Several studies have demonstrated that low-dimensional structures (e.g., two-dimensional (2D)) associated with three-dimensional (3D) perovskite films enhance the efficiency and stability of perovskite solar cells. Here, we aim to track the formation sites of the 2D phase on top of the 3D perovskite and to establish correlations between molecular stiffness and steric hindrance of the organic cations and their influence on the formation and crystallization of 2D/3D. Using cathodoluminescence combined with a scanning electron microscopy technique, we verified that the formation of the 2D phase occurs preferentially on the grain boundaries of the 3D perovskite. This helps explain some passivation mechanisms conferred by the 2D phase on 3D perovskite films. Furthermore, by employing in situ grazing-incidence wide-angle X-ray scattering, we monitored the formation and crystallization of the 2D/3D perovskite using three cations with varying molecular stiffness. In this series of molecules, the formation and crystallization of the 2D phase are found to be dependent on both steric hindrance around the ammonium group and molecular stiffness. Finally, we employed a 2D/3D perovskite heterointerface in a solar cell. The presence of the 2D phase, particularly those formed from flexible cations, resulted in a maximum power conversion efficiency of 21.5%. This study provides insight into critical aspects related to how bulky organic cations' stiffness and steric hindrance influence the formation, crystallization, and distribution of 2D perovskite phases.