Metal halide semiconductors are promising materials for large-area flexible electronics owing to their processability in solution.^1,2 In particular, toxic element-free copper-based iodides have attracted attention in photonics and electronics owing to their wide bandgap, p-type conductivity, sharp optical absorption, and excellent luminescence properties. We reported blue photoluminescent Cs₃Cu₂I₅ was attracted attention because of high PLQY we reported in 2018.³ It is inferred that the near-unity PL quantum efficiency of bright blue luminescence in Cs₃Cu₂I₅ originates from the unique local structure of the photoactive site; an asymmetric [Cu₂I₅]³⁻ dimer composed of an edge-shared CuI₃ triangle and a CuI₄ tetrahedron is isolated by Cs⁺ ions to form a pseudo-quantum dot. This photoactive site transforms into face-shared CuI₄ tetrahedrons upon photoexcitation and forms a self-trapped exciton, giving rise to PL with large Stokes shift and high PL quantum efficiency. However, why such a rare local structure exists in the ground state remained obscure to date. Recently, we found that solid-state reactions between CsI and CuI occur near room temperature (RT) to form Cs₃Cu₂I₅ and/or CsCu₂I₃ phases.⁴ 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 result in the RT synthesis of Cs₃Cu₂I₅. The unique structure formation of the photoactive site 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 pattering of the luminous regions on thin films was demonstrated.                                                                   

1)Grundman, M. et al. physica status solidi (a) 210, 1671 (2013), 2) Jun, T., Kim, J., Sasase, M., & Hosono, H. (2018). Advanced Materials, 30(12), 1706573. 3) Jun, T., Sim, K., Iimura, S., Sasase, M., Kamioka, H., Kim, J., & Hosono, H. (2018). Advanced Materials, 30(43), 1804547. 4) Tsuji, M., Sasase, M., Iimura, S., Kim, J., & Hosono, H. (2023). J. Am. Chem. Soc. 145, 21, 11650