Bay‐Area Fluorobenzene‐Substituted Perylene Diimide Cathode Interlayer Enables Organic Solar Cells Exceeding 20% Efficiency

Abstract Rational molecular engineering of cathode interlayers (CILs) is critical for elevating the overall performance of organic solar cells (OSCs). Herein, two novel perylene diimide (PDI)‐based CILs, PDINN‐B2F and PDINN‐B3F, are designed. The fluorobenzene substituents at the bay positions of the PDI core effectively suppress excessive aggregation and improve film‐forming ability. Moreover, their strong electron‐withdrawing nature downshifts the frontier molecular orbital energy levels, enhancing intrinsic n‐type doping and enabling favorable energy alignments. The high electronegativity of fluorine also promotes robust interfacial interactions with active layer components, resulting in intimate contact and a more ordered molecular arrangement at the contact interface. These synergistic effects collectively promote efficient electron extraction, transport, and collection at the interface. Consequently, both PDINN‐B2F and PDINN‐B3F function as high‐performance CILs across diverse binary and ternary active layer systems. Remarkably, when 2PACz is used as the hole transport layer, non‐fullerene OSCs based on PDINN‐B2F and PDINN‐B3F achieve outstanding power conversion efficiencies (PCEs) of 19.56% and 20.36%, respectively, outperforming the PDINN‐based control device (PCE = 18.49%) in the PM6:D18:L8‐BO ternary system. Furthermore, the fluorinated CILs also endow the devices with excellent air and operational stability, offering a promising design strategy for high‐performance OSCs.