Structure and Device-Operando Photostability of Quasi-2D Ruddlesden–Popper Perovskites: Engineering the Spacer Cation Matters

Despite the attractive properties of two-dimensional (2D) perovskites, the structural origin of their photostability remains elusive, especially extending to device scales. This work systematically investigates spacer engineering in quasi-2D single crystals ( n = 2) using para-substituted phenylethylamine derivatives (XPEA), establishing critical correlations between the spacer conformation and the structural/electronic properties of hybrid lattices. We find that the BrPEA cation is conducive to strengthening the organic–inorganic interface and suppressing the structural fluctuations of the inorganic framework, thereby stabilizing the overall lattice. Integrated experiments and simulations confirm the optimal photostability of the BrPEA-based lattice. In photovoltaic devices, BrPEA promotes optimized film morphology, homogeneous phase distribution, and improved charge-carrier dynamics, yielding a high device efficiency. Operational stability analysis reveals that device degradation is initially governed by spacer-related structural robustness, while photoactivated trap states dominate at excessive defect densities. This work provides a guideline for engineering organic spacers to enhance 2D perovskite photostability for cutting-edge optoelectronic applications.