Ultralow Dark Current\nin Lead Sulfide Colloidal Quantum\nDots Near-Infrared Photodetectors Achieved through Trap Passivation\nwith Efficient Surface Ligands Attachment

Dark current plays a significant role in deciding various\ndevice\nparameters like the on/off ratio, responsivity, detectivity, etc.,\nin photodetectors. A lower dark current is desirable for a superior\nphotodetector performance. Lead sulfide (PbS) colloidal quantum dots\n(CQDs)-based photodetectors suffer from trap-induced leakage current,\nresulting in a high dark current and significantly lowering the devices’\nperformance. Here, we report a simple and efficient method to reduce\nthe dark current of PbS CQDs-based photodetectors through trap passivation\nby forming bulk heterojunctions (BHJs) with zinc oxide (ZnO) nanocrystals\n(NCs) and ligand engineering. The suitable arrangement of the BHJ\nensemble leads to a better ligand arrangement on the CQDs surface,\nresulting in a low dark current. Optimized mixing of ZnO NCs with\nPbS CQDs resulted in a dark current reduction from 15.92 to 0.12 nA/cm<sup>2</sup> with trap state density reduction from 2 × 10<sup>16</sup> to 5 × 10<sup>15</sup> cm<sup>–3</sup>. The reduction\nresults in nearly 2 orders of magnitude improvement in the on/off\nratio and 1 order of magnitude improvement in the detectivity (∼10<sup>12</sup> to ∼10<sup>13</sup> Jones).