Tailoring Interlayer Spacers for Efficient and Stable Formamidinium‐Based Low‐Dimensional Perovskite Solar Cells

2D Dion-Jacobson (DJ) perovskite solar cells generally show mediocre device performances as they are restrained by their defective film quality. The rigid diammonium organic interlayer spacers are intolerant to lattice mismatches, which induces defects and distortions and ultimately deteriorates the optoelectronic properties. Herein, a secondary interlayer spacer is introduced into formamidinium (FA)-based low-dimensional perovskite, which substantially improves the film quality. The flexible monovalent spacer cations effectively alleviate lattice distortions and reduce crystal defects, providing perovskite films with desirable microscopic morphology, preferable crystal orientation, reduced defect states, and improved charge transport capability. As a result, the optimized perovskite solar cell based on the (PDA0.9 PA0.2 )(FA)3 Pb4 I13 (PDA = propane-1,3-diammonium, PA = propylammonium) film exhibits the exceptional power conversion efficiency of 16.0%, the highest reported value in its class. In addition, the device demonstrates the enhanced thermal stability, retaining 90% of its initial efficiency after aging at 85 °C for 800 h.