Methoxy Position Engineering in Self‐Assembled Interlayer for High‐Performance Organic Solar Cells

Comprehensive Summary Self‐assembled interlayers are widely used to improve electrode/semiconductor contacts in optoelectronic devices and their properties can be fine‐tuned through rational molecular design. In molecular semiconductors and interfacial modifiers, substituent position is known to profoundly influence molecular conformation, packing motif, and dipole orientation. Here, we adopt this strategy and introduce different methoxy substituent positions in self‐assembled interlayers. Two positional isomers, o ‐DMPCz and m ‐DMPCz, are synthesized and employed as hole‐transporting interlayers (HTLs). Their impact on interfacial energetics and charge transport is explored through density functional theory, molecular dynamics simulations, together with surface‐potential, electrochemical, and conductive‐probe measurements. These investigations show that the ortho ‐methoxy isomer ( o ‐DMPCz) exhibits a larger dipole moment, stronger adsorption, denser packing, and higher interfacial conductivity, leading to more efficient hole extraction and reduced interfacial recombination. In PM6:BTP‐eC9 OSCs, the o ‐DMPCz interlayer yields a power conversion efficiency of 19.53%, outperforming m ‐DMPCz (14.48%) and PEDOT:PSS (18.45%), due to simultaneous gains in short‐circuit current and fill factor. Similar efficiency improvements in PM6:Y6 and PM6:L8‐BO systems highlight the generality of this positional‐isomer strategy. This study reveals the key role of substitution sites in self‐assembled interlayers and provides a design framework for high‐performance interfacial materials in OSCs and other emerging optoelectronic devices.