A Flexible Side-Chain Dispersant Enables Uniform Self-Assembled Monolayers for 18.67% Organic Solar Cells

Self-assembled monolayers (SAMs) on indium tin oxide (ITO) surfaces tend to undergo molecular aggregation, resulting in non-uniform interfacial coverage and thus limiting the charge extraction efficiency and device performance in organic solar cells (OSCs). To address this issue, a novel dispersant molecule, 3,6-Diiodo-9-[2-(2-methoxyethoxy) ethyl]-9H-carbazole (2ICzMPE), featuring dipolar characteristics and flexible side chains, is designed and synthesized to regulate the interfacial distribution of 2PACz molecules on ITO electrodes. With a large dipole moment and steric hindrance, 2ICzMPE suppresses 2PACz aggregation, while the oxygen atoms in the side chains interact with the ITO surface, enabling a more uniform interfacial structure. Upon introducing 2ICzMPE, the interfacial energy level alignment is optimized, leading to more efficient charge extraction. Device physics analysis reveals suppressed carrier recombination and enhanced charge transport. As a result, the power conversion efficiency (PCE) of OSCs based on the PM6:L8-BO system is improved from 17.51% to 18.67%. This work provides a simple and effective molecular design strategy for constructing high-quality SAM interfaces and promoting the scalable application of OSCs.