The development of lead-free perovskite is seen as a windfall for the future of photovoltaic (PV) technology. The use of double perovskite as a solution to the problem of lead toxicity that plagues the PV sector is an intriguing possibility. Lead-free perovskite (Cs2AgBi0.75Sb0.25Br6) solar cells with a large bandgap of 1.8 eV have been modeled using SCAPS-1D device simulation software. In this simulation work, firstly, the device is calibrated to reach the experimental work. We studied the optimal thickness of the absorber layer, and we examined the diffusion length as a measure of absorber quality by varying the bulk defect density (1 × 1013 cm−3 to 1 × 1017cm−3) of the absorber layer. Both of these analyses were performed in conjunction with one another. The thickness of the absorber layer is a primary aspect of a high-performing device. The results revealed that the high carrier diffusion length of 1 µm and carrier lifetime of 200 ns is enough for high conversion efficiency. We have achieved 10.10% PCE (Power conversion efficiency), a high Voc of 1.123 V and Jsc of 15.28 mA-cm−2 with optimized parameters of the proposed model.