This study employed solar-cell capacitance simulator (SCAPS-1D) simulation to investigate the effects of bulk defect concentration, absorber thickness, and operating temperature on the carrier lifetime, current density–voltage (J–V) characteristics, quantum efficiency (QE), and photovoltaic parameters (Voc, Jsc, FF, and η) of SnS₂/ZnSnN₂/Cu₂O solar cells. Increasing the bulk defect concentration from 10¹² to 10¹⁶ cm⁻³ reduced the carrier lifetime from 10⁵ ns to 10 ns and decreased the efficiency from 33.38% to 19.91% due to enhanced SRH recombination. The optimum ZnSnN₂ absorber thickness was found to be 0.8 µm, yielding Voc = 0.981 V, Jsc = 31.71 mA cm⁻², FF = 84.90%, and η = 26.44%. Thermal analysis (273–333 K) showed a decrease in efficiency from 27.89% to 24.63%, while QE remained nearly unchanged. These findings identify the critical design limits and demonstrate the potential of SnS₂/ZnSnN₂/Cu₂O solar cells for stable high-performance photovoltaic applications.