Large perovskite crystals and improved interfacial engineering have been achieved simultaneously through an effective and universal sequential solution deposition strategy for planar perovskite solar cells (PSCs). The structure of the proposed device, see article number 1800027, is realized by adding DMSO, which is used as a cosolvent for the perovskite layer, into a hole transport material solution. After deposition onto the perovskite film surface, the hole transport material penetrates the perovskite layer during spin casting, resulting in an interdiffusion structure. In addition, DMSO captures vacant CH3NH3+ groups at the boundaries of perovskite grains, resulting in the growth of large‐grain crystals. The two new changes achieve enhanced optical absorption, reduced recombination, and effective carrier extraction. PSCs fabricated via this process exhibit 26.9% increment, compared to a control device.