High-performance organic photodetectors (OPDs) require a simultaneous minimization of dark current and maximization of specific detectivity. In this study, we investigate the impact of cathode interlayers (CILs) on the device physics of OPDs, focusing on perylene diimide (PDI) derivative PDI3. We demonstrate that the interface dipole moments induced by these CILs determine the work function of the Ag cathode and the resulting charge injection barriers. Our results indicate that PDI3 induces a significantly lower effective WF (~3.35 eV) on the Ag surface. This shift creates a massive hole injection barrier at the cathode interface, leading to a substantial suppression of leakage current under reverse bias and a marked increase in shunt resistance. By analyzing the exceptional blocking capability of PDI3, this work provides critical design guidelines for interface engineering to achieve high-detectivity OPDs.