One of the most widely discussed solar cells lately is the perovskite solar cell, due to its high PCE, adjustable bandgap, and low-cost. Conversely, the intrinsic and interfacial defect densities may potently affect the stability and functionality of these cells since they act as recombination centers and restrict the carrier transport extremely. This paper gives a numerical performance analysis of a lead-free perovskite solar cell (ITO/LBSO/FASnI₃/CuO) structure via SCAPS-1D software. The FASnI₃ was chosen as absorber material because of its desirable band gap and non-toxicity, whereas LBSO and CuO were used as the electron and hole transport layers, respectively, owing to their desirable energy level alignment. We examine the effects of the density of defects in the absorber layer, the transport layers, and the variations in the absorber thickness. According to our results, the drastic enhancements in final PCE, from 19.20 % to 29.46 %, can be achieved by adjusting bulk defect densities to the range of 1x10⁻¹⁷ to 1x10⁻¹³. Better performance occurs during optimization of absorber thickness up to 1000 nm. These observations underline the ability to manipulate the level of defects and thickness to design and develop stable and efficient PSCs technologies.