Size Effects of 1,2-Ethanedithiol-Treated PbS Quantum Dots on Short-Wave Infrared Photodetector Hole Transport Layers

Colloidal quantum dots (QDs), notably lead sulfide (PbS) QDs, represent a promising platform for short-wave infrared (SWIR) photodetection, offering a cost-effective and scalable alternative to conventional indium gallium arsenide (InGaAs) systems. This study investigates the pivotal role of PbS QD size in optimizing the hole transport layer (HTL) for SWIR photodetectors, addressing the interplay among film morphology, electronic structure, and device performance. Through the precise synthesis of monodisperse PbS QDs (3.33-4.14 nm) and solid-state ligand exchange with 1,2-ethanedithiol (EDT), we reveal that smaller QDs, while benefiting from strong quantum confinement and superior electron blocking, suffer from pronounced volumetric shrinkage and microcracking due to high ligand-to-QD ratios. Conversely, larger QDs enhance film integrity but introduce surface-facet-dependent defects and increase dark current density. Combining transmission electron microscopy, absorption spectroscopy, photoluminescence quenching, and space-charge-limited current analysis, we elucidate the size-dependent trade-offs governing HTL functionality. Devices with intermediate-sized QDs (e.g., 4.04 nm) achieve peak external quantum efficiency (55.74%), responsivity (0.54 A/W), and specific detectivity (5.50 × 1012 Jones), while smaller QDs (3.33 nm) excel in trap state suppression and faster response speed (1.0 μs rise and 1.3 μs fall). These findings establish a materials-by-design framework for tailoring QD size to balance mechanical stability and optoelectronic performance, advancing solution-processed SWIR imaging technologies.