Self-Assembled Monolayers as Hole Transport Layers in Organic Solar Cells: Progress in Molecular Design and Device Engineering
Organic solar cells (OSCs) have achieved certified power conversion efficiencies (PCEs) exceeding 19% through the integration of self-assembled monolayers (SAMs) as hole transport layers (HTLs). Unlike PEDOT:PSS, SAMs eliminate corrosive sulfonic acid groups, enhancing device stability, while their ultrathin nature minimizes parasitic absorption and material consumption. This review systematically analyzes SAM molecular design—anchoring group selection, linker length, and end-group nature—and its impact on self-assembly quality and charge transport. Key experimental benchmarks include 18.9% PCE for n-doped bulk-heterojunction OSCs with halogen-substituted SAMs (Adv Energy Mater, 2022), 19.67% for push–pull substituent SAMs (Energy Environ Sci, 2025), and 19.63% for oligomeric carbazole phosphonic acid HTLs (Adv Funct Mater, 2025). Asymmetric SAMs enable binary PM6:Y6 OSCs with over 19% efficiency (CCS Chem, 2025). Despite these gains, surface coverage and operational stability remain critical bottlenecks. This review proposes optimization strategies addressing molecular design and device processing, and highlights future research directions to overcome SAM limitations in large-area OSC fabrication.