Despite two-dimensional (2D) Ruddlesden–Popper-phase layered perovskites (RPLPs) exhibiting excellent environmental stability, most solar cells based on 2D RPLP films are fabricated in a controlled inert atmosphere. Meanwhile, the poor charge transport of 2D RPLP films owing to the unfavorable phase arrangement and defects limits the efficiency of 2D RPLP solar cells. Here, we fabricate high-efficiency 2D RPLP solar cells in ambient air assisted by a zwitterion (ZW) additive. We show that the ZW additive suppresses the formation of the bottom 2D phases (n ≤ 2) and the top 3D-like phases in 2D RPLP films. These 2D phases usually grow parallel to the substrate and act as trap sites that inhibit charge transport in the vertical direction. The 3D-like phases, on the other hand, aggravate the long-term stability due to the intrinsic instability of MA+ cations. With improved phase distribution, crystal orientation, and reduced trap states in 2D RPLP films, efficient charge transport is obtained. Finally, a record-high open-circuit voltage (V oc) of 1.19 V and a power conversion efficiency of 17.04% with an enhanced stability are achieved for (BA0.9PEA0.1)2MA3Pb4I13-based (n = 4) solar cells fabricated under high humidity (∼65% RH).