Ambient stability remains a critical hurdle for commercialization of perovskite solar cells. Two-dimensional Ruddlesden-Popper (RP) perovskite solar cells exhibit excellent stability but suffer from low photovoltaic performance so far. Herein, a RP/3D heterostructure passivated by semiconducting molecules is reported, which systematically addresses both charge dynamics and degradation mechanisms in concert for cesium-free FAPbI solar cells, delivering a power-conversion efficiency as high as 20.62% and remarkable long-term ambient stability with a t lifetime exceeding 2880 hours without encapsulation. In situ characterizations were carried out to gain insight into structural evolution and crystal growth mechanisms during spin coating. Comprehensive film and device characterizations were performed to understand the influences of the RP perovskite and molecule passivation on the film quality, photovoltaic performance and degradation mechanisms. This enables fabrication of a superior quality film with significantly improved optoelectronic properties, which lead to higher charge collection efficiency. The underlying mitigated degradation mechanisms of the passivated RP/3D devices were further elucidated. The understanding of the necessity of addressing both the charge dynamics and degradation mechanisms of solar cells will guide the future design and fabrication of chemically stable, high-efficiency photovoltaic devices.