Two-dimensional lead-free mixed-halide perovskites with tunable optical properties

Chemically tuned organic–inorganic layered (C4H9NH3)2CuClxBr4−x perovskites have been synthesized and characterized. The crystal structure was investigated by X-ray diffraction (XRD). The morphology was explored by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The chemical structure was studied by Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The thermal stability was determined by thermogravimetric analysis (TGA). The optical properties were investigated by UV-Vis absorption spectroscopy and spectroscopic ellipsometry. Thermochromism in (C4H9NH3)2CuClxBr4−x crystals was observed and the mechanism was determined by a combination of differential scanning calorimetry (DSC), in situ temperature-dependent Raman spectroscopy, and in situ temperature-dependent XRD. This study paves the way to explore the structure–property relationships in two-dimensional perovskites and offers inspiration for the development of eco-friendly thermochromic devices.