Lead‐free halide double perovskites have emerged as sustainable alternatives to toxic lead‐based materials for next‐generation photovoltaic (PV) and optoelectronic applications. In this study, a comprehensive first‐principles investigation is conducted on A 2 GeX 6 (A = K, Rb; X = Cl, Br) double perovskites to explore their structural, X‐ray diffraction, electronic, optical, mechanical, charge density, phonon, molecular dynamics (MD), thermal and energetic stability, thermodynamic, vibrational, and thermoelectric properties using density functional theory combined with SCAPS‐1D device simulations. All compounds exhibit excellent thermodynamic, mechanical, and dynamic stability, with negative formation energies and phonon spectra confirming the absence of imaginary frequencies. The bandgap values, ranging from 0.83 to 3.52 eV, can be tuned via halide and alkali substitutions, enabling both visible and UV light absorption. Optical analysis reveals strong absorption coefficients (∼10 5 cm −1 ), high dielectric constants, and favorable refractive indices, indicating efficient light‐harvesting capability. Charge density and MD analyses confirm robust Ge–X bonding and thermal resilience, maintaining structural integrity up to 1000 K. Thermodynamic and vibrational studies further validate their energetic stability, while thermoelectric evaluation suggests good potential for energy conversion. SCAPS‐1D simulations demonstrate promising device performance, achieving up to 22.3% power conversion efficiency for Rb 2 GeBr 6 with optimized absorber parameters. These findings establish A 2 GeX 6 double perovskites as nontoxic, structurally stable, and optoelectronically efficient candidates for future PV, thermoelectric, and optoelectronic devices.
Rahman et al. (Thu,) studied this question.