The stability behavior of perovskite-based solar cells (PSCs) has recently been enhanced by the addition of 2D perovskite materials to conventional 3D perovskite films as a practical approach. In this research, tetrabutylammonium tetrafluoroborate (TBABF₄) salt was used to fabricate a 2D/3D structure over the 3D perovskite film. It improved the efficiency and stability behavior of PSCs. To further boost the properties of the 2D/3D bi-layer, methylammonium chloride as an additive was incorporated into a 2D TBABF₄-based capping layer. As the results showed, this additive engineering approach could improve the optical and morphological properties of the 2D/3D perovskite layers and the photovoltaic performance of the reference 2D/3D PSCs. A champion power conversion efficiency (PCE) of 18.09% was obtained for the 2D/3D devices modified with the additive; it was 14% higher than that of the fabricated reference cells (15.81%). The proposed approach is facile and low-cost, and it can effectively increase the PCE and stability of hybrid 2D/3D PSCs. Additive engineering dramatically reduced the nonradiative recombination of the perovskite film and facilitated the charge transport processes at the interface of perovskite-HTL. After modification, the hydrophobicity and the (001) crystalline orientation of the 2D/3D perovskite layers enhanced. This, in turn, led to the improvement of the ambient and irradiance stability of the modified 2D/3D PSCs.