Comprehensive first-principles analysis and device simulations of vacancy-ordered D2CeX6 double perovskites for high-efficiency lead-free solar cells.

The quest for efficient and durable absorber materials has steered attention toward vacancy-ordered double perovskites, which exhibit tunable band gaps and strong optical absorption, making them promising candidates for next-generation solar cell architectures. In particular, lead-free vacancy-ordered halide double perovskites have emerged as viable alternatives to toxic Pb-based counterparts. In this study, we systematically investigate the structural, electronic, charges density, mechanical, optical, phonon stability, molecular dynamics (MD), population analysis and photovoltaic properties of D2CeX6 (D = Ga, In, Tl; X = Cl, Br) compounds by employing first-principles calculations in conjunction with SCAPS-1D device simulations. Our results reveal that all compounds crystallize in a stable cubic phase with negative formation enthalpies, confirming their thermodynamic stability. Within GGA-PBE, the calculated direct band gaps are 1.733 eV (Ga2CeCl6), 1.276 eV (Ga2CeBr6), 1.555 eV (In2CeCl6), 0.859 eV (In2CeBr6), 1.755 eV (Tl2CeCl6), and 1.364 eV (Tl2CeBr6), placing all but In2CeBr6 within or near the optimal 1.1 to 1.8 eV range for single-junction solar cells. HSE06 hybrid functional results yield wider gaps of 1.776 eV, 2.843 eV, 2.261 eV, 2.170 eV, 3.632 eV, and 1.418 eV, respectively, suggesting suitability for both single-junction and tandem architectures. Optical analyses demonstrate high absorption coefficients (>105 cm-1), strong dielectric responses, and large refractive indices, particularly in In2CeBr6 and Tl2CeBr6. Mechanical evaluations confirm ductile behavior, with Tl2CeBr6 and In2CeBr6 exhibiting superior stiffness and near-isotropic mechanical stability. Molecular dynamics simulations (NPT ensemble, 50 ps) at room temperature confirm the excellent thermal robustness of the studied compounds. The phonon dispersion analysis further indicates full dynamical stability for Ga2CeCl6, In2CeCl6, and Tl2CeCl6, while Ga2CeBr6, In2CeBr6, and Tl2CeBr6 exhibit minor soft or near-zero modes that are likely stabilized by finite-temperature effects. Device-level simulations predict power conversion efficiencies (PCE) up to 25.29% for Ga2CeBr6, with In2CeCl6 and Tl2CeBr6 also exceeding 24%. These findings position the D2CeX6 family, especially bromide-based compounds, as promising candidates for efficient and stable lead-free perovskite solar cells.