## 3 Results and Discussion
Indeed, a subtle change at the interfacial energy level by electrical bias stress causes an exponential difference in the magnitude of the current, assuming the electric-field enhanced thermionic emission model [29; 30]. An interfacial energy level-dependent diode current was confirmed in the I-V curve of the ITO/PEDOT:PSS/PbS/ZnO/Al CQD LED below the turn-on voltage in Figure 4 in which the forward and reverse bias current was clearly observed. The formation of the forward and reverse bias regions suggests that the test structure we used consists of energetically well-defined interfaces for asymmetric carrier transfer. In the test structure with distinct electron and hole energy barriers, under the application of a positive or negative voltage to the ITO side, for example, carriers are selectively accumulated and depleted at the PEDOT:PSS/PbS and/or PbS/ZnO interfaces, modifying the interfacial energy level. During the charging process in Figure 3b at a high voltage (10 V), electrons are injected through Al and passivate the surface trap sites on the ZnO and PbS CQDs. The surface of ZnO CQDs are depleted of electrons due to adsorption of oxygen molecules as well as a high density of surface traps [31; 32; 33; 34]. A high injection of electrons fills and passivates the surface traps on the ZnO CQDs close to the PbS/ZnO interface. According to [PERSON] et al., indeed, reductive passivation of surface traps through electron injection over the trap energy level of ZnSe CQDs led to large photoluminescence electro-brightening in the ZnSe CQDs, implicating interfacial carrier charging in determining luminescence intensity [35]. On the other hand, PbS CQDs are very well-known to form dynamic traps through which electrons are depleted, as will be discussed later. In contrast to the charging process, the discharging process under application of \(-\)10 V depletes the charged electrons at the PbS/ZnO interface. Therefore, the subsequent bias at 1.5 V, after the charging process, causes a far higher current in comparison with that after the discharging process, as shown in Figure 3b. It is important to note that the diode current increases with time at a higher voltage (10 V) while the current decreases at a lower voltage (5 V, Data not shown). At a high electric field, injected electrons fill the trap states fast and begin to passivate the interface  
Figure 3: Voltage polarity (\(+\)10 V and \(-\)10 V) dependent EL intensity change. EL intensity in (**a**) was measured after application of \(+\)10 V and \(-\)10 V. The diode current in (**b**) was measured right immediately after application of \(+\)10 V and \(-\)10 V as a function of time under application of 1.5 V.  
while, at a low electric field, injected carriers are still in the process of being trapped. On the basis of the framework, the current decrease during the increase in the EL intensity at a low voltage (1.5 V) implicates electron trapping at the PbS/ZnO interface, achieving charge balance. In our previous studies, we investigated interfacial charging of PbS CQDs, finding that the time scale of carrier trapping associated with surface traps corresponds to tens of seconds which is consistent with that of the current and EL intensity change in Figure 2b [31; 32]. The long time scale over tens of seconds and the significant current change over an order of magnitude in Figure 3a in combination with the charging/discharging experiments in Figure 3b allow us to infer that the increase in the EL intensity during light emission is ascribed to the interfacial trapping of carriers at the PbS/ZnO interface. Indeed, suppression of the injection current due to interfacial charge trapping should be considered in articulating the origin of the EL intensity increase during emission. The net injection current in the disordered material systems embedding CQDs is determined by the difference between the injected current across interface and the surface recombination current [36; 37; 38; 39].