Photoluminescence properties of Cs3Cu2Cl5 and CsCu2Cl3 with 1D-polyhedron-chain structure

Copper-based metal halides, as a metal halide polyhedral structure similar to perovskite, have garnered attention due to their superior optoelectronic properties and abundant resource reserves in the earth. Among these, Cs₃Cu₂I₅ has been applied in X-ray scintillators, X-ray imaging systems, and light-emitting diodes due to its outstanding luminescent properties. However, luminescence studies on its homologues—Cs₃Cu₂Cl₅ and CsCu₂Cl₃, which feature unique one-dimensional polyhedral chains—remain controversial. This is evident in the orthorhombic structure adopted by Cs₃Cu₂X₅ (X = Br/I), with the space group Pnma, while the space group of Cs₃Cu₂Cl₅ remains uncertain. Furthermore, conflicting reports on the fluorescence spectra and luminescent properties of CsCu₂Cl₃ crystals have hindered comprehensive investigation of this compound. In this study, we synthesized single crystals of Cs₃Cu₂Cl₅ and CsCu₂Cl₃ via vapor-assisted precipitation and compared their luminescent properties. The Cs₃Cu₂Cl₅ and CsCu₂Cl₃ crystals exhibit green and orange emission, respectively. Despite their similar one-dimensional structures, their photoluminescence quantum yields differ by more than two orders of magnitude (96.7% and 0.7%). CsCu₂Cl₃ crystals exhibit lower radiative transitions and higher non-radiative transitions compared to Cs₃Cu₂Cl₅. The experiment in combination with the density functional theory calculation reveals that their 1D-polyhedron-chains have distinct bonding structures and degrees of distortion. This leads to different distributions of electron wave functions and different concentrations of carrier-trapping chlorine vacancies, which account for their highly contrasted quantum efficiencies. The CsCu₂Cl₃ and Cs₃Cu₂Cl₅ crystals exhibit easy phase transition between each other driven by the changed temperature or ethanol erosion owing to their resembling skeleton structures of 1D polyhedral chain.

Ruonan Miao is a PhD student in the school of physics at Southeast University, China, specializing in the fluorescence properties of semiconductor copper-based metal halides.