## 3 Results and Discussion
### The Roles of HMDS Coating of the QD on the Improvement of QD-LED Characteristics  
To clarify the effects of the HMDS-coated QDs on the electrical and optical characteristics of the QD-LEDs, we prepared the test samples without the glass capping layer (i.e., no encapsulation) as shown in Figure 2a. Patterned indium tin oxide (ITO) and aluminum (Al) were used as the anode and the cathode, respectively. The functions of poly(ethylenedioxythiophene): polystyrene sulphonate (PEDOT:PSS), poly(9-vinylcarbazole) (PVK) and ZnO are the hole injection layer (HIL), the hole transport layer (HTL), and the electron transport layer (ETL), respectively. Additionally, they were all spin-coated to make the multi-layered structure. All manufacturing processes were carefully performed under a controlled environment in the N\({}_{2}\) glove box to avoid the unwanted contamination or oxidation of QD-LEDs except for HIL PEDOT:PSS coating. The hole mobility of PVK employed as HTL is known to be \(2.5\times 10^{-6}\) cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\)[4]. On the other hand, the mobility of electron in ETL of ZnO is known to be \(1.8\times 10^{-3}\) cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\)[17], which is approximately 1000 times faster than the hole mobility in PVK. This significant discrepancy in mobility can cause the problem of charge imbalance in QD-LEDs: we attempted to find the solution of the charge imbalance in the QD-LEDs. The results of the charge imbalance in the QD  
for this type of problem via the emission profiling study and describe it later in this paper. Figure 2b demonstrates the energy levels of QD-LED shown in Figure 2a. In PVK, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are 5.5 and 2.0 eV, respectively. In contrast, the HOMO and the LUMO energy level for ZnO are 7.5 and 4.2 eV, respectively. In this scheme, an energy barrier for electron injection from the cathode (i.e., Al electrode) to the ETL layer of ZnO with the LUMO level of 4.2 eV is small enough, and therefore electrons can easily reach to CdSe/ZnS QDs. Now, electrons can be confined within CdSe/ZnS QDs since PVK provides a high enough energy barrier to block the injection of electrons to ITO. The efficiency of hole injection from ITO to CdSe/ZnS QDs is high since the energy barrier between the HOMO of PEDOT:PSS and PVK is small enough for hole injection, although a relatively high energy band offset exists between PVK and CdSe/ZnS QDs. The energy level for CdSe/ZnS QDs can effectively confine the charges (i.e., injected electrons and holes) within the QD emitting layer, resulting in the efficiency improvement of charge recombination. Data shown in Figure 2c-f show the effects of an HMDS treatment of QD-LEDs on the reliability, J-V characteristics, luminance, and efficiency, respectively. Note that the test samples used for these measurements were fabricated without the glass capping layer to monitor the roles of the HMDS treatment as quickly as possible. Results in Figure 2c illustrate the initial stage of luminance degradation with time: the change in luminance characteristics of QD-LED samples was investigated as a function of time while applying a constant current that set to obtain the initial luminance (L0) of 1000 cd/m\({}^{2}\). The test samples with HMDS-coated quantum dots (i.e., denoted by HMDS1 and HMDS2) exhibits a significant improvement in reliability. For instance, the luminance more rapidly degrades below 50% of the initial value with the stress time in the QD-LED samples without the additional HMDS coating process (i.e., denoted by Ref1 and Ref2). We suggest that this significant luminance improvement by the simple HMDS treatment is the curing effect of surface defects of CdSe/ZnS QDs by the SiO(CH\({}_{3}\))\({}_{3}\). In Figure 2d-f, the test samples made of CdSe/ZnS QDs without the HMDS treatment are denoted as Ref, while HMDS1 and HMDS2 are the test samples processed by the different concentrations of HMDS (refer to Table 1).