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Planar perovskite solar cells (PSCs) have exhibited a promising alternative for high efficiency recently because of a new record of certified power conversion efficiency (PCE) of 21.5%. At the present stage of PSCs, achieving further improvements for long-term stability and efficiency are the key issues. It is necessary to fabricate PSCs with good interface and film crystallinity. Here, planar PSCs are fabricated by a synergistic effect of dimethyl sulfoxide additive and a dropwise addition method in a two-step deposition method. Under the optimizing procedure, a large-grain perovskite film with well-packed flat crystals is induced with a largest grain size of about 5 μm, which reveals decreased defect states owing to reduced grain boundaries. Moreover, the film has a highly compact and flat surface, leading to high crystallinity, enhanced light absorption, and low carrier recombination rates. Corresponding PCE is significantly promoted to 20.63% from the original 16.02%. More importantly, the PSCs even without encapsulation exhibit excellent humidity stability, retaining 93% of its initial efficiency after 500 h in air with 40% humidity, whereas the pristine devices degrade by approximately 65% after only surviving for 256 h. Therefore, the result provides a promising route for preparation of large-grain perovskite films and high-performance and stable PSCs.

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