Reversible Thermochromism in Lead-Free 2D Copper-Based Perovskites for Smart Solar Applications

The present study has focused on toxic lead-free low-dimensional hybrid perovskites due to their impressive optoelectronic properties, highlighting concerns over their environmental stability and toxicity. In light of this, versatile 2D lead-free (CH₃NH₃)₂CuCl₄ (CuP), Co-infused (CH₃NH₃)₂CuCl₄ (CCuP), and Ni-infused (CH₃NH₃)₂CuCl₄ (NCuP) hybrid perovskites are synthesized via the sono-coprecipitation method with a potential compositional engineering approach and dopant effects studied using different characterization tools. The diffractograms reveal that the partial dopant Co and Ni insertion at the “B”- site of the (CH₃NH₃)₂CuCl₄ crystal effectively modified the optical bandgaps from 2.29 to 2.16 eV. The perovskites display a spectrum of colors (yellow, orange, red, and dark brown) accompanied by thermal-induced lattice expansion, phase transition, and electron–phonon interactions. Lattice dynamics, structural and optical features are explored to gain insights into the effects of cation infusion on thermochromism and other characteristics. The in situ temperature-dependent powder VT-XRD, VT-Raman, and VT-UV–vis spectra were analyzed in heating and cooling cycles. Notably, it is discovered that the degree of thermochromism is possibly depending on the dopant used. Repetitive heating–cooling cycles showed minimal fluctuation in VT-XRD, demonstrating excellent stability with thermochromic properties. Cyclic voltammetry was performed for the first time on these copper-based perovskites to investigate the electrochemical activity with plausible oxidation and reduction reactions. Dielectric characteristics and AC conductivity at different temperatures have been studied. A sustainable post-treatment technique resulted in excellent stability, crystallinity, and tailored grain boundaries at the nanoscale using a solvent with a lower Gutmann donor number and Kamlet–Taft parameter. The present study provides insights into the basic principles governing thermochromic behavior and possibilities for the thoughtful development of perovskites. These are believed to be the optimal results for the systems that have been reported to date.