In this study, a novel Ge-Sn based perovskite solar cell (PSC) with the structure FTO/WS2/ FA0.75MA0.25Sn0.95Ge0.05I3/MoO3/Ag has been designed and thoroughly analyzed employing SCAPS-1D. Drawing attention from the work of Ito et al where a similar perovskite-based PSC displayed a poor performance of ∼ 4.48% PCE, in which a large conduction band offset (CBO) acts as a critical factor contributing to interfacial recombination and device deterioration. To address this issue, we presented WS2 as an electron transport layer (ETL) along with MoO3 as a hole transport layer (HTL), both possessing compatible CBO and valence band offset (VBO) with perovskite material. Through systematic simulations and optimizations, remarkable improvements in the PSC's performance have been acquired, getting a power conversion efficiency (PCE) of 18.97%. The optimized structure involved a 50 nm MoO3 HTL, 350 nm FA0.75MA0.25Sn0.95Ge0.05I3 light-harvesting layer (LHL), and a 50 nm WS2 ETL. Bulk defect densities for the LHL and ETL were optimized to 1 × 1015 cm−3 and 1 × 1018 cm−3, respectively, significantly superior values than that of reported value in the literature. Particularly, the tolerable defect density of ETL has increased 1000 times more than the published literature. The interfacial tolerable trap density for MoO3/perovskite increased from 1 × 1014 cm−2 to 1 × 1016 cm−2. The study also explored the impact of defects on quantum efficiency, revealing a severe negative influence beyond a perovskite bulk defect density of 1 × 1017 cm−3. Light intensity analysis demonstrated a correlation between incident light reduction and device performance decay. Capacitance–Voltage (C-V) and Mott–Schottky (M-S) have been analyzed during the study. Finally, the total recombination of the optimized device concerning thickness has been analyzed along with the dark J-V characteristics. The comprehensive insights gained from this work are anticipated to accelerate the fabrication of mixed Ge-Sn based PSCs with improved efficiency, paving the way for commercialization in the photovoltaic industry.