Rational Design of a Perylene Diimide Cathode Interlayer for Enhanced Signal Detection in Organic Photodetectors

In this work, we demonstrate a rational cathode interlayer engineering strategy based on a newly designed perylene diimide derivative (PDI3) for high‐sensitivity organic photodetectors (OPDs). Unlike conventional amine‐functionalized cathode interlayers such as PDINN, PDI3 incorporates bulky branched alkyl chains and rigid fluorene units that enable controlled interfacial electronic interactions while maintaining efficient electron extraction. Electronic structure analyses reveal that PDI3 induces a strong interfacial dipole, significantly reducing the work function of the Ag electrode to −3.36 eV. This energy‐level realignment increases the hole‐blocking barrier to 2.32 eV, which effectively controls leakage current and suppresses externally injected holes. In addition, solvent‐dependent wettability and improved film‐forming characteristics of PDI3 lead to more uniform interfacial morphology and reduced trap density, thereby improving interfacial uniformity and signal stability. As a result, the PDI3‐based OPD exhibits a low dark current of 3.63 × 10 −9 A cm −2 and a high detectivity of 1.52 × 10 13 Jones. These findings provide insight into the coupled electronic and morphological roles of CILs, offering design guidelines for high‐sensitivity organic optoelectronic devices.