Temperature-Optimized Liquid-Phase Iodide Ligand Exchange Enables Low-Trap Solution-Processed PbS Quantum Dot Photodetection at 940 nm

PbS quantum dots (QDs) synthesized with oleic acid (OA) ligands suffer from poor charge transport in solid films, necessitating ligand exchange to shorter halide ligands for optoelectronic applications. This study investigates how ligand-exchange temperature governs OA-to-iodide substitution in PbS QDs. At 40 °C, the QD surface shows maximized halide passivation (I/Pb = 0.60) and minimized oxygen-related species (O/Pb = 0.23), suggesting reduced oxygen-associated defect formation and enabling n-type band alignment and reduced trap-mediated losses. PbS QD photodetectors fabricated from the 40 °C-treated QDs have 52% external quantum efficiency (EQE) at 940 nm (vs. 39% at 25 °C), with a responsivity of 0.394 A/W and an estimated detectivity of 2.1 × 10¹³ Jones. Temperature optimization of ligand-exchange provides a straightforward lever to improve device performance and reproducibility.