Temperature‐Dependent Structural Evolution and Near‐Unity Yellow Emission in 0D Hybrid Copper Iodides for White‐Light Emitting Diodes and X‐ray Scintillators

ABSTRACT Recently, low‐dimensional metal halides have gained considerable attention in the field of solid‐state lighting owing to their excellent luminescence properties. However, understanding the structure‐property relationship is still a significant challenge. Herein, we studied the structural and photoluminescence evolution of a 0D hybrid Cu(I) iodide (ETMA) ₃ Cu ₂ I ₅ (ETMA: ethyltrimethylammonium) at low temperatures. At room temperature, the compound crystallizes in the orthorhombic P n m a space group as α‐(ETMA) ₃ Cu ₂ I ₅ . It exhibits efficient yellow self‐trapped exciton (STE) emission centered at 584 nm with a quantum yield of 99.7%. As temperature decreases, a phase transition occurs at 199 K, leading to asynchronous distortion of the two Cu‐I tetrahedra in the [Cu ₂ I ₅ ] ³⁻ dimer of (ETMA) ₃ Cu ₂ I ₅ and thereby inducing excitation‐wavelength‐dependent emission behavior. α‐(ETMA) ₃ Cu ₂ I ₅ exhibits high X‐ray scintillation performance with a light yield of 31700 photons/MeV and a low detection limit of 47.8 nGy/s. The flexible scintillator film based on α‐(ETMA) ₃ Cu ₂ I ₅ also demonstrates excellent X‐ray imaging performance. Furthermore, an optically pumped white‐light‐emitting diode (WLED) was fabricated using α‐(ETMA) ₃ Cu ₂ I ₅ as yellow phosphors with a correlated color temperature (CCT) of 5973 K and a high color rendering index (CRI) of 88.