## 3 Results and Discussion
### The Emission Profiling Study for Characterizing the Flow of Charge
Regarding the case of Ref, we expect that the electron-hole recombination occurs mainly near the EML-PVK interface since the electron mobility in ZnO was 1000 times faster than the hole mobility in PVK. Furthermore, the energy difference between the LUMO level of PVK (2.0 eV) and the one of EML (4.3 eV) is big enough to block the electron injection into PVK. Consequently, PVK acts as an electron blocking layer (EBL), and therefore, the electrons accumulate near the EML-PVK interface, and start to emit as soon as the holes with relatively low mobility are finally transferred to EML. Regarding the case of HOD, the electron injected into EML directly from the cathode without assisting by ETL, resulting in the reduced electron injection. This phenomenon caused the decrease of EL intensity by three times. The emission occurs within EML regardless of the existence of ZnO in HOD because PVK still acts as EBL. In the case of EOD without PVK, we shall observe the charge-imbalance problem since the ZnO layer with high electron mobility effectively helps to inject the large number of electrons into EML. On the other hand, hole injection into EML decreases due to the lack of hole transfer from the anode without HTL. This charge-imbalance phenomenon reduced emission efficiency by 22 times as compared to the one of Ref. Additionally, we may expect the exciton formation does not strictly restrict within EML because of no PVK in EOD. As shown in the EL spectra, however, luminescence occurs mostly in EML, although the EL intensity is very weak. We interpret that both the fast electron mobility in ZnO and the low injection efficiency of the hole causes this observation. The majority of injected electrons that did not convert into light within EML then dissipated via a Joule heat consumption. In the case of ND, the light emission occurs at both ETL (i.e., the blue emission with the 410 nm wavelength in PVK) and HTL (i.e., the white emission with the 400-700 nm wavelength in ZnO) because we did not use QDs to make the samples. However, the intensities of all EL peaks were very weak, and especially the EL peak observed in the region of white wavelength of 400-700 nm was barely distinguishable. We anticipate that this phenomenon is due to the difference in carrier mobility between PVK and ZnO. The exciton recombination mainly occurred at the PVK region because the electron mobility in ZnO is faster than that of injected holes in PVK, indicating that the injected electrons from the cathode arrived at PVK very fast to recombine with the slowly moving holes. Data in Figure 4 and Table 2 represent luminance as a function of voltage and the maximum luminance of the test samples, respectively. Based on the experimental results in Figure 4 and Table 2, the emission profiling study using the various types of devices (i.e., Ref, HOD, EOD, and ND) helps us \begin{table}
\begin{tabular}{c c} \hline \hline
**Unit (cd/m2)** & **Max. Luminance** \\ \hline Reference (Ref) & 12,488 \\ Hole only device (HOD) & 4074 \\ Electron only device (EOD) & 562 \\ No-QD device (ND) & 25 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The maximum luminance for four devices. to understand how electrons and holes flow through the layers. First, PVK used as HTL has more significant roles in the luminous efficiency than that of ZnO used as ETL: the values of maximum luminance of both EOD without PVK and HOD without ZnO reduced up to the 1/22 and the 1/3 of Ref, respectively. Second, in the test samples except ND, the recombination of injected electron and holes via an exciton formation occurs mainly in the EML region, but very near the EML-PVK interface since the electron mobility in ZnO is 1000 times faster than the hole mobility in PVK that also acts as EBL. For the high efficiency of luminance, therefore, we recommend controlling the centroid of electron-hole recombination.