Ion Beam Deposition from Colloidal CdSe/ZnS Nanocrystals Solutions for Fabricating Electroluminescent Device
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1030-G07-03
Ion Beam Deposition from Colloidal CdSe/ZnS Nanocrystals Solutions for Fabricating Electroluminescent Device Yuki Tani1, Satoshi Kobayashi2, and Hiroshi Kawazoe3 1 R&D center, HOYA Corporation, 3-3-6 Musashino, Akishima-shi, Tokyo, 196-8510, Japan 2 HOYA Corporation, Tokyo, 196-8150, Japan 3 Kawazoe Frontier Technologies Corporation, Tokyo, 196-8510, Japan ABSTRACT We report on alternating current (AC)-driven CdSe/ZnS quantum dot (QD) inorganic electroluminescent (EL) devices prepared from a QD colloidal solution. The EL spectra of the devices reproduce the photoluminescent spectra of the source QD solutions. The time-resolved luminescent properties of the devices in ambient atmosphere at room temperature are analyzed for brightness and efficiency. The results suggest that the lifetimes of the excited states in QDs are affected by the electric field in the EL devices. INTRODUCTION Semiconductor nanocrystal quantum dots (QDs) have useful characteristics [1, 2], such as high luminescent quantum yield, spectrum tunability, and durability. Semiconductor nanocrystal QDs, which are prepared by colloidal nanosynthesis, are supplied as a solution. It is absolutely necessary that QD deposition methods for producing a solid luminescent layer of electroluminescent (EL) devices from a QD solution are carried out in vacuum. We have developed a new QD deposition method, called liquid-dispersed quantum-dots ion-beam deposition (LIQUID), which is compatible with a vacuum system. QD-EL devices have been successfully fabricated using the LIQUID technique [3, 4]. CdSe/ZnS QD-EL devices are different from QD-activated organic light-emitting diodes (OLEDs) and QD light-emitting diodes (LEDs) [5–8] because QD-EL devices are made of inorganic materials and driven by an alternating current (AC). The device structure is shown in Fig. 1. One of the features of the devices is their low process temperature. It is possible to deposit films on plastic substrates such as polycarbonate. A second feature is the electrode selectability. The device is categorized as an inorganic EL, and charge injection from the electrodes is unnecessary. Totally transparent QD-EL devices can be fabricated by using transparent conductive oxide as the electrodes. A third feature is the durability of the devices because of the use of inorganic materials. The devices are humidity- and temperatureresistant [9]. In this report, we study the transient luminescent properties and the frequency dependence of the EL to improve the QD-EL devices.
EL DEVICES Figure 1 shows the structure of the QD-EL device. The devices are made of all inorganic materials. A typical device consists of a stack of thin films, e.g., electrode / tantalum oxide (TaOx) (200 nm) / ZnS (5–10 nm) / QDs (10–100 nm) / ZnS (5–10 nm) / TaOx (200 nm) / indium tin oxide (ITO) electrode (100 nm) / substrate. The light-emitting layer of QDs is sandwiched between buffer layers of ZnS and insulator layers of TaOx. They are driven by an AC voltage. The layer of QDs is prepared from a solution of CdSe/
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