Enhancing Efficiency and Stability of Perovskite Solar Cells through Synergistic Guanidine–Oxysalt‐Mediated Surface Engineering

Three‐dimensional organic–inorganic perovskite solar cells show continuous improvement in power conversion efficiency. However, the defects present on the perovskite surface affect the device performance and long‐term stability. In this study, we introduced N‐(2‐phenoxyethyl) guanidine nitrate salt (NPEGN) as a surface passivator to effectively engineer surface defects and reduce nonradiative recombination at the interface. The NPEGN introduction on the perovskite surface results in large grains with fewer grain boundaries, leading to the formation of low‐dimensional 2D phase on the perovskite surface. Furthermore, NPEGN treatment passivates defects through ionic and hydrogen bonding with perovskite and inhibits perovskite degradation by preventing ion migration. Additionally, improved energy‐level alignment at the perovskite/electron transport layer interface enhances charge transport capacity and reduces charge recombination. Consequently, the efficiency of perovskite solar cells with NPEGN treatment increases to 21.02%, while the unencapsulated devices retained 100% of their initial power conversion efficiency for 2200 h in nitrogen atmosphere and 90% of their initial efficiency for 450 h at 65°C.