## 3 Results and Discussion
### The Charge Balance Through the Thickness Adjustment of Both ETL and HTL  
Data in Figure 5 and Table 3 represent the effects of thickness adjustment of PVK and ZnO by adjusting the time of a spin coating process. As compared to the values of Ref, the thickness of ZnO in Sample 1 increased to delay the electron transport by reducing the spin-coating time from 60 to 30 s. On the other hand, when the spin coating time for PVK was increased from 30 to 60 s, the thickness of the PVK layer became thinner, thereby reducing the hole transport time from HTL to EML. Even though Sample 3 had the same thickness conditions as Sample 2, it did not blend QD and HMDS, resulting in low luminance and efficiency. The effectiveness of HMDS was again confirmed. Data in Figure 5a indicate that all four samples have both the similar EL peaks at the wavelength of 523-529 nm and the sharp emission spectra (e.g., FWHM of 22 nm). To measure the bandgap of QD, a Ultraviolet-visible (UV) spectrometry technique was used, and the wavelength from the absorption spectra in Figure 10 of the Appendix A was identified as 525 \(\sim\) 530 nm, and the calculated bandgap value was 2.3\(\sim\)2.4 eV. The frequency and bandgap energy of the absorption spectrum of QD with or without HMDS are summarized in Table 14.  
Figure 4: Luminance for the test devices employed for the emission profiling study. The inset shows green emission light observed in dark chamber with power supply and spectroradiometer and is also confirmed by the CIE 1931 chart in Figure 5b. As we expected, Sample 2 with both a thinner PVK and a thicker ZnO results in the highest EL intensity peak since the problem of charge imbalance was minimized as compared to the one of Ref. Additionally, it is interesting to note that the EL intensity of Sample 1 is higher than Ref but lower than Sample 2: the thickness of ZnO in Sample 1 was thicker than that of Ref so that we intentionally obtained the time delay for the electron transport to EML. However, the thicknesses of PVK in both Ref and Sample 1 were the same, suggesting that the difference in the EL intensity between Sample 1 and Sample 2 is due to the thickness difference of PVK. HTL and ETL of Sample 2 and Sample 3 have the same thickness, \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Item** & **Spin-Coating Conditions** & **Turn-On** & **Luminance** & **Efficiency** \\  & **(s)** & **Voltage (V)** & **(cd/m\({}^{2}\))** & **(cd/A)** \\ \hline Ref & PVK(30) and ZnO(60) & 5.5 & 12,005 & 11.9 \\ Sample 1 & PVK(30) and ZnO(30) & 5.0 & 35,270 & 29.8 \\ Sample 2 & PVK(60) and ZnO(30) & 5.0 & 53,125 & 32.4 \\ Sample 3 & PVK(60) and ZnO(30) & 5.0 & 41,179 & 21.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The effects of thickness change of both Poly 9-vinylcarbazole (PVK) and ZnO on current density, luminance, and efficiency. The best results were obtained from Sample 2 although the measured current density was the lowest one. QD without HMDS was used for Sample 3. Figure 5: The effects of thickness change of both PVK and ZnO: (**a**) the EL spectra, (**b**) CIE 1931 chart, (**c**) electroluminescence, (**d**) current density as a function of the driving voltage, (**e**) luminance, and (**f**) efficiency. The EL spectra showed a wavelength of 523–529 nm regardless of the sample, and it was found that it was in a similar position on the CIE 1931 diagram. When power is applied to the metal terminal, it can be seen that the green color has a wavelength of 520–530 nm from the electroluminescence. We designed this experiment to find the optimal thickness combination for minimizing the problems caused by the charge imbalance phenomenon. The efficiency was obtained by dividing the luminance by the current density, and was best when the PVK thickness was thin and the ZnO thickness was increased. but QD without HMDS was used for Sample 3. Electroluminescence in Figure 5c is consistent with experimental results of Figure 5a,b. Figure 5d,f shows the experimental results of current density, luminance, and efficiency, respectively.