Manipulating a Bilayer Transport Layer to Improve the Charge‐Transport Pathway Enables Efficient Organic Solar Cells †

Comprehensive Summary PEDOT:PSS has been extensively employed as a hole transport layer in solution‐processed organic solar cells. However, its imperfect interfacial contact and energy level mismatch with the active layer restrict charge extraction and limit device performance. Herein, we demonstrate a novel interfacial engineering strategy by depositing an ultrathin D18‐Cl interlayer atop the conventional PEDOT:PSS hole transport layer. This strategy significantly improves the contact between the hole transport layer and the active layer, thereby forming a more favorable energy level alignment, thus enhancing charge extraction and reducing recombination losses. As a result, the optimized devices exhibit improved charge extraction, suppressed dark current density, and reduced charge recombination. Therefore, the binary OSCs based on PM6:Y6, PM6:BTP‐eC9, and PM6:L8‐BO treated with D18‐Cl, achieved power conversion efficiencies of 18.93%, 19.35%, and 19.92%, respectively. In summary, this study provides a practical approach to boosting OSC performance via rational interfacial design, paving the way for high‐efficiency photovoltaic technologies.