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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]

• Cesium acetate (CsAc) is introduced to promote the conversion of PbI 2 to perovskite. • CsAc optimizes perovskite quality, reduces defects and non-radiative recombination. • CsAc optimized devices achieves a PCE of 22.01% with excellent stability. Although the best research-cell efficiency (>25%) of perovskite solar cells (PSCs) is comparable with that of silicon solar cells, the industrial application of PSCs is limited by the further improvement of performance and stability. Here, cesium acetate (CsAc) is introduced into lead iodide (PbI 2) precursor, which tunes the morphology and structure of PbI 2 and promotes the conversion of PbI 2 into stable large perovskite grains. Moreover, the introduction of Cs+ and Ac− can fill up the FA+, I− vacancy and combine with uncoordinated Pb2+, which reduces the trap density. The CsAc introduction not only optimizes the quality of perovskite films, but also reduces the defects in the bulk phase, which facilitates the transportation of carriers and mitigates non-radiative recombination. Consequently, the CsAc optimized PSCs achieve a maximum power conversion efficiency (PCE) of 22.01%. Meanwhile, the unsealed device still maintains an initial PCE of 96% after being exposed in the air for 45 days. [ABSTRACT FROM AUTHOR]