## 3 Results and Discussion
### The Emission Profiling Study for Characterizing the Flow of Charge  
By adding the HMDS coating step for minimizing the surface defects on QDs to the conventional fabrication process, we obtained the substantial improvement of QD-LED characteristics as discussed above. In addition to improving the QD properties themselves, we conducted the emission profiling study using the various types of devices (e.g., hole only device, electron only device, etc.) to understand how electrons and holes flow through the layers. Based on the data obtained from the emission profiling study, we can research how to improve current efficiency by achieving charge balance through the structural adjustment of QD-LEDs. We prepared various types of test samples for this purpose: for instance, in Figure 3, \"reference (Ref)\" represents the QD-LED device with the HMDS treatment process, \"hole only device (HOD)\" is the HMDS-treated QD-LED device without a ZnO ETL layer, \"electron only device (EOD)\" is the HMDS-treated QD-LED device without a PVK HTL layer, and finally \"no-QD device (ND)\" represents the test samples without the HMDS-treated QDs. Figure 3: A step for the emission profiling study for characterizing the charge flow through the layers consisting of the newly proposed QD-LEDs: the first, second, and third columns represent the Electroluminescence (EL) spectra, the Commission internationale de l’ éxiarage (CIE) 1931 chart, and the emission profile, respectively, for the various test samples. (**a**) is the measurement of the electroluminescence according to the voltage, and the emission color can be predicted from the peak value. (**b**) is a diagram that defines the wavelength distribution of the electromagnetic visible spectrum and the color perceived physiologically in human color vision. (**c**) shows the position where electrons and holes from the electrode move to the emission layer to form electron hole pairs and emit light through recombination. Ref, HOD, EOD, and ND are the abbreviations for the reference devices, the hole only devices, the electron only devices, and the devices without QDs, respectively. Note that the structure of HOD was the same as Ref except the ZnO layer that acts as both ETL and HBL. On the other hand, the structure of EOD was the same as Ref except the PVK layer that acts as both HTL and EBL. Using these schemes, we can study the roles of PVK and ZnO in the emission profile via the exciton formation. Where, Ref, HOD, EOD, and ND are the abbreviations for reference, hole only device, electron only device, and no-QD device, respectively. Data shown in the first column and the second column in Figure 3 are the electroluminescence (EL) spectra and the Commission internationale de l' eclairage (CIE) 1931 chart (CIE 1931) color space chromaticity diagram, respectively, of the test samples employed in this work for the emission profiling study. Based on the experimental results of the first two columns, we analyzed the emission profiles shown in the third column in Figure 3. Data shown in the first column in Figure 3 show the EL spectra of the various samples at the driving voltage from 4.25 to 9 V. All test devices except ND produced the EL spectra of 515-520 nm wavelength (i.e., green light emission). Nonetheless, the maximum luminance observed from the HOD and EOD samples was approximately 1/3 and 1/22, respectively, as compared to the one recorded from the Ref sample (refer to Table 2). The characteristics of ND were different from the other samples (i.e., Ref, HOD, and EOD): for instance, we observed the EL peaks with the wavelength of 410 nm (i.e., blue light emission), but the intensity was very weak. We also found a broad EL peak with the wavelength of 400-700 nm. The EL spectra of Ref, HOD, and EOD represent the same green light emission, suggesting that the exciton recombination occurred within EML.