Perovskite CsSnI₃ is a highly promising material in optoelectronics, particularly for solar cells. This research focuses on enhancing the efficiency of CsSnI₃-based solar cells by investigating the effects of key parameters such as absorber layer thickness, defect density, and parasitic resistances on their performance. Utilizing the SCAPS-1D software, a solar cell with a layered architecture consisting of SPIRO/CsSnI₃/CH₃NH₃SnI₃/PCBM/ITO was simulated, where SPIRO functions as the hole transport layer, CsSnI₃ as the absorber layer, CH₃NH₃SnI₃ as the intermediate layer, and PCBM as the electron transport layer. Simulation results identified an optimal CsSnI₃ absorber thickness of 300 nm, yielding a maximum power conversion efficiency of 18.67%, with high values for open-circuit voltage (Voc = 0.85 V), short-circuit current density (Jsc = 31.092 mA/cm²), and fill factor (FF = 71.02%). However, the study also highlighted the adverse impact of parasitic resistances on cell efficiency, with the efficiency decreasing to 13.32% when realistic series resistance values were considered, underscoring the importance of minimizing these resistances to improve the stability of the device.Additionally, a notable efficiency of 19.47% was achieved when the shunt resistance was set at Rshunt = 108Ω⋅cm².