Interfacial Trap State Suppression through a Highly Ordered Chlorinated Cathode Interlayer Strategy toward Efficient Organic Solar Cells

Balancing ordered molecular packing in cathode interlayers with the suppression of trap states along the electron transport pathway continues to be a major hurdle for high‐performance organic solar cells (OSCs). Here, a molecular design strategy that enables vertical ordering across the entire interlayer in a chlorinated perylene diimide (PDI‐2Cl) cathode interlayer is reported. Compared with a naphthalene diimide analogue (NDI‐2Cl), PDI‐2Cl possesses a larger conjugated core and stronger intermolecular π‑‐π interactions, leading to highly ordered packing that persists uniformly from the interface to the interior of the interlayer, as directly revealed by depth‑dependent GIWAXS. This structural superiority reduces energetic disorder and suppresses trap‑assisted recombination. Consequently, OSCs incorporating the PDI‐2Cl interlayer achieve a power conversion efficiency of 20.0%, substantially outperforming the NDI‐2Cl‑based counterpart (18.4%). This work highlights that engineering depth‑stable full‑layer ordering in cathode interlayers is an effective strategy to minimize transport‑pathway trap states and enhance device performance.