Rb₂BX₆ double perovskites: unlocking 22% efficiency through structural, electronic, mechanical, and optical insights

Developing stable and efficient perovskite-inspired materials has become a key focus in the pursuit of next-generation solar energy technologies, with recent advances in material design highlighting the potential of novel halide structures as sustainable alternatives to conventional silicon-based solar absorbers. This study presents a comprehensive first-principles investigation of novel Rb-based double perovskites, Rb₂BX₆ (B = Sn/Pb; X = Cl/Br), highlighting their potential for photovoltaic applications. All compounds exhibit negative formation enthalpies, indicating thermodynamic stability, and tolerance factors around 0.80 confirm structural feasibility. Mechanical stability is validated through Born criteria and elastic constants. Band structure calculations reveal direct band gaps of 2.646 eV (Rb₂SnCl₆), 1.451 eV (Rb₂SnBr₆), 1.379 eV (Rb₂PbCl₆), and 0.357 eV (Rb₂PbBr₆), suggesting semiconducting behavior, with Rb₂SnBr₆ and Rb₂PbCl₆ falling within the optimal range for visible-light absorption. Partial density of states (PDOS) analyses show that the valence bands are mainly composed of halide p-orbitals, while conduction bands are dominated by B-site s- and p-orbitals. ELATE tensor analysis reveals moderate elastic anisotropy, with anisotropy indices of 0.22 for Rb₂SnBr₆ and 0.18 for Rb₂PbCl₆. Optical studies indicate absorption coefficients exceeding 10⁵ cm^-1 in the visible region for both materials. Mulliken population analysis confirms strong ionic bonding and moderate charge transfer between atoms. Moreover, Rb₂SnCl₆ and Rb₂PbCl₆ are dynamically stable, whereas Rb₂SnBr₆ and Rb₂PbBr₆ exhibit dynamic instability. SCAPS-1D device simulations yield a power conversion efficiency (PCE) of 20.44% for Rb₂SnBr₆, accompanied by a short-circuit current density (J _SC) of 22.3 mA cm^-2, an open-circuit voltage (V _OC) of 1.01 V, and a fill factor (FF) of 89.7%. For Rb₂PbCl₆, a slightly higher PCE of 21.84% is achieved (J _SC of 23.1 mA cm^-2, V _OC of 1.05 V, FF of 90.1%). Although Rb₂PbCl₆ demonstrates superior efficiency, its toxicity due to lead content poses environmental concerns. In contrast, Rb₂SnBr₆ offers a highly efficient, non-toxic alternative, positioning it as a viable candidate for eco-friendly and sustainable photovoltaic devices.