Facile, Scalable, and High‐Yield Synthesis of a Near‐Unity Quantum Yield Zero‐Dimensional Copper Halide Cs 3 Cu 2 Cl 5 Scintillator for X‐Ray Imaging

Metal halide perovskites, particularly copper halide variants, have emerged as promising scintillators owing to their large Stokes shift, high photoluminescence quantum yield (PLQY), and efficient radioluminescence. Here, we demonstrate a dioxolane‐mediated reaction synthesis as a simple, scalable, and high‐yield strategy for producing Cs 3 Cu 2 Cl 5 powders with near‐complete conversion. By employing low‐polarity yet water‐miscible dioxolane as the reaction medium together with an aqueous H 3 PO 2 additive, the solubility imbalance between precursors inherent to low‐polarity solvents is effectively addressed. This enables stoichiometry‐controlled dissolution–precipitation under a low‐polarity environment and the formation of phase‐pure Cs 3 Cu 2 Cl 5 powders with near‐unity PLQY. The powders show bright green emission at ∼530 nm and retain over 80% of their initial efficiency after 300 days of ambient storage without encapsulation, demonstrating remarkable intrinsic stability. Cs 3 Cu 2 Cl 5 ‐based composite films deliver an ultrahigh light yield of ∼98,000 photons MeV −1 , surpassing commercial scintillators such as Gd 2 O 2 S and CsI:Tl, and exhibiting excellent dose linearity and irradiation stability. When integrated into commercial flat‐panel detectors, the Cs 3 Cu 2 Cl 5 ‐based composite films enabled high‐quality 2D radiographic and 3D computed tomography imaging, confirming compatibility with existing X‐ray detection systems. This facile and efficient synthesis, together with the demonstrated device‐level imaging performance, highlights Cs 3 Cu 2 Cl 5 as a scalable and practical candidate for next‐generation X‐ray scintillators.