Lead sulfide quantum dots (QDs) have become one of the most promising new materials for photodetector fabrication due to their low fabrication cost, solution processability, and tunable spectral range. Small-sized PbS QDs have yielded excellent performance in optoelectronic devices by the mixed lead halide ligand exchange method. However, as the diameter of the PbS colloidal QDs (CQDs) increases, the proportion of the originally employed small-sized mixed lead halide ligands cannot fully passivate the (100) crystal plane of the large-sized QDs, which introduces deep defects and degrades the device performance. We propose a ligand exchange method with higher PbBr₂ concentration to fully passivate large-sized PbS CQDs with absorption peaks at 1550 nm. As the PbBr₂ concentration (0.01?0.06 mol/L) increases, the defects are effectively passivated, with the decreased dark current. At PbBr₂ concentration of 0.04 mol/L, the prepared device shows the best performance, with the responsivity and specific detectivity at 1550 nm of 324.259 mA/W and 2.03×10¹¹ Jones respectively, the external quantum efficiency of 25.77%, increase time of 240 μs, and decrease time of 180 μs for the device. This ligand exchange method provides an opportunity to obtain high-performance solutions for infrared photodetectors.

We focus on the synthesis of PbS QDs, liquid-phase ligand exchange, and photodetector preparation. In the synthesis, lead sulfide QDs are synthesized by the thermal injection method, in which 0.45 g of PbO, 16.2 g of oleic acid, and 10 g of 1-octadecene are added to a triplex flask to obtain the lead source precursor. Then the triplex is evacuated at room temperature for 60 min, and then the flask is heated to 110 ℃ with an overnight stir. Next, 210 μL of bis (trimethylsilyl) sulfide and 10 mL of 1-octadecene solution are rapidly injected into the three-necked flask at 120 ℃, and then acetone is injected into the three-necked flask after lowering temperature to room temperature to purify the QDs. Then, the QD solution is washed three times and then filtered, with hexane added next to configure the n-hexane solution of PbS QDs. In the liquid-phase ligand exchange process, we dissolve 0.23 g of lead iodide, 0.01835 g (0.01 mol/L), 0.0367 g (0.02 mol/L), 0.05505 g (0.03 mol/L), 0.0734 g (0.04 mol/L), 0.09175 g (0.05 mol/L), and 0.1101 g (0.06 mol/L) of lead bromide and 0.009 g of sodium acetate (NaAc) in 5 mL of N,N-dimethylformamide (DMF), and add 35 mg of PbS QDs to the above solution. Then we add 10 mL of n-hexane, and after the ligand exchange, n-hexane is adopted to conduct washing for three times. Meanwhile, toluene is added to make the QDs precipitate by centrifugation and then fabricated into a 300 mg/mL QD ink standby. During the photodetector preparation, the device structure is ITO/ZnO/PbS QDs/PbS-EDT/Au, ITO is selected as the bottom electrode, and a ZnO film with a 30 nm thickness is prepared on ITO, after which liquid-phase ligand-exchanged QDs with a 300 nm thickness are spin-coated. Then a PbS-EDT layer with a thickness of about 70 nm is adopted for the preparation, and finally, a layer with an 80 nm thickness is deposited by thermal evaporation, with a gold electrode of an 80 nm thickness deposited by thermal evaporation.

The roughness statistics of the QD films after liquid-phase ligand exchange are shown in Fig. 3(g), which reveals that the roughness of the films shows a tendency to decrease and then increase with the rising concentration of PbBr₂, with the lowest roughness of the 0.04 mol/L PbBr₂ film being 0.851 nm. The results of the tests on XPS are shown in Fig. 4, and the detected iodide ions and bromide ions indicate that the short-chain ligand successfully replaces the long-chain ligand with the successful ligand exchange. Meanwhile, the higher Br^- concentration of the lower carbon content of the QD film implies that the replacement of the original long-chain ligand is more appropriate, with more successful ligand exchange. We further fabricate photodiode devices and investigate the effect of light absorbing layers passivated by different concentrations of PbBr₂ on the device performance. By counting the dark current at -0.5 V, as shown in Fig. 5(c), it can be seen that the dark current of the devices can be effectively reduced by modulating PbBr₂, and the devices reach external quantum efficiency of 25.77% at a PbBr₂ concentration of 0.04 mol/L. Specifically, the response degree R reaches 324.259 mA/W and the specific detectivity D^* is 2.03×10¹¹ Jones, as shown in Figs. 5(e)?(g).

The incomplete passivation of the first exciton absorption peak in the liquid-phase ligand exchange of large-sized PbS QDs at 1550 nm is solved by increasing the PbBr₂ concentration in the liquid-phase ligand exchange and adjusting the lead bromide concentration. Additionally, the device is prepared with a detectivity as high as 2.03×10¹¹ Jones, an external quantum efficiency as high as 25.77%, and the response degree of 324.259 mA/W. This shows excellent optoelectronic performance and provides a feasible solution for obtaining high-performance PbS QD photodetectors.