TiN as an Anode Material for Organic Light-Emitting Diodes

Abstract

Organic light-emitting diodes (OLEDs) are currently receiving a great deal of attention, both academically and commercially. These devices have promise in applications ranging from low information content alphanumeric displays, to high resolution, large area flat panel displays. The most common OLED structure consists of a substrate coated with a transparent conductor (indium tin oxide, ITO) coated with a thin film of organic material(s), followed by a vapor deposited metal cathode. When a potential is applied to the device, holes are injected into the organic material(s) from the ITO electrode and electrons from the metal cathode. The holes and electrons recombine in the organic material, giving excitons, which radiatively relax to give off light. The efficiency of the device is best if multiple organic layers are used, i.e. separate hole and electron transporting layers.The choice of anode material for OLEDs is based on several criteria. The anode in a conventional OLED must have good optical transparency, good electrical conductivity and chemical stability, and a work function that lies near the HOMO levels of the organic materials to which it will inject holes. ITO fits these criteria and is the most widely used anode material in OLEDs. ITO films combine high transparency (» 90 %) with low resistivity (1´10 ±3 ±71 0 ±5 W cm). [1,2] Thin films of ITO can be prepared by a variety of methods, including sputtering, chemical vapor deposition (CVD) and sol-gel techniques.[2] The work function of ITO (4.4±4.7 eV based on ultraviolet photoemission spectroscopy measurements) [3±4] lies near the HOMO levels of typical OLED hole transporting or injecting material, which leads to a small barrier for hole injection into the organic material. It has been found that the ITO electrode can be treated to reduce this energy barrier. For example, devices with lower drive voltage and higher brightness have been prepared by treating the ITO surface with an oxygen plasma [5] or aqua regia. [6] The authors stressed the relationship between the surface morphology of the ITO and the device behavior. The device stability and efficiency strongly depend on the nature of the anode/organic interface. [7] One of the causes of long term OLED degradation involves the diffusion of metal ions or oxygen from the ITO into the organic film.[8±10]By introducing an ultra-thin interlayer of different organic materials at the interface, the efficiency of the devices could be improved.[11] Application of ITO covered with polyaniline film in the same type of devices also gives better results in terms of carrier injection and electroluminescence efficiency. [12,13] Polyaniline film serves as a barrier that prevents oxygen from diffusing out of the ITO and thus stabilizes the electrode±organic interface. In addition to polymers, copper phthalocyanine (CuPc) [3] and aluminum oxide [14] have been used as hole-injecting buffer layers between the ITO electrode and hole-transporting materials in OLEDs. Although the ITO electrode can be used efficiently with m...

White organic light-emitting diode (OLED) microdisplay with a tandem structure

“…Instead, an aluminium (Al)/titanium nitride (TiN) bilayer compatible with the CMOS process is used as the bottom electrode for the OLED microdisplay [11]. Al contributes to low sheet resistance and high reflectance, and TiN contributes to the proper work function for using an anode in OLEDs [12]. To lower the operating voltage of tandem OLEDs, the selection of a hole injection layer (HIL) is important to efficiently inject holes from TiN to the HTL.…”

Autonomously Self‐Reporting Hard Coatings: Tracking the Temporal Oxidation Behavior of TiN by In Situ Sheet Resistance Measurements

“…Furthermore, this property needs to be measurable during application and application‐induced changes in structure and/or composition are required to result in a significant change in magnitude of the self‐reporting property. TiN was selected as a reference system since it is widely applied as protective coatings,6 diffusion barriers,7 or in microelectronics8 and has recently raised attention as plasmonic material 9. It shows a high thermal stability10 and metallic electrical properties,11 while its oxide TiO 2 exhibits a bandgap of ≈3.1 eV 12…”

ZnS/Ag/ZnS coating as transparent anode for organic light emitting diodes

“…However, ITO also has some disadvantages including the complex sputtering and high temperature production process and the diffusion nature of indium ions which is harmful to the long-term performance of OLEDs [4,5]. Aluminum-doped or Zirconium-doped zinc oxide [6][7][8], layered-transition metal dichalcogenides [9], Titanium nitride [10], semi-transparent gold [11], and conducting polymer [12,13] have been reported as potential candidates for replacement of ITO.…”