A Fluorinated Phenethylammonium‐Based Spacer Cation Prompts the Spontaneous Formation of Gradient 2D/3D Perovskites for Efficient and Stable Solar Cells

Despite the prominent photovoltaic performance and broad application prospect of perovskite solar cells (PVSCs), a significant challenge for their commercialization is the stability issue. Herein, a fluorinated organic ammonium salt, 4‐trifluoromethyl phenethyl ammonium iodide (4FPAI), is utilized for the spontaneous formation of gradient 2D/3D perovskite which can be prepared by a simple one‐step spin‐coating process with low‐dimensional phases on the surface and high‐dimensional phases at the bottom. The introduction of 4FPAI leads to improved charge transport as well as reduced nonradiative recombination. The optimal 4FPAI‐based PVSC exhibits a power conversion efficiency (PCE) of 22.12% which is higher than that of the control PVSC without the 4FPAI treatment (20.87%). Meanwhile, the encapsulated 4FPAI‐based device remains at about 88% of its initial PCE under ambient air exposure for 80 days, demonstrating its improved moisture resistance compared to the reference device. This work provides an important strategy for fabricating high efficiency and long‐term stable 2D/3D PVSCs.