## 3 Results and Discussion
Many studies have reported that the electrical contact resistance at the interface involving CQDs is governed by the charge carrier mobility as well as the interfacial energy level [40; 41]. PbS CQDs possess a high density of surface charge trapping states originating from unpassivated cationic lead sites and oxide species such as PbSO\({}_{3}\) and PbSO\({}_{4}\) (electron traps), and anionic sulfur sites (hole traps) [42; 43; 44; 45; 46]. It is important to emphasize that Auger recombination as well as CQD charging is mitigated due to the reduction in the net injection current. According to [PERSON] et al., the onset of the EQE roll-off in the CQDs with reduced Auger recombination rate starts at higher currents with increased emission efficiency [12; 19]. They demonstrated that the EQE roll-off in the CdSe/CdS structure is not affected by electric field-induced luminescence quenching due to spatial separation between electrons and hole. In other words, the optimum electric field at which the degree of charge balance is maximized exists, and a higher value of EQE is obtained, as the degree of CQD charging is reduced. Indeed, charge balance state was improved by increasing the interfacial energy barrier through the introduction of a thin outer shell (Zn\({}_{0.5}\)Cd\({}_{0.5}\)S) between the CdS (shell) and ZnO. Suppression of electron injection into the emission layer by the outer shell exhibited far improved roll-off behavior. To elucidate the origin of the EL intensity increase, we traced the emission spectra in Figure 5a. The change in the EL intensity peak wavelength is plotted as a function of time under the application of a voltage in Figure 5b. The wavelength at the emission peak was, importantly, blue-shifted from 1450 to 1350 nm in 2 min, ruling out the possibility of electric-field-induced quantum-confined Stark effect [47]. The observed blue-shift could be a signature of multi-carrier emission, contributing to an increase in the EL intensity [12]. In a previous study, for example, the core/alloy layer/shell QD  
Figure 4: PbS infrared LED I-V characteristic curve. device showed a blue-shift of the EL spectra, which is translated into the contribution arising from multi-carrier emission increased [12; 19]. In the CdSe/CdS QDs, a fraction of electrons and holes are separated in the core/shell structure. In case that multi-carrier emission takes place in the structure, spatial distribution of carriers causes repulsion, increasing the energy of the multi-carrier system. We eliminate the Joule-heating effect as a possible origin because the EL intensity increased despite a blue-shift [48; 49; 50]. The presence of competing mechanisms decreased the EL intensity in 2 min, as seen in Figure 6a. Suppression of the blue-shift occurred with a decrease in the EL intensity, as seen in Figure 6b, indicating that electric field-induced luminescence quenching mechanisms were activated, involving spatial separation of the electrons and holes, QD charging, and carrier coupling to surface states. In this regime, we speculate that trapping of the injected carriers into the localized surface states, i.e., carrier coupling to surface states, has a significant effect on the decrease in the EL intensity. Importantly, PbS CQDs have been reported to form dynamic traps under high electric field, originating from field-induced ionization of capping molecules and nano morphology change of the CQD surface [43]. In our previous study [31], indeed, we observed a time constant of tens of seconds in the time-domain measurements for pentacene/PbS CQD bilayer field-effect transistor devices, allowing us to connect the EL decay under high electric field to dynamic trap formation. From a far greater time constant than that with static electronic traps of fs to ms, we argued that dynamic traps originating from impurities associated with hydroxyl and hydrogen are present on the PbS CQD surface.