Optimizing Thin Films and Device Structures for the Design of PbS Colloidal Quantum Dots Near-Infrared Photodetectors

Lead sulfide (PbS) colloidal quantum dots (CQDs) have exhibited significant potential in the fabrication of near-infrared photodetectors owing to their robust and adjustable absorption. However, the high density of surface states in PbS CQDs exacerbates their intrinsic photocarrier dynamics, such as increased photocarrier recombination and charge trapping, thereby resulting in low detectivity, slow response speed, and heightened noise of devices. To achieve high-performance PbS-CQD photodetectors, this study focuses on optimizing the photocarrier dynamics through materials and device fabrication techniques such as surface engineering, energy band engineering, and device structures. The PbS CQDs were synthesized via hot injection, while the thin-film devices were fabricated through spin-coating followed by solid-state ligand exchange. Different ligands were employed to modulate the charge carrier transport, surficial defect states, and energy band structures of the PbS-CQD films. By managing energy band structures, bilayer PbS-CQD films with type-II heterojunctions were fabricated to elucidate the role of heterojunctions in photocarrier separation and device performance. Two types of device structures, photoconductors and phototransistors, were explored for their applications as photodetectors while studying their operation mechanisms. A theoretical analysis was conducted on photocarrier dynamics within these devices by a comprehensive comparative investigation on electronic characteristics of PbS-CQD films such as defect states, conductance, and energy band alignments at heterojunctions along with device performances including 1/f noise behavior, sensitivity, responsivity, detectivity, and operation speed. The experimental findings, combined with theoretical insights from this study, serve as essential references for optimizing material selection and designing efficient devices for photodetection based on PbS-CQDs.