Tunable Optical Properties of Two-Dimensional Copper-Based Halide Perovskites with Mixed Organic Cations

Copper-based halide perovskites are evolving as alternative materials to lead-based perovskites. Herein, we report the synthesis of double organic cation copper-halide perovskites (C4H9NH3)x(3-BrC3H6NH3)2–xCuCl2+xBr2–x (x = 0.7, 1.0, 1.3, 1.6, and 1.9). The crystal structures were studied by powder X-ray diffraction (XRD). The optical properties of the perovskite thin films were investigated by UV–vis absorption spectroscopy and spectroscopic ellipsometry. It was demonstrated that the use of multiple cations is an effective compositional strategy to control the structural properties of two-dimensional (2D) perovskites. In addition, the thermochromism was also investigated by differential scanning calorimetry and in situ temperature-dependent powder XRD. This topic opens up a path for 2D copper-based halide perovskites to adjust their optical properties via spacer cation engineering. This research inspires future research interests in designing environmentally friendly 2D metal halide perovskites.