Vibrational Spectroscopic, Optical Properties and Electrical Conduction Mechanism of Lead-Free Perovskite Cs₂ZnCl₄

In recent years, the metallic halide perovskite materials without lead have drawn a lot of interest as incredibly fascinating materials for photovoltaic applications. These materials offer exceptional stability, lower toxicity, and convenient process ability making them attractive alternatives to traditional lead-based perovskites. Recent studies have demonstrated its successful utilization in indoor energy applications, thereby suggesting its potential for integration into building-integrated devices. Using the slow evaporation solution growth technique, we have successfully synthesized a non-toxic Cs₂ZnCl₄ metal halide compound. The X-ray diffraction measurement at room temperature that the Cs₂ZnCl₄ crystallizes in the orthorhombic phase of the Pnma space group. The morphological characteristics and the ratios of the constituents of Cs₂ZnCl₄ were determined by the SEM–EDX study. The assignment of the Raman spectrum was based on DFT calculations. The optimized geometries, vibrational frequencies, and Raman intensities were calculated using the CAM-B3LYB technique with the 6-31G(d,p) basis set. Furthermore, the direct band gap of Cs₂ZnCl₄ was determined to be \(4.04 {\text{eV}}\) $4.04\text{eV}$ based on UV–Visible spectroscopy measurement and DFT calculations. In order to model the electrical response of the grain and the grain boundary, we have chosen an equivalent circuit composed by a series combination of two cells which are composed by a parallel combination of resistance (R) and constant phase element (CPE). The conductivity frequency dependence is interpreted using the non-overlapping small polar tunneling model (NSPT).