Perovskite solar cells (PSCs) have attracted considerable attention as next-generation photovoltaic technologies owing to their solution processability, low weight, and mechanical flexibility. Despite rapid progress, defect-induced nonradiative recombination remains a major obstacle, hindering further improvements in the device efficiency and operational stability. In this study, we introduce 1H-indole-3-carbohydrazide (1H-CBH) as a multifunctional molecular additive that effectively mitigates these issues through synergistic defect passivation. Specifically, 1H-CBH simultaneously coordinates with uncoordinated Pb²⁺ ions and forms hydrogen bonds with uncoordinated I⁻ ions and formamidinium cations. This dual interaction strategy promotes the growth of larger grains, reduces the density of grain boundary defects, and enhances the interfacial compatibility with the electron-transport layer, thereby enabling improved charge transport. Consequently, the incorporation of 1H-CBH into mixed-cation PSCs yields a remarkable enhancement in the power conversion efficiency from 21.18% to 23.59%. Moreover, the 1H-CBH-modified devices demonstrated exceptional environmental stability, retaining their initial morphology after 8 months under ambient conditions (25°C, 50%‒80% relative humidity), whereas their unpassivated counterparts underwent complete degradation. Under inert N₂ atmosphere, PSCs incorporating 1H-CBH maintained >80% of their initial power conversion efficiency after 600 h continuous storage. These results highlight the critical role of multifunctional additive engineering in achieving highly efficient and durable perovskite solar cells, paving the way toward scalable and reliable photovoltaic technologies.