Computational Engineering of Direct-Bandgap Double Perovskites for Solar Cells

The pursuit of sustainable, lead-free perovskites has fueled the growing interest in halide double perovskites. This study utilizes first-principles calculations to explore the structural, elastic, electronic, and optical properties of Rb2NaInX6 (X=Br, I) and to evaluate their photovoltaic potential. The structural stability and mechanical strength of Rb2NaInBr6 have been substantiated through the application of Goldschmidt's tolerance factors, elastic constants, and bulk moduli. These findings collectively indicate that Rb2NaInBr6 exhibits superior mechanical performance. Electronic analysis indicates direct band gaps of 2.879 eV and 1.35 eV (TB-mBJ) for Rb2NaInBr6 and Rb2NaInI6, respectively, thereby enabling robust absorption across the ultraviolet (UV) to visible (Vis) spectrum. The simulated solar cell architectures (FTO/TiO2/Rb2NaInX6/Cu2O/Au) demonstrate efficiencies of 4.81% for the bromide and 26.6% for the iodide compound, with an absorber thickness of 0.8 μm. These findings identify Rb2NaInI6 as a promising eco-friendly material for high-efficiency, lead-free solar cells.