Lead-Free (TEA) ₂ Cu ₂ I ₄ 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 (C 8 H 20 N) 2 Cu 2 I 4 ((TEA) 2 Cu 2 I 4 ), the iodide analogue of (TEA) 2 Cu 2 Br 4 . Although (TEA) 2 Cu 2 Br 4 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 [CuI 2 ] − 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) 2 Cu 2 I 4 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) 2 Cu 2 I 4 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.