Tin-based halide perovskites have drawn considerable scientific attention as promising candidates for next-generation photovoltaic devices, primarily because they circumvent the toxicological and regulatory challenges associated with lead-containing counterparts. In the present work, a lead-free n-i-p type perovskite heterostructure solar cell was numerically designed and systematically optimized via the one-dimensional Solar Cell Capacitance Simulator (SCAPS-1D). The proposed device adopts the configuration FTO/CH₃NH₃SnCl₃/CH₃NH₃SnBr₃/CH₃NH₃SnI₃/Au, in which the tin chloride perovskite functions as the electron transport layer (ETL), the tin bromide perovskite serves as the intrinsic absorber, and the tin iodide perovskite acts as the hole transport layer (HTL). A systematic parametric investigation was carried out, covering layer thickness, electron affinity, shallow doping concentration, and trap-state density for each functional layer. Simulation outcomes demonstrate that an absorber thickness of 750 nm yields the best balance between photon harvesting and charge-carrier collection. Furthermore, elevated doping in the absorber was found to reduce device performance through enhanced Shockley–Read–Hall (SRH) recombination, while a reduced trap density markedly improved all photovoltaic figures of merit. Following comprehensive optimization, the device achieved a power conversion efficiency (PCE) of 31.11%, a short-circuit current density (J_SC) of 33.89 mA/cm², an open-circuit voltage (V_OC) of 1.0410 V, and a fill factor (FF) of 88.18%. The influence of ambient temperature was also studied, revealing that 300 K represents the optimal operating condition. The findings underline the viability of all-tin halide perovskite architectures for sustainable, high-efficiency photovoltaics.