## 3 Results and Discussion  
The size of the synthesized PbS CQDs was estimated to be \(\sim\)3 nm from the transmission electron microscopy (TEM) image, as seen in Figure 1b. An energy band gap of 0.9 eV and a highest occupied molecular orbital (HOMO) energy level of 5.1 eV were determined from the optical absorption and PESA characterizations, respectively, as seen in Figure 1c,d, which is consistent with the size determined from the TEM image. During PESA measurements to measure the HOMO level of the PbS CQDs, the photocurrent was recorded in the photon energy ranging between 4.2 and 6.2 eV. We plotted the emission yield as a function of the incident photon energy to estimate the HOMO level, which is determined from the incident photon energy at which the effective emission is negligible. To prevent charging, we deposited PbS CQDs films on ITO-coated glass substrates. The temporal evolution of the EL intensity and current during light emission was simultaneously investigated through sampling measurements in which a constant voltage (1.5 V) over the turn-on voltage is applied. Figure 2a shows plots of the EL intensity as a function of the emission wavelength. Importantly, the EL intensity significantly increased during electrical biasing at the entire emission wavelength between 1100 and 1600 nm, while the current decreased, as seen in Figure 2b. We also observed that the abrupt increase in the EL intensity immediately after application of the electric field occurred with a significant decrease in the current in Figure 2b, which is addressed later. Simultaneous change in the magnitude of the current and the EL intensity provides a hint of the origin of the rise in the EL intensity during emission. We hypothesize that charge balance in the emission layer can be attained by electrical bias-induced modification of the interfacial electrical contacts, altering the magnitude of the injection current across the interface between PbS and ZnO CQD layers. It is well-known that non-radiative Auger recombination can be mitigated by suppressing QD charging, achieving charge balance in the emission layer [10,19]. To address the origin of the current variation during light emission, we carried out experiments in which the ITO/PEDOT:PSS/PbS/ZnO/Al device is charged and discharged by applying positive and negative voltages to the ITO side, respectively, for tens of seconds, followed by measurement of the EL intensity at a driving voltage of 10 V. Prior to the charging/discharging experiments, we confirmed that, at a high voltage over 10 V, the EL intensity increased with voltage and time (data not shown), indicating that electrical biasing at such a high voltage for the charging/discharging experiments has not damaged the device. In Figure 3a, the PbS QLED device was charged by applying a high  
Figure 2: (**a**) Plots of electroluminescence (EL) spectra as a function of time during light emission under application of 1.5 V to the indium tin oxide (ITO) side. (**b**) Plots of current and EL intensity as a function of time. positive (10 V) or negative voltage (\(-\)10 V) for 2 min. Notably, the measured EL intensity under application of 1.5 V, after applying a negative voltage, was far lower than that after applying a positive voltage, indicating that the effect of electrical pre-biasing on emission is significant. It is noted that reproducible results were obtained after applying smaller voltages (5 V and \(-\)5 V) for charging and discharging, respectively. A significant difference in the current over an order of magnitude was observed depending on the polarity of the electrical pre-biasing (10 V and \(-\)10 V for 2 min), as seen in Figure 3b. This suggests that the charge injection energy level between the transport (PEDOT:PSS or ZnO) and the emission (PbS) layer was altered, depending on the polarity of the electrical pre-biasing.