Characteristics of Ni-Doped IZO Layers Grown on IZO Anode for Enhancing Hole Injection in OLEDs

Abstract

The preparation and characteristics of a Ni-doped indium zinc oxide ͑NIZO͒ layer were investigated to enhance hole injection in organic light emitting diodes ͑OLEDs͒. A thin NIZO layer with a thickness of 5 nm was cosputtered onto an indium zinc oxide ͑IZO͒ anode using tilted Ni and IZO dual targets dc magnetron sputtering at room temperature in a pure Ar atmosphere. Using 3 W of Ni dc power, we can obtain a NIZO ͑5 nm͒/IZO ͑135 nm͒ double-layer anode with a sheet resistance of 30.04 ⍀/ᮀ and an optical transmittance of 83.8% at a wavelength of 550 nm. In addition, it was found that the work function of the NIZO layer was higher than that of a pure IZO anode due to the presence of a NiO x phase in the NIZO layer. An increase of Ni dc power above 7 W significantly degrades the electrical and optical properties in the NIZO layer. X-ray diffraction examination demonstrated that the NIZO layer consisted of an amorphous structure regardless of the Ni dc deposition power due to low substrate temperature. Furthermore, an OLED fabricated on the NIZO layer exhibited a higher current density, luminance, and efficiency due to improved hole injection by the high work function NIZO. These results indicate that the NIZO/IZO anode scheme is a promising anode material system for enhancing hole injection from the anode into the active layer of OLEDs. Indium tin oxide ͑ITO͒ films deposited on glass substrate have been academically and industrially used as transparent electrodes for organic light emitting diodes ͑OLEDs͒ and flexible OLEDs due to their visible transparency ͑ϳ90%͒ and low resistivity ͑2-4 ϫ 10 −4 ⍀ cm͒. 1-5 High-performance OLEDs and flexible OLEDs require a high-quality ITO anode layer with low resistance, high transparency, chemical stability, and high work function because hole injection and light-emitting properties are critically impacted by the ITO anode layer. 6 Unfortunately, conventional ITO anodes have several drawbacks, such as an imperfect work function alignment with organic materials, chemical instability, high process temperature, and easy deterioration of the ITO target. In particular, the high energy barrier between ITO and the highest occupied molecular orbit of organic materials prevents effective hole injection from the ITO into the organic layer. For these reasons, several approaches have been employed to increase the work function of ITO, including wet treatment, 7,8 plasma treatment, 9,10 UV ozone treatment, 11 selfassembly monolayer coating treatment, 12 and Ni doping of ITO. 13,14In particular, several groups have reported that Ni cosputtering with ITO or Ni doping on the surface region of an ITO can potentially enhance hole injection into OLEDs due to the effect of the p-type transparent conductive NiO x layer. [13][14][15] Because the NiO x layer is a well-known p-type conductor with high work function, it is more desirable for hole injection into the organic layer than ITO.16 Similar to ITO anode films, the investigation of Ni doping or cosputtering with indium zinc oxide ͑IZO͒ is therefor...

Stabilizing of Mechanical Property of Amorphous In−Zn−O Thin Films with Hydrogen Flow

“…There have been many attempts to improve the mechanical properties of TCOs, using dopants, heat treatment, and amorphization [6,[11][12][13][14][15]. Microcracks, which deteriorate the mechanical properties of polycrystalline structure, mainly depend on the grain boundary that generate microcracks that comes from defects [16][17][18][19].…”

“…Despite the higher mechanical stability of a-IZO, the mechanical stability of a-IZO needs improvement. To this end, many kinds of dopants such as heavy or alkali elements have been doped in IZO, such as W, Zr, Ga, Ni, Li, Na, and K [4,13,14,27,28].…”

Conductive properties of transparent Ni–In–Zn–O films deposited by combinatorial RF magnetron cosputtering

“…[6][7][8][9][10] Recently, a nickel-indium-zinc oxide (Ni-In-Zn-O) film has been reported to be a promising anode material for enhancing hole injection from the anode into the active layer of organic light-emiting diodes (OLEDs) because it possesses an unusually high work function (5.0-5.25 eV compared with 4.7 eV for ITO). 11) It also has a distinct metallic Ni luster when the Ni content in the Ni-In-Zn-O system is increased, enabling it to be applied as a TCO in smart windows in automobiles. In addition, a Ni-In-Zn-O system with the optimum composition could reduce the amount of heat rays and block solar radiation indoors and outdoors by cutting off infrared rays effectively.…”

Effects of rapid thermal annealing on electrical, optical, and structural properties of Ni-doped In2O3 anodes for bulk heterojunction organic solar cells

“…In our previous work, a very thin NiO film on an indium zinc oxide (IZO) film enhanced the performance of OLEDs due to the high work function of the NiO layer covering the IZO anode. 20 Although the electrical and optical properties of Ni-doped ITO films used as transparent electrodes were investigated, detailed studies of Ni-doped In 2 O 3 (INO) films are lacking. In addition, applications of INO films in OSCs as anode have not yet been reported.…”