Suppressing Aggregation in Self-Assembled Monolayers via an Environmentally Friendly Co-solvent Strategy for High-Performance Organic Solar Cells

Self-assembled monolayers (SAMs) have emerged as highly efficient hole transport layers for organic solar cells (OSCs). Nevertheless, the majority of SAM molecules are intrinsically amphiphilic and prone to aggregation in conventional alcohol-based processing solvents, leading to the formation of micellar nanoparticles. Such aggregation hampers the formation of dense and uniform SAM films on indium tin oxide (ITO) electrodes, thereby limiting charge extraction and device performance. Herein, we report a simple and broadly applicable cosolvent strategy to regulate the aggregation behavior of SAMs during solution processing. By introducing environmentally friendly cyclohexanol (CyOH) as a cosolvent with ethanol (EtOH), the aggregation of (2-(9H-carbazole-9-yl)) phosphonic acid (2PACz) is effectively suppressed, enabling fine control over its solution-state organization and interfacial assembly. This cosolvent system promotes the formation of dense, uniform, and well-ordered SAM films on ITO, resulting in improved interfacial energetics and enhanced device reproducibility and stability. As a result, OSCs based on PM6:BTP-eC9 achieves a champion power conversion efficiency (PCE) of 19.33%, compared to 18.26% for devices processed from pure EtOH. Notably, this strategy is compatible with multiple interfacial layer materials and photoactive systems, delivering a PCE exceeding 20% in the ternary D18:AQx-2F:BTP-eC9 system. This work demonstrates a green, versatile, and effective solvent-engineering approach for the design of high-performance OSCs.