Conjugation‐Engineered Self‐Assembled Monolayers for Multifunctional Interfaces Regulation Enabling High‐Performance Organic Solar Cells

Organic solar cells (OSCs) critically depend on interfacial layers that simultaneously align energy levels, enable efficient charge extraction, and regulate active layer morphology; however, these functions are typically optimized independently. Here, we demonstrate that π‑conjugation of the terminal functional group in self‑assembled monolayers (SAMs) based anode interface layers (AILs) acts as a unified molecular strategy to coordinate these interdependent processes. Through a targeted molecular comparison, we designed two carbazole‑based SAMs, namely 2DPA‑PACz and 2Cz‑PACz, which share an identical phosphonic acid anchor but differ in functional group: one bearing a flexible diphenylamine unit, the other is a rigid, fully π‑conjugated carbazole extension. We show that the conjugated structure in 2Cz‑PACz simultaneously strengthens intermolecular π–π stacking, enlarges the molecular dipole and ITO substrate work function, enhances lateral hole transport, and promotes donor‑enriched vertical phase separation near the anode. As a result, OSCs employing 2Cz‐PACz as the AIL achieved a power conversion efficiency (PCE) of 20.64% with a short‐circuit current density ( J sc ) of 28.06 mA cm −2 and a superior fill factor (FF) of 81.74% in D18:L8‐BO:BTP‐eC9 based devices. Our work demonstrates that π‐conjugation of functional groups is a generalizable molecular strategy for engineering multifunctional organic semiconductor interfaces.