Two-step ligand exchange to promote charge transfer in PbSe quantum dot photodetectors for pulse monitoring

Quantum dots (QDs) have emerged as promising materials for next-generation infrared semiconductors due to their facile solution processing, low-cost, tunable bandgap and superior optoelectronic properties. However, organic long-chain ligands that modify the surface of QDs hinder charge transfer, thus impairing the performance of QD infrared photodetectors. Here, we report a two-step ligand exchange strategy that decouples the native long-chain ligands from the QDs using specific molecules and then attaches the short-chain ligands, resulting in high response for lead-rich lead selenide (PbSe) QD photodetectors. During the layer-by-layer film deposition process, 1-octanethiol is first used for primary ligand exchange to remove stable ligands, followed by 3-mercaptopropionic acid for secondary exchange to ensure thorough passivation of surface defects. The two-step processing PbSe QD photodetector has a responsivity of up to 1.28 A W^-1, a detectivity of 9.65 × 10¹² Jones and a record high external quantum efficiency of 144.4% at a bias voltage of 0.5 V at 1100 nm. Benefitting from the high performance, the PbSe QD photodetector can be integrated into a pulse monitoring platform, achieving a physiological sign monitoring by capturing real-time pulse signals of human superficial arteries.