Synergistic Conformation Locking and Terminal Conjugation Engineering of Carbazole-Based Self-Assembled Monolayers for High-Performance Organic Solar Cells
Self-assembled monolayers (SAMs) are critical for optimizing electrode interfaces in organic solar cells (OSCs), as their molecular conformation and ordering govern interfacial quality. Conventional carbazole-based SAMs (e.g., 2PACz) rely on flexible alkyl linkers whose conformational freedom often results in disordered packing, incomplete coverage, and limited environmental robustness, especially under air processing. Here, we design and systematically study a series of well-defined carbazole SAM homologues (Cz-PPA, Cz-HPA, PCz-HPA, and PCz-PPA) to elucidate the role of synergistic conformational locking achieved through linker rigidification and terminal conjugation extension. PCz-HPA, which integrates a rigid cyclohexane linker with a strongly conjugated 3,6-diphenylcarbazole end group, enables effective conformational locking. It forms a highly ordered, densely packed monolayer on ITO, delivering high surface coverage, a strengthened interfacial dipole, and improved energy-level alignment. The rigid framework and ordered interface enhance air-process stability and interfacial contact, thereby suppressing recombination and facilitating exciton dissociation and charge collection. Using PCz-HPA as SAM for the devices based on PM6:L8-BO reaches 19.75% efficiency and it demonstrates consistent gains across multiple systems. These results identify conformational locking via linker rigidification as a general design rule for durable, high-performance SAM interlayers in organic optoelectronics.