Mixed-Cation Engineering for Orientation Control and Defect Suppression in Quasi-2D Dion–Jacobson-Type Tin Perovskites for Solar Cells

Low-<i>n</i>-member quasi-two-dimensional (quasi-2D) Sn perovskites, particularly those based on a Dion-Jacobson (DJ) phase, have attracted attention for their enhanced stability and Pb-free composition. However, their practical application is limited by poor out-of-plane crystallographic orientation and high defect densities, which hinder efficient charge transport. To address these challenges, we introduce a mixed-cation engineering strategy combining <i>N</i>,<i>N</i>-dimethyl-<i>p</i>-phenylenediammonium (DMPhDA²⁺) and phenylethylammonium (PEA⁺) spacers. PEA⁺ acts as an orientation-directing spacer that reduces steric hindrance in the DMPhDA-based framework, facilitating vertical growth and enhancing crystallographic alignment. This approach effectively modulates crystallization, enhances vertical orientation, and suppresses both structural defects and Sn²⁺ oxidation. As a result, the optimized low-<i>n</i> quasi-2D DJ/RP perovskite solar cells exhibit improved performance, achieving an outstanding power conversion efficiency (PCE) of 6.76%. This work provides a viable route to overcome orientation- and defect-related limitations in low-<i>n</i> Sn-based perovskites, advancing the development of stable and efficient Pb-free solar cells.