Highly Efficient and Stable 2D/3D Heterojunction Perovskite Solar Cells by In Situ Interface Modification with [(p-Fluorophenyl)ethyl]ammonium Acetate

2D/3D heterojunction perovskites, meaning a rationally prepared 2D capping layer on 3D perovskite films, have been demonstrated as an effective avenue for simultaneously enhancing the efficiency and stability in perovskite solar cells (PSCs). However, the mechanism of the 2D perovskite induced by organic agents is still not extensively studied. Here, we report 2D/3D heterojunction PSCs by in situ fabricating a 2D modified layer on 3D perovskite films with [(p-fluorophenyl)ethyl]ammonium acetate (FPEAAc). During the annealing process, FPEAAc melts and uniformly covers the 3D perovskite films. Then, the excess acetate salt is volatilized, eventually forming a compact 2D perovskite thin layer. On the one hand, the organic agents can effectively rivet onto the 3D perovskite surface, ensuring formation of the necessary 2D perovskites with hydrophobic FPEA+ ions. On the other hand, the reaction generates some PbI2, which passivates the defects on 3D perovskite films and improves the interface contact, significantly enhancing the open-circuit voltage (VOC) and fill factor (FF) in 2D/3D PSCs. The highest power conversion efficiency of 22.53% is achieved compared with 20.16% in 3D PSCs. The 2D/3D-heterojunction-structured PSCs modified by FPEAAc exhibit high stability, retaining about 90% of the initial device efficiency after 500 h at 85 °C and 40 ± 5% relative humidity. Our research provides a simple method to control the 2D perovskite layer formation and effectively enhance the performance and stability in 2D/3D heterojunction perovskite cells.