## 1 Introduction  
An organic light emitting diode (OLED), in general, has a multilayered structure in which an electric field is applied to a light emitting layer sandwiched between an electron and a hole transport layer [1; 2; 3; 4; 5; 6; 7]. It has been issued that OLED must tackle many problems such as a burn-in phenomenon originated from the use of organic materials, the oxidation problem of an organic layer [8], the recrystallization of hole transport layer [9; 10; 11], the observation of black dot at cathode [12; 13; 14], and the yield issues in large area fabrication. Several strategies to overcome the problems observed in OLED have been suggested for various device architectures and mainly focused to highlight the high performance and efficiency of the system [15; 16]. Recently, various studies using quantum dots (QDs) as the light emitting layer has attracted intensive attention [17; 18; 19; 20; 21]. The emission wavelength is dependent on the size of QD which has a narrow full width at half maximum (FWHM), resulting in both the controllability of color modulation and the high intensity of color purity. Nevertheless, the performance and reliability issues relating to QDs must be clarified prior to their wide adoption in the industry. A recent trend in quantum dot light emitting diodes (QD-LED) research has mainly focused on improving the properties of the hole transport layer (HTL) and the electron transport layer (ETL) for improving the performance of devices. For example, [PERSON] et al. reported the way to make the charge balance both by introducing the stepped HTL layer to enhance the hole injection and by inserting an insulating layer between the emission layer (EML) and ETL to delay the electron transport in ETL [17]. On the other hand, [PERSON] et al. inserted polyethylenimine ethoxylated (PEIE) between HTL and EML as the surface modifier of QDs, and reported the improvement of the hole injection efficiency by directly controlling the level of the valance band [18]. Besides, it is known that the doping of Ca ion [19], Al [20], or Mg [21] to the ZnO layer helps to improve the electron injection efficiency and the exciton lifetime. [PERSON] et al. reported that the 8% doping of Ca ion into ZnO improves the efficiency of QD-LEDs [19]. Specifically, the doping of Ca ion not only improves the efficiency of electron injection by controlling both the energy level and the electrical conductivity of ZnO but also effectively suppresses the separation of excitons at the interface between QDs and ZnO. [PERSON] et al. [20] and [PERSON] et al. [21] also reported the similar effects from the QD-LEDs with the ZnO ETL doped using Al and Mg, respectively. In this study, unlike the previous research effort to focus mainly on the property improvement of either HTL or ETL by other research groups, we propose two new approaches to obtain highly efficient and charge-balanced QD-LEDs. The first approach is to improve the property of QDs itself used for the EML layer by a simple mixing process of CdSe/ZnS core-shell QDs with hexamethyldisilazane (HMDS) under atmospheric conditions. Additionally, the second approach is to optimize the structure of QD-LEDs to achieve the charge balance through the emission profiling analysis.