The defects located at the interfaces and grain boundaries (GBs) of perovskite films are detrimental to the photovoltaic performance and stability of perovskite solar cells. Manipulating the perovskite crystallization process and tailoring the interfaces with molecular passivators are the main effective strategies to mitigate performance loss and instability. Herein, a new strategy is reported to manipulate the crystallization process of FAPbI(3)-rich perovskite by incorporating a small amount of alkali-functionalized polymers into the antisolvent solution. The synergic effects of the alkali cations and poly(acrylic acid) anion effectively passivate the defects on the surface and GBs of perovskite films. As a result, the rubidium (Rb)-functionalized poly(acrylic acid) significantly improves the power conversion efficiency of FAPbI(3) perovskite solar cells to approaching 25% and reduces the risk of lead ion (Pb2+) leakage continuously via the strong interaction between C(sic)O bonds and Pb2+. In addition, the unencapsulated device shows enhanced operational stability, retaining 80% of its initial efficiency after 500 h operation at maximum power point under one-sun illumination.