Computational study of mixed halide perovskites CsSnX3 (X = Br, Cl, mixed halides) for optoelectronic and thermoelectric applications: A DFT and Boltzmann transport methods

Abstract

Density Functional Theory (DFT) is a powerful tool in materials science, widely used to predict key properties such as mechanical, electronic, optical, and thermoelectric characteristics. This study employs DFT method with both the Generalized Gradient Approximation (GGA-PBE) and the Hybrid Functional (HSE) approaches to explore the potential of CsSnBr_3-xCl_x mixed halide perovskites for photovoltaic and thermoelectric applications. The structural analysis indicates that CsSnBr₃ and CsSnCl₃ crystallize in a cubic phase with the Pm-3 m space group, whereas the mixed halide compounds CsSnBr₂Cl and CsSnCl₂Br adopt a tetragonal structure belonging to the P4/mmm space group. The negative formation energy values confirm the thermodynamic stability of these materials. Mechanical property evaluations indicate that the materials are mechanically stable and ductile, with bonding predominantly ionic in nature. Furthermore, electronic band structure calculations show that all compounds exhibit p-type semiconducting behavior with direct band gaps. Notably, the HSE approach provides more accurate electronic properties compared to the GGA-PBE approximation, with band gap values of 1.22 eV, 1.41 eV, 1.50 eV, and 1.62 eV for CsSnBr₃, CsSnBr₂Cl, CsSnCl₂Br, and CsSnCl₃, respectively. Optical properties, including dielectric function, absorption, refractive index, optical loss, and reflectivity, were analyzed across the 0–10 eV energy range, highlighting significant absorption in the visible and ultraviolet regions, crucial for solar cell applications. Thermoelectric performance was assessed using the figure of merit (ZT), considering the Seebeck coefficient, thermal conductivity, and electrical conductivity. At 1000 K, CsSnBr₃, CsSnBr₂Cl, CsSnCl₂Br, and CsSnCl₃ show ZT values of 0.46, 0.47, 0.48, and 0.52, respectively, indicating improved thermoelectric efficiency with increasing Cl content. These findings suggest that CsSnBr₂Cl, CsSnCl₂Br, and CsSnCl₃ are promising materials for high-temperature thermoelectric applications.