First-principles insights into lead-free Ruddlesden–Popper Cs 2 BX 4 (B = Ge, Sn; X = Br, I) perovskites for solar energy conversion and water splitting

Two-dimensional (2D) Ruddlesden–Popper (RP) lead-free halide perovskites offer superior stability and tunable optoelectronic features compared to their 3D counterparts. Using first-principles density functional theory, we systematically investigated all-inorganic Cs 2 BX 4 (B = Ge, Sn; X = Br, I) compounds to establish comprehensive trends across Ge/Sn and Br/I compositions. Structural optimization, phonon spectra, and ab initio molecular dynamics at 300 K confirm both dynamic and thermal stability, while formation energies and tolerance factors support the RP-phase geometry. HSE06+SOC calculations reveal direct band gaps of 1.43–2.00 eV, ideal for visible-light absorption. The optical response shows strong in-plane absorption ( ∼ 105 cm−1), low reflectivity ( < 30%), and refractive indices of 2.1–2.5. Elastic constants and Poisson’s ratios (0.24–0.29) indicate composition-dependent ductility and anisotropy, suggesting suitability for flexible devices. Band-edge positions relative to the normal hydrogen electrode identify Br-based systems as promising photocatalysts for water splitting. Overall, Cs 2 BX 4 RP phases emerge as stable, non-toxic, and tunable materials for next-generation optoelectronic and solar energy applications.