Room-Temperature Solid-State Synthesis of Cs3Cu2I5Thin Films and Formation Mechanism for Its Unique Local Structure

Blue-emitting Cs3Cu2I5has 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 CuI3triangle and the CuI4tetrahedron dimer, which is isolated by Cs+ions. We found that solid-state reactions between CsI and CuI occur near room temperature (RT) to form Cs3Cu2I5and/or CsCu2I3phases. 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.