Surface imperfections in perovskite films upon crystallization may trigger trap-assisted non-radiative recombination which is a dominant recombination mechanism that potentially restricts the performance of solar devices. In this work, 2D alkylammonium halide perovskites are formed on the 3D perovskite structure to passivate interfacial defects and vacancies and enhance moisture tolerance. The hybrid 3D/2D perovskite films possess longer photoluminescence lifetimes, as well as lower trap state densities, indicating the passivation of cationic and halide vacancies on the surface or grain boundaries, thereby reducing the non-radiative recombination pathways. More importantly, the hybrid 3D/2D perovskite exhibits higher ambient stability than a pure 3D perovskite where the hydrophobic nature of the long aliphatic carbon chains in the 2D perovskite provide an additional moisture repelling effect to the entire perovskite film. With this approach, the power conversion efficiency of perovskite solar cells was improved from 14.17% to 15.74% along with improved device stability. The hybrid 3D/2D perovskite solar cell retained 86% of its initial power conversion efficiency whereas the control device lost almost 40% of its overall efficiency. Thus, the hybrid 3D/2D perovskite structure is an alternative solution for modulating defects and trap-state densities in high efficiency perovskite solar cells with simultaneously enhanced moisture stability.