Characteristics of Ni-doped IZO layers grown on IZO anode for enhancing hole injection in OLEDs

The preparation and characteristics of a Ni-doped indium zinc oxide (NIZO) layer were investigated to enhance hole injection in organic light emitting diodes (OLEDs). A thin NIZO layer with a thickness of 5 nm was cosputtered onto an indium zinc oxide (IZO) 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 nm)/IZO (135 nm) double-layer anode with a sheet resistance of 30.04 Omega/square 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. (C) 2008 The Electrochemical Society.