This study examines the structural, electronic, mechanical, optical, phonon, and population properties of novel In2GeX6 (X = Cl, Br) double perovskite materials using density functional theory (DFT). The In2GeBr6 compound shows the largest unit cell volumes, lattice constants, and densities among the materials studied. The structural stability of all compounds is confirmed through tolerance factor analysis, while their chemical and mechanical stability is supported by formation energy and Born stability criteria. The band gap energies of In2GeX6 perovskites are found to be direct at the high-symmetry M point when using the GGA-PBE functional. To gain a better understanding of their electrical behavior, the partial density of states (PDOS) and total density of states (TDOS) are analyzed. Strong interatomic bonds, high resistance, superior ductility, machinability, hardness, and a significant amount of elastic anisotropy are among the other mechanical properties, anisotropy factors, and elastic constants that are evaluated for In2GeCl6 and In2GeBr6. These anisotropic characteristics are also visualized using three-dimensional contour maps. A thorough analysis is conducted of the materials' optical properties, such as their absorption coefficient, optical conductivity, dielectric function, refractive index, reflectivity, and energy loss function. Phonon studies show that both compounds are dynamically stable. Additionally, population analysis reveals insights into their bonding nature. Regarding their potential for solar cell applications, the materials' performance is assessed based on parameters such as thickness, shallow acceptor density, total defect density, and interface defect density. The research also discloses the temperature-dependent behavior and JV-QE characteristics through SCAP-1D simulation. The power conversion efficiency (PCE) is about 18.24 % for In2GeCl6 and 26.68 % for In2GeBr6 with the optimization of solar cell device (FTO/CdS/In2GeX6). Among perovskite compounds, In2GeX6 displays more promising potential for efficient operation in multijunction solar cells and optoelectronic devices.