When the Substrate Matters in Solar Cells: Photoinduced Force Microscopy Uncovers How ITO Pretreatment Governs Self-Assembled Monolayers.

Self-assembled monolayers (SAMs) have emerged as promising hole-selective layers in perovskite and organic solar cells. While previous studies have mainly focused on preventing micelle formation in SAMs precursor solutions, the influence of substrate anchoring during film deposition has received far less attention. Here, we demonstrate that SAM aggregation can still occur in situ during spin coating when anchoring sites on ITO are insufficient, primarily due to the rapid increase in solute concentration upon solvent evaporation. Photoinduced force microscopy (PiFM) measurement provides spatially correlated structural information that UV-ozone-treated ITO promotes micelle aggregation and face-on adsorption, whereas O2 plasma treatment provides denser anchoring sites, suppressing micelle nucleation and favoring edge-on orientation. These interfacial differences translate directly into device performance: perovskite and organic solar cells fabricated on O2 plasma-treated substrates consistently outperform their UV-ozone counterparts. This finding challenges the conventional view that UV-ozone treatment yields a smoother and more ordered framework for SAMs. Instead, we propose a competitive mechanism in which molecular self-aggregation and the density of available anchoring sites jointly govern SAM formation, highlighting substrate pretreatment as a critical yet often overlooked factor in solar cell performance.