Room-Temperature Solid-State Synthesis of Cs₃Cu₂I₅ Thin Films and Formation Mechanism for Its Unique Local Structure

Blue-emitting Cs₃Cu₂I₅ has attracted attention owing to its near-unity PL quantum yield and applications in DUV photodetectors and scintillators. Its PL properties originate from the unique local structure around the luminescent center, the [Cu₂I₅]^3- polyhedron iodocuprate anion consisting of the edge-shared CuI₃ triangle and the CuI₄ tetrahedron dimer, which is isolated by Cs⁺ ions. We found that solid-state reactions between CsI and CuI occur near room temperature (RT) to form Cs₃Cu₂I₅ and/or CsCu₂I₃ phases. High-quality thin films of these phases were obtained by the sequential deposition of CuI and CsI by thermal evaporation. We elucidated that the formation of interstitial Cu⁺ and the antisite of I^- at the Cs⁺ site in the CsI crystal through Cu⁺ and I^- diffusion results in the RT synthesis of Cs₃Cu₂I₅. The unique structure formation of the luminescent center was revealed using a model based on the low packing density of the CsCl-type crystal structure, similar sizes of Cs⁺ and I^- ions, and the high diffusivity of Cu⁺. The self-aligned patterning of the luminous regions on thin films was demonstrated.