Upscaling large‐area formamidinium (FA)‐based perovskite solar cells (PSCs) has been considered as one of the most promising routes for the commercial applications of this rising photovoltaics technology. Here, a natural amino acid, phenylalanine (Phe), is introduced to regulate the nucleation and crystal growth process of the large‐scale coating of FA‐based perovskite films. Better film coverage and larger grain sizes are observed after adding Phe. Moreover, it is found that Phe can effectively passivate defects within perovskite films and suppress the nonradiative recombination due to the strong interaction with under‐coordinated Pb2+ ions in the perovskite films. Rigid PSCs based on the blade‐coated perovskite films containing Phe obtain a champion efficiency of 21.95%. The corresponding unencapsulated devices also exhibit excellent ambient stability, retaining 95% of their initial efficiencies after storage in the glovebox at 20 °C for 1000 h. Further, the strategy is applied to fabricate flexible PSCs and modules on polyethylene terephthalate/indium doped tin oxide substrates via slot‐die coating. Phe modified flexible devices achieve outstanding efficiencies of 20.21%, 12.1%, and 11.2% with aperture areas of 0.10, 185, and 333 cm2, respectively. The strategy here has paved a promising way for the large‐scale production of flexible PSCs.

Chemical bath deposition (CBD) has been demonstrated as a remarkable technology to fabricate high‐quality SnO2 electron transport layer (ETL) for large‐area perovskite solar cells (PSCs). However, surface defects always exist on the SnO2 film coated by the CBD process, impairing the devices’ performance. Here, a facile periodic acid post‐treatment (PAPT) method is developed to modify the SnO2 layer. Periodic acid can react with hydroxyl groups on the surface of SnO2 films and oxidize Tin(II) oxide to Tin(IV) oxide. With the help of periodic acid, a better energy level alignment between the SnO2 and perovskite layers is achieved. In addition, the PAPT method inhibits interfacial nonradiative recombination and facilitates charge transportation. Such a multifunctional strategy enables to fabricate PSC with a champion power conversion efficiency (PCE) of 22.25%, which remains 93.32% of its initial efficiency after 3000 h without any encapsulation. Furthermore, 3 × 3 cm2 perovskite mini‐modules are presented, achieving a champion efficiency of 18.10%. All these results suggest that the PAPT method is promising for promoting the commercial application of large‐area PSCs.