Performance Promotion of Inverted FA x Cs 1– x PbI ₃ Solar Cells through Dual Molecular Passivation Strategy

The high density of defects at the perovskite electron-transporting layer (ETL) interface remains a critical factor limiting further performance improvements in perovskite solar cells (PSCs). Conventional passivation strategies predominantly employ single bulky ammonium cations as passivators, which often introduce two-dimensional (2D) perovskite phases while passivating surface defects, potentially impeding efficient photogenerated carrier transport and extraction. In this work, we propose an effective dual molecular passivation strategy. By synergistically passivating the surface with an optimized n-BABr/PEAI ratio, a stable and dense passivation monolayer forms, reducing the defect density from 1.027 × 10¹⁶ to 4.073 × 10¹⁵ cm⁻³. Simultaneously, steric competition between passivators suppresses the 2D perovskite formation. Furthermore, this strategy optimizes the energy-level alignment between the perovskite and the ETL, enhancing carrier extraction efficiency at the heterojunction. Consequently, both the device efficiency and the operational stability are substantially improved. This study introduced a novel interface passivation approach that effectively inhibits 2D perovskite phase growth, offering a promising pathway toward high-performance PSCs.