Multiple‐Ring Aromatic Spacer Cation Tailored Interlayer Interaction for Efficient and Air‐Stable Ruddlesden–Popper Perovskite Solar Cells

2D Ruddlesden–Popper (2DRP) perovskites with hydrophobic bulky cations are proven to improve the environmental stability in photovoltaic devices significantly. However, these spacer cations lead to a weak interplay in the 2DRP perovskites, severely impacting the charge carrier transport and require a systematic understanding of the interaction between spacer cations and inorganic slabs. Herein, a series of novel perovskites (BA 1− x NMA x ) 2 MA 3 Pb 4 I 13 (NMA = 1‐naphthalenemethylammonium, BA = n ‐butylammonium) are successfully fabricated to reveal the interaction of mixed spacers and inorganic slabs. Incorporating NMA cations enhances the NH⋯I − hydrogen‐bonding interaction between the spacer cations and [PbI 6 ] 4− slabs, resulting in a preferentially crystal vertical orientation of smoothed perovskite films with larger crystal grains. Thus, a significant reduction in the density of trap states of the resulting 2DRP perovskites is achieved which leads to highly efficient charge carrier transport. Consequently, the champion (BA 0.9 NMA 0.1 ) 2 MA 3 Pb 4 I 13 device yields a power conversion efficiency (PCE) of 14.21%, along with the unencapsulated devices that can retain 85% of their initial PCE for 1200 h under 35–65% relative humidity conditions. This work provides a simple and original method to modulate the interlayer interplay in 2DRP perovskites for highly efficient and air‐stable perovskite solar cells.