Manipulating the Formation of 2D/3D Heterostructure in Stable High‐Performance Printable CsPbI₃ Perovskite Solar Cells

Abstract Manipulating the formation process of the 2D/3D perovskite heterostructure, including its nucleation/growth dynamics and phase transition pathway, plays a critical role in controlling the charge transport between 2D and 3D crystals, and consequently, the scalable fabrication of efficient and stable perovskite solar cells. Herein, the structural evolution and phase transition pathways of the ligand‐dependent 2D perovskite atop the 3D surface are revealed using time‐resolved X‐ray scattering. The results show that the ligand size and shape have a critical influence on the final 2D structure. In particular, ligands with smaller sizes and more reactive sites tend to form the n = 1 phase. Increasing the ligand size and decreasing the reactive sites promote the transformation from 3D to n = 3 and n < 3 phases. These findings are useful for the rational design of the phase distribution in 2D perovskites to balance the charge transport and stability of the perovskite films. Finally, solar cells based on ambient‐printed CsPbI₃ with n‐butylammonium iodide treatment achieve an improved efficiency of 20.33%, which is the highest reported value for printed inorganic perovskite solar cells.