AbstractIn perovskite solar cells (PSCs), a defective perovskite (PVK) surface and cliff‐like energy offset at the interface always slow down the charge extraction; meanwhile, interface ion diffusion causes oxidation of the metal electrode, inducing device instability. Here, the in situ grown 2D‐(CH₃NH₂)₃Sb₂I₉ (MA₃Sb₂I₉) on the back surface of MAPbI₃ results in a more robust interface. MA₃Sb₂I₉ changes the MAPbI₃ surface to p‐type and thus acts like a back surface field to drive charge extraction and suppress recombination, resulting in an obviously higher fill factor (FF) = 0.8 and power conversion efficiency (PCE) = 20.4% of SnO₂/MAPbI₃/MA₃Sb₂I₉/Spiro‐OMeTAD (2,2′,7,7′‐Tetrakis[N,N‐di(4‐methoxyphenyl)amino]‐9,9′‐spirobifluorene) PSC than the pure MAPbI₃ device. More importantly, strong chemical bonding of SbI prohibits ion diffusion, largely enhancing the thermal stability and longtime stability. Here, special 2D‐MA₃Sb₂I₉ constructs’ robust band alignment and chemical environment at the interface are highlighted for efficient and stable PSCs.