In this talk, I will discuss a new linear polar polymer additive, polycaprolactone (PCL), that can functionalize grain boundaries (GBs) in methylammonium lead triiodie (MAPbI3) hybrid perovskites via direct backbone attachment, enabling significantly enhanced perovskite solar cell (PSC) performance and environmental stability. The champion PSC with optimal PCL concentration shows a power conversion efficiency (PCE) of 20.1%, which is reduced by only 10% after 400-h, ambient air storage. 500-h continuous thermal aging at 85 °C still preserves ~65% of the initial PCE. The PCL GB passivation is visualized directly via lateral scanning force and transmission electron microscopies, supporting the roles of PCL in suppressing moisture ingression and ion migration. Secondary ion mass spectrometry and ion conduction measurements confirm the PCL GB functionalization significantly retards I- ion migration in MAPbI3 PSCs while without PCL, I- ions spontaneously accumulate at the organic hole transport layer interface even without externally applied electric field.