Perovskite solar cells are increasing attention due to their unique characteristics in the field of photovoltaic technology. Lead-based perovskite solar cells are particularly notable for their high efficiency. However, their commercial production is limited because of the use of lead-based absorbers. As a result, research has shifted towards exploring lead-free alternatives in the realm of perovskite materials. This study utilizes SCAPS-1D simulation software to optimize the performance of a lead-free flexible solar cell. Lead (Pb), a group 14 element, is proposed to be replaced by bismuth (Bi), a group 15 element. We investigate how the selection of configurations for the Electron Transport Layer (ETL), Hole Transport Layer (HTL), and absorber layer impacts solar cell performance. This study represents a comprehensive examination of this material. The device optimization involves using a fluorine-doped tin oxide (FTO) substrate, cadmium sulfide (CdS), tungsten disulfide (WS2), titanium dioxide (TiO2), and fullerene (C60) as ETL components, methyl ammonium bismuth iodide (CH3NH3BiI3) as the absorber, molybdenum disulfide (MoS2) as the HTL, and platinum (Pt) as the electrode. In addition to optimizing ETL and HTL configurations, this study explores the effects of various factors such as absorber, ETL, and HTL thickness, shunt and series resistance, temperature variations, Mott-Schottky behavior, capacitance, recombination, generation rates, J-V characteristics, and quantum energy. The results show that the solar cell configuration using FTO/CdS/CH3NH3BiI3/MoS2/Pt achieves an efficiency of 26.60 %, a current density (JSC) of 32.02 mA/cm2, an open circuit voltage (VOC) of 0.974 V, and a fill factor (FF) of 85.24 %. This represents a significant improvement compared to previous research. This detailed simulation analysis enables researchers to develop cost-effective and highly efficient perovskite solar cells (PSCs), driving advancements in solar technology.