21 µmol g −1 h −1 , apparent quantum yield 1.42%), nearly 1.6× higher than defect‐free analogues, consistent with DFT‐predicted shallow V Br states enhancing charge availability. These findings establish a scalable, defect‐engineered mechanochemical route for multifunctional perovskites, uniting robust luminescence and efficient photocatalytic energy conversion.