| Summary: | 碩士 === 正修科技大學 === 電子工程研究所 === 100 === Recently, organic light-emitting diodes (OLEDs) have attracted a great attention due to their applications in full color displays, backlights, and solid-state lighting sources. In order to improve the optical and electrical properties of OLEDs, the hole injection layer (HIL) is necessary for enhancement of hole injected efficiency between the indium tin oxide (ITO) anode and the organic hole transport layer (HTL). In this study, the metal oxide with the appropriate thickness was utilized to increase the surface flatness and then improve the efficiency of hole injected from ITO films. Thus, the insertion of the HIL induces an enhancement of the characteristic of OLEDs. Several metal oxides, for example, NiO, RuO, MoO3, and WO3 have been used as the HIL in OLEDs in the literature.
In this study, we examine hole injection performance in the OLED structure and apply the nickel oxide (NiO) as a HIL due to its superior performance, such as low driving voltage, low sheet resistance, and low surface roughness. In addition, with its excellent chemical stability, optic, and electrical characteristics, NiO can be a good injection layer for OLEDs. First, the Ni thin films were prepared by using thermal evaporation system. After the deposition, the Ni thin films were then annealed using furnace by controlling the various annealed time and temperature. The transmittances of the NiO thin film were investigated by using spectrophotometer. Furthermore, OLEDs with the insertion of NiO as the HIL were fabricated and the structure is ITO/NiO/NPB/Dpvbi/Alq3/ LiF/Al. For comparison, OLED devices without NiO as the HIL were also fabricated as control and the structure is ITO/NPB/ Dpvbi/Alq3/LiF/Al. As expected, with NiO as a HTL of optimized thickness and annealed temperature, the driving voltage can be effectively decreased as compared to that without NiO HIL. With a 5 V applied voltage, the current density for OLEDs without NiO HIL is 177 mA/cm2, while the current density for OLEDs with NiO HIL is 563 mA/cm2. With the same current density, the driving voltage is reduced from 5 to 3 V when a thin layer of NiO is inserted between the ITO and NPB as the HIL.
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