2D\nPhase Formation on 3D Perovskite: Insights from\nMolecular Stiffness

Several studies have\ndemonstrated that low-dimensional structures\n(<i>e.g</i>., two-dimensional (2D)) associated with three-dimensional\n(3D) perovskite films enhance the efficiency and stability of perovskite\nsolar cells. Here, we aim to track the formation sites of the 2D phase\non top of the 3D perovskite and to establish correlations between\nmolecular stiffness and steric hindrance of the organic cations and\ntheir influence on the formation and crystallization of 2D/3D. Using\ncathodoluminescence combined with a scanning electron microscopy technique,\nwe verified that the formation of the 2D phase occurs preferentially\non the grain boundaries of the 3D perovskite. This helps explain some\npassivation mechanisms conferred by the 2D phase on 3D perovskite\nfilms. Furthermore, by employing <i>in situ</i> grazing-incidence\nwide-angle X-ray scattering, we monitored the formation and crystallization\nof the 2D/3D perovskite using three cations with varying molecular\nstiffness. In this series of molecules, the formation and crystallization\nof the 2D phase are found to be dependent on both steric hindrance\naround the ammonium group and molecular stiffness. Finally, we employed\na 2D/3D perovskite heterointerface in a solar cell. The presence of\nthe 2D phase, particularly those formed from flexible cations, resulted\nin a maximum power conversion efficiency of 21.5%. This study provides\ninsight into critical aspects related to how bulky organic cations’\nstiffness and steric hindrance influence the formation, crystallization,\nand distribution of 2D perovskite phases.