Printed non-fullerene acceptor (NFA) organic photovoltaics (OPVs) are rapidly approaching record power conversion efficiencies (PCEs), but further progress towards manufacturing-relevant modules requires simultaneous control of interfacial losses, process tolerance, and operational stability. A major bottleneck resides at the transparent electrode/active-layer interface, where non-ideal energy-level alignment, interfacial disorder, and defect-mediated recombination reduce fill factor (FF) and accelerate degradation, especially when devices are translated from small-area lab cells to printed, larger-area architectures. Carbazole–phosphonic acid self-assembled monolayers (SAMs) have emerged as an attractive route to create ultrathin hole-selective contacts on ITO, because they can tune the ITO work function, improve wetting/film formation of the organic stack, and be compatible with low-temperature processing. However, single-component SAMs may form quasi-monolayers with incomplete coverage and microscopic defects; in addition, the molecular geometry of the anchoring group, spacer, and substituents can impose a trade-off between dense packing (coverage/passivation) and charge transport (tunneling distance and series resistance). These issues become increasingly consequential for printed OPVs, where coating dynamics and large-area uniformity amplify sensitivity to interfacial imperfections.

Here we summarize and connect three complementary studies that establish a molecular design–assembly framework for dense, defect-tolerant carbazole SAM interfaces in high-efficiency NFA OPVs, with a focus on scalability relevant to Symposium L. First, systematic monosubstituent engineering in 1X-2PACz (X = F, Cl, Br, I, CF3) reveals that modest chemical modifications can strongly influence packing, interface dipoles, and ultimately device performance. In representative NFA systems, Cl/Br/I substitutions favor improved packing and more favorable interfacial energetics, enabling PCEs up to 19.03% in PM6:L8-BO and 20.12% in D18:L8-BO devices, while F and CF3 tend to increase interfacial disorder and reduce performance. These results highlight that “near-identical” SAMs can behave very differently at buried interfaces, emphasizing the need for chemistry-informed selection when targeting robust printed fabrication.

Second, spacer-length engineering in chlorinated nPACz derivatives (2Cl-nPACz, n = 2–5) decouples interfacial coverage/passivation from charge-transport limitations by explicitly controlling tunneling distance. Structural and spectroscopic analysis indicates that shorter spacers strengthen intermolecular interactions and increase ITO coverage while reducing series resistance; conversely, longer spacers increase the hole-tunneling barrier and progressively degrade FF and Voc. In this series, 2Cl-2PACz yields the most favorable balance and achieves up to 18.95% PCE. Together, these two studies provide quantitative guidance: dense packing and defect passivation are crucial, but they must be achieved without introducing excessive transport resistance at the buried contact.

Guided by these insights, our third contribution introduces a co-assembled multilayer SAM (coSAMu) strategy designed for scalable processing. Instead of relying on a single molecule to simultaneously set the work function, maximize coverage, and maintain low resistance, we combine 2PACz and a longer-spacer chlorinated analogue (2Cl-4PACz) using blend casting and sequential casting to control interfacial composition. The resulting layered interface comprises a 2PACz-rich chemisorbed bottom layer that primarily sets the ITO work function and an upper region enriched in 2Cl-4PACz that fills voids and passivates defects. This defect-filling architecture suppresses trap-assisted recombination, improves charge extraction, and delivers 20.1% efficiency (0.042 cm2) with FF above 81%. Importantly for printed and manufacturing-relevant OPVs, the interfacial strategy translates to scale: a 17.14 cm2 six-subcell mini-module reaches 17.0% efficiency, compared to 12.2% for the 2PACz control. Overall, substituent tuning, spacer engineering, and multilayer co-assembly provide a general pathway to dense, defect-tolerant buried contacts that support high-efficiency, printed, and potentially more stable NFA OPVs by reducing the sensitivity to interfacial disorder and coverage gaps.