Phosphonic Acid Group Positions Dependent Face-on and Edge-on Configurations in Carbazole-Based SAMs and Impacts on Perovskite Solar Cell Performance

Self-assembled molecules (SAMs) are promising hole transport materials for perovskite solar cells owing to their tunability and low-cost synthesis. We designed and synthesized a series of carbazole-based SAMs with phosphonic acid (PA) groups at the 2- or 3-position to investigate how structural variation influences energy level alignment and molecular stacking on transparent conducting oxide (TCO) substrates. The positioning of PA groups strongly affects the work function of the SAMs, thereby impacting charge collection and device efficiency. Flexible ethyl linkers promote a face-on orientation, leading to thinner interlayers than vinyl linkers, which favor edge-on alignment. This change in molecular stacking alters the interfacial electrical properties and stability. In particular, the face-on configuration reduces series resistance and enhances thermophotostability through stronger bonding at the perovskite interface. These results highlight the importance of molecular design in optimizing SAM orientation and interfacial properties for improved efficiency and durability of perovskite solar cells.