Improving Phase Homogeneity of Formamidinium Cesium Lead Iodide Perovskites Solar Cells via Densely Packed Amorphous Self‐Assembled Multilayer

Formamidinium‐cesium lead iodide (FACsPbI 3 ) perovskites with methylammonium‐free compositions have demonstrated improved intrinsic stability and hold promise for the commercialization of perovskite solar cells (PSCs). However, phase inhomogeneity caused by the segregation of A‐site cations limits further improvements in device efficiency. In this work, a self‐assembled molecule (4‐(3,6‐bis(1,3‐benzodioxole‐5‐yl)‐9H‐carbazole‐9‐yl)butyl)phosphonic acid (PhDo4PACz) was designed by introducing a benzodioxole group into the carbazole unit. The benzodioxole group provides additional steric hindrance and non‐coplanar van der Waals interactions, forming an amorphous self‐assembled multilayer on the substrate. Additionally, there is a strong intermolecular interaction between the benzodioxole group and FA + , which reduces the nucleation barrier and anchors FA at the bottom of the perovskite. Consequently, the perovskite film deposited on this self‐assembled multilayer exhibits improved phase homogeneity. Furthermore, PhDo4PACz enhances energy level alignment and defect passivation at the buried interface, resulting in minimized bandgap‐to‐voltage loss (0.091 eV) and maximized fill factor (86.10%). As a result, inverted PSCs based on PhDo4PACz/FTO substrates achieved a champion power conversion efficiency (PCE) of 26.75% (certified 26.27% for reverse‐scan), retaining 95.15% of their initial efficiency after 1500 h of illumination under the ISOS‐L‐2 protocol, demonstrating great potential for excellent stability and industrial applications.