Defective and mechanically weak interfaces substantially undermine both the efficiency and stability of perovskite solar cells (PSCs). Here we introduce a linear polymer, heparin sodium, as a multifunctional interface bridge layer in n–i–p PSCs. Unlike commonly employed small-molecule interface modification/passivation materials, heparin sodium features functional groups and ions including COO−, SO3− and Na+ distributed along the top and bottom sides of its backbone. It thus serves as a bridge connecting the SnO2 electron transport layer and the perovskite film through robust chemical bonding, mitigating defects and enhancing heterointerface bonding in PSCs. The power conversion efficiency (PCE) of the resulting rigid devices is 26.61% (certified 26.54%), positioning it among the highest-efficiency PSCs. We also fabricate flexible SnO2/heparin sodium-based PSCs that achieve a PCE of 25.23%. The heparin sodium-based devices demonstrate excellent operational and thermal stability. After 1,800 h under maximum power point tracking under simulated 1-Sun conditions, 94.9% of the initial PCE is retained. The devices also maintain 95.2% of their initial PCE after ageing at 85 °C for 1,800 h.