PbS Quantum Dots Passivated by High Concentration of PbBr2 and Their Photovoltaic Properties

Objective 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 2.03x1011 Jones respectively, the external quantum efficiency of 25.77%, increase time of 240 mu s, and decrease time of 180 mu s for the device. This ligand exchange method provides an opportunity to obtain high-performance solutions for infrared photodetectors. Methods 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 degrees C with an overnight stir. Next, 210 mu L of bis (trimethylsilyl) sulfide and 10 mL of 1-octadecene solution are rapidly injected into the three-necked flask at 120 degrees C , 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. Results and Discussions 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 2.03x10(11) Jones, as shown in Figs. 5(e)-(g). Conclusions 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 2.03x1011 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.