Passivation engineering has been recognized as a brilliant strategy to obtain stable and efficient perovskite solar cells (PSCs). The natural alkene lycopene (LP) extracted from tomatoes is the first heteroatom-free passivator that evidently increased the stability of PSCs in recent experiments, although the mechanism remains unclear. Herein, using density functional theory calculations and ab initio molecular dynamics simulations, we demonstrated that LP successfully passivates Pb with alleviatived charge localization by donating π-electrons, which exhibit stable bonding components. LP eliminates detrimental trap states induced by oxygen and suppresses nonradiative electron–hole recombination. Moreover, LP spatially blocks water from perovskite in an effective manner while it also weakens atomic fluctuations triggered by water, thus maintaining an optimum moisture tolerance. More importantly, the horizontal adsorption of LP limits the movement of I atoms, leading to a boosted activation barrier for I migration and a reduced migration rate that was decreased by four orders of magnitude. The satisfactory planarity of π–π-conjugated LP provides tight adsorption with perovskite in parallel, which results in outstanding passivation performance. Delocalized electrons move in the skeleton of LP, leading to distributed and flexible interactions with perovskite that resemble those of glue, and preventing lattice mismatch and regional destruction that results when thumbtack-like heteroatom-containing passivators are used. The established mechanism emphasizes the predominant contribution of conjugated CC bonds to stability and ion migration for green and efficient PSCs.