## 2 Materials and Methods  
PbS CQDs were synthesized by the hot injection method [28]; 1.45 mL of oleic acid (Sigma Aldrich, 90%) and 18.5 mL of 1-octadecene (ODE, Sigma Aldrich, 90%) were added to 0.45 g of lead (II) oxide (Sigma Aldrich, 99.999%) in a three-neck flask until the solute (PbO) is completely dissolved and becomes transparent. Temperature is increased to the target reaction temperature followed by injection of a hexamethyldisilathiane (TMS, Sigma Aldrich)-ODE solution containing 110 \(\upmu\)L of TMS into 15 mL of ODE. The three-neck flask is immersed into ice water immediately after the reaction is completed, followed by a centrifugation process. For CQD washing, we centrifuged the synthesized solution, separating unreacted material. The supernatant was transferred to new tubes and centrifuged at 9000 rpm for 10 min. The precipitate is re-dispersed in hexane. We repeated the washing process several times and the sediments were re-dispersed in chlorobenzene (10 mL) to obtain a PbS CQD (-40 mg/mL).  
To fabricate PbS CQD LEDs as structured in Figure 1a, PEDOT:PSS (70 nm, M124 HTL Solar from Ossila) was spin-coated onto the patterned indium tin oxide (ITO, 70 \(\Omega\Omega\)) substrate at 6000 rpm for 60 s. PbS CQD solution in chlorobenzene (40 mg/mL) was spin-coated onto the PEDOT:PSS film at 1000 rpm for 30 s. As an electron transport layer, ZnO CQDs were synthesized. Zinc acetate dehydrate (5 mmol) in Dimethyl sulfoxide (DMSO) was stirred for 10 min at room temperature. Simultaneously, 5 mmol of a tetramethylammonium hydroxide pentahydrate (TMAH) (0.968 g) in ethanol was prepared followed by being added to the zinc acetate solution. After the injection process, the mixed solution was cooled in air. For washing process, we added acetone (35 mL) into the solution (10 mL), followed by centrifugation at 9000 rpm for 10 min. After centrifugation, the solvent was removed and ZnO CQDs were dried for 1 min. We added ethanol (10 mL) into the conical tube containing ZnO CQDs for another centrifugation. After washing, ZnO solution was spin-coated at 1000 rpm for 60 s, producing a thickness of ~60 nm followed by thermal evaporation of aluminum (Al) electrodes [5]. Using photoelectron spectroscopy in air (PESA) and optical absorption spectroscopy, the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) energy levels were calculated.  
The EL intensity of a PbS NIR QLED device was monitored at a driving voltage applied to the ITO side. With a photo-spectrometer ([PERSON] TG-cooled series) and a semiconductor parameter analyzer (Keithley 2400), the EL intensity and the electrical current were plotted as a function of time, respectively, during light emission at a particular voltage over the turn-on voltage.  
Figure 1: (**a**) Schematics of PbS infrared light-emitting diodes (LED) structure and the energy band diagram. (**b**) transmission electron microscopy (TEM) image of the lead sulfide (PbS) colloidal quantum dot (CQDs) synthesized. (**c**) Tauc plot of a PbS film. An energy band gap of 0.9 eV was calculated. (**d**) A Plot of emission yield as a function photon energy for a PbS CQD film. A highest occupied molecular orbital (HOMO) energy level of 5.1 eV was calculated.  
Interfacial charging/discharging experiments were carried out by applying positive and negative voltages to the ITO side.