Lead-Free (TEA)₂Cu₂I₄ Scintillators with Ultrahigh Light Yield Enabling Flexible Fiber-Based Signal Transmission

X-ray detection and imaging are crucial in many fields, yet most conventional scintillators suffer from brittleness, limited light yield, and incompatibility with flexible or confined-space applications. Here, we report the first synthesis of transparent single crystals of (C₈H₂₀N)₂Cu₂I₄ ((TEA)₂Cu₂I₄), the iodide analogue of (TEA)₂Cu₂Br₄. Although (TEA)₂Cu₂Br₄ has shown remarkable scintillation performance, its iodide counterpart, expected to deliver stronger X-ray absorption and higher efficiency, had remained inaccessible. Single-crystal diffraction reveals that TEA⁺ cations fully encapsulate [CuI₂]^- units, forming spatially isolated 0D motifs that promote efficient self-trapped exciton recombination. The crystals emit deep-blue light at 450 nm with a photoluminescence quantum yield of 96% and a microsecond STE lifetime. Under X-ray excitation, (TEA)₂Cu₂I₄ achieves a record light yield of 115,000 Photons/MeV, a detection limit of 167 nGy_air/s, and an imaging resolution of 11.9 lp/mm. These findings establish (TEA)₂Cu₂I₄ as the first structurally well-defined iodide analogue in this family, and they highlight structural design as a viable strategy for developing high-performance lead-free Cu(I)-halide scintillators.