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The efficiency of perovskite solar cells (PSCs) is hindered by substantial defects within the grain boundaries (GBs) of polycrystalline perovskite films. Conventional post‐treatment strategies struggle to precisely repair these defects at GBs. Here, a targeted grain boundary passivation strategy through solvent effects by incorporating symmetrical biphenyl molecules is proposed, 4,4′‐diaminodiphenyl sulfone (DDS) and 4,4′‐sulfodiphenol (SDP), aiming to mitigate defects at GBs and optimize energy level arrangements through their electric dipole effects. Compared to the pristine device, the SDP‐modified device exhibits significant improvements, including a champion efficiency of 24.39% and an impressive fill factor, along with excellent operation stability. This work provides an effective and straightforward solution for improving the performance of PSCs. [ABSTRACT FROM AUTHOR]

Driven by their many unique features, perovskite solar cells (PSCs) have become one of the most promising candidates in the photovoltaic field. Two‐step preparation of perovskite film is advantageous for its higher stability and reproducibility compared to the one‐step method, which is more suitable for practical application. However, the incomplete conversion of the dense lead iodide (PbI2) layer during the sequential spin‐coating of formamidinium/methylammonium (FA+/MA+) organic amine salts severely affect the performance of PSCs. Herein, sodium bicarbonate (NaHCO3) is used to induce the formation of porous PbI2, which facilitates the penetration of the FA+/MA+ ions and the formation of a perovskite film with high crystallinity and large grain microstructure. Meanwhile, the introduction of Na+ not only improves the energetic alignment of the PSC, but also increases the conductivity via p‐doping. As a result, the optimized NaHCO3‐modified PSC achieves a champion power conversion efficiency of 24.0% with suppressed hysteresis. Moreover, the significant reduction in defect density and ion migration as well as a mild alkaline environment enhance the stability of device. The unencapsulated NaHCO3‐modified PSCs maintain over 90% of their original efficiency upon storage in ambient air (30%–40% relative humidity) for 2160 h. We have demonstrated an ingenious strategy for controlling the quality of perovskite and improving the performance of device by low‐temperature foaming of simple inorganic molecules of NaHCO3. [ABSTRACT FROM AUTHOR]