Non-Fused Conjugated Alkylfluorene Bridged Naphthalimides Cathode Interlayer Enables Low-Noise Near-Infrared Organic Photodiodes

The cathode interface layers (CILs) play a crucial role in enhancing the performance of organic photodiodes (OPDs). However, the poor film formation, the tendency of orientation and aggregation, and contact in the interface of the bulk heterojunction (BHJ)/CILs/electrode increase the dark current, leading to a limitation of light detection. In this study, we designed and synthesized a nonfused conjugated small molecule (FNA) with dioctyl fluorene (F) as the π-bridge and N-dimethylaminopropyl-1,8-naphthalimide (NA) as terminal groups. As the CIL, it significantly modifies the interfaces of BHJ/CILs/electrode and minimizes dark current density (J_d). Using PTB7-Th:IEICO-4F as the BHJ, FNA-based OPDs achieve a remarkably low J_d of 1.12 × 10⁻⁸ A cm⁻² (at −0.5 V) and a peak specific detectivity of 8.23 × 10¹² Jones (940 nm), outperforming conventional devices with PDINN as CIL. These improvements are attributed to lower J_d by modified interfacial contact and increased photoresponse by reducing depletion region width in FNA-based photodiodes. Furthermore, uniform photoresponse in 5 × 5 OPD pixel arrays further validates its potential for imaging applications.