Multifunctional SAM Engineering Breaks the Efficiency‐Stability Trade‐Off in Inverted Organic Solar Cells

ABSTRACT The inverted architecture of organic solar cells offers great promise for scalable manufacturing and enhanced operational stability, yet its efficiency still lags behind that of conventional counterparts. A critical and often overlooked challenge lies in the instability of the hole‐transport interface, particularly the diffusion of MoO 3 into the photoactive layer, which leads to progressive performance degradation. Here, we demonstrate a multifunctional interface engineering strategy using a series of tailored SAM‐derived ultra‐thin films (SAMs)‐2PACz, MeO‐2PACz, and Br‐2PACz‐to simultaneously enhance efficiency and stability. By systematically increasing the electronegativity of the terminal group from ─H to ─Br, we strengthen the out‐of‐plane interfacial dipole verified by ultraviolet photoelectron spectroscopy (UPS), thereby improving energy level alignment and charge extraction. More importantly, the phosphonic acid anchoring groups in the SAMs form strong chemical bonding with evaporated MoO 3 and a robust barrier that effectively inhibits MoO 3 diffusion, as confirmed by defect density of states and XPS analysis. The resulting Br‐2PACz‐based devices achieve a champion PCE of 19.31% and retain 95.5% of their initial efficiency after 1000 h of continuous illumination. This work provides a universal and scalable interfacial design strategy to break the efficiency‐stability trade‐off in inverted OSCs, paving the way for their commercial realization.