Although perovskite solar cells have been widely investigated to improve the photovoltaic conversion efficiency so far, perovskite light absorbers have a severe disadvantage for commercialization due to their facile degradation when exposed to humidity and oxygen. Herein, we suggest a synergic anti-solvent dripping strategy to improve the performance and stability of a photovoltaic device using a mixture of PMMA/sodium borate salt (borax) and perovskite materials. We characterized its unique features, i.e., the enhanced hydrogen bond formation between the PMMA/sodium borate mixture and the perovskite light absorber, which induced the lateral grain growth of perovskite films and efficient passivation of the surface and grain boundaries. Consequently, the PMMA/borax-treated devices exhibited significantly improved efficiency from 20.93% to 22.05% and retained 80% of the initial device performance over 1084 h at 22 °C and 50% RH. We believe that this technology will be useful for commercializing various optoelectronic devices.