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.