Ultralow Dark Current in Lead Sulfide Colloidal Quantum Dots Near-Infrared Photodetectors Achieved through Trap Passivation with Efficient Surface Ligands Attachment

Dark current plays a significant role in deciding various device parameters like the on/off ratio, responsivity, detectivity, etc., in photodetectors. A lower dark current is desirable for a superior photodetector performance. Lead sulfide (PbS) colloidal quantum dots (CQDs)-based photodetectors suffer from trap-induced leakage current, resulting in a high dark current and significantly lowering the devices' performance. Here, we report a simple and efficient method to reduce the dark current of PbS CQDs-based photodetectors through trap passivation by forming bulk heterojunctions (BHJs) with zinc oxide (ZnO) nanocrystals (NCs) and ligand engineering. The suitable arrangement of the BHJ ensemble leads to a better ligand arrangement on the CQDs surface, resulting in a low dark current. Optimized mixing of ZnO NCs with PbS CQDs resulted in a dark current reduction from 15.92 to 0.12 nA/cm2 with trap state density reduction from 2 × 1016 to 5 × 1015 cm–3. The reduction results in nearly 2 orders of magnitude improvement in the on/off ratio and 1 order of magnitude improvement in the detectivity (∼1012 to ∼1013 Jones).