## 3 Results  
A TEM image of the synthesized ZnO NPs is shown in Figure 1a. The ZnO NPs are nearly spherical and have an average diameter of 4 nm. Figure 1b shows the XRD pattern of the ZnO NPs together with that of the bulk phase ZnO. A comparison of the diffraction peaks suggests that the ZnO NPs are crystalline and adopt the wurtzite structure of the bulk ZnO. The small particle size leads to a significant broadening of the characteristic diffraction peaks from the ZnO NPs. Figure 2a shows an illustration of two different spin-coating processes for the ETL using ZnO NPs. Generally, the ETL is formed by a one-step spin-coating process (1 SC, 4000 rpm for 60 s). However, in this study, one more step at a faster speed was added before the maximum rpm. Thus, the devices were fabricated with a two-step spin-coating process (2 SC, 500 rpm for 5 s \(\rightarrow\) 4000 rpm for 60 s). The transmittance spectra for the ZnO layers with different step schedules are shown in Figure 2b. The transmittance of both ZnO layers is more than 80% [11] and there is no significant difference between the layers throughout the entire visible region. Figure 3 shows the AFM images of the ZnO layer on patterned ITO glass with different step schedules of the spin-coating process. The morphology of the ZnO layer was investigated to determine its effect on the optical and electrical performance of the QLEDs. The surface roughness of the ZnO layer using the 2 SC was increased due to the addition of a second step. Figure 4 presents a schematic device structure of the inverted QLEDs, which consist of ITO (cathode)/ZnO NPs (70 nm)/green QDs (12 nm)/CBP (40 nm)/MoO\({}_{3}\) (10 nm)/Al (anode) [12]. There was no significant difference in the thickness of both ZnO layers with 1 SC and 2 SC (Figure S2). In contrast to the standard QLEDs, electrons are injected from the ITO, then transported to the QDs (EML) through the ZnO NPs (ETL) and holes are injected from the Al, then transported to the QDs (EML) through the MoO\({}_{3}\) and CBP (HTL). Due to the low conduction band and valence band edge (\(-4.3\) and \(-7.6\) eV, respectively) of ZnO, the electrons move easily from the ITO to the EML while the holes are blocked at the interface between the EML and the ETL [3]. This is a huge advantage for highly efficient QLEDs because a higher rate of radiative recombination can occur in the EML. Figure 1: (a) transmission electron microscopy (TEM) image and (b) X-ray diffraction (XRD) pattern of ZnO nanoparticles (NPs). Figure 2: (a) Schematic description of two different spin-coating processes for the electron transport layer (ETL) using ZnO NPs and (b) transmittance spectra of ZnO layers on patterned ITO glass substrate. Figure 5a represents the voltage-dependent variations of the current density of QLEDs with different step schedules of the spin-coating process. The current density is significantly reduced by the additional step of the spin-coating process. At a voltage of 5 V, which is close to the turn-on voltage, the current density of the device with 1 SC is 19.71 mA/cm\({}^{2}\), while that with 2 SC is only 0.27 mA/cm\({}^{2}\). Figure 5b shows the current efficiency as a function of the luminance of QLEDs with different step schedules. Interestingly, the devices with 1 SC are brighter and less efficient than those with 2 SC (maximum luminance: 160,030 vs. 85,083 cd/m\({}^{2}\) and current efficiency: 23.33 vs. 55.23 cd/A, respectively). This is because the difference in the current density is much greater than the difference in the luminance. In detail, the maximum luminance of the devices with 1 SC is approximately twice as bright but the current density is also more than ten times higher compared to the devices with 2 SC. Figure 6 presents the current density-voltage characteristics of electron-only devices (EODs). The current density of EODs with 2 SC was also reduced like whole devices.