Why do similar 1D-polyhedron-chain copper chloride semiconductors have 2-order-distinct luminescence quantum efficiencies?

The "green" copper halides with one-dimensional polyhedron chains are very interesting novel semiconductors. These weakly interacting parallel quantum wires (1D polyhedron chains) play key roles in their photophysical properties. Unlike Cs3Cu2I5, which has been much investigated, its homologous compounds Cs3Cu2Cl5 and CsCu2Cl3 remain less studied and their properties are controversial. Both of them are composed of specific 1D-polyhedron-chains. We report the synthesis and comparatively study the photophysical properties of the single crystals of Cs3Cu2Cl5 and CsCu2Cl3. They exhibit green and orange emissions, respectively. Surprisingly, their luminescence quantum efficiencies have a giant difference of over two orders of magnitude (96.7% vs 0.7%). The CsCu2Cl3 crystals exhibit much slower radiative transition and substantially faster nonradiative transition. 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.