Molecular spacer engineering of carbazole–phosphonic acid SAMs: Probing interfacial energetics in low-light organic optoelectronic devices

Self-assembled monolayers (SAMs) have significantly advanced the performance of indoor organic photovoltaics (OPVs) and photodetectors (OPDs), accelerating their path toward commercialization. Among them, carbazole phosphonic acid SAMs are particularly promising due to their strong interfacial adhesion, tunable electronic properties, and excellent stability. However, the relationship between their molecular structure and the resulting device physics remains poorly understood, hindering the development of rational design strategies for performance optimization. Here, we introduce two new carbazole-based SAMs—5PACz, with an extended alkyl spacer, and DEPACz, incorporating an ether-linked spacer—and compare them to the widely used 2PACz and PEDOT:PSS. The results demonstrate that spacer engineering introduces a critical trade-off between current gain and voltage loss. Specifically, the ether-linked spacer in DEPACz enhanced surface wettability and packing density, which suppressed interfacial recombination. This resulted in a boosted short-circuit current density, but the longer spacer slightly reduced the open-circuit voltage in indoor OPV mode. Conversely, OPD performance was robustly unaffected, as it is governed primarily by the shared molecular backbone rather than the spacer modifications. These findings establish a key design principle: optimizing the SAM spacer structure is essential for balancing the current generation and voltage loss in dual-functional OPV–OPD devices.