Suppressing 2D Interfacial Reconstruction via Tetrafluoroborate Surface Engineering for Stable Perovskite Solar Cells

Surface passivation using two-dimensional (2D) perovskites has been widely employed to improve the efficiency and stability of perovskite solar cells (PSCs). However, conventional 3D/2D heterointerfaces often suffer from interfacial reconstruction caused by spacer-cation migration during operation, leading to performance degradation. Here, we investigate pseudo-halogen-modified phenethylammonium salts (PEAX, X = SCN and BF4) for surface passivation. Unlike the commonly used phenethylammonium iodide (PEAI), which forms a low-dimensional interfacial phase, PEABF4 does not induce 2D perovskite formation but instead enables non-reconstructive surface passivation. PEABF4 treatment improves energy-level alignment, carrier lifetime, charge extraction, and crystallinity and suppresses surface traps and nonradiative recombination. As a result, PSCs achieve a power conversion efficiency of 24.72% with a VOC of 1.16 V, outperforming untreated and PEAI-treated devices. Furthermore, unencapsulated PEABF4-based devices retain 85.96% of their initial efficiency after 320 h of maximum power point tracking, demonstrating that pseudo-halogen engineering offers an effective route toward efficient and operationally stable PSCs.