Single crystal of two-dimensional mixed-halide copper-based perovskites with reversible thermochromism

Two-dimensional (2D) lead-based hybrid perovskites have shown strong photochemical stability. However, the high toxicity of lead hinders their commercialization. Here, we report the synthesis of 2D Cu-based mixed-halide perovskite (CH3NH3)(2)CuClxBr4-x thin films by the exfoliation of centimeter-sized single crystals and study their structural, thermal, and optical properties, and thermochromism. A strong orientation along the c-axis direction with preferable (00l) diffraction peaks in the XRD spectrum and no grain boundary in the SEM image of the bulk crystal confirm single crystal formation. As the Br content increases, mixed-halide Cu-perovskites exhibit tunable band gap energy ranging from 2.24 eV to 1.43 eV. Upon temperature variation, the Cl-rich perovskites show a reversible thermochromic behavior with a color change between yellow, orange, red, and brown, but no observable color change is observed for Br-rich perovskites. The in situ temperature-dependent X-ray diffraction reveals that thermochromism originates from structural changes in the 2D crystal.