Organic Solar Cells with 20.12% Efficiency Enabled by Monosubstituted Carbazole-Based Self-Assembled Monolayers

Carbazole-derived self-assembled monolayer (SAM) materials as hole transport layers are widely used in organic photovoltaics, yet the role of subtle substituent effects on interfacial structure and device performance remains underexplored. Here, we systematically investigate monosubstituted carbazole SAMs (1X-2PACz, X = F, Cl, Br, I, CF<inf>3</inf>) on indium tin oxide (ITO) and their device performance. Fluorine substitution achieves the highest surface coverage but poor interfacial order, while bulky 1CF<inf>3</inf>-2PACz introduces interfacial disorder. In contrast, Cl-, Br-, and I-substituted SAMs exhibit favorable packing and work-function alignment, enabling efficiencies of up to 19.03% in PM6:L8-BO and 20.12% in D18:L8-BO based solar cells. Crucially, cross-sectional scanning transmission electron microscopy provides the first direct visualization of the nonideal SAM morphologies on ITO, revealing molecular aggregates in ITO valleys, mono- or multilayers on flat regions, and incomplete surface coverage. These findings establish how substituents and processing critically govern interfacial packing and photovoltaic efficiency, guiding SAM-based interfacial design.