<p>Germanium-based perovskite solar cells have emerged as promising lead-free alternatives to conventional perovskite photovoltaic technologies due to reduced toxicity and relative earth abundance. This study presents a comprehensive computational investigation of four germanium-based perovskite absorbers MAGeI₃, FAGeI₃, CsGeI₃, and RbGeI₃ using SCAPS-1D numerical simulations to evaluate their potential for high-efficiency solar cells. Although germanium possesses favorable properties including lower spin–orbit coupling and better charge-carrier transport characteristics than lead-based perovskites, the strong tendency of Ge²⁺ to oxidize to Ge⁴⁺ results in elevated defect densities exceeding 10¹⁵&#xa0;cm⁻³, significantly limiting device performance through enhanced trap-assisted recombination. A planar <i>n-i-p</i> device architecture comprising FTO/TiO₂/Ge-based perovskite/Spiro-OMeTAD/Au was simulated under identical conditions to enable systematic comparison. Key parameters including absorber thickness, bulk defect density, and interface defect density were systematically optimized to determine their influence on photovoltaic characteristics. Results demonstrate that an optimal absorber thickness of approximately 400&#xa0;nm provides the best balance between light absorption and recombination losses across all four materials. Analysis of current density–voltage characteristics, external quantum efficiency spectra, and generation-recombination profiles reveals that RbGeI₃ exhibits superior performance with the highest power conversion efficiency, open-circuit voltage, and fill factor, while FAGeI₃ displays lower efficiency due to increased recombination losses. These findings highlight the critical importance of defect control in developing efficient germanium-based perovskite solar cells and provide valuable insights for future experimental optimization strategies toward environmentally benign, high-performance photovoltaic devices.</p>