A green perovskite-quantum-dot (Per-QD) light-emitting diode with high efficiency and color purity was fabricated. This device exhibited a narrow spectrum with a half width of 19 nm and covered the BT.2020 color gamut. Improving an electron-transport layer increased the external quantum efficiency to 6.2%.

Light emission of a quantum-dot (QD)-based device using the new buffer layer has been successfully demonstrated through the injection of the carriers directly into a QD layer. As the energy of the HOMO level of a material used for the buffer layer becomes deep, the emission with higher intensity and efficiency is observed. Author KeywordsQuantum-Dot; OMOx; QLED; Injection barrier; Hole transporting layer Objective and BackgroundAn organic light-emitting diode (OLED) display, which exhibits a high luminance, a high contrast, and an extremely high response speed, has already become commercially practical; for example, OLED-based smartphones and televisions have been manufactured. Recently, displays that take advantages of the unique features of OLEDs, such as bendable displays, have also been reported [1][2][3][4][5].Despite the impressive progress, further improvement in device performance, cost reduction, and increase in size are still necessary to enable efficient mass production of OLED displays. In particular, the resolution has shifted from 4K to 8K in the research stage, and a wide color gamut such as BT.2020 must be rapidly achieved to provide a more realistic image. Due to intrinsically broad spectra of organic emitters, including phosphorescent emitters, OLED devices have to face hard challenges. Therefore, new materials that allow easy color adjustment and exhibit an emission spectrum with a narrow half-width must be developed.Quantum dots (QD) have recently attracted increased attention as potential materials for OLED applications. A QD is a semiconductor nanocrystal with a size-dependent band gap and optical and electrical properties that are significantly different from those of the other organic materials [6][7][8][9]. Previous studies have found that QDs exhibit the following excellent properties: emission color is adjustable by changing the grain diameter, the half-width is narrow, and the stability is high. QD layers can be easily formed using solution methods; thus, QD-based devices are advantageous from the standpoint of cost reduction and the easy fabrication of large displays. These favorable features have motivated the current effort to develop a QD-based lightemitting diode (QLED) as next-generation light-emitting devices that are highly reliable and suitable for mass production.The design of QLED structures has been based on OLED device architecture. It must be noted, however, that the energy of the top of valence band (E VB ) and the bottom of conduction band (E CB ) of a QD are located at lower energies than the HOMO and LUMO levels of a typical OLED material, respectively. This leads to easy electron injection but makes hole injection difficult in a carrier-injection-type devices such as the OLED. Therefore, the typical OLED architecture is not suitable for QLEDs. Although many ZnObased inverted-structure QLEDs have been reported, there are still some problems regarding the design of the hole transport layer (HTL) and the hole injection layer (HIL) in this type of device structur...