<p>Recent advancements in low-cost fabrication techniques have intensified research interest in three-dimensional (3D) perovskite solar cells (PSCs) due to their exceptional photoconversion efficiencies. However, their practical deployment remains limited by poor long-term stability under ambient conditions. In contrast, two-dimensional (2D) perovskites offer superior environmental stability, attributed to the incorporation of bulky, hydrophobic organic cations in their crystal structure. Nevertheless, their wider optical bandgaps often result in reduced power output. To address these issues, the integration of 2D and 3D perovskites into a bilayered architecture has emerged as a promising strategy, combining the efficiency of 3D perovskites with the stability of 2D materials. In this study, a novel 2D/3D PSC architecture—Glass/FTO/SnO₂/3D perovskite/2D perovskite/Cu₂O/Ni—was modeled and optimized using the SCAPS-1D simulation tool. The model’s physical parameters were calibrated based on experimental data to ensure realistic performance predictions. Simulation results demonstrate that proper energy band alignment between the 2D and 3D perovskite layers, along with the judicious selection of charge transport layers, substantially enhances device performance. Further optimization of structural parameters, including absorber thickness, acceptor doping level, interface trap densities at the 2D/3D boundary, and the work function of the back contact, yielded a maximum power conversion efficiency of 23.10%, nearly 24% higher than the initial setup. This performance enhancement primarily results from increased quantum efficiency, higher photogeneration rates, and significant suppression of interfacial recombination. These findings highlight the potential of 2D/3D perovskite structures to simultaneously improve efficiency and stability, thereby accelerating the commercial viability of next-generation perovskite solar cells.</p>