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Spontaneous Formation of Upper Gradient 2D Structure for Efficient and Stable Quasi‐2D Perovskites

Highly efficient and stable quasi‐2D hybrid perovskite solar cells (PSCs) using hydrophobic 4‐(trifluoromethyl) benzylamine (4TFBZA) as the spacer cation are successfully demonstrated. It is found that the incorporation of hydrophobic 4TFBZA into MAPbI3 can effectively induce a spontaneous upper gradient 2D (SUG‐2D) structure, passivate the trap states, and restrain the ion motion. Meanwhile, the strong hydrogen bonding of F···HN between 4TFBZA ions and methylamine ions can effectively suppress the decomposition of perovskite, which gives the device a better thermal stability. Besides, due to the SUG‐2D structure with hydrophobic 4TFBZA, the device also exhibits a better moisture stability. The SUG‐2D‐structure‐based device exhibits a power conversion efficiency of 17.07% with a high open‐circuit voltage of 1.10 V and a notable fill factor of 71%. This work provides a new strategy for constructing efficient and stable quasi‐2D PSCs, and it is an inspiration for the packaging strategy of perovskites.