First-Principles Study of Lead-Free Double Perovskites Rb₂SiX₆ (X = Cl, Br, and I) for Solar Cell Applications

The double perovskites play an important role in renewable energy. They are a potential alternative to fulfill the energy demand for the sustainable growth and development of lead-free solar cells. This work investigates the electronic, thermoelectric, mechanical, and optical properties of the new double perovskites Rb₂SiX₆ (X = Cl, Br, and I). To know about the structural and thermodynamic stability, we studied the tolerance factor and enthalpy of formation, ensuring its thermodynamic stability. Poisson's and Pugh's ratios are examined from the elastic constants to know about the ductility and brittleness of the studied material. From the study of band structure, the direct band gaps of 3.19, 1.68, and 0.17 eV are obtained for Rb₂SiX₆ (X = Cl, Br, and I), respectively, ensuring such materials' applicability in ultraviolet, visible, and infrared regions for solar cell applications. Optical properties are investigated by analyzing the optical loss, refractive index, absorption of light, and dielectric constants. Lattice thermal conductivity is investigated using Slack's model. Thermoelectric performance is evaluated using the figure of merit (ZT), with room temperature values of 0.77, 0.76, and 0.44 for Rb₂SiX₆ (X = Cl, Br, and I), respectively, highlighting their thermoelectric potential.