Interface engineering has significantly boosted perovskite solar cell efficiency and stability. However, numerous approaches focus on addressing defects at the interfaces between transport layers while neglecting potential issues within the bulk perovskite material. Here, a multifunctional molecule, sodium lignosulfonate (SL), containing three types of functional groups, was introduced as a chemical bridge at the perovskite/SnO2 interface. The introduced SL bridges promote energy level alignment at the perovskite/SnO2 interface and regulate the perovskite crystallization process. Meanwhile, the coordinated interactions between the perovskite components with −OH and −SO3- groups on SL, coupled with Na+ diffusion, effectively passivate defects at the buried interface and within the perovskite bulk. As a result, the champion SnO2-SL based n-i-p PSC achieved power conversion efficiencies of 25.73% and 25.13% on rigid and flexible substrates, respectively. Unencapsulated devices maintained 92.9% of their initial efficiency after 2,550 h of maximum power point-tracking under 1-sun illumination in an inert atmosphere.