The Interplay of Organic Spacers and Small Cations for Efficient Dion–Jacobson Perovskite Solar Cells

2D halide perovskites have displayed wide tunability in structure and physicochemical properties through the diversity and versatility of interlayer spacers. However, the interaction between the large spacer and small cation inside the inorganic framework has not been elucidated yet. Herein, the impact of small‐organic cation on the crystallization kinetics, carrier behavior, and the structure distortion of the low‐dimensional Dion–Jacobson (DJ) perovskite materials and solar cells is systematically studied. The formamidium (FA)‐based DJ film exhibits higher crystal orientation and larger crystal domain induced by slower crystallization as compared to methylammonium‐based one, accompanied with lower trap density, more efficient carrier transfer, and higher photovoltaic performance. As a result, the BDAFA 2 Pb 3 I 10 perovskite solar cell delivers a champion power conversion efficiency up to 15.34%, which is the highest among FA‐based small‐ n DJ series. Theoretical simulation results uncover that the smaller tilting and higher rigidity of the octahedra in the FA‐based DJ systems render more outstanding environmental stability, leading to well‐maintained 85% of its efficiency after prolonged storing time of 1020 h at 65 °C.