Steric hindrance-induced configuration-locking in self-assembled interfacial layer enables higher surface coverage and 20.14 % efficiency binary organic solar cells

Steric effects play a crucial role in determining molecular configuration and orientation, significantly impacting interfacial properties and enabling precise control of surface functionality. A major challenge lies in balancing these effects to prevent excessive distortion or disruption of π-conjugation, while still promoting optimal solid state packing and efficient charge transport. Overcoming this trade-off requires careful molecular design combined with advanced computational modelling, as well as comprehensive interface and device engineering. In this work, two new interfacial molecules, namely Ph-DIACz and Ph-DIBCz, with adjacent group restrictions are designed and systematically studied in organic solar cells (OSCs). Among them, the sterically hindered Ph-DIACz exhibits higher absorption energy, improved surface coverage, and stronger non-covalent interactions. These attributes synergistically enhance electrical conductivity and charge extraction, and the self-assembled monolayers/multilayers (SAMs) surface thereby templates a more ordered active layer morphology. This interfacial and active layer synergy facilitates suppressed recombination throughout the device. As a result, OSC devices employing Ph-DIACz achieve a remarkable power conversion efficiency (PCE) of 20.14 % in the PM6:BTP-eC9 binary system, with a significantly elevated fill factor (FF) and short-circuit current density (J_SC) compared to PEDOT:PSS and Ph-DIBCz-based devices (PCE 18.39 % and 18.70 %, respectively). Notably, Ph-DIACz demonstrates universal applicability in other systems, such as PM6:Y6 and PM6:L8-BO active layer. This work establishes a steric hindrance-induced configuration-locking strategy as a powerful interfacial engineering approach, paving a new way for the development of highly efficient OSCs. © 2025 Elsevier B.V.